Orientation and Polarization Dependence of Ground- and Excited-State FSRS in Crystalline Betaine-30

Charge transfer reactions are frequently accompanied by large changes in dipole and are of instrumental importance in many biological and chemical processes. Understanding the mechanism and dynamics associated with charge transfer reactions has been the focus of decades of investigations; however, most studies have been done with solvated analytes in which the measurements can be significantly impacted by environmental broadening or solvent-mediated effects. Here, we study photoinduced intramolecular charge transfer in crystalline betaine-30, thus dramatically reducing environmental heterogeneities and solvent contributions to the charge transfer process. We use femtosecond stimulated Raman spectroscopy in a reflective microscope configuration to probe the ground and excited state structures in a well-characterized betaine-30 crystal, with a unit cell consisting of four nearly orthogonal betaine-30 molecules and six water molecules. This study highlights how changing the crystal orientation and laser polarization can impact the excited- and ground-state Raman signals due to differences in the unit cell net dipole moment along different crystal axes. We find that the forward charge transfer process (τ ∼500 fs) is unaffected by the crystalline environment, while the back electron transfer process (τ ∼20 ps) is delayed in the crystalline environment as compared to lifetimes obtained in a range of solvents. This work highlights the role of the net transition dipole orientation on signals resulting from charge transfer in an anisotropic system and should aid in guiding the rational design of solid-state photoactivated devices.