External electronic effect enhances room-temperature phosphorescence and photochromism in organophosphine oxide

Introducing both locally excited and charge transfer (CT) featured single-molecule long-lived excited states in energy transfer can optimize room-temperature phosphorescence (RTP) at molecular level, but this requires accurate energetic modulation and sequential populations of these states. Herein, we utilize intermolecular interactions between donors (D) and/or acceptors (A), namely, the external electronic effect, to modulate intramolecular CT in a linear system named POMPDPAC containing N-(3-methylphenyl)-9,9-diphenylacridine (MPDPAC) donor and diphenylphosphine oxide (DPPO) acceptor. Theoretical simulation and time-resolved photophysical results show that intermolecular D-A or D-D/A-A interactions give rise to two stabilized ππ* and CT-featured triplet states on a single POMPDPAC molecule in its two different dimers, whose incorporation in the RTP process establishes a step-by-step energy transfer. As a result, compared to its Br or diphenylphosphine (DPP)-modified congeners only with ππ* featured triplet states, RTP quantum yield and lifetime of POMPDPAC powder are improved as high as ∼3 and 5-11 folds, respectively, accompanied by 77% RTP ratio in photoluminescence and typical photochromic properties. This work demonstrates not only the possibility of coexistence of multiple stabilized single-molecule triplet states in organic solids, but also the feasibility of developing multifunctional "RTP+" materials.