The nature of singlet exciton fission in carotenoid aggregates.

Andrew J Musser,Richard H Friend,Daniele Brida,Jenny Clark,Margherita Maiuri,Giulio Cerullo

doi:10.1021/jacs.5b01130

Abstract

Singlet exciton fission allows the fast and efficient generation of two spin triplet states from one photoexcited singlet. It has the potential to improve organic photovoltaics, enabling efficient coupling to the blue to ultraviolet region of the solar spectrum to capture the energy generally lost as waste heat. However, many questions remain about the underlying fission mechanism. The relation between intermolecular geometry and singlet fission rate and yield is poorly understood and remains one of the most significant barriers to the design of new singlet fission sensitizers. Here we explore the structure–property relationship and examine the mechanism of singlet fission in aggregates of astaxanthin, a small polyene. We isolate five distinct supramolecular structures of astaxanthin generated through self-assembly in solution. Each is capable of undergoing intermolecular singlet fission, with rates of triplet generation and annihilation that can be correlated with intermolecular coupling strength. In contrast with the conventional model of singlet fission in linear molecules, we demonstrate that no intermediate states are involved in the triplet formation: instead, singlet fission occurs directly from the initial 1Bu photoexcited state on ultrafast time scales. This result demands a re-evaluation of current theories of polyene photophysics and highlights the robustness of carotenoid singlet fission.

Highlights

Singlet exciton fission is the quantum mechanical process by which a singlet exciton splits into two distinct spin triplet excitons
The triplets are initially coupled into an overall singlet state, conserving spin and allowing for extremely fast and highly efficient triplet formation.[1−3] This phenomenon has recently become the object of intense study due to its proposed use for carrier multiplication in solar cells.[4]
The record external quantum efficiency in these systems of 135% is the highest achieved for any photovoltaic technology, demonstrating the great potential of singlet fission devices

Summary

■ INTRODUCTION

Singlet exciton fission is the quantum mechanical process by which a singlet exciton splits into two distinct spin triplet excitons. It has been shown in amorphous films of diphenyltetracene that long-range order is not required for efficient triplet formation,[12] but other studies on disordered systems highlight the importance of specific local interactions.[13,14] These local interactions, and the strength of intermolecular coupling, have been proposed to play a central role in determining the rate and dominant mechanism of singlet fission nonadiabatic or adiabatic in a new model covering all acenes.[15] This significant progress has occurred almost exclusively within a relatively narrow class of materials: tetracene, pentacene, and their derivatives. Hightime-resolution measurements were performed on a similar setup, in which both the pump and probe beams were generated by homebuilt NOPAs generating sub-30 fs (pump) and sub-10 fs (probe) pulses using chirped mirror compression

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES