Controlling the Triplet Dynamics for Efficient Room Temperature Phosphorescence by Molecular Engineering

Abstract

Fine-tuning room-temperature phosphorescence (RTP) through careful molecular design has been an active research area over the past few decades. Here, we report a series of naphthalene–borane (NB) conjugates that differ in the functional group (Br, OMe, OH, NH2, COOH) attached to the naphthalene unit. Fine-tuning triplet energy levels and emission kinetics was achieved in the condensed state through this functional group engineering. Detailed PL studies in the crystalline state, revaled that NB-Br and NB-OMe, exhibited RTP, with NB-Br showing PL bands at 530 and 572 nm (τₐᵥ = 552.6 μs), which are highly resistant to oxygen quenching, reflecting heavy-atom-induced triplet stabilization. Meanwhile, NB-OMe displayed a red-shifted band at 590 nm with a shorter lifetime (τₐᵥ = 160.5 μs), indicating the absence of heavy atom assistance. All derivatives exhibit long-lived RTP in PMMA matrices (1 wt%), highlighting the crucial role of host-induced rigidification in suppressing nonradiative decay pathways. In PMMA, the NB-Br compound emits at 570 nm with a lifetime of 4.53 ms, while NB-OMe shows emissions at 517 and 550 nm, with an additional shoulder at 603 nm (550 nm, τₐᵥ = 716 ms). The NB-OH derivative displays emissions at 560 and 524 nm (560 nm, τₐᵥ = 396.3 ms), whereas NB-NH2 and NB-COOH exhibit bands at 550, 585 nm (550 nm, τₐᵥ = 186 ms) and 550, 590 nm (550 nm, τₐᵥ = 196.8 ms), respectively. Third-order nonlinear optical coefficients at 800 nm were also measured using femtosecond laser pulses. These findings reveal the synergistic effects of substituents and polymer rigidification, establishing naphthalene–boron frameworks as a versatile platform for purely organic phosphorescent emitters in optoelectronic, photonic, and security applications.