PRIMARY PROCESSES OF THE ELECTRONIC EXCITED-STATES OF TRANS-UROCANIC ACID

The primary photoreactivity of the excited states of trans-urocanic ac id (t-UA) is investigated by ultrafast transient-absorption spectrosco py. Fundamentally different photophysics were observed when t-UA is ex cited at 266 nm, near the peak of the absorption spectrum, and 306 nm, in the red tail of the absorption spectrum. The data support the conc lusion that the wavelength-dependent photophysics of t-UA is due to th e presence of two different closely spaced electronic states. Excitati on at 266 nm populates a pi pi state that is localized on the imidazo le ring. The transient data following photoexcitation of t-UA at 266 n m in both a pH 5.6 and a pH 7.2 solution are similar, even though the protonation state of the tertiary nitrogen on the imidazole ring is di fferent at these two pH values. The data therefore support that the ph otophysics at pH 5.6 and pH 7.2 must involve a common excited state. S teady-state excitation spectra suggest that a proton transfer process from t-UA to the solvent occurs following the excitation at 266 nn at pH 5.6, which generates an electronically excited singlet state of the deprotonated molecule. This state is directly accessed by the 266 nm excitation of t-UA at pH 7.2. The population in this singlet state dec ays by intersystem crossing with a rate constant of 1.4 x 10(11) s(-1) . Isomerization is not believed to occur from this triplet state. Exci tation of t-UA at 306 nm populates an entirely different state, which leads to isomerization. From the observed ground state repopulation dy namics, the minimum rate for the excited state isomerization is 1.2 x 10(10) s(-1).