Carbazole donor-carbazole linker-based compounds: preparation, photophysical properties, and formation of fluorescent nanoparticles.

A new class of highly soluble and stable compounds (1-4) has been synthesized and characterized. Compounds 1, 2, and 3 have 3,6-disubstituted carbazole as electron-donor-linked through the 2,7-positions of N-substituted carbazole with variably substituted phenyl acetylenes as electron acceptors. Compound 4 has two identical donors linked through the 2,7-position of N-substituted carbazole. These compounds absorb from UV to visible region and emit intensely from blue to green. The effect on the photophysical properties of these compounds by changing acceptors while keeping the donor and linker constant has been studied. A study of solvent effects on their photophysical properties has shown that an increase in polarity of the solvent causes a reduction of fluorescence quantum yields. With a decrease in temperature, these compounds showed increased emission intensity accompanied by a red shift of emission. Edge excitation red shift showed as much as 128 nm for compound 2 in isopropanol. Solid-state fluorescence quantum yields vary from 0.27 to 0.68 for these compounds. The fluorescence lifetimes of these compounds are solvent dependent. Compound 2 showed remarkable change in emission with concentration. Compounds 2 and 3 form highly stable fluorescent organic nanoparticles (FONs) in tetrahydrofuran/water mixtures. The general and specific solvent effect on the emission properties of these compounds are investigated by Lipert-Mataga plots. The potentiality of these compounds for use in dye-sensitized solar cells and in organic light-emitting diodes is under study in our laboratories.