ENERGETICS AND COUPLINGS IN OLIGOACENE-BASED SINGLET FISSION: EFFICIENT AND ACCURATE DENSITY FUNCTIONAL THEORY TELLS THE STORY

Abstract

In the sub-picosecond singlet fission (SF) process, two coherent triplet excitons ($^1$(T$_1$T$_1$)) are generated from one singlet exciton (S$_1$) through a mysterious ``multi-exciton'' ($^1$ME) intermediate: S$_1$ $\longleftrightarrow$ $^1$ME $\longleftrightarrow$ $^1$(T$_1$T$_1$) $\longrightarrow$ 2T$_1$.\footnote{M. B. Smith and J. Michl, {\it Chem. Rev.} {\bf 110}, 6891 (2010).}

Organic semiconducting materials that undergo exothermic SF double the efficiency associated with high-energy incident photons and allow the photovoltaics to exceed the Shockley--Queisser limit of 33.7\%.\footnote{W. Shockley and H. J. Queisser, {\it J. Appl. Phys.} {\bf 32}, 510 (1961).}

Understanding the character of $^1$ME serves as the key to deciphering the ultrafast SF mechanism.

Based on a popular hypothesis, the complete SF procedure involves two steps of charge transfer (CT) and $^1$ME is a superposition of localized and charge-separated excited states.\footnote{T. C. Berkelbach, M. S. Hybertsen, and D. R. Reichman, {\it J. Chem. Phys.} {\bf 141}, 074705 (2014).} However, the straightforward experimental and theoretical evidence to support this hypothesis is still in darkness due to the challenges to measure ultrafast photochemistry and to describe charge-separated and multiply-excited energy levels.

Herein we modeled the local SF reactions occurring in hexacene, pentacene and bis(6,13-bis(triisopropylsilylethynyl)pentacene)benzene in the crystalline or solution phase. The efficient and accurate density functional theory (DFT) based approaches developed in the earlier\footnote{S. R. Yost, J. Lee, M. W. B. Wilson, T. Wu, D. P. McMahon, R. R. Parkhurst, N. J. Thompson, D. N. Congreve, A. Rao, K. Johnson, M. Y. Sfeir, M. G. Bawendi, T. M. Swager, R. H. Friend, M. A. Baldo and T. Van Voorhis, {\it Nat. Chem.}, {\bf 6}, 492 (2014).} and present study were utilized to evaluate energies of S$_1$, $^1$ME and $^1$(T$_1$T$_1$), as well as the non-adiabatic coupling between each pair of them.

Our results shed light on the roles of localized and charge-separated excited states in the complete SF mechanism and propose the strategy for the rational design of oligoacene-based SF materials.