Vibronic coupling in asymmetric bichromophores: Experimental investigation of diphenylmethane-d5

Abstract

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)].

Simulation of Multi-Dimensional Signals in the Optical Domain: Quantum-Classical Feedback in Nonlinear Exciton Propagation

“…CASPT2 (8,8) Given the moderate quality of the employed basis set (6-31G*) we assign this behavior to error cancelation effects also observed in other molecules. 53 Finally, SS-CASPT2 (12,12) calculations are in almost quantitative agreement with experiment.…”

“…Energy gap fluctuations originating from fast C−H stretching vibrations around 3300−3400 cm −1 are responsible for the ultrafast modulation of the peak shape. This ultrafast spectral diffusion can be identified similarly on both, the CASSCF (8,8) and CASSCF (12,12) level of theory.…”

“…The initial energy gaps are found in the range 45000−52000 cm −1 (CASSCF (8,8)) and 37000−42000 cm −1 (CASSCF (12,12)) characterized approximately by a Gaussian distribution with the mean indicated by red and blue lines in Figure 7a and b. The trajectories quickly relax due to nuclear motion on the excited PESs which leads to a shift of the energy gap distribution followed by an oscillatory fluctuation.…”

“…The trajectories quickly relax due to nuclear motion on the excited PESs which leads to a shift of the energy gap distribution followed by an oscillatory fluctuation. For the ensemble of trajectories evaluated on the CASSCF (8,8) level the shift is substantially larger than for the CASSCF (12,12) ensemble (c.f., Figure 7a and b), while the oscillatory behavior can be identified for both. Optimized minima of S 1 and S 2 are characterized by increased aromatic C−C bond lengths ( Figure SI.4b,c).…”

“…However, because of the restricted active space and lack of dynamic electron correlation the absolute excitation energies are about 1.6 eV to high. Increasing the active space by considering all π orbitals of both phenyl (CASSCF (12,12)) accounts for 75% of electron correlation with respect to experiments (S 1 /S 2 splitting: 159 cm −1 ). CASPT2 (8,8) Given the moderate quality of the employed basis set (6-31G*) we assign this behavior to error cancelation effects also observed in other molecules.…”