Synthesis of Tropylium Acenes as Singlet Fission Materials for Applications in Photovoltaics

Abstract

Singlet fission is a process whereby one singlet excited state is converted into two triplet states, with prominent examples including acenes, namely tetracene and pentacene. These singlet fission chromophores have been widely exploited as materials for photovoltaics (PVs), as it is believed that singlet fission can help PVs overcome the Shockley−Queisser limit and increase the maximum efficiency of the devices from 33% to 45%.

This project aims to develop a synthetic route for tropylium acenes, so that they can be explored as a novel class of singlet fission chromophores with potential applications in PVs. To date, there have not been any reports which takes advantage of fused tropylium rings in functional materials. However, tropyliums possess unique characteristics which could set them apart from more common organic motifs; they exhibit remarkable stabilities relative to traditional carbocations, while their positive charges allow better charge delocalisation across the material. This fundamental area of organic materials will therefore be explored for the first time.

Previous DFT calculations has shown that tetracene tropylium may have singlet and triplet energies suitable for singlet fission. Several synthetic pathways to tropylium acenes were therefore designed and tested in this work, in hopes of accessing the target molecule. At the time of writing, however, tetracene tropylium has not been successfully generated.
Nevertheless, one promising pathway has provided access to anthracene tropone which should be able to convert to anthracene tropylium via relatively common reduction and dehydration steps. This iterative pathway should allow longer tropylium acenes to be synthesised using similar methods. Once generated, the electronic and photophysical properties of these compounds, as well as their singlet and triplet energies, can be investigated spectroscopically to provide insight into these novel structures.