A cationic naphthyl derivative defies the non-equilibrated excited rotamers principle

The ground and excited state properties of 1-methyl-2-[(E)-2-(2-naphthyl) vinyl] pyridinium iodide have been investigated in solvents of different polarities and viscosities using stationary and ultrafast time resolved spectroscopic techniques supported by density functional theory (DFT) calculations. The investigated compound shows an important negative solvatochromism, which serves as evidence of a certain push-pull character exhibited upon photoexcitation, but the most remarkable feature is an extremely large absorption spectrum, as opposed to the narrower emission band. Interestingly, both experiments and calculations have revealed a conformational disorder in the ground state between four quasi-isoenergetic rotamers which contribute to the broad absorption spectrum. These equilibria are shifted towards one prevailing species in the excited state, pointing out an unexpected and efficient rotamer interconversion during the S-1 lifetime, manifestly against the non-equilibrated excited rotamers principle. The rotamer interconversion has been found to be very fast and only hindered in a rigid matrix at low temperatures.