Uncovering intramolecular singlet fission at the root of the dual fluorescence of 1,4-bis( <i>p</i> -nitro-β-styryl)benzene in solution

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(<i>p</i>-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 <i>ab initio</i> 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 (<i>ca.</i> 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.