Transient absorption spectroscopy of the electron transfer step in the photochemically activated polymerizations of N-ethylcarbazole and 9-phenylcarbazole.
Transient absorption spectroscopy of the electron transfer step in the photochemically activated polymerizations of N-ethylcarbazole and 9-phenylcarbazole.
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DOI：
10.1039/d1cp03137f
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发表时间：
2021-09-14
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影响因子：
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光激发的N-乙基咔唑（N-EC）在电子受体存在下的聚合开始于电子转移（ET）步骤以产生N-EC的自由基阳离子（N-EC stec+）。在这里，生产N-EC stec+的皮秒到纳秒的时间尺度上研究后，N-EC光激发在波长λex = 345 nm，使用瞬态电子和振动吸收光谱。在二氯甲烷（DCM）和乙腈（ACN）溶液中检测ET生成二苯基碘鎓六氟磷酸盐（Ph 2 I + PF 6 −）或对位烷基化变体的动力学和机制。基于Smoluchowski理论的扩散动力学模型很好地描述了N-EC stec+的生成：对于Ph 2 I + PF 6 −，在DCM中导出的ET双分子速率系数为kET =（1.8 ± 0.5）× 1010 M−1 s−1，这与扩散限制动力学一致。这种ET发生在N-EC的第一激发单重态（S1），与系统间穿越竞争以填充三重态（T1），ET也可能由此产生。一个更快的组件的ET反应表明预形成的基态复合物之间的N-EC和电子受体。在ACN中，预反应络合物的贡献较小，导出的ET速率系数为kET =（1.0 ± 0.3）× 1010 M−1 s−1。光激发的9-苯基咔唑（9-PC）和Ph 2 I + PF 6 −溶液的相应测量给出DCM中的kET =（5 ± 1）× 109 M−1 s−1。对电子受体进行结构修饰以增加其空间体积，从而降低kET的大小：甲基和叔丁基加成到苯环的帕拉（帕拉Me 2 Ph 2 I + PF 6 −和叔丁基-Ph 2 I + PF 6 −）分别给出kET =（1.2 ± 0.3）× 1010 M−1 s−1，kET =（5.4 ± 1.5）× 109 M−1 s−1。这些减少kET归因于较慢的扩散速率或ET反应中的空间限制。咔唑和碘鎓盐之间的电子转移反应的瞬态吸收光谱揭示了结构和溶剂依赖的动力学和机械的细节重要的聚合引发。
The polymerization of photoexcited N-ethylcarbazole (N-EC) in the presence of an electron acceptor begins with an electron transfer (ET) step to generate a radical cation of N-EC (N-EC˙+). Here, the production of N-EC˙+ is studied on picosecond to nanosecond timescales after N-EC photoexcitation at a wavelength λex = 345 nm using transient electronic and vibrational absorption spectroscopy. The kinetics and mechanisms of ET to diphenyliodonium hexafluorophosphate (Ph2I+PF6−) or para-alkylated variants are examined in dichloromethane (DCM) and acetonitrile (ACN) solutions. The generation of N-EC˙+ is well described by a diffusional kinetic model based on Smoluchowski theory: with Ph2I+PF6−, the derived bimolecular rate coefficient for ET is kET = (1.8 ± 0.5) × 1010 M−1 s−1 in DCM, which is consistent with diffusion-limited kinetics. This ET occurs from the first excited singlet (S1) state of N-EC, in competition with intersystem crossing to populate the triplet (T1) state, from which ET may also arise. A faster component of the ET reaction suggests pre-formation of a ground-state complex between N-EC and the electron acceptor. In ACN, the contribution from pre-reaction complexes is smaller, and the derived ET rate coefficient is kET = (1.0 ± 0.3) × 1010 M−1 s−1. Corresponding measurements for solutions of photoexcited 9-phenylcarbazole (9-PC) and Ph2I+PF6− give kET = (5 ± 1) × 109 M−1 s−1 in DCM. Structural modifications of the electron acceptor to increase its steric bulk reduce the magnitude of kET: methyl and t-butyl additions to the para positions of the phenyl rings (para Me2Ph2I+PF6− and t-butyl-Ph2I+PF6−) respectively give kET = (1.2 ± 0.3) × 1010 M−1 s−1 and kET = (5.4 ± 1.5) × 109 M−1 s−1 for reaction with photoexcited N-EC in DCM. These reductions in kET are attributed to slower rates of diffusion or to steric constraints in the ET reaction. Transient absorption spectroscopy of electron transfer reactions between a carbazole and an iodonium salt reveals structure and solvent-dependent kinetic and mechanistic details important to initiation of polymerization.
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影响因子：
3.6
作者：
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