Singlet and Triplet State Back Electron Transfer from Photogenerated Radical Ion Pairs Studied by Time-Resolved CIDNP

Competition of singlet and triplet recombination of radical pairs in photoreactions of carboxy benzophenones and aromatic amino acids

Time-resolved chemically induced dynamic nuclear polarization (CIDNP) and transient absorption (TA) were applied to reveal the branching ratio of the singlet and triplet recombination channels in the reaction of short-lived radicals of carboxy benzophenones and the aromatic amino acids histidine, tryptophan, and tyrosine in neutral aqueous solution. It was established that the share of triplet recombination increases with increasing number of carboxylic groups: no triplet recombination was found for 4-carboxy benzophenone, whereas ∼13% of radicals of 4,4'-dicarboxy benzophenone (DCBP) and ∼27% of radicals of 3,3',4,4'-tetracarboxy benzophenone (TCBP) react with histidine radicals from the triplet state of radical pairs. The main idea is that the protonated (π,π*) triplet state of TCBP or DCBP is populated via back electron transfer from the ketyl radical of TCBP or DCBP to the radical of the amino acid. The protonated triplet state of the ketone decays with the formation of a metastable hydroxylated product, which is detected by TA. Taking into account triplet recombination provides excellent coincidence between experimental data and the simulated CIDNP kinetics.

Photoinduced Electron Transfer in Dyads with (R)-/(S)-Naproxen and (S)-Tryptophan

Short-lived intermediates arising from the donor-acceptor interaction of non-steroidal anti-inflammatory drug (NSAID) – (S)-naproxen (NPX) and its (R)-enantiomer with the tryptophan amino acid residue (Trp) have been studied by spin chemistry and photochemistry methods. The donor-acceptor interaction has caried out in a model linked system – dyad under the UV-irradiation. Interest in the NPX-Trp dyad diastereomers is connected with the possibility of using them as models of ligand-enzyme binding as long as amino acid residues are located at the enzyme’s active centers. It is these residues that interact with NSAID during the binding. It is widely thought that charge transfer processes are involved in the process of drug-enzyme binding. Withing this framework the role of charge transfer in NPX-Trp excited state quenching have been investigated. The analysis of the chemically induced dynamic nuclear polarization (CIDNP), as well as fluorescence kinetics and quantum yield in different polarity media has shown that the main channel of NPX fluorescence quenching is the intramolecular electron transfer between NPX and Trp fragments. Electron transfer rate constants and fluorescence quantum yields of diastereomers have demonstrated stereodifferentiation.

Elucidating organic reaction mechanisms using photo-CIDNP spectroscopy

CIDNP (chemically induced dynamic nuclear polarization) arises in radical pairs or biradicals but is detected in the diamagnetic reaction products. Hence, it can be used not only to identify and characterize both types of species but also to establish the pathways connecting precursors, paramagnetic intermediates and products, and to employ the polarizations as labels to individual nuclei. Recent applications of CIDNP to elucidate the mechanisms of photochemical reactions are reviewed, which illustrate all these facets.

ESR-Investigations on the Dynamic Solvent Effects of Degenerate Electron Exchange Reactions. Part I: Cyanobenzenes

The rates of degenerate electron exchange (electron self-exchange) of various cyanobenzenes have been measured by EPR line broadening technique in nine different solvents at room temperature. The molecules studied comprise besides benzene-1,2-dicarbonitrile, benzene-1,4-dicarbonitrile and benzene-1,2,4,5-tetracarbonitrile, the two isomeric tricyanobenzenes, benzene-1,2,3-tricarbonitrile and benzene-1,2,4-tricarbonitrile, the anion radicals of which have not been characterized before.

The experimentally observed rates vary from 4.5 × 108 to 44.0 × 108 M−1 s−1 and show the pronounced dependence on the longitudinal relaxation times, τL, of the solvents. The solvent dynamical effect so manifested is confirmed with remarkable clarity using solvents spanning a wide range of τL-values, which comprise acetonitrile (0.2 ps) and o-dichlorobenzene (6.0 ps) at its extremes. The rate constants are compared with Marcus theory using the continuum model (CM) and the mean spherical approximation (MSA) for the outer sphere reorganization energies and Nelson’s method for the inner sphere reorganization energies. Furthermore, an estimation of the resonance splitting energies, V RP, is given based on the experimental rates.

Charge Shift and Triplet State Formation in the 9-Mesityl-10-methylacridinium Cation

The target donor-acceptor compound forms an acridinium-like, locally excited (LE) singlet state on illumination with blue or near-UV light. This LE state undergoes rapid charge transfer from the acridinium ion to the orthogonally sited mesityl group in polar solution. The resultant charge-transfer (CT) state fluoresces in modest yield and decays on the nanosecond time scale. The LE and CT states reside in thermal equilibrium at ambient temperature; decay of both states is weakly activated in fluid solution, but decay of the CT state is activationless in a glassy matrix. Analysis of the fluorescence spectrum allows precise location of the relevant energy levels. Intersystem crossing competes with radiative and nonradiative decay of the CT state such that an acridinium-like, locally excited triplet state is formed in both fluid solution and a glassy matrix. Phosphorescence spectra position the triplet energy well below that of the CT state. The triplet decays via first-order kinetics with a lifetime of ca. 30 micros at room temperature in the absence of oxygen but survives for ca. 5 ms in an ethanol glass at 77 K. The quantum yield for formation of the LE triplet state is 0.38 but increases by a factor of 2.3-fold in the presence of iodomethane. The triplet reacts with molecular oxygen to produce singlet molecular oxygen in high quantum yield. In sharp contradiction to a recent literature report, there is no spectroscopic evidence to indicate the presence of an unusually long-lived CT state.