Ultrafast processes in photoexcitedN-salicylideneaniline have been investigated with femtosecondtime-resolved resonance-enhanced multiphoton ionization spectroscopy. The ion signals via theS 1 (n,π * ) state of the enol form as well as the proton-transferred cis-keto form emerge within a few hundred femtoseconds after photoexcitation to the firstS 1 (π,π * ) state of the enol form. This reveals that two ultrafast processes,excited-state intramolecular proton transfer (ESIPT) reaction and an internal conversion (IC) to theS 1 (n,π * ) state, occur on a time scale less than a few hundred femtoseconds from theS 1 (π,π * ) state of the enol form. The rise time of the transient corresponding to the production of the proton-transferred cis-keto form is within 750 fs when near the red edge of the absorption is excited, indicating that the ESIPT reaction occurs within 750 fs. The decay time of theS 1 (π,π * ) state of the cis-keto form is 8.9 ps by exciting the enol form at 370 nm, but it dramatically decreases to be 1.5–1.6 ps for the excitation at 365–320 nm. The decrease in the decay time has been attributed to the opening of an efficient nonradiative channel; an IC fromS 1 (π,π * ) toS 1 (n,π * ) of the cis-keto form promotes the production of the trans-keto form as the final photochromic products. The two IC processes may provide opposite effect on the quantum yield of photochromic products: IC in the enol form may substantially reduce the quantum yield, but IC in the cis-keto form increase it.

This work was supported in part by Grant-in-Aid for Scientific Research No. 15350015 from the Ministry of Education, Sports and Culture in Japan, and by the Joint Studies Program (Grant No. 2002-2003) of Institute for Molecular Science.