Elucidating the mechanisms of acidochromic spiropyran-merocyanine interconversion
Elucidating the mechanisms of acidochromic spiropyran-merocyanine interconversion
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DOI：
10.1021/jp068575r
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发表时间：
2007-04-05
影响因子：
2.9
通讯作者：
Buncel, Erwin
中科院分区：
文献类型：
--
作者：
Wojtyk, James T. C.;Wasey, Adnaan;Buncel, Erwin
用紫外维斯分光光度法研究了6-硝基BIPS（SP-1）在丙酮中酸诱导转化生成质子化汞（MC-OH+）的热和光化学过程。结果表明，三氟乙酸（TFA）诱导SP开环的机理和MC-OH+形成的速率遵循一般的酸催化机理。与该机理雅阁，随着培养基中TFA浓度的增加，酸诱导开环形式（MC-OH+）的热生长受到抑制。N-质子化SP，即，SP-NH+在竞争性侧平衡过程中作为非反应性“汇”形成，其中氮孤对不再可用于驱动开环过程，并导致作为线性1/k（obs）vs [HA]图的逆速率依赖性。将叔胺添加到MC-OH+再生的MC，其经历热环闭合到SP，从而恢复其作为分子开关的功能。SP样品的NMR滴定显示在增加TFA浓度时N-取代基峰的低场位移。然而，与模型化合物N，N-二甲基苯胺（N，N-DMA）不同，在高达IM酸的SP-1下不能观察到饱和行为，这表明碱强度顺序为N，N-DMA > SP-1。此外，我们已经证明，在溶剂丙酮中，在酸化时，正常的光致变色和热致变色行为被逆转;现在MC-OH+被光化学转化为SP-H+，SP-H+经历热开环为MC-OH+。
The thermal and photochemical processes associated with the acid-induced conversions of 6-nitroBIPS, SP-1, to form the protonated merocyanine (MC-OH+) were investigated via UV/vis spectrophotometric studies in acetone. It was found that the mechanism of trifluoroacetic acid (TFA)-induced ring-opening of the SP and the rate of MC-OH+ formation follows a general acid catalysis mechanism. In accord with this mechanism, the thermal growth of the acid-induced ring-opened form (MC-OH+) was retarded as the concentration of TFA in the medium was increased. The N-protonated SP, i.e., SP-NH+, is formed in a competing side-equilibrium process as an unreactive "sink", with the nitrogen lone-pair no longer available to drive the ring-opening process and resulting in the inverse rate dependence as a linear 1/k(obs) vs [HA] plot. Addition of a tertiary amine to MC-OH+ regenerated MC which underwent thermal ring closure to the SP, thus restoring its function as a molecular switch. NMR titration of SP samples showed a downfield shift of the N-substituent peak upon increasing the TFA concentration. However, a saturation behavior could not be observed with SP-1 up to 1 M acid, unlike the model compound, N,N-dimethylaniline (N,N-DMA), which indicates a base strength order of N,N-DMA > SP-1. Further, we have demonstrated that in solvent acetone, on acidification, the normal photo- and thermochromic behavior is reversed; now MC-OH+ is photochemically transformed into SP-H+, which undergoes thermal ring-opening to MC-OH+.