Pillsbury Nathan R, Kidwell Nathanael M, Nebgen Benjamin, Slipchenko Lyudmila V, Douglass Kevin O, Cable John R, Plusquellic David F, Zwier Timothy S
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, USA.
Quantum Electronics and Photonics Division, Physical Measurement Laboratory, National Institute of Standards and Technology, Boulder, Colorado 80305-3328, USA.
J Chem Phys. 2014 Aug 14;141(6):064316. doi: 10.1063/1.4892344.
Vibrationally and rotationally resolved electronic spectra of diphenylmethane-d5 (DPM-d5) are reported in the isolated-molecule environment of a supersonic expansion. While small, the asymmetry induced by deuteration of one of the aromatic rings is sufficient to cause several important effects that change the principle mechanism of vibronic coupling between the close-lying S1 and S2 states, and spectroscopic signatures such coupling produces. The splitting between S1 and S2 origins is 186 cm(-1), about 50% greater than its value in DPM-d0 (123 cm(-1)), and an amount sufficient to bring the S2 zero-point level into near-resonance with the v = 1 level in the S1 state of a low-frequency phenyl flapping mode, ν(R) = 191 cm(-1). Dispersed fluorescence spectra bear clear evidence that Δv(R) = 1 Herzberg-Teller coupling dominates the near-resonant internal mixing between the S1 and S2 manifolds. The fluorescence into each pair of Franck-Condon active ring modes shows an asymmetry that suggests incorrectly that the S1 and S2 states may be electronically localized. From rotationally resolved studies, the S0 and S1 states have been well-fit to asymmetric rotor Hamiltonians while the S2 state is perturbed and not fit. The transition dipole moment (TDM) orientation of the S1 state is nearly perpendicular to the C2 symmetry axes with 66(2)%:3(1)%:34(2)% a:b:c hybrid-type character while that of the S2 origin contains 50(10)% a:c-type (S1) and 50(10)% b-type (S2) character. A model is put forward that explains qualitatively the TDM compositions and dispersed emission patterns without the need to invoke electronic localization. The experimental data discussed here serve as a foundation for a multi-mode vibronic coupling model capable of being applied to asymmetric bichromophores, as presented in the work of B. Nebgen and L. V. Slipchenko ["Vibronic coupling in asymmetric bichromophores: Theory and application to diphenylmethane-d5," J. Chem. Phys. (submitted)].
本文报道了在超声速膨胀的孤立分子环境中，二苯基甲烷 - d5（DPM - d5）的振动和转动分辨电子光谱。虽然由一个芳环的氘代引起的不对称性很小，但足以产生几个重要影响，这些影响改变了紧邻的S1和S2态之间振动 - 电子耦合的主要机制，以及这种耦合产生的光谱特征。S1和S2态起源之间的分裂为186 cm⁻¹，比其在DPM - d0中的值（123 cm⁻¹）大约大50%，这一数值足以使S2零点能级与低频苯基摆动模式ν(R) = 191 cm⁻¹的S1态中的v = 1能级接近共振。色散荧光光谱清楚地表明，Δv(R) = 1的赫兹伯格 - 泰勒耦合主导了S1和S2流形之间的近共振内混合。进入每对弗兰克 - 康登活性环模式的荧光显示出不对称性，这错误地表明S1和S2态可能是电子局域化的。通过转动分辨研究，S0和S1态已很好地拟合到不对称转子哈密顿量，而S2态受到扰动且无法拟合。S1态的跃迁偶极矩（TDM）方向几乎垂直于C2对称轴，具有66(2)%:3(1)%:34(2)%的a:b:c杂化类型特征，而S2态起源的TDM包含50(10)%的a:c型（S1）和50(10)%的b型（S2）特征。提出了一个模型，该模型无需调用电子局域化就能定性地解释TDM组成和色散发射模式。这里讨论的实验数据为一个能够应用于不对称双色团的多模式振动 - 电子耦合模型奠定了基础，如B. Nebgen和L. V. Slipchenko的工作中所述["不对称双色团中的振动 - 电子耦合：理论及对二苯基甲烷 - d5的应用"，《化学物理杂志》（已提交）]。
