Spectroscopic study of nonamphiphilic 2,2'-p-phenylenebis(5-phenyloxazol) (POPOP) assembled in supramolecular Langmuir-Blodgett films

This paper reports the incorporation of a nonamphiphilic blue laser dye 2,2'-p-phenylenebis(5-phenyloxazol), abbreviated as POPOP, in Langmuir-Blodgett (LB) films mixed with stearic acid (SA) and their spectroscopic properties. The surface pressure versus area per molecule isotherms of POPOP mixed with SA at different molar ratios reveal a plateau-like region that suggests reorientation of the POPOP molecules from lying flat at the air-water interface to an almost vertical alignment. Miscibility studies indicate a repulsive type of interaction between the components in the mixture, resulting in the formation of microcrystallites in the LB films that is confirmed by spectroscopic studies. Perhaps the most interesting feature of this study is the appearance of a new band at about 400 nm in absorption spectra of the mixed LB film of POPOP and SA that is assigned to a low-lying hidden (1)L(b) state. The appearance of the same band in a mixture of ethanol and water with the volume fraction of water in the mixture exceeding 76% confirms that the manifestation of this band is aggregation induced. Fluorescence emission studies of POPOP in solution reveal a structured emission that originates from the (1)L(a) state, while the emission studies from the LB films exhibit two bands, one at 420 nm originating from the low-lying (1)L(b) State of the monomer and the other at 460 nm corresponding to a (1)L(b) dimer. Excitation and polarized anisotropy measurements confirm these findings and indicate that the above observations may be attributed to a deformed geometry of the POPOP molecules in the LB films.