Vibrational Coherence in Polar Solutions of Zn~(II) tetrakis(N-methyIpyridyl)porphyrin with Soret-Band Excitation: Rapidly Damped Intermolecular Modes with Clustered Solvent Molecules and Slowly Damped Intramolecular Modes from the Porphyrin Macrocyc

摘要

Ground-state coherent wavepacket motions arising from intermolecular modes with clustered, first-shell solvent molecules were observed using the femtosecond dynamic absorption technique in polar solutions of Zn~(II) meso-tetrakis(N-mefhylpyridyl)porphyrin (ZnTMPyP) with excitation in the Soret absorption band. As was observed previously in bacteriochlorophyll a solution, the pump-probe transients in ZnTMPyP solutions are weakly modulated by slowly damped (effective damping time γ > 1 ps) features that are assigned to intramolecular modes, the skeletal normal modes of vibration of the porphyrin. The 40 cm~(-1) and 215 cm~(-1) modes from the metal-doming and metal-solvent-ligand modes, respectively, are members of this set of modulation components. A slowly damped 2-4 cm~(-1) component is assigned to the internal rotation of the N-mefhylpyridyl rings with respect to the porphyrin macrocycle; this mode obtains strong resonance Raman intensity enhancement from an extensive delocalization of π-electron density from the porphyrin in the ground state onto the rings in the π~* excited states. The dominant features observed in the pump-probe transients are a pair of rapidly damped (γ < 250 fs) modulation components arising from intermolecular modes with solvent molecules. This structural assignment is supported by an isotope-dependent shift of the average mode frequencies in methanol and perdeuterated methanol. The solvent dependence of the mean intermolecular mode frequency is consistent with a van der Waals intermolecular potential that has significant contributions only from the London dispersion and induction interactions; ion-dipole or ion-induced-dipole terms do not make large contributions because the π-electron density is not extensively delocalized onto the N-methylpyridyl rings. The modulation depth associated with the intermolecular modes exhibits a marked dependence on the electronic structure of the solvent that is probably related to the degree of covalency; the strongest modulations are observed in acetonitrile and dimethylsulfoxide. The results strongly support a structural assignment of the low-frequency modes that are coupled to the primary and secondary electron-transfer reactions in photosynthetic reaction centers to intermolecular modes between the redox-active chromophores and first-solvation shell groups from the surrounding protein, and an important additional function of the intermolecular modes in the stabilization of charged intermediates is suggested.