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Low-Lying Excited States of 7-Aminocoumarin Derivatives: A Theoretical Study
Abstract
Excited states of three 7-aminocoumarin derivatives, coumarin 120 (7-amino-4-methylcoumarin), pyrrolyl coumarin 7-(1H-pyrrol-1-yl)-4-methyl-2H-1-benzopyran-2-one, and carbazole-coumarin hybrid 10H-4-methyl-2H-2-oxopyrano[5,6-b]carbazole, have been studied using B3LYP time-dependent density functional theory (TDDFT). The solvent effect has been taken into account using the polarizable continuum model. The spectra calculated using TDDFT agree well with the experimental absorption spectra. The electronic structures and the solvent effect for the low-lying singlet excited states have been investigated. The HOMO of the pyrrolyl coumarin is localized on the pyrrolyl ring, while the HOMO in the other 7-aminocoumarins is delocalized over the entire molecule. This leads to the weak fluorescence of the pyrrolyl coumarins found in experiments. The HOMO and next HOMO in carbazole-coumarin hybrids have similar orbital energy values, which is not the case in the other 7-aminocoumarin derivatives. This leads to the additional peaks found in the 30,000–40,000 cm−1 region of the observed absorption spectra, which are specific for carbazole-coumarin hybrids. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
... However, coumarin molecules seem to be challenging systems for time-dependent DFT [198]. Nevertheless, the S 1 state was associated with a ππ * excitation and S 2 (or higher) as nπ * over a wide range of solvents and substituents although they can be close in energy (e. g., [196,203,206]). ...
Processes initiated by photoexcitation play an important role in many biological systems as well as in technical applications. A whole variety of quantum chemical methods for the treatment of such excited states has been developed over the past years. However, many are either restricted to small or medium-sized systems or only applicable to certain types of electronic excitations. Therefore, the development of efficient quantum chemical excited states methods is one of the central aspects of modern theoretical chemistry.
In this work, different excited state approaches within the algebraic diagrammatic construction (ADC) family of methods were derived and implemented. First, the scaled-opposite spin approximation was used to develop a variant of the extended ADC(2) methods that allows for an improved treatment of doubly excited states at reduced computational cost. Additionally, the generation of spin-orbit coupling elements based on an atomic mean-field approach was implemented for the whole hierarchy of ADC methods up to third order. Test calculations and comparison with existing methods revealed very good results. Last but not least, a scaling approach for the identification of plasmons in molecules previously introduced for TDDFT has been adopted to the ADC methods. Such plasmons are of great importance in the field of organic electronics. Here, the scaling approach was shown to work efficiently for a series of linear polyenes. All three theoretical methods were implemented in a development version of the adcman module of the Q-Chem program package. Thereby, the functionality of this module has been further extended making it applicable to a wider range of molecular systems and photochemical problems. Finally, ADC methods were used in combination with experimental results to successfully unravel the photochemical relaxation network of coumarin derivatives which turned out to incorporate two parallel radiationless relaxation pathways.
... Coumarin (2H-chromen-2-one), a benzo-a-pyrone, consists of fused pyrone and benzene rings with a pyrone carbonyl group at position 2, and is well known as fluorescent material because of its high photostability and photoluminescence efficiency [44][45][46]. Coumarins are an important group of organic compounds that are used as additives to food and cosmetics, optical brightening agents, and dispersed fluorescent and laser dyes [47], [48]. A number of 7-aminocoumarin derivatives bearing an aromatic or heteroaromatic radical in the 3-position serve as valuable fluorescent dyes [49]. ...
Rapid in-field diagnosis is very important to prevent the outbreak of various infectious and contagious diseases. Highly sensitive and quantitative detection of diseases can be performed using fluorescent immunochemical assay with specific antigen-antibody binding and a good quality fluorophore. This can lead to the development of a small, portable, quantitative biosensor to transmit diagnostic results to a control center in order to systematically prevent disease outbreaks. In this study, we developed a novel fluorophore, coumarin-derived dendrimer, with high emission intensity, strong signal brightness, and high photostability. It is easily coupled with biomolecules and emits strong and stable fluorescence at 590 nm with excitation at 455 nm. Application to fluorescent immunochromatographic test (FICT) showed that the novel coumarin-derived dendrimer bioconjugate could detect antigens at amount as low as 0.1 ng. The clinical results and the spectral characteristics of the novel coumarin-derived dendrimer open, for the first time, the possibility of developing a cost/energy efficient LED-based portable quantitative biosensor for point-of-care (POC) disease diagnosis, which can permit real time monitoring (U-healthcare system) by a disease control center.
... In the past decades, in fact, stable ground states and pure electronic absorption spectra of coumarin derivatives in gas phase and in solution have been studied by using quantum chemical methods. [32][33][34][35][36][37][38][39][40][41][42][43] For instance, in the framework of density functional theory (DFT), Preat et al. 33 investigated the UV electronic absorption spectra of substituted coumarins with various basis sets and functionals. It turns out that the Becke-Lee-Yang-Parr functional (B3LYP) together with the polarized continuum model (PCM) provides valid ground-state geometries and the consistent UV spectra with experimental measurements. ...
The vibrationally resolved spectral method and quantum chemical calculations are employed to reveal the structural and spectral properties of Coumarin 343 (C343), an ideal candidate for organic dye photosensitizers, in vacuum and solution. The results manifest that the ground-state energies are dominantly determined by different placements of hydrogen atom in carboxylic group of C343 conformations. Compared to those in vacuum, the electronic absorption spectra in methanol solvent show a hyperchromic property together with the redshift and blueshift for the neutral C343 isomers and their deprotonated anions, respectively. From the absorption, emission, and resonance Raman spectra, it is found that the maximal absorption and emission come from low-frequency modes whereas the high-frequency modes have high Raman activities. The detailed spectra are further analyzed for the identification of the conformers and understanding the potential charge transfer mechanism in their photovoltaic applications.
