Theoretical studies on structures and spectroscopic properties of bis-cyclometalated iridium complexes

The series of iridium(III) complexes Ir((CN)-N-and)(2)LX ((CN)-N-and = bzq, LX = acac, 1; (CN)-N-and = thp, LX = acac, 2; (CN)-N-and = tpy, LX = acac, 3; (CN)-N-and = bzq, LX = dbm, 4; acac = acetoylacetonate, bzq = benzoquinoline, tpy = 2-(4-tolyl)pyridine, thp = 2-(2'-thienyl)pyridine and dbm = dibenzoylmethanate) have been investigated theoretically to explore their electronic structures and spectroscopic properties. Their structures in the ground and excited states have been optimized at the B3LYP/LANL2DZ and CIS/LANL2DZ levels, respectively. The calculated bond lengths of Ir-N, Ir-C, and Ir-O in the ground state agree well with the corresponding experimental results. Upon excitation, the bond lengths of Ir-N and Ir-C lengthen by 0.03 A and that of Ir-O shortens by 0.01 A compared with those in the X(1)A ground states. This is consistent with the variation trend of corresponding vibration mode in the ground and excited states. At the TD-DFT and PCM levels, 1-4 give rise to absorptions at 484, 462, 452, and 536 nm and phosphorescent emissions at 541, 589, 499, and 540 nm, respectively. The transitions of 1-3 are attributed to [d(Ir) + pi((CN)-N-and)] -> [pi*((CN)-N-and)] charge transfer, whereas those of 4 are related to [d(Ir) + pi((CN)-N-and)] -> [pi*(LX) + pi*((CN)-N-and)]. It is shown that the emissions are significantly dominated by the metal participating in the frontier molecular orbitals and affected by the (CN)-N-and ligands but are hardly perturbed by the LX ligands.