Kinetically Blocked Stable Heptazethrene and Octazethrene: Closed-Shell or Open-Shell in the Ground State?

Polycyclic aromatic hydrocarbons with an open-shell singlet
biradical
ground state are of fundamental interest and have potential applications
in materials science. However, the inherent high reactivity makes
their synthesis and characterization very challenging. In this work,
a convenient synthetic route was developed to synthesize two kinetically
blocked heptazethrene (HZ-TIPS) and octazethrene (OZ-TIPS) compounds with good stability. Their ground-state
electronic structures were systematically investigated by a combination
of different experimental methods, including steady-state and transient
absorption spectroscopy, variable temperature NMR, electron spin resonance
(ESR), superconducting quantum interfering device (SQUID), FT Raman,
and X-ray crystallographic analysis, assisted by unrestricted symmetry-broken
density functional theory (DFT) calculations. All these demonstrated
that the heptazethrene derivative HZ-TIPS has a closed-shell
ground state while its octazethrene analogue OZ-TIPS with
a smaller energy gap exists as an open-shell singlet biradical with
a large measured biradical character (y = 0.56).
Large two-photon absorption (TPA) cross sections (σ(2)) were determined for HZ-TIPS (σ(2)max = 920 GM at 1250 nm) and OZ-TIPS (σ(2)max = 1200 GM at 1250 nm). In addition, HZ-TIPS and OZ-TIPS show a closely stacked 1D
polymer chain in single crystals.