Coumarins are widely applied in sensing devices due to their diverse fluorescence properties. In this paper, we consider the C4‐substituted 7‐aminocoumarins to investigate the structure‐property relationship via examining the effect of the various substitutions on their optical properties. Absorption and fluorescence measurements together with time‐dependent density functional theory calculations are described. We find that the significant red shift in the fluorescence spectra is associated with the cyclic carbonyl group of coumarins due to a substantial increase in the dipole moment in the photoexcited state. The apparent correlation between experimental and calculated results provides an atomistic‐level insight into the role of the functionality at the C4 of coumarins and sets the basis for using computational analysis to predict the fluorescence properties of coumarins. The computational analysis of the ground state and excited state properties of the fluorophores of the coumarin family in the attempt of predicting absorption and fluorescence properties is described. The experiment and theory correlate well with the overall impact on the absorbance, and emission maxima by the substitution at the C4‐position of the analyzed focused coumarin library, providing the theoretical basis for tuning coumarin fluorescence.
A new approach toward chromeno[4,3-b]pyrrol-4(1H)-ones, by annulation of 4-phenylaminocoumarins with β-nitroalkenes, is reported and its conditions were optimized. The transformations under the fine-tuned conditions took place under promotion by TsOH.H2O, in the absence of solvent. The scope and limitations of the process were explored, by systematic modification of two diversification points. A reaction mechanism, triggered by a Michael addition of the nitrogen moiety of the 4-phenylaminocoumarins to the β-nitroalkene, was also proposed. Further functionalization of a selected tricycle was performed; the photophysical (UV and fluorescence spectra; 1O2 generation) and electrochemical (cyclic voltammetry) properties of some heterocycles were studied.
The time-dependent density functional theory (TDDFT) is used to investigate the effects of solvent polarity and hydrogen bond on coumarin 500 (C500). Both the absorption maximum and the hydrogen bond strength of C500 hydrogen bonded complex are confirmed to have a linear relationship with the solvent polarity function Δf. The type B (at carbonyl group position) complex whose absorption spectrum is extremely agreement with the experimental results is demonstrated to be preponderant among all the hydrogen bonded complexes. The investigation of C500-phenol complex (type B) shows that the strengthening of hydrogen bond in the S1 state induces the fluorescence intensity of C500 weakened. The solvent polarity is the key influencing factor on intermolecular hydrogen bond by comprising the influence of solvent polarity and substituent on C500.
Recent advances in theoretical photophysics and photochemistry derive from the improved capabilities of ab initio quantum-chemical methods to deal with different types of excited states phenomena in molecules of increasing size and complexity. Whereas the widespread use of time-dependent density functional (TD-DFT) based techniques for the excited state have extended the study of absorption and emission processes to large molecular systems and coupled-cluster (CC) methods have increased the accuracy of spectroscopic studies in medium-size compounds, multiconfigurational ab initio approaches such as CASPT2 and MRCI are now able to cope accurately with all types of photochemical processes in medium to relatively large systems, including nonadiabatic processes involving conical intersections, i.e., energy degeneracies, between potential energy hypersurfaces (PEHs), which are out of reach for the other single reference approaches. The coupling of accurate electronic structure calculations based on PEHs with reaction dynamic procedures is starting to make available the theoretical determination of both static and dynamic, time-dependent and statistical, photoinduced properties in systems of different type and complexity. Examples of the studied processes and the most commonly used approaches are given below.
Red emitting dyes are of interest in various technological applications. Coumarins, though being an important class of fluorescent molecules, those with red emission, have been rarely studied theoretically. The structural and electronic aspects of three novel red emitting coumarins were studied using DFT and TD-DFT methods. The functionals employed were the hybrid functionals B3LYP, CAM-B3LYP, PBE0 and the highly parameterized empirical functional M06. The geometry at ground state reveals the electron donor N,N-diethylamino group is coplanar with the chromophoric system and the nitrile group induces a red shift to the absorption and emission. The electronic energies and dipole moments were solvent dependent. The basis sets and functionals were benchmarked for their performance with these molecules. B3LYP has been proved to be more efficient in computations whereas the basis sets do not have noticeable effect on the electronic properties. However, adding a polarization function to the basis set has improved the calculation of vertical excitation. The B3LYP functional gives maximum absolute deviation of 0.20 eV in calculating the vertical excitations and 0.18 eV for emission.
A layer of a coumarin derivative\multiwall carbon nanotube (MWCNT) composite was sandwiched between an indium tin oxide (ITO) layer and a top metal electrode. This layer acted as the active hetero-junction (middle hetero-junction) in a triple hetero-junction organic solar cell. The composition of the organic material in the middle hetero-junction component, second from the ITO layer, was varied, keeping the weight percentage of the MWCNTs constant, to investigate the photovoltaic properties of the cell. The choice of amino-coumarin and its derivatives were dictated by the need for a high efficiency organic up-converter. Amino-coumarin is a two photon absorbing organic donor whereas CNTs are acceptors and ballistic charge transport media. For those reasons, amino-coumarin-CNT composite materials were selected as a photo active layer to will absorb the sub-band photons effectively. Additionally, the radial self-assembling of the CNTs inside the organic matrix (during synthesis) enhances the photovoltaic characteristics of the material. The photo induced charge transport mechanism between the MWCNTs and organics was analyzed using photo luminescence (PL) measurements. By the use of 15 wt% of CNTs with the organics, more than 90% of the PL signal was quenched, indicating an ultrafast transport mechanism between the donor and acceptor materials.
