Morphological Study on the Gelation-Induced Phosphorescence of All-Organic Phosphors Based on Substituted Benzil

Abstract We report gelation-induced phosphorescence at room temperature based on 4,4’-disubstituted benzils with alkyloxy and ester groups. The title compounds include five 4,4’-dialkyloxybenzils with varying alkyl chain lengths (from octyloxy to dodecyloxy) and two 4,4’-diesterbenzil (nonylester and phenyl ester). All of the title compounds showed excellent gelation properties in hydrocarbon, alcohol, and acetonitrile solvents. The room-temperature phosphorescence was turned on upon gelation, whereas no emission was observed in solution. The investigation of the photophysical properties of the dried gels (xerogels) revealed that the length of the alkyloxy side group significantly influenced the phosphorescence lifetime (τ) and quantum yield (Φp). BZL-OC8, -OC9, and -OC10 showed better phosphorescence performance than their longer chain counterparts (BZL-OC11 and -OC12). The maximum τ of 1.28 ms and Φp of 43% were achieved for 4,4’-dinonyloxybenzil (BZL-OC9). Powder X-ray diffraction patterns suggested that BZL-OC11 and -OC12 may adopt different molecular packing modes that may not facilitate the restriction of intramolecular motion as efficiently as in the shorter-chain molecules. The ester-substituted benzils showed inferior phosphorescence to BZL-OC9. Between the two ester-substituted benzils, nonylester (BZL-EstC10) outperformed phenylester (BZL-EstPh). Interestingly, the title compounds showed strong gelation solvent-dependent phosphorescence. This indicates that the appropriate choice of gelation solvent is crucial for efficient fiber growth. For BZL-OC8 and -OC9, ethanol and decane gels were superior to the acetonitrile gel. In the case of BZL-EstC10, ethanol was a better solvent than hexane. 1,1,1-Trichloroethane was found to be the best gelation solvent for phosphorescence in BZL-EstPh. Polarized optical microscopy and scanning electron microscopy confirmed the presence of fibers in the xerogels. In addition, the dependence of phosphorescence on the alkyloxy length and gelation solvent was strongly correlated with the fiber morphology. In general, longer and thicker fibers were associated with better phosphorescence performance. This study successfully proved that properly substituted benzils can turn on room-temperature phosphorescence via gelation, which could be used as an application-friendly processing method.