Ultrafast and long-time excited state kinetics of an NIR-emissive vanadium(iii) complex I: synthesis, spectroscopy and static quantum chemistry

Matthias Dorn,Markus Gerhards,Chahinez Dab,Katja Heinze,Gereon Niedner-Schatteburg,Jens Kalmbach,Stefan Lochbrunner,Michael Seitz,Pit Boden,Christian Reber,Ayla Kruse

doi:10.1039/d1sc02137k

Abstract

In spite of intense, recent research efforts, luminescent transition metal complexes with Earth-abundant metals are still very rare owing to the small ligand field splitting of 3d transition metal complexes and the resulting non-emissive low-energy metal-centered states. Low-energy excited states decay efficiently non-radiatively, so that near-infrared emissive transition metal complexes with 3d transition metals are even more challenging. We report that the heteroleptic pseudo-octahedral d2-vanadium(iii) complex VCl3(ddpd) (ddpd = N,N′-dimethyl-N,N′-dipyridine-2-yl-pyridine-2,6-diamine) shows near-infrared singlet → triplet spin–flip phosphorescence maxima at 1102, 1219 and 1256 nm with a lifetime of 0.5 μs at room temperature. Band splitting, ligand deuteration, excitation energy and temperature effects on the excited state dynamics will be discussed on slow and fast timescales using Raman, static and time-resolved photoluminescence, step-scan FTIR and fs-UV pump-vis probe spectroscopy as well as photolysis experiments in combination with static quantum chemical calculations. These results inform future design strategies for molecular materials of Earth-abundant metal ions exhibiting spin–flip luminescence and photoinduced metal–ligand bond homolysis.

Highlights

The control of photophysical properties of transition metal complexes by chemical means, especially for applications in lighting, imaging, sensing, photonics, dye sensitised solar cells, phototherapy or photocatalysis, is a very active research eld.[1,2,3,4,5,6,7,8] most applications rely on noble metal complexes with d6 or d8 electron con gurations such as ruthenium(II), iridium(III) or platinum(II).[9]
We report that the heteroleptic pseudo-octahedral d2-vanadium(III) complex VCl3(ddpd) shows near-infrared singlet / triplet spin–flip phosphorescence maxima at 1102, 1219 and 1256 nm with a lifetime of 0.5 ms at room temperature
Several factors are relevant for this emission from a 3d transition metal complex to occur: (i) The ligand eld splitting in VCl3(ddpd) is large enough to place the emissive singlet states 1E/1T2 below the distorted metal-centred triplet excited states 3T2

Summary

Introduction

The control of photophysical properties of transition metal complexes by chemical means, especially for applications in lighting, imaging, sensing, photonics, dye sensitised solar cells, phototherapy or photocatalysis, is a very active research eld.[1,2,3,4,5,6,7,8] most applications rely on noble metal complexes with d6 or d8 electron con gurations such as ruthenium(II), iridium(III) or platinum(II).[9]. Ligand deuteration, excitation energy and temperature effects on the excited state dynamics will be discussed on slow and fast timescales using Raman, static and timeresolved photoluminescence, step-scan FTIR and fs-UV pump-vis probe spectroscopy as well as photolysis experiments in combination with static quantum chemical calculations.

Results

Conclusion