Zheng Yilong, Giordano Anthony J, Marder Seth R, Saavedra S Scott
Department of Chemistry & Biochemistry, University of Arizona, Tucson, Arizona 85721, United States.
School of Chemistry & Biochemistry and the Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332, United States.
Langmuir. 2020 Jun 23;36(24):6728-6735. doi: 10.1021/acs.langmuir.0c00817. Epub 2020 Jun 8.
An electroreflectance method to determine the electron transfer rate constant of a film of redox-active chromophores immobilized on an optically transparent electrode when the surface coverage of the film is very low (<0.1 monolayer) is described herein. The method, potential-modulated total internal reflection fluorescence (PM-TIRF) spectroscopy, is a fluorescence version of potential-modulated attenuated total reflection (PM-ATR) spectroscopy that is applicable when the immobilized chromophores are luminescent. The method was tested using perylene diimide (PDI) molecules functionalized with -phenylene phosphonic acid (PA) moieties that bind strongly to indium-tin oxide (ITO). Conditions to prepare PDI-phenyl-PA films that exhibit absorbance and fluorescence spectra characteristic of monomeric (i.e., nonaggregated) molecules were identified; the electrochemical surface coverage was approximately 0.03 monolayer. The tilt angle of the long axis of the PDI molecular plane is 58° relative to the ITO surface normal, 25° greater than the tilt angle of aggregated PDI-phenyl-PA films, which have a surface coverage of approximately one monolayer. The more in-plane orientation of monomeric films is likely due to the absence of cofacial π-π interactions present in aggregated films and possibly a difference in PA-ITO binding modes. The electron transfer rate constant () of monomeric PDI-phenyl-PA films was determined using PM-TIRF and compared with PM-ATR results obtained for aggregated films. For PDI monomers, = 3.8 × 10 s, which is about 3.7-fold less than for aggregated films. The slower kinetics are attributed to the absence of electron self-exchange between monomeric PDI molecules. Differences in the electroactivity of the binding sites on the ITO electrode surface also may play a role. This is the first demonstration of PM-TIRF for determining electron transfer rate constants at an electrode/organic film interface.
本文描述了一种用于测定固定在光学透明电极上的氧化还原活性发色团薄膜的电子转移速率常数的电光反射法，该方法适用于薄膜表面覆盖率非常低（<0.1单层）的情况。该方法即电位调制全内反射荧光（PM-TIRF）光谱法，是电位调制衰减全反射（PM-ATR）光谱法的荧光版本，适用于固定的发色团具有发光性的情况。该方法使用了用亚苯基膦酸（PA）部分官能化的苝二酰亚胺（PDI）分子进行测试，这些分子与氧化铟锡（ITO）强烈结合。确定了制备具有单体（即非聚集）分子特征吸收光谱和荧光光谱的PDI-苯基-PA薄膜的条件；电化学表面覆盖率约为0.03单层。PDI分子平面长轴相对于ITO表面法线的倾斜角为58°，比表面覆盖率约为一个单层的聚集PDI-苯基-PA薄膜的倾斜角大25°。单体薄膜更多的面内取向可能是由于聚集薄膜中不存在共面π-π相互作用，也可能是由于PA-ITO结合模式的差异。使用PM-TIRF测定了单体PDI-苯基-PA薄膜的电子转移速率常数（），并与聚集薄膜的PM-ATR结果进行了比较。对于PDI单体，= 3.8 × 10 s，约为聚集薄膜的3.7倍。较慢的动力学归因于单体PDI分子之间不存在电子自交换。ITO电极表面结合位点的电活性差异也可能起作用。这是首次证明PM-TIRF可用于测定电极/有机薄膜界面的电子转移速率常数。
