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

Dielectric response of modified Hubbard models with neutral-ionic and Peierls transitions

The dipole P(F) of systems with periodic boundary conditions (PBC) in a static electric field F is applied to one-dimensional Peierls-Hubbard models for organic charge-transfer (CT) salts. Exact results for P(F) are obtained for finite systems of N = 14 and 16 sites that are almost converged to infinite chains in deformable lattices subject to a Peierls transition. The electronic polarizability per site, \alpha_{el} = (\partial P/\partial F)_0, of rigid stacks with alternating transfer integrals t(1 +/- \delta) diverges at the neutral-ionic transition for \delta = 0 but remains finite for \delta > 0 in dimerized chains. The Peierls or dimerization mode couples to charge fluctuations along the stack and results in large vibrational contributions, \alpha_{vib}, that are related to \partial P/\partial \delta and that peak sharply at the Peierls transition. The extension of P(F) to correlated electronic states yields the dielectric response \kappa of models with neutral-ionic or Peierls transitions, where \kappa peaks >100 are found with parameters used previously for variable ionicity \rho and vibrational spectra of CT salts. The calculated \kappa accounts for the dielectric response of CT salts based on substituted TTFs (tetrathiafulvalene) and substituted CAs (chloranil). The role of lattice stiffness appears clearly in models: soft systems have a Peierls instability at small \rho and continuous crossover to large \rho, while stiff stacks such as TTF-CA have a first-order transition with discontinuous \rho that is both a neutral-ionic and Peierls transition. The transitions are associated with tuning the electronic ground state of insulators via temperature or pressure in experiments, or via model parameters in calculations.Comment: 10 pages, 9 figures; J.Chem.Phys., in pres

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 molecular aggregates and crystals: testing approximation schemes

The interplay between exciton delocalization and molecular vibrations profoundly affects optical spectra of molecular aggregates and crystals. The exciton motion occurs on a similar timescale as molecular vibrations, leading to a complex and intrinsically non-adiabatic problem that has been handled over the years introducing several approximation schemes. Here we discuss systems where intermolecular distances are large enough so that only electrostatic intermolecular interactions enter into play and can be treated in the dipolar approximation. Moreover, we only account for interactions between transition dipole moments, as relevant to symmetric molecules, with negligible permanent (multi)polar moments in the ground and low-lying excited states. Translational symmetry is fully exploited to obtain numerically exact solutions of the relevant Hamiltonian for systems of comparatively large size. This offers a unique opportunity to assess the reliability of different approximation schemes. The so-called Heitler–London approximation, only accounting for the effects of intermolecular interactions among degenerate electronic states, leads to the celebrated exciton model, widely adopted to describe optical spectra of molecular aggregates and crystals. We demonstrate that, mainly due to a cancellation of errors, the exciton model approximates well the position of exciton bands and reasonably well the bandshapes, but it fails to predict spectral intensities, leading to underestimated intensities in J-aggregates and overestimated intensities in H-aggregates. This general result is validated against an exact sum-rule. Finally, we address the validity of several approximation schemes adopted to reduce the dimension of the vibrational basis

Effective models for TADF: the role of the medium polarizability

A novel approch to estimate intersystem and reverse intersystem crossing rates (ISC and RISC rates, respectively) is proposed. We build on an effective model Hamiltonian recently parametrized ab initio and validated experimentally for a prototypical dye for thermally activated delayed fluorescence (TADF). The model describes the relevant physics in terms of a few diabatic states coupled to an effective vibrational coordinate and an effective conformational mode. A complete, numerically exact, non- adiabatic solution of the problem opens the way to the calculation of ISC and RISC rates fully accounting for the anharmonic and non-adiabatic nuclear dynamics. The model is further extended to address the role of the environmental polarizability, as described by the medium refractive index. The marginal variability of the refractive index in organic media results in marginal effects on the rates in different media. However, large variations of the rates are predicted when moving from the gas phase to an organic medium, suggesting that a meaningful analysis of experimental data must rely on computational analysis properly accounting for the dielectric properties of the surrounding medium

Supramolecular chirality: a caveat in assigning the handedness of chiral aggregates

The handedness of a supramolecular chiral aggregate is often assigned based on the sign of circular dichroism spectra, adopting the exciton chirality method. However, the method does not properly account for the nature of intermolecular interactions. We introduce a generalized picture on the use of the sign of chiral signals in determining the helicity of chiral aggregates, rooted in the exciton model, supported by TD-DFT results

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