Article

Intramolecular charge transfer of π-conjugated push–pull systems in terms of polarizability and electronegativity

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Abstract

We have derived a simple expression to evaluate the amount of intramolecular charge transfer ICT of -conjugate push–pull systems from the properties of electronegativity and polarizability of the corresponding push and pull systems. This simple model is verified from ab initio calculations of disubstituted benzenes, stilbenes, and butadienes push–pull systems and their monosubstituted push or pull systems with various donors and acceptors. The bond length alternation BLA is often used as a good structural parameter to describe the amount of ICT; however, it is not a complete parameter because the amounts of ICT for the same sets of donor/acceptor pairs are different for different bridge systems. Here, we report a parameter composed of polarizability and electronegativity to give a consistent amount of ICT for different bridge systems. In particular, when a highly electropositive donor is used, the polarizability of an acceptor is the most determining factor for ICT. On the basis of this model, we find a very strong acceptor with large polarization effect, CHvC 5 H 4 , which gives a large nonlinear optical NLO response when a highly electropositive donor is used. The model would be a very useful utility to design various types of new functional molecular systems involving ICT optimization. © 2001 American Institute of Physics.

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... Recently [2,3] it was experimentally demonstrated that changes in the end group of an oligo(p-phenylene vinylene) molecule strongly influence optical properties of the oligomer. Although this phenomenon is not novel, and functionalized conjugated molecules, especially of phenylene vinylene family, have been extensively investigated both theoretically (see general references [4,5,6,7], optical spectra [8,9,10], nonlinear optical response [11,12], cations [13], functionalized oligomers [14,15,16,17]) and experimentally [18,19,20,21,22], the underlying mechanisms of how functionalization changes molecular properties have been rarely analyzed on the basis of first-principle calculations [23,24]. The present paper aims to fill this gap for the case of linearly extended π-conjugated systems. ...
... Existing studies in this direction are rather limited. For example, in Ref. [23] a simple formula for predicting the dipole moment of donor-bridge-acceptor systems is proposed based on electronegativity and polarizability of the donor and acceptor; however, in the original formulation it is not applicable to a one-side functionalization, predicting zero dipole. ...
Article
A comprehensive DFT study of a set of oligo(p-phenylene vinylene) molecules is performed to understand the structural and electronic changes upon functionalization. These changes are rationalized within a model considering frontier molecular orbitals of the -conjugated system and -bonding orbital by which the functional group is attached to the host molecule. Two simple scalar quantum chemical descriptors are shown to correlate with optoelectronic properties of the functionalized molecule: the electronegativity and the relative electric dipole moment of the smallest -closed shell subsystem containing the functional group and the terminal segment of the host molecule (phenyl). Both descriptors correlate linearly with the empirical Hammett constant for a set of 24 functional groups. Comparison with available experimental data on UV-Vis absorption and cyclic voltammetry is made. Observed structural changes reflect changes in the electronic density.
... The nature of functional groups in chromophores principally impact the ICT within D-π -A systems [28,29]. The non-linearity in chromophores mainly results by the unsymmetrical electronic distribution in the 'excited'/'ground' states [29,30]. ...
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Linear and nonlinear optical (NLO) properties of three ESIPT-rhodol derivatives, spirocyclic and open-forms of 10, 11, and 12 estimated by spectroscopic based mathematical calculations and compared with computed values by using density functional theory (DFT) calculations. The group of global and range-separated hybrid functionals were used for theoretical estimation of NLO properties. Spectroscopic based mathematical calculations were used for estimation of polarizabilty (α), and second-order hyperpolarizability (γ) in series of solvents. Benchmarking strategies were applied to used functionals for investigation of NLO characteristics. BHHLYP and ωB97 functionals performed well in the company of GHs and RSHs functionals to evaluate the NLO characteristics. The resulting NLO characteristics for open 10, 11, and 12 are higher than the consequences observed for close 10, 11, and 12. The computed polarizabilty (α), and second-order hyperpolarizability (γ) follow the trend 10 > 11 > 12, while the first-order hyperpolarizability (β) and molecular hyperpolarizability (µβ) follow the trend 12 > 10 > 11 for both forms. The observed trends in NLO parameters for compounds 10, 11, and 12 are supported by electron correlation analysis.
... Therefore, the UV-vis absorption spectrum of TN+1-d has an obvious blue shift with decreasing absorption strength in comparison with T2N+1-d, but has a small red shift and absorption enhancement in comparison with TN-d due to the introduction of N + . For TN+2-d and T2N+2-d, the locations of N + are on double bond of π-linkers, which is the main path of intramolecular charge transfer [23]. This can prevent the intramolecular charge transfer somewhat due to the strong electron-withdrawing ability of N + . ...
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On the basis of thieno(3,2-b)thiophene and dithieno[3,2-b:2′,3′-d]thiophene (T2 and T3 moieties) as π-linker, the A, D and S series dyes were designed to investigate the effect of the introducing N+ as an “electron trap” into T2 and T3 on the properties of the dyes. The optimized structures, electronic and optical properties were investigated by the density functional theory (DFT) and time-dependent DFT (TD-DFT). The results show that the properties of the dyes are sensitive to the N+ position in π-linkers. D series dyes with electron-withdrawing units located near the donor have better properties than the corresponding A series with the electron-withdrawing units located near the acceptor. For A and D series, the N+ modified dye named T2N+1-d displays the largest red shift of the UV–vis absorption, the maximum integral values of the adsorption-wavelength curves over the visible light, the highest light harvesting efficiency (LHE, 0.996), and the strongest adsorption energy (−44.33 kcal/mol). T2N+1-d also has a large driving force of hole injection (ΔGinj, −0.74 eV), which results in a more efficient hole injection. Bearing a lengthier π-linker than T2N+1-d, the properties of T2N+1-s are further improved. T2N+1-d moiety or its increased conjugated derivatives may be a promising π-linker.
... Large application possibilities of such push–pull systems in various photonic technologies deserve the structures of this type of compounds to be carefully studied [2]. Therefore, intramolecular interactions in such systems have been studied for a long time34567891011. Among various more common electron-donors and electron-acceptors, Me 2 N and NO 2 groups are the extreme cases and thus a molecule bearing both of them, Me 2 N-G- NO 2 , is a classical push–pull system. ...
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1H, 13C, 15N and 17O NMR chemical shifts are used for the characterization of the intramolecular interactions in several nitramines of the Me2N-G-NO2 type. The charge of lone electron pair of the amino group in N,N-dimethylnitramine, N,N-dimethyl-2-nitroethenamine, N,N-dimethyl-p-nitroaniline, 4-nitro-β-dimethylaminostyrene, 4-N,N-dimethylamino-β-nitrostyrene, 4-(N,N-dimethylamino)-4′-nitrobiphenyl, and 4-(N,N-dimethylamino)-4′-nitrostilbene is transferred not only to the nitro oxygens, but also to the vinylene and benzene carbons of the G spacer and to N-methyl carbons as well. Decreased nuclear shielding is found to be qualitatively related to the decreased atomic charge around a nucleus. This finding was further verified and quantified by comparison of the NMR data with those obtained by ab initio quantum chemical calculations. 17O NMR chemical shift changes seem to be more significant when the interacting NMe2 and NO2 groups are separated by a short spacer. On the other hand, 15N NMR chemical shifts suggest that a decrease of the charge at the amino nitrogen is not related to the length of the spacer alone. A lack of the linear dependence between the 17Onitro and 15Namino chemical shifts suggests that the charge lost by the amino nitrogen was only partially gained by the oxygens in the nitro group. The increased shieldings of the aryl carbons in 4-(N,N-dimethylamino)-4′-nitrobiphenyl indicate that atoms of the p,p-biphenylene spacer also gain some charge originating from the amino nitrogen. 3J H,H spin–spin coupling constant shows that among different vinylene compounds, the charge transfer to the nitro group is practically effective only in N,N-dimethyl-2-nitroethenamine where the bond between the vinylene carbons is significantly of low order by character. The calculated Natural Population Analysis (NPA) data confirms that except the nitro oxygens, other atoms that receive the negative charge lost by NMe2 in the compounds studied are the aryl and N-methyl carbons.
