Time-resolved spectra of polar-polarizable chromophores in solution

A recently proposed model for steady-state spectra of polar-polarizable chromophores is extended to describe time-resolved spectra. The model, based on a two-state picture for the solute and on a continuum overdamped description for the (polar) solvent, grasps the essential physics of solvation dynamics, as demonstrated by the comparison with experimental spectra. The solute (hyper)polarizability is responsible for spectroscopic features that cannot be rationalized within the standard picture based on a linear perturbative treatment of the solute-solvent interaction. In particular, the temporal evolution of band-shapes and the appearance of temporary isosbestic points, two common puzzling features of observed spectra, are natural consequences of the molecular hyperpolarizability and of the consequent coupling between solvation and vibrational degrees of freedom.Comment: 14pages, including 7 figure

Conjugated donor-acceptor chromophores in solution: non-linearity at work

We propose a model that, accounting for the intrinsic non-linearity of the electronic system, is able to rationalize steady-state electronic and vibrational spectra of polar chromophores in solution, as well as time-resolved experiments.Comment: 5 pages, including 2 figure

Static polarizability of molecular materials: environmental and vibrational contributions

Modeling the dielectric behavior of molecular materials made up of large pi-conjugated molecules is an interesting and complex task. Here we address linear polarizabilities, and the related dielectric constant, of molecular crystals and aggregates made up of closed-shell pi-conjugated molecules with either a non-polar or largely polar ground-state, and also examine the behavior of mixed-valence (or charge-transfer) organic salts. We recognize important collective phenomena due to supramolecular interactions in materials with large molecular polarizabilities, and underline large vibrational contributions to the polarizability in materials with largely delocalized electrons.Comment: 18 pages, including 9 figure

Static NLO susceptibilities: testing approximation schemes against exact results

The reliability of the approximations commonly adopted in the calculation of static optical (hyper)polarizabilities is tested against exact results obtained for an interesting toy-model. The model accounts for the principal features of typical nonlinear organic materials with mobile electrons strongly coupled to molecular vibrations. The approximations introduced in sum over states and finite field schemes are analyzed in detail. Both the Born-Oppenheimer and the clamped nucleus approximations turn out to be safe for molecules, whereas for donor-acceptor charge transfer complexes deviations from adiabaticity are expected. In the regime of low vibrational frequency, static susceptibilities are strongly dominated by the successive derivatives of the potential energy and large vibrational contributions to hyperpolarizabilities are found. In this regime anharmonic corrections to hyperpolarizabilities are very large, and the harmonic approximation, exact for the linear polarizability, turns out totally inadequate for nonlinear responses. With increasing phonon frequency the role of vibrations smoothly decreases, until, in the antiadiabatic (infinite vibrational frequency) regime, vibrations do not contribute anymore to static susceptibilities, and the purely electronic responses are regained.Comment: 9 pages, including 3 figure

Optical spectra of organic dyes in condensed phases: the role of the medium polarizability

When designing molecular functional materials, the properties of the active specie, the dye, must be optimized fully accounting for the presence of a surrounding medium (a solvent, a polymeric matrix, etc) that may largely alter the dye behavior. Here we present an effective model to account for the spectroscopic effects of the medium electronic polarizability on the properties of charge-transfer dyes. Different classes of molecules are considered and the proposed antiadiabatic approach to solvation is contrasted with the adiabatic approach, currently adopted in all quantum chemical approaches to solvation. Transition frequencies and band-shapes are addressed, and the role of the medium polarizability on symmetry-breaking phenomena is also discussed

Enhanced two-photon absorption of organic chromophores: theoretical and experimental assessments

C. Katan present address: CNRS UMR6082 FOTON, INSA de Rennes, 20 avenue des Buttes de Coësmes, CS 70839, 35708 RENNES cedex 7, FranceInternational audienceFunctional organic materials with enhanced two-photon absorption (TPA) lead to new technologies in the fields of chemistry, biology, and photonics. In this article we review experimental and theoretical methodologies allowing detailed investigation and analysis of TPA properties of organic chromophores. This includes femtosecond two-photon excited fluorescence (TPEF) experimental setups and quantum-chemical methodologies based on time-dependent density functional theory (TD-DFT). We thoroughly analyze physical phenomena and trends leading to large TPA responses of a few series of model chromophores focusing on the effects of symmetric and asymmetric donor/acceptor substitution and branching

