"Impurity"-driven Tunable Organic Room Temperature Phosphorescence via Conformational Regulation in Multi Host/Guest Systems

Abstract

The presence of trace amounts of impurities can have unprecedented effects on the luminescence features of organic room temperature phosphorescent (ORTP) materials, requiring conscientious investigation. In this study, we have compared the photoluminescence properties of biphenyl-4-carboxylic acid (BCA) and biphenyl-4,4′-dicarboxylic acid (BDCA), synthesized via two distinct synthetic routes-Friedel-Crafts (FC-BCA or FC-BDCA) and cross-coupling (cc-BCA or cc-BDCA) pathways and observed remarkable orange phosphorescence in FC-BCA or FC-BDCA which were absent in cc-BCA or cc-BDCA. Our investigations identified traces (<0.3 mol%) of diphenylbenzil based impurities, formed as byproducts during Friedel-Crafts acylation of biphenyl, responsible for the RTP activation in FC-BCA or FC-BDCA. Bicomponent solids prepared by deliberately doping traces of DPB into various organic matrices ensued tunable RTP color (green to red) with high quantum yield (26.4 %) and lifetime up to 1.6 ms. Comprehensive experimental investigations substantiated with theoretical studies revealed that photoexcited conformational dynamics of guest DPB are responsible for RTP color variation concertedly involving multiple energy transfer channels, e.g., singlet-to-singlet (SSET), triplet-to-triplet (TTET). It presents a novel trace doping strategy for developing RTP materials with tunable optical features by synergistically controlling the ground and excited state geometries of a single guest molecule, which is rarely reported in the literature. Furthermore, by employing a suitable host matrix, we successfully stabilized a linear conformer of guest DPB in the ground state, which is otherwise unstable, resulted in improved quantum yield. Simultaneously, we report an unusual RTP from commercial BDCA, which we suspect to be caused by the presence of diphenylbenzil-based impurities, reiteratively emphasizing the importance of exercising caution whenever a system exhibits unusual properties.