Singlet Fission in Aza-Derivatives of Diphenylhexatriene

As an exciton multiplication process, singlet fission (SF) potentially increases the energy conversion efficiency of solar cells. (E,E,E)-1,6-Diphenylhexa-1,3,5-triene (DPH) is an SF-active short polyene having a triplet state of higher energy (around 1.5 eV) and longer lifetime (>50 μs) compared with those of tetracenes and pentacenes. However, the functionalization of DPH molecule by N-substitution has not been fully studied yet. Here, we investigate the excited states and SF dynamics in DPH aza-derivatives [(E,E,E)-1-phenyl-6-(4-pyridyl)hexa-1,3,5-triene (1) and (E,E,E)-1,6-di(4-pyridyl)hexa-1,3,5-triene (2)]. The increase in steady-state fluorescence intensity upon application of a magnetic field indicates that 1 and 2 are new SF materials. Both trienes, 1 in particular, exhibit good photostability at room temperature in air. Crystal 1 exists in two polymorphic (monoclinic and orthorhombic) forms, as shown by X-ray diffraction analysis. A combination of time-resolved fluorescence measurements and lifetime analysis reveals that in the monoclinic crystal (and powder) of 1, the photogenerated singlet state [S*(LE)] converts into an intramolecular charge transfer state [S*(CT)] within 0.2 ns, from which SF [S*→ correlated triplet state (TT)] occurs in 1–2 ns. In contrast, in the orthorhombic crystal of 1 and powder of 2, S*(LE) → S*(CT) transition does not take place, and SF occurs from S*(LE) in 0.7 and 5.0 ns, respectively. In monoclinic 1, S*(LE) → S*(CT) may be facilitated by the large dipole moment of the molecular chain formed by linearly extended CH···N hydrogen bonds in the structure.