Photoisomerization and adiabatic tautomerization of the α-pyridylpyrrolinones

The spectroscopy and photochemical behavior of E- and Z-2,3-diethyl-5-(2-pyridylmethylidene)-3-pyrrolin-2-one have been reinvestigated. In accord with earlier reports, E,Z photoisomerization of the E isomer is more efficient than that of the Z isomer, resulting in photostationary states enriched in the Z isomer. The low reactivity of the Z isomer was previously attributed to the presence of an intramolecular hydrogen bond, which is responsible for the greater thermodynamic stability of the Z vs. the E isomer. The Z isomer displays red-shifted fluorescence, attributed to the adiabatic formation of a tautomer which reverts to the Z isomer in the ground state. Both the forward (excited state) and reverse (ground state) hydrogen transfer processes have very low activation energies, thus providing a route for the decay of the excited Z isomer which competes effectively with isomerization. Short-lived fluorescence attributed to the π, π^* singlet states of both the E and Z isomers can be observed at 77 K. The short lifetimes may result from rapid internal conversion to lower energy n,π^* states, whose presence is indicated by INDO/S-SCF-CI calculations.