Holm Ann-Kathrin, Mohammed Omar F, Rini Matteo, Mukhtar Emad, Nibbering Erik T J, Fidder Henk
Department of Physical Chemistry, Uppsala University, P.O. Box 579, S-751 23 Uppsala, Sweden.
J Phys Chem A. 2005 Oct 13;109(40):8962-8. doi: 10.1021/jp053543+.
The ring-opening dynamics of the photochromic switch 1',3'-dihydro-1',3',3'-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2'-(2H)-indole] in tetrachloroethene is studied with both femtosecond time-resolved ultraviolet (UV)/visible and UV/mid-infrared (IR) pump-probe spectroscopy. During the first picosecond we identify two new transient features in the UV/vis experiments, the first of which we assign to spiropyran S1 --> S(n) absorption (lifetime < or = 0.2 ps). The second feature (lifetime 0.5 +/- 0.2 ps) we tentatively assign to the merocyanine T2 state. After 1 ps both probing methods show biexponential merocyanine formation kinetics, with average time constants of 17 +/- 3 and 350 +/- 20 ps. In the UV/IR experiments, the initial dynamics show more dispersion in formation times than in the UV/vis measurements, whereas the slower time constant is the same in both. A weak transient IR signal at approximately 1360 cm(-1) demonstrates that this biexponentiality is caused by a sequential isomerization between two merocyanine species. Lifetimes provide evidence that the merocyanine S1 state is not involved in the photochemical reaction.
采用飞秒时间分辨紫外（UV）/可见光谱和紫外/中红外（IR）泵浦 - 探测光谱研究了光致变色开关1',3'-二氢-1',3',3'-三甲基-6-硝基螺[2H-1-苯并吡喃-2,2'-(2H)-吲哚]在四氯乙烯中的开环动力学。在最初的皮秒时间内，我们在紫外/可见实验中识别出两个新的瞬态特征，其中第一个我们归因于螺吡喃S1→S(n)吸收（寿命≤0.2 ps）。第二个特征（寿命0.5±0.2 ps）我们初步归因于部花青T2态。1 ps后，两种探测方法均显示出部花青形成的双指数动力学，平均时间常数分别为17±3和350±20 ps。在紫外/红外实验中，初始动力学显示形成时间的分散性比紫外/可见测量中的更大，而较慢的时间常数在两者中是相同的。在约1360 cm(-1)处的一个弱瞬态红外信号表明这种双指数性是由两种部花青物种之间的顺序异构化引起的。寿命提供了证据表明部花青S1态不参与光化学反应。
