Böhnke Hendrik, Bahrenburg Julia, Ma Xiaonan, Röttger Katharina, Näther Christian, Rode Michał F, Sobolewski Andrzej L, Temps Friedrich
Institute of Physical Chemistry, Christian-Albrechts-University Kiel, Olshausenstr. 40, 24098 Kiel, Germany.
Phys Chem Chem Phys. 2018 Jan 24;20(4):2646-2655. doi: 10.1039/c7cp06145e.
Molecular switches based on proton transfer that are photochromic and can be interconverted by light at different wavelengths back and forth between two thermodynamically stable tautomeric states in solution at room temperature are rare to date. We report on a study of the ultrafast conversion of the bistable proton transfer switch N-(3-pyridinyl)-2-pyridinecarboxamide (NPPCA) to a corresponding iminol after photoexcitation at λ ≈ 265 nm by means of femtosecond time-resolved broad-band and single-colour transient electronic absorption spectroscopy (TEAS), transient fluorescence spectroscopy (TFLS), and transient vibrational absorption spectroscopy (TVAS) in acetonitrile solution. The interpretation of the data was accompanied by ab initio quantum chemical calculations of the excited electronic states and the vibrational frequencies of the reactant and product in their ground electronic state. The TEAS experiments provided four time constants, τ = 0.09 ± 0.01 ps, τ = 0.61 ± 0.01 ps, τ = 5.10 ± 0.80 ps, and τ = 20.0 ± 1.0 ps. The first two agree well with the measured TFLS lifetimes, τ < 0.18 ps and τ = 0.50 ± 0.01 ps. τ is related to the relaxation of the initially excited Franck-Condon (FC) state of the pyridinecarboxamide, followed by the excited-state intramolecular proton transfer (ESIPT) step to the neighbouring pyridine. The subsequent return of the molecules to the electronic ground state takes place within τ, mediated by a conical intersection (CI) at a twisted configuration of the pyridinecarboxamide moiety. The main components in all TEAS time profiles feature a rise with τ and a decay with τ and describe subsequent molecular transformations in the electronic ground state. τ is ascribed to vibrational cooling of the molecules. The final iminol exhibits a permanent UV absorption at λ = 247 nm, where its absorbance is stronger than that of the carboxamide reactant. The iminol structure is unambiguously identified by the TVA spectra, which show the build-up of corresponding vibrational bands with τ = 23 ± 2 ps after the initial bleach of the reactant vibrational bands, in excellent agreement with the TEAS data. Its lifetime is >10 ns.
基于质子转移的分子开关具有光致变色特性，在室温下的溶液中，能在两个热力学稳定的互变异构状态之间，通过不同波长的光来回相互转换，此类分子开关目前还很罕见。我们报告了一项研究，在乙腈溶液中，通过飞秒时间分辨宽带和单色瞬态电子吸收光谱（TEAS）、瞬态荧光光谱（TFLS）以及瞬态振动吸收光谱（TVAS），对双稳态质子转移开关N-(3-吡啶基)-2-吡啶甲酰胺（NPPCA）在λ≈265nm光激发后超快转换为相应亚胺醇的过程进行了研究。对数据的解释辅以对反应物和产物基态激发电子态及振动频率的从头算量子化学计算。TEAS实验给出了四个时间常数，τ = 0.09 ± 0.01 ps、τ = 0.61 ± 0.01 ps、τ = 5.10 ± 0.80 ps和τ = 20.0 ± 1.0 ps。前两个与测量的TFLS寿命τ < 0.18 ps和τ = 0.50 ± 0.01 ps吻合良好。τ与吡啶甲酰胺初始激发的弗兰克-康登（FC）态的弛豫有关，随后是激发态分子内质子转移（ESIPT）步骤至相邻吡啶。分子随后返回电子基态在τ内发生，由吡啶甲酰胺部分扭曲构型处的锥形交叉（CI）介导。所有TEAS时间曲线中的主要成分呈现出以τ上升和以τ衰减的特征，并描述了电子基态下随后的分子转变。τ归因于分子的振动冷却。最终的亚胺醇在λ = 247 nm处呈现永久性紫外吸收，其吸光度强于羧酰胺反应物。亚胺醇结构由TVA光谱明确鉴定，该光谱显示在反应物振动带初始漂白后，相应振动带在τ = 23 ± 2 ps时逐渐形成，与TEAS数据高度吻合。其寿命>10 ns
