Vibrational quenching of excitonic splittings in H-bonded molecular dimers: the electronic Davydov splittings cannot match experiment.
Vibrational quenching of excitonic splittings in H-bonded molecular dimers: the electronic Davydov splittings cannot match experiment.
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DOI：
10.1063/1.4705119
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发表时间：
2012-05
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影响因子：
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通讯作者：
Philipp Ottiger;S. Leutwyler;H. Köppel
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文献类型：
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作者：
Philipp Ottiger;S. Leutwyler;H. Köppel
对称双氢键气相二聚体的S(1)/S(2)态激子分裂为紫外发色团之间的激发态电子耦合提供了光谱基准。这对于从光合作用捕光天线到光合作用反应中心、共轭聚合物、分子晶体和核酸的多发色系统中的电子能量传递具有重要的意义。我们提供了双氢键邻氰苯酚二聚体的S(1)/S(2)激子分裂Δ(EXP)的激光光谱数据，并与(2-氨基吡啶)(2)，[(2AP)(2)]，(2-吡啶酮)(2)，[(2PY)(2)]，(苯甲酸)(2)，[(BZA)(2)]和(苯腈)(2)，[(BN)(2)]的二聚体的分裂进行了比较。S(1)/S(2)的实验激子劈裂为：(OCp)(2)的Δ(Exp)=16.4 cm(-1)，(2Ap)(2)的11.5 cm(-1)，(2PY)(2)的43.5 cm(-1)，(Bza)(2)的<；1 cm(-1)。相反，用近似二阶耦合团簇方法计算的S(1)/S(2)垂直能隙Δ(计算)比Δ(EXP)值大10-40倍。这种和其他从头算方法重现激子分裂Δ(EXP)的定性失败源于Born-Oppenheimer(BO)近似，该近似隐含地假设了强耦合情况，不能用于评估处于弱耦合极限的系统的激子分裂。给定典型的氢键距离和振子强度，大多数氢键二聚体处于弱耦合极限。在这种情况下，必须考虑电子激发时的单体电子-振动耦合；激子分裂发生在振动(而不是电子)跃迁之间。通过考虑单体S(1)ΔS(0)激发中分子内振动耦合对BO分裂的猝灭，解决了基于BO的分裂Δ(Calc)和实验值←(EXP)之间的差异。五个二聚体(OCp)(2)、(2Ap)(2)、(2Ap)(2)、(BN)(2)和(BZA)(2)的振动猝灭因子Γ在Γ=0.030.2范围内。激子猝灭分裂Γ[中间点]Δ(Calc)与观察到的分裂Δ(Exp)符合得很好。振动猝灭方法预测了所研究的二聚体的可靠Δ(EXP)值，证实了电子Davydov分裂的振动猝灭的重要性，并为预测多发色体系中实际的激子分裂提供了可靠的基础。
The S(1)/S(2) state exciton splittings of symmetric doubly hydrogen-bonded gas-phase dimers provide spectroscopic benchmarks for the excited-state electronic couplings between UV chromophores. These have important implications for electronic energy transfer in multichromophoric systems ranging from photosynthetic light-harvesting antennae to photosynthetic reaction centers, conjugated polymers, molecular crystals, and nucleic acids. We provide laser spectroscopic data on the S(1)/S(2) excitonic splitting Δ(exp) of the doubly H-bonded o-cyanophenol (oCP) dimer and compare to the splittings of the dimers of (2-aminopyridine)(2), [(2AP)(2)], (2-pyridone)(2), [(2PY)(2)], (benzoic acid)(2), [(BZA)(2)], and (benzonitrile)(2), [(BN)(2)]. The experimental S(1)/S(2) excitonic splittings are Δ(exp) = 16.4 cm(-1) for (oCP)(2), 11.5 cm(-1) for (2AP)(2), 43.5 cm(-1) for (2PY)(2), and <1 cm(-1) for (BZA)(2). In contrast, the vertical S(1)/S(2) energy gaps Δ(calc) calculated by the approximate second-order coupled cluster (CC2) method for the same dimers are 10-40 times larger than the Δ(exp) values. The qualitative failure of this and other ab initio methods to reproduce the exciton splitting Δ(exp) arises from the Born-Oppenheimer (BO) approximation, which implicitly assumes the strong-coupling case and cannot be employed to evaluate excitonic splittings of systems that are in the weak-coupling limit. Given typical H-bond distances and oscillator strengths, the majority of H-bonded dimers lie in the weak-coupling limit. In this case, the monomer electronic-vibrational coupling upon electronic excitation must be accounted for; the excitonic splittings arise between the vibronic (and not the electronic) transitions. The discrepancy between the BO-based splittings Δ(calc) and the much smaller experimental Δ(exp) values is resolved by taking into account the quenching of the BO splitting by the intramolecular vibronic coupling in the monomer S(1) ← S(0) excitation. The vibrational quenching factors Γ for the five dimers (oCP)(2), (2AP)(2), (2AP)(2), (BN)(2), and (BZA)(2) lie in the range Γ = 0.03-0.2. The quenched excitonic splittings Γ[middle dot]Δ(calc) are found to be in very good agreement with the observed splittings Δ(exp). The vibrational quenching approach predicts reliable Δ(exp) values for the investigated dimers, confirms the importance of vibrational quenching of the electronic Davydov splittings, and provides a sound basis for predicting realistic exciton splittings in multichromophoric systems.