Paul I. P. Elliott

The development of sustainable photosensitiser complexes based on Earth-abundant metallic elements is important for sustainable solar energy conversion and green synthesis through photocatalysis. We report the synthesis and characterisation of a new bis(tripodal) iron(III) N-heterocyclic carbene complex [Fe(PhB{impy}3)2]+ (PhB{impy}3 = phenyltris(imidazo-[1,5-a]-pyridinylidene)borate) featuring -extended imidazo-[1,5-a¬]-pyridinylidene donors. This complex exhibits dual charge transfer absorption through well-resolved 2LMCT and 2MLCT absorption bands at 600-800 and 400-500 nm wavelengths, respectively. The dual charge transfer absorber motif combines ligands with a comparatively narrow -* gap, together with a central Fe(III) metal ion with a set of partially filled metal t2g levels capable of acting as both electron acceptors and donors. Ultrafast transient absorption spectroscopy reveals that ground state recovery is dominated by conversion from the lower 2LMCT state with a 13 ps time constant, independently of which of the two charge transfer bands is initially excited. The measurements further reveal that the initial dynamics following excitation into the higher 2MLCT state through 400 nm excitation results in efficient internal conversion into the lower 2LMCT state on a sub-ps timescale. This ultrafast conversion involves a two-electron internal conversion process that changes the orbital configuration from (L)2(t2g)4(L*)1 in the 2MLCT state to (L)1(t2g)6(L*)0 in the 2LMCT state. Excitation into the 2MLCT band thus drives a rare case of double reduction of the metal from Fe(IV) to Fe(II) that efficiently outcompetes alternative ultrafast decay mechanisms via metal centered (4MC) scavenger states. The presence of two charge transfer bands with opposite charge transfer directionality provides an opportunity to provide detailed insight into the excited state dynamics of a prototype dual charge transfer absorber, and the broader implications of the results for further development and potential utilization of dual charge transfer absorbers are finally discussed.

The exploration of the novel photochemistry offered by Earth-abundant first row transition metal complexes as replacements for photosensitiser complexes based on rare and expensive 4d and 5d metals is a highly topical area of contemporary inorganic chemistry. Here we report the synthesis and optoelectronic characterisation of the cyclometalated ‘abnormal’ carbene complex [Co(btzph)2]+ (btzphH = 1,3-bis(1-butyl-3-methyl-1,2,3-triazol-4-yl)benzene) which exhibits an intense 1MLCT absorption band (l = 421 nm, e = 7550 dm3 mol-1 cm-1) and a Co(III)/Co(IV) oxidation process at +0.46 V vs Fc/Fc+ in acetonitrile. Ultrafast transient absorption spectroscopic measurements reveal that the 3MLCT excited state is depopulated on timescale of 650 fs to populate a 3MC state, which then decays to the ground state with a lifetime of 1.5 ns. The complex is emissive from the 3MC state at 77 K (max = 700 nm) in EtOH/MeOH (4:1) glass. Using this wavelength to approximate a value for E0-0 of 1.77 eV, an excited state oxidation potential Eox* of ca. -1.3 V can be estimated, indicating that the complex may be able to act as a competent photoreductant in stark contrast to recently highlighted potently photooxidising Co(III) diimine complexes.

Facial and meridional isomerism of metal complexes is known to result in fundamental differences in photophysical properties. One may also envisage differences in their photochemical reactivity and therefore predict different outcomes of their light-triggered transformations. The fac- and mer-isomers of the complex [Ru(pytz)₃]²⁺ (fac-1 & mer-1, pytz = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole) were separated and isolated. mer-1 undergoes a predicted pytz photodechelation process in acetonitrile to yield trans-[Ru(κ²-pytz)₂(κ¹-pytz)(NCMe)]²⁺ (2) whereas unfavorable interligand steric interactions are predicted to, and indeed do prevent comparable photoreactivity for fac-1. Reversible photoisomerization of fac-1 and mer-1 is also observed, however. The differences in photochemical reactivity of the two isomers can be rationalized based on structural programming of the preferential accessibility of particular ³MC excited states due to differences in their interligand steric interactions. Here we present an initial predictive thought experiment, subsequent experimental verification, and computational rationalization of the differences in photochemical reactivity of these two isomeric complexes.

