Unified model for singlet fission within a non-conjugated covalent pentacene dimer

Bettina S Basel,S Rajagopala Reddy,Brian T Phelan,Ryan M Young,Rik R Tykwinski,Fraser J White,Matthew D Krzyaniak,Pedro B Coto,Noah E Horwitz,Timothy Clark,Frank Hampel,Michael R Wasielewski,Dirk M Guldi,Constantin Hetzer,Michael Thoss,Johannes Zirzlmeier

doi:10.1038/ncomms15171

Abstract

When molecular dimers, crystalline films or molecular aggregates absorb a photon to produce a singlet exciton, spin-allowed singlet fission may produce two triplet excitons that can be used to generate two electron–hole pairs, leading to a predicted ∼50% enhancement in maximum solar cell performance. The singlet fission mechanism is still not well understood. Here we report on the use of time-resolved optical and electron paramagnetic resonance spectroscopy to probe singlet fission in a pentacene dimer linked by a non-conjugated spacer. We observe the key intermediates in the singlet fission process, including the formation and decay of a quintet state that precedes formation of the pentacene triplet excitons. Using these combined data, we develop a single kinetic model that describes the data over seven temporal orders of magnitude both at room and cryogenic temperatures.

Highlights

When molecular dimers, crystalline films or molecular aggregates absorb a photon to produce a singlet exciton, spin-allowed singlet fission may produce two triplet excitons that can be used to generate two electron–hole pairs, leading to a predicted B50% enhancement in maximum solar cell performance
This contrasts with recent observations of quintet states formed by singlet fission (SF) in tetracene films and conjugated pentacene dimers[15,16], and allows us to use complementary time-resolved optical and time-resolved electron paramagnetic resonance (TREPR) spectroscopy to observe the intermediates and kinetics for the entire 1(S1S0) Ð 1(T1T1) Ð 5(T1T1)-(T1 þ T1) sequence
Lithiated tri-isobutylsilylacetylene was added to a suspension of 1 in THF at low temperature, and after reaction for 5 h, the reaction was quenched by the addition of MeI to afford ketone 2 in 75% yield. 1,3-Diethynyladamantane was lithiated by a reaction with LiHMDS in THF at low temperature to provide 3, and ketone 2 was added to the solution of 3

Summary

Introduction

Crystalline films or molecular aggregates absorb a photon to produce a singlet exciton, spin-allowed singlet fission may produce two triplet excitons that can be used to generate two electron–hole pairs, leading to a predicted B50% enhancement in maximum solar cell performance. The pentacenes in NC are coupled just strongly enough to allow efficient SF before any other excited state processes occur, but weakly enough that the 1(T1T1) state formed by SF can undergo spin-mixing to form the 5(T1T1) on a timescale sufficiently slow to allow time-resolved electron paramagnetic resonance (TREPR) spectroscopy to observe both its formation and decay into independent triplet states (T1 þ T1) This contrasts with recent observations of quintet states formed by SF in tetracene films and conjugated pentacene dimers[15,16], and allows us to use complementary time-resolved optical and TREPR spectroscopy to observe the intermediates and kinetics for the entire 1(S1S0) Ð 1(T1T1) Ð 5(T1T1)-(T1 þ T1) sequence. We develop a single kinetic model that describes the data over seven temporal orders of magnitude both at room and cryogenic temperatures

Methods

Results

Conclusion