The structural and spectral properties of coumarin derivatives in complex environments were investigated within the time-dependent density functional theory (TD DFT). Absorption spectra calculations were obtained at TD PBE0/6-31+G(d,p) level of theory for coumarin47 in the gas-phase and in various polar and non-polar organic solvents. The geometries of coumarins 6, 30, 47 and 522 in the gas phase and in inclusion complexes with the β-cyclodextrin (βCD) were determined by PM3 and DFT (HCTH/6-31G) calculations. Encapsulation of coumarin in βCD and associated changes in electronic structure produced either a red or blue shift in the absorption spectra of coumarins. A proposed cavity model for βCD-coumarin complex in water solution allowed identification of various contributions to the overall shift in the absorption spectra of coumarin upon complex formation in a solvent environment
In the present work we have explored the ultrafast relaxation network of coumarin and umbelliferone (7-hydroxy-coumarin) using time-resolved femtosecond spectroscopy and quantum chemical calculations. Despite the importance of the photophysical properties of coumarin derivatives for applications in biomedicine, the low fluorescence quantum yield of coumarin itself has not been fully understood so far. On the basis of our combined experimental and theoretical results we suggest a model for the ultrafast decay after photoexcitation incorporating two parallel radiationless relaxation pathways: one within the initially excited state via ring opening and the other one by transition into a dark state along the carbonyl stretching mode. The fluorescence quantum yield is determined by the position of the branching point relative to the Franck-Condon region which is strongly influenced by interactions with the environment and the substitution pattern. This model is finally capable of giving a comprehensive account of the striking differences observed in the photophysical behavior of coumarin as opposed to umbelliferone.
This paper presents the results of a spectral (absorption and emission) and photophysical study of 6-aminocoumarin (6AC) in the solvents with which this molecule interacts only nonspecifically (n-alkanes, tetrachloromethane and 1-chloro-n-alkanes) and in nitriles. The strong effects of the solvents on the emission spectra, fluorescence quantum yield and lifetime of 6AC were observed. The results of the steady-state and time-resolved photophysical study suggest the presence of very fast nonradiative deactivation processes. It is concluded that besides fluorescence, the efficient S(1)-ICT → S(0) internal conversion in nonpolar aprotic solvents arises from vibronic interactions between close-lying S(1)-ICT(π,π*) and S(2)(n,π*) states. Moreover, unexpectedly efficient triplet state formation occurs. In nitriles the intermolecular hydrogen-bonding interactions with solvent molecules also facilitate the nonradiative decay process involving the S(1)-exciplex.
The solvent effect on the absorption spectra of coumarin 120 (C120) in water was studied utilizing the combined quantum mechanical∕molecular mechanical (QM∕MM) method. In molecular dynamics (MD) simulation, a new sampling scheme was introduced to provide enough samples for both solute and solvent molecules to obtain the average physical properties of the molecules in solution. We sampled the structure of the solute and solvent molecules separately. First, we executed a QM∕MM MD simulation, where we sampled the solute molecule in solution. Next, we chose random solute structures from this simulation and performed classical MD simulation for each chosen solute structure with its geometry fixed. This new scheme allowed us to sample the solute molecule quantum mechanically and sample many solvent structures classically. Excitation energy calculations using the selected samples were carried out by the generalized multiconfigurational perturbation theory. We succeeded in constructing the absorption spectra and realizing the red shift of the absorption spectra found in polar solvents. To understand the motion of C120 in water, we carried out principal component analysis and found that the motion of the methyl group made the largest contribution and the motion of the amino group the second largest. The solvent effect on the absorption spectrum was studied by decomposing it in two components: the effect from the distortion of the solute molecule and the field effect from the solvent molecules. The solvent effect from the solvent molecules shows large contribution to the solvent shift of the peak of the absorption spectrum, while the solvent effect from the solute molecule shows no contribution. The solvent effect from the solute molecule mainly contributes to the broadening of the absorption spectrum. In the solvent effect, the variation in C-C bond length has the largest contribution on the absorption spectrum from the solute molecule. For the solvent effect on the absorption spectrum from the solvent molecules, the solvent structure around the amino group of C120 plays the key role.
Excited states of two 7-aminocoumarin derivatives, coumarin 120 (7-amino-4-methylcoumarin) and coumarin 151 (7-amino-4-trifluoromethylcoumarin), were investigated using generalized multiconfigurational quasidegenerate perturbation theory (GMC-QDPT), multiconfigurational quasidegenerate perturbation theory (MC-QDPT) and time-dependent density functional theory (TDDFT) with the B3LYP and CAM-B3LYP functionals. The absorption and fluorescence spectra of C120 and C151 were calculated. We elucidated the characters of the low-lying states of C120 and C151. The absorption spectra calculated with GMC-QDPT and TDDFT B3LYP agreed well with the experimental data, while for the fluorescence spectra, the TDDFT calculations overestimated the fluorescence spectra compared to GMC-QDPT calculations. Utilizing active spaces with large numbers of electrons and orbitals for reference functions, GMC-QDPT showed a better performance than MC-QDPT with a complete active space self-consistent field (CASSCF) reference of active space with smaller number of electrons and orbitals. In our gas phase calculation, we found that the optimized structures for the first excited states have a planar amino group with a CN single bond, while the amino group is pyramidal in the ground state.
Time-dependent density-functional response theory (TD-DFRT) is presented from the point of view of quantum chemistry. The extension of density-functional theory (DFT) into the time-domain is reviewed from the point of view of Runge, Gross, and Kohn. The basic working equations of TD-DFRT are then derived in a form analogous to the time-dependent Hartree–Fock (TDHF) equations used for molecular calculations. This is the first practical formulation of TD-DFRT for molecular applications, and the equations are presented in a more general form than has been the case for either atoms or solids. In particular, the present TD-DFRT equations anticipate applications to open-shell molecules based on spin-unrestricted DFT equations with fractional occupation numbers, and are general enough to accept time-dependent exchange-correlation functionals beyond the adiabatic approximation. The use of auxiliary function techniques to eliminate the four-center integrals that arise in TD-DFRT is discussed. The simple example of H2 is used to illustrate the TD-DFRT method and its relationship to TDHF and to the usual ad hoc ΔSCF-based DFT treatment of excited states. The TD-DFRT method set forth here provides a powerful DFT technique for the calculation of such optical properties as dynamic polarizabilities and electronic excitation spectra, and can be readily extended to treat a number of other properties such as hyperpolarizabilities and intermolecular forces.