... The heteroaromatic moieties added into the donor-acceptor skeleton can act as auxiliary donor or acceptor and further enhance nonlinear optical properties [8]. In general intramolecular charge transfer (ICT) is possible in donor-p-acceptor system to generate dipolar pushpull systems [26,27] in which polarizability of the whole system can be easily tuned by varying the strength of donor or acceptor moiety and conjugation length [28e30]. A variety of organic chromophores has been utilized for the study of their non-linear optical behaviour. ...
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Rigid pyrazino-phenanthroline based donor-π-acceptor-π-auxiliary acceptor type compounds have been studied for their linear and non-linear optical properties. The non-linear optical (NLO) behavior of these dyes was studied by calculating the values of staticα, β and γ using solvatochromic as well as computational methods. The results obtained by solvatochromic method are correlated theoretically with Density Functional Theory (DFT) using B3LYP/6-31G (d), CAM B3LYP/6-31 G(d), B3LYP/6-31++ g(d,P) and CAM B3LYP/6-31++ g(d,P) methods. The results reveal that, among all four computational methods CAM-B3LYP/6-31++ g(d,P) performs well for the calculation of linear polarizability (α) and first order hyperpolarizability (β), while CAM-B3LYP/6-31 g(d,P) for the calculation of second order hyperpolarizability (ϒ). Overall TPA depends on the molecular structure variation with increase in complexity and molecular weight, which implies that both the number of branches and the size of π-framework are important factors for the molecular TPA in this chromophoric system. Generalized Mulliken-Hush (GMH) analysis is performed to study the effective charge transfer from donor to acceptor.
... Planarity of the π-conjugated system, which largely depends on the dihedral properties of rotatable bonds, enhances the mobility of π-electrons [39], and thus makes the drug more active chemically. In the case of NI-5, the planarity is severely affected, which could be the reason for its lowest value of α, <Δα>, and its diagonal components. ...
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... Molecular complexity is the criterion that can be related with Δα (Chen et al., 2017;Aihara et al., 1999;Obot et al., 2009;Ghanadzadeh et al., 2000;Zhan et al., 2003;Xue et al., 2004;Xue et al., 1999;Lim et al., 1999;Hansch et al., 2003;Boger et al., 2003;Lee et al., 2001). More the complexity of structure more is the anisotropy of polarizability (Δα). ...
... As the geometry of a charged molecule changes to accommodate that charge, the charge equilibrates along molecular bonds by ICT. We may understand the charge balance along these bonds by comparing them to diatomic molecules of elements A and B. In the so-called electronegativity equalization approximation 41,42 (EEA), the equilibrium charge Q AB due to ICT in such a molecule can be approximated as ...
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The tunability of high-mobility organic semi-conductors (OSCs) holds great promise for molecular spintronics. In this study, we show this extreme variability - and therefore potential tunability - of the molecular gyromagnetic coupling ("g-") tensor with respect to the geometric and electronic structure in a much studied class of OSCs. Composed of a structural theme of phenyl- and chalcogenophene (group XVI element containing, five-membered) rings and alkyl functional groups, this class forms the basis of several intensely studied high-mobility polymers and molecular OSCs. We show how in this class the g-tensor shifts, $\Delta g$, are determined by the effective molecular spin-orbit coupling (SOC), defined by the overlap of the atomic spin-density and the heavy atoms in the polymers. We explain the dramatic variations in SOC with molecular geometry, chemical composition, functionalization, and charge life-time using a first-principles theoretical model based on atomic spin populations. Our approach gives a guide to tuning the magnetic response of these OSCs by chemical synthesis.
... Therefore, the UV-vis absorption spectrum of TN+1-d has an obvious blue shift with decreasing absorption strength in comparison with T2N+1-d, but has a small red shift and absorption enhancement in comparison with TN-d due to the introduction of N + . For TN+2-d and T2N+2-d, the locations of N + are on double bond of π-linkers, which is the main path of intramolecular charge transfer [23]. This can prevent the intramolecular charge transfer somewhat due to the strong electron-withdrawing ability of N + . ...
... Collectively, these observed results indicate that the excited state has a prominently higher dipole-dipole moment than the ground state, which leads to solvatochromic properties, a crucial characteristic of fluorophores exhibiting ICT. [42] Furthermore, BZ1 and BZ2 exhibited an almost similar behavior in the relation between fluorescence quantum yield and the solvent dielectric constant. As shown in Fig. 4, a decreasing fluorescence quantum yield was observed with increasing the solvent dielectric constant, except for BZ3. ...
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... 27 The electron transference from the D to A unit through -conjugation in molecules leads to intramolecular charge transfer (ICT), [28][29][30][31][32][33][34] which is primarily directed by D and A substituents. [35][36][37][38][39][40][41] The inequality among these two substituents leads to the asymmetric distribution of electrons either in ground or in excited or in both the states which results in the nonlinearity in optical properties. 36,42 The presence of extended -bridge among the electron-donating and electron-withdrawing substituents is decisive for the enhancement in optical nonlinearity. ...
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We have investigated the chiroptical, linear, and second-order nonlinear optical (NLO) properties of seven binaphthol derivatives and elucidated structure-property relationships from the micromechanism for the first time. The excitation energies, oscillator strengths, and rotational strengths of the 150 lowest energy electron excitations for the most stable conformers have been calculated at TDB3LYP/cc-pVDZ level of theory. The experimental UV-vis absorption energies were reproduced well by our calculations. The simulated circular dichroism (CD) spectra and calculated optical rotation (OR) values are in reasonable agreement with experimental ones. These results demonstrate that TDDFT calculations can not only describe the electron transition property but also can be used to assign the absolute configurations (ACs) of binaphthol derivatives with high confidence. Whereas OR values are more sensitive to the molecular structures than CD spectra. The electron transition property and chiroptical origin have been assigned and analyzed. These derivatives possess remarkably large molecular first hyperpolarizabilities, especially compound 7 which has a value of 241.65 × 10(-30) esu. This value is about 60 times as large as that of highly π-delocalized phenyliminomethyl ferrocene complex. Moreover, compound 6 exhibits pronounced different second-order NLO response values from neutral state to the two cationic states (6(2+) and 6(4+)), which indicates that this compound could act as a potential NLO switch material. The cooperativity of intramolecular charge transfer of the studied compounds was also discussed.
