This work reports the first structure-properties relationship study of ortho [2,1-c]-, meta [1,2-a]-, and para [1,2-b]dihydroindenofluorenes, highlighting the influence of bridge rigidification on the electronic properties. This study has made it possible to devise an extended π-conjugated molecule with both a high triplet state energy level and excellent thermal and morphological stability. As a proof of concept, dihydroindenofluorenes were used as the host in sky-blue phosphorescent organic light-emitting diodes (PhOLEDs) with high performance.

The future of organic electronics is driven by the synthesis and the study of novel molecular fragments for the construction of highly efficient polymers or oligomers. [1] In this context, poly-and oligophenylene derivatives constitute an important class of highly promising molecules, which have been widely studied for the last two decades. [2,3] Of particular interest in the chemistry and physics of oligophenylenes is the bridged-para-terphenyl unit, namely, 6,12dihydroindeno[1,2-b]fluorene (Scheme 1). Although it has been known since the 1950s, [4] investigations of the dihydroindeno[1,2-b]fluorenyl core only started a decade ago thanks to the pioneering work of Müllen, which made this molecule a key building block for electronics. [2] There are nowadays numerous examples of efficient dihydroindeno[1,2-b]fluorenylbased semiconductors that have found application in various fields, such as fluorescent [2,3,[5][6][7][8] and phosphorescent [9] organic light-emitting diodes (OLEDs), organic field-effect transistors, [10][11][12] and organic solar cells. [13] This wide range of applications clearly shows the high potential of this building block, but also its versatility. However, the dihydroindeno[1,2-b]fluorene is not the only member of the bridged-terphenyl family, since it possesses four other positional isomers with different phenyl linkages (para/meta/ortho) and different ring-bridging positions (anti vs. syn; Scheme 1). There are hence five dihydroindenofluorene positional isomers, each possessing its own ring topology, which in turn has structural and electronic consequences. However, in contrast to the dihydroindeno[1,2-b]fluorene, other positional isomers remain very scarce in the literature owing to synthetic difficulties. For example, the dihydroindeno[2,1-a]fluorenyl (syn para-terphenyl) unit (Scheme 1) has only been investigated for organic electronics very recently, [14] and thanks its particular syn geometry has emerged as a promising scaffold for a new generation of excimer-based OLEDs. [15] Similarly, antiaromatic fully conjugated indenofluorene derivatives have recently attracted particular attention; [16][17][18][19] Haley and co-workers have for example reported a new class of (2,1-c)indenofluorenes with high electron affinities. [20] However, the anti and syn meta-terphenyl isomers, that is, dihydroindeno-[1,2-a]fluorene and dihydroindeno[2,1-b]fluorene, although known for 60 years, [21] are almost absent from the literature, [22] and their intrinsic properties have never been studied. As the design of novel molecular fragments is of key importance for the future of organic electronics, we report herein the first examples of the use of dihydroindeno[1,2b]fluorene (1) and dihydroindeno[2,1-a]fluorene (2; Scheme 1. The five positional dihydroindenofluorene isomers.