Electronic structure evolution of an organic dye coumarin 120 coupled to polar solvation dynamics is examined by combining ab initio electronic structure calculations and molecular dynamics (MD) simulations. Sets of nonorthogonal Hartree–Fock molecular orbitals optimized in vacuo and in dielectric continuum are utilized for a quantum mechanical description of the solute electronic polarization coupled to the solvent fluctuation. The adiabatic MD simulation for methanol solution is performed to evaluate the equilibrium and nonequilibrium dynamics of the (S0–S1) energy gap coordinate and the dipole moments. The absorption and fluorescence spectra are computed via the spectral density functions obtained from the simulation analysis. The results for the quantum polarizable (Q-Pol) model of the solute probe are compared with those for a nonpolarizable fixed-charge (Fix-Z) model. It is shown that the solute electronic polarization notably affects the solvent-induced key quantities such as the reorganization energy, the spectral linewidth, and the Stokes shift (which are mutually related): For example, the computed Stokes shifts are ∼2500 and ∼4000 cm−1 for Fix-Z and Q-Pol, respectively. On the other hand, the solute polarization tends to slightly slow down the methanol solvation, which is not necessarily attributed to reduction of the “solvent force constant” because the effective mass of the coordinate is reduced as well.
We report the absorption spectrum of Coumarin 153 in the gas phase at 383 K and its fluorescence spectra in a supersonic jet. At excess vibrational energy above 200 cm-1 in the first excited state S1, intramolecular vibrational redistribution leads to emission from interconverting conformers. For the hot isolated molecules, the emission spectrum is dominated by active modes of 350, 810, and 1150 cm-1 and the distributions of oscillator strength for absorption and emission cross at 25420±40 cm-1. This value should be used in solvation studies for the electronic S1 energy of the bare molecule. The oscillator distributions change with temperature indicating a dependence of Franck-Condon factors for high-frequency modes on a low-frequency coordinate. Calculations of electronic structure with density-functional theory show that the second excited state is well separated from the first. It is proposed that spectral changes observed during nonequilibrium polar solvation are caused by intramolecular reorganisation of the julolidine group in the S1 state on the 200 fs time scale.
The valence π → π
* excited states of anthracene and naphthacene are studied with multireference perturbation theory with complete active space
self-consistent field reference functions. The predicted spectra provide a consistent assignment of all one- and two-photon
spectra and T-T spectra of low-lying valence π → π
* excited states of anthracene and naphthacene. The present theory predicts the valence π → π
* excitation energies with an accuracy of 0.15 eV for anthracene and of 0.25 eV or better for naphthacene. The excited states
of anthracene and naphthacene are compared with those of benzene and naphthalene studied previously. The present calculations
predict that, going from anthracene to naphthacene, there is a symmetry reversal of the two lowest singlet state transitions,
but not for the triplet, just as indicated by the experimental data. Some general trends of polyacene excited states are discussed
based on the calculated results for benzene to naphthacene. Conclusive results obtained for anthracene and naphthacene can
be used as a model for understanding the excited states of larger polyacenes.
In the past, basis sets for use in correlated molecular calculations have largely been taken from single configuration calculations. Recently, Almlöf, Taylor, and co‐workers have found that basis sets of natural orbitals derived from correlated atomic calculations (ANOs) provide an excellent description of molecular correlation effects. We report here a careful study of correlation effects in the oxygen atom, establishing that compact sets of primitive Gaussian functions effectively and efficiently describe correlation effects if the exponents of the functions are optimized in atomic correlated calculations, although the primitive (sp) functions for describing correlation effects can be taken from atomic Hartree–Fock calculations if the appropriate primitive set is used. Test calculations on oxygen‐containing molecules indicate that these primitive basis sets describe molecular correlation effects as well as the ANO sets of Almlöf and Taylor. Guided by the calculations on oxygen, basis sets for use in correlated atomic and molecular calculations were developed for all of the first row atoms from boron through neon and for hydrogen. As in the oxygen atom calculations, it was found that the incremental energy lowerings due to the addition of correlating functions fall into distinct groups. This leads to the concept of correlation consistent basis sets, i.e., sets which include all functions in a given group as well as all functions in any higher groups. Correlation consistent sets are given for all of the atoms considered. The most accurate sets determined in this way, [5s4p3d2f1g], consistently yield 99% of the correlation energy obtained with the corresponding ANO sets, even though the latter contains 50% more primitive functions and twice as many primitive polarization functions. It is estimated that this set yields 94%–97% of the total (HF+1+2) correlation energy for the atoms neon through boron.
We report one- and two-photon absorption excitation energies and cross sections for a series of 7-aminocoumarins using time-dependent density functional theory with various basis sets and functionals, including exchange-correlation functionals using the Coulomb-attenuating method, to evaluate their performance in the gas phase and in solvents. Except for the results of one functional, the computed one-photon excitation energies and transition dipole moments are in good agreement with experiment. The range of errors obtained from various functionals is discussed in detail. The relationship of donor and acceptor groups with the one- and two-photon resonances and intensities is also discussed.
Photophysical parameters have been determined for coumarin laser dyes in a variety of organic solvents, water, and mixed media. The response of fluorescence emission yield and lifetime to changes in solvent polarity was a sensitive function of coumarin substitution pattern. Most important were substituent influences which resulted in enlarged excited-state dipole moments for the fluorescent state. For dyes displaying sharp reductions in emission yield and lifetime with increased solvent polarity, protic media and particularly water were most effective in inhibiting fluorescence. The temperature dependence of emission yield and lifetime was measured for two solvent-sensitive dyes in acetonitrile and in a highly viscous solvent, glycerol. The quenching of coumarin fluorescence by oxygen for dyes with lifetimes > 2 ns was also observed. The dominant photophysical features for coumarin dyes are discussed in terms of emission from an intramolecular charge-transfer (ICT) excited state and an important nonradiative decay path involving rotation of the amine functionality (7-position) leading to a twisted intramolecular CT state (TICT). The role of excited-state bond orders involving the rotating group in determining the importance of interconversions of the type ICT ..-->.. TICT is discussed. 73 references, 1 figure, 3 tables.
