• AIM
• atoms in molecules
• internal coordinates
• push-pull
• solvatochromic
• solvent
• TD-DFT
• TDDFT
• Time Dependent Density Functional Theory

Nowadays, organic push-pull chromophores fulfil a pivotal role in the generation of nonlinear optical (NLO) materials, due to their unique physical and chemical behavior. The key to design high performances compounds is the understanding of structure-property relationship, as confirmed by the benefits provided from copious theoretical studies. Generally, molecules of technological interest are charge-transfer medium-to-large flexible compounds, strongly sensible to the environment. All these features make the Time-Dependent Density Functional Theory (TD-DFT) the most promising computational scheme for the theoretical investigation. The first step toward the evaluation of NLO properties and spectra should be the accurate reproduction of ground state features and the simplest one-photon absorption (OPA) spectrum. The majority of research works aims to reproduce the maximum absorption wavelength of the electronic transition, including a symmetric phenomenological distribution function in order to reproduce the band-broadening. Here an alternative, more accurate approach is presented, where the vibro-electronic structure of the bandshape is explicitly calculated from first principles. A prototypical conjugate dye (2-((E)-2-[2,20]bithiophenyl-5-yl-vinyl)-1-methylquinolinium cation) in acetonitrile solvent has been chosen as a test case to demonstrate the maturity of the model. Indeed this compound is particularly challenge from a computational point of view, due to the flexibility of the structure, the charge transfer nature of the first excited state and the sensitivity of the spectral features to the environment. A detailed investigation of the ground and excited state properties have been performed, with particular care to the physical effects introduced by different electronic methods, the solvent presence and thermal contributions. Moreover, the investigation has permitted to define the best approach in order to simulate the vibronic spectra. The comparison of the calculation with the experiment confirm the reliability of the results.