5,5'-Biindeno[2,1-c]fluorene: importance of C5-C5' linkage for an indeno[2,1-c]fluorene dimer exhibiting 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, which results in its far-red UV-vis absorption, redox amphoterism, partially para-quinoidal as-indacene core, and temperature-dependant EPR response due to a small singlet-triplet energy gap.