Monobenzopentalenes have received moderate attention compared to dibenzopentalenes,
yet their accessibility as stable, non‐symmetric structures with diverse substituents
could be interesting for materials applications, including molecular photonics. Recently,
monobenzopentalene was considered computationally as a potential chromophore for singlet
fission (SF) photovoltaics. To advance this compound class towards photonics applications,
the excited state energetics must be characterized, computationally and experimentally.
In this report we synthesized a series of stable substituted monobenzopentalenes and
provided the first experimental exploration of their photophysical properties. Structural
and opto‐electronic characterization revealed that all derivatives showed 1H NMR shifts
in the olefinic region, bond length alternation in the pentalene unit, low‐intensity
absorptions reflecting the ground‐state antiaromatic character and in turn the symmetry
forbidden HOMO‐to‐LUMO transitions of ~2 eV and redox amphotericity. This was also
supported by computed aromaticity indices (NICS, ACID, HOMA). Accordingly, substituents
did not affect the fulfilment of the energetic criterion of SF, as the computed excited‐state
energy levels satisfied the required E(S1)/E(T1) > 2 relationship. Further spectroscopic
measurements revealed a concentration dependent quenching of the excited state and
population of the S2 state on the nanosecond timescale, providing initial evidence
for unusual photophysics and an alternative entry point for singlet fission with monobenzopentalenes.
