Disentangling Time Scales of Vibrational Cooling, Solvation, and Hydrogen Bond Reorganization Dynamics Using Ultrafast Transient Infrared Spectroscopy of Formylperylene.

Abstract

Unraveling dynamics of solvation and hydrogen bond (H-bond) reorganization between a solute and solvent is crucial to understand the importance of specific and nonspecific interactions in a solution-phase chemical reaction. Ultrafast time-resolved infrared (TRIR) spectroscopy provides direct opportunity to monitor site-specific intermolecular dynamics on a real-time scale by probing vibrational marker bands in the excited state of a solute. Herein, we report the real-time dynamics of vibrational cooling, solvation, and hydrogen bond reorganization of formylperylene (FPe) through TRIR spectroscopy of carbonyl (C═O) stretching mode in nonpolar, polar aprotic, and polar protic solvents. High sensitivity of the C═O stretch frequency (υ̅C═O) to photoinduced intramolecular charge transfer processes induced by specific and nonspecific solvent interactions led us to monitor the dynamics of dipolar solvation, site-specific H-bond formation, and reorganization processes by the TRIR method. In nonpolar cyclohexane, the υ̅C═O stretch band appears at 1610 cm-1 and exhibits negligible spectral shift over several tens of picoseconds. In acetonitrile, the υ̅C═O peak shifts to 1594 cm-1 and exhibits a further temporal red shift of about 5 cm-1 with a characteristic solvation time scale of acetonitrile (τ ∼ 0.5 ps). In methanol, υ̅C═O exhibits two bands corresponding to free and H-bonded FPe in early time scale. The free FPe population converts to the hydrogen-bonded population with a lifetime of about 10 ps. Vibrational cooling (τvc ∼ 12-20 ps) in the excited electronic state of FPe could independently be monitored from the temporal dynamics of the ring vibration mode, which is less sensitive to solvation and hydrogen bonding. The present study provides insight into the specific and nonspecific solvation-controlled charge transfer dynamics in aprotic and protic solvents using FPe as a probe.