D. M. Martino

Changes in excited-state character of [M(L1)(L2)(CO)2(α-diimine)] (M=Ru, Os) induced by variation of L1 and L2

Abstract The character of the lowest excited state of the d 6 -complexes [M(L 1 )(L 2 )(CO) 2 (α-diimine] (M=Ru, Os) varies with L 1 and L 2 . In the case of [Ru(Cl)(Me)(CO) 2 (α-diimine)] it has predominant metal-to-ligand charge transfer (MLCT) character. For [M(SnPh 3 ) 2 CO) 2 (α-diimine)] (M=Ru, Os), having two axial SnPh 3 ligands, it has σ (Sn M Sn)π*(α-diimine) (Sigma-Bond-to-Ligand Charge Transfer, SBLCT) character. In a glass at 90 K the emission lifetimes of the [Ru(SnPh 3 ) 2 (CO) 2 (α-diimine)] complexes are nearly a thousand times longer than those of the corresponding [Ru(Cl)(Me)(CO) 2 (α-diimine)] compounds, although their emission energies are very similar. In fact, these lifetimes are extremely long (e.g. τ =1070 μs for [Ru(SnPh 3 ) 2 (CO) 2 (4,4′-dimethyl-2,2′-bipyridine)]) for a CT state. Although replacement of Ru by Os decreases both the emission energy and lifetime of complexes such as [M(bpy) 3 ] 2+ (M=Ru, Os), only the emission lifetime decreases in the case of the [M(SnPh 3 ) 2 CO) 2 (α-diimine)] compounds. The latter effect is due to the increase of spin-orbit coupling (SOC), the influence of which could thus be studied without taking into account any energy-gap-law effect. Preliminary photophysical data are presented for [Pt(SnPh 3 ) 2 (Me) 2 ( i -Pr-DAB)], which has very similar excited state properties as the corresponding Ru- and Os-complexes. Replacement of a SnPh 3 ligand by a methyl group to give [Ru(Me)(SnPh 3 )(CO) 2 ( i -Pr-DAB)] increases the photoreactivity of the complex. The weaker Ru Me bond is broken homolytically and the formation of methyl radicals was studied in detail with FT-EPR spectroscopy.

FT-EPR study of alkyl radicals formed in the photochemical reaction of Re(alkyl)(α-diimine) and Ru(alkyl)(α-diimine) complexes

A Fourier transform EPR (FT-EPR) study was made of the photochemistry of [Re(R)(CO)3 (α-diimine)] and [Ru(E)(R)(CO)2(α-diimine)] complexes, where R = alkyl or benzyl, E = I or SnPh3, and α-diimine = 4,4′-dimethyl-2,2′-bipyridine (DMB) orN,N′-diisopropyl-1,4-diazabutadiene (iPr-DAB). Photoexcitation of these complexes leads to homolysis of the metal-alkyl (benzyl) bonds as evident from the detection of the spectra of the alkyl (benzyl) radicals. FT-EPR spectra display strong spin polarization effects attributed to Triplet Mechanism (TM) and Radical Pair Mechanism (RPM) Chemically Induced Dynamic Electron Polarization (CIDEP). CIDEP patterns point to bond dissociation via a triplet state precursor. For a number of complexes, spin polarization was found to exhibit unusually large solvent effects, whereas for one complex the CIDEP pattern proved to be sensitive to the wavelength of laser light used to initiate bond dissociation. These effects reflect the strong dependence of CIDEP on the character of the excited states involved in the photochemical reactions and contribute to the understanding of the reaction mechanism.

FT-EPR and HPLC study of the mechanism of 4-chlorophenol photolysis

The mechanism of 4-chlorophenol (4CP) photolysis was investigated with the aid of Fourier Transform Electron Paramagnetic Resonance (FT-EPR) and pulsed-laser photolysis combined with High Performance Liquid Chromatography (HPLC) detection-of stable (diamagnetic) products. With FT-EPR transient free radicals produced by pulsed-laser excitation of solutions of 4CP in alcohols could be identified. Time profiles of the FT-EPR spectra provided information on reaction kinetics and Chemically Induced Dynamic, Electron Polarization (CIDEP) effects. It was found that 4CP photolysis in alcohols leads to the simultaneous formation of the phenoxyl radical and radicals produced by hydrogen abstraction from the solvent. CIDEP patterns establish that these radicals are formed in a reaction sequence involving a triplet state precursor and radical pair intermediate. Results of earlier transient optical absorption measurements indicate that the triplet precursor must be the carbene 4-oxocyclohexa-2,5-dienylidene. This assignment is supported by the finding that photolysis of quinone diazide in a hydrogen-donating solvent gives the same free radical products as those obtained from 4CP. The formation of the phenoxyl radical intermediate accounts for the finding that photolysis of deoxygenated solutions of 4CP in alcohols gives phenol as stable diamagnetic product. By contrast, photolysis of aerated and deoxygenated aqueous solutions of 4CP produces benzoquinone and hydroquinone as primary products, respectively.

Time-resolved EPR study of 3C60 in solid matrices

Using time-resolved cw EPR and pulsed EPR techniques, a study was made of the characteristics of the photoexcited triplet state of C60 adsorbed in silica gel pores and embedded in a polymethylmethacrylate matrix. It was found that the time-resolved spectra from 3C60 in these matrices at room temperature retain the absorption/emission features of spectra from 3C60 in frozen solution (≤120 K). Apparently, the combination of molecular rotation and pseudo rotation (resulting from interconversion between Jahn-Teller states) is not fast enough, to lead to complete averaging of the dipole-dipole interaction between the unpaired electrons. The study explored the effect of introduction of solvent molecules in the silica gel pores on the 3C60 spectrum and quenching of the triplet by electron donors.

A closer look at photochemical reactions of transition-metal compounds by time-resolved EPR

A review is given of applications of time-resolved electron paramagnetic resonance (TREPR) in the field of photochemistry of transition-metal compounds. The two main TREPR techniques used in these studies are described. A brief overview is given of chemically induced dynamic electron polarization mechanisms that can affect TREPR spectra and that can give insights into the mechanism of photochemical reactions. Following these background sections, experimental results are presented. The discussion focuses in particular on the Fourier-transform EPR studies of photoinduced metal-alkyl bond homolysis reactions of a series of transition-metal (Co, Ru, Re, Pt) complexes carried out by the authors.

FT-EPR study of photoionization in micellar solutions

Fourier Transform EPR (FT-EPR) was used to study the formation and decay of free radicals produced by photoionization of phenothiazine (PTH) solubilized in aqueous SDS and Triton X-100 micellar solutions in the absence and presence of electron acceptors. CIDEP spectra produced by PTH photoionization in micellar solution differ from those found in homogeneous solution. The effect is attributed to changes in relative importance of single-photon, singlet excited state, and biphotonic, triplet excited state, photoionization. With quinone acceptors present in the bulk aqueous phase, photoionization of PTH in SDS, results in instantaneous formation of quinone anion radicals that carry the spin polarization of the precursor hydrated electrons. If the acceptor is anchored in the micelle, electron capture cannot compete with electron escape into the aqueous phase. Instead, anion radicals are formed primarily by reductive quenching of3PTH*. This process gives rise to a spectrum that is attributed to long-lived spin-correlated radical pairs, [PTH+…Q−].