A "twist" on the interpretation of the multifluorescence patterns of DASPMI

In this computational study, we describe the decay mechanism of DASPMI, providing robust and documented answers to some crucial questions of still open debates on the photophysical behavior of this cationic dye. After the initial excitation, the system evolves along a torsional motion, characterized by a quite flat potential energy surface, which crosses an intramolecular charge transfer (ICT) excited state with higher energy. A nonemissive twisted-ICT (TICT) minimum is populated, and this enhances the radiationless deactivation to the ground state. Additionally, during the twisting motion path toward the TICT minima, the system can emit in a quite wide range of angles, which should lead to a red shift of the locally excited (LE) emission and asymmetric broadening of fluorescence. This picture is fully supported by experimental evidence of the multifluorescence of DASPMI. Three twisted minima are found with different energies (namely, T1, T2, and T3). The extension of the work to charge properties shows that, in the GS, the positive charge of the molecule is mainly localized on the acceptor moiety (i.e., methyl-pyridinium), and after the excitation, the charge delocalizes over the whole molecule with a slight preference for the acceptor moiety. Because of the subsequent deactivation via twisting motions, the positive charge moves from the acceptor to the donor moiety (dimethylaminophenyl moiety) so that in TICT minima the positive charge is localized in the donor part. These large differences between charge localization in LE and TICT minima are responsible for a larger population of twisted forms in solvents of increasing polarity and the enhancement of radiationless deactivation.