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We report the synthesis and physical study of a series of 1,1-dicyano-4-[4-(diethylamino)phenyl]buta-1,3-dienes in which the number and position of additional CN substituents along the 1,1-dicyanobuta-1,3-dienyl fragment is systematically varied. While X-ray analysis provided unambiguous information about molecular geometries in the crystal, UV/Vis and electrochemical measurements, by cyclic voltammetry (CV) and rotating disk voltammetry (RDV), revealed that introduction of additional cyano groups in the C2- and C4-positions most affected the optical properties of these molecules in solution, in terms of intramolecular charge-transfer absorption energy and intensity. A comparison with structurally related chromophores indicates that the shift of the anilino donor from position 2/3 to 4 along the butadiene scaffold results in a remarkable bathochromic shift of the ICT absorption maxima, mainly due to the higher planarity in the present series. These findings are further corroborated by density functional theory calculations. Preliminary nonlinear optical (NLO) measurements confirm the promise of the new push-pull chromophores as third-order nonlinear-optical molecular materials.
Article
We present the large single crystal growth of a series of donor‒π‒acceptor (D‒π‒A) push-pull chromophores; 4-N,N-dimethylamino-β-nitrostyrene (1), 2-(4-(dimethylamino) benzylidene) malononitrile (2), ethyl 2-cyano-3-(4-(dimethylamino)phenyl) acrylate (3) and methyl 2-cyano-3-(4-(dimethylamino)phenyl)acrylate (4). Single crystals with good optical transparency were grown from a mixed organic solvent (1:1 acetone-methanol) applying slow evaporation technique, and crystal dimensions up to 17×4×1 mm3 has been successfully achieved in the case of chromophore 4. Single crystal X-ray diffraction study revealed that two (3 and 4) of them belong to triclinic crystal system with space group P-1, while 1 and 2 crystallize in orthorhombic and monoclinic crystal systems with space group Pbca and P21, respectively. The packing in all the chromophores (except 2) is stabilized by C‒H...π interaction. Powder X-ray diffraction pattern reflect the well crystalline nature of all the chromophores. Thermal analysis shows higher thermal stability for 3 (Td = 365 oC) and 4 (Td = 365 oC) than 1 (Td = 240 oC) and 2 (Td = 340 oC). The thermodynamic and kinetic parameters, such as heat and entropy of fusions; and activation energy have been determined by exploiting DSC and TGA data, respectively. Lippert-Mataga plot reflect the highest change in dipole moment on excitation from ground state to excited state for chromophore 3 (4.80 D) followed by 4 (4.45 D), 2 (3.59 D) and 1 (3.17 D). We also report the quantum yield of all the chromophores in different organic solvents. In acetonitrile, it has been found 0.066, 0.015, 0.026 and 0.021 for chromophores 1, 2, 3 and 4; respectively. These studies indicate the potential opto-electronic application of these push-pull chromophores.
Article
Thirteen new, stable, push–pull systems featuring dimethylamino and pyrazine-2,3-dicarbonitrile moieties as the donor and acceptor, respectively, and systematically extended and varied π-linkers were prepared and investigated. Evaluation of the measured UV/Vis spectra, electrochemical data (cyclic voltammetry, rotating disc voltammetry, and polarography), X-ray data, and experimentally determined and calculated hyperpolarizability values enabled structure–property studies; these revealed some important structural features that affected the efficiency of intramolecular charge-transfer and nonlinear optical properties in this class ofheterocyclic push–pull chromophores. The charge-transfer transition was most significantly affected by structuralfeatures such as π-linker length, chromophore planarity, and the number of 1,4-phenylene/ethynylene subunits in the π-linker.
Article
Ethyl 2-cyano 3-(4-dimethylaminophenyl) acrylate crystals were grown by slow cooling method by the condensation reaction of 4-dimethylaminobenzaldehyde and methyl cyanoacetate. The molecular structure of the crystal was confirmed by single crystal X-ray diffraction, ¹H Nuclear Magnetic Resonance (¹H NMR), ¹³C Nuclear Magnetic Resonance (¹³C NMR) and Mass spectrum. The single crystal X-ray diffraction study revealed that the crystal belongs to the triclinic crystal system with space group P. The functional groups of the crystal were confirmed by Fourier transform infrared spectrum. The absorption properties of the crystal were analysed by Ultraviolet–Visible absorption spectrum. The Molar extinction coefficient of the crystal was also calculated using Beer Lamberts law. The emission behaviour of the compound was analysed by Photoluminescence spectrum. Cyclic voltammetry of the compound was analysed through three electrode system at the scan rate of 100 mV/s and HOMO-LUMO of the crystal was calculated by cyclic voltammetry. The thermal behaviour of the crystal was analysed by Thermal gravimetric and differential thermal analysis (TG-DTA). The photo conductivity of the crystal was measured and the result shows that the crystal exhibits positive conductivity.
Article
Intramolecular charge transfer (ICT) derived from a donor‐acceptor system has been applied for years to enhance the charge mobility in the field of organic photovoltaics and chemical sensing. Similar strategies have gradually been developed in polymeric graphitic carbon nitride (GCN) photocatalytic systems for promoting the light absorption and charge separation. However, there are no reviews focusing on the effects of ICT after the modification of GCN so far. Herein, we summarize some typical literature on GCN engineering to expound profoundly the roles of ICT in electronic properties regulation in terms of in‐situ formation and molecular coupling. At last, some important perspectives are also proposed. This review will deepen understanding of the traditional theory for a new recognition among such many methods to improve the performance of GCN. This article is protected by copyright. All rights reserved.
Article
To investigate the effect of substituents in azo dyes on their nonlinear optical properties, four azo dyes were synthesized using ethyl‐3‐hydroxy‐2‐naphthoate as a coupler and characterized. The polarizability (α) and hyperpolarizability (β and γ) constants were determined by using solvatochromic and computational method. A group of global hybrid and range seperated hybrid functionals having different Hartree‐Fock exchange percentage were used to evaluate the NLO properties. The functional ωB97 and ωB97XD performed better in the determination of polarizability (α) and hyperpolarizability (β and γ), respectively. The observed results for polarizability and hyperpolarizability parameters were supported by highest occupied molecular orbital‐lowest unoccupied molecular orbital (HOMO‐LUMO) energy gap, Hammett‐coefficient (σ), excited‐state coefficients (σex), and hyper‐hardness (Γ) values. Third‐order NLO properties were analyzed by using Z‐scan technique. The –OCH3 substituted dye 4d shows higher value of molecular susceptibility (χ(3)), polarizability (α), and hyperpolarizability (β and γ) parameters among the studied azo dyes. They are likely to have the potential applications in optoelectronics and photonics. The linear and nonlinear optical properties of four napthol ester azo dyes were examined by solvatochromic, computational, and Z‐scan methods. The functional ωB97 performed better in the determination of linear polarizability (α), and ωB97XD performed better in the determination of the first‐order hyperpolarizability (β) and second‐order hyperpolarizability (γ) among the global hybrid and range separated hybrid functionals. Third‐order susceptibility (χ(3)) was investigated by using Z‐scan technique. From the observed results for azo dyes 4a‐4d have potential applications in opto‐electronics
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Experimental and theoretical insights into the nature of intermolecular interactions and their effect on optical properties of 1‐allyl‐4‐(1‐cyano‐2‐(4‐dialkylaminophenyl)vinyl)pyridin‐1‐ium bromide salts (I and II) are reported. A comparison of optical properties in solution and in the solid‐state of the salts (I and II) with their precursors (Ia and IIa) is made. The experimental absorption maxima (λmax) in CHCl3 is at 528 nm for I and at 542 nm for II, and a strong bathochromic shift of ∼110 nm is observed for salts I and II compared with their precursors. The absorption bands in solid‐state at ∼627 nm for I and at ∼615 nm for II that are assigned to charge transfer (CT) effect. The optical properties and single crystal structural features of I and II are explored by experimental and computational tools. The calculated λmax and the CT are in good agreement with the experimental results. The intermolecular interactions existing in the crystal structures and their energies are quantified for various dimers by PIXEL, QTAIM and DFT approaches. Three types of interactions, (i) the cation⋅⋅⋅cation interactions, (ii) cation⋅⋅⋅anion interactions and (iii) anion⋅⋅⋅anion interactions are observed. The cationic moiety is mainly destabilized by C−H⋅⋅⋅N/π and π⋅⋅⋅π interactions whereas the cation and anion moiety is predominantly stabilized by strong C−H⋅⋅⋅Br− interactions in both structures. The existence of charge transfer between cation and anion moieties in these structures is established through NBO analysis. Inside the salt: Experimental and theoretical insights into the nature of optical, charge transfer and intermolecular interactions in 1‐allyl‐4‐(1‐cyano‐2‐(4‐(dimethylamino)phenyl)vinyl)pyridin‐1‐ium bromide salts (I and II) are compared with their precursors (Ia–IIa).
