PHOTOISOMERIZATION AND ADIABATIC TAUTOMERIZATION OF THE ALPHA-PYRIDYLPYRROLINONES

The spectroscopy and photochemical behavior of E- and Z-2,3-diethyl-5- (2-pyridylmethylidene)-3 have been reinvestigated. In accord with earl ier reports, E,Z photoisomerization of the E isomer is more efficient than that of the Z isomer, resulting in photostationary states enriche d in the Z isomer. The low reactivity of the Z isomer was previously a ttributed 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 t o the adiabatic formation of a tautomer which reverts to the Z isomer in the ground state. Both the forward (excited state) and reverse (gro und state) hydrogen transfer processes have very low activation energi es, thus providing a route for the decay of the excited Z isomer which competes effectively with isomerization. Short-lived fluorescence att ributed to the pi pi singlet states of both the E and Z isomers can b e observed at 77 K. The short lifetimes may result from rapid internal conversion to lower energy n,pi pi states, whose presence is indicat ed by INDO/S-SCF-CI calculations.