Studies on synthesis, structures, and photophysics have been carried out for a series of luminescent copper(I)
halide complexes with the chelating ligand, 1,2-bis[diphenylphosphino]benzene (dppb). The complexes studied are
halogen-bridged dinuclear complexes, [Cu(μ-X)dppb]2 (X = I (1), Br (2), Cl (3)), and a mononuclear complex,
CuI(dppb)(PPh3) (4). These complexes in the solid state exhibit intense blue-green photoluminescence with
microsecond lifetimes (emission peaks, λmax = 492−533 nm; quantum yields, Φ = 0.6−0.8; and lifetimes, τ =
4.0−10.4 μs) at 298 K. In 2-methyltetrahydrofuran (2mTHF) solutions at 298 K, only 1 and 4 show weaker emission
(Φ = 0.009) with shorter lifetimes (τ = 0.35 and 0.23 μs) and red-shifted spectra (λmax = 543 and 546 nm). The
emission in the solid state originates from the (M + X)LCT excited state with a distorted-tetrahedral conformation,
in which emissive excited states, 1(M + X)LCT and 3(M + X)LCT, are in equilibrium with an energy difference of
∼2 kcal/mol. On the other hand, the complexes in the 2mTHF solutions emit from the MLCT excited state with an
energetically favorable flattened conformation in the temperature range of 298−130 K. The flattened geometry with
equilibrated 1MLCT and 3MLCT states has a nonradiative rate at least 2 orders of magnitude larger than that of the
distorted-tetrahedral geometry, leading to a much smaller emission quantum yield (Φ = 0.009) at 298 K. Since the
flattening motion is markedly suppressed below 130 K, the emission observed in 2mTHF below 130 K is considered
to occur principally from the (M + X)LCT state with a distorted-tetrahedral geometry. To interpret the photophysics
of 1 and 4 in both the solid and solution states, we have proposed the “2-conformations with 2-spin-states model
(2C × 2S model)”. The electroluminescence device using (1) as a green emissive dopant showed a moderate EL
efficiency; luminous efficiency = 10.4 cd/A, power efficiency = 4.2 lm/W at 93 cd/m2, and maximum external
quantum efficiency = 4.8%.