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.

Substituent effects on the electronic structure of a novel class of blue-green emitting aryl and diaryl phenanthroimidazoles: The tuning of excited state intramolecular proton transfer reactions and optical properties of the aryl and diaryl substituted phenanthroimidazoles comprising an arylene unit having adjacent substituents, said substituents being oriented such as to affect the electronic structure of the compound sufficiently to cause a drastic change in the photoluminescence and/or electroluminescence of the compound. Photoluminescence (PL) images of PMMA film doped with 5 wt% dye on the wedged quartz plate and its absolute photoluminescence quantum yield (ΦPL) in the thin film (I) and in the solution (II) (aCHCl3 and b1-propanol). Structures of photoreactive hydrogen-bonded phenanthroimidazole species in the excited state intramolecular proton transfer processes, ESIPT and intramolecular charge transfer processes, ICT (III).Figure optionsDownload as PowerPoint slideHighlights
► Synthesized a novel class of blue-emitting phenanthroimidazole dyes in high yields and report on their optical, electrochemical, and thermal stabilities.
► Studied electronic structures and substituent effects on tuning the excited state intramolecular proton transfer reactions and optical properties of the aryl and diaryl substituted phenanthroimidazoles.
► Detailed quantum chemical calculations confirmed the intramolecular charge transfer characteristics of the ESIPT tautomers, also provided a rational for the observed emission shifts based on the differential change in the HOMO and LUMO energies.
► The high photoluminescence quantum yield (ΦPL ∼ 39–68%) in the solid state is ascribed to rotated chromophores due to phenyl substituents at 1,2-position of the imidazole ring which restricted intramolecular motion, leading to an optically allowed lowest optical transition.