The photophysics of fac-[Re(CO)3(NN)(bpa)]+ complexes: a theoretical/experimental study

Abstract

The influence of the polypyridyl ligand on the photophysics of fac-[Re(CO)₃(NN)(bpa)]⁺, bpa = 1,2-bis-(4-pyridyl)ethane and NN = 1,10-phenanthroline (phen), pyrazino[2,3-f][1,10]-phenanthroline (dpq), and dipyrido[3,2-a:2′3′-c]phenazine (dppz) has been investigated by steady state and time-resolved emission spectroscopy combined with theoretical calculations using time-dependent density functional theory (TD-DFT). The fac-[Re(CO)₃(phen)(bpa)]⁺ is a typical MLCT emitter in acetonitrile with ϕ = 0.11 and τ = 970 ns. The emission lifetime and quantum yield decrease significantly in fac-[Re(CO)₃(dpq)(bpa)]⁺ (ϕ = 0.05; τ = 375 ns) due to the presence of a close lying dark charge transfer state located at the pyrazine ring of dpq, as indicated by TD-DFT data. The luminescence of these complexes is quenched by hydroquinone with k_q = (2.9 ± 0.1) × 10⁹ and (2.6 ± 0.1) × 10⁹ L mol⁻¹ s⁻¹, respectively, for NN = phen or dpq. These values are increased respectively to (4.6 ± 0.1) × 10⁹ and (4.2 ± 0.1) × 10⁹ L mol⁻¹ s⁻¹ in the 1 : 1 H₂O–CH₃CN mixture. In this medium Stern–Volmer constants determined by steady-state and time-resolved measurements differ from each other, which is indicative of static quenching, i.e. the pre-association of hydroquinone and the complexes through hydrogen bonding between the remote N-atom in the bpa ligand (K_A ≅ 1–2 × 10¹ L mol⁻¹), followed by a concerted proton–electron transfer. In contrast to other investigated complexes, fac-[Re(CO)₃(dppz)(bpa)]⁺ is weakly emissive in acetonitrile at room temperature (ϕ ≅ 10⁻⁴) and does not exhibit a rigidochromic effect. This photophysical behaviour as well as TD-DFT data indicate that the lowest lying triplet excited state can be described as ³IL_dppz. The results provide additional insight into the influence of the polypyridyl ligand on the photophysical properties of Re(I) complexes.