Multireference perturbation theory with complete active space self-consistent field (CASSCF) reference functions was applied to the study of the valence π↠π* excited states of benzene and naphthalene. The eigenvectors and eigenvalues of CASSCF with valence π active orbitals satisfy pairing properties for the alternant hydrocarbons to a good approximation. The excited states of polyacenes are classified into the covalent minus states and ionic plus states with the use of the alternancy symmetry. The present theory satisfactorily describes the ordering of low-lying valence π↠π* excited states. The overall accuracy of the present approach is surprisingly high. We were able to predict the valence excitation energies with an accuracy of 0.27 eV for singlet u states and of 0.52 eV or better for singlet g states of naphthalene. Our predicted triplet states spectrum provides a consistent assignment of the triplet–triplet absorption spectrum of naphthalene. For benzene we were able to predict the valence excitation energy with an accuracy of about 0.29 eV. The covalent minus states and ionic plus states exhibit different behavior as far as the electron correlation is concerned. The ionic plus states are dominated by the single excitations but covalent minus states include a large fraction of doubly excited configurations. The covalent minus states always give lower energy than the corresponding ionic plus states. This is true for triplet states. The dynamic σ–π polarization effects introduced by perturbation theory are significant for the ionic plus states while those on covalent excited states are usually of the same order as in the covalent ground state. The enlargement of the active space of the reference functions represents a great improvement of the description of the ionic states. The present approach with the pairing properties has proved to be of great value in understanding and predicting the experimental data of the alternant hydrocarbons.
The photophysical properties of 7-amino-4-trifluoromethyl-1,2-benzopyrone (coumarin-151; C151) in nonpolar (NP) solvents are drastically different than those in other solvents of moderate to higher polarities. Thus, the absorption and fluorescence maxima are largely blue-shifted and the Stokes' shifts unusually low in NP solvents compared to those in the other solvents. The fluorescence quantum yields (Φf) in NP solvents are also exceptionally lower. While in all other solvents the fluorescence decays of C151 are single-exponential, in NP solvents the decays show non-single-exponential behavior. The average fluorescence lifetimes (τf) in NP solvents are also substantially shorter. Low Φf and τf values in NP solvents indicate very fast nonradiative deactivation for the S1 state of C151. Investigating the triplet characteristics of C151 by using picosecond laser flash photolysis and pulse radiolysis techniques, it has been established that the fast nonradiative deactivation in NP solvents is not due to an enhancement in the intersystem crossing rate. It is inferred from the present results that in NP solvents the 7-NH2 group of C151 exists as a free substituent without participating in resonance with the benzopyrone moiety and the flip-flop motion of the NH2 group introduces the fast deactivation channel for the S1 state of the dye. In other solvents of moderate to higher polarities, there is an intramolecular charge transfer (ICT) from the 7-NH2 group to the benzopyrone moiety, resulting in a planar structure for the S1 state in which the flip-flop motion of the 7-NH2 group is restricted. Involvement of the ICT structure in moderate to higher polarity solvents causes very high Φf and τf values and large Stokes' shifts for the dye compared to those in NP solvents.
The absorption and fluorescence spectra of 7-aminocoumarins and 7-aminocarbostyrils with different degrees of alkylation were studied in 2-propanol (IP), polyfluorinated alcohols and water. The spectral properties of substituted 7-aminocoumarins and 7-aminocarbostyrils in hexafluoro-2-propanol (HFP) are very different from those in 2-propanol due to the strong hydrogen-bonding (HB) interaction between the solute and the solvent (HFP). The spectral behaviour can be explained in terms of the strength of the HB interaction which depends on the degree of alkylation of the amino group and the electron affinity of the electron-accepting moiety. The absorption spectra indicate that a structural change at the amino nitrogen is induced on formation of strong hydrogen bonds.
The photophysical properties of fluorinated 7-aminocoumarin derivatives with different alkylation degree at the amino group are studied in water/ethanol mixtures. The effect of the molecular structure of the chromophore and solute-protic solvent interactions are considered to explain the observed spectral shifts and the internal conversion process. The radiationless deactivation process is analyzed on the basis of the formation of a twisted intramolecular charge transfer state and a planar-to-pyramidal structural change of the amino group.
A procedure based on the polarizable continuum model (PCM) has been applied to reproduce solvent effects on electronic spectra in connection with the time-dependent density functional theory (TD-DFT). To account for solute-solvent interactions, a suitable operator has been defined, which depends on the solute electronic density and can be used to modify the TD-DFT equations for the calculation of molecular polarizabilities and of electronic transition energies. The solute-solvent operator has been derived from a PCM approach depending on solute electrostatic potential: Recently, it has been shown that such an approach also provides an excellent treatment of the solute electronic charge lying far from the nuclei, being particularly reliable for this kind of applications. The method has been tested for formaldehyde in water and in diethyl-ether, and then applied to the calculation of solvent effects on the n→π∗ transition of diazabenzenes in different solvents. The computed transition energies are in fairly good agreement with experimental values. © 2001 American Institute of Physics.
Time-dependent density-functional (TDDFT) methods are applied within the adiabatic approximation to a series of molecules including C70. Our implementation provides an efficient approach for treating frequency-dependent response properties and electronic excitation spectra of large molecules. We also present a new algorithm for the diagonalization of large non-Hermitian matrices which is needed for hybrid functionals and is also faster than the widely used Davidson algorithm when employed for the Hermitian case appearing in excited energy calculations. Results for a few selected molecules using local, gradient-corrected, and hybrid functionals are discussed. We find that for molecules with low lying excited states TDDFT constitutes a considerable improvement over Hartree–Fock based methods (like the random phase approximation) which require comparable computational effort. © 1998 American Institute of Physics.
