Research

Research in our group centres on the photophysics and photochemistry of transition metal coordination complexes, with much of our work having focused on those bearing 1,2,3-triazole-based ligand systems.

Photochemistry and Photoreactive Excited States

Through our investigations of 1,2,3-triazole-based ligands as oligopyridyl analogues we found that the 1,2,3-triazole moiety promotes photochemical ligand ejection reactivity in their ruthenium(II) complexes through an electronic destabilisation of triplet MLCT states rather than more conventional steric strain induced triplet MC state stabilisation. Moreover, we have shown that 4,4'-bi-1,2,3-triazolyl (btz) complexes such as [Ru(bpy)(btz)2]2+ (bpy = 2,2'-bipyridyl, Figure 1a) undergo extremely intriguing photochemical reactivity with loss of one btz ligand and coplanarisation of the remaining bidentate ligands to yield trans-[Ru(bpy)(btz)(NCMe)2]2+ in acetonitrile. Remarkably this proceed with the observation of the monodentate btz ligand loss intermediate which we were able to crystallographically characterise. The use of btz ligand in the complex [Os(btz)3]2+ extraordinarily confers photochemical lability to the supposedly photoinert osmium(II) centre with observation of the formation of both cis- and trans-[Os(btz)2(NCMe)2]2+ in aceotnitrile along with observation of their corresponding ligand loss intermediates. 

Figure 1. a) Photochemical reactivity  of [Ru(bpy)(btz)2]2+ in acetonitrile, b) calculated geometries of the key triplet metal centred states involved, and c) calculated geometries of lowest triplet potential energy surface of [Ru(bpy)3]2+. 

Computational investigations in collaboration with Isabelle Dixon and Fabienne Alary (Universite Paul Sabatier, Toulouse, France) on these systems have revealed the lowest triplet excited state potential energy surface to be far more complex than originally envisaged with the existence of a number of readily accessible triplet metal-centred excited states identified as either quenchers of both photophysics and photochemistry (favouring ground state recovery) or favouring photoproductive pathways. In particular we were able to identify novel 3MC states exhibiting elongation of both Ru-N bonds to the same btz ligand with flatterning of the rest of the complex and for the first time fully elucidated the excited state mechanism for photodechelation for a metal complex for our bis-btz systems (Figure 1b). We have since been able to demonstrate existence of a comparable flattened 3MC state for the iconic archetypal complex [Ru(bpy)3]2+ and shown that it will play a major role in 3MC state mediated excited state deactivation (Figure 1c).

Our current work continues to explore the interplay between photoexcited 3MLCT and photoreactive 3MC states to better understand the topology of the lowest triplet potential energy surface in these systems, understand the impact this has on experimentally observed photochemistry and gain deeper insight in to the particular roles given 3MC state play. 

Luminescent Complexes and Excited State Dynamics

As structural analogues of more conventional oligopyridyl ligands 1,2,3-triazole chemistry has enables us facile access to a range of luminescent complexes of kinetically inert ions such as rhenium(I), osmium(II) and iridium(III). We have explored the luminescence and photophysics of a range of triazole-containing osmium(II) complexes which are emissive between the organe/red and near-infrared. In particular, for a series of bistridentate complexes containing triazole, pyridine and pyrazine donors we have been able to effect facile tuning of the luminescent emission and been able to capture a tipping point at which increased electron withdrawing character of the ligand (with increased pyrazine content) yields an inversion of photophysical tuning as increasing stabilisation of the highest occupied molecular orbital out-competes stabilisation of the lowest unoccupied molecular orbital leading to counter-intuitive blue-shifting of emission maxima (Figure 2).

For a series of iridium(III) complexes of the form [Ir(C^N)2(N^N)]+ we have shown that variation of the C^N and N^N ligand enables spanning of the regimes emissi0n derives from 3MLCT admixed with C^N localised 3LC or N^N localised 3LL'CT state character. Importantly by variation of the energies of these two states we can observed dual emission from both 3LC and 3LL'CT admixed states. Through femto- to picosecond transient absorption experiments we have been able to observe that all complexes are photoexcited to the 3LC state and that steady-state 3LL'CT emitters undergo energy transfer from the 3LC state on the picosecond timescale after excitation. Where dual emission is observed, these experiments so equilibration of the two state within 10 ps (Figure 3). Through detailed computational calculations in collaboration with Alary and Dixon we have been able to locate optimised minima for both 3LC and 3LL'CT admixed states in individual complexes for the first time and in doing so have identified the key bond vibrations responsible for these energy transfer processes. 

Application of Luminescent Complexes in Biological Imaging

Phosphorescent metal complexes with long-lived emissive excited state offer significant advantages for luminescent imaging by confocal microscopy. Osmium(II) complexes in particular have attractive properties in that the strong spin-orbit coupling associated with the metal centre results in direct triplet MLCT absorption bands of appreciable intensity to be observed in the visible absorption spectrum. Appearing at wavelengths in the optimum biological transparent region osmium(II) complexes therefore offer the potential for greater tissue penetration depth for excitation at longer wavelength, lower energy and less damaging light. We have therefore investigated a range of osmium(II) complexes which exhibit cellular uptake and their localisation within cancer cell lines (Figure 4). These materials therefore present significant potential for the development of future cellular imaging agents with highly attractive photophysical properties.