Excited-state vibrational coherence in methanol solution of Zn(II) tetrakis(N-methylpyridyl)porphyrin: charge-dependent intermolecular mode frequencies and implications for electron-transfer dynamics in photosynthetic reaction centers.
The nature of the intermolecular vibrational modes between the redox-active chromophores and the protein medium in the photosynthetic reaction center is central to an understanding of the structural origin of the quantum efficiency of the light-driven charge-separation reactions that result in storage of solar energy. In recent work on this issue, we have characterized the low-frequency vibrational coherence of Zn(II) meso-tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP) and compared it to that from bacteriochlorophyll a in polar solution and in the small light-harvesting subunits B820 and B777. The charge-transfer character of ZnTMPyP's π* excited states afford us the opportunity to characterize how the intermolecular vibrational modes and potential with the surrounding medium are affected by the charge on the porphyrin macrocycle. The excited-state vibrational coherence observed with Q-band (S(1) state) excitation at 625 nm of ZnTMPyP in methanol solution contains dominant contributions from a pair of rapidly damped (effective damping time γ < 400 fs) components that are assigned to the hindered translational and librational porphyrin-solvent intermolecular modes. The 256 cm(-1) mean frequency of the intermolecular modes is significantly higher than that observed previously in the ground state, 79 cm(-1), with Soret-band excitation at 420 nm [Dillman et al., J. Phys. Chem. B. 2009, 113, 6127-6139]. The increased mode frequency arises from the activation of the ion-dipole and ion-induced-dipole terms in the intermolecular potential. In the ground state, the π-electron density of ZnTMPyP is mostly confined to the region of the porphyrin macrocycle. In the excited state, the π-electron density is extensively delocalized from the porphyrin out to two of the peripheral N-methylpyridyl rings, each of which carries a single formal charge. The charge-dependent terms contribute to a significant stabilization of the equilibrium geometry of the porphyrin-solvent complex in the excited state. In the photosynthetic reaction center, these terms will play an important role in trapping the charged products of the forward, charge-separation reactions, and the location of the bacteriopheophytin acceptor in a nonpolar region of the structure enhances the rate of the secondary charge-separation reaction.
Dillman
Kevin L
KL
Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, USA.
Beck
Warren F
WF
D004579:Electron Transport; D008665:Metalloporphyrins / Q000737:chemistry*; D000432:Methanol / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry; D014732:Vibration
D008665:Metalloporphyrins; D045322:Photosynthetic Reaction Center Complex Proteins; D012997:Solvents; C060088:zinc tetra(4-N-methylpyridyl)porphine; D000432:Methanol