Vibronic Coupling in Asymmetric Bichromophores: Experimental Investigation of Diphenylmethane-d5
Nathan Pillsbury, Nathanael M. Kidwell, Benjamin Nebgen, Lyudmila V. Slipchenko, Kevin O. Douglass, John R. Cable, David F. Plusquellic
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, R=191 cm-1. Dispersed fluorescence spectra bear clear evidence that v(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 companion paper.