Photophysical processes such as excitation energy
transfer and electron transfer are expected to occur very fast in organized
molecular systems in which reactant molecules are arranged with close proximity
and optimal orientation, and they can be coupled to an adjacent molecule
with relatively strong molecular interaction. In an extreme of such
strongly interconnected molecular systems, the photoinduced reaction can
compete with or exceed the vibrational relaxation from vibrational levels
photoexcited. Moreover, if the reaction system consists of several molecules which are arranged linearly along the reaction pathway, a sequential reaction can occur among non-equilibrium excited states of reactant molecules. In 1960's, Forster [1, 2] formulated the excitation energy transfer taking place under relatively strong dipole-dipole interaction denoted as the medium interaction case. Recently one can see such quantum coherent processes of sequential exciton transfer in natural photosynthetic reaction systems in which an exciton propagates as a quantum wave packet through a molecular channel.
We have investigated ultrafast excitation migration and relaxation in (1) circular arrange-ment of zinc porphyrin (ZnP) dimer and trimer (Fig. 1) and (2) sequentially stacked LB multilayer films (Fig. 2), with a fs fluorescence up-conversion method and a ps time-correlated photon-counting method. The time-resolved fluorescence spectra revealed non-equilibrium processes of excitation delocalization and transport in femtosecond and picosecond time scales.
Figure 1. Zinc porphyrin (ZnP) monomer, dimer and trimer in circular arrangement.
The center-to-center distance between ZnP rings is 1.1 nm The angle between molecular planes of a ZnP ring and a phenyl ring is 7.0 nm
Figure 2. Schematic illustration of sequential excitation energy transport in LB multilayers and measurement by time-resolved fluorescence spectra. The distance between layers containing different dye molecules is 2.5 nm. .
 Th. Forster, in Modern Quantum Chemistry, ed.by O. Sinanoglu, Part III, Action of Light and Organic Crystals, Academic Press, New York (1965), pp. 93-137.
 Th. Forster, in Comparative Effects of Radiation, eds.by M. Burton, J. S. Kirby-Smith and J. L. Magee, Wiley, New York (1960), pp. 300-341.
 I. Yamazaki, N. Ohta, Pure and Appl. Chem., 67, 209 (1995).
 I. Yamazaki, S. Okazaki, T. Minami, N. Ohta, Appl. Opt., 33, 7561 (1994).
 I. Yamazaki, M. Yamaguchi, N. Okada, S. Akimoto, T. Yamazaki and N. Ohta, J. Luminesc. 72-74, 71 (1997).