Jan Kroon

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.

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.

Through-bond modulation of intramolecular electron transfer in rigidly linked donor-acceptor systems

Abstract Photoinduced electron transfer rates are reported for a pair of rigid bichromophoric molecules, 1 (8) and 3 (8), in a variety of solvents. It was found that intramolecular electron transfer in 1 (8) is up to five times faster than in 3 (8). X-ray crystallography revealed that the cyclopentyl ring attached to the dicyanoethylene group is “flatter” in 3 (8) compared to 1 (8), and this results in diminished through-bond coupling in 3 (8) compared to 1 (8). This conclusion is reinforced by the observation that the calculated through-bond π, π and π * interaction energies in dienes 7 (6) and 9 (6) are larger than those in the dienes 8 (6) and 10 (6), respectively.

Rigid donor—bridge—acceptor systems: photoinduced charge transfer in solution and in a supersonic jet

Abstract In this paper the fluorescent behaviour is studied under jet-cooled conditions as well as in solution of two donor—bridge—acceptor molecules ( 1 and 2 ) in which the donor (D) and acceptor (A) are held rigidly apart by an extended saturated hydrocarbon bridge with an effective length of three sigma-bonds. It was found that even in the isolated molecules no excess excitation energy at all is needed to induce charge separation and that charge transfer (CT) fluorescence can be observed from both these molecules. Furthermore, broad red-shifted excitation bands are observed indicating that excitation can take place directly from the ground state into the CT state, even at energies considerably lower than that required to reach any of the locally (in D or A) excited states. Comparison of the radiative rate constants for CT fluorescence in media of different polarity indicates important intensity borrowing from local transitions in media of low polarity. Nevertheless, the admixture of locally excited configurations in the CT state is too small to diminish significantly the CT nature of the emissive state even in the gas phase.

(Sub)nanosecond intramolecular hole transfer through a rigid, 10-sigma-bond, saturated-hydrocarbon bridge

Abstract Hole transfer from the naphthalene radical cation to dimethoxynaphthalene occurs on a timescale of nanoseconds or less even when the aromatic moieties are held apart by a rigid, saturated-hydrocarbon spacer 10 σ bonds and at least 12 A long.

Kinetic and Spectroscopic Investigation of The Influence of Conformation and Orbital-Symmetry on Long-Range Intramolecular Donor-Acceptor Interaction

In conformationally well defined systems where an electron-donor (D) and acceptor (A) are interconnected by one ore more arrays of saturated carbon-carbon bonds the D/A interaction is influenced by changes in the length as well as in the configuration of these arrays. This not only affects the kinetics of (photoinduced) electron transfer between D and A, but also the electronic absorption and emission spectra are modified as a result of the presence of a new transition of the charge-transfer type. It is shown that especially charge-transfer emission can yield detailed information about the conformational and orbital-symmetry dependence of D/A interaction, while the effect of these parameters on electron transfer kinetics tends to be masked by concomitant changes in Franck-Condon factors.