2,2′-Bipyridine derived doubly B ← N fused bisphosphine-chalcogenides, [C5H3N(BF2){NCH2P(E)Ph2}]2 (E = O, S, Se): tuning of structural features and photophysical studies

Abstract

2,2′-Bipyridine based bisphosphine [C₅H₃N{N(H)CH₂PPh₂}]₂ (1) and its bischalcogenide derivatives [C₅H₃N{N(H)CH₂P(E)Ph₂}]₂ (2, E = O; 3, E = S; 4, E = Se) were synthesized, and further reacted with BF₃·Et₂O/Et₃N to form doubly B ← N fused compounds [C₅H₃N(BF₂){NCH₂P(E)Ph₂}]₂ (5, E = O; 6, E = S; 7, E = Se) in excellent yields. The influence of the PE bonds on the electronic properties of the doubly B ← N fused systems and their structural features were investigated in detail, supported by extensive experimental and computational studies. Compound 6 exhibited a very high quantum yield of ϕ = 0.56 in CH₂Cl₂, whereas compound 7 showed a least quantum yield of ϕ = 0.003 in acetonitrile. Density functional theory (DFT) calculations demonstrated that the LUMO/HOMO of compounds 5–7 mostly delocalized over the entire π-conjugated frameworks. The involvement of PE bonds in the HOMO energy level of these compounds follows the order: PO < PS < PSe. Time-correlated single photon counting (TCSPC) experiments of compounds 5–7 revealed the singlet lifetime of 4.26 ns for 6, followed by 4.03 ns for 5 and a lowest value of 2.18 ns (τ₁) and 0.47 ns (τ₂) with a double decay profile for 7. Our findings provide important strategies for the design of highly effective B ← N bridged compounds and tuning their photophysical properties by oxidizing phosphorus with different chalcogens. Compounds 5 and 6 have been employed as green emitters (λ_em = 515 nm) in fluorescent organic light-emitting diodes (OLEDs). For compound 5, doped into the poly(9-vinylcarbazole) (PVK) matrix with 5 wt% doping concentration, nearly 90 Cd m⁻² luminance with 0.022% external quantum efficiency (EQE) was achieved. The best performance was observed for compound 6 doped into PVK by 1 wt% having a maximum luminance of 350 Cd m⁻² and a similar EQE value.