Shedding light on unusual photophysical properties of bis-cyclometalated iridium(iii) complexes containing 2,5-diaryl-1,3,4-oxadiazole-based and acetylacetonate ligands

Abstract

A DFT/TDDFT investigation was conducted on a series of cyclometalated iridium(iii) complexes with 2,5-diaryl-1,3,4-oxadiazole (oxd(n)) derivatives to shed light on the effects of the stereoisomeric and steric factors on the photophysical properties. On the basis of the results reported herein, we attempt to explain the experimental observations according to which complexes N,N-trans [Ir(oxd(0))(2)(acac)] (1a) and N,N-trans [Ir(oxd(1))(2)(acac)] (2a) [with oxd(0) = 2,5-diphenyl-1,3,4-oxadiazole, oxd(1) = 2,5-bis(4-fluorophenyl)-1,3,4-oxadiazole and acac = acetylacetonate] show high quantum phosphorescence efficiencies (Φ(PL)) of 35 and 44%, while an extremely low Φ(PL) (<1%) was observed for a number of oxd(n) based complexes including N,N-cis [Ir(oxd(3))(2)(acac)] (4b) [with oxd(3) = 2-(4-fluorophenyl)-5-(2,4,6-triisopropylphenyl)-1,3,4-oxadiazole]. While new insights were gained on structural and electronic properties, the unusual photophysical properties recently reported for 4b were found to be not inherent to spin-orbit coupling (SOC) effects, but determined by both the S(1)-T(1) splitting energy (ΔE(S1-T1)) and the transition dipole moment (μ(S1)) upon the S(0)→ S(1) transition. Drastically large ΔE(S1-T1) and small μ(S1) for 4b (0.70 eV and 0.23 D, respectively) comparative to those for 2a (0.38 eV and 2.76 D, respectively) and 1a (0.58 eV and 2.44 D, respectively) were found to be tightly linked to the twisting degree of the oxd(n) ligand and to the trans-cis structural isomerism. On the basis of these parameters, the unusual physical properties of 4b were interpreted with respect to 1a and 2a, and the higher Φ(PL) of 2a with respect to that for 1a was explained.