PHOTOINDUCED ELECTRON-TRANSFER AND MOLECULAR-ORIENTATION OF ZINC PORPHYRIN-IMIDE DYADS IN LANGMUIR-BLODGETT MONOLAYER FILMS

Photoinduced electron transfer has been studied for bridged donor/acce ptor compounds incorporated in Langmuir-Blodgett (LB) films of phospho lipid matrix, dioleoylphosphatidylcholine (DOPC). The compounds under investigation are zinc porphyrin (ZnP)-pyromellitimide (D1), ZnP-napht halenetetracarboxamide (D2), and the derivatives of D1 and D2 with lon g alkyl chains in the ZnP ring periphery (D3 and D4, respectively). Th e kinetics of intra- or intermolecular electron transfer was investiga ted by probing fluorescence quenching of photoexcited ZnP with the pi cosecond time-resolved fluorescence measurement. A fast intramolecular electron transfer occurs only in D4-LB film similarly to that in solu tion (tau(f) = 16 ps), but not in D1 and D3. In D2-LB film where D2 mo lecules are distributed as dimer and higher aggregates as well as mono mer, an intermolecular electron transfer (tau(f) = 48 ps) occurs at di mer sites instead of the intramolecular reaction. To examine the spati al conformation of D2 dimer in LB monolayer films, the polarized absor ption spectra were measured, suggesting a slightly slipped head-to-tai l dimerization of two ZnP rings in which ZnP rings are tilted (15 degr ees) to the substrate plane.