Photochemical ligand release from metal complexes may be exploited in the development of novel photoactivated chemotherapy agents for the treatment of cancer and other diseases. Highly intriguing photochemical behavior is reported for two ruthenium(II) complexes bearing conformationally flexible 1,2,3-triazole-based ligands incorporating a methylene spacer to form 6-membered chelate rings. [Ru(bpy)(2)(pictz)](2+) (1) and [Ru(bpy)(2)(btzm)](2+) (2) (bpy = 2,2 '-bipyridyl; pictz = 1-(picolyl)-4-phenyl-1,2,3-triazole; btzm = bis(4-phenyl-1,2,3-triazol-4-yl)methane) exhibit coordination by the triazole ring through the less basic N2 atom as a consequence of chelation and readily undergo photochemical release of the pictz and btzm ligands (phi = 0.079 and 0.091, respectively) in acetonitrile solution to form cis-[Ru(bpy)(2)(NCMe)(2)](2+) (3) in both cases. Ligand-loss intermediates of the form [Ru(bpy)(2)(kappa(1)-pictz or kappa(1)-btzm)(NCCD3)](2+) are detected by H-1 NMR spectroscopy and mass spectrometry. Photolysis of 1 yields three ligand-loss intermediates with monodentate pictz ligands, two of which form through simple decoordination of either the pyridine or triazole donor with subsequent solvent coordination (4-tz((N2)) and 4-py, respectively). The third intermediate, shown to be able to form photochemically directly from 1, arises through linkage isomerism in which the monodentate pictz ligand is coordinated by the triazole N3 atom (4-tz((N3))) with a comparable ligand-loss intermediate with an N3-bound kappa(1)-btzm ligand also observed for 2.

Photoluminescent coordination complexes of Cr(III) are of interest as near-infrared spin-flip emitters. Here, we explore the preparation, electrochemistry, and photophysical properties of the first two examples of homoleptic N-heterocyclic carbene complexes of Cr(III), featuring 2,6-bis(imidazolyl)pyridine (ImPyIm) and 2-imidazolylpyridine (ImPy) ligands. The complex [Cr(ImPy)₃]³⁺ displays luminescence at 803 nm on the microsecond time scale (13.7 μs) from a spin-flip doublet excited state, which transient absorption spectroscopy reveals to be populated within several picoseconds following photoexcitation. Conversely, [Cr(ImPyIm)₂]³⁺ is nonemissive and has a ca. 500 ps excited-state lifetime.

To unravel the role of driving force and structural changes in directing the photoinduced pathways in donor–bridge–acceptor (DBA) systems, we compared the ultrafast dynamics in novel DBAs which share a phenothiazine (PTZ) electron donor and a Pt(II) trans-acetylide bridge (–CC–Pt–CC–), but bear different acceptors conjugated into the bridge (naphthalene-diimide, NDI; or naphthalene-monoimide, NAP). The excited state dynamics were elucidated by transient absorption, time-resolved infrared (TRIR, directly following electron density changes on the bridge/acceptor), and broadband fluorescence-upconversion (FLUP, directly following sub-picosecond intersystem crossing) spectroscopies, supported by TDDFT calculations. Direct conjugation of a strong acceptor into the bridge leads to switching of the lowest excited state from the intraligand ³IL state to the desired charge-separated ³CSS state. We observe two surprising effects of an increased strength of the acceptor in NDI vs. NAP: a ca. 70-fold slow-down of the ³CSS formation—(971 ps)⁻¹vs. (14 ps)⁻¹, and a longer lifetime of the ³CSS (5.9 vs. 1 ns); these are attributed to differences in the driving force ΔG_et, and to distance dependence. The 100-fold increase in the rate of intersystem crossing—to sub-500 fs—by the stronger acceptor highlights the role of delocalisation across the heavy-atom containing bridge in this process. The close proximity of several excited states allows one to control the yield of ³CSS from ∼100% to 0% by solvent polarity. The new DBAs offer a versatile platform for investigating the role of bridge vibrations as a tool to control excited state dynamics.

Guest Editors Paul I. Elliott, Katja Heinze and Thomas Teets introduce the spotlight collection, “Photoinduced redox chemistry”.