Photophysical properties of coumarin-120 (C120; 7-amino-4-methyl-1,2-benzopyrone) dye have been investigated in different solvents using steady-state and time-resolved fluorescence and picosecond laser flash photolysis (LFP) and nanosecond pulse radiolysis (PR) techniques. C120 shows unusual photophysical properties in nonpolar solvents compared to those in other solvents of moderate to higher polarities. Where the Stokes shifts (Δ = abs−fl), fluorescence quantum yields (Φf), and fluorescence lifetimes (τf) show more or less linear correlation with the solvent polarity function Δf = {(ε−1)/(2ε+1)−(n2−1)/(2n2+1)}, all these parameters are unusually lower in nonpolar solvents. Unlike in other solvents, both Φf and τf in nonpolar solvents are also strongly temperature dependent. It is indicated that the excited singlet (S1) state of C120 undergoes a fast activation-controlled nonradiative deexcitation in nonpolar solvents, which is absent in all other solvents. LFP and PR studies indicate that the intersystem crossing process is negligible for the present dye in all the solvents studied. Photophysical behavior of C120 in nonpolar solvent has been rationalized assuming that in these solvents the dye exists in a nonpolar structure, with its 7-NH2 group in a pyramidal configuration. In this structure, since the 7-NH2 group is bonded to the 1,2-benzopyrone moiety by a single bond, the former group can undergo a fast flip-flop motion, which in effect causes the fast nonradiative deexcitation of the dye excited state. In moderate to higher polarity solvents, it is indicated that the dye exists in an intramolecular charge-transfer structure, where the bond between 7-NH2 group and the 1,2-benzopyrone moiety attains substantial double bond character. In this structure, the flip-flop motion of the 7-NH2 group is highly restricted and thus there is no fast nonradiative deexcitation process for the excited dye. © 2003 American Institute of Physics.
© 2004 American Institute of Physics.
The solvatochromism of several polar solutes, including some that contain both hydrogen bond-donating and -accepting properties (coumarins 1, 102, 120, 151, 152, and 153; nile red; and 4-aminofluorenone), is analyzed in terms of three models: the Reichardt single parameter ETN polarity scale, the multiparameter Kamlet−Taft equation, and the reaction field model. We use a “step-forward” procedure to determine which terms of the Kamlet−Taft equation are statistically significant in fitting the data. These equations provide the best fits to the data in almost all cases. We also find a correlation between the parameters s and a, which quantify the effects on the transition energy related to the solvatochromic parameters π* and α, respectively. This relationship suggests that the magnitude of a is not indicative of the strength of the hydrogen-bonding interaction, but rather reflects the additional field produced from the dipole moment of a hydrogen bond-donating molecule that is held in an orientation that roughly parallels the solute dipole.
We report our calculations on a series of coumarin molecules. Using semiempirical methods with an AMI parametrization, we have calculated the ground-state, first excited triplet state, and first excited singlet state energies and dipole moments as well as their dependence on the geometries of different labile side groups. We find that for all of the coumarins there are excited triplet states in close energetic proximity to the excited singlet states, and the relative ordering of these states depends on the substituents attached to the coumarin chromophore.
Among several newly synthesized coumarins are 7-pyrrolyl coumarins and carbazole-coumarin hybrids (2-pyranone-condensed carbazoles). Their spectral properties and crystal structures are discussed by comparing with those of related 7-aminocoumarin derivatives. Carbazole-coumarin hybrids exhibit absorption properties common to those of 7-aminocoumarins and simple carbazoles.
We present calculations of various properties of the ground and excited states of Coumarins 151 and 120. These and related coumarins are important in investigating ultrafast solvation processes in liquids and complex solutions as well as being important acceptors in model electron-transfer systems. We calculate the following: (1) the electronic excitation energies to several low-lying singlet states, (2) ground and excited-state dipole moments, (3) solvation effects on excitation energies, and (4) the properties of single Coumarin 151-water complexes. We test our Time-Dependent Density Functional Theory (TDDFT) calculations against CASSCF, CASPT2 (both single and multistate versions), CIS, and ZINDO. Using TDDFT, we find excellent agreement with experimental S 1 r S 0 excitation energies. On the basis of these results, we address several outstanding questions for these systems and find: (1) that TICT-formation is unlikely upon photexcitation for gas-phase C151, (2) a greater tendency toward a planar amine group for the S 1 state than for the ground state, (3) significant differences between our gas-phase ground-state dipole moment and the experimental value, and (4) TDDFT results for water-Coumarin 151 complexes are in good agreement with the experimental results of Topp and co-workers.
The optical properties of molecules in complex environments were investigated within hybrid time-dependent density functional theory / molecular mechanics (TDDFT/MM) simulation studies. The potential energy surface in the excited state is described within time-dependent density functional theory (TDDFT). The solvent is described through a molecular mechanics approach and the effects due to the inhomogeneities of the electric field of the solvent molecules are fully included. The results for different systems including both n → π* and π → π* transitions are discussed. We apply this TDDFT/MM technique to the study of the properties of the ground state and of the first excited singlet state of two different systems: acetone in water and aminocoumarins in water and acetonitrile. Our approach yields quantitative information on the solvent-induced shifts, both batho- and hypsochromic, of the electronic absorption spectra, and on the effect of a protic and an aprotic solvent on the spectral shift. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005
Time-resolved fluorescence spectra of three amino-substituted coumarin dyes have been recorded in methanol and dimethyl sulfoxide using the fluorescence upconversion technique with an apparatus response function of 200 fs fwhm. The three fluorinated coumarins are the 7-amino-4-trifluoromethylcoumarin (C151), the 7-diethylamino-4-trifluoromethylcoumarin (C35), and the rigidified aminocoumarin with a julolidine structure (C153). The dynamic Stokes shifts are found to be dominated by an ultrafast component with a characteristic time shorter than the present time resolution of 50 fs. The dynamic Stokes shifts are compared to estimations based on a ""Kamlet and Taft"" analysis of steady-state data in 20 solvents. It is found that the ultrafast component can be assigned mainly to intramolecular relaxation. The influences of photoinduced changes of solute-solvent hydrogen bonds on the observed spectral shifts are discussed. The breaking of hydrogen bonds at the amino group is very fast in both solvents and embedded in the ultrafast solvent inertial relaxation, while the reformation of hydrogen bonds at the carbonyl group is believed to occur on the 10-20 ps time scale in the hydrogen bond donating (HBD) solvent methanol. However, it is impossible to unambiguously correlate a particular experimental time constant with the breaking or the formation of a hydrogen bond.
