Formation of planarized intramolecular charge-transfer state in dichlorotriazinyl-pyrene fluorescent probe: TD-DFT and resonance Raman study

Abstract This study is focused on explanation of the remarkable photophysical behaviour of the 1-(4,6-dichloro-1,3,5-triazin-2-yl)-pyrene (PyTC2) compound which has been introduced as a fluorescent polarity probe. This compound exhibits large solvatochromic red-shift of fluorescence emission band while maintaining high fluorescence quantum yield and monoexponential decay kinetics throughout the whole solvent polarity scale. As the semi-empirical calculations reported in the original paper have not revealed any excited state possessing a high dipole moment, it has been suggested that the red-shift originates from planarization of the emitting excited state in polar solvents in contrast to unchanged twisted geometry in non-polar solvents. However, both the extent of the red-shift and the disappearance of the vibronic structure in polar solvents indicate that the emission originates from an excited state with high dipole moment and that the semi-empirical methods may not be sufficient to describe the emitting state of this molecule correctly. Thus, we have performed TD-DFT calculations including the potential energy surface scans. According to these calculations and scans, the emission takes place from a planarized intramolecular charge-transfer (ICT) state. This is in good agreement with all aspects of the observed fluorescence behaviour of PyTC2. Independent experimental evidence for the ICT has been provided by analysis of resonance Raman intensities where bands corresponding to enhanced normal modes residing on triazine and the stretching mode between pyrene and triazine moieties have been identified. The formation of the photoinduced ICT together with easy and inexpensive preparation make this compound and its derivatives candidate as push-pull building blocks for the design of advanced functional materials.