Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Knowledge City, Sector 81, SAS Nagar, Punjab 140 306, India.
J Phys Chem B. 2020 Aug 6;124(31):6825-6834. doi: 10.1021/acs.jpcb.0c01333. Epub 2020 Jul 27.
Photophysical properties of tricarbocyanine dyes in various solvents have been widely investigated using a variety of spectroscopic tools. However, the presence of several ground-state isomers and interconversion between these isomers on an ultrafast timescale upon photoexcitation render unambiguous assignment of spectral features quite difficult. In this work, ultrafast excited-state dynamics of two tricarbocyanine dyes in two solvents, DNTTCI and IR140, in ethanol and ethylene glycol, are studied by two-dimensional electronic spectroscopy (2DES). We present a detailed discussion on design and calibration of the 2DES apparatus and on the method for data processing by phase-cycling. For DNTTCI we report a method to obtain solvation correlation function, the nature of which is found to be strongly dependent on the excitation frequencies; a blue-shifted spectrum at early time is observed and explained based on preferential emission from a subset among various isomers having overlapping spectral features. For IR140 in ethanol, four isomers with distinct spectral features are identified, and most importantly, three of these isomers were found to interconvert upon photoexcitation which completes within 100 fs and is explained based on a kinetic model of consecutive chemical reaction. Density functional theory calculations show the presence of several ground-state isomers for both these dyes. Through this work we demonstrate how 2DES can help us to decipher distinct excited-state photophysics in two carbocyanine dyes, polar solvation and photoisomerization, by resolving spectral congestion without sacrificing time resolution.
已经使用各种光谱工具广泛研究了各种溶剂中三碳菁染料的光物理性质。然而，在光激发下，几个基态异构体的存在以及这些异构体之间的超快转换使得明确分配光谱特征变得非常困难。在这项工作中，通过二维电子光谱（2DES）研究了两种三碳菁染料在两种溶剂（DNTTCI 和 IR140）中的超快激发态动力学，即乙醇和乙二醇。我们详细讨论了 2DES 仪器的设计和校准以及通过相循环进行数据处理的方法。对于 DNTTCI，我们报告了一种获得溶剂化相关函数的方法，其性质强烈依赖于激发频率；在早期观察到并解释了光谱蓝移，这是基于具有重叠光谱特征的各种异构体中一组的优先发射。对于乙醇中的 IR140，鉴定出了四个具有不同光谱特征的异构体，最重要的是，发现其中三个异构体在光激发下可以相互转换，这在 100 fs 内完成，并基于连续化学反应的动力学模型进行了解释。密度泛函理论计算表明，这两种染料都存在几个基态异构体。通过这项工作，我们展示了 2DES 如何通过解析光谱拥挤而不牺牲时间分辨率来帮助我们破译两种碳菁染料的不同激发态光物理性质，即极性溶剂化和光异构化。
