Triplet pathways in the cis⇆trans photoisomerization of 4-acetyl-and 4-benzoyl-stilbene
作者:Helmut Görner
DOI:10.1039/ft9938904027
日期:——
The photochemical properties of 4-acetylstilbene (AS) and 4-benzoylstilbene (BS) have been studied in various solvents using steady-state irradiation and ns laser flash photolysis. The quantum yields of trans â cis photoisomerization (Φt â c, Φc â t) are substantial (0.3) over a wide temperature range, while the quantum yield of fluorescence is small even at â196 °C (e.g.Φf⩽ 0.04). Similar transient absorption spectra, showing maxima at ca. 380 and ca. 415 nm in fluid and frozen media, respectively, were recorded for trans-AS and also for cis-AS. The transient is assigned to the triplet state in either case; its lifetime between 25 and â196 °C varies by a factor of ca. 105. These two virtually identical TâT absorption spectra are suggested to originate from the trans(3t*) and the cis(3c*) triplet state, respectively, since at â196 °C intersystem crossing occurs without significant configurational changes. Apart from small spectral differences BS behaves similarly. Quenching measurements by ferrocene of Φt â c, in comparison with those of the triplet decay, support that the triplet state is intermediate in the trans â cis photoisomerization pathway. The spectral and kinetic features of the triplet state are interpreted on the basis of the intermediacy of the twisting step from 3c* to the perpendicular conformation (3p*) in the cis â trans photoisomerization pathway, the 3t*â3p* equilibrium and decay to the ground states via the 3p*â1p intersystem-crossing step.
利用稳态辐照和ns激光闪烁光解技术,研究了4-乙酰基二苯乙烯(AS)和4-苯甲酰基二苯乙烯(BS)在各种溶剂中的光化学特性。在很宽的温度范围内,反式-顺式光异构化的量子产率(δtâc,δcât)都很可观(0.3),而荧光的量子产率即使在â196 °C时也很小(例如δf⩽ 0.04)。类似的瞬态吸收光谱在约 380 纳米和约 415 纳米处显示最大值。在流体介质和冷冻介质中,反式-AS 和顺式-AS 都记录到了类似的瞬态吸收光谱,最大值分别为 380 纳米和 415 纳米。在这两种情况下,瞬态都属于三重态;在 25 °C到 196 °C之间,瞬态的寿命相差约 105 倍。这两种几乎完全相同的 TâT 吸收光谱被认为分别源自反式(3t*)和顺式(3c*)三重态,因为在 196 °C时,系统间的交叉不会发生显著的构型变化。除了光谱上的微小差异,BS 的表现也很相似。用二茂铁对δt αc进行的淬灭测量与三重态衰变的测量结果相比较,证明三重态是反式α-顺式光异构化途径的中间态。三重态的光谱和动力学特征是根据顺式-反式光异构化途径中从 3c* 到垂直构象(3p*)的扭转步骤的中间性、3t*â3p* 平衡以及通过 3p*â1p 系统间交叉步骤衰变到基态来解释的。