Article
We synthesized random copolymers using 6-(2-thienyl)-4H-thieno[3,2-b]indole (TTI) and N-alkyl-2,2′-bithiophene-3,3′-dicarboximide (BTI) as the electron-rich and deficient units for the polymer solar cells (PSCs). Herein, by using different feed ratios of electron-deficient unit, the push-pull type copolymers were synthesized by Stille polymerization of TTI, BTI and thiophene linkage. Out of the series PBTI3 with 7:3 ratio of TTI and BTI showed the highest molar absorption coefficient and mobility, which is favorable for efficiently harvesting solar photons and higher photo-induced carriers to improve the JSC. Among devices of synthesized copolymers, PBTI3/PC71BM (1:2) (with 3% DIO additive) blend-based PSCs showed the best performance with a JSC of 6.78 mA/cm², a VOC of 0.72 V, a FF of 0.54 and a PCE of 2.64%.
Article
Based on the 2 (N-piperidinyl)thiophene central donor, 32 model push-pull molecules with systematically varied malonic acid-derived peripheral acceptors have been prepared. Further property tuning has been achieved by modifying the π-linker and the structural arrangement (linear vs. quadrupolar D-π-A systems). Malonic acid derivatives such as cyanoacetic acid, malondinitrile, diethyl malonate, Meldrum´s acid, and N,N´-dibutyl(thio)barbituric acid as well as 1,3-diketo analogues dimedone and indan-1,3-dione were employed as acceptor moieties. Knoevenagel condensation with four thiophene aldehydes afforded target chromophores in satisfactory yields. Withdrawing abilities of malonic acid acceptors were examined both by experiment including X-ray analysis, differential scanning calorimetry, electrochemistry, and UV Vis absorption spectroscopy and by DFT calculations. Thorough structure-property relationships have been elucidated. According the increasing electron withdrawing ability, the widely used malonic acid acceptor units can be ordered as follows: diethyl malonate ≤ cyanoacetic acid ˂ malondinitrile ˂ Meldrum's acid ˂ dimedone ≤ N,N'-dibutylbarbituric acid ˂ indan-1,3-dione ≤ N,N' dibutylthiobarbituric acid.
Article
For decades, host-guest nonlinear optical (NLO) polymers have been studied because of their versatilities. NLO polymers do not display second-order nonlinearities in as-prepared conditions because the guest chromophores are randomly oriented in the host polymers and they are centro-symmetric. We report non-electrical poling behaviors of push-pull guest chromophores that consist of a π-conjugated phenylene vinylene bridged by an electron-accepting tricyanofuran moiety and electron-donating phenyl amine derivatives in poly (methyl methacrylate). When the phenyl amine moieties were substituted either by hydroxyl or carboxyl groups, enhanced emissions of second harmonic generations (SHG) were observed after annealing at temperatures above the glass transition point of the host polymers in the absence of external electric fields. The number of hydroxyl or carboxylic groups exerted significant influence on the polar ordering of the chromophores. The orientation behaviors of the chromophores were examined using heterodyne SHG, polarized linear absorption and polarized SHG spectroscopies.
Article
A series of model push-pull molecules with linear, quadrupolar, and tripodal arrangement, variously substituted amino donor, two acceptors, and partially extended π-system has been prepared. Two peripheral electron acceptors, namely N,N´-dibutylbarbituric acid or dicyanovinyl, were employed. The fundamental properties of 24 push-pull chromophores were investigated by differential scanning calorimetry, electrochemistry, one photon absorption spectra, photoinduced piezooptics, and were completed by DFT calculations. Thorough structure-property relationships were elucidated, while a significant influence of the structural arrangement/branching on the electronic and optical properties has been revealed. The fundamental optoelectronic properties of push-pull molecules are affected by their arrangement (linear/quadrupolar/tripodal), the peripheral acceptor attached, extension and planarization of the π-system, and also by the type of auxiliary N-substituent.
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Hydroxytriphenylmethane dyes are organic compounds whose properties suggest a potential use in nonlinear optics. They are also widely used in chemical analysis. The most commonly used is C.I. Mordant Blue 29. However, in the literature there are no complete spectroscopic characteristics of the dye and there is a lack of detailed information on the electrical properties of C.I. Mordant Blue 29 dye. The optimum geometry of C.I. Mordant Blue 29 was determined by minimizing the energy with respect to all geometrical parameters without imposing molecular symmetry constraints. The UV-VIS spectra of C.I. Mordant Blue 29 and its charged species were computed with time-dependent density functional theory (TDDFT) by employing the B3LYP functional together with the 6-31++G(d,p) basis set at optimized geometries. The calculations of UV-VIS spectra also considered the presence of a solvent in terms of the polarized continuum model (PCM). The calculated spectra were compared with those obtained experimentally. The dipole moment (μ), polarizability (α), and first hyperpolarizability (β) of C.I. Mordant Blue 29 were calculated using the time-dependent Hartree–Fock method, and the electron correlation contribution to the electric properties was introduced by DFT using the functionals B3LYP, PBE0, and HCTH. Calculations were performed using several basis sets from 6-31G to 6-311++G(d,p).
Article
Strategies to construct zwitterionic anions from the parent anions are proposed. Two principles are employed; the cationic counterpart is (a) attached as a substituent or (b) inserted as an integral part at a remote location in the assembly. The optimized geometries reveal that a striking similarity exists between the zwitterions and the respective precursor parent anion. The computed vibrational frequencies emphasize that these novel entities are minima on their respective potential energy surfaces. A substantial HOMO-LUMO gap indicates that the proposed structures do not show instability in their respective electronic states, and that the higher energy configuration states do not contribute to the ground state viability. The separation of charge between the monopoles in these zwitterions is demonstrated by moderately large non-zero dipole moments. Significant large energy barriers for rearrangement to the closely related positional isomers, demonstrated in few cases, advocate the thermal stability (associated with spectroscopic viability) of the novel molecules. The donor capacity (basicity) of the anionic subunit in these zwitterions is comparable to that of the respective parent anions. Since the qualitative and quantitative features in the designed charged compensated complexes are conserved as anions, these molecules may perhaps be employed in synthetic organic or organometallic chemistry.
