Tracking Twisted Intramolecular Charge Transfer and Isomerization Dynamics in 9-(2,2-Dicyanovinyl) Julolidine Using Femtosecond Stimulated Raman Spectroscopy

Understanding the mechanism of how micro-environments affect molecular rotors helps the design and development of molecular sensors. Here, we utilized femtosecond stimulated Raman spectroscopy, helped by quantum chemical calculation, to study the structural dynamics of 9-(2,2-dicyanovinyl) julolidine in cyclohexane, THF, and DMSO solvents. The obtained hydrogen out-of-plane (HOOP) mode and symmetric/anti-symmetric stretching of two nitriles (C≡N) indicate the rotation of the C₇=C₈ double bond and C₄−C₇ single bond in the excited-state which provide two non-radiative decay channels to effectively quench the excited-state population on local excited (LE) state via isomerization and twisted intramolecular charge transfer (TICT). In nonpolar solvent, the excited molecule in the LE state radiatively relaxes to the ground state or performs rotation motions via isomerization and TICT to deactivate fluorescence in the LE state. In the polar solvent, the isomerization plays a role to quench the LE state population; simultaneously, an ultrafast intramolecular charge transfer (ICT) from LE state to emissive ICT state was followed by an TICT between ICT state and dark ICT′ state.