Imidazole-Based Excited-State Intramolecular Proton-Transfer Materials: Synthesis and Amplified Spontaneous Emission from a Large Single Crystal

We have synthesized a novel class of imidazole-based excited-state intramolecular proton-transfer
(ESIPT) materials, i.e., hydroxy-substituted tetraphenylimidazole (HPI) and its derivative HPI-Ac, which
formed large single crystals exhibiting intense blue fluorescence and amplified spontaneous emission (ASE).
Transparent, clear, and well-defined fluorescent single crystals of HPI-Ac as large as 20 mm × 25 mm ×
5 mm were easily grown from its dilute solution. From the X-ray crystallographic analysis and semiempirical
molecular orbital calculation, it was deduced that the four phenyl groups substituted into the imidazole ring
of HPI and HPI-Ac allowed the crystals free from concentration quenching of fluorescence by limiting the
excessive tight-stacking responsible for intermolecular vibrational coupling and relevant nonradiative
relaxation. Fluorescence spectral narrowing and efficient ASE were observed in the HPI-Ac single crystal
even at low excitation levels attributed to the intrinsic four-level ESIPT photocycle.