A versatile approach to the preparation of [Ir(LL)2Cl2](PF6) type complexes is reported, in which LL is an N^N bound polypyridyl ligand [X2bpy, X2phen, where X = H-, CH3-, (CH3)3C-, or phenyl-, and bpy = 2,2′-bipyridyl, phen = 1,10-phenanthroline] as well as their deuterated analogues. In the synthesis of [Ir(bpy)2Cl2]PF6 (I) and its deuterated analogue (III), the cyclometallated complexes [Ir(bpy)2(bpy-C–N)](PF6)2 (II) and [Ir([D8]-bpy)2([D8]-bpy-C–N)](PF6)2 (IV) were also obtained. The complexes are characterised by 1H NMR, UV/Vis absorption and emission spectroscopy. The effect of deuteration on emission quantum yields and emission lifetimes is discussed. The ready availability of these complexes through efficient scaleable synthetics routes opens new opportunities for their use, especially in energy related applications.

•Cyclisisation/cycloreversion.•Photoswitches metal carbonyls.•Transition metal compounds.•Photochemistry.•Electrochemistry.In this review recent progress in combining metal carbonyl and bipyridyl transition metal complexes with dithienylethene photochromic switches is discussed. A key challenge in designing such systems is to allow for interaction between the various components, but without loss in photochemical activity, i.e. the cyclization/cycloreversion of the dithienylethene unit. Examples of where the properties of the switching unit can be fine-tuned either photochemically or electrochemically by the addition of either bipyridyl transition metals or organometallic moieties to the switch will be highlighted.In this review recent progress in combining metal carbonyl and bipyridyl transition metal complexes with dithienylethene photochromic switches is discussed.

The photophysics and photochemistry of [(CO)5MC(OMe)Me] (M = Cr or W) were investigated using picosecond time-resolved infrared spectroscopy (M = Cr or W), low-temperature matrix isolation techniques (M = Cr), and time-dependent density functional calculations (M = Cr or W). These studies provide unambiguous evidence for the photochemical formation of a long-lived, 18-electron metallaketene species capable of acting as a synthetically useful intermediate. For the Cr complex, an intermediate metallacyclopropanone singlet excited state was detected on the reaction path to the metallaketene species. This metallacyclopropanone excited state species has a lifetime of less than 100 ps and a characteristic bridging carbonyl band at 1770 cm−1. The tungsten ketene species was also detected but in contrast to the chromium system, this forms directly from a low-lying triplet excited state. The electrochemical release of CO showed a greater efficiency for the chromium complex when compared to the tungsten.

In this contribution the synthesis and characterisation of a series of novel mixed ligand iridium(III) complexes, functionalised with a carboxy ester or phosphonate groups are reported. These groupings are introduced on the 4-position of either the phenyl pyridine or the 2,2′-bipyridyl ligands. A low temperature high yield synthesis for the precursor [Ir(ppy-COOEt)2(μ-Cl)]2 was developed. The photophysical and electrochemical properties of these compounds are also described, together with their behaviour as photosensitisers for the generation of hydrogen from water.

A high yield synthetic route for the preparation of N6 coordinated heteroleptic Ir(III) complexes using bidentate polypyridyl type ligands is described. The complexes are near-blue emitters and show microsecond emission lifetimes, high emission quantum yields and have two quasi-reversible reduction processes between −1.0 and −1.3 V vs. Ag/AgCl.

Two porphyrin–cobaloxime complexes; [{Co(dmgH)2Cl}{MPyTPP}] (1a) and [{Co(dmgH)2Cl}{ZnMPyTPP}] (2a) (dmgH = dimethylglyoxime, MPyTPP = 5-(4-pyridyl)-10,15,20-triphenylporphyrin) have been synthesised as model systems for the generation of hydrogen from water. Although initially envisaged as photocatalytic systems neither complex catalysed the reduction of water to hydrogen following irradiation. However, both complexes are molecular precursors for hydrogen evolution under electrochemical conditions. Turnover numbers for hydrogen production of 1.8 × 103 and 5.1 × 103 were obtained for 1a and 2a respectively following potentiostatic electrolysis at −1.2 V vs. Ag/AgCl while cobaloxime alone produced a turnover-number of 8.0 × 103. The photophysical properties of 1a and 2a were examined to provide an explanation for the lack of photochemical activity. These results, coupled with quantum chemical calculations, confirm that porphyrins fail to act as light-harvesting units for these systems and that the lowest energy excited states are in fact cobaloxime-based rather than porphyrin based.

Multimodal photo, thermal and electrochemical approaches toward CO release from the amino carbene complex [(CO)5CrC(NC4H8)CH3] is reported. Picosecond time resolved infrared spectroscopy was used to probe the photo-induced early state dynamics leading to CO release, and DFT calculations confirmed that CO release occurs from a singlet excited state.

