Electron transfer (ET) from donor to acceptor is often mediated by nuclear-electronic (vibronic) interactions in molecular bridges. Using an ultrafast electronic-vibrational-vibrational pulse-sequence, we demonstrate how the outcome of light-induced ET can be radically altered by mode-specific infrared (IR) excitation of vibrations that are coupled to the ET pathway. Picosecond narrow-band IR excitation of high-frequency bridge vibrations in an electronically excited covalent trans-acetylide platinum(II) donor-bridge-acceptor system in solution alters both the dynamics and the yields of competing ET pathways, completely switching a charge separation pathway off. These results offer a step toward quantum control of chemical reactivity by IR excitation.

Nuclear-electronic (vibronic) coupling is increasingly recognized as a mechanism of major importance in controlling the light-induced function of molecular systems. It was recently shown that infrared light excitation of intramolecular vibrations can radically change the efficiency of electron transfer, a fundamental chemical process. We now extend and generalize the understanding of this phenomenon by probing and perturbing vibronic coupling in several molecules in solution. In the experiments an ultrafast electronic-vibrational pulse sequence is applied to a range of donor-bridge-acceptor Pt(II) trans-acetylide assemblies, for which infrared excitation of selected bridge vibrations during ultraviolet-initiated charge separation alters the yields of light-induced product states. The experiments, augmented by quantum chemical calculations, reveal a complex combination of vibronic mechanisms responsible for the observed changes in electron transfer rates and pathways. The study raises new fundamental questions about the function of vibrational processes immediately following charge transfer photoexcitation, and highlights the molecular features necessary for external vibronic control of excited-state processes.

A linear asymmetric Pt(ii) trans-acetylide donor-bridge-acceptor triad designed for efficient charge separation, NAP[triple bond, length as m-dash]Pt(PBu3)2[triple bond, length as m-dash]Ph-CH2-PTZ (), containing strong electron acceptor and donor groups, 4-ethynyl-N-octyl-1,8-naphthalimide (NAP) and phenothiazine (PTZ) respectively, has been synthesised and its photoinduced charge transfer processes characterised in detail. Excitation with 400 nm, ∼50 fs laser pulse initially populates a charge transfer manifold stemming from electron transfer from the Pt-acetylide centre to the NAP acceptor and triggers a cascade of charge and energy transfer events. A combination of ultrafast time-resolved infrared (TRIR) and transient absorption (TA) spectroscopies, supported by UV-Vis/IR spectroelectrochemistry, emission spectroscopy and DFT calculations reveals a self-consistent photophysical picture of the excited state evolution from femto- to milliseconds. The characteristic features of the NAP-anion and PTZ-cation are clearly observed in both the TRIR and TA spectra, confirming the occurrence of electron transfer and allowing the rate constants of individual ET-steps to be obtained. Intriguingly, has three separate ultrafast electron transfer pathways from a non-thermalised charge transfer manifold directly observed by TRIR on timescales ranging from 0.2 to 14 ps: charge recombination to form either the intraligand triplet (3)NAP with 57% yield, or the ground state, and forward electron transfer to form the full charge-separated state (3)CSS ((3)[PTZ(+)-NAP(-)]) with 10% yield as determined by target analysis. The (3)CSS decays by charge-recombination to the ground state with ∼1 ns lifetime. The lowest excited state is (3)NAP, which possesses a long lifetime of 190 μs and efficiently sensitises singlet oxygen. Overall, molecular donor-bridge-acceptor triad demonstrates excited state branching over 3 different pathways, including formation of a long-distant (18 Å) full charge-separated excited state from a directly observed vibrationally hot precursor state.

The copper-catalysed cycloaddition of alkynes and azides to form 1,2,3-triazoles has emerged as a powerful tool in ligand design and the synthesis of novel transition metal complexes. In this review we focus on the photophysical properties of metal complexes bearing 1,2,3-triazole-based ligands with a particular emphasis on those of d 6 metals including rhenium(I), iron(II), ruthenium(II), osmium(II) and iridium(III). We also highlight key examples of triazole complexes of platinum(II) and palladium(II) as well as the lanthanides and coinage metals. 1. Introduction 2. Photophysical properties of d 6 metal triazole-based complexes 2.1 Rhenium(I) complexes 2.2 Iridium(III) complexes 2.3 Ruthenium(II) complexes 2.4 Iron(II) complexes 2.5 Osmium(II) complexes 2.6 Photochemistry of 1,2,3-triazole-based d 6 metal complexes 3. Platinum(II) and palladium(II) complexes 4. Coinage metal complexes 5. Lanthanide complexes 6. Triazole-based sensors for metal ions 7. Conclusions & outlook.

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%.

Fundamental insights into the mechanism of triplet excited state interligand energy transfer dynamics and origin of dual emission for phosphorescent iridium(III) complexes are presented. The complexes [Ir(C^N) 2 (N^N)] + (HC^N = 2-phenylpyridine (1a-c), 2-(2,4-difluorophenyl)pyridine (2a-c), 1-benzyl-4-phenyl-1,2,3-triazole (3a-c); N^N = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (pytz, a), 1-benzyl-4-(pyrimidin-2-yl)-1,2,3-triazole (pymtz, b), 1-benzyl-4-(pyrazin-2-yl)-1,2,3-triazole (pyztz, c)) are phosphorescent in room temperature fluid solutions from triplet metal-to-ligand charge transfer

The photophysical properties of transition metal complexes have long attracted interest in the literature with significant research activity during the past two to three decades due to the potential exploitation of these materials in solar energy conversion, light-emitting technology, luminescence biological imaging and photodynamic therapeutic applications to name but a few. Since the advent of the facile preparation of 1,2,3-triazole-based compounds through copper(i)-catalysed cycloaddition, ligands based on this heterocycle have received widespread attention in coordination chemistry. Inevitably, their ability to be used as pyridine-like analogues has resulted in significant attention on the photophysical properties of their resultant complexes. There are, however, two sides to this tale; on the one hand, routes to 1,2,3-triazoles have enabled the realisation of highly tunable and efficient phosphors and photosensitisers. On the other hand, 1,2,3-triazole-based complexes have allowed highly novel photochemical processes to be explored offering insights into hitherto unappreciated excited state dynamics. This Perspective review covers the developments of photophysically active triazole-based complexes over the last decade, highlighting some of the key discoveries from our own laboratory as well as seminal contributions from other groups who are active in the area. We also identify possible new avenues for investigation and exploitation stemming from the work so far.

We report the synthesis, characterisation and photochemical reactivity of the triazolecontaining complex [Ru(pytz)(btz) 2 ] 2+ (1, pytz = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole, btz = 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl). The UV-visible absorption spectrum of 1 exhibits pytz- and that the T 1 state from a single point triplet state calculation at the S 0 geometry suggests 3 MC character. Optimisation of the T 1 state of the complex starting from the ground state geometry leads to elongation of the two Ru-N(btz) bonds cis to the pytz ligand to 2.539 and 2.544 Å leading to a psuedo 4-coordinate 3 MC state rather than the 3 MLCT state. The work therefore provides additional insights into the photophysical and photochemical properties of ruthenium triazole-containing complexes and their excited state dynamics.2