Dual fluorescence of 4-(dimethylamino)-pyridine: a comparative linear response TDDFT versus state-specific CASSCF study including solvent with the PCM model

Abstract

Two different theoretical approaches have been used for the description of the experimentally observed dual luminescence of 4-(dimethylamino)pyridine (DMAP), introducing solvent effects with the polarizable continuum model (PCM), which seems needed to represent the dual fluorescence in polar media. These approaches are the linear response time-dependent density functional theory (TDDFT) and the state-specific complete active space self-consistent field. Both levels of theory represent the expected planar high-energy and the twisted intramolecular charge transfer (ICT) low-energy excited-state structures in the presence of solvent (toluene and acetonitrile). The comparison between both approaches shows that the main distortion of the ICT state is similar for both cases, i.e. twisting to almost 90o of the pyridine ring and the dimethylamino planes, but that other secondary distortions are slightly different. In the case of the TDDFT approach, the geometry optimizations of DMAP in the ground and excited states have been carried out using the conventional linear response approach (LR-PCM) for the solvent inclusion. The LR-PCM and the specific state (SS-PCM) approaches have been used for the prediction of the excitation and emission energies of DMAP in toluene and acetonitrile. The prediction of the emission energies at TDDFT/LR-PCM and CASPT2/PCM (complete active space perturbation theory) levels agrees with the experimental ones.