New insights into the photochemistry of [CpFe(CO)2]2 using picosecond through microsecond time-resolved infrared spectroscopy (TRIR)

[Fp(CH4)]+, [η5-CpRu(CO)2(CH4)]+, and [η5-CpOs(CO)2(CH4)]+: A Complete Set of Group 8 Metal-Methane Complexes

Here, we report the NMR spectroscopic analysis of the group 8 transition metal methane σ-complexes [η⁵-CpM(CO)₂(CH₄)][Al(OC(CF₃)₃)₄] (M = Fe, Ru) at -90 °C in the weakly coordinating solvent 1,1,1,3,3,3-hexafluoropropane. The iron(II)-methane complex has a ¹H resonance at δ -4.27, a ¹³C resonance at δ -53.0, and ¹J_C-H = 126 Hz for the bound methane fragment. The ruthenium(II)-methane complex has a ¹H resonance at δ -2.10, a ¹³C resonance at δ -48.8, and a ¹J_C-H = 126 Hz for the bound methane fragment. DFT and ab initio calculations support these experimental observations and provide further detail on the structures of the [η⁵-CpM(CO)₂(CH₄)]⁺ (M = Fe, Ru) complexes of the Group 8 metals. Both the iron centered methane complex, [η⁵-CpFe(CO)₂(CH₄)][Al(OC(CF₃)₃)₄], and the ruthenium centered methane complex, [η⁵-CpRu(CO)₂(CH₄)][Al(OC(CF₃)₃)₄], are significantly less stable than the previously reported osmium-methane complex [η⁵-CpOs(CO)₂(CH₄)][Al(OC(CF₃)₃)₄].

Photochemistry of transition metal carbonyls

The purpose of this Tutorial Review is to outline the fundamental photochemistry of metal carbonyls, and to show how the advances in technology have increased our understanding of the detailed mechanisms, particularly how relatively simple experiments can provide deep understanding of complex problems. We recall some important early experiments that demonstrate the key principles underlying current research, concentrating on the binary carbonyls and selected substituted metal carbonyls. At each stage, we illustrate with examples from recent applications. This review first considers the detection of photochemical intermediates in three environments: glasses and matrices; gas phase; solution. It is followed by an examination of the theory underpinning these observations. In the final two sections, we briefly address applications to the characterization and behaviour of complexes with very labile ligands such as N₂, H₂ and alkanes, concentrating on key mechanistic points, and also describe some principles and examples of photocatalysis.

Picosecond to millisecond tracking of a photocatalytic decarboxylation reaction provides direct mechanistic insights

The photochemical decarboxylation of carboxylic acids is a versatile route to free radical intermediates for chemical synthesis. However, the sequential nature of this multi-step reaction renders the mechanism challenging to probe. Here, we employ a 100 kHz mid-infrared probe in a transient absorption spectroscopy experiment to track the decarboxylation of cyclohexanecarboxylic acid in acetonitrile-d₃ over picosecond to millisecond timescales using a photooxidant pair (phenanthrene and 1,4-dicyanobenzene). Selective excitation of phenanthrene at 256 nm enables a diffusion-limited photoinduced electron transfer to 1,4-dicyanobenzene. A measured time offset in the rise of the CO₂ byproduct reports on the lifetime (520 ± 120 ns) of a reactive carboxyl radical in solution, and spectroscopic observation of the carboxyl radical confirm its formation as a reaction intermediate. Precise clocking of the lifetimes of radicals generated in situ by an activated C-C bond fission will pave the way for improving the photocatalytic selectivity and turnover.

A lifetime in photochemistry; some ultrafast measurements on singlet states

We describe here the development of time-correlated single-photon counting techniques from the early use of spark discharge lamps as light sources through to the use of femtosecond mode-locked lasers through the personal work of the author. We used laser-excited fluorescence in studies on energy migration and rotational relaxation in synthetic polymer solutions, in biological probe molecules and in supersonic jet expansions. Time-correlated single-photon counting was the first method used in early fluorescence lifetime imaging microscopy (FLIM), and we outline the development of this powerful technique, with a comparison of techniques including wide-field microscopy. We employed these modern forms of FLIM to study single biological cells, and applied FLIM also to gain an understanding the distribution in tissue, and fates of photosensitizer molecules used in photodynamic therapy. We also describe the uses and instrumental design of laser systems for the study of ultrafast time-resolved vibrational spectroscopy.