Blue Emitting 3 π-2 Spiro Terfluorene-Indenofluorene Isomers: A Structure-Properties Relationship Study

Dihydroindenofluorenes as building units in organic semiconductors for organic electronics

This review aims to discuss organic semiconductors constructed on dihydroindenofluorene positional isomers, which are key molecular scaffolds in organic electronics. Bridged oligophenylenes are key organic semiconductors that have allowed the development of organic electronic technologies. Dihydroindenofluorenes (DHIFs) belong to the family of bridged oligophenylenes constructed on a terphenyl backbone. They have proven to be very promising building blocks for the construction of highly efficient organic semiconductors for all OE devices, namely organic light emitting diodes (OLEDs), phosphorescent OLEDs, organic field-effect transistors (OFETs), solar cells, etc.

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.

Electron-Deficient Dihydroindaceno-Dithiophene Regioisomers for n-Type Organic Field-Effect Transistors

In this work, we wish to report the first member of a new family of organic semiconductors constructed on a meta dihydroindacenodithiophene core, that is, 2,2'-(2,8-dihexyl-4,6-dihydro-s-indaceno[1,2-b:7,6-b']dithiophene-4,6-diylidene)dimalononitrile (called meta-IDT(═C(CN)₂)₂). The properties of this molecule were studied in detail through a structure-properties relationship study with its regioisomer, that is, 2,2'-(2,7-dihexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-4,9-diylidene)dimalononitrile (para-IDT(═C(CN)₂)₂) (see isomer structures in blue in Chart 2). The influence of the bridge functionalization was also investigated by comparison with their diketone analogues meta-IDT(═O)₂ and para-IDT(═O)₂. This study sheds light on the impact of regioisomerism on the electronic properties at the molecular level (electrochemistry, absorption spectroscopy, molecular modeling) and also on the supramolecular arrangement, and finally on the organic field-effect transistors (OFET) performances and stabilities. The significant effect of self-assembled monolayers of 4-(dimethylamino)benzenethiol grafted on the gold drain and source electrodes or of the use of flexible substrate (polyethylene naphtalate) instead of glass on the OFET performances and stabilities are also reported. In the light of these results (maximum mobility reaching 7.1 × 10^-2 cm² V^-1 cm^-1, high I_Don/I_Doff of 2.3 × 10⁷, and subthreshold swing of 1.2 V/dec), we believe that the present OFETs can be further used to construct electronic circuits.

Electron-Rich 4-Substituted Spirobifluorenes: Toward a New Family of High Triplet Energy Host Materials for High-Efficiency Green and Sky Blue Phosphorescent OLEDs

We report herein a detailed structure-properties relationship study of the first examples of electron-rich 4-substituted spirobifluorenes for organic electronic applications, namely, 4-phenyl-N-carbazole-spirobifluorene (4-PhCz-SBF) and 4-(3,4,5-trimethoxyphenyl)-spirobifluorene (4-Ph(OMe)₃-SBF). The incorporation of the electron-rich moieties in the ortho position of the biphenyl linkage (position C4) induces unique properties, very different from those previously described in the literature for this family of semiconductors. Both dyes can be readily synthesized, possess high triplet energies and excellent thermal stability, and their HOMO energy levels are highly increased compared to those of other 4-substituted SBFs. We also provide in this work the first rationalization of the peculiar fluorescence of 4-substituted SBFs. Finally, the present dyes have been successfully incorporated as host in green and blue phosphorescent organic light-emitting diodes with high performance either for the green (EQE of 20.2%) or the blue color (EQE of 9.6%). These performances are, to the best of our knowledge, among the highest reported to date for 4-substituted SBF derivatives.

Incorporation of spirobifluorene regioisomers in electron-donating molecular systems for organic solar cells

We report herein the synthesis and characterizations of new small molecules for organic photovoltaics including diketopyrrolopyrrole and spirobifluorene.

Broadband spectra with fluorescence and phosphorescence dual emission from bichromophoric platinum metallomesogens containing a 6,12-dihydro-indeno[1,2-b]fluorene linkage

Two bichromophoric platinum metallomesogens, FJ-1 and FJ-2, exhibited fluorescence (¹π,π*) and phosphorescence (³π,π*) dual emissions in solution. Their photophysical properties were detailedly investigated by experiment and theory calculation.

An efficient AIE-active blue-emitting molecule by incorporating multifunctional groups into tetraphenylsilane

A multifunctional AIE-active molecule, CzPySiTPE, in which carbazole (Cz) and pyridine (Py) were attached to tetraphenylsilane to facilitate carrier injection has been designed and synthesized. Tetraphenylethene (TPE) was adopted to maintain efficient blue emission. Blue electroluminescent (EL) emission of CzPySiTPE was obtained with CIE coordinates of (0.16, 0.17) and an external quantum efficiency of 1.12%.

Synthesis and properties of B,N-bridged p-terphenyls

We describe the efficient synthesis and intriguing photophysical properties of a new class of ladder π-conjugated molecules, B,N-bridged p-terphenyls.

Indenofluorenes and derivatives: syntheses and emerging materials applications

The growing demand for flexible electronic devices and hydrogen storage materials has spurred a resurgence of interest in polyaryl hydrocarbons including graphene, acenes, fullerenes, polythiophenes, etc. Indenofluorenes are another polyaryl molecular scaffold that has shown utility in the organic and hybrid materials arena, with polymers incorporating the indeno[1,2-b]fluorene moiety being common in organic light emitting diodes. This review examines the syntheses and properties of the five distinct indenofluorene regioisomers, with a focus on