Luminescent osmium(ii) bi-1,2,3-triazol-4-yl complexes: photophysical characterisation and application in light-emitting electrochemical cells.

Daniel A W Ross,Antonio Pertegás,Paul A Scattergood,Henk J Bolink,Paul I P Elliott,Azin Babaei

doi:10.1039/c6dt00830e

Abstract

The series of osmium(ii) complexes [Os(bpy)3-n(btz)n][PF6]2 (bpy = 2,2'-bipyridyl, btz = 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl, n = 0, n = 1, n = 2, n = 3), have been prepared and characterised. The progressive replacement of bpy by btz leads to blue-shifted UV-visible electronic absorption spectra, indicative of btz perturbation of the successively destabilised bpy-centred LUMO. For , a dramatic blue-shift relative to the absorption profile for is observed, indicative of the much higher energy LUMO of the btz ligand over that of bpy, mirroring previously reported data on analogous ruthenium(ii) complexes. Unlike the previously reported ruthenium systems, heteroleptic complexes and display intense emission in the far-red/near-infrared (λmax = 724 and 713 nm respectively in aerated acetonitrile at RT) as a consequence of higher lying, and hence less thermally accessible, (3)MC states. This assertion is supported by ground state DFT calculations which show that the dσ* orbitals of to are destabilised by between 0.60 and 0.79 eV relative to their Ru(ii) analogues. The homoleptic complex appears to display extremely weak room temperature emission, but on cooling to 77 K the complex exhibits highly intense blue emission with λmax 444 nm. As complexes to display room temperature luminescent emission and readily reversible Os(ii)/(iii) redox couples, light-emitting electrochemical cell (LEC) devices were fabricated. All LECs display electroluminescent emission in the deep-red/near-IR (λmax = 695 to 730 nm). Whilst devices based on and show inferior current density and luminance than LECs based on , the device utilising shows the highest external quantum efficiency at 0.3%.

Highlights

The photophysics of d6 oligopyridine based complexes have been extensively investigated over the past four decades.[1,2] This interest stems from the potential application of these complexes from light-harvesting and solar energy conversion[3,4,5] to artificial lighting.[6,7,8] The control and optimisation of the excited state energies of these complexes in relation to their use within the aforementioned applications has been a focus of innumerable studies on ligand design for these systems.Complexes of Os(II) in particular have been of significant interest since the 1980s, with the photophysical properties of the parent complex of the family, Os(bpy)32+, having been orbit coupling constant for Os permitting the occurrence of both spin-allowed 1MLCT and spin-forbidden direct 3MLCT electronic transitions.[22]
3 was prepared via an alternative procedure making use of a bipyridine-containing intermediate (Scheme 1), a strategy which has been employed in our previous work concerning the synthesis of the Ru analogue of 3.40,41 refluxing [Os(η6-C6H6)(bpy)(Cl)][PF6] in ethanol/ water with two equivalents of btz yields 3, which was isolated as a dark green powder as its hexafluorophosphate salt
Progressing across the series both the HOMO and LUMO are destabilised (Table 3), more so for the latter resulting in an increased HOMO–LUMO energy gap in agreement with the experimental spectroscopic and electrochemical data; from complex 1 to 3 the HOMO–LUMO gap increases by 0.29 eV as bpy ligands are replaced by btz

Summary

Introduction

The photophysics of d6 oligopyridine based complexes have been extensively investigated over the past four decades.[1,2] This interest stems from the potential application of these complexes from light-harvesting and solar energy conversion[3,4,5] to artificial lighting.[6,7,8] The control and optimisation of the excited state energies of these complexes in relation to their use within the aforementioned applications has been a focus of innumerable studies on ligand design for these systems.Complexes of Os(II) in particular have been of significant interest since the 1980s, with the photophysical properties of the parent complex of the family, Os(bpy)32+, having been orbit coupling constant for Os permitting the occurrence of both spin-allowed 1MLCT and spin-forbidden direct 3MLCT electronic transitions.[22]. For the homoleptic complex 4 no reduction potential is observed within the available electrochemical solvent window, indicative of the much higher energy LUMO of btz over that of bpy.

Results

Conclusion