Interactions of electronic and vibrational degrees of freedom, namely vibronic couplings, are essential for understanding excited states relaxation pathways of molecular systems in bulk and at interfaces and surfaces. In this talk, I will give two examples of vibronic couplings applied to singlet fission and interfacial charge transfer.

Vibronic coupling is believed to play an important role in singlet fission, wherein a photo-excited singlet exciton is converted into two triplet excitons. In the first part, we examined the role of vibronic coupling in singlet fission using polarized transient absorption microscopy and ab initio simulations on single-crystalline pentacene. It was found that singlet fission in pentacene is greatly facilitated by the vibrational coherence of a 35.0 cm^-1 phonon, which anisotropic coherence persists extensively for a few picoseconds. This coherence-preserving phonon that drives the anisotropic singlet-fission is made possible by a unique cross-axial charge transfer intermediate state. In the same fashion, this phonon was also found to predominantly drive the quantum decoherence of a correlated triplet pair to form a decoupled triplet dimer. Moreover, our transient kinetic experimental data illustrates notable directional anisotropic nature of singlet fission rate in single crystalline pentacene.

In the second part, we presented the development of interface-specific two-dimensional electronic-vibrational sum frequency spectroscopy (2D-EVSFG) for electronic-vibrational couplings for excited states at interfaces and surfaces. We demonstrate this 2D-EVSFG technique by investigating photoexcited interface-active molecules of Malachite Green (MG) at the air/water interface as an example. Our 2D-EVSFG experiments show strong vibronic couplings of interfacial MG molecules upon photoexcitation and subsequent relaxation of a locally excited (LE) state. We believe that this development of 2D-EVSFG opens up a new avenue of understanding excited state dynamics related to interfaces and surfaces.

Research Lab at Website (Utah State): https://www.yirao.org/