A Solution- and Solid-State Investigation of Medium Effects on Charge Separation in Metastable Photomerocyanines

The effects of solution-state dielectric and intermolecular interactions on the degree of charge separation in metastable spirooxazine photomerocyanines (PMCs) is investigated. We report the first X-ray diffraction (XRD) analyses of an open form, a metastable photomerocyanine, of the spirooxazine class of photochromic molecules in two derivatives: spiro[azahomoadamantane-isoquinolinoxazine] (1) and spiro[azahomoadamantane-phenanthrolinoxazine] (2). Using the results of XRD analysis of the open photomerocyanine forms, in conjunction with computation, solvatochromism, and solution NMR studies, we have investigated the effect of the medium on the ground-state structure of these photomerocyanines. Solvatochromism and NMR chemical shift studies of 1 and 2 support the assignment of a quinoidal structure in nonpolar solvents and a zwitterionic structure in high-polarity solvents. The effect of azahomoadamantyl substitution is explored by comparing 1 and 2 with the analogous indolyl derivatives, spiro[indoline-isoquinolinoxazine] (3) and spiro[indoline-phenanthrolinoxazine] (4) through XRD analysis of the closed spirooxazine (SO) forms, solution-state kinetic experiments, solvatochromism, and NMR studies. Longer Cspiro−O bond lengths in the SO form and slower rates of thermal PMC → SO isomerization for the azahomoadamantyl derivatives are associated with greater zwitterionic character in the PMC form, as found in the solvatochromism studies. XRD analysis of photomerocyanines 1 and 2 indicate a greater contribution from the canonical zwitterionic resonance form relative to the quinoidal form in the solid state. Structural differences observed in two pseudopolymorphs of 2-PMC suggest that the degree of charge-separated character is influenced by the crystal packing environment. These results provide direct structural evidence for the effects of the medium polarity on charge-separated states of photomerocyanines.

本研究探究了溶液态介电常数与分子间相互作用对亚稳态螺恶嗪光致变色花菁（metastable spirooxazine photomerocyanines, PMCs）中电荷分离程度的影响。本文首次完成了两类螺恶嗪类光致变色分子的开环形式（亚稳态光致变色花菁）的X射线衍射（X-ray diffraction, XRD）分析，这两类衍生物分别为螺[氮杂高金刚烷-异喹啉恶嗪]（1）与螺[氮杂高金刚烷-菲咯啉恶嗪]（2）。结合开环光致变色花菁形式的XRD分析结果、计算模拟、溶剂致变色效应（solvatochromism）以及溶液核磁共振（solution NMR）研究，我们系统考察了介质环境对这类光致变色花菁基态结构的影响。针对化合物1和2开展的溶剂致变色效应与核磁共振化学位移研究证实：在非极性溶剂中其分子呈现醌式结构，而在高极性溶剂中则为两性离子结构。为探究氮杂高金刚烷基取代的影响，我们将化合物1、2与对应的吲哚基衍生物螺[吲哚啉-异喹啉恶嗪]（3）和螺[吲哚啉-菲咯啉恶嗪]（4）进行对比分析，所用手段包括闭环螺恶嗪（closed spirooxazine, SO）形式的XRD分析、溶液态动力学实验、溶剂致变色效应以及溶液核磁共振研究。研究结果表明，正如溶剂致变色效应研究所揭示的，闭环SO形式中更长的Cspiro−O键长，以及氮杂高金刚烷基衍生物更慢的热致PMC→SO异构化速率，均与PMC形式更强的两性离子特性相关。光致变色花菁1和2的XRD分析显示，固态下相较于醌式结构，典型两性离子共振式对其分子结构的贡献更为显著。2-PMC的两种假多晶型（pseudopolymorphs）之间观测到的结构差异表明，电荷分离程度受晶体堆积环境的调控。上述研究结果为介质极性对光致变色花菁电荷分离态的影响提供了直接的结构实验证据。