Bridged Donor-acceptor Systems Research Articles

Hopping versus Tunneling Mechanism for Long-Range Electron Transfer in Porphyrin Oligomer Bridged Donor-Acceptor Systems.

Achieving long-range charge transport in molecular systems is interesting to foresee applications of molecules in practical devices. However, designing molecular systems with pre-defined wire-like properties remains difficult due to the lack of understanding of the mechanism for charge transfer. Here we investigate a series of porphyrin oligomer-bridged donor-acceptor systems Fc-Pn-C60 (n = 1-4, 6). In these triads, excitation of the porphyrin-based bridge generates the fully charge-separated state, Fc(•+)-Pn-C60(•-), through a sequence of electron transfer steps. Temperature dependence of both charge separation (Fc-Pn*-C60 → Fc-Pn(•+)-C60(•-)) and recombination (Fc(•+)-Pn-C60(•-) → Fc-Pn-C60) processes was probed by time-resolved fluorescence and femtosecond transient absorption. In the long triads, two mechanisms contribute to recombination of Fc(•+)-Pn-C60(•-) to the ground state. At high temperatures (≥280 K), recombination via tunneling dominates for the entire series. At low temperatures (<280 K), unusual crossover from tunneling to hopping occurs in long triads. This crossover is rationalized by the increased lifetimes of Fc(•+)-Pn-C60(•-), hence the higher probability of reforming Fc-Pn(•+)-C60(•-) during recombination. We demonstrate that at 300 K, the weak distance dependence for charge transfer (β = 0.028 Å(-1)) relies on tunneling rather than hopping.

The dependence of the electronic coupling on energy gap and bridge conformation – Towards prediction of the distance dependence of electron transfer reactions

The attenuation factor, β, for the distance dependence of electron exchange reactions is a sensitive function of the donor–bridge energy gap and bridge conformation. In this work the electronic coupling for electron and triplet excitation energy transfer has been investigated for five commonly used repeating bridge structures. The investigated bridge structures are OF (oligo fluorene), OP (oligo phenylene), OPE (oligo p-phenyleneethynylene), OPV (oligo phenylenevinylene), and OTP (oligo thiophene). Firstly, the impact of the donor–bridge energy gap was investigated by performing calculations with a variety of donors appended onto bridges that were kept in a planar conformation. This resulted in, to our knowledge, the first presented sets of bridge specific parameters to be inserted into the commonly used McConnell model. Secondly, since at experimental conditions large conformational flexibility is expected, a previously developed model that takes conformational disorder of the bridge into account has been applied to the investigated systems [M.P. Eng, T. Ljungdahl, J. Mårtensson, B. Albinsson, J. Phys. Chem. B 110 (2006) 6483]. This model is based on Boltzmann averaging and has been shown to describe the temperature dependence of the attenuation factor through OPE-bridges. Together, the parameters describing the donor–bridge energy gap dependence, for planar bridge structures, and the Boltzmann averaging procedure, describing the impact of rotational disorder, have the potential to a priori predict attenuation factors for electron and excitation energy transfer reactions through bridged donor–acceptor systems.

Are donor–acceptor self organised aromatic systems NLO (non-linear optical) active?

This contribution to the understanding of non-linear optical (NLO) properties of organic systems is concentrated on donor–acceptor aromatic systems such as [2]catenanes. We report accurate ab initio quantum chemical calculations of the first static hyperpolarizability ( β) of donor–acceptor aromatic systems containing naphthalene and anthracene as electron donors and pyromellitimide and naphthadiimide as electron acceptors. We investigate the NLO effect when the donor acceptor units are not connected through a bridge. In this kind of system the electronic communication between donor and acceptor is solely due to through space charge transfer mechanism. Geometries of all molecular systems were optimised at the Hartree–Fock level with STO-3G minimal basis set and with the 3-21G split valence basis and finally with 6-31G basis set using gaussian 98W. The first static hyperpolarizabilities of these molecular systems were calculated using Hartree–Fock level using 6-31G basis set using gaussian 98W. To understand the possibility of developing these systems as NLO materials we have also calculated the linear and nonlinear optical properties of bridged donor–acceptor systems. The study suggested that these unconnected donor–acceptor systems equivalent to some [2]catenanes reported in literature in general have little influence on the first static hyperpolarizability, however, the linked macro cyclic systems may have potential applications in the development of non-linear optical materials.

A continuum theory of solvation in quadrupolar solvents. II. Solvation free energetics, dynamics, and solvatochromism

The continuum theory of solvation developed in the preceding paper [J. Chem. Phys. 119, 8606 (2003)] is applied to study free energetics and dynamics relevant to charge shift processes in polarizable quadrupolar solvents in the Born–Oppenheimer regime of solvent electronic degrees of freedom. The outer-sphere solvent reorganization free energy and free energy of reaction for electron transfer systems are investigated in both one and two-sphere cavity descriptions. The quadrupolar susceptibility needed in the theory is determined using recent Stokes shift measurements by Reynolds et al. [J. Phys. Chem. 100, 10337 (1996)]. A good accord with measurements and molecular theory estimates of the solvent reorganization free energy for bridged donor–acceptor systems is obtained. The solvatochromic shifts associated with chromophores are examined with a first-order perturbation method. The application of the theory to electronic absorption of betaine in the quadrupolar solvents yields a good agreement with the ET(30) solvent polarity scale. By incorporating the collective motions associated with solvent quadrupoles, the theory is extended to study inertial solvation dynamics. In contrast with dipolar liquids, the solvent frequency in the quadrupolar solvents increases as the multipole character of the solute charge distribution increases. This is in accord with a recent molecular dynamics simulation result for supercritical CO2.

Pathways and mobile ? electrons in biological electron transfer

Electron transfer in proteins is discussed and compared to electron transfer in bridged donor–acceptor systems. Results from extensive ab initio calculations for the latter type of system are summarized. The electronic factor Δ is often nonexponential as a function of number of bonds in the bridge. Δ may even change sign as the number of bonds in the bridge is increased in a repetitive way. Correlation effects, including the “broken symmetry” correction, are studied and are found to be important in some cases. The variational problem is formulated as an interaction between a donor and an acceptor via “pathways” through the protein. © 1993 John Wiley & Sons, Inc.

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.

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.