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An efficient route to synthesize, for the first time, a series of small molecules based on the [A`(D`AD)2] architecture was developed using selective direct heteroarylation of the C-H bond with a Pd(AcO)2/Bu4NBr simple catalytic system. The C-H arylation of the unsymmetrical compound 4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-7-(thiophen-2yl)benzo[c][1,2,5] thiadiazole (5), showed that, the (C5) in the ethyldioxythiophene moiety is more reactive towards C-H arylation than its counterpart in the thiophene unit. The new small molecules bear two different acceptors and/or donors which led to decrease the electron band gap and improve the strength of push-pull system as well as the amount of intramolecular charge transfer from donor to acceptor units. Lactams and imide containing acceptors are used as second electron withdrawing units due to their well-conjugated structure, strong π-π interaction and the lactam and high electron affinity as well as their potential as electron withdrawing units in photovoltaics conjugated materials. All small molecules showed broad absorption spectra with the optical band gaps which were estimated to be in the range of 1.72–1.29 eV. From the Cyclic voltammetry, the high occupied molecular orbital (HOMO) energy level could be tuned with the changing of the second acceptor, suggesting high open circuit voltage (Voc). The EHID(EDBTT)2:PC71BM-based solar cells reached a maximum PCE of 3.24%.
Chapter
This chapter discusses how the insights obtained from theoretical investigations of various cluster systems have enabled a researcher to predict structures and properties of novel functional molecular systems. Clusters are self-assembled structures comprised of a number of monomers under the given condition. Apart from aiding the development of novel materials, clusters are very useful for understanding the intrinsic and fundamental nature of molecular recognition and self-assembling phenomena. This is amply illustrated in a number of publications on a wide variety of atomic and molecular clusters, ranging from H-bonded clusters, p-system-containing clusters, and metal clusters. These investigations not only provide pertinent information useful for nanomaterial design but also highlight some of the important similarities and differences in their physical characteristics. These characteristics include structures, magnitudes of both attractive and repulsive interaction energies, vibrational frequencies, and charge redistributions. Additionally, one also obtains an insight into the contributions of cooperative and competitive forces, both of which govern self-assembly and molecular recognition.
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A series of twelve tripodal push-pull molecules with central triphenylamine donor and peripheral cyano substituted acceptors has been prepared. These molecules possess systematically altered π-linker as well as cyano acceptors. Based on the experimental properties measured by differential scanning calorimetry, electrochemistry, one and two photon absorption/emission spectra supported by the DFT calculations, thorough structure-property relationships were elucidated.
Article
A promising organic third-order nonlinear optical crystal of the benzo[e]indole family 2-(2-(4-(4-hydroxy) phenyl)vinyl)-1, 1,3-trimethyl-1H-benzo[e] indolium iodide (H-BI) has been synthesized and grown by slow evaporation method. The grown crystal’s physicochemical properties have been characterized by several analytical techniques to ensure the suitability for industrial applications. Single-crystal x-ray diffraction has been employed to confirm the crystal structure and space group of the grown crystal and indicated that the title crystal belongs to a monoclinic crystal system with centrosymmetric space group P21/c. The optical absorption of the title crystal has been analyzed by an ultraviolet–visible (UV–Vis) spectrometer, and no significant absorption was found in the entire visible region. The test crystal linear refractive index value has been calculated using standard optical formulations. The dielectric constant and dielectric losses of the test crystal were measured using an impedance analyzer in the temperature range of 40°C to 100°C; no significant changes were found with respect to temperature, and it is highly favorable for device fabrications. The thermal stability assessment of the title crystal was performed by thermogravimetric analysis (TGA) and differential thermal analysis (DTA), which revealed the grown crystal has moderate thermal stability and that the melting point is 266.4°C. Third-order nonlinear optical properties were investigated by the Z-scan technique and determining the refractive index (n2), absorption coefficient (β), and susceptibility (χ (3)). Based on the corresponding results, the titled crystal is well suited for photonic device applications.
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Property tuning in selected examples of D-p-A molecules has been discussed and summarized in this review article. The tuning and structure-property relationships have been demonstrated on the particular A, p and D parts of the push-pull molecule. A special emphasis has been put on the tuning of FMO’s levels and optical properties. Further prospective applications of the given chromophore have also been considered.
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Fifteen model charge-transfer chromophores with ferrocene donors, NO2 acceptors, and 1,4-phenylene and 2,5-thienylene π-linkers were synthesized and their properties investigated by X-ray analysis, electrochemistry, absorption spectra, and DFT calculations. Structure–property relationships were further evaluated to answer the question of suitability of the π-linkers in the construction of chromophore backbone. Gradual replacement of 1,4-phenylene with 2,5-thienylene and π-linker extension resulted in a lower HOMO–LUMO gap, a bathochromic shift of both charge-transfer bands, and overall planarization of the molecule. Hence, thiophene, when incorporated into a chromophore backbone, is a more polarizable moiety that allows very efficient intramolecular charge transfer. Bylo syntetizováno patnáct modelových chromoforů s vnitřním přenosem náboje, které obsahují ferrocen donor, NO2 skupinu jako akceptor a 1,4-fenylenové a 2,5-thienylenové π-můstky. Tyto sloučeniny byly dále zkoumány pomocí rentgenostrukturní analýzy, elektrochemie, absorpčních spekter a DFT výpočty. Dále byly vyvozeny vztahy struktura-vlastnosti s cílem zodpovědět otázku vhodnosti obou π-můstků pro konstrukci základního π-konjugovaného systému obecného chromoforu. Postupná náhrada 1,4-fenylenových můstků za 2,5-thienylenové a prodloužení π-konjugované cesty vedlo ke snižování rozdílu HOMO–LUMO, bathochromnímu posunu obou pozorovaných CT-pásů a k celkové planarizaci chromoforu. Molekula thiofenu, začleněná do π-konjugovaného systému chromoforu, se tak ukázala jako více polarizovatelná jednotka umožňující velice efektivní vnitřní přenos náboje.
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Density functional theory calculations have been carried out on nonconjugated D-π-A chromophores to investigate the different electron donors and conjugated bridges effects on the molecular nonlinear optical response. The results show that the large second-order polarizability values can be achieved through careful combination of available electron donors, conjugated bridges for our studied nonconjugated D-π-A chromophores. The calculations also provide a clear explanation for the second-order polarizability changes from the standpoint of transition energies, oscillator strengths, electron density difference, and bond length alternation. Solvent effect has great influence on the second-order polarizability and electronic absorption spectrum. It is hoped that the results presented in this article will give some hints to the interrelated studies. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
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A simple model is developed for predicting solvation effects on the nonlinear optical properties of charge transfer organic materials such as 1,1 dicyano,6-(di-butyl amine) hexatriene. This model is based on the valence-bond charge-transfer (VB-CT) framework, using a continuum description of the solvent. The resulting VB-CT solvation model leads to analytic formulas for the absorption frequency (Eg), the polarizability (α), the hyperpolarizabilities (β,Γ,δ), and the bond length alternation with only one solvent dependent parameter (&egr;, the dielectric constant of the solution). The theory involves just four solvent-independent parameters, V0, t, SF, and Q which are related to the band gap, bandwidth, geometry, and dipole moment of the CT molecule [plus a length (RDA) and force constant (k) derivable from standard force fields]. The results are in good agreement with experiment.