Infrared-optical double-resonance measurements have been made for electronically excited, hydrogen-bonded clusters involving Coumarin 151 molecules under jet-cooled conditions. Two main differences are reported between the ground and excited states:1.Infrared resonances attributed to “donor”-OH and -NH bonds, as well as the symmetric-stretch modes of non-hydrogen-bonded NH2 tend to downshift in energy and are usually intensified in the excited state. Also, “donor”-NH groups tend to develop an additional strong infrared resonance in the excited state, which is observed for clusters of C151 with methanol, ethanol, di-isopropyl ether, and for the “B” conformers of the water n=1, 2 and acetone n=1 clusters. The effect evidently requires no activation energy, since it is observed following electronic-origin excitation. A similar effect is also observed in the doubly-hydrogen-bonded dimers of some aminophthalimide molecules. This result is discussed in terms of possible excited-state proton transfer and Fermi resonance interactions.2.Several clusters have shown changes in the infrared spectra consistent with vibronically-induced conformational rearrangement. The driving force is the relative stabilization of the NH2 proton-donor site in the excited state and the formation of a strong NH⋯O hydrogen bond. The cases noted here are C151/(H2O)2, C120/(H2O)2 and C151/((CH3)2CO)1. One surprising observation here is that the activation energy for the conformational relaxation of C151/(H2O)2 is measured to be only 60 cm−1, and yet the displacement of the water dimer within the cluster exceeds 7 Å.
The multireference Møller–Plesset perturbation (MRMP) theory with complete active space self-consistent field (CASSCF) reference functions is applied to the study of the singlet and triplet valence π–π* excited states and Rydberg excited states of benzene in the ultraviolet region. The overall accuracy of MRMP is surprisingly high. The average deviations of the excitation energies from the available experimental values are 0.1 eV for the valence excited states and 0.15 eV for the Rydberg states. A comparison is made with recent results of single reference-based methods. It is concluded that MRMP is able to describe satisfactorily excited states with a double excitation character, while single reference-based methods are not.
Time dependent density functional methods are applied in the adiabatic approximation to compute low-lying electronic excitations of N2, ethylene, formaldehyde, pyridine and porphin. Out of various local, gradient-corrected and hybrid (including exact exchange) functionals, the best results are obtained for the three-parameter Lee-Yang-Parr (B3LYP) functional proposed by Becke. B3LYP yields excitation energies about 0.4 eV too low but typically gives the correct ordering of states and constitutes a considerable improvement over HF-based approaches requiring comparable numerical work.
A density-functional formalism comparable to the Hohenberg-Kohn-Sham theory of the ground state is developed for arbitrary time-dependent systems. It is proven that the single-particle potential $v(\stackrel{$\rightarrow${}}{\mathrm{r}}t)$ leading to a given $v$-representable density $n(\stackrel{$\rightarrow${}}{\mathrm{r}}t)$ is uniquely determined so that the corresponding map $v$\rightarrow${}n$ is invertible. On the basis of this theorem, three schemes are derived to calculate the density: a set of hydrodynamical equations, a stationary action principle, and an effective single-particle Schr\"odinger equation.
Despite the remarkable thermochemical accuracy of Kohn–Sham density-functional theories with gradient corrections for exchange-correlation [see, for example, A. D. Becke, J. Chem. Phys. 96, 2155 (1992)], we believe that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional containing local-spin-density, gradient, and exact-exchange terms is tested on 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total atomic energies of first- and second-row systems. This functional performs significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.
This work presents theory, implementation, and validation of excited state properties obtained from time-dependent density functional theory (TDDFT). Based on a fully variational expression for the excited state energy, a compact derivation of first order properties is given. We report an implementation of analytic excited state gradients and charge moments for local, gradient corrected, and hybrid functionals, as well as for the configuration interaction singles (CIS) and time-dependent Hartree-Fock (TDHF) methods. By exploiting analogies to ground state energy and gradient calculations, efficient techniques can be transferred to excited state methods. Benchmark results demonstrate that, for low-lying excited states, geometry optimizations are not substantially more expensive than for the ground state, independent of the molecular size. We assess the quality of calculated adiabatic excitation energies, structures, dipole moments, and vibrational frequencies by comparison with accurate experimental data for a variety of excited states and molecules. Similar trends are observed for adiabatic excitation energies as for vertical ones. TDDFT is more robust than CIS and TDHF, in particular, for geometries differing significantly from the ground state minimum. The TDDFT excited state structures, dipole moments, and vibrational frequencies are of a remarkably high quality, which is comparable to that obtained in ground state density functional calculations. Thus, yielding considerably more accurate results at similar computational cost, TDDFT rivals CIS as a standard method for calculating excited state properties in larger molecules. (C) 2002 American Institute of Physics.
We present a hybrid time-dependent density functional/molecular mechanics (TDDFT/MM) simulation study on the optical properties of aminocoumarins in gas phase and solution. As solvation is described through a molecular approach, the effects due to the inhomogeneities of the electric field of the solvent molecules are fully included. We focus on the ground state and first excited singlet state properties of C151, C35 and C153, three aminocoumarins for which a homogeneous set of experimental data is available. Our approach is able to give quantitative information on the redshifts in water and acetonitrile, two solvents which show different H-bonding properties. In addition, it is able to quantify the effects of chemical substituents, such as the spectral redshift due to the increased alkylation at the amino position.
Current gradient-corrected density-functional approximations for the exchange energies of atomic and molecular systems fail to reproduce the correct 1/r asymptotic behavior of the exchange-energy density. Here we report a gradient-corrected exchange-energy functional with the proper asymptotic limit. Our functional, containing only one parameter, fits the exact Hartree-Fock exchange energies of a wide variety of atomic systems with remarkable accuracy, surpassing the performance of previous functionals containing two parameters or more.
