Abstract
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 1(1)Bu singlet state to the 2(1)Ag 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 1(1)B and 2(1)A states. A sensitization bracketing experiment with ns-TA is used to analyze the T(1) 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. 1(1)B states are predominantly described by ordinary HOMO-LUMO excitation. 2(1)A states can be projected in localized frontier molecular orbitals as an intramolecular strongly coupled triplet biexciton [(1)(T(1)T(1))] with the inclusion of intramolecular charge-transfer states. Moreover, the experimental resemblance of 2(1)A and T(1) absorption is elucidated. The fluorescence temperature-dependence experiment further corroborates the XMCQDPT2 model accurate prediction of the 1(1)B and 2(1)A 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)(T(1)T(1)) formation followed by intermolecular triplet separation aided by entropy and spatial separation.