Boosting the Phosphorescence Efficiency in Doped Organic Crystals: Critical Role of Hydrogen Bonding

Host–guest doping systems with phthalimides (BI) and N-methylphthalimide (NMeBI) as the host and 1,8-naphthalimide (NI) and 4-bromo-1,8-naphthalimide (4BrNI) as the guest have been developed. The 0.2% NI/BI (molar ratio) with a strong C=O···H–N hydrogen bond exhibited a phosphorescence quantum efficiency (29.2%) higher than that of NI/NMeBI with a weak C=O···H–C hydrogen bond (10.1%). A similar trend was observed in the 4BrNI guest system. A remarkable phosphorescent efficiency of 42.1% was achieved in a 0.5% 4BrNI/BI composite, which represents the highest value in NI-based phosphors. This research indicates stronger hydrogen bonding may have a greater contribution in boosting the phosphorescence efficiency.