The effectiveness of essential-state models in the description of optical properties of branched push-pull chromophores

In this paper we present the synthesis, spectroscopic characterization and theoretical modelling of two pairs of correlated dipolar and octupolar donor-acceptor conjugated chromophores, based on the triphenylamine branching centre. The two pairs of chromophores differ for the electron withdrawing end-groups. Linear absorption, fluorescence and two-photon absorption of all the compounds in different solvents can be well described by the use of charge-resonance theoretical models based on essential-state descriptions of the electronic structure, and taking into account the coupling to effective molecular vibrations and to polar solvation degrees of freedom. On the contrary, the alternative Frenkel exciton model does not provide a good description of the observed behavior. The robustness of the proposed theoretical models is demonstrated for the first time by the fact that the modulation of a single molecular parameter (the one linked to the electron-withdrawing ability of the end groups) is enough to describe the evolution of the spectroscopic properties along the whole series of chromophores, both "intra-pair" and "inter-pair". The effectiveness of the approach suggests that this kind of theoretical modelling can be very useful to predict different properties of the compounds at hand or of correlated structures of increasing complexity, such as dendrons and dendrimers, giving a guide to the synthesis of (macro) molecules for applications in light-emitting and nonlinear optical devices, artificial light-harvesting systems or optical imaging of living tissues.