Two BODIPY–cobaloxime complexes; [{Co(dmgH)2Cl}{3-[bis-(4-ethyl-3,5-dimethyl-1H-pyrrol-2-yl)-methyl]-pyridine-borondiflouride}] (1a) and [{Co(dmgH)2Cl}{4-[bis-(4-ethyl-3,5-dimethyl-1H-pyrrol-2-yl)-methyl]-pyridine-borondiflouride}] (2a) (BODIPY = boron dipyrromethene), (dmgH = dimethylglyoxime) have been synthesised and studied as model catalytic systems for the generation of hydrogen gas in aqueous media. Under photochemical conditions, neither complex catalysed the reduction of water to hydrogen. However, both complexes showed considerable activity under electrochemical conditions. Turn-over-numbers for hydrogen production of 1.65 × 104 and 1.08 × 104 were obtained for 1a and 2a respectively following potentiostatic electrolysis at −1.2 V vs. Ag/AgCl after 1 hour. Quantum chemical calculations were performed to provide an explanation for the lack of photochemical activity.

The synthesis and characterization of a series of dithienyl perhydro- and perfluorocyclopentene photochromic molecular switches appended with cobalt carbonyl binding 3-ethynylthiophene and phenyl-3-ethynylthiophene substituents are reported. Their photochromic properties, fatigue resistance, and thermal stability were examined to establish the effect of substituents on their performance as molecular photoswitches. The photochemical properties of the dithienylethene core were retained to the greatest extent by the inclusion of phenyl units and a hexafluorocyclopentene ring. The alkyne units of the switches were used to coordinate cobalt carbonyl moieties: i.e., Co2(CO)6 and Co2(CO)4(dppm). The cobalt carbonyl moieties were found to reduce the efficiency of cyclization and cycloreversion of the dithienylethene unit. Density functional theory was used to identify the excited states responsible for cyclization.

The redox behavior of dithienyl perhydro- and perfluorocyclopentene photochromic molecular switches, modified with 3-ethynylthiophene and phenyl-3-ethynylthiophene substituents, is explored by cyclic voltammetry and UV/vis-NIR and IR spectroelectrochemistry. The extent of electrochemical oxidation induced cyclization was depedent on whether a perhydro- or perfluorocyclopentene unit was present, with the former favoring ring closure, and on the nature of the substituents on the thienyl ring. The inclusion of a phenyl spacer between the alkynyl and thienyl moieties increased the stability of the molecular switches when addressed electrochemically. Binding of Co2(CO)6 and Co2(CO)4dppm moieties to the alkyne units is shown to destabilize the cationic closed form and, in one example, inhibit oxidative cyclization for the 1,2-bis(5′-(4″-phenyl-3‴-ethynylthiophene)-2′-methylthien-3′-yl)perfluorocyclopentene [Co2(CO)6]2 complex (4Fo). However, the electrochemical cyclization observed for the Co2(CO)6 and Co2(CO)4dppm complexes of 1,2-bis(5′-(3″-ethynylthiophene)-2′-methylthien-3′-yl)cyclopentene (3Ho and 5Ho, respectively) was induced following oxidation of the cobalt carbonyl moieties (i.e., at lower potentials than oxidation of the open form of the dithienylethene), possibly via an intramolecular electron transfer mechanism and thereby providing an alternative route to control the electrochromic behavior of the switch.

The regioselective methylation of a ruthenium polypyridyl complex bearing both a 1,2,4-triazolato and a pyrazine moiety is reported. In contrast to previous studies in which methylation of the 1,2,4-triazolato ring was observed, in the present system methylation takes place exclusively at the non-coordinated nitrogen of the pyrazine ring. The monomethylation is confirmed by 1H NMR spectroscopy and ESI-MS and the electronic properties of the methylated complexes are studied by UV/vis absorption, emission, surface enhanced, resonance and transient resonance Raman spectroscopy. Ligand deuteriation is used to simplify the 1H NMR spectra and to assign definitively the Raman spectra. Acid–base studies show that the triazolato ring of the N-methylated complexes can be protonated at low pH and that at high pH the N-methyl group can be deprotonated reversibly. Furthermore it is shown that under conditions where the methyl group is deprotonated, demethylation occurs to recover the initial complex.

This review focuses on the synthesis, photophysical and electrochemical properties of thienyl porphyrins where processes such as electron transfer, energy transfer and electropolymerisation are discussed. The purpose of this review is to examine the influence of the thienyl ring, whether it be directly connected (via meso and β positions) or indirectly connected (via a covalent linker or axial coordination) on the ground and excited state electronic properties of the porphyrin macrocycle. Additionally, the importance of the electronic properties of a bridging oligothiophene between the porphyrin and another centre in supramolecular systems is discussed. Also included are applications of thienyl porphyrins in such areas as catalysis, therapeutics, (opto)electronics and electron-transfer/light-harvesting systems.

