Effects of protonation on the lowest triplet state of trans-4,4′-dipyridylethylene. A time-resolved electron paramagnetic resonance study

Time-resolved electron paramagnetic resonance, ultraviolet absorption and fluorescence spectra of trans-4,4′-dipyridylethylene (DPE) and its singly protonated cation have been observed in methanol–water (4:1 by volume) and in stretched poly(vinyl alcohol) films at 77 K. The single protonation appears to have little effect on the zero-field splitting parameters and the anisotropic sublevel populating rates of the lowest excited triplet (T1) state of DPE. However, the fluorescence lifetimes indicate that the intersystem crossing rates for the three T1 sublevels decrease on single protonation.