Indenofluorene congeners: Biradicaloids and beyond

Abstract

Abstract Singlet biradicaloids are certainly one of the most frequently discussed topics at ISNA-15. In this review our own work on the synthesis, molecular structures, and physical properties relevant to biradical character of indenofluorene isomers are described. These include indeno[2,1-a]fluorene with an o-quinodimethane substructure, benz[c]indeno[2,1-a]fluorene which is the first 2,3-naphthoquinodimethane derivative isolated, and indeno[2,1-b]fluorene with an m-quinodimethane (m-xylylene) subunit. The observed physical properties are explained in terms of the theoretically derived biradical factors (y), HOMO-LUMO energy gaps, and singlet-triplet energy gaps in the ground state electronic configurations. In addition, we discuss here two perspectives in the biradicaloid chemistry, (i) two- and three-dimensional indenofluorene congeners arranged in a macrocyclic form and (ii) macrocyclic indenofluorenes having multiple radical characters.