Issue 46, 2020

Quantitative calculations of the non-radiative rate of phosphorescent Ir(iii) complexes

Abstract

It has recently been proposed that the dominant non-radiative decay mechanism in blue Ir(III) phosphors at room temperature is due to the low-lying non-radiative metal-centred triplet states. These are populated thermally via an activated transition from the highly radiative metal-to-ligand-charge-transfer states that are initially populated due to intersystem crossing following the radiative or electronic excitation of the phosphor. We apply transition state theory to quantitatively calculate the non-radiative decay rate of a family of Ir(III) complexes containing N-heterocyclic carbene (NHC) ligands. We compare the, computationally inexpensive, one-dimensional theory with the, more accurate, multi-dimensional theory. Both methods find a non-radiative rate with an Arrhenius form (knr = kae−ΔE/kBT). The pre-exponential factors, ka, and activation energies, ΔE, are evaluated via density functional theory (DFT). The multi-dimensional theory shows that there is an order of magnitude variation in ka within this family of materials (between 3 × 1011 s−1 and 3 × 1012 s−1). This is not captured by the one-dimensional theory, which predicts very uniform rate constants in the middle of this range (∼1012 s−1). Nevertheless, the activated process involved, and the linear relationship between ka and knr, mean that ka plays a subtle role in determining knr. Consistent with this we find that both methods capture the trend observed experimentally in the non-radiative rates. Furthermore, the magnitude of the calculated knr is similar in both methods and in good agreement with experimental values [except for one complex with a very shallow activation barrier (<0.1 eV)]. It has previously been demonstrated that radiative decay rates can be accurately calculated from DFT. Combined with our results for the non-radiative rates, this implies that DFT methods can accurately predict the emission efficiency in Ir(III) phosphors. Therefore, DFT calculations are both fast and accurate enough to play a significant role in the design of new deep blue Ir(III) phosphors with high emission efficiency. Even the one-dimensional theory provides reasonable agreement with experiment. This suggests that a funneling approach – where only the best performing molecules, according to the one-dimensional theory, are studied in the more laborious multi-dimensional framework – could be a powerful strategy for designing active materials for phosphorescent organic light-emitting diodes (PHOLEDs) from first principles.

Graphical abstract: Quantitative calculations of the non-radiative rate of phosphorescent Ir(iii) complexes

Supplementary files

Article information

Article type
Paper
Submitted
07 sep 2020
Accepted
05 nov 2020
First published
06 nov 2020

Phys. Chem. Chem. Phys., 2020,22, 27348-27356

Quantitative calculations of the non-radiative rate of phosphorescent Ir(III) complexes

X. Zhou and B. J. Powell, Phys. Chem. Chem. Phys., 2020, 22, 27348 DOI: 10.1039/D0CP04709K

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