Chemistry

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16 October 2019

Research Seminar: Unravelling the mechanism of excited state interligand energy transfer and the engineering of dual-emission in iridium complexes

Presented By Dr Paul I. P. Elliot (University of Huddersfield)

About this event

Unravelling the mechanism of excited state interligand energy transfer and the engineering of dual-emission in [Ir(C^N)2(N^N)]+complexes

Heteroleptic bis-cyclometalated iridium complexes of the form [Ir(C^N)2(N^N)]+have attracted enormous interest in the literature and form one of the corner stones of modern photophysics. This interest primarily stems from their application in light-emitting electrochemical cells, luminescent confocal imaging microscopy and solar catalysis. Complexes of this family typically emit from either a 3MLCT/3LC state where the excited electron resides on the cyclometalated ligand or a 3MLCT/3LL’CT state where charge transfer occurs to the neutral ancillary ligand. Structural variation of the cyclometalated and ancillary ligands and thus tuning of the energies of the 3MLCT/3LC and 3MLCT/3LL’CT states then determines which state ultimately emits.1, 2Dependent on the ligand set present complexes can display switching of emission of these states between fluid solutions and cryogenic glass samples, but the exact nature of the factors that underpin the switching mechanism are not clear.

In a thorough survey of complexes [Ir(C^N)2(N^N)]+bearing a asymmetric ancillary ligand (1a-c(C^N = 2-phenylpyridine) & 2a-c(C^N = dfppy); N^N = 2-(1-benzyl-1,2,3-triazol-4-yl)pyridine (a), 2-(1-benzyl-1,2,3-triazol-4-yl)pyrimidine (b), 2-(1-benzyl-1,2,3-triazol-4-yl)pyrazine (c)) we tune the relative energies of both 3MLCT/3LC and 3MLCT/3LL’CT states. Complexes 1a2a2bexhibit exclusively 3MLCT/3LC-based emission at both 77 K and room temperature whereas 1cexhibits exclusively 3MLCT/3LL’CT-based emission. Complex 2cexhibits switching from 3MLCT/3LC to 3MLCT/3LL’CT-based emission between frozen and fluid solutions. Significantly we are able to show that the energies of these two states can be tuned such that they are nearly isoenergetic with 1b, thus displaying very rare dual emission from both 3MLCT/3LC and 3MLCT/3LL’CT states over a broad range of solvents and temperatures. Computational DFT calculations have enabled us to optimise geometries for both 3LC- and 3LL’CT-admixed 3MLCT states and to compute minimum energy paths connecting the various triplet states. This data reveal for the first time the key structural changes and key vibrations involved in the population transfer, and highlight the mechanism by which these states interconvert. Furthermore the theoretical data fully rationalizes the observed photophysical behaviour. This work therefore provides significant new insights into this important class of transition metal phosphor.  

  

1.         Bevernaegie, R; et al.PCCP 201820, 27256-27260.

2.         Cho, Y.-J.; et alPCCP 201719, 8778-8786.