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ABSTRACT: An extensive spectroscopic analysis is presented of an elongated polycondensed dye with a donor-acceptor substitution. The charge-transfer (CT) state, polarized along the long molecular axis, is close in energy to a local excitation (LE) of the polycondensed system, roughly polarized along the short molecular axis, which makes this system particularly suitable to investigate the subtle LE/CT interplay. An essential-state model is presented that quantitatively reproduces absorption and fluorescence spectra, as well as fluorescence emission and excitation anisotropy spectra collected in solvents of different polarity and viscosity, which sets a sound basis for the understanding of how solvent polarity and solvent relaxation affect the nature of low-lying excitations. The markedly different fluorescence emission and excitation anisotropy spectra measured in glassy and liquid polar solvents unambiguously demonstrate the major role played by solvent relaxation in the definition of fluorescence properties of the dye.
Chemistry 11/2012; · 5.93 Impact Factor
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ABSTRACT: A four essential-state model is proposed for a detailed description of optical spectra of a family of anionic polymethine dyes. Extensive spectroscopic data from the literature are quantitatively described within a model that accounts for non-adiabatic electron-vibration coupling and for polar solvation.
ChemPhysChem 06/2012; 13(11):2795-800. · 3.41 Impact Factor
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ABSTRACT: Joint experimental and theoretical work is presented on two quadrupolar D-π-A-π-D chromophores characterized by the same bulky donor (D) group and two different central cores. The first chromophore, a newly synthesized species with a malononitrile-based acceptor (A) group, has a V-shaped structure that makes its absorption spectrum very broad, covering most of the visible region. The second chromophore has a squaraine-based core and therefore a linear structure, as also evinced from its absorption spectra. Both chromophores show an anomalous red shift of the absorption band upon increasing solvent polarity, a feature that is ascribed to the large, bulky structure of the molecules. For these molecules, the basic description of polar solvation in terms of a uniform reaction field fails. Indeed, a simple extension of the model to account for two independent reaction fields associated with the two molecular arms quantitatively reproduces the observed linear absorption and fluorescence as well as fluorescence anisotropy spectra, fully rationalizing their nontrivial dependence on solvent polarity. The model derived from the analysis of linear spectra is adopted to predict nonlinear spectra and specifically hyper-Rayleigh scattering and two-photon absorption spectra. In polar solvents, the V-shaped chromophore is predicted to have a large HRS response in a wide spectral region (approximately 600-1300 nm). Anomalously large and largely solvent-dependent HRS responses for the linear chromophores are ascribed to symmetry lowering induced by polar solvation and amplified in this bulky system by the presence of two reaction fields.
The Journal of Physical Chemistry B 04/2012; · 3.70 Impact Factor
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ABSTRACT: An extensive spectroscopic study is presented of two asymmetrically substituted fluorene dyes and of the related spiro dimers in both liquid and glassy solvents. Essential-state models are developed to accurately reproduce linear absorption and fluorescence spectra and their complex dependence on solvent polarity. The same models are exploited to quantitatively calculate fluorescence excitation and anisotropy spectra in rigid matrixes. Impressive red-edge effects observed for spiro dimers in glassy polar solvents are accurately reproduced and understood. Interchromophore interactions in the spiro dimers are very small, leading to marginal effects in absorption and fluorescence spectra, but they effectively promote energy transfer between the two chromophores, as best evidenced comparing anisotropy spectra of the substituted fluorenes and of corresponding spiro dimers dissolved in glassy solvent matrixes.
The Journal of Physical Chemistry B 08/2011; 115(39):11420-30. · 3.70 Impact Factor
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ABSTRACT: Fluorescence excitation and emission spectra and corresponding anisotropies of branched intramolecular charge-transfer (ICT) chromophores are reported. To unravel the role of disorder related to polar solvation, we collected spectra of molecules with octupolar (C(3)) symmetry in glasses obtained from solvents of different polarity. Thermal disorder related to polar solvation is demonstrated by the dependence of fluorescence excitation/emission spectra and anisotropy spectra on the emission/excitation wavelength. In particular, for excitation in the red edge of the absorption band, the fluorescence anisotropy signal deviates from the 0.1 value expected for octupolar chromophores, approaching the limiting 0.4 value. A qualitatively different result is observed in nonpolar solvents. Based on essential-state models for ICT chromophores, we develop an original approach for the calculation of anisotropy spectra that quantitatively reproduces experimental data. The model, accounting for the coupling between electrons and molecular vibrations and polar solvation, leads to a thorough understanding of the phenomena at the basis of solute-solvent interactions in branched ICT chromophores. A clear distinction is made between symmetry-breaking phenomena in the excited state and disorder-induced lowering of the molecular symmetry. Their relation with red-edge effects is pointed out.
