The photochemical formation of the open merocyanine forms of several naphthoxazine-spiro-indolines in different solvents have been studied using both picosecond transient absorption (PTA) and picosecond time-resolved resonance Raman (PTR3) methods. The PTA studies have established the presence of several metastable species in the photochemical formation of the coloured merocyanine form of these photochromic compounds. The primary photochemical step occurs on the sub-ps timescale and is followed by the formation of a cisoid intermediate over the next 6–30 ps. This cisoid species then isomerises to the equilibrated distribution of transoid isomers of the merocyanine form with a lifetime that is dependent upon both solvent viscosity and polarity as well as the nature of the substituents on the naphthalene part of the molecule. However, the rate of this cis → trans isomerisation is unaffected on changing the N-alkyl group on the indoline part of the molecule from a methyl to an isobutyl group. The PTR3 studies have demonstrated that, in butan-1-ol, there are at least three different transient species with characteristic vibrational spectra which evolve with different lifetimes to give the final isomeric distribution over the first few ns of the reaction. In cyclohexane it is likely that the formation of a single species is being probed, which is fully developed after the first 200 ps of the reaction. This single species has a spectrum which is the same as the equilibrated steady-state resonance Raman merocyanine spectrum. It is likely that the evolution in the PTR3 spectra obtained here, in butan-1-ol, results from an equilibration of initially formed transoid merocyanine isomers to give a more stable distribution in this polar hydrogen-bonded solvent. Such an equilibration appears to be unnecessary in a non-polar solvent such as cyclohexane and it is suggested that this is because the transoid isomer, initially formed, is already in its most stable form.
利用皮秒瞬态吸收(
PTA)和皮秒时间分辨共振拉曼(
PTR3)方法,研究了几种
萘并恶嗪-螺
吲哚在不同溶剂中形成开放的梅花菁形式的光
化学过程。皮秒瞬时吸收研究证实,在这些光致变色化合物的彩色梅花苷形式的光
化学形成过程中,存在几种可转移的物种。主要的光
化学步骤发生在亚 ps 时间尺度上,然后在接下来的 6-30 ps 内形成顺式中间体。然后,这种顺式异构体会异构化为平衡分布的美
花青素形式的横向异构体,其寿命取决于溶剂的粘度和极性以及分子中
萘部分取代基的性质。不过,当分子中
吲哚啉部分的 N-烷基从甲基变为异丁基时,顺式 → 反式异构化的速率不会受到影响。
PTR3 研究表明,在丁-1-醇中,至少有三种不同的瞬态物种具有特征性的振动光谱,它们以不同的生命周期演变,从而在反应的最初几毫秒内形成最终的异构体分布。在
环己烷中,可能正在检测的是单一物种的形成,该物种在反应的前 200 ps 后得到充分发展。这种单一物种的光谱与平衡稳态共振拉曼美拉曼光谱相同。这里获得的
PTR3 光谱在丁-1-醇中的演变很可能是最初形成的横向美拉曼异构体在这种极性氢键溶剂中进行平衡以获得更稳定分布的结果。在
环己烷等非极性溶剂中,这种平衡似乎是不必要的,这是因为最初形成的横轴异构体已经是最稳定的形式。