Tuning the role of charge-transfer states in intramolecular singlet exciton fission through side-group engineering.

Steven Lukman,David H P Turban,Kai Chen,Shaoqiang Dong,Andrew J Musser,Nicholas D M Hine,Jishan Wu,Neil C Greenham,Justin M Hodgkiss

doi:10.1038/ncomms13622

Abstract

Understanding the mechanism of singlet exciton fission, in which a singlet exciton separates into a pair of triplet excitons, is crucial to the development of new chromophores for efficient fission-sensitized solar cells. The challenge of controlling molecular packing and energy levels in the solid state precludes clear determination of the singlet fission pathway. Here, we circumvent this difficulty by utilizing covalent dimers of pentacene with two types of side groups. We report rapid and efficient intramolecular singlet fission in both molecules, in one case via a virtual charge-transfer state and in the other via a distinct charge-transfer intermediate. The singlet fission pathway is governed by the energy gap between singlet and charge-transfer states, which change dynamically with molecular geometry but are primarily set by the side group. These results clearly establish the role of charge-transfer states in singlet fission and highlight the importance of solubilizing groups to optimize excited-state photophysics.

Highlights

Understanding the mechanism of singlet exciton fission, in which a singlet exciton separates into a pair of triplet excitons, is crucial to the development of new chromophores for efficient fission-sensitized solar cells
The dimer S1 states can essentially be interpreted as symmetric linear combinations of the corresponding monomer states, the transition dipoles of which are polarized along the short pentacene axis[23,24]
This reduction is expected to result in a blue-shift, strong in DP-TIPS, which competes with the red-shift due to coupling of transition dipoles

Summary

Introduction

Understanding the mechanism of singlet exciton fission, in which a singlet exciton separates into a pair of triplet excitons, is crucial to the development of new chromophores for efficient fission-sensitized solar cells. To expand the pool of suitable chromophores, it is necessary to better understand the underlying mechanism of triplet formation This is complicated in typical solid-state materials, where morphology and crystal packing can play a significant role and are difficult to control[6,7]. Many leading theoretical models of SF assign a crucial mediating role to CT states[13,14,15] These are generally calculated to exhibit strong coupling to singlet and triplet-pair states, even when they are energetically inaccessible, and could drive efficient triplet formation. These CT states are difficult to observe directly, but evidence of solvent-polarity-dependent SF rates in two classes of pentacene dimer[8,11,12] and dimers of. The population of this state can be suppressed by restricting the molecular geometry: in a DP-TIPS: Singlet fission via real CT

Methods

Results

Conclusion