Phosphorescence or Delayed Fluorescence? Fate of Excitons in Core-Substituted Naphthalimide-Derived Emitters and Ratiometric Optical Oxygen Sensors

Competing rates of phosphorescence and thermally activated delayed fluorescence (TADF) need essential balance to yield efficient molecular emitters with color purity falling in either of the categories mentioned above. The large spin–orbit coupling (SOC) matrix element and the small singlet–triplet energy offset (ΔES–T) facilitate the reverse intersystem crossing (RISC) and efficient TADF. In contrast, the SOC constant and the radiative rate of triplets determine the efficiency of phosphorescence. Herein, we rationalize the above-mentioned radiative mechanisms in perspective of the chemical structure for a pair of naphthalimide derivatives (Br-NMI and NMI-Cz). With experimental evidence and computational support, we prove that the natural selection of these pathways is determined by the existence and nature of the higher-lying triplet states (Tn, n > 1) of suitable energy. Eventually, a highly radiative T1 state and a poor RISC rate led to room-temperature phosphorescence in Br-NMI. In contrast, the higher-lying 3LE facilitated fast RISC, leading to efficient TADF in NMI-Cz. Additionally, the Br-NMI-doped polymethyl methacrylate (PMMA) film (10 wt %), with enhanced phosphorescence quantum yield (ϕP) compared to its monomeric and pristine films, is also described. C═O···Br noncovalent interactions between phosphors and the polymer matrix were responsible for the enhancement in ϕP and were proven by IR spectroscopic techniques. Interestingly, ϕP of the doped films was insensitive to the sample temperature (77–300 K) but highly susceptible to the sample O2 partial pressure. Utilizing these characteristics, we developed a self-referenced optical oxygen sensor with high sensitivity (KSV = 8.53 kPa–1) and reversibility.