Ultrafast and Coherent Transport of Excitons in Sequential Molecular Arrangements

I. Yamazaki, S. Akimoto, T. Yamazaki and A. Osuka+

Graduate School of Molecular Chemistry, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan, e-mail: yamiw@eng.hokudai.ac.jp, FAX: 81-11-709-2037
+Graduate School of Chemistry, Kyoto University, Sakyo-ku, Kyoto 606-8224, Japan

    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 initially
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.

Excitation delocalization in circular arrangement of ZnP dimer and trimer

    Following 80-fs excitation at Soret band (420 nm) or Q band (580 nm) of ZnP dimer and trimer, the fluorescence emissions decay with 430 fs and 230 fs lifetimes, respectively.  The rapid decays correspond to disappearance of monomer-like emission in the time-resolved spectra followed by red-shifted and broaden spectra after 1 ps.  These ultrafast processes are assigned as due to excitation transfer among monomers, or in other words, delocalization of excitation localized initially at a ZnP ring.  During the processes, the exciton is trapped at slightly lower sites of different conformations of the dimer and trimer.

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. .

Sequential and cooperative excitation energy transfer in LB multilayers [3-4]

    The sequential excitation transfer was studied with Langmuir-Blodgett multilayer films which were prepared by successive deposition of monolayers containing different dye molecules in each layer.  Four kinds of cyanine dyes in respective layers, as denoted by N1, N2 --, are stacked in the order of decreasing S1 energy levels (Fig. 2).  The distance between adjacent layer is 2.5 nm while the Forster critical transfer distance are in 5.0~7.0 nm.
    From analyses of the ps time-resolved fluorescence spectra taken at 10 K and 298 K, it is found that the time courses of constituent emission of N1, N2 -- are superposition of the fast (less than 10 ps) and slow (longer than 200 ps) transfer processes.  On lowering temperature down to 10 K, the fast kinetic components remain unchanged, while the slow kinetic component becomes much slower.  Striking features are that the fast fluorescence decay and anisotropy decay of N1 become much faster in going from two (N1-N2) to four (N1-N2-N3-N4) layer systems.  These observation can be accounted for by assuming participation of a reversible transfer process in the medium interaction case.  The fast excitation transfer should compete with the vibrational relaxation (1~10 ps) within the vibrational manifolds in S1 initially populated, and involves a transfer from vibrationally unrelaxed levels.

[1] 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.
[2] 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.
[3] I. Yamazaki, N. Ohta, Pure and Appl. Chem., 67, 209 (1995).
[4] I. Yamazaki, S. Okazaki, T. Minami, N. Ohta, Appl. Opt., 33, 7561 (1994).
[5] I. Yamazaki, M. Yamaguchi, N. Okada, S. Akimoto, T. Yamazaki and N. Ohta,  J. Luminesc. 72-74, 71 (1997).