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We develop an original methodology to calculate analytically the long-range (LR) Coulombic effects to be included in Hartree–Fock forces computed on stereoregular polymers. The technique presented, based on multiple Taylor series expansions, is completely general and can be easily extended to all orders of expansion and to geometrical derivatives of higher order. In the McMurchie Davidson or similar schemes, the LR terms are added directly to Hermite integrals and LR effects are naturally considered during the computation of the energy and its derivatives. Each type of derivative (nuclear-repulsion energy, overlap and kinetic energy integrals, electron–nuclear attraction and two-electron integrals) is examined to quantify the impact of LR corrections. It turns out that the lattice sums of the gradients evaluated on a model macromolecule converge much faster when including those corrections. In addition, the dependence of the energy with respect to the unit cell length is for the first time considered in full details. © 1999 American Institute of Physics.
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Correlated semi-empirical quantum-chemical calculations are performed to rationalize the origin of the large first- (α), second- (β), and third- (γ) order molecular polarizabilities in carotenoidlike molecules capped at a single end by an acceptor group. The degree of ground-state polarization of the chromophores is made to vary via the application of an external homogeneous electric field in the range 107–108 V/cm. The most interesting feature is a stepwise evolution of the longitudinal component of the dipole moment with field, caused by charge transfer toward the acceptor end of the molecule. Since the steps are abrupt, this results in very large values of the molecular polarizabilities, in accordance with the derivative relationships among the dipole moment and α, β, and γ. The longitudinal components of the molecular polarizabilities are analyzed as a function of both the external field and the bond-order alternation (BOA) parameter. The ability of simple models to describe the evolution of the molecular polarizabilities is assessed. © 2000 American Institute of Physics.
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The performance of two exact exchange methods is tested in the calculation of ionization energies, electron affinities, electronegativities, and hardnesses using Dunning's correlation consistent basis sets. Comparison is made to experiment and other density functional methods, including the local density approximation and two gradient corrected functionals. The obtained electronegativities and hardnesses are also compared with high level coupled cluster results. Both the exact exchange methods show an excellent performance in the calculation of all four properties, yielding mean absolute deviations from experiment below 0.20 eV for all basis sets. (C) 1997 American Institute of Physics.
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Conjugated organic compounds with 3-phenyl-5-isoxazolone or N,N'-diethylthiobarbituric acid acceptors have large first molecular hyperpolarizabilities (beta) in comparison with compounds with 4-nitrophenyl acceptors. For example, julolidinyl-(CH=CH)(3)-CH=N,N'- diethylthiobarbituric acid, which has 12 atoms between the donor and acceptor, has a beta(0) of 911 x 10(-30) electrostatic units, whereas (CH(3))(2)NC(6)H(4),-(CH=CH)(4)-C(6)H(4)NO(2), with 16 atoms between its donor and acceptor, has a beta(0) of 133 x 10(-30) electrostatic units. The design strategies demonstrated here have resulted in chromophores that when incorporated into poled-polymer electrooptic modulators exhibited significant enhancements in electrooptic coefficients relative to polymers containing the commonly used dye Disperse Red-1. Poled polymer devices based on these or related chromophores may ultimately lead to high-speed electrooptic switching elements with low drive-power requirements, suitable for telecommunications applications.
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The solvent dependence of the second hyperpolarizability, γ, of a variety of unsaturated organic compounds has been measured by third harmonic generation at 1907 nanometers. It is seen that the measured y is a function of solvent polarity. These solvent-dependent hyperpolarizabilities are associated with changes in molecular geometry from a highly bond-length alternated, polyene-like structure for a formyl-substituted compound in nonpolar solvents, to a cyanine-like structure, with little bond-length alternation, for a dicyanovinyl-substituted compound in polar solvents. By tuning bond-length alternation, γ can be optimized in either a positive or negative sense for polymethine dyes of a given conjugation length.
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Static electronic and vibrational longitudinal first hyperpolarizabilities (βLe(0) and βLv(0)) of a series of mono- and disubstituted benzenes were calculated at the Hartree–Fock 6-31G level by using coupled Hartree–Fock and the double harmonic oscillator approximations, respectively. Although the βLv(0)/βLe(0) ratio is slightly larger than unity and rather constant with respect to the substituent(s), it turns out, for the monosubstituted compounds, that the variations of βLe(0) upon substitution can be most accounted for by the mesomeric effects, whereas for βLv(0), the inductive effects are also of importance. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 689–696, 1997
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Precision is given to the concept of electronegativity. It is the negative of the chemical potential (the Lagrange multiplier for the normalization constraint) in the Hohenberg–Kohn density functional theory of the ground state: χ=−μ=−(∂E/∂N)v. Electronegativity is constant throughout an atom or molecule, and constant from orbital to orbital within an atom or molecule. Definitions are given of the concepts of an atom in a molecule and of a valence state of an atom in a molecule, and it is shown how valence‐state electronegativity differences drive charge transfers on molecule formation. An equation of Gibbs–Duhem type is given for the change of electronegativity from one situation to another, and some discussion is given of certain relations among energy components discovered by Fraga.
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The asymptotic pairwise interaction between nonbonded atoms in planar dimers is analyzed. It is pointed out that the second derivative of the energy with respect to the perpendicular coordinates of the two nonbonded atoms enables to extract the parameters of the pairwise electrostatic interaction from the total energy. In the limit of large intermolecular separations the coupling second derivatives of the energy are shown to be determined by the atomic polar and quadrupolar tensors of the isolated monomers. It is further shown that the dipole moment of planar molecules can be expressed in terms of the out of plane Cartesian derivatives of its perpendicular component 〈z〉, and that the molecular quadrupole moments can be expressed in terms of these derivatives and the out of plane Cartesian derivatives of the second moments 〈xz〉 and 〈yz〉. These derivatives form a unique and well defined decomposition of the molecular multipoles into atomic quantities that can be formally interpreted as atomic point charges and point dipoles. These atomic point charges and point dipoles completely determine the asymptotic total and pairwise interactions in planar and rigid dimers.
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We have investigated the influence of donor and acceptor substituents on the second and third order hyperpolarizabilities β and Γ of large conjugated molecules such as stilbene and styrene derivatives. This was performed by two independent measurements of β and Γ using the techniques of dc electric-field induced second-harmonic generation and tunable four-wave mixing in liquids and solutions. For trans−stilbene derivatives, β and Γ are typically 10 times larger than for the corresponding benzene compounds, and are strongly correlated with the mesomeric effect of the substituents. A series of disubstituted molecules with strong donor–acceptor intramolecular charge transfer exhibit very large β, and it is shown that this enhancement can be predicted from the basic properties of the first electronic excited state.
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[GRAPHICS] Given the recent report of a novel pyrazole receptor exhibiting a high selectivity for NH4+ over K+, it would be interesting to investigate the origin of this selectivity and affinity so that better receptors could be designed. On the basis of extensive theoretical studies, we conclude that the origin arises from a subtle interplay of charged H-bonds and cation-pi interactions. The approach employed herein would be very useful in the rational design of novel functional molecular systems.
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An analysis of the second-order optical nonlinearities of crystalline substituted benzene is presented. A new irreducible tensor formalism for the decomposition of a crystal tensor as the sum of the contributions of microscopic units is developed and applied to the study of the second-order susceptibility in four benzene derivatives with related structures and substituent radicals (m-dinitrobenzene, m-nitroaniline, m-aminophenol, and resorcinol). It is shown that an additivity approach can precisely account for the crystal nonlinearities and enables one to calculate the contributions of each radical to the nonlinearity of the molecule. these contributions are found to be strongly related to the donor or acceptor character of the radical. A very simple model is proposed to explain this relation.