Excited-State Symmetry Breaking in an Aza-nanographene Dye

The photophysics of a structurally unique aza-analogue of polycyclic aromatic hydrocarbons characterized by 12 conjugated rings and a curved architecture is studied in detail. The combined experimental and computational investigation reveals that the lowest excited state has charge-transfer character in spite of the absence of any peripheral electron-withdrawing groups. The exceptionally electron-rich core comprised of two fused pyrrole rings is responsible for it. The observed strong solvatofluorochromism is related to symmetry breaking occurring in the emitting excited state, leading to a significant dipole moment (13.5 D) in the relaxed excited state. The anomalously small fluorescence anisotropy of this molecule, qualitatively different from what is observed in standard quadrupolar dyes, is explained as due to the presence of excited states being close in energy but having different polarization directions

Effects of dipolar interactions on linear and nonlinear optical properties of multichromophore assemblies: A case study

Claudine Katan ‘s present address : CNRS UMR6082 FOTON, INSA de Rennes, 20 avenue des Buttes de Coësmes, CS 70839, 35708 RENNES cedex 7, FranceInternational audienceInterchromophore interactions in flexible multidipolar structures for nonlinear optics were addressed by a combined experimental and theoretical study on two series of one-, two-, and three-chromophore systems in which identical push-pull chromophores are assembled through covalent and flexible linkers in close proximity. The photophysical and nonlinear optical properties (quadratic hyperpolarizability) of the multichromophore systems were investigated and compared to those of the monomeric chromophores. Multimers have larger dipole moments than their monomeric analogues, that is, the dipolar subchromophores self-orientate within the multimeric structures. This effect was found to depend on the intersubchromophore distance in a nontrivial manner, which confirms that molecular engineering of such flexible systems is more complex than in completely geometrically controlled systems. Electric-field-induced second-harmonic generation (EFISHG) measurements in solution revealed increased figures of merit as compared to the monomeric analogue. This effect increases with increasing number and polarity of the individual subchromophores in the nanoassembly and increasing spacing between dipolar subchromophores. Experimental results are interpreted by a theoretical model for interacting polar and polarizable chromophores. The properties of multidipolar assemblies are shown to be related to the relative orientation of chromophores, which is imposed by interchromophore interactions. The supramolecular structure is thus a result of self-organization. The proposed theoretical model was also used to predict the properties of multichromophore structures made up of more polar and polarizable push-pull chromophores, and showed that stronger interchromophore interactions can heavily affect the individual optical responses. This suggests new routes for engineering highly NLO responsive multichromophore systems

Aggregates of Cyanine Dyes: When Molecular Vibrations and Electrostatic Screening Make the Difference

Aggregates of cyanine dyes are currently investigated as promising materials for advanced electronic and photonic applications. The spectral properties of aggregates of cyanine dyes can be tuned by altering the supramolecular packing, which is affected by the length of the dye, the presence of alkyl chains, or the nature of the counterions. In this work, we present a joint experimental and theoretical study of a family of cyanine dyes forming aggregates of different types according to the length of the polymethinic chain. Linear and nonlinear optical spectra of aggregates are rationalized here in terms of an essential-state model accounting for intermolecular interactions together with the molecular polarizability and vibronic coupling. A strategy is implemented to properly account for screening effects, distinguishing between electrostatic intermolecular interactions relevant to the ground state (mean-field effect) and the interactions relevant to the excited states (excitonic effects). To the best of our knowledge, this is the first attempt to simulate nonlinear spectral properties of aggregates of symmetric dyes accounting for molecular vibrations

Thermally activated delayed fluorescence: A critical assessment of environmental effects on the singlet–triplet energy gap

The effective design of dyes optimized for thermally activated delayed fluorescence (TADF) requires the precise control of two tiny energies: the singlet–triplet gap, which has to be maintained within thermal energy, and the strength of spin–orbit coupling. A subtle interplay among low-energy excited states having dominant charge-transfer and local character then governs TADF efficiency, making models for environmental effects both crucial and challenging. The main message of this paper is a warning to the community of chemists, physicists, and material scientists working in the field: the adiabatic approximation implicitly imposed to the treatment of fast environmental degrees of freedom in quantum–classical and continuum solvation models leads to uncontrolled results. Several approximation schemes were proposed to mitigate the issue, but we underline that the adiabatic approximation to fast solvation is inadequate and cannot be improved; rather, it must be abandoned in favor of an antiadiabatic approach