Probing organometallic reactions by time-resolved infrared spectroscopy in solution and in the solid state using quantum cascade lasers

Can aliphatic anchoring groups be utilised with dyes for p-type dye sensitized solar cells?

A series of novel laterally anchoring tetrahydroquinoline derivatives have been synthesized and investigated for their use in NiO-based p-type dye-sensitized solar cells. The kinetics of charge injection and recombination at the NiO-dye interface for these dyes have been thoroughly investigated using picosecond transient absorption and time-resolved infrared measurements. It was revealed that despite the anchoring unit being electronically decoupled from the dye structure, charge injection occurred on a sub picosecond timescale. However, rapid recombination was also observed due to the close proximity of the electron acceptor on the dyes to the NiO surface, ultimately limiting the performance of the p-DSCs.

Photochemistry of framework-supported M(diimine)(CO)3X complexes in three-dimensional lithium carboxylate metal-organic frameworks: monitoring the effect of framework cations

The structures and photochemical behaviour of two new metal-organic frameworks (MOFs) are reported. Reaction of Re(2,2'-bipy-5,5'-dicarboxylic acid)(CO)₃Cl or Mn(2,2'-bipy-5,5'-dicarboxylic acid)(CO)₃Br with LiCl or LiBr, respectively, produces single crystals of {Li₂(DMF)₂ [(2,2'-bipy-5,5'-dicarboxylate)Re(CO)₃Cl]}_n ( RELI: ) or {Li₂(DMF)₂[(2,2'-bipy-5,5'-dicarboxylate)Mn(CO)₃Br]}_n ( MNLI: ). The structures formed by the two MOFs comprise one-dimensional chains of carboxylate-bridged Li(I) cations that are cross-linked by units of Re(2,2'-bipy-5,5'-dicarboxylate)(CO)₃Cl ( RELI: ) or Mn(2,2'-bipy-5,5'- dicarboxylate)(CO)₃Br ( MNLI: ). The photophysical and photochemical behaviour of both RELI: and MNLI: are probed. The rhenium-containing MOF, RELI: , exhibits luminescence and the excited state behaviour, as established by time-resolved infrared measurements, is closer in behaviour to that of unsubstituted [Re(bipy)(CO)₃Cl] rather than a related MOF where the Li(I) cations are replaced by Mn(II) cations. These observations are further supported by density functional theory calculations. Upon excitation MNLI: forms a dicarbonyl species which rapidly recombines with the dissociated CO, in a fashion consistent with the majority of the photoejected CO not escaping the MOF channels.This article is part of the themed issue 'Coordination polymers and metal-organic frameworks: materials by design'.

From curiosity to applications. A personal perspective on inorganic photochemistry

Over the past several decades, the photochemistry and photophysics of transition metal compounds has blossomed from a relatively niche topic to a major research theme. Applications arising from the elucidation of the fundamental principles defining this field now range from probing the rates and mechanisms of small molecules with metalloproteins to light activated molecular machines. Offered here is a personal perspective of metal complex photochemistry drawn from this author's long involvement with this field. Several examples are described. Topics include characterizing key excited states and tuning these to modify chemical reactivity and/or photoluminescence properties, as well as using photoreactions as an entry to reactive intermediates relevant to homogeneous catalysts. This is followed by discussions of applying these concepts to developing precursors and precursor-antenna conjugates for the photochemical delivery of small molecule bioregulators to physiological targets.

Synthesis and Photophysical Study of a [NiFe] Hydrogenase Biomimetic Compound Covalently Linked to a Re-diimine Photosensitizer

