Photoionization of tris(2-phenylpyridine)iridium

C. Nemirow, J. Fine, Z. Lu, K. Diri, A.I. Krylov, and C. Wittig
Mol. Phys. 110, 1893 – 1908 (2012)

An experimental study of the photoionization of gas phase tris(2-phenylpyridine)iridium, hereafter referred to as Ir(ppy)3, is presented. One- and two-photon ionization studies yield a conservative estimate for the upper bound to the vertical ionization energy of 6.4 eV. The one- photon experiment was carried out using 193 nm radiation, and the latter experiments used tunable doubled dye laser radiation to excite the ligand-centered 1LC-S0 transition, followed by photoionization. An accompanying paper presents a theoretical study of Ir(ppy)3 excited singlets and triplets and low energy states of Ir(ppy)3+. The calculated ionization energy is ~5.9 eV. Taking the experimental and calculated results together, including a calculation by Hay (5.94 eV), it is concluded that the ionization energy is in the vicinity of 6 eV. This is favorable from the perspective of organometallic light-emitting diode applications. An undulation in the ion yield spectrum (~ 270 cm–1 spacing) was observed that is due possibly to structure in either the 1LC-S0 transition or the ionization transition that originates from the T1 state populated through radiationless decay of 1LC. It is interesting that the parent cation Ir(ppy)3+ is produced with no discernible fragmentation, despite the fact that a large amount of vibrational energy is present in the T1 state that undergoes photoionization. Specifically, this vibrational energy is the sum of the energy difference between the photon and T1 energies (i.e., hv-ET1 ) and the thermal energy of the 177 Ir(ppy)3 vibrational degrees of freedom at 500 K, i.e., the temperature at which the experiments were carried out. As photoionization is mainly vertical, vibrational energy is transported efficiently to the cation.

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