Article
The electronic and vibrational hyperpolarizabilities of polyenes NH2–(HC�CH)n–NO2 and polyynes NH2–(C�C)n–NO2 have been investigated. As the chain length increases, the first hyperpolarizabilities increase drastically for polyenes, while they increase moderately for polyynes. Furthermore, in polyenes the vibrational first hyperpolarizabilities are almost the same as the electronic first hyperpolarizabilities, while in polyynes the former is much smaller than the latter. An analysis of the electron densities of HOMOs of polyenes and polyynes confirms that the polyene π-frame is more efficient for long pathways of nonlinear optical molecules, while the polyyne π-frame is slightly better for very short pathways. In both polyenes and polyynes, the IR intensities arise mainly from the terminal carbon sites, while the Raman intensities, from the central carbon sites. © 2000 American Institute of Physics.
Article
Ab initio molecular dynamics simulation is performed on a small polyacetylene chain with a positive soliton defect. The dynamics is initialized by an external electric field. The collective motion of the carbon and hydrogen atoms are compared to some low frequency vibrational modes of positively charged polyacetylene chains of varying lengths having the characteristics of the soliton displacement. The soliton effective mass estimated using a variety of schemes is found to be about 10 electron masses. The static linear polarizability of singly charged polyacetylene chains of varying lengths is computed and compared with that of undoped chains. The electronic contributions to the polarizability are computed at the level of the coupled Hartree–Fock or the random phase approximation, and the vibrational contributions are estimated by invoking the double harmonic oscillator approximation. The soliton defect causes some enhancement of the electronic term, which covers 10–15 carbon–carbon double bonds, and it generates a large increase of the vibrational part, which in the absence of counter ions show no sign of leveling off even for the longest chains considered.© 1997 American Institute of Physics.
Article
We have measured the second and third order hyperpolarizabilities of the three nitroaniline isomers and of two related molecules. For some isomers the intramolecular charge transfer is found to cause a very large enhancement of the second order polarizabilities. We present a theory relating this contribution to the first excited state of energy, oscillator strength, and dipole moment of the molecules. Experimental results are accounted for with an excellent accuracy.
Article
A new approximate ``absolute'' scale of electronegativity, or electroaffinity, is set up. The absolute electroaffinity on this scale is equal to the average of ionization potential and electron affinity. These quantities must, however, in general, be calculated not in the ordinary way, but for suitable ``valence states'' of the positive and negative ion. Also, the electroaffinity of an atom has different values for different values of its valence; in general the electroaffinity as here calculated (in agreement with chemical facts) is larger for higher valences. Electroaffinity values have been calculated here for H, Li, B, C, N, O, F, Cl, Br, I. They show good agreement in known cases with Pauling's electronegativity scale based on thermal data, and with the dipole moment scale. The present electronegativity scale (like the others) is rather largely empirical, especially as to its quantitative validity; and it remains to be seen whether or not the latter will be more than very rough when tested for a wider range of cases. Nevertheless the new scale has a degree of theoretical background and foundation which throws some new light on the physical meaning of the concept of electronegativity (or electro‐affinity). The basis of the present scale, it should be mentioned, is simpler and more certain for univalent than for polyvalent atoms.—The nature of valence states of atoms is briefly discussed. It is hoped that the tabulations of atomic valence state energies and valence state ionization potentials and electron affinities given at the end of this paper may be useful in problems of molecular structure.
Article
Single-walled carbon nanotubes (SWNTs) are predicted to be metallic for certain diameters and pitches of the twisted graphene ribbons that make up their walls. Chemical doping is expected to substantially increase the density of free charge carriers and thereby enhance the electrical (and thermal) conductivity. Here we use Raman spectroscopy to study the effects of exposing SWNT bundles to typical electron-donor (potassium, rubidium) and electron-acceptor (iodine, bromine) dopants. We find that the high-frequency tangential vibrational modes of the carbon atoms in the SWNTs shift substantially to lower (for K, Rb) or higher (for Br2) frequencies. Little change is seen for I2 doping. These shifts provide evidence for charge transfer between the dopants and the nanotubes, indicating an ionic character of the doped samples. This, together with conductivity measurements, suggests that doping does increase the carrier concentration of the SWNT bundles.
Article
It is not clearly understood how and why the Raman intensity increases drastically with an increasing chain length of polyenes. We therefore investigated the vibrational intensities of the CC stretching vibrational modes of four polyene systems of X–HCCH n –Y, where X/YH/H, H/NH 2 , H/NO 2 , and NH 2 /NO 2 . The investigation was done using nodal mode analysis based on the number of nodes formed by the alternations of stretches and contractions combined with ab initio frequency calculations. The CC stretching/contracting mode without node is found to have the strongest Raman intensities regardless of polyene systems because of the long-range cooperation effect by the concurrent stretch/contraction motion of all CC bonds. The corresponding IR spectra have also the strongest intensities for the nonsymmetric polyene systems, whereas are inactive for the symmetric polyenes by the exclusion rule. The intensities of the nonconcurrent CC stretching/contracting modes particularly for the Raman spectra tend to decrease drastically in proportion to n/(m 1) 4) with increasing node number m, though weak or zero intensities appear somewhat alternately because the molecules have approximate or exact centrosymmetry. © 1997 American Institute of Physics. S0021-96069750135-8
Article
No theoretical and experimental information has been available for the role of each atom in infrared IR and Raman spectra of polyene derivative systems. Analyzing atomic polar and polarizability tensors at each carbon site in the CC bond fixed frame, we find that in IR intensities, the motions of carbon atoms in the proximity of a substituent are important, while in Raman intensities the carbon motions in the central region are important. The strong IR and Raman intensities appear for the concurrent CC stretch/contraction motion in nonsymmetric polyenes. © 1997 American Institute of Physics. S0021-96069751537-6
Article
The relationship between hyperpolarizability and spectral intensities has been investigated for polyene or oligoene systems. The vibrational polarizability, first hyerpolarizability, and second hyperpolarizability have been derived for the geometrical and electrical potentials with anharmonic terms included. This relationship has been tested by performing ab initio calculations on polyene derivative systems of X-HCvCH n Y, where 1n5 for X/YH/NH 2 , H/NO 2 , and NH 2 /NO 2 . Particular attention has been given to the first hyperpolarizability. The electronic hyperpolarizability shows a linear relationship with the square root of the product of IR and Raman intensities of the concurrent CvC stretching vibrational mode. The spectral intensities represent the vibrational hyperpolarizabilities which are of equivalent magnitude to the electronic hyperpolarizabilities. In the presence of an external electric field, the CvC bond stretches are responsible for the electronic hyperpolarizability, while the effects by other bond stretches are not significant. Thus the hyperpolarizability can be estimated by obtaining the IR and Raman intensities of CvC stretching vibration. This relationship would be valid for poly -conjugated molecules. © 1997 American Institute of Physics. S0021-96069700141-4
Article
The introduction, within a π-conjugated donor−acceptor molecule, of an intermediate barrier to electron tunneling and its size scaling and influence on electronic polarization properties have remained so far elusive issues of great potential interest toward the fine-tuning of the linear and nonlinear optical properties of molecular materials. Paracyclophane (pCP) provides a most relevant cornerstone for more elaborate compounds where donor and acceptor substituents are made to interact through a sterically constrained π−π stack. A first attempt in this direction is reported here with the synthesis of a model dipolar 4-(4-dihexylaminostyryl)-16-(4-nitrostyryl)[2.2]paracyclophane and the subsequent experimental and theoretical study of its quadratic nonlinear optical properties. A major outcome of this investigation is the evidence of a significant “through-space” charge transfer as unambiguously designated by the strong departure of the β quadratic hyperpolarizability tensor of the full doubly substituted molecule (60 × 10-30 esu) from the additive β value (18 × 10-30 esu) expected for strictly noninteracting singly substituted pCP moieties. This desired increase of nonlinear efficiency upon substitution is not offset by the usual red-shift of the absorption spectrum which generally curtails application perspectives in more common uninterrupted conjugated chains. The collective nonlinear polarization behavior involving the full end-to-end molecular structure is confirmed by theoretical calculations using the Collective Electron Oscillator (CEO) approach which furthermore indicates a significantly enhanced role of electron−hole pair delocalization in the higher order nonlinear response, compared to the linear polarizability or the static dipole moment.