Time-resolved infrared spectroscopy was used to probe the photochemistry of three (μ2-alkyne)Co2(CO)6 complexes. The data indicate the formation of a triplet diradical species, with lifetimes in the range 38−71 ps. Theoretical calculations support these experimental findings. No evidence for the CO loss species, (μ2-alkyne)Co2(CO)5, was observed, and this is rationalized by the low quantum yield for this process at the excitation wavelengths used.

The femto- and picosecond time resolved spectroscopy of (η6-cis-stilbene)Cr(CO)3 provides evidence for three optically accessible excited states one leading to cis-trans isomerisation of the coordinated stilbene ligand and another which results in a slow release of CO.

Visible light driven decarbonylation of [(μ2-alkyne)Co2(CO)6] complexes enables the formation of cyclopentenones in good yields at ambient temperatures.

A series of pyrene based dyad systems together with their dicobalt hexacarbonyl complexes (1b−6b) were synthesized. The pyrene-thiophene dyads are luminescent in room temperature solution with luminescence lifetimes on the nanosecond time scale. At room temperature the dyad emission is quenched by coordination to a Co2(CO)6 moiety via an acetylene bridge. However, at 77 K this emission is not fully quenched following complexation. Electrochemical studies suggest that an intraligand state is responsible for the emission. Photochemical studies in the presence of PPh3 indicate that CO loss occurs following broadband irradiation with λexc > 400 nm, resulting in the formation of both -pentacarbonyl and -tetracarbonyl photoproducts.

The influence of the thiophene ring on the ground and excited state properties of the porphyrin ring is investigated, when substituted at the meso-position. A series of mono-, di-, tri-, and tetra-meso-thien-2-yl porphyrins are studied and discussed with respect to the reference compounds zinc(II)-5,10,15,20-tetra(thien-2′-yl)porphyrin (1a) and zinc(II)-5,10,15,20-tetraphenylporphyrin (ZnTPP). The extended conjugated system zinc(II)-5-(5′-(5′′-ethynyl-2′′-thiophenecarboxaldehyde)thien-2′-yl)-10,15,20-triphenylporphyrin (4d) is also studied and shows enhanced charge transfer character due to the presence of the terminal aldehyde accepting group. A detailed analysis of ground and excited state UV−vis absorption, steady-state and time-resolved fluorescence, laser flash photolysis, and electrochemical data all point toward substantial electronic communication between the central Zn(II) porphyrin ring and the meso-thien-2-yl substituents, which is evident from excited state charge transfer character.

The photochemistry of (η5-C4H4Se)Cr(CO)3 was investigated by matrix isolation, time-resolved infrared spectroscopy, and steady-state photochemical methods. Density functional theory (DFT) was used to assist in the identification of the photoproducts. Irradiation (λexc= 406 nm) of (η5-C4H4Se)Cr(CO)3 in either an Ar or CH4 matrix at 20 K produced the selenophene ring-opened insertion product (C,Se-C4H4Se)Cr(CO)3. Further irradiation of this matrix produced the CO-loss species (C,Se-C4H4Se)Cr(CO)2. Pulsed irradiation at 400 nm produced the CO-loss species (η5-C4H4Se)Cr(CO)2(S) in n-heptane (S) along with the insertion products (C,Se-C4H4Se)Cr(CO)3 and (C,Se-C4H4Se)Cr(CO)2, both of which may have triplet character. Time-resolved measurements on the microsecond time scale confirmed that the CO-loss species (η5-C4H4Se)Cr(CO)2(S) reacts with CO (k2 = 5.8 × 106 dm3 mol−1 s−1 at 298 K), while (C,Se-C4H4Se)Cr(CO)3 and (C,Se-C4H4Se)Cr(CO)2 do not react on this time scale. DFT calculations provide an explanation of the stability of the triplet (C,Se-C4H4Se)Cr(CO)3 species in terms of a chromaselanabenzene structure, which is consistent with previously observed metal insertion into coordinated selenophene ligands.

Two porphyrin–cobaloxime complexes; [{Co(dmgH)2Cl}{MPyTPP}] (1a) and [{Co(dmgH)2Cl}{ZnMPyTPP}] (2a) (dmgH = dimethylglyoxime, MPyTPP = 5-(4-pyridyl)-10,15,20-triphenylporphyrin) have been synthesised as model systems for the generation of hydrogen from water. Although initially envisaged as photocatalytic systems neither complex catalysed the reduction of water to hydrogen following irradiation. However, both complexes are molecular precursors for hydrogen evolution under electrochemical conditions. Turnover numbers for hydrogen production of 1.8 × 103 and 5.1 × 103 were obtained for 1a and 2a respectively following potentiostatic electrolysis at −1.2 V vs. Ag/AgCl while cobaloxime alone produced a turnover-number of 8.0 × 103. The photophysical properties of 1a and 2a were examined to provide an explanation for the lack of photochemical activity. These results, coupled with quantum chemical calculations, confirm that porphyrins fail to act as light-harvesting units for these systems and that the lowest energy excited states are in fact cobaloxime-based rather than porphyrin based.

