Singlet Fission in Pechmann Dyes and Molecular Oligomers

Singlet fission is an ultrafast photophysical process in organic systems whereby a high-energy photoexcited singlet exciton converts to two low-energy triplet excitons through a spin-conserving pathway. When integrated into photovoltaics, singlet fission offers the opportunity to mitigate thermalization losses and possesses the potential to improve single-junction photovoltaic efficiencies, surpassing the Shockley-Queisser limit. Two major challenges, however, are (i) finding suitable material systems and (ii) improving the triplet exciton lifetimes for harvesting. This thesis addresses these questions and presents a new molecular design strategy, introduces a new family of singlet fission material system, elucidates their ultrafast photophysics, and investigates triplet hopping in singlet fission oligomers. First, we present a novel screening approach that leverages the concept of ground and excited state aromaticity combined with double-bond conformation to establish new design rules for singlet fission chromophores. By investigating Pechmann dye isomers, we demonstrate that although their planarity and degree of charge transfer are similar, singlet fission is active only in the trans-isomer, while the cis-isomer exhibits greater favourability for polaronic processes. Experimentally validation is established using spectroscopic techniques including ultrafast transient absorption and electron spin resonance. Thereafter, we engineer sidechains of Pechmann dyes and investigate their solid-state structure-function relationship and spin physics in these derivatives. We show that one can effectively tailor singlet fission dynamics by engineering polyene side chains. Finally, we address the challenge of decoupling singlet fission triplets by employing methods to improve their lifetimes. Utilizing DPH molecular dimers and TIPS trimers dispersed in polystyrene matrices at varying concentrations, we explore how to manipulate intermolecular triplet hopping, providing insights into the efficient harvesting of singlet-fission triplets so that they can be transferred to emissive entities for use in singlet fission-based photon multiplier devices.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2023

Author dc:creator

• Girija, Aswathy

Advisor dc:contributor.advisor

• Rao, Akshay

• Pechmann Dyes
• Singlet Fission

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/8a181de7-8daa-498e-aef8-af83956c59ee/download
• https://creativecommons.org/licenses/by/4.0/

Language dc:language

eng

Author Identifier

0000-0002-5586-2818

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/377894