The Primary Processes of the Electronic Excited States of
trans-Urocanic Acid
Bulang Li, Kerry M. Hanson, and John D. Simon
The primary photoreactivity of the excited states of trans-urocanic acid (t-UA)
is investigated by ultrafast transient-absorption spectroscopy. Fundamentally
different photophysics were observed when t-UA is excited 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 conclusion that the wavelength-dependent photophysics
of t-UA is due to the presence of two different closely spaced electronic states.
Excitation at 266 nm populates a pi pi* state that is localized on the imidazole
ring. The transient data following photoexcitation of t-UA at 266 nm in both a
pH 5.6 and pH 7.2 solution are similar, even though the protonation state of the
tertiary nitrogen on the imidizole ring is different at these two pHs. The data
therefore support that the photophysics at pH 5.6 and pH 7.2 must involve a common
excited state. Steady-state excitation spectra suggest that a proton transfer
process from t-UA to the solvent occurs following the excitation at 266 nm 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 decays 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. Excitation of t-UA at 306 nm populates an entirely different
state, which leads to isomerization. From the observed ground-state repopulation
dynamics, the minimum rate for the excited state isomerization is 1.2 x 10^10
s-1.
The Journal of Physical Chemistry, volume 101A, p. 969 (1997).