In this paper we present the theory and implementation of analytic derivatives of time-dependent density functional theory (TDDFT) excited states energies, both in vacuo and including solvent effects by means of the polarizable continuum model. The method is applied to two case studies: p-nitroaniline and 4-(dimethyl)aminobenzonitrile. For both molecules PCM-TDDFT is shown to be successful in supporting the analysis of experimental data with useful insights for a better understanding of photophysical and photochemical pathways in solution.

Geometries and properties of excited states in the gas phase and in solution: theory and application of a time-dependent density functional theory polarizable continuum model.

Property tuning in selected examples of D–π–A molecules has been discussed and summarized in this review article. The tuning and structure–property relationships have been demonstrated on the particular A, π and D parts of the push–pull molecule. Special emphasis has been put on the tuning of the FMO levels and optical properties. Further prospective applications of the given chromophore have also been considered.

https://pubs.rsc.org/en/content/articlepdf/2014/RA/C4RA11264D

Fundamental aspects of property tuning in push-pull molecules†

Functional organic materials with enhanced two-photon absorption 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 two-photon absorption properties of organic chromophores. This includes femtosecond two-photon excited fluorescence experimental setups and quantum-chemical methodologies based on time-dependent density functional theory. We thoroughly analyze physical phenomena and trends leading to large twophoton absorption responses of a few series of model chromophores focusing on the effects of symmetric and asymmetric donor/acceptor substitution and branching.

/pdf/enhanced-two-photon-absorption-of-organic-chromophores-59smnlo107.pdf

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

We present a joint theoretical and experimental work aimed to understand the spectroscopic behavior of multipolar dyes of interest for nonlinear optics (NLO) applications. In particular, we focus on the occurrence of broken-symmetry states in quadrupolar organic dyes and their spectroscopic consequences. To gain a unified description, we have developed a model based on a few-state description of the charge-transfer processes characterizing the low-energy physics of these systems. The model takes into account the coupling between electrons and slow degrees of freedom, namely, molecular vibrations and polar solvation coordinates. We predict the occurrence of symmetry breaking in either the ground or first excited state. In this respect, quadrupolar chromophores are classified in three different classes, with distinctively different spectroscopic behavior. Cases of true and false symmetry breaking are discriminated and discussed by making resort to nonadiabatic calculations. The theoretical model is applied to three representative quadrupolar chromophores: their qualitatively different solvatochromic properties are connected to the presence or absence of broken-symmetry states and related to two-photon absorption (TPA) cross-sections. The proposed approach provides useful guidelines for the synthesis of dyes for TPA application and represents a general and unifying reference frame to understand energy-transfer processes in multipolar molecular systems, offering important clues to understand basic properties of materials of interest for NLO and energy-harvesting applications.

/pdf/charge-instability-in-quadrupolar-chromophores-symmetry-aapqfltt0p.pdf

Charge instability in quadrupolar chromophores: symmetry breaking and solvatochromism.

To investigate the effect of branching on linear and nonlinear optical properties, a specific series of chromophores, epitome of (multi)branched dipoles, has been thoroughly explored by a combined theoretical and experimental approach. Excited-state structure calculations based on quantum-chemical techniques (time-dependent density functional theory) as well as a Frenkel exciton model nicely complement experimental photoluminescence and one- and two-photon absorption findings and contribute to their interpretation. This allowed us to get a deep insight into the nature of fundamental excited-state dynamics and the nonlinear optical (NLO) response involved. Both experiment and theory reveal that a multidimensional intramolecular charge transfer takes place from the donating moiety to the periphery of the branched molecules upon excitation, while fluorescence stems from an excited state localized on one of the dipolar branches. Branching is also observed to lead to cooperative enhancement of two-photon absorption (TPA) while maintaining high fluorescence quantum yield, thanks to localization of the emitting state. The comparison between results obtained in the Frenkel exciton scheme and ab initio results suggests the coherent coupling between branches as one of the possible mechanisms for the observed enhancement. New strategies for the rational design of NLO molecular assemblies are thus inferred on the basis of the acquired insights.

/pdf/effects-of-multi-branching-of-dipolar-chromophores-on-4l3ovs8gv3.pdf

Effects of (Multi)branching of Dipolar Chromophores on Photophysical Properties and Two-Photon Absorption

The linear absorption spectra and absolute resonance Raman excitation profiles of the “push-pull” chromophore julolidinemalononitrile have been measured in cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol solution at excitation wavelengths spanning the strong visible charge-transfer absorption band Numerical simulation of the spectra using time-dependent wave-packet propagation methods yields the excited-state geometry changes along the ∼15 strongly Raman-active vibrations as well as the solvent reorganization energies The distribution of the total vibrational reorganization energy among the various normal modes is solvent dependent, indicating solvent polarity effects on the electronic structure These results are compared with those previously obtained for two other push-pull chromophores, p-nitroaniline and julolidinyl-n-N,N′-diethylthiobarbituric acid The frequency dispersion of the molecular first hyperpolarizability, β, is also calculated in each solvent using a time-domain form of the standard Oudar–Chemla two-state model modified to incorporate solvent reorganization, inhomogeneous broadening, and the vibronic structure of the charge-transfer state We show that accurate extrapolation of β measured at frequencies in the near-infrared to zero frequency requires a realistic description of the excited state as the measuring wavelength approaches a two-photon resonance This is particularly relevant to the high chromophore concentrations needed for device applications, where intermolecular interactions can strongly perturb the electronic transitions

Vibronic effects on solvent dependent linear and nonlinear optical properties of push-pull chromophores: Julolidinemalononitrile

http://cnls.lanl.gov/~serg/postscript/dacpl.pdf

Excited state molecular dynamics simulations of nonlinear push-pull chromophores

Resonance Raman spectra and cross sections of a “push−pull” chromophore containing a julolidine donor and a thiobarbituric acid acceptor have been measured in dilute solution in five solvents having a wide range of polarities (cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol) at excitation wavelengths spanning the strong visible charge-transfer absorption band. The absolute Raman excitation profiles and absorption spectra are simulated using time-dependent wave packet propagation techniques to determine the excited-state geometry changes along the ∼30 Raman-active vibrations as well as the solvent reorganization energies. Several vibrational modes undergo significant (5−15 cm-1) frequency changes as the solvent is varied, signaling solvent polarity effects on the ground-state electronic structure. The excited-state geometry changes are solvent dependent for some vibrational modes but not for others. The total vibrational reorganization energy decreases, and the solvent reorganization ...

/pdf/solvent-effects-on-ground-and-excited-electronic-state-1uznd1krcp.pdf

Andrew M. Moran

Papers

Vibronic effects on solvent dependent linear and nonlinear optical properties of push-pull chromophores: Julolidinemalononitrile

Excited state molecular dynamics simulations of nonlinear push-pull chromophores

Solvent Effects on Ground and Excited Electronic State Structures of the Push-Pull Chromophore Julolidinyl-n-N,N‘-diethylthiobarbituric Acid

Aromatic versus polyenic linkers in push-pull chromophores: Electron-phonon coupling effects