Abstract

Over the past decade there has been a renaissance of interest in the processes by which electronic energy can be transported around an organized molecular array with minimal loss. The motivation for this research stems from a desire to apply, at a molecular level, the lessons acquired from our ever-deepening understanding of the natural light-harvesting machinery that powers bacterial and green-plant photosynthesis and other biological processes. A key requirement of all such functional units is the need to move the photonic energy to a site where chemical reactions are initiated. Paramount to the successful design of artificial prototypes able to operate in this way is the logical positioning of individual units in a way that favours vectorial electronic energy transfer (EET) along the molecular axis or by way of some other preferred pathway. An obvious, and indeed enviable, extension for these materials is to devise a simple means by which the EET flow can be reversed, while maintaining very high efficiency. In designing new molecular systems for capable of performing specific EET it is necessary to establish a thorough understanding of the underlying mechanism and, in particular, to distinguish between through-space and bridge-mediated routes. The arrays need to retain the capacity to sensitise solar cells, such as those based on amorphous silicon, in a beneficial manner.

We have examined many different molecular arrays built around boron dipyrromethene dyes of varying conjugation length and of quite different topology. The disparate spatial arrangements and the application of high pressure to curtail low-frequency torsional motions allow screening of the EET mechanism in these systems. By combining many such arrays into a coherent network, it becomes possible to create artificial light-harvesting complexes with some modest degree of self regulatory function. Furthermore, the EET direction is easily switched by photochemical means, in certain cases. Of particular interest are those molecular systems where EET occurs predominantly via Förster-type interactions since the theory can be tested over short separations and in different media.