Anna M. Oliver

Strong effects of the bridge configuration on photoinduced charge separation in rigidly linked donor-acceptor systems

Abstract Photoinduced electron-transfer rates are reported for two pairs of rigid bichromophoric molecules 1 (6)/ 2 (6) and 1 (8)/ 2 (8). In the first pair electron donor and acceptor are separated by six, in the second pair by eight, carbon—carbon σ bonds. While these σ bonds provide an all-trans coupling path in 1 (6) and 1 (8), that path contains s-cis elements in 2 (6) and 2 (8), which - as shown by X-ray structure data and by spectroscopic evidence - leads to a slight decrease in the effective, spatial donor-acceptor separation. Nevertheless, photoinduced electron transfer in each of the “stretched” compounds is about one order of magnitude faster than in the corresponding “bent” compound. This remarkable effect is interpreted as resulting from the unique ability of an all-trans array of σ bonds to mediate electronic through-bond interaction (TBI). Interestingly the solvent dependence of the rate of photoinduced electron transfer is significantly larger in the “bent” systems, thus indicating that superexchange via solvent molecules becomes competitive with TBI if an all-trans array is not available.

Donor, acceptor, and self-quenching of the giant-dipole state of a rigid, σ-bond separated, donor-acceptor molecular assembly

Abstract Photoexcitation of a molecular assembly consisting of the donor and acceptor moieties dimethoxynaphthalene and dicyanoethylene rigidly separated by an approximately 15 A long, saturated hydrocarbon bridge results in a charge-separated state with a dipole moment of 77 D and a lifetime of 740 ns in benzene. This giant dipole state is discharged by ground-state molecules and by the individual donor and acceptor moieties with close to diffusion-controlled rate constants.

Orbital symmetry effects on intramolecular charge recombination

Abstract Quantum yields and lifetimes were determined for charge transfer fluorescence accompanying the charge recombination, that follows photoinduced charge separation, in a series of four rigid D-bridge-A systems (1–4) each with overall molecular Cs symmetry and an effective bridge length of eight sigma bonds. From these data the electronic coupling (Hcr) between the ground state and the charge separated state was calculated. In 2–4 charge recombination is “symmetry allowed” and for these systems Hcr is consistently larger than for the “symmetry forbidden” charge recombination in 1. The results obtained indicate that such symmetry effects could modify the overall rate of intramolecular electron transfer by about one order of magnitude.

Intramolecular charge separation and recombination in non-polar environments via long-distance electron transfer through saturated hydrocarbon barriers

Abstract Time-resolved microwave conductivity and fluorescence spectroscopy techniques have been used to monitor the kinetics of charge separation and recombination following photo-excitation of donor-spacer-acceptor (DSA) molecules in which the spacer is a rigid saturated hydrocarbon bridge of length varying from 4.6 to 13.5 A. The solvents used were all completely non-polar saturated hydrocarbons with relative dielectric constants varying from 1.8 to 2.3. The lifetimes of the highly dipolar, charge-separated states formed increase initially with increasing length of the spacer but eventually decrease for distances longer than approximately 9 A. At that point the lifetime becomes sensitive to the dielectric constant of the medium and the temperature which was varied between 175 and 375 K. At the transition distance delayed donor fluorescence is observed. The results are explained in terms of the decrease in the Coulomb energy with increasing distance which raises the energy level of the charge separated state and eventually brings it close to the energy level of the locally excited donor (LED) state. Under these conditions charge recombination occurs preferentially via the LED state by thermally activated back electron transfer. The enegetics underlying this change in recombination mechanism are discussed.

A synthetic strategy for the construction of a novel series of buckminsterfullerene (C60) Ball-and-Chain molecules containing the porphyrin chromophore

Abstract A general method is described for synthesizing rigid “Ball-and-Chain” bichromophoric systems of the type C 60 -{rigid bridge}-porphyrin 2 .

Field dependent CIDNP from photochemically generated radical ion pairs in rigid bichromophoric systems

Abstract Field dependent chemically induced dynamic nuclear polarization (CIDNP) spectroscopy has been used to measure the exchange interaction, J , in photochemically generated radical ion pairs of rigid donor-bridge-acceptor molecules in benzene and dioxane solution. The positive and surprisingly large values of J show an exponential dependence on the bridge length. The magnetic field dependence of CIDNP is explained via the radical pair theory and yields, besides J , the rate constants for the back electron transfer from radical ion pair singlet and triplet states. Intersystem crossing is promoted by the weak magnetic interactions of the radical ion pair and hindered by the singlet–triplet energy gap. Therefore, the overall lifetime of the species is dominated by back electron transfer from the singlet state.

Charge recombination kinetics of giant dipoles in saturated hydrocarbon solvents

Abstract Long-distance, intramolecular charge recombination following photoexcitation of a series of rigid donor-insulator-acceptor molecules dissolved in n-hexane, cyclohexane and trans-decalin has been monitored using the time-resolved microwave conductivity (TRMC) technique. Two recombination pathways are found to be operative; a direct transition to the ground state which is temperature and solvent independent and an indirect route via the local excited donor which is thermally activated and dependent on the polarisability of the medium. The latter pathway becomes important when the charge-separated state and the local excited state are close to being degenerate.

A New Class of Giant Tetrads for Studying Aspects of Long-Range Intramolecular Electron Transfer Processes: Synthesis and Computational Studies

Theroleofsolvent-mediatedelectrontransfer can be studied in multichromophoric systems that possess “U-shaped” cavities (see scheme). One approach to building these geometries is through the use of Diels–Alder chemistry on norbornylogous scaffolds. 1H NMR and computational AM 1 and HF/3–21G studies indicate that the major isomer formed within this sequence has a “U-shape”. This conclusion is consistent with recently reported photophysical studies on the U-shaped system.

Synthesis and structural characterization of a novel pair of rigid diastereomeric triads

Abstract A method is described for constructing totally rigid triad (nichromophoric) systems, D 2-B 1-D 1-B 2-A, in which the chromophores D 2 (= dimethylaniline), D 1 (= 1,4-dimethoxynaphthalene), and A (= dicyanovinyl) are fused to rigid hydrocarbon bridges, B 1 and B 2, comprising linearly fused norbornane and bicyclo[2.2.0]hexane units.

Synthesis of totally rigid covalently linked bis-porphyrin systems for studying long-range electron transfer and energy transfer processes

Abstract The synthesis of novel rigid covalently linked bis-porphyrin systems, 3 – 5 , and the monoporphyrin model, 6 , is described. The synthetic strategy entails formation of o -phenylenediamine units, 11 , and their reaction with the dioxotetraphenylchlorin, 12 .