Anna Painelli

Vibronic contribution to static NLO properties: exact results for the DA dimer

Abstract The standard 2-state model for push–pull polyenes is extended to include electron–phonon coupling. Exact adiabatic results are obtained for the vibronic contributions to static non-linear-optical polarizabilities. These contributions are very large. The Herzberg–Teller expansion offers a well-controlled approximation and naturally relates these vibronic contributions to vibrational intensities. The model parameters can be extracted from experiment: the analysis of available experimental data and the comparison with results of quantum chemistry calculations prove that the proposed model is very simple yet very accurate. The important role of electron–phonon coupling in linear and non-linear optical properties of other classes of compounds is also discussed.

Symmetry Breaking in Octupolar Chromophores: Solvatochromism and Electroabsorption

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.

A non-perturbative approach to solvatochromic shifts of push–pull chromophores

Abstract The two-state model for push–pull chromophores (the DA dimer model) is extended to account for the coupling of electrons to molecular vibrations and for the interaction with the solvent. The resulting self-consistent two-state model is solved exactly. In this Letter, we present exact results on linear absorption and emission spectra. Standard perturbative treatments of both vibronic and solvent effects on spectral properties are proved to fail in highly non-linear systems as push–pull chromophores. The use of solvatochromic data to extract mesomeric dipole moments as well as to investigate the dynamics of the solvent relaxation is critically reviewed.

Amplification of NLO responses: vibronic and solvent effects in push–pull polyenes

Abstract The two-state model for push–pull polyenes is extended to account for the coupling of electrons with adiabatic vibrations, and for the interaction with the solvent, modeled in terms of a reaction field. Both interactions introduce a self-consistent term in the dimer Hamiltonian and therefore lead to a highly non-linear behavior. Exact results on the self-consistent dimer demonstrate a large amplification of static NLO responses due to the coupling of charge oscillations with the slow degrees of freedom associated to molecular vibrations and/or solvent reorientation.

First hyperpolarizability dispersion of the octupolar molecule crystal violet: multiple resonances and vibrational and solvation effects.

The first hyperpolarizability (β) dispersion curve is measured for the first time for an octupolar nonlinear optical (NLO) molecule (crystal violet, CV) and modeled theoretically, yielding an in-depth understanding of the electronic structure and vibronic and solvation effects on such octupolar conjugated systems. Tunable wavelength hyper-Rayleigh scattering (HRS) measurements were performed on this prototypical octupolar molecule in the broad fundamental wavelength range of 620-1580 nm, showing significant shortcomings of the commonly used β dispersion models. Three well-separated β resonances involving the lowest-energy state and several higher excited states are clearly observed, including a significant contribution from a nominally one-photon forbidden transition. The experimental results for second-harmonic wavelengths above 330 nm are successfully modeled by means of a vibronically coupled essential-state description for octupolar chromophores, developed by Terenziani et al. (J. Phys. Chem. B 2008, 112, 5079), which takes into account polar solvation effects. The relative intensities of the various resonances, including the one below 330 nm, are also quantified by quantum chemical calculations. Furthermore, interesting effects of inhomogeneous broadening due to polar solvation of the two-dimensional chromophore are recognized in both linear and nonlinear spectra, allowing us to quantitatively address the long-standing problem of the band shape of the linear absorption spectrum of CV. This clearly demonstrates that extensive wavelength-dependent HRS measurements, as presented in this work, are essential to the characterization and design of NLO materials and represent a powerful tool to gain valuable information on molecular excitations and environmental effects in general.

Bistability in Fc-PTM crystals: the role of intermolecular electrostatic interactions.

