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

Abstract

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 x 25 mm x 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.

Molecular crystals based on 9,10-distyrylanthracene derivatives with high solid state fluorescence efficiency and uniaxial orientation induced by supramolecular interactions

“…The torsional angles of the double bonds   (2,3,4,5) is 179.9° in the CNDSA crystal and 179.3° in the t-BUDSA (Table 2), larger than that of DSA (172.2°), resulting in relatively less twisted molecular structure. The torsional angles   (1,2,3,4) of CNDSA and t-BUDSA are 54.4° and 60.7°, larger than that of DSA (4.6°) and   (3,4,5,6) between the double bonds and the adjacent phenyl ring are 20.6° for CNDSA and 4.6° for t-BUDSA, smaller than that of DSA (75.0°) due to the effect of internal steric hindrance between the vinylene moieties and the substituents moieties (CN and t-BU). The analyses demonstrate that the two compounds have nonplanar conformation in their crystals.…”

“…So they are prospectively applied in optical waveguides [5], solid-state laser [6], and many other high-tech areas [7,8]. As usual, molecular packing models can affect the fluorescence properties of organic crystals, as well as their practical applications.…”

Low-frequency Raman spectrum of crystalline 2-(2�-hydroxyphenyl)benzoxazole and density-functional based tight-binding phonon calculations

“…when tailoring laser dyes and electroluminescent materials [1]. As an example, a photoreactive compound based on excited-state intramolecular proton transfer in a crystalline state has been probed as a possible lasing material [2]. Low-frequency intramolecular vibrations have been considered to play a prominent role in the proton transfer process, changing the distance between a proton donor and an acceptor [3].…”

Optical Properties of Molecular Crystals: The Effect of Molecular Packing and Polymorphism

“…Optical gain can also result in amplified spontaneous emission (ASE), which is characterized by a spectrally narrowed emission upon increasing the excitation fluence above a threshold. Gainnarrowing has been demonstrated in a number of molecular crystalline materials, such as oligothiophenes, oligo(p-phenylene), oligo(p-phenylene vinylene), trans-1,4-distyrylbenzene, cyano derivatives, fluorene/phenylene co-oligomers, hydroxy-substituted tetraphenylimidazole, and 1,1,4,4-tetraphenyl-1,3-butadiene (Fichou et al 1997(Fichou et al , 1999Hibino et al 2002;Horowitz et al 1999;Ichikawa et al 2003Ichikawa et al , 2005Losio et al 2007;Nagawa et al 2002;Park et al 2005;Polo et al 2008;Tavazzi et al 2006cTavazzi et al , 2007Tavazzi et al , 2008Tavazzi et al , 2010bXie et al 2007;Zhu et al 2003). Relatively low thresholds have been recently reported for p-sexiphenyl samples.…”