Electronic spectroscopy and photoisomerization of trans-urocanic acid in asupersonic jet

trans-Urocanic acid (trans-UA), a component of the epidermal layer of skin, exhibits wavelength-dependent photochemistry. The quantum efficiency of is omerization to cis-UA is greatest when the molecule is excited on the long wavelength tail of its absorption profile in solution (300-320 nm). However , exciting the molecule where it absorbs UV light most efficiently (260-285 nm) causes almost no isomerization. We have used fluorescence excitation a nd dispersed emission methods in a supersonic jet to investigate the electr onic states involved in this complex and interesting photochemistry. Three distinct regions are present in the excitation spectrum. Region I, which is below the isomerization barrier, contains sharp, well-resolved peaks that upon excitation emit from the S-1 state of trans-UA. Region II exhibits pea ks that increase in broadness and decrease in intensity with increasing exc itation energy. Upon excitation these peaks produce dual emission from the SI states of both trans- and cis-UA. The trans to cis isomerization barrier is estimated to be 1400 cm(-1). Region III exhibits excitation to the S-2 electronic state and has a broad structure that spans 3000 cm(-1) and occur s 4000 cm(-1) above S-1. S-2 excitation results in essentially no trans to cis isomerization.