title: Intermolecular vibrational coherence in bacteriochlorophyll a with clustered polar solvent molecules.
abstract: We show that resonant impulsive excitation of the Qy absorption band of bacteriochlorophyll a (BChl) launches a rapidly damped (gamma < 200 fs) ground-state coherent wave-packet motion that arises from intermolecular modes with clustered solvent molecules. Femtosecond pump-probe, dynamic-absorption signals were obtained at room temperature with BChl solutions in pyridine, acetone, and 1-propanol. The vibrational coherence observed in the 0-800-fs regime is modeled in the time domain by two (or three, in the case of 1-propanol) modulation components with asymmetric, inhomogeneously broadened line shapes and frequencies in the 100-200-cm(-1) range. The mean frequency of the vibrational coherence exhibits at least a quadratic dependence on the dipole moment of the solvent molecules and a y-intercept in the 100-cm(-1) regime. This trend is modeled by an expression for the natural frequency of a "6-12" potential composed of attractive terms from van der Waals forces and a repulsive term from the exchange (Pauli exclusion) force. The model suggests that comparable contributions to the potential are provided by the dipole-dipole and London dispersion interactions. These results support the hypothesis that the low-frequency vibrational modes in the 100-cm(-1) regime that are coupled to the light-driven charge-separation reactions in the reaction center from purple bacteria are derived from intermolecular vibrational modes between the chromophores and the surrounding protein medium.
authors.affiliation: Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, USA.
authors.forename: Katherine R
authors.initials: KR
authors.lastname: Shelly
authors.forename: Elizabeth C
authors.initials: EC
authors.lastname: Golovich
authors.forename: Warren F
authors.initials: WF
authors.lastname: Beck
mesh_terms: D000433:1-Propanol / Q000737:chemistry; D000042:Absorption; D000096:Acetone / Q000737:chemistry; D025541:Bacteriochlorophyll A / Q000737:chemistry*; D001703:Biophysics / Q000379:methods*; D002627:Chemistry, Physical / Q000379:methods*; D015233:Models, Statistical; D008968:Molecular Conformation; D009991:Oscillometry; D011725:Pyridines / Q000737:chemistry; D012997:Solvents / Q000737:chemistry; D013053:Spectrophotometry; D013696:Temperature; D013997:Time Factors
chemical_list: D025541:Bacteriochlorophyll A; D011725:Pyridines; D012997:Solvents; D000096:Acetone; D000433:1-Propanol; C023666:pyridine