Synthesis, characterization and optical properties of aryl and diaryl substituted phenanthroimidazoles

This article presents a facile synthesis of novel class of blue-green fluorescent phenanthroimidazoles and report on their optical, electrochemical, and thermal properties. Detailed photophysical and quantum chemical studies with a series of hydroxy-, methoxy-, nitro-, amino- and tosylaminophenyl substituted derivatives of 2-phenyl-1H-phenanthro[9,10-d]imidazole and 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole have been performed to elucidate the origin of the surprisingly divergent emission shifts. Out of these, three dyes (HPhI, HPPhI, and TsPPhI) undergo excited state intramolecular proton transfer (ESIPT) reaction leading a large Stokes’ shifted fluorescence emission from the phototautomer. The results of quantum chemical investigations not only confirmed the intramolecular charge transfer characteristics of the ESIPT tautomers but also provided a rational for the observed high fluorescence quantum efficiency in the solid state. The high photoluminescence quantum yield (ΦPL ∼ 39–68%) in the solid state is ascribed to twisted chromophores due to phenyl substituents at 1,2-position of the phenanthroimidazole ring which restricted intramolecular motion, leading to an optically allowed lowest optical transition without self quenching.