The S-1<--S-0 transition of 1-phenylimidazole (1PI) has been studied in a supersonic jet expansion by resonant two-photon ionization. The origin band at 36 075 cm(-1) is accompanied by a low frequency progression associated with torsion about the bond connecting phenyl and imidazole groups. Torsional potentials have been determined for both states. In S-0, phi(min)=37.2+/-0.5degrees and the planar barrier is 339+/-20 cm(-1), while in S-1, phi(min)=17.6+/-0.5degrees and the planar barrier is 57+/-2 cm(-1). The transition moment alignment is observed to be consistent with an excited state of L-b character, in spite of the "off-axis" conjugation provided by the imidazole ring. These results are compared with ab initio calculations on both states, performed using Hartree-Fock, Moller-Plesset second-order perturbation, density functional theory with the Becke3-Lee-Yang-Parr functional, time-dependant density functional theory, configuration interaction singles, and complete active space self-consistent field methods. Solution-phase UV spectra of neutral and protonated 1PI are also reported.(C) 2004 American Institute of Physics.