RESUMO
4-(Dimethylamino)benzonitrile (DMABN) has been one of the most studied photoinduced charge-transfer (CT) compounds for over 50 years, but due to the complexity of its excited electronic states and the importance of both intramolecular and solvent reorganization, the detailed microscopic mechanism of the CT is still debated. In this work, we have probed the ultrafast intramolecular CT process of DMABN in methanol using broad-band transient absorption spectroscopy from 280 to 620 nm and ultraviolet femtosecond stimulated Raman spectroscopy (FSRS) incorporating a 330 nm Raman pump pulse. Global analysis of the transient absorption kinetics revealed dynamics occurring with three distinct time constants: relaxation from the Franck-Condon L(a) state to the lower locally excited (LE) L(b) state in 0.3 ps, internal conversion in 2-2.4 ps that produces a vibrationally hot CT state, and vibrational relaxation within the CT state occurring in 6 ps. The 330 nm FSRS spectra established the dynamics along three vibrational coordinates: the ring-breathing stretch, ν(ph), at 764 cm(-1) in the CT state; the quinoidal CâC stretch, ν(CC), at 1582 cm(-1) in the CT state; and the nitrile stretch, ν(CN), at 2096 cm(-1) in the CT state. FSRS spectra collected with a 400 nm Raman pump probed the dynamics of the 1174 cm(-1) CH bending vibration, δ(CH). Spectral shifts of each of these modes occur on the 2-20 ps time scale and were analyzed in terms of the vibrational anharmonicity of the CT state, calculated using density functional theory. The frequencies of the δ(CH) and ν(CC) modes upshift with a 6-7 ps time constant, consistent with their off-diagonal anharmonic coupling to other modes that act as receiving modes during the CT process and then cool in 6-7 ps. It was found that the spectral down-shifts of the δ(CH) and ν(CN) modes are inconsistent with vibrational anharmonicity and are instead due to changes in molecular structure and hydrogen bonding that occur as the molecule relaxes within the CT state potential energy surface.
RESUMO
Femtosecond stimulated Raman spectroscopy (FSRS) and femtosecond transient absorption have been used to probe the photoinduced charge transfer (CT) dynamics of 4-(dimethylamino)benzonitrile in methanol and n-hexane. Through a combined analysis of temporal changes in the Raman modes and transient absorption kinetics, a more complete picture of the reaction coordinate of the intramolecular charge transfer process has been established. FSRS spectra of the phenyl CâC stretching mode (Wilson mode 8a) at 1607 cm(-1), which shifts to 1581 cm(-1) in the CT state, and transient absorption measurements ranging from 360 to 700 nm support internal conversion from the locally excited to the charge transfer state in 4-5 ps and then a subsequent vibrational relaxation within the CT state manifold on a 6-8 ps time scale. Dramatic shifting and narrowing of the 1581 cm(-1) quinoidal CâC stretch (ν(8a)) on the â¼7 ps time scale indicates that the quinoidal distortion is an important probe of the CT reaction dynamics. The cause of the spectral shifts is determined by comparing the observed shifts in the vibrational spectrum to anharmonic couplings computed for the benzonitrile radical anion by density functional theory (DFT) and with quantitative theoretical models of the solvent induced vibrational peak shifts. The DFT calculations indicate that the 10 cm(-1) downshift of the CâC stretch is most likely attributable to significant vibrational excitation in nontotally symmetric modes that are strongly anharmonically coupled to the CâC stretch.
