The exploration of singlet fission (SF) promises a pathway to many leaps forward including more efficient solar energy extraction and, more recently, organic-based quantum computing. Our study, through a joint experimental and computational approach, revolves around 1,4-bis(p-nitro-β-styryl)benzene (1) as the smallest molecule where the intramolecular transformation of the initially allowed 11Bu singlet state to the 21Ag excited state stops being ordinary internal conversion and becomes the first half of the SF process. Herein, we experimentally observe explicit breaking of the Kasha rule. Using femtosecond broadband fluorescence upconversion, we measure a dual fluorescence of 1 in solution from its two lowest singlet excited states of different symmetry. Femtosecond transient absorption (TA) and fluorescence upconversion spectroscopy of 1 in toluene reveal ultrafast (17 ± 5 ps), almost quantitative interconversion between 11B and 21A states. A sensitization bracketing experiment with ns-TA is used to analyze the T1 state of 1. Employing high-level ab initio extended multi-configuration quasi-degenerate 2nd-order perturbation theory (XMCQDPT2) calculations, we accurately model ground- and excited-state potential energy surfaces. 11B states are predominantly described by ordinary HOMO-LUMO excitation. 21A states can be projected in localized frontier molecular orbitals as an intramolecular strongly coupled triplet biexciton [1(T1T1)] with the inclusion of intramolecular charge-transfer states. Moreover, the experimental resemblance of 21A and T1 absorption is elucidated. The fluorescence temperature-dependence experiment further corroborates the XMCQDPT2 model accurate prediction of the 11B and 21A low barrier of crossing (ca. 600 cm-1). The concentration-dependent experiment shows a dramatic increase in triplet yield: up to 200% yield is obtained by ns-TA quantitative measurements. All the obtained results suggest the occurrence of an SF mechanism for the triplet production: intramolecular 1(T1T1) formation followed by intermolecular triplet separation aided by entropy and spatial separation.

Department of Chemistry Biology and Biotechnology University of Perugia Via dell' Elce di Sotto 8 06123 Perugia PG Italy

Department of Chemistry University of Colorado Boulder Cristol Chemistry and Biochemistry Boulder CO 80309 USA

Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences Prague 6 160 00 Czech Republic

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