Article
The dependence of the product of the first hyperpolarizability, ~, and the ground-state dipole moment, p, for a series of donor-acceptor polyenes with a large range of ground-state polarization, was measured in a variety of solvents by electric field induced second harmonic generation. The observed behavior of ~~ as a function of ground-state polarization agrees well with theoretical predictions, In particular, as a function of increasing polarization, ~0~ was found to first increase, peak in a positive sense, decrease, pass through zero, become large and negative and eventually peak in a negative sense.
Article
The electrostatic forces exerted by external point charges on model compounds are calculated ab initio and analyzed. It is shown that in general molecules cannot be represented as collections of isolated point charges interacting through vacuum. The distortion of the molecular geometry, caused by the external source, leads to an intramolecular charge redistribution (''charge flux'') that modifies the static pairwise force, in some cases drastically. In general, charge flux introduces a dynamical anisotropy into electrostatic forces that is usually much larger than the static anisotropy due to atomic dipoles and higher atomic multipoles. The effect of charge flux is also larger than that of polarizability. Because of charge flux, electrostatic forces on nuclei in molecules are, in general, nonlocal and nonpairwise. Rather, electrostatic forces on nuclei in molecules are determined by the response of all other nuclear sites to the external source, and particularly the sites that are closest to the perturbing source. Consequently, flux forces may oppose the forces from the static multipoles and even override them. The calculation of charge flux parameters is not straightforward with common methods of determining atomic charges. This problem is solved by using the recently introduced force related (FR) atomic multipoles which are well-defined derivatives of quantum mechanical expectation values. The FR atomic multipoles and their flux successfully reproduce the ab initio forces in the molecules discussed in this work.
Article
An analytical expression for the atomic charge fluxes in planar molecular systems is given in terms of specific second derivatives of the molecular dipole moments. This, together with previously derived expressions for the atomic charges and point dipoles, offers a formal and rigorous basis for the analysis of IR intensities in planar systems in terms of well-defined quantum-mechanical observables (expectation values). The suggested formalism is consistent with the ECCF model.
Article
Ab initio quantum-mechanical investigations have been carried out to examine the effects of substituents on structural deformations and electronic properties. We have studied eight substituent groups: two electron donor groups (–NH2, –N(CH3)2) and six electron acceptor groups (–NO2, –BCl2, –BF2, –BH2, –CN, and –NC). These groups have been used to provide 12 different donor–acceptor combinations. As the donor strength increases from –NH2 to –N(CH3)2, the change of the electronic properties shows the same trend for all acceptors. It has been shown that a constant like a Hammett constant exists and it can be used for the prediction of electronic properties.
Article
Optical second-harmonic generation has been measured in some organic liquids in presence of an electric field. Our preliminary results on the molecular hyperpolarizabilities of eleven benzene derivatives and a disubstituted stilbene are presented. These data show that the magnitude and sign of the π-electron contribution to the quadratic polarizabilities are strongly correlated with the mesomeric effect of the substituent and increase drastically with the size of the aromatic system.
Article
We present a very simple model which describes the nonlinear susceptibilities of benzene substitutes as arising from the electromeric coupling of the π-electrons of the benzene ring with inductive radicals. Agreement between theory and experiment is excellent.
Article
Vibrational spectra of push–pull polyenes show a peculiar feature namely, in infrared and Raman spectra strong, coincident bands appear, arising from vibrations localised on the polyene bridge. A simple model, based on the introduction of an effective internal field due to the charge transfer between end groups allows the infrared and Raman intensities of these bands to be related.
Article
Fluorescent analogs of adenine that selectively oxidize guanine were used to investigate photoinduced electron transfer through the DNA pi-stack as a function of reactant stacking and energetics. Small variations in these factors led to profound changes in the kinetics and distance dependences of DNA-mediated electron-transfer reactions. Values of beta, a parameter reflecting the dependence of electron transfer on distance, ranged from 0.1 to 1.0 per angstrom. Strong stacking interactions result in the fastest electron-transfer kinetics. Electrons are thus transported preferentially through an intrastrand rather than interstrand pathway. Reactant energetics also modulate the distance dependence of DNA-mediated charge transport. These studies may resolve the range of disparate results previously reported, and paradigms must now be developed to describe these properties of the DNA pi-stack, which can range from insulator- to "wire"-like.
Article
Organic molecular Mott insulators, in which carriers are localized as a result of the electron correlation, showed nonlinear electric conduction upon application of a high electric field along the molecular stacking axis. The current-driven low-resistive state of potassium 7,7,8,8-tetracyanoquinodimethanane was stabilized down to 2 kelvin, where a metallic path was visible with a microscope. The current flow caused a stripe-like periodic phase-segregation into the carrier-rich and carrier-poor regions along the current path.
Article
Ketosteroid isomerase (KSI) is one of the most proficient enzymes catalyzing an allylic isomerization reaction at a diffusion-controlled rate. In this study of KSI, we have detailed the structures of its active site, the role of various catalytic residues, and have explained the origin of the its fast reactivity by carrying out a detailed investigation of the enzymatic reaction mechanism. This investigation included the X-ray determination of 15 crystal structures of two homologous enzymes in free and complexed states (with inhibitors) and extensive ab initio calculations of the interactions between the active sites and the reaction intermediates. The catalytic residues, through short strong hydrogen bonds, play the role of charge buffer to stabilize the negative charge built up on the intermediates in the course of the reaction. The hydrogen bond distances in the intermediate analogues are found to be about 0.2 A shorter in the product analogues both experimentally and theoretically.
Article
A two-state, four-orbital, independent electron analysis of the first optical molecular hyperpolarizability, β, leads to the prediction that |β| maximizes at a combination of donor and acceptor strengths for a given conjugated bridge. Molecular design strategies that focus on the energetic manipulations of the bridge states are proposed for the optimization of β. The limitations of molecular classes based on common bridge structures are highlighted and more promising candidates are described. Experimental results supporting the validity of this approach are presented.
14 We find that, on the basis of our present and previous ͑Ref. 15͒ ab initio calculations, charge transferred polarizabilities of various push–pull sys-tems are found to be less than 10% the total polarizabilities
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