Department of Chemistry, Temple University, Philadelphia, PA 19122, USA.
Department of Chemistry and Biochemistry, University of Oregon, Eugene, OR 97403, USA.
Molecules. 2022 Jun 24;27(13):4062. doi: 10.3390/molecules27134062.
Accurate modeling of optical spectra requires careful treatment of the molecular structures and vibronic, environmental, and thermal contributions. The accuracy of the computational methods used to simulate absorption spectra is limited by their ability to account for all the factors that affect the spectral shapes and energetics. The ensemble-based approaches are widely used to model the absorption spectra of molecules in the condensed-phase, and their performance is system dependent. The Franck-Condon approach is suitable for simulating high resolution spectra of rigid systems, and its accuracy is limited mainly by the harmonic approximation. In this work, the absorption spectrum of the widely used cyanine Cy3 is simulated using the ensemble approach via classical and quantum sampling, as well as, the Franck-Condon approach. The factors limiting the ensemble approaches, including the sampling and force field effects, are tested, while the vertical and adiabatic harmonic approximations of the Franck-Condon approach are also systematically examined. Our results show that all the vertical methods, including the ensemble approach, are not suitable to model the absorption spectrum of Cy3, and recommend the adiabatic methods as suitable approaches for the modeling of spectra with strong vibronic contributions. We find that the thermal effects, the low frequency modes, and the simultaneous vibrational excitations have prominent contributions to the Cy3 spectrum. The inclusion of the solvent stabilizes the energetics significantly, while its negligible effect on the spectral shapes aligns well with the experimental observations.
准确建模光学光谱需要仔细处理分子结构以及振动态、环境和热贡献。用于模拟吸收光谱的计算方法的准确性受到其能够考虑所有影响光谱形状和能量学的因素的限制。基于集合的方法广泛用于模拟凝聚相分子的吸收光谱，其性能取决于系统。 Franck-Condon 方法适用于模拟刚性体系的高分辨率光谱，其准确性主要受谐波近似的限制。在这项工作中，使用集合方法通过经典和量子采样以及 Franck-Condon 方法模拟了广泛使用的花青 Cy3 的吸收光谱。测试了限制集合方法的因素，包括采样和力场效应，还系统地检查了 Franck-Condon 方法的垂直和绝热谐波近似。我们的结果表明，所有垂直方法，包括集合方法，都不适合模拟 Cy3 的吸收光谱，并推荐绝热方法作为具有强烈振动态贡献的光谱建模的合适方法。我们发现热效应、低频模式和同时的振动激发对 Cy3 光谱有显著贡献。溶剂的包含显著稳定了能量学，而其对光谱形状的影响可以忽略不计，这与实验观察结果一致。
