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

摘要

A computational study of tris(2-phenylpyridine)iridium, Ir(ppy)₃, is presented. The perspective is that of using organo-transition-metal complexes as phosphorescent species in light-emitting diodes (OLED's). Quantum yields approaching 100% are possible through a triplet harvesting mechanism. Complexes such as Ir(ppy)₃ are amenable to exacting 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. Because fac-Ir(ppy)₃ dominates in most environments, 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 . The calculated T₁ - S₀ energy gap (2.30 eV) is in reasonable agreement with the experimental value of 2.44 eV. Only a few percent of singlet character in T₁ is needed to explain so short a phosphorescence lifetime as 200 ns, because of the large and absorption cross-sections. Equilibrium geometries are calculated for S₀, T₁, and the lowest cation state (D₀), and several ionization energies are obtained: adiabatic (5.86 eV); vertical from the S₀ equilibrium geometry (5.88 eV); and vertical ionization of T₁ at its equilibrium geometry (5.87 eV). These agree with a calculation by Hay (5.94 eV), and with the conservative experimental upper bound of 6.4 eV. Molecular orbitals provide qualitative explanations. A calculated UV absorption spectrum, in which transitions are vertical from the S₀ equilibrium geometry, agrees with the room temperature experimental spectrum. This is consistent with Franck-Condon factors dominated by , as expected given the delocalized nature of the orbitals. Ir(ppy)₃ vibrational frequencies were calculated and used to estimate the probability density for 500 K, i.e. the temperature at which the experiments were carried out. In combination with the vibrational energy imparted through photoexcitation, it is seen that a large amount of vibrational energy appears in without causing its fragmentation. Specifically, for  = 15,000 cm−1, the probability density for total vibrational energy peaks at 31,000 cm−1 with a 7800 cm−1 width.View full textDownload full textKeywordsionization, TDDFT, long-range-corrected functionals, internal conversion, intersystem crossing, phosphorescence, iridium complexes, spin-orbit coupling, OLEDRelated var addthis_config = { ui_cobrand: "Taylor & Francis Online", services_compact: "citeulike,netvibes,twitter,technorati,delicious,linkedin,facebook,stumbleupon,digg,google,more", pubid: "ra-4dff56cd6bb1830b" }; Add to shortlist Link Permalink http://dx.doi.org/10.1080/00268976.2012.685899