Reaction coordinate analysis of the S2-S1 internal conversion of phenylacetylene

The reaction coordinate of the S-2-S-1 internal conversion (IC) of phenylacetylene (PA) was analyzed using the ab initio complete active space self-consistent field (CASSCF) method. In the first process after electronic excitation into S-2, the aromatic benzene ring is transformed into a nonaromatic quinoid structure. The ethynyl part (-C equivalent to CH) takes an incomplete allenoid structure in which the CC bond elongates to an intermediate value between typical C equivalent to C triple and C=C double bonds, but the bend angle of -CCH is 180 degrees. In the second process, PA takes a complete allenoid structure with an out-of-plane location of the beta-H atom (i.e., the H atom of the ethynyl part) and a further elongation of the CC bond so that PA is most stable in S2 (S-2-bent). The conical intersection between S2 and S, (S-2/S-1-CIX) is located near the S2-bent geometry and is slightly unstable energetically. After transition at S-2/S-1-CIX, PA quickly loses both quinoid and allenoid structures and recovers the aromaticity of the benzene ring in S-1. Analysis of the dipole moment along the reaction coordinate shows that the weak electron-withdrawing group of the ethynyl part in So suddenly changes into an electron-donating group in S2 after the main transition of S-0-S-2. The photoinduced change of the dipole moment is a driving force to the formation of a quinoid structure in S2. Regarding the benefit of the reaction coordinate analysis of the multidimensional potential energy surfaces of PA, the present picture of the IC process is much more elaborate than our previous representation (Amatatsu, Y.; Hasebe, Y. J. Phys. Chem. A 2003 107, 11169-11173). Vibrational analyses along the reaction coordinate were also performed to support a time-resolved spectroscopic experiment on the S-2-S-1 IC process of PA.