Dihydroindenofluorene Positional Isomers.

Bridged oligophenylenes are very important organic semiconductors (OSCs) in organic electronics (OE). The fluorene unit, which is a bridged biphenyl, is the spearhead of this class of materials and has, over the last 20 years, led to fantastic breakthroughs in organic light-emitting diodes. Dihydroindenofluorenes belong to the family of bridged terphenyls and can be viewed as the fusion of a fluorene unit with an indene fragment. Dihydroindenofluorenes have also appeared as very promising building blocks for OE applications. In the dihydroindenofluorene family, there are five positional isomers, with three different phenyl linkages ( para/ meta/ ortho) and two different ring bridge arrangements ( anti/ syn). We have focused on the concept of positional isomerism. Indeed, the structural differences of the dihydroindenofluorenyl cores lead to unusual electronic properties, which our group has described since 2006, thanks to the five dispirofluorene-indenofluorene positional isomers (dihydroindenofluorenes substituted on the bridges by fluorenyl units). 6,12-Dihydroindeno[1,2- b]fluorene (the para-anti isomer) is constructed on a p-terphenyl core and possesses an anti geometry. Although this isomer has been widely investigated over the last 20 years, studies of the four other isomers remain very scarce. 11,12-Dihydroindeno[2,1- a]fluorene (the para-syn isomer) is also built on a bridged p-terphenyl core but possesses a syn geometry. This particular geometry has been advantageously used by our group to drastically tune the electronic properties, and this isomer has emerged as a promising scaffold to obtain stable blue emission arising from conformationally controllable intramolecular excimers. These preliminary studies have shown the crucial influence of the geometry on the electronic properties of the dihydroindenofluorenes. Modification of the arrangement of the phenyl linkages from para to meta provides the meta isomers, namely, 7,12-dihydroindeno[1,2- a]fluorene (the meta-anti isomer) and 5,7-dihydroindeno[2,1- b]fluorene (the meta-syn isomer). With these two regioisomers, the strong impacts of both the linkage and the geometry on the electronic properties have been particularly highlighted over the years. The last positional isomer of the family is 5,8-dihydroindeno[2,1- c]fluorene, which possesses a central o-terphenyl backbone and a syn geometry. This isomer is unique because of its ortho linkage, which induces a particular helicoidal turn of the dihydroindenofluorenyl core. Using a structure-property relationship approach, in the present Account we describe the molecular diversity of the five dispirofluorene-indenofluorene positional isomers and the consequences both in terms of their organic synthesis and electronic properties. This Account shows how positional isomerism can be a powerful tool to tune the electronic properties of OSCs.