Picosecond Fluorescence Spectroscopy on Molecular Association in Langmuir-Blodgett Films

The Langmuir-Blodgett (LB) film is a highly-ordered molecular assembly of multilayered architechture which is prepared by transferring a compact monolayer spread on a water surface onto a quartz substrate. In this paper, we report new aspects of molecular association of pyrene in LB multilayers studied by means of a synchronously pumped, cavity-dumped dye laser and a single-photon counting apparatus [1]. LB films with different numbers of layers were prepared from stearic acid and small amounts of 16-(1-pyrenyDhexadecanoic acid (PHA). Dependence of the layered structure and of the concentration of PHA on molecular association was investigated. Figure 1 shows an typical example of time-resolved fluorescence spectra of the LB film. Four kinds of fluorescence bands appear depending on the time region. (1) F 1 band at 0-200 ps with three peaks located at 380, 400 and 421 nm. This band is shifted by 3 nm to longer wavelength relative to F2 band. (2) E 1 band at 100 ps-7.0 ns. A broad band with a peak at 422 nm. (3) E 2 band after 1 ns with a peak at 470 nm, corresponding to the well-known fluorescence of pyrene excimer in solution. (4) F 2 band after 3 ns with peaks at 377, 397 and 421 nm. This band is assigned to the monomer fluorescence of pyrene. The fluorescence excitation spectra are different depending on the monitoring wavelength: the excitation spectrum of the monomer region exhibits peaks at 345 and 329 nm for 1La absorption band of pyrene, while that of the excimer E2 at 347 and 331 nm. This means that the pyrene chromophores in the LB film form an associated complex in the ground state. The fluorescence decay curves for various concentrations of PHA are reasonably analyzed in terms of (1) rapid excimer formation in the dimer sites and (2) energy transfer and trapping in the isolated-monomer site. The dimer fluorescence F1 rapidly decays, and its decay (150 ps) is associated with a rise of the excimer fluorescence E2 indicating that the excimer (E2) forms through the dimer (F1). The excimer fluorescence E1 also shows a rise with much shorter rise time than that of E2. From the spectral difference between E1 and E2, the conformation of the excimer (E1) should be significantly different from that of E2 which is known to be of a sandwich type. Its conformation is of a type of partially overlapping pyrene rings.