5,5′-Biindeno[2,1-c]fluorene: importance of C5–C5′ linkage for an indeno[2,1-c]fluorene dimer exhibiting an open-shell ground state

Abstract

Mesityl-protected indeno[2,1-c]fluorene and its C2–C2′-linked dimer 2,2′-biindeno[2,1-c]fluorene are closed-shell molecules. Herein, we report the synthesis and characterization of a mesityl-protected 5,5′-biindeno[2,1-c]fluorene displaying 26.4% diradical and 0.9% tetraradical characters. The C5–C5′ linkage between two [2,1-c]IFs is crucial for its open-shell character, resulting in its far-red UV-vis absorption, redox amphoterism, partially para-quinoidal as-indacene core, and temperature-dependent EPR response due to a small singlet–triplet energy gap.