Reppert Mike, Dutta Rajesh, Slipchenko Lyudmila
Department of Chemistry, Purdue University, West Lafayette, Indiana 47920, USA.
J Chem Phys. 2024 Oct 21;161(15). doi: 10.1063/5.0225083.
The dynamics of molecular excitonic systems are complicated by a competition between electronic coupling (which drives delocalization) and vibrational-electronic (vibronic) interactions (which tend to encourage electronic localization). A particular challenge of molecular systems is that they typically possess a large number of independent vibrations, with frequencies often spanning the entire spectrum of relevant electronic energy gaps. Recent spectroscopic observations and numerical simulations on a water-soluble chlorophyll-binding protein (WSCP) reveal a transition between two regimes of vibronic behavior, a Redfield-like regime in which low-frequency vibrations respond to a delocalized excitonic state, and a Förster-like regime where high-frequency vibrations act as incoherent excitations on individual pigments. Although numerical simulations can reproduce these effects, there is a need for a simple, systematic theory that accurately describes the smooth transition between these two regimes in experimental spectra. Here we address this challenge by generalizing the variational polaron transform approach of [Bloemsma et al., Chem. Phys. 481, 250 (2016)] to include arbitrary bath densities for systems with or without symmetry. We benchmark this theory against both numerical matrix-diagonalization methods and experimental 77 K fluorescence spectra for two WSCP variants, obtaining quite satisfactory agreement in both cases. We apply this theory to offer an explanation for the large loss in apparent electronic coupling in the WSCP Q57K mutant and to examine the likely impact of the interplay between excitonic delocalization and vibrational localization on vibrational sideband shapes and apparent coupling strengths in high-resolution optical spectra for chlorophyll-protein complexes such as WSCP.
分子激子系统的动力学因电子耦合（驱动离域）和振动 - 电子（振子）相互作用（倾向于促进电子定域）之间的竞争而变得复杂。分子系统的一个特殊挑战是它们通常拥有大量独立振动，其频率常常跨越相关电子能隙的整个光谱范围。最近对一种水溶性叶绿素结合蛋白（WSCP）的光谱观测和数值模拟揭示了振子行为的两种状态之间的转变，一种是类似雷德菲尔德（Redfield）状态，其中低频振动响应离域激子态；另一种是类似福斯特（Förster）状态，其中高频振动对单个色素起非相干激发作用。尽管数值模拟可以重现这些效应，但需要一种简单、系统的理论来准确描述实验光谱中这两种状态之间的平滑转变。在这里，我们通过将[Bloemsma等人，《化学物理》481, 250 (2016)]中的变分极化子变换方法进行推广，以包括具有或不具有对称性的系统的任意浴密度，来应对这一挑战。我们将该理论与数值矩阵对角化方法以及两种WSCP变体的77 K实验荧光光谱进行基准测试，在两种情况下都获得了相当令人满意的一致性。我们应用该理论来解释WSCP Q57K突变体中表观电子耦合的大幅损失，并研究激子离域和振动定域之间的相互作用对叶绿素 - 蛋白复合物（如WSCP）的高分辨率光谱中的振动边带形状和表观耦合强度可能产生的影响。
