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

Abstract

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 (ΔE S−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 (T n , n > 1) of suitable energy. Eventually, a highly radiative T 1 state and a poor RISC rate led to roomtemperature phosphorescence in Br-NMI. In contrast, the higher-lying 3 LE 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 (ϕ P ) 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 ϕ P and were proven by IR spectroscopic techniques. Interestingly, ϕ P of the doped films was insensitive to the sample temperature (77−300 K) but highly susceptible to the sample O 2 partial pressure. Utilizing these characteristics, we developed a self-referenced optical oxygen sensor with high sensitivity (K SV = 8.53 kPa −1 ) and reversibility.

Engineered solid-state aggregates in brickwork stacks of n-type organic semiconductors: a way to achieve high electron mobility

“…In contrast, the substituted derivatives EHNDIBr 2 and EHNDIBr 4 displayed non-luminescent behavior in solution, likely due to intersystem crossing and non-radiative triplets under ambient conditions, a phenomenon associated with the presence of heavy atoms like bromine, which can induce spin crossover in the excited state. 32 a , b Further, PL spectra of deaerated toluene solutions were also recorded, revealing intense emission bands at 665 nm and 715 nm for EHNDIBr 2 and EHNDIBr 4 , respectively (Fig. S8 † ).…”

Facile Fabrication and Analysis of Highly Sensitive PtTFPP/Carbon Black/Polystyrene Oxygen-Sensitive Composite Films for Optical Dissolved-Oxygen Sensor

“…An appropriate substrate should exhibit adequate breathability, great photochemical stability, excellent chemical and sensing properties, good processability, and commercial availability. 35 Polymers or copolymers such as polystyrene (PS), 36 polypropylene, 37 and poly(methyl methacrylate) 38 are often used as substrates. PS is a thermoplastic polymer with an amorphous molecular structure; therefore, it does not form a typical crystalline structure in its solid state.…”