Efficient energy transfer in Langmuir–Blodgett films containing J-aggregated antenna cyanine dye

The light-harvesting function of the J-aggregate of the cyanine dye, N,N′-dioctadecyloxacyanine (Cy), in Langmuir–Blodgett (LB) monolayers containing the amphiphilic perylene derivative (Pe) as an energy acceptor was examined by varying the Cy:Pe molar ratio from 10:1 to 2000:1. A characteristic J-band considerably red-shifted from the corresponding monomer absorption band of Cy was intensely observed in the absorption spectra of the mixed Cy LB monolayers with Pe with the molar mixing ratios Cy:Pe=200:1, 500:1, 1000:1, and 2000:1. Although the contents of Pe in the mixed LB monolayers are such small fractions, the emission from excited Pe induced by energy transfer (ET) from photo-excited Cy was observed clearly. The highest sensitized emission intensity of Pe was observed on the mixed LB monolayer with the molar mixing ratio Cy:Pe=50:1 in which all excited energies of Cy were transferred to Pe. In this molar ratio, however, the ET from monomer or dimer Cy to Pe seemed to be dominant, and the antenna effect of Cy in this state is as low as ca. 50. In contrast, the antenna effect of J-aggregated Cy estimated for the mixed LB monolayers with low Pe ratios (Cy:Pe=500:1, 1000:1, and 2000:1) was such that ca. 250 molecules of Cy would harvest light for excitation of one Pe molecule. This can be attributed to the high rate of exciton migration in the J-aggregates, by which ET probability was increased.