Pillsbury Nathan R, Müller Christian W, Meerts W Leo, Plusquellic David F, Zwier Timothy S
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA.
J Phys Chem A. 2009 Apr 30;113(17):5000-12. doi: 10.1021/jp8098686.
Laser-induced fluorescence, single-vibronic level fluorescence (SVLF), UV hole burning, and fluorescence dip infrared (FDIR) spectroscopy have been carried out on bis-(2-hydroxyphenyl)methane in order to characterize the ground-state and first excited-state vibronic spectroscopy of this model flexible bichromophore. These studies identified the presence of two conformational isomers. The FDIR spectra in the OH-stretch region determine that conformer A is an OH...O H-bonded conformer, while conformer B is a doubly OH...pi H-bonded conformer with C(2) symmetry. High-resolution ultraviolet spectra ( approximately 50 MHz resolution) of a series of vibronic bands of both conformers confirm and refine these assignments. The transition dipole moment (TDM) direction in conformer A is consistent with electronic excitation that is primarily localized on the donor phenol ring. A tentative assignment of the S(2) origin is made to a set of transitions approximately 400 cm(-1) above S(1). In conformer B, the TDM direction firmly establishes C(2) symmetry for the conformer in its S(1) state and establishes the electronic excitation as delocalized over the two rings, as the lower member of an excitonic pair. The S(2) state has not been clearly identified in the spectrum. Based on CIS calculations, the S(2) state is postulated to be several times weaker than S(1), making it difficult to identify, especially in the midst of overlap from vibronic bands due to conformer A. SVLF spectra show highly unusual vibronic intensity patterns, particularly in conformer B, which cannot be understood by simple harmonic Franck-Condon models, even in the presence of Duschinsky mixing. We postulate that these model flexible bichromophores have TDMs that are extraordinarily sensitive to the distance and orientation of the two aromatic rings, highlighting the need to map out the TDM surface and its dependence on the (up to) five torsional and bending coordinates in order to understand the observations.
为了表征这种典型的柔性双发色团的基态和第一激发态振动态光谱，对双（2-羟基苯基）甲烷进行了激光诱导荧光、单振动能级荧光（SVLF）、紫外空穴烧蚀和荧光凹陷红外（FDIR）光谱研究。这些研究确定了两种构象异构体的存在。OH伸缩区域的FDIR光谱表明构象异构体A是一种OH...O氢键构象异构体，而构象异构体B是具有C(2)对称性的双OH...π氢键构象异构体。两种构象异构体的一系列振动态带的高分辨率紫外光谱（分辨率约为50 MHz）证实并完善了这些归属。构象异构体A中的跃迁偶极矩（TDM）方向与主要定域在供体酚环上的电子激发一致。对S(2)起源进行了初步归属，认为是一组比S(+)高出约400 cm(-1)的跃迁。在构象异构体B中，TDM方向明确确定了该构象异构体在S(1)态时的C(2)对称性，并确定电子激发在两个环上离域，是激子对的较低成员。光谱中尚未明确识别出S(2)态。基于CIS计算，推测S(2)态比S(1)弱几倍，这使得其难以识别，尤其是在存在来自构象异构体A的振动态带重叠的情况下。SVLF光谱显示出非常不寻常的振动态强度模式，特别是在构象异构体B中，即使存在杜什金斯基混合，简单的简谐弗兰克-康登模型也无法解释这种模式。我们推测这些典型的柔性双发色团的TDM对两个芳香环的距离和取向异常敏感，这突出表明需要绘制TDM表面及其对（多达）五个扭转和弯曲坐标的依赖性，以便理解这些观测结果。
