Daniel A. W. Ross

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.

Three ferrocene‐based rotary switches featuring 2‐pyridyl‐1,2,3‐triazole (pytri) “click” chelates have been developed. The rotors can be switched from a folded (syn) to extended (anti) conformation by the addition of [Cu(CH₃CN)₄](PF₆) and 6,6′‐dimesityl‐2,2′‐bipyridine. This extension process could be reversed by the addition of 1,4,8,11‐tetraazacyclotetradecane (cyclam) to sequester the Cu(I) ions. Disappointingly, the weakness of the pytri chelates prevented the development of an electrochemically switchable system.

Fundamental insights into the mechanism of triplet-excited-state interligand energy transfer dynamics and the 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 (³MLCT) states admixed with either ligand-centered (³LC) (1a, 2a, and 2b) or ligand-to-ligand charge transfer (³LL′CT) character (1c, 2c, and 3a–c). Particularly striking is the observation that pyrimidine-based complex 1b exhibits dual emission from both ³MLCT/³LC and ³MLCT/³LL′CT states. At 77 K, the ³MLCT/³LL′CT component is lost from the photoluminescence spectra of 1b, with emission exclusively arising from its ³MLCT/³LC state, while for 2c switching from ³MLCT/³LL′CT- to ³MLCT/³LC-based emission is observed. Femtosecond transient absorption data reveal distinct spectral signatures characteristic of the population of ³MLCT/³LC states for 1a, 2a, and 2b which persist throughout the 3 ns time frame of the experiment. These ³MLCT/³LC state signatures are apparent in the transient absorption spectra for 1c and 2c immediately following photoexcitation but rapidly evolve to yield spectral profiles characteristic of their ³MLCT/³LL′CT states. Transient data for 1b reveals intermediate behavior: the spectral features of the initially populated ³MLCT/³LC state also undergo rapid evolution, although to a lesser extent than that observed for 1c and 2c, behavior assigned to the equilibration of the ³MLCT/³LC and ³MLCT/³LL′CT states. Density functional theory (DFT) calculations enabled minima to be optimized for both ³MLCT/³LC and ³MLCT/³LL′CT states of 1a–c and 2a–c. Indeed, two distinct ³MLCT/³LC minima were optimized for 1a, 1b, 2a, and 2b distinguished by upon which of the two C^∧N ligands the excited electron resides. The ³MLCT/³LC and ³MLCT/³LL′CT states for 1b are very close in energy, in excellent agreement with experimental data demonstrating dual emission. Calculated vibrationally resolved emission spectra (VRES) for the complexes are in excellent agreement with experimental data, with the overlay of spectral maxima arising from emission from the ³MLCT/³LC and ³MLCT/³LL′CT states of 1b convincingly reproducing the observed experimental spectral features. Analysis of the optimized excited-state geometries enable the key structural differences between the ³MLCT/³LC and ³MLCT/³LL′CT states of the complexes to be identified and quantified. The calculation of interconversion pathways between triplet excited states provides for the first time a through-space mechanism for a photoinduced interligand energy transfer process. Furthermore, examination of structural changes between the possible emitting triplet excited states reveals the key bond vibrations that mediate energy transfer between these states. This work therefore provides for the first time detailed mechanistic insights into the fundamental photophysical processes of this important class of complexes.

Four new heteroleptic copper(I) complexes bearing either 2-pyridyl-1,2,3-triazole (pytri) or the related triphenylamine (TPA) substituted (TPApytri) ligands and the ancillary ligands 6,6′-dimesityl-2,2′-bipyridine (diMesbpy) or bis[(2-diphenylphosphino)phenyl] ether (POP) were synthesized in good yields (75-95%). All the complexes were extensively characterized using nuclear magnetic resonance (NMR) spectroscopies and electrospray ionization mass spectrometry (ESIMS) and in the case of the two pytri compounds the solid state structures were determined via X-ray crystallography. The pytri complexes showed MLCT absorption bands which shift from 433 nm for the diMesbpy complex to 347 nm for POP. TPA-pytri complexes introduce an ILCT band resulting in improved visible absorption (376 nm, 26400 M⁻¹ cm⁻¹ for [Cu(TPA-pytri)(diMesbpy)](PF₆)). Emission from this ILCT state (470 nm, Φ = 0.08) was redshifted compared to the free ligand with negligible effects from ancillary ligands. Band assignments were confirmed with resonance Raman spectroscopy and TD-DFT calculations.

A new “click” ligand, 2,6-bis(1-(pyridin-4-ylmethyl)-1H-1,2,3-triazol-4-yl)pyridine (L) featuring a tridentate 2,6-bis(1,2,3-triazol-4-yl)pyridine (tripy) pocket and two pyridyl (py) units was synthesized in modest yield (42%) using the copper(I) catalyzed azide-alkyne cycloaddition (CuAAC) reaction. The coordination chemistry of the ligand with silver(I) and iron(II) ions was examined using a battery of solution (¹H and DOSY (diffusion ordered spectroscopy) nuclear magnetic resonance (NMR), infrared and absorption spectroscopies, high-resolution electrospray ionization mass spectrometry (HR-ESI-MS)), and solid state (X-ray crystallography, elemental analysis) techniques. When treated with silver(I) ions, the ligand forms discrete [Ag(L)]⁺ (X⁻, where X⁻ = BF₄⁻, NO₃⁻ or SbF₆⁻) complexes in dimethyl sulfoxide (DMSO) solution but these complexes crystallize as coordination polymers. The addition of [Fe(H₂O)₆](BF₄)₂ to an acetonitrile solution of the ligand forms the expected monomeric octahedral [Fe(L)₂]²⁺ complex and treatment of the iron(II) complex with AgBF₄ generates a heterometallic linear coordination polymer.

Whilst [Os(N^N)₃]²⁺ complexes are supposed to be photochemically inert to ligand loss, the complex [Os(btz)₃]²⁺ (btz=1,1′‐dibenzyl‐4,4′‐bi‐1,2,3‐triazolyl) undergoes unprecedented photolytic reactivity to liberate free btz (Φ₃₆₃≈1.2 %). Further, both cis and trans isomers of the photodechelated ligand‐loss solvento intermediate [Os(κ²‐btz)₂(κ¹‐btz)(NCMe)]²⁺ are unambiguously observed and characterized by NMR spectroscopy and mass spectrometry.

Whilst [Os(N^N)₃]²⁺ complexes are supposed to be photochemically inert to ligand loss, the complex [Os(btz)₃]²⁺ (btz=1,1′‐dibenzyl‐4,4′‐bi‐1,2,3‐triazolyl) undergoes unprecedented photolytic reactivity to liberate free btz (Φ₃₆₃≈1.2 %). Further, both cis and trans isomers of the photodechelated ligand‐loss solvento intermediate [Os(κ²‐btz)₂(κ¹‐btz)(NCMe)]²⁺ are unambiguously observed and characterized by NMR spectroscopy and mass spectrometry.

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,1 n= 0,2 n= 1,3 n= 2,4 n= 3), have been prepared, characterised and used as phosphors within light-emitting electrochemical cells.