Laboratoire de Spectrochimie Infrarouge et Raman (UMR 8516 du CNRS), Centre d'études et de recherches Lasers et Applications (FR 2416 du CNRS), Université des Sciences et Technologies de Lille, Bat C5, 59655 Villeneuve d'Ascq Cedex, France.
J Am Chem Soc. 2010 Jun 2;132(21):7379-90. doi: 10.1021/ja910813x.
Two photochromic diarylethenes blocked by alkyl bridges in an ideal conformation for photocyclization are studied by stationary and femtosecond transient spectroscopy in order to depict the photocyclization processes: the bistable 1,2-dicyano[2.n]metacyclophan-1-ene with n = 2, abbreviated as [2.2], and its non-bistable analogue with n = 4, abbreviated as [2.4]. The data are interpreted in the light of AM1-CIS calculations and state correlation diagrams based on conclusive TD-DFT calculations. For [2.2], a solvent-sensitive excitation wavelength threshold governing the photocyclization yield is clearly evidenced between the S(1) and S(2) singlet states. Excitation above and beyond this threshold induces two distinct photochemical pathways. The S(1) vertical excitation induces direct efficient (phi approximately = 0.9-1), and ultrafast (approximately 120 fs) photocylization from S(1) open form that leads to a ground-state transition structure, probably through a conical intersection, then to a hot cyclized ground state that relaxes by vibrational cooling. Upon higher excitation energy, the system undergoes internal conversion to the hot S(1) state, then evolves toward the cyclized S(1) state and relaxes by ultrafast S(1)-S(0) internal conversion. Alternatively, the possibility for a second conical intersection near hot S(1) state is discussed. This second photoclosure reaction is less efficient and both the photocylization yield and overall kinetics depend on solvent polarity (phi = 0.49, tau = 2.5 ps in nonpolar solvent; phi = 0.7, tau = 1.5 ps in polar solvent). In the case of [2.4], for which the distance between the two reactive carbons is larger, a unique photoclosure mechanism is found and a structural effect is reported. Indeed, this mechanim is similar to the above second reaction of [2.2] but characterized by much slower kinetics ranging from 12 to 20 ps (depending on the excitation wavelength and solvent polarity). All polarity effects are rationalized in terms of stabilization of the transient states of charge-transfer character involved in the photocyclization process.
两种用烷基桥阻断的光致变色二噻吩并[2,3-b:2',3'-d]噻吩，在理想的光环化构象下，通过稳态和飞秒瞬态光谱进行研究，以描绘光环化过程：双稳定的 1,2-二氰基[2.n]并环戊二烯-1-烯，n = 2，缩写为[2.2]，及其非双稳定类似物，n = 4，缩写为[2.4]。数据根据 AM1-CIS 计算和基于明确 TD-DFT 计算的态相关图进行解释。对于[2.2]，在 S(1)和 S(2)单重态之间，明显存在一个控制光环化产率的溶剂敏感激发波长阈值。激发超过这个阈值会诱导两种不同的光化学途径。S(1)垂直激发诱导直接有效的（phi 约为 0.9-1）和超快（约 120 fs）光环化，从 S(1)开环形式直接有效地进行，这导致了一个基态过渡态结构，可能通过一个锥形交叉，然后到一个热的环化基态，通过振动冷却来松弛。在更高的激发能量下，系统经历内部转换到热 S(1)态，然后向环化 S(1)态演化并通过超快的 S(1)-S(0)内部转换来松弛。或者，讨论了在热 S(1)态附近存在第二个锥形交叉的可能性。这个第二次光闭合反应效率较低，光环化产率和整体动力学都取决于溶剂极性（在非极性溶剂中 phi = 0.49，tau = 2.5 ps；在极性溶剂中 phi = 0.7，tau = 1.5 ps）。对于[2.4]，由于两个反应碳之间的距离较大，发现了一种独特的光闭合机制，并报告了结构效应。实际上，这种机制类似于[2.2]的第二个反应，但具有从 12 到 20 ps 的慢得多的动力学（取决于激发波长和溶剂极性）。所有的极性效应都根据涉及光环化过程的电荷转移特征的瞬态态的稳定化来合理化。
