Cyclometalated Ir-III Complexes with Substituted 1,10-Phenanthrolines: A New Class of Efficient Cationic Organometallic Second-Order NLO Chromophores

Cyclometalated cationic Ir-III complexes with substituted 1,10-phenanthrolines (1,10-phen), such as [Ir-(ppy)(2)(5-R-1,10-phen)]Y (ppy=cyclometalated 2-phenylpyridine; R = NO2, H, Me, NMe2; Y- = PF6-, C12H25SO3-, I-) and [Ir(ppy)(2)(4-R,7-R-1,10-phen)]Y (R = Me, Ph) are characterized by a significant second-order optical non linearity (measured by the electrical field induced second harmonic generation (EFISH) technique). This nonlinearity is controlled by MLCT processes from the cyclometalated Ir-III, acting as a donor push system, to pi* orbitals of the phenanthroline, acting as an acceptor pull system. Substitution of cyclometalated 2-phenylpyridine by the more pi delocalized 2-phenylquinoline (pq) or benzo[h]quinoline (bzq) or by the sulfur-containing 4,5-diphenyl-2-methyl-thiazole (dpmf) does not significantly affect the mu beta absolute value, which instead is affected by the nature of the R substituents on the phenanthroline, the higher value being associated with the electron-withdrawing NO2 group. By using a combined experimental (the EFISH technique and H-1 and F-19 PGSE NMR spectroscopy) and theoretical (DFT, time-dependent-DFT (TDDFT), sum over states (SOS) approach) investigation, evidence is obtained that ion pairing, which is controlled by the nature of the counterion and by the concentration, may significantly affect the mu beta values of these cationic NLO chromophores. In CH2Cl2, concentration-dependent high absolute values of mu beta are obtained for [Ir-(ppy)(2)(5-NO2-1,10-phen)]Y if Y is a weakly interacting anion, such as PF6-, whereas with a counterion, such as C12H25SO3- or I-, which form tight ion-pairs, the absolute value of mu beta is lower and quite independent of the concentration. This mu beta trend is partially due to the perturbation of the counterion on the LUMO pi* levels of the phenanthroline. The correlation between the mu beta value and dilution shows that the effect of concentration is a factor that must be taken into careful consideration.