The Journal of Physical Chemistry B 06/2011; 115(21):7009-20. · 3.70 Impact Factor
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ABSTRACT: In this paper we present the synthesis, spectroscopic characterization and theoretical modelling of two pairs of correlated dipolar and octupolar donor-acceptor conjugated chromophores, based on the triphenylamine branching centre. The two pairs of chromophores differ for the electron withdrawing end-groups. Linear absorption, fluorescence and two-photon absorption of all the compounds in different solvents can be well described by the use of charge-resonance theoretical models based on essential-state descriptions of the electronic structure, and taking into account the coupling to effective molecular vibrations and to polar solvation degrees of freedom. On the contrary, the alternative Frenkel exciton model does not provide a good description of the observed behavior. The robustness of the proposed theoretical models is demonstrated for the first time by the fact that the modulation of a single molecular parameter (the one linked to the electron-withdrawing ability of the end groups) is enough to describe the evolution of the spectroscopic properties along the whole series of chromophores, both "intra-pair" and "inter-pair". The effectiveness of the approach suggests that this kind of theoretical modelling can be very useful to predict different properties of the compounds at hand or of correlated structures of increasing complexity, such as dendrons and dendrimers, giving a guide to the synthesis of (macro)molecules for applications in light-emitting and nonlinear optical devices, artificial light-harvesting systems or optical imaging of living tissues.
Physical Chemistry Chemical Physics 10/2010; 12(37):11715-27. · 3.57 Impact Factor
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The Journal of Physical Chemistry B 02/2010; · 3.70 Impact Factor
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ABSTRACT: Two dimers of a heteroaromatic quadrupolar (acceptor-donor-acceptor) chromophore have been synthesized with different interchromophoric distances. Optical spectra of dimers in solution show a red shift of the linear absorption band upon decreasing the interchromophore distance, while fluorescence and two-photon absorption spectra are only marginally affected by the interactions. A bottom up approach is adopted to describe the spectra: via a detailed spectroscopic analysis of the monomeric species in solution, we define an essential-state model for the isolated chromophore and use this information to set up a model for the dimers also accounting for interchromophore electrostatic interactions. To discriminate between static screening governed by the static dielectric constant and dynamical screening at optical frequencies, we first solve the problem in the mean-field approximation and then define the excitonic Hamiltonian on the resulting best excitonic basis. Along this line, the evolution of spectral properties with the interchromophore distance is properly rationalized.
The Journal of Physical Chemistry B 01/2010; 114(2):882-93. · 3.70 Impact Factor
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Luca Grisanti, Cristina Sissa,
Francesca Terenziani,
Anna Painelli,
Dominique Roberto,
Francesca Tessore,
Renato Ugo,
Silvio Quici,
Ilaria Fortunati,
Eleonora Garbin,
Camilla Ferrante,
Renato Bozio
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ABSTRACT: The intensity of the two-photon absorption (TPA) spectrum of a terpyridine ligand acting as a D-pi-A chromophore (D = donor and A = acceptor) is enhanced by a factor of about 2 upon coordination to ZnCl(2). Based on an analysis of linear absorption and fluorescence spectra of both the ligand and its Zn(II) complex, we have defined essential-state models for the two species. Linear and TPA spectra of the ligand are well reproduced in terms of a two-state model accounting for the D-pi-A <--> D(+)-pi-A(-) charge resonance. However, the enhancement of the TPA response of its Zn(II) complex can only be understood by extending the model to account for the active role of the "ZnCl(2)" moiety acting as a virtual A(v) acceptor group of a D-pi-AA(v) structure. The virtual D(+)AA(v)(-) state of the relevant three-state model has negligible weight in the ground state but contributes to the first excited state. The resulting increase of the excited-state dipole moment is responsible for the enhancement of the TPA cross section, and also explains the increase of the second order nonlinear optical response upon coordination.
Physical Chemistry Chemical Physics 11/2009; 11(41):9450-7. · 3.57 Impact Factor
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ABSTRACT: We present theoretical models for electroabsorption (EA) spectra of polar and multipolar (quadrupolar and octupolar) organic chromophores in solutions. Based on essential-state models for the electronic structure, we obtain closed expressions for the EA spectra of these dyes. For polar dyes, we regain the well-known Liptay result, which relates the EA spectrum to the linear absorption spectrum and its first and second derivatives. For multipolar dyes, an additional contribution is found, which is due to the field-induced absorption toward dark states: when the dark states are close in energy to allowed states, this term leads to spurious second-derivative-like structures in the EA spectrum. This is particularly well-apparent for quadrupolar dyes where second-derivative contributions to the EA signal are symmetry-forbidden. The extension of essential-state models to account for slow degrees of freedom, including molecular vibrations and polar solvation, leads to a more-realistic description of optical spectra but hinders the analytical treatment of EA. However, numerically exact EA spectra can be obtained along the same lines that have been recently developed for linear and two-photon absorption spectra of (multi)polar organic chromophores in solution. Essential-state models offer the possibility for a joint analysis of linear and nonlinear spectra (including EA) of (multi)polar dyes: reliable information can then be extracted also on molecular properties for systems where overlapping signals from allowed and dark states would hinder the standard analysis of EA spectra.
