Okabe Chie, Nakabayashi Takakazu, Inokuchi Yoshiya, Nishi Nobuyuki, Sekiya Hiroshi
Department of Chemistry, Faculty of Science, Kyushu University, 6-1-10 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan.
J Chem Phys. 2004 Nov 15;121(19):9436-42. doi: 10.1063/1.1801991.
Ultrafast processes in photoexcited N-salicylideneaniline have been investigated with femtosecond time-resolved resonance-enhanced multiphoton ionization spectroscopy. The ion signals via the S(1)(n,pi( *)) state of the enol form as well as the proton-transferred cis-keto form emerge within a few hundred femtoseconds after photoexcitation to the first S(1)(pi,pi( *)) state of the enol form. This reveals that two ultrafast processes, excited-state intramolecular proton transfer (ESIPT) reaction and an internal conversion (IC) to the S(1)(n,pi( *)) state, occur on a time scale less than a few hundred femtoseconds from the S(1)(pi,pi( *)) 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 the S(1)(pi,pi( *)) 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 from S(1)(pi,pi( *)) to S(1)(n,pi( *)) 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.
利用飞秒时间分辨共振增强多光子电离光谱研究了光激发N-水杨醛苯胺中的超快过程。光激发至烯醇形式的第一S(1)(π,π*)态后，经由烯醇形式的S(1)(n,π*)态以及质子转移的顺式酮形式的离子信号在几百飞秒内出现。这表明，从烯醇形式的S(1)(π,π*)态开始，在小于几百飞秒的时间尺度上发生了两个超快过程，即激发态分子内质子转移（ESIPT）反应和向S(1)(n,π*)态的内转换（IC）。当激发吸收的红边附近时，对应于质子转移顺式酮形式产生的瞬态上升时间在750飞秒以内，表明ESIPT反应在750飞秒内发生。通过在370 nm激发烯醇形式，顺式酮形式的S(1)(π,π*)态的衰减时间为8.9 ps，但在365 - 320 nm激发时，衰减时间显著缩短至1.5 - 1.6 ps。衰减时间的缩短归因于一个有效的非辐射通道的开启；顺式酮形式从S(1)(π,π*)到S(1)(n,π*)的内转换促进了反式酮形式作为最终光致变色产物的产生。这两个内转换过程可能对光致变色产物的量子产率产生相反的影响：烯醇形式的内转换可能会大幅降低量子产率，但顺式酮形式的内转换会提高量子产率。
