Investigations into the charge transfer mechanism of 4-(dimethylamino)benzonitrile using ultrafast spectroscopy

Abstract

Thesis (Ph. D.)--University of Rochester. Dept. of Chemistry, 2012. Chapter 2 was co-authored with Randy Mehlenbacher and David W. McCamant.Femtosecond stimulated Raman spectroscopy (FSRS), in conjunction with femtosecond transient absorption (fsTA) and density functional theory (DFT) calculations, have been used to investigate the photoinduced charge transfer (CT) dynamics and structural evolution in 4-(dimethylamino)benzonitrile (DMABN). Excited state vibrational modes corresponding to the ring breathing (764 cm-1), CH in-plane bending (1168-1174 cm-1), quinoidal C=C stretching (1575-1582 cm-1) and nitrile stretching (2096 cm-1) modes were observed in the CT state for time delays between 0 and 30-40 ps in various solvents. The CT reaction dependence on solvent polarity was also investigated through the use of a range of polar protic and aprotic solvents. The role of anharmonic coupling between energy receiving modes and observed spectral shifts in the CT state was investigated through density functional theory (DFT) calculations conducted on the isotopically labeled aminobenzonitrile analog (ABN15H2). This revealed a collection of vibrational modes potentially coupled to the CT reaction coordinate that could account for the experimentally observed spectral shifts upon formation of the equilibrated CT state. These results were then placed into the context of the structure of DMABN as it proceeds through the CT reaction pathway and the validity of the prevailing models for this process is discussed