Langmuir-Blodgett (LB) assemblies of a novel electron donor, 16-ferrocenylhexadecanoic acid [q5-(CsHs)Fe(y5-CsH4))(CHz)lsCOOH], have been prepared on a solid substrate. The structures of these LB assemblies have been characterized by polarized infrared (F'T-IR) spectroscopy, ellipsometry, small-angle X-ray scattering, and UV-vis spectroscopy. The polarized FT-IR experimental results show that the LB assemblies are densely packed into microcrystalline structures with a triclinic or pseudohexagonal subcell packing form. A detailed analysis of the FT-IR dichroism suggests that the axis of the all-trans hydrocarbon chains of the functionalized ferrocene fatty acid is tilted approximately 27" from the surface normal and the cyclopentadienyl ring of the ferrocene chromophore in the LB assembly has a tilt angle of 53" with respect to the substrate surface normal. It is also shown that these terminal ferrocene chromophores have a preferential orientation perpendicular to the film transfer direction. This is presumably due to the formation of aggregates between ferrocene chromophores in the LB assembly; aggregate formation is detectable from shifts of the UV-vis absorption in the films compared with the spectrum in solution.

monomer-like, but a moderate number of dimers and trimers with the J-aggregate configuration have formed from statistical associations. In addition, a few defect states, possibly having a sandwich dimer configuration, have formed. Energy transfer from the monomer-like species to the dimers, trimers, and defect states causes the bulk of the fluorescence to come from these states. The defect-state emission dominates at 90 K, where it has a reasonably high quantum yield, while the room-temperature emission comes mainly from the small aggregates, since the defect state relaxes nonradiatively at this temperature. At 0.50 mole fraction, most the PIC molecules have formed small J-aggregates and extensive energy migration among these aggregates causes significant energy transfer to the defect states, resulting in a low yield of J-aggregate emission at room temperature. At 90 K, emission occurs from both the J-aggregates and the defect states, but an increased rate of nonradiative relaxation for the defect states formed at this concentration causes the overall yield of fluorescence to be lower than for the 0.125 mole fraction sample. For the undiluted PIC J-aggregates, the increased range of energy migration results in further quenching of the room-temperature J-aggregate fluorescence by the nonradiative defect states. At 90 K, the quenching persists and some emission from the defect states can be seen. The overall yield of fluorescence at this temperature remains quite low, presumably because the defect states associated with the undiluted J-aggregate have significant nonradiative relaxation at this temperature. One focus of our future work will be development of a better understanding of the nature of the defect states and a more quantitative model for their effect on the excitation dynamics of PIC J-aggregates.While energy transfer from the J-aggregates to the defect state is the dominant factor controlling the size-dependent excitation dynamics, a weak superradiant enhancement of the J-aggregate radiative decay is also present for the samples containing the larger aggregates. The values obtained for this enhancement are much smaller than those observed for PIC J-aggregates in solution but can be rationalized by postulating strong coupling of the J-aggregate exciton to a low-frequency phonon. While the much larger disorder present in the surface-adsorbed J-aggregates may also be a factor in limiting the superradiance, disorder alone is not sufficient to explain the differences in enhancement observed between these aggregates and the solution aggregates.Both the energy transfer to the defect states and the enhanced radiative rate of the large J-aggregates compete with the desired process of electron transfer from the aggregate excited state to the AgBr conduction band. Consequently, the relative quantum efficiency of spectral sensitization by the dye decreases as the aggregate size increases. However, this decrease is not as large as would be expected because the electron-transfer rate constant from the aggregate excite...

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