Henk Oevering

Distance dependence of photoinduced electron transfer through non-conjugated bridges

Abstract Picosecond time-resolved emission studies of a series of molecules containing an electron donor-acceptor pair interconnected by a series of rigid non-conjugated bridges reveal the occurrence of very fast photoinduced intramolecular electron transfer. The length of the bridge was varied to provide donor-acceptor centre-to-centre separations ranging from 8.1 to 13.3 A (edge-to-edge 5 to 10.2 A). At centre-to-centre separations up to 10.7 A the rate of photoinduced electron transfer exceeded 5×10 10 s −1 (τ 10 s −1 (τ = 68 ps).

Charge-transfer absorption and emission resulting from long-range through-bond interaction; exploring the relation between electronic coupling andelectron-transfer in bridged donor-acceptor systems.

Abstract The electronic absorption- and emission spectra of seven (D)onor-(A)cceptor systems are studied with the general structure D-bridge-A, where the bridge consists of an extended, rigid, saturated hydrocarbon skeleton that separates D and A by distances ranging from 3 to 12 CC σ-bonds. Across bridges with a length up to six σ-bonds sufficient electronic interaction occurs to cause a detectable perturbation of the electronic absorption spectra and for lower homologues this leads to the appearance of discrete intramolecular charge-transfer absorptions with an intensity that is strongly enhanced by intensity borrowing from symmetry-matched local transitions. In the fluorescence spectra discrete charge-transfer (CT) type emission has been detected for bridge lengths up to ten σ-bonds. The radiative transition probability of this CT emission provides a direct measure for the electronic coupling matrix element (H da ) between the charge-separated- and the groundstate. The magnitude of H da is found to decrease exponentially with the number of intervening σ-bonds from 850 cm −1 at 3-bond separation to 17.6 cm −1 at 10-bond separation. Furthermore the rate of charge-recombination in the compounds studied is found to be proportional to the square of H da , thus providing an experimental verification of this often implied “golden rule” relation.

Long-range exchange contribution to singlet-singlet energy transfer in a series of rigid bichromophoric molecules

Abstract Intramolecular singlet-singlet energy transfer is reported in a series of compounds containing a 1,4-dimethoxy-naphthalene chromophore as the energy donor and a cyclic ketone as the energy acceptor connected by rigid, elongated, saturated hydrocarbon bridges with an effective length of 4, 6, and 8 CC σ bonds. The rate of energy transfer is found to be proportional to the spectral overlap - as varied by solvent variation - and to show an exponential distance dependence while its magnitude significantly exceeds that predicted for a dipole-dipole coupling mechanism. From this it is concluded that energy transfer occurs predominantly via an exchange mechanism. Exchange integrals of 60, 10, and 2.5 cm −1 across 4, 6, and 8 σ bonds are calculated. The magnitude of these is proposed to signify through-bond exchange interaction between symmetry-matched donor (ππ*) and acceptor (nπ*) states.

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.

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.

On a long-range exchange mechanism for energy transfer in rigid bichromophoric molecules

Abstract The comment of Speiser and Rubin regarding the impossibility of long-range, intramolecular exchange energy transfer is refuted by comparison with recent data for other phenomena, e.g. electron transfer, that depend upon electronic interaction between nonconjugatively connected chromophores.