A correlation-energy formula due to Colle and Salvetti [Theor. Chim. Acta 37, 329 (1975)], in which the correlation energy density is expressed in terms of the electron density and a Laplacian of the second-order Hartree-Fock density matrix, is restated as a formula involving the density and local kinetic-energy density. On insertion of gradient expansions for the local kinetic-energy density, density-functional formulas for the correlation energy and correlation potential are then obtained. Through numerical calculations on a number of atoms, positive ions, and molecules, of both open- and closed-shell type, it is demonstrated that these formulas, like the original Colle-Salvetti formulas, give correlation energies within a few percent.
Absorption and fluorescence emission of 4 and 7 substituted coumarins viz. C 440, C 490, C 485 and C 311 have been studied in various polar and non-polar organic solvents. These coumarin dyes are substituted with alkyl, amine and fluorine groups at 4- and 7-positions. They give different absorption and emission spectra in different solvents. The study leads to a possible assignment of energy level scheme for such coumarins including the effect on ground state and excited state dipole moments due to substitutions. Excited state dipole moments of these dyes are calculated by solvetochromic data experimentally and theoretically these are calculated by PM 3 method. The dipole moments in excited state, for all molecules investigated here, are higher than the corresponding values in the ground state. The increase in dipole moment has been explained in terms of the nature of excited state and resonance structure.
For Abstract see ChemInform Abstract in Full Text.
The steady-state spectral properties (absorption and emission) of three structurally similar Coumarin dyes, C151, C500, and C35 were investigated in 13 different solvents. A Kamlet-Taft (KT) analysis of the spectral peak frequencies reveals that, in addition to polarity, hydrogen bonding between the carbonyl oxygen and a protic solvent in the excited state imparts maximum stabilization for C151 and minimum for C35, while that for C500 lies in between. The spectral properties of the three dyes in two solvents, chloroform and THF, which have similar polarity in the KT scale but have only hydrogen-bond donor (chloroform) and hydrogen-bond acceptor (THF) properties, are seen to be sensitive to the substitution pattern at the 7-amino position. In addition, a slow emission spectral relaxation is observed for C151 and C500 having a time constant of approximately 500 ps in chloroform. For C35 this was too fast to be detected by the time resolution of our setup. The exact reason for this slow spectral relaxation in chloroform is unclear at present, and further studies are needed to understand clearly the structural effects on the hydrogen bonding dynamics of these dyes.
The absorption spectra of aminocoumarin C151 in water and n-hexane solution are investigated by an explicit quantum chemical solvent model. We improved the efficiency of the frozen-density embedding scheme, as used in a former study on solvatochromism (J. Chem. Phys. 2005, 122, 094115) to describe very large solvent shells. The computer time used in this new implementation scales approximately linearly (with a low prefactor) with the number of solvent molecules. We test the ability of the frozen-density embedding to describe specific solvent effects due to hydrogen bonding for a small example system, as well as the convergence of the excitation energy with the number of solvent molecules considered in the solvation shell. Calculations with up to 500 water molecules (1500 atoms) in the solvent system are carried out. The absorption spectra are studied for C151 in aqueous or n-hexane solution for direct comparison with experimental data. To obtain snapshots of the dye molecule in solution, for which subsequent excitation energies are calculated, we use a classical molecular dynamics (MD) simulation with a force field adapted to first-principles calculations. In the calculation of solvatochromic shifts between solvents of different polarity, the vertical excitation energy obtained at the equilibrium structure of the isolated chromophore is sometimes taken as a guess for the excitation energy in a nonpolar solvent. Our results show that this is, in general, not an appropriate assumption. This is mainly due to the fact that the solute dynamics is neglected. The experimental shift between n-hexane and water as solvents is qualitatively reproduced, even by the simplest embedding approximation, and the results can be improved by a partial polarization of the frozen density. It is shown that the shift is mainly due to the electronic effect of the water molecules, and the structural effects are similar in n-hexane and water. By including water molecules, which might be directly involved in the excitation, in the embedded region, an agreement with experimental values within 0.05 eV is achieved.
An effective state specific (SS) model for the inclusion of solvent effects in time dependent density functional theory (TD-DFT) computations of excited electronic states has been developed and coded in the framework of the so-called polarizable continuum model (PCM). Different relaxation time regimes can be treated thus giving access to a number of different spectroscopic properties together with solvent relaxation energies of paramount relevance in electron transfer processes. SS and conventional linear response (LR) models have been compared for two benchmark systems (coumarin 153 and formaldehyde in different solvents) and in the limiting simple case of a dipolar solute embedded in a spherical cavity. The results point out the complementarity of LR and SS approaches and the advantages of the latter model especially for polar solvents. The favorable scaling properties of PCM-TD-DFT models in both SS and LR variants and their availability in effective quantum mechanical codes pave the route for the computation of reliable spectroscopic properties of large molecules of technological and/or biological interest in their natural environments.
Shapes changed by solvent: An ab initio method for calculating the absorption spectra of large molecules including solvent effects and molecular vibrations shows how the solvent can shift the spectra and modulate their shapes (see picture; black lines: stick representation of absorption spectrum). The computed spectra of coumarin C153 in various solvents agree with the experimental ones. (Figure Presented).
Using time-dependent density functional theory (TD-DFT), configuration interaction single (CIS) method, and approximate coupled cluster singles and doubles (CC2) method, we investigated the absorption spectra of coumarin derivative dyes (C343, NKX-2388, NKX-2311, NKX-2586, and NKX-2677), which have been synthesized for efficient dye-sensitized solar cells. The CC2 calculations are found in good agreement with the experimental results except for the smallest coumarin dye (C343). TD-DFT underestimates the vertical excitation energy of the larger coumarin dyes (NKX-2586 and -2677). Solvents (methanol) are found to induce a red shift of the vertical excitation energies, and their effects on the molecular geometry and the electronic structure are examined in detail. The deprotonated form of coumarin is also investigated, where a blue shift of the vertical excitation energies is observed.
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