Linear and Nonlinear Optical Properties of Cationic Bipyridyl Iridium(III) Complexes: Tunable and Photoswitchable?

A series of cationic Ir(III) substituted bipyridyl (N∧N (N ∧ N-bpy) complexes incorporating electron-donor and -acceptor substituents, [Ir(C ∧ N-ppy-R′)2(N ∧ N-bpy-CHCH-C6H4-R)][X] (X– = PF6 – or C12H25SO3 –), 2 (a, R = NEt2 and R′ = Me; b, R = O-Oct and R′ = Me; c, R = NO2 and R′ = C6H13; C ∧ N-ppy = cyclometalated 2-phenylpyridine, [Ir(C ∧ N-ppy-Me)2(N ∧ N-bpy-CHCH-thienyl-Me)][PF6], 2d, and the dithienylethene (DTE)-containing complex 2e have been synthesized and characterized, and their absorption, luminescence, and quadratic nonlinear optical (NLO) properties are reported. Density functional theory (DFT) and time-dependent-DFT (TD-DFT) calculations on the complexes facilitate a detailed assignment of the excited states involved in the absorption and emission processes. All five complexes are luminescent in a rigid glass at 77 K, displaying vibronically structured spectra with long lifetimes (14–90 μs), attributed to triplet states localized on the styryl-appended bipyridines. The second-order NLO properties of 2a–d and related complexes 1a–d with 1,10-phenanthrolines have been investigated by both electric field induced second harmonic generation (EFISH) and harmonic light scattering (HLS) techniques. They are characterized by high negative EFISH μβ values which decrease when the ion pair strength between the cation and the counterion (PF6 –, C12H25SO3 –) increases. The EFISH response is mainly controlled by metal-to-ligand charge-transfer/ligand-to-ligand charge-transfer (MLCT/L′LCT) processes. A combination of HLS and EFISH techniques is used to evaluate both the dipolar and octupolar contributions to the total quadratic hyperpolarizability, demonstrating that the major contribution is controlled by the octupolar part. The incorporation of a photochromic DTE unit into the N ∧ N-bpy ligand (complex 2e) allows the luminescence to be switched ON or OFF. The photocyclisation of the DTE unit can be triggered by using either UV (365 nm) or visible light (430 nm), leading to an efficient quenching of the ligand-based 77 K luminescence, which can be restored upon irradiation of the closed form at 715 nm. In contrast, no significant modification of the EFISH μβ value is observed upon photocyclization, suggesting that the quadratic NLO response is dominated by the MLCT/L′LCT processes, rather than by the intraligand excited states localized on the substituted bipyridine ligand.