A CASSCF study on the lowest ???* excitation of urocanic acid

Abstract

ABSTRACT:The photochemical cisrtrans isomerization of urocanic acid UCA,. . E -3-1 H-imidazol-4 -yl propenoic acid was investigated using complete active space Ž . SCF CASSCF ab initio calculations. The singlet ground state and the triplet and the U Ž . singlet manifolds of the lowest-lying ª HOMO ª LUMO excitation of the neutral and the anionic UCA were calculated using the 6-31G U and the 6-31 q G U basis sets, respectively. The torsional barrier of the double bond of the propenoic acid moiety in UCA is observed to be considerably lower in the T and S excited states of the neutral energy difference is larger in the excited states than in the ground state, probably due to strengthening of the intramolecular hydrogen bond of cis-UCA as the molecule is excited. The results of the calculations, interpreted in terms of the idea that UCA is deprotonated upon electronic excitation, led to construction of a new model for the photoisomerization mechanisms of UCA. According to this model, the trans-to-cis isomerization proceeds via both the triplet and the singlet manifolds in the deprotonated form of UCA. This isomerization may occur in the S state of the neutral UCA as well. The cis-to-trans 0 isomerization is suggested to proceed only in the S state of the neutral UCA. ᮊ 1999

Photochemical Mechanisms of Radiationless Deactivation Processes in Urocanic Acid

“…Vertical excitation energies of urocanic acid have been computed in the past using a number of electronic-structure methods. 19,23,24,26,28,29,32 The results obtained at the CASPT2 and the CC2 levels agree surprisingly well. At both levels, the lowest ππ* state, which in all cases is the HOMO→LUMO transition, possesses the largest oscillator strength.…”

“…Computational Methods section) are given in Table . Vertical excitation energies of urocanic acid have been computed in the past using a number of electronic-structure methods. ,,,,,, …”

“…A number of publications have reported on the results of computational investigations. − Various protonated forms of ( E )- and ( Z )-urocanic acid have been considered, and a detailed investigation of an intramolecularly hydrogen-bonded Z isomer has been performed . Molecular-dynamics simulations of the conformational dynamics in aqueous solution were reported .…”

“…Molecular-dynamics simulations of the conformational dynamics in aqueous solution were reported . A number of studies reported on the vertical excitation energies of the neutral species of urocanic acid, ,,,, while two studies reported on the vertical excitation energies of ionic forms. , Further studies dealt with the photosensitization mechanism (the ability of triplet-excited urocanic acid to react with molecular oxygen to produce singlet molecular oxygen) and with the hydroxyl-radical-scavenging ability of urocanic acid . The reaction paths for the photoisomerization process have been investigated .…”

Some photophysical studies of cis- and trans-urocanic acid

“…They proposed an energy level diagram for the processes showing excited cis (Z*) and trans (E*) states with several intermediate states involved in the photoisomerisation mechanism. A recent theoretical study 23 suggests that a more complex series of states is involved in the photoisomerisation process. However, Simon and co-workers 12 using ultrafast but weaker energy laser pulses at 266 nm excitation (sub picosecond, 1 µJ pulses), found a very short-lived transient absorption (lifetime ≈ 0.5 ps) at 340 nm and a long-lived depletion or bleaching transient at 266 nm with no change in the signal of the latter transient for delays of up to 8 ns.…”