Electronic structure of tris(2-phenylpyridine)iridium: Electronically excited and ionized states

J. Fine, K. Diri, A.I. Krylov, C. Nemirow, Z. Lu, and C. Wittig
Mol. Phys. 110, 1849 – 1862 (2012)

A computational study of tris(2-phenylpyridine)iridium, hereafter referred to as Ir(ppy)₃, is presented. The perspective is that of using organo-transition-metal complexes as phosphorescent species in light-emitting diodes (OLED's). These species enable quantum yields approaching 100% to be achieved through a mechanism referred to as triplet harvesting, which offers advantage over organic counterparts, whose maximum quantum yield is 25%. Complexes such as Ir(ppy)₃ are amenable to exacting and symbiotic experimental and theoretical studies: small enough to accommodate rigor, yet large enough to support bulk phenomena in a range of host materials. The facial and meridional isomers differ by ~ 220 meV, with fac-Ir(ppy)₃ having the lower energy. Thus, fac-Ir(ppy)3 dominates in most environments, and therefore focus is on this species. Time dependent density functional theory using long-range-corrected functionals (BNL and ωB97X) is used to calculate excited states of Ir(ppy)₃ and a few low energy states of Ir(ppy)₃⁺. The calculated T₁–S₀ energy gap (2.30 eV) is in reasonable agreement with the ex- perimental value of 2.44 eV. It is shown that only a few percent of singlet character mixed into T1 is needed to explain the short phosphorescence lifetime of 200 ns for one of the T1 sublevels, because ofthe large ¹LC-S₀ and ¹MLCT-S₀ absorption cross sections (~10–16 cm²). Equilibrium geometries are calculated for S0, T1, and the lowest cation state (D0), and ionization energies are obtained: adiabatic (5.86 eV); vertical from the S0 equilibrium geometry (5.88 eV); and vertical ionization energy of the T1 state (5.87 eV). These agree with a calculation by Hay, and with the conservative experimental upper bound of 6.4 eV reported in the companion paper (Paper I) that precedes this one. Molecular orbitals provide qualitative explanation of the results. A calculated ultraviolet absorption spectrum, in which transitions are vertical from the S0 equilibrium geometry, is in agreement with the room temperature experimental spectrum. This is consistent with Franck-Condon factors dominated by vi = 0 , as expected for the delocalized nature of the orbitals. The B3LYP functional was used to calculate the 177 vibrational frequencies. These were used to estimate the probability density P(Evib) for finding the molecule in a small energy interval at the "thermal" vibrational energy Evib at 500 K, i.e., the approximate temperature at which the experiments reported in Paper I were carried out. In combination with the vibrational energy imparted through ¹LC-S₀ photoexcitation (accompanied by radiationless decay that results ultimately in population of T₁), it is seen that a large amount of vibrational energy appears in Ir(ppy)₃⁺ without causing its fragmentation. Specifically, for an hv-ET1 value of 15 000 cm^–1, the probability density for total vibrational energy peaks at ~ 31 000 cm^–1 with a full width at half maximum of 7800 cm^–1.

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