A detailed understanding of the dynamics of photoinduced processes occurring in the electronic excited state is essential in informing the rational design of photoactive transition-metal complexes. Here, the rate of intersystem crossing in a Cr(III)-centered spin-flip emitter is directly determined through the use of ultrafast broadband fluorescence upconversion spectroscopy (FLUPS). In this contribution, we combine 1,2,3-triazole-based ligands with a Cr(III) center and report the solution-stable complex [Cr(btmp)₂]³⁺ (btmp = 2,6-bis(4-phenyl-1,2,3-triazol-1-yl-methyl)pyridine) (1³⁺), which displays near-infrared (NIR) luminescence at 760 nm (τ = 13.7 μs, ϕ = 0.1%) in fluid solution. The excited-state properties of 1³⁺ are probed in detail through a combination of ultrafast transient absorption (TA) and femtosecond-to-picosecond FLUPS. Although TA spectroscopy allows us to observe the evolution of phosphorescent excited states within the doublet manifold, more significantly and for the first time for a complex of Cr(III), we utilize FLUPS to capture the short-lived fluorescence from initially populated quartet excited states immediately prior to the intersystem crossing process. The decay of fluorescence from the low-lying ⁴MC state therefore allows us to assign a value of (823 fs)^-1 to the rate of intersystem crossing. Importantly, the sensitivity of FLUPS to only luminescent states allows us to disentangle the rate of intersystem crossing from other closely associated excited-state events, something which has not been possible in the spectroscopic studies previously reported for luminescent Cr(III) systems.

G‐quadruplexes are emerging targets in cancer research and understanding how diagnostic probes bind to DNA G‐quadruplexes in solution is critical to the development of new molecular tools. In this study the binding of an enantiopure NIR emitting [Os(TAP)₂(dppz)]²⁺ complex to different G‐quadruplex structures formed by human telomer (hTel) and cMYC sequences in solution is reported. The combination of NMR and time‐resolved infrared spectroscopic techniques reveals the sensitivity of the emission response to subtle changes in the binding environment of the complex. Similar behaviour is also observed for the related complex [Os(TAP)₂(dppp2)]²⁺ upon quadruplex binding.

Ruthenium(II) complexes feature prominently in the development of agents for photoactivated chemotherapy; however, the excited-state mechanisms by which photochemical ligand release operates remain unclear. We report here a systematic experimental and computational study of a series of complexes [Ru(bpy)₂(N^∧N)]²⁺ (bpy = 2,2'-bipyridyl; N^∧N = bpy (1), 6-methyl-2,2'-bipyridyl (2), 6,6'-dimethyl-2,2'-bipyridyl (3), 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (4), 1-benzyl-4-(6-methylpyrid-2-yl)-1,2,3-triazole (5), 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl (6)), in which we probe the contribution to the promotion of photochemical N^∧N ligand release of the introduction of sterically encumbering methyl substituents and the electronic effect of replacement of pyridine by 1,2,3-triazole donors in the N^∧N ligand. Complexes 2 to 6 all release the ligand N^∧N on irradiation in acetonitrile solution to yield cis-[Ru(bpy)₂(NCMe)₂]²⁺, with resultant photorelease quantum yields that at first seem counter-intuitive and span a broad range. The data show that incorporation of a single sterically encumbering methyl substituent on the N^∧N ligand (2 and 5) leads to a significantly enhanced rate of triplet metal-to-ligand charge-transfer (³MLCT) state deactivation but with little promotion of photoreactivity, whereas replacement of pyridine by triazole donors (4 and 6) leads to a similar rate of ³MLCT deactivation but with much greater photochemical reactivity. The data reported here, discussed in conjunction with previously reported data on related complexes, suggest that monomethylation in 2 and 5 sterically inhibits the formation of a ³MC_cis state but promotes the population of ³MC_trans states which rapidly deactivate ³MLCT states and are prone to mediating ground-state recovery. On the other hand, increased photochemical reactivity in 4 and 6 seems to stem from the accessibility of ³MC_cis states. The data provide important insights into the excited-state mechanism of photochemical ligand release by Ru(II) tris-bidentate complexes.