Fc-PTM is a valence tautomeric radical, where the ferrocene (Fc) group, a good electron donor, is linked by an ethylenic spacer to a perchlorotriphenylmethyl radical (PTM(*)), a good electron acceptor. In solution this compound exists mainly in the neutral Fc-PTM(*) form which can be photoexcited through an intramolecular electron transfer to the zwitterionic Fc(+*)-PTM(-) form. By contrast, in crystals of Fc-PTM at room temperature both the neutral and the zwitterionic forms coexist, pointing to a true bistability phenomenon. We rationalize these findings accounting for the role of intermolecular electrostatic interactions in Fc-PTM crystals. In fact the energy of the zwitterionic Fc(+*)-PTM(-) form is lowered in the crystal by attractive electrostatic intermolecular interactions and the cooperative nature of these interactions explains the observed coexistence of neutral Fc-PTM(*) and zwitterionic Fc(+*)-PTM(-) species. The temperature evolution of Mossbauer spectra of Fc-PTM is quantitatively reproduced adopting a bottom-up modeling strategy that combines a molecular model, derived from optical spectra of Fc-PTM in solution, with a model for intermolecular electrostatic interactions, supported by quantum-chemical calculations. Fc-PTM then offers the first experimental demonstration of bistability induced by electrostatic interactions in crystals of valence tautomeric donor-acceptor molecules.

Accurate electron-molecular vibration coupling constants from powders optical spectra: TCNQ and TTF

Abstract A simple but accurate method is proposed for the experimental determination of electron — molecular vibration (e-mv) coupling constants from the optical spectra of powdered samples of dimerized, segregated stack organic quasi-one-dimensional radical salts. By re-examining various TCNQ-based compounds a transferable set of coupling constants is obtained for this key molecular structure. The application of our method to TTF Br salt leads to TTF e-mv coupling constants which nicely compare with those determined from TTF-Chloranil.

Intra- and Intermolecular Charge Transfer in Aggregates of Tetrathiafulvalene-Triphenylmethyl Radical Derivatives in Solution

An extensive investigation of aggregation phenomena occurring in solution for a family of electron donor-acceptor derivatives, based on polychlorotriphenylmethyl radicals (PTM) linked via a vinylene-bridge to tetrathiafulvalene (TTF) units, is presented. A large set of temperature and/or concentration dependent optical absorption and electron spin resonance (ESR) spectra in a solution of dyads bearing different number of electrons and/or with a hydrogenated PTM residue offer reliable information on the formation of homo dimers and mixed valence dimers. The results shed light on the reciprocal influence of intramolecular electron transfer (IET) within a dyad and the intermolecular charge transfer (CT) occurring in a dimer between the TTF residues and are rationalized based on a theoretical model that describes both interactions.

Supramolecular interactions in clusters of polar and polarizable molecules

(Dated: February 2, 2008)We present a model for molecular materials made up of polar and polarizable molecular units.A simple two state model is adopted for each molecular site and only classical intermolecular in-teractions are accounted for, neglecting any intermolecular overlap. The complex and interestingphysics driven by interactions among polar and polarizable molecules becomes fairly transparent inthe adopted model. Collective effects are recognized in the large variation of the molecular polaritywith supramolecular interactions, and cooperative behavior shows up with the appearance, in at-tractive lattices, of discontinuous charge crossovers. The mf approximation proves fairly accurate inthe description of the gs properties of MM, including static linear and non-linear optical suscepti-bilities, apart from the region in the close proximity of the discontinuous charge crossover. Sizeabledeviations from the excitonic description are recognized both in the excitation spectrum and inlinear and non-linear optical responses. New and interesting phenomena are recognized near thediscontinuous charge crossover for non-centrosymmetric clusters, where the primary photoexcitationevent corresponds to a multielectron transfer.

Vibronic contributions to resonant NLO responses: two-photon absorption in push–pull chromophores

Abstract Two-photon absorption (TPA) spectra of push–pull chromophores are described in terms of a two-state model accounting for electron-vibration coupling. Vibrations have two main effects in TPA spectra. The most obvious one is the appearance of a vibrational structure in the spectrum; in this respect we find large Herzberg–Teller (HT) contributions. The second effect was not recognized so far: vibrational states contribute a new channel to TPA process, that shows up with a blue-shift and a distortion of the spectrum.