The Journal of Physical Chemistry A 10/2008; 112(37):8697-705. · 2.95 Impact Factor
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ABSTRACT: In this contribution, we adopt an essential-state description for octupolar (AD3 or DA3) chromophores (where A is an electron-acceptor and D is an electron-donor) that also accounts for the coupling of electrons to molecular vibrations and for solvation effects. The first excited state of octupolar chromophores is always multistable and can therefore support symmetry breaking. In particular, symmetry is always broken in the relaxed excited state of octupolar dyes in polar solvents, with consequent localization of the excitation on one of the dipolar molecular branches. This rationalizes the common observation of strongly solvatochromic fluorescence spectra for octupolar chromophores. The model is validated through the comparison with experimental data. The essential-state model is also adopted to derive a perturbative expression for the electroabsorption spectrum: if compared with the formalism derived for dipolar molecules, a new term appears for octupolar chromophores, due to the field activation of an otherwise dark transition. The importance and implications of this term are discussed.
The Journal of Physical Chemistry B 05/2008; 112(16):5079-87. · 3.70 Impact Factor
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ABSTRACT: We report visible, Raman, and infrared spectra of self-assembled monolayers (SAMs) formed by the donor-(pi-bridge)-acceptor chromophore, Z-beta-[N-(omega-acetylthioalkyl)-4-quinolinium]-alpha-cyano-4-styryldicyanomethanide (CH3CO-S-CnH2n-Q3CNQ where n=8, 10), on gold-coated substrates. The data are compared with the spectra collected for the same compound in solution and in the solid state, and with those obtained for a Langmuir-Blodgett (LB) monolayer of C16H33-Q3CNQ deposited on gold. Spectral analysis confirms that in solution, in the solid state and in the LB film the chromophore has a zwitterionic (D+-pi-A-) ground state. At variance with this well-known result, our data show that in SAMs deposited on gold the chromophore has a more neutral, quinoid ground state. We relate this difference to the different packing of the molecules in the two different films: in SAMs in fact the chromophores stand almost vertical with respect to the substrate, whereas in LB films they make an angle of about 45 degrees. The Q3CNQ molecule is a well-known molecular rectifier, and for SAMs we were able to check the direction of electron flow at forward bias on the same samples that have been characterized spectroscopically, shedding light on the rectification mechanism.
ChemPhysChem 11/2007; 8(15):2195-201. · 3.41 Impact Factor
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Cristina Sissa
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ABSTRACT: This thesis presents an extensive study of charge-transfer (CT) dyes, an interesting class of organic chromophores for advanced applications in the fields of nonlinear optics, energy harvesting, LEDs and solar cells, just to cite a few. The study combines theoretical and experimental work to offer a reliable description of optical properties and spectra of CT chromophores in different environments. CT dyes constitute excellent model systems for investigating charge- and energy-transfer processes, that represent key phenomena in physics, chemistry and biology. The work starts with the description of essential-state models that are at the heart of the theoretical approach that we have developed for CT dyes. The low-energy physics of CT chromophores is governed by charge resonance between electron-donor (D) and electron-acceptor (A) groups and their optical spectra and properties are dominated by CT transitions. The behavior of CT dyes can be described in terms of comparatively simple models that just account for the few relevant degrees of freedom. These essential-state models are general and allow for the rationalization of optical spectra and properties of families of CT chromophores, and to set up reliable structure-properties relationships as required to guide the chemical synthesis. A thorough understanding of the physics of CT chromophores requires their detailed spectroscopic characterization. Essential-state models allow for the calculation of linear and nonlinear spectra of polar and multipolar chromophores accounting for molecular vibrations and for polar solvation, making possible a detailed analysis of the spectral position, intensities and bandshapes. In this thesis, we develop the calculation of electroabsorption spectra, and we discuss, both at the experimental and theoretical level, fluorescence thermochromism and low-temperature fluorescence anisotropy spectra of different families of CT dyes. The models developed for CT dyes in solution naturally lent themselves to be extended to the description of the same dyes in more complex and challenging environments. Specifically, collective and cooperative phenomena are expected in aggregates, films, crystals and more generally in clusters where different chromophores interact via electrostatic interactions. Two families of systems are studied: the first one composed by a zwitterionic DA-chromophore, its covalent dimer and its self-assembled film on gold; the second one composed by a quadrupolar CT chromophore and its covalent dimers. These examples allow to demonstrate the power of the bottom-up approach in this context, and to fully exploit the advantages of essential-state models. The same electrostatic interactions that are responsible for cooperative and collective phenomena in multichromophoric assemblies drive the phenomenon of energy transfer in heterochromophoric macromolecules. We are able to prove that the extended-dipole approximation for interchromophore interactions leads to qualitatively different results than the more commonly adopted point-dipole approximation. Specifically, the extended-dipole approximation opens new channels for energy transfer through optically dark states, that, in the point-dipole approximation are strictly forbidden.
