Empirical Formula of Exciton Coherent Domain in Oligomers and Application to LH2

Intramolecular and intermolecular energy transfers in donor-acceptor linear porphyrin arrays

We present highly time-resolved spontaneous fluorescence spectra of a porphyrin array system that consists of an energy donor and an acceptor linked by a phenyl group. The donors are meso-meso directly linked zinc(II) porphyrin arrays and the acceptor is a zinc(II) 5,15-di(phenylethynyl)porphyrin. The spectra over the entire Q (S1) emission band following the excitation of the donor B (S2) state have been measured directly without the conventional spectral reconstruction method. The time-resolved fluorescence spectra revealed detailed energy relaxation processes within the donor and subsequent energy transfer to the acceptor. The observed energy transfer rates to the acceptor are consistent with the Forster energy transfer rates calculated on the assumption that the energy is localized in the Q state of each porphyrin unit of the donor prior to the energy transfer. The passage of the energy deposited initially on one porphyrin unit of the donor to the acceptor illustrates a sequence of energy delocalization and localization processes before it finally reaches the acceptor.

Giant Porphyrin Wheels with Large Electronic Coupling as Models of Light-Harvesting Photosynthetic Antenna

Starting from a 1,3-phenylene-linked diporphyrin zinc(II) complex 2ZA, repeated stepwise Ag(I)-promoted coupling reactions provided linear oligomers 4ZA, 6ZA, 8ZA, and 12ZA. The intramolecular cyclization reaction of 12ZA under dilute conditions (1x10(-6) M) gave porphyrin ring C12ZA with a diameter of approximately 35 A in 60% yield. This synthetic strategy has been applied to a 1,3-phenylene-linked tetraporphyrin 4ZB to provide 8ZB, 12ZB, 16ZB, 24ZB, and 32ZB. The intramolecular coupling reaction of 24ZB gave a larger 24-mer porphyrin ring C24ZB with a diameter of approximately 70 A in 34% yield. These two large porphyrin rings were characterized by means of 1H NMR spectroscopy, matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy, UV-visible spectroscopy, gel permeation chromatography (GPC) analysis, and scanning tunneling microscopy (STM) techniques. The STM images of C12ZA reveal largely circular structures, whereas those of C24ZB exhibit mostly ellipsoidal shapes, indicating more conformational flexibility of C24ZB. Similar to the case of C12ZA, the efficient excitation energy transfer along the ring has been confirmed for C24ZB by using the time-correlated single-photon counting (TCSPC) and picosecond transient absorption anisotropy (TAA) measurements, and occurs with a rate of (35 ps)(-1) for energy hops between neighboring tetraporphyrin subunits. Collectively, the present work provides an important step for the construction of large cyclic-arranged porphyrin arrays with ample electronic interactions as a model of light-harvesting antenna.

Effect of Conformational Heterogeneity on Excitation Energy Transfer Efficiency in Directly meso−meso Linked Zn(II) Porphyrin Arrays

We have investigated the overall excitation energy relaxation dynamics in linear porphyrin arrays as well as the energy transport phenomena by attaching an energy acceptor to one end of a linear porphyrin array by using steady state and time-resolved spectroscopic measurements. We have revealed that the solvation dynamics as well as the conformational dynamics contributes significantly to the energy relaxation processes of linear porphyrin arrays. Consequently, long porphyrin arrays no longer serve as good energy transmission elements in donor-acceptor linked systems due to conformational heterogeneities which provide the non-radiative deactivation channels as energy quenchers.