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Combined Effects of Stereoisomeric and Steric Factors on Electronic and Photophysical Properties of Bis-cyclometalated Ir(III) Complexes Containing 2,5-Diaryl-1,3,4-oxadiazole Based and Picolinate Ligands

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journal contribution
posted on 12.07.2012, 00:00 by Gahungu Godefroid, Su Juanjuan, Liu Yuqi, Qu Xiaochun, Si Yanling, Shang Xiaohong, Wu Zhijian
Electronic structure methods based on DFT and TDDFT were applied to understand the recently reported unusual photophysical properties for a series of cyclometalated iridium complexes of 2,5-diaryl-1,3,4-oxadiazole (oxdn) based and picolinate ligands. Under the hypotheses of possible steric and stereoisomeric effects on the electronic and photophysical properties, both the N,N-trans and N,N-cis types of complexes bearing oxdn ligands of different twisting degree were considered. On the basis of the results herein, an explanation of the experimental observations was attempted, according to which complexes Ir­(oxd1)2(pic) (1) [oxd1 = 2-(4-fluorophenyl)-5-(2,4,6-tri-tert-butylphenyl)-1,3,4-oxadiazole and pic = picolinate] and Ir­(oxd2)2(pic) (2) [oxd2 = 2-(4-fluorophenyl)-5-(2,4,6-triisopropylphenyl)-1,3,4-oxadiazole] show an extremely poor quantum phosphorescence efficiency (ΦPL < 1%). The poor ΦPL was shown not to be inherent to spin–orbit coupling effects, but determined by the intersystem crossing (ISC) rate, which was found to be tightly linked to the twisting degree of the oxdn ligands, and to the stereoisomeric factor. The results evidenced a large S1–T1 splitting energy (ΔES1‑T1) estimated at 0.81 and 0.72 eV (for 1 and 2, respectively) and a drastically small S0→S1 transition dipole (μS1) estimated at 0.26 and 0.32 D, comparatively to 0.38 eV and 2.76 D of ΔES1‑T1 and μS1 values for the N,N-trans Ir­(oxd3)2(acac) complex [with oxd3 = 2-(4-fluorophenyl)-5-(2,4,6-triisopropylphenyl)-1,3,4-oxadiazole and acac = acetyl acetone] showing a ΦPL of ∼44% [Chem. Commun. 2007, 1352], due to the structural isomerism and to the twisting degree of the oxdn ligand, respectively. Accordingly, we were able to explain the reported unusual photophysical properties of 2 and 1.