Intramolecular Charge Transfer of Push-Pull Pyridinium Salts in the Triplet Manifold

The solvent effect on the triplet state properties of the iodides of three trans (E) isomers of 2-D-vinyl-1-methylpyridinium, where D is a donor group (4-(dimethylamino)phenyl, 3,4,5-trimethoxyphenyl, and 1-pyrenyl), was studied by nanosecond transient absorption techniques. The results obtained allowed us to complete a previous study on the negative solvatochromism and the role of the solvent-controlled intramolecular charge transfer (ICT) relaxation pathways in the excited singlet state, carried out by ultrafast absorption techniques and DFT calculations ( J. Phys. Chem. A 2014, 118, 3580-3592). The solvent effect on the intersystem crossing to the triplet manifold gave information on the competition of the relaxation processes in the singlet and triplet states and the extent and mechanism of the photoreactive deactivation, particularly operative for the pyrenyl derivative. For the latter the results showed that the ICT process also occurs in the triplet manifold. In fact, the formation of two triplet states with different nature and affected differently by oxygen was observed for the pyrenyl derivative in water and assigned to the (LE)-L-3* and (ICT)-I-3* states. The related structure and solvent effects on the trans --> cis photoisomerization are briefly discussed.