The synthesis, photophysics, and photochemistry of a linked dyad ([Re]-[NiFe₂]) containing an analogue ([NiFe₂]) of the active site of [NiFe] hydrogenase, covalently bound to a Re-diimine photosensitizer ([Re]), are described. Following excitation, the mechanisms of electron transfer involving the [Re] and [NiFe₂] centers and the resulting decomposition were investigated. Excitation of the [Re] center results in the population of a diimine-based metal-to-ligand charge transfer excited state. Reductive quenching by NEt₃ produces the radically reduced form of [Re], [Re]⁻ (k_q = 1.4 ± 0.1 × 10⁷ M^–1 s^–1). Once formed, [Re]⁻ reduces the [NiFe₂] center to [NiFe₂]⁻, and this reduction was followed using time-resolved infrared spectroscopy. The concentration dependence of the electron transfer rate constants suggests that both inter- and intramolecular electron transfer pathways are involved, and the rate constants for these processes have been estimated (k_inter = 5.9 ± 0.7 × 10⁸ M^–1 s^–1, k_intra = 1.5 ± 0.1 × 10⁵ s^–1). For the analogous bimolecular system, only intermolecular electron transfer could be observed (k_inter = 3.8 ± 0.5 × 10⁹ M^–1 s^–1). Fourier transform infrared spectroscopic studies confirms that decomposition of the dyad occurs upon prolonged photolysis, and this appears to be a major factor for the low activity of the system toward H₂ production in acidic conditions.

Excitation of the [Re] center in the linked-dyad complex ([Re]-[NiFe₂]) populates the ³MLCT excited state, and reductive quenching by NEt₃ produces [Re]⁻. [Re]⁻ reduces the [NiFe₂] center to [NiFe₂]⁻ via inter- and intramolecular electron transfer pathways (k_inter = 5.9 ± 0.7 × 10⁸ M⁻¹ s⁻¹, k_intra = 1.5 ± 0.1 × 10⁵ s⁻¹). For the analogous bimolecular system, where only intermolecular electron transfer could be observed, k_inter = 3.8 ± 0.5 × 10⁹ M⁻¹ s⁻¹.

Comparison of rhenium-porphyrin dyads for CO2 photoreduction: photocatalytic studies and charge separation dynamics studied by time-resolved IR spectroscopy

We report a study of the photocatalytic reduction of CO₂ to CO by zinc porphyrins covalently linked to [Re^I(2,2'-bipyridine)(CO)₃L]^+/0 moieties with visible light of wavelength >520 nm. Dyad 1 contains an amide C₆H₄NHC(O) link from porphyrin to bipyridine (Bpy), Dyad 2 contains an additional methoxybenzamide within the bridge C₆H₄NHC(O)C₆H₃(OMe)NHC(O), while Dyad 3 has a saturated bridge C₆H₄NHC(O)CH₂; each dyad is studied with either L = Br or 3-picoline. The syntheses, spectroscopic characterisation and cyclic voltammetry of Dyad 3 Br and [Dyad 3 pic]OTf are described. The photocatalytic performance of [Dyad 3 pic]OTf in DMF/triethanolamine (5 : 1) is approximately an order of magnitude better than [Dyad 1 pic]PF₆ or [Dyad 2 pic]OTf in turnover frequency and turnover number, reaching a turnover number of 360. The performance of the dyads with Re-Br units is very similar to that of the dyads with [Re-pic]⁺ units in spite of the adverse free energy of electron transfer. The dyads undergo reactions during photocatalysis: hydrogenation of the porphyrin to form chlorin and isobacteriochlorin units is detected by visible absorption spectroscopy, while IR spectroscopy reveals replacement of the axial ligand by a triethanolaminato group and insertion of CO₂ into the latter to form a carbonate. Time-resolved IR spectra of [Dyad 2 pic]OTf and [Dyad 3 pic]OTf (560 nm excitation in CH₂Cl₂) demonstrated electron transfer from porphyrin to Re(Bpy) units resulting in a shift of ν(CO) bands to low wavenumbers. The rise time of the charge-separated species for [Dyad 3 pic]OTf is longest at 8 (±1) ps and its lifetime is also the longest at 320 (±15) ps. The TRIR spectra of Dyad 1 Br and Dyad 2 Br are quite different showing a mixture of ³MLCT, IL and charge-separated excited states. In the case of Dyad 3 Br, the charge-separated state is absent altogether. The TRIR spectra emphasize the very different excited states of the bromide complexes and the picoline complexes. Thus, the similarity of the photocatalytic data for bromide and picoline dyads suggests that they share common intermediates. Most likely, these involve hydrogenation of the porphyrin and substitution of the axial ligand at rhenium.