Long-lived, near-IR emission from Cr(iii) under ambient conditions

Sawicka, Natalia, Craze, Chloe J., Horton, Peter N., Coles, Simon J., Richards, Emma ORCID: https://orcid.org/0000-0001-6691-2377 and Pope, Simon J. A. ORCID: https://orcid.org/0000-0001-9110-9711 2022. Long-lived, near-IR emission from Cr(iii) under ambient conditions. Chemical Communications 58 (38) , pp. 5733-5736. 10.1039/D2CC01434C

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Abstract

Bis-terdentate (N^N^N) ligands coordinated to Cr(III) yield complexes that display near-IR emission under aerated solvent conditions at room temperature. Graphical abstract: Long-lived, near-IR emission from Cr(iii) under ambient conditions Cr(iii) complexes have attracted interest due to their optical,1 (photo)redox and magnetic properties.2 Cr(iii) photoluminescence,3 from a discreet coordination complex was first reported in the 1960s.4 The photophysics of Cr(iii) complexes is generally dominated by metal-centred excited states.5–7 A strong, pseudo-octahedral ligand field at Cr(iii) yields the possibility for populating doublet excited states (2E and 2T), while simultaneously preventing 4T2/2E back intersystem crossing. As relaxation from these excited states to the 4A2 ground state is spin forbidden, long-lived phosphorescence can be observed. However, Cr(iii) species are often hampered by poor emissivity. Deoxygenated, deuterated solvents and deuteriation of ligands has been employed to help minimise quenching from non-radiative multiphonon relaxation pathways.8 In contrast to well-known precious metal lumophores, earth-abundant Cr(iii) complexes of archetypal diimine ligands typically show Cr-centred phosphorescence in the 650–760 nm range. Related heteroleptic complexes provide some rational tuning of emission within similar wavelength ranges.9 Scheme 1 describes some important luminescent Cr(iii) complexes, most of which have been reported in the last few years. Consideration of the N–Cr–N bite angles of the coordinated ligands is key. Thus, larger six-membered chelate rings are well suited in optimizing N–Cr–N bite angles towards 90° and this in turn reduces non-radiative deactivation.7 Recent studies have clearly demonstrated the use of ligands such as N,N′-dimethyl-N-N′-dipyridine-2-yl-pyridine-2,6-diamine (ddpd),10 2,6-bis(2-pyridylmethyl)pyridine (bpmp),11 and 1,1,1-tris(pyrid-2-yl)ethane (tpe)12 that yield Cr(iii) complexes with more intense emission properties and longer lifetimes.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Chemistry
Additional Information:	This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Publisher:	Royal Society of Chemistry
ISSN:	1359-7345
Date of First Compliant Deposit:	28 April 2022
Date of Acceptance:	12 April 2022
Last Modified:	11 Aug 2023 18:58
URI:	https://orca.cardiff.ac.uk/id/eprint/149345

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