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

2,2 '-Bipyridine based bisphosphine [C5H3N{N(H)CH2PPh2}](2) (1) and its bischalcogenide derivatives [C5H3N{N(H)CH2P(E)Ph-2}](2) (2, E = O; 3, E = S; 4, E = Se) were synthesized, and further reacted with BF3 center dot Et2O/Et3N to form doubly B <- N fused compounds [C5H3N(BF2){NCH2P(E)Ph-2}](2) (5, E = O; 6, E = S; 7, E = Se) in excellent yields. The influence of the P = E 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 phi = 0.56 in CH2Cl2, whereas compound 7 showed a least quantum yield of phi = 0.003 in acetonitrile. Density functional theory (DFT) calculations demonstrated that the LUMO/HOMO of compounds 5-7 mostly delocalized over the entire pi-conjugated frameworks. The involvement of P E bonds in the HOMO energy level of these compounds follows the order: P = O < P = S < P = Se. 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 (tau(1)) and 0.47 ns (tau(2)) 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 (lambda(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.