Visible light driven decarbonylation of [(μ2-alkyne)Co2(CO)6] complexes enables the formation of cyclopentenones in good yields at ambient temperatures.

Multimodal photo, thermal and electrochemical approaches toward CO release from the amino carbene complex [(CO)5CrC(NC4H8)CH3] is reported. Picosecond time resolved infrared spectroscopy was used to probe the photo-induced early state dynamics leading to CO release, and DFT calculations confirmed that CO release occurs from a singlet excited state.

Two BODIPY–cobaloxime complexes; [{Co(dmgH)2Cl}{3-[bis-(4-ethyl-3,5-dimethyl-1H-pyrrol-2-yl)-methyl]-pyridine-borondiflouride}] (1a) and [{Co(dmgH)2Cl}{4-[bis-(4-ethyl-3,5-dimethyl-1H-pyrrol-2-yl)-methyl]-pyridine-borondiflouride}] (2a) (BODIPY = boron dipyrromethene), (dmgH = dimethylglyoxime) have been synthesised and studied as model catalytic systems for the generation of hydrogen gas in aqueous media. Under photochemical conditions, neither complex catalysed the reduction of water to hydrogen. However, both complexes showed considerable activity under electrochemical conditions. Turn-over-numbers for hydrogen production of 1.65 × 104 and 1.08 × 104 were obtained for 1a and 2a respectively following potentiostatic electrolysis at −1.2 V vs. Ag/AgCl after 1 hour. Quantum chemical calculations were performed to provide an explanation for the lack of photochemical activity.

The femto- and picosecond time resolved spectroscopy of (η6-cis-stilbene)Cr(CO)3 provides evidence for three optically accessible excited states one leading to cis-trans isomerisation of the coordinated stilbene ligand and another which results in a slow release of CO.

The regioselective methylation of a ruthenium polypyridyl complex bearing both a 1,2,4-triazolato and a pyrazine moiety is reported. In contrast to previous studies in which methylation of the 1,2,4-triazolato ring was observed, in the present system methylation takes place exclusively at the non-coordinated nitrogen of the pyrazine ring. The monomethylation is confirmed by 1H NMR spectroscopy and ESI-MS and the electronic properties of the methylated complexes are studied by UV/vis absorption, emission, surface enhanced, resonance and transient resonance Raman spectroscopy. Ligand deuteriation is used to simplify the 1H NMR spectra and to assign definitively the Raman spectra. Acid–base studies show that the triazolato ring of the N-methylated complexes can be protonated at low pH and that at high pH the N-methyl group can be deprotonated reversibly. Furthermore it is shown that under conditions where the methyl group is deprotonated, demethylation occurs to recover the initial complex.

In this contribution the synthesis and characterisation of a series of novel mixed ligand iridium(III) complexes, functionalised with a carboxy ester or phosphonate groups are reported. These groupings are introduced on the 4-position of either the phenyl pyridine or the 2,2′-bipyridyl ligands. A low temperature high yield synthesis for the precursor [Ir(ppy-COOEt)2(μ-Cl)]2 was developed. The photophysical and electrochemical properties of these compounds are also described, together with their behaviour as photosensitisers for the generation of hydrogen from water.

The photophysics and photochemistry of [(CO)5MC(OMe)Me] (M = Cr or W) were investigated using picosecond time-resolved infrared spectroscopy (M = Cr or W), low-temperature matrix isolation techniques (M = Cr), and time-dependent density functional calculations (M = Cr or W). These studies provide unambiguous evidence for the photochemical formation of a long-lived, 18-electron metallaketene species capable of acting as a synthetically useful intermediate. For the Cr complex, an intermediate metallacyclopropanone singlet excited state was detected on the reaction path to the metallaketene species. This metallacyclopropanone excited state species has a lifetime of less than 100 ps and a characteristic bridging carbonyl band at 1770 cm−1. The tungsten ketene species was also detected but in contrast to the chromium system, this forms directly from a low-lying triplet excited state. The electrochemical release of CO showed a greater efficiency for the chromium complex when compared to the tungsten.

A high yield synthetic route for the preparation of N6 coordinated heteroleptic Ir(III) complexes using bidentate polypyridyl type ligands is described. The complexes are near-blue emitters and show microsecond emission lifetimes, high emission quantum yields and have two quasi-reversible reduction processes between −1.0 and −1.3 V vs. Ag/AgCl.