William B. Davis

Molecular-wire behaviour in p-phenylenevinylene oligomers

Electron transfer from electron-donor to electron-acceptor molecules via a molecular ‘bridge’ is a feature of many biological andchemical systems. The electronic structure of the bridge component in donor–bridge–acceptor (DBA) systems is known to play a critical role in determining the ease of electron transfer,. In most DBA systems, the rate at which electron transfer occurs scales exponentially with the donor–acceptor distance — effectively the length of the bridge molecule. But theory predicts that regimes exist wherein the distance dependence may be very weak, the bridge molecules essentially acting as incoherent molecular wires. Here we show how these regimes can be accessed by molecular design. We have synthesized a series of structurally well-defined DBA molecules that incorporate tetracene as the donor and pyromellitimide as the acceptor, linked by p -phenylenevinylene oligomers of various lengths. Photoinduced electron transfer in this series exhibits very weak distance dependence for donor–acceptor separations as large as 40 Å, with rate constants of the order of 1011 s−1. These findings demonstrate the importance of energy matching between the donor and bridge components for achieving molecular-wire behaviour.

Dynamics and relaxation in interacting systems: Semigroup methods

The dynamical study of chemical systems whose evolution is governed by quantum mechanics can now be computed fairly effectively for small systems in which the evolution is entirely Hamiltonian. When such Hamiltonian systems interact with their environment, however, relaxation and dephasing terms are introduced into the evolution. To include the effect of these terms, several methods are in current use. This paper is devoted to an exposition, analysis, and several simple applications of the semigroup technique for dealing with these non-Hamiltonian evolution terms. We discuss the nature of the semigroup terms, how they arise and how they are applied, and some of their advantages and disadvantages compared to other methods including dissipation. Specific applications to three simple two-site problems are given.

Dependence of electron transfer dynamics in wire-like bridge molecules on donor-bridge energetics and electronic interactions

Abstract Electron transfer (ET) reactions in donor–bridge–acceptor (DBA) molecules that occur by means of superexchange interactions between the donor (D) and the bridge (B) molecules depend on the vertical energy gap separating D and B. This dependence modulates the electronic coupling matrix element for ET, and hence the ET rate. However, when the energy gap between D and B is small, the assumptions intrinsic to the simplest superexchange model break down and charge injection from D to B may occur. To investigate this range of possibilities, we synthesized a family of DBA molecules based on a 2,5-bis(2 ′ -ethylhexyloxy)-1,4-distyrylbenzene (OPV3) bridge. Three electron donors, zinc 5,10,15,20-tetraphenylporphyrin (ZnTPP), perylene (PER) and tetracene (TET) as well as two electron acceptors naphthalene-1,8:4,5-bis(dicarboximide) (NI) and pyromellitimide (PI) were attached to the OPV3 bridge. The observed ET dynamics of these molecules are sensitive not only to the donor–bridge energy gap, but also to the excited state torsional dynamics between the donor and bridge.

Influence of substituents and chain length on the optical properties of poly(p‐phenylenevinylene) oligomers

Electronic-structure calculations based upon the Pariser-Parr-Pople semiempirical Hamiltonian were applied to short-chain oligomers of the conducting polymer poly(p-phenylenevinylene) (PPV) to study the effects of appending methoxy groups along the backbone as well as push-pull nitro and amino groups at the ends of the oligomers. Optical absorption spectra were calculated utilizing configuration interaction methods at the single-excitation level, and the nonlinear optical properties of the push-pull oligomers were studied using the sum-over-states approach. In short-chain oligomers, the calculated electronic and optical properties are controlled by the nature of the pendant groups. However, in the longest oligomers studied, these molecular properties are relatively insensitive to any of the studied modifications.