Limitations of Generic Chromophore Concept for Femtosecond Vibrational Coherences

Marcin Andrzejak,Tomasz Skóra,Piotr Petelenz

doi:10.1021/acs.jpcc.9b11772

Abstract

The spectrum of vibrational coherences modulating transient triplet–triplet absorption of bis-triisopropylsilylethynylpentacene that propagate in the course of the fission process from the singlet state of this molecule to the triplet manifold has been lately reproduced by ab initio calculations. This has led to the conclusion that, contrary to a recent hypothesis, spontaneous generation of coherent vibrational wave packets is not a constitutive feature of singlet exciton fission. Here we demonstrate by calculations that the corresponding spectrum of unsubstituted pentacene is significantly different, so that the two systems cannot be viewed as different models of the same generic chromophore. This shows that femtosecond pump–probe spectra of vibrational coherences are sensitive to subtle features of molecular structure, which we identify here, thereby rationalizing the registered spectral differences. The crucial factor is the substituent-induced coupling between some pentacene normal modes that in other...

Highlights

Singlet exciton fission is presently extensively investigated because of its prospective application in increasing the efficiency of photovoltaic devices by charge carrier multiplication
In order to attain ultimate understanding of the process and optimize its conditions, considerable effort is invested in synthesizing and examining new compounds endowed with fissionable singlet state.[1−9] Their majority derive from tetracene and pentacene, which at interpretational level are treated as generic chromophores in the sense that the relevant photophysical mechanisms operative in the acenes are tacitly assumed to be unaffected by chemical modifications
The mission of a generic chromophore is to serve as an idealized proxy of the actual chromophore, providing a more general view of a problem by ignoring those details of the molecular structure that are deemed insignificant in the pertinent context

Summary

INTRODUCTION

Singlet exciton fission is presently extensively investigated because of its prospective application in increasing the efficiency of photovoltaic devices by charge carrier multiplication. The fission process, consisting in the decay of an intramolecular singlet state into an entangled pair of triplets located at different molecules, is a very fast one For this reason, major progress in this field that has been registered in recent years is largely due to application of femtosecond spectroscopy, enabling one to probe in real time the progress of the radiationless transition in hand, which can be accomplished, for instance, by measuring transient absorption of the emergent triplet states in the pump−probe regime. The differences between the calculated spectra of the two compounds will be exposed and rationalized to pinpoint the underlying physical mechanisms

THEORY

RESULTS AND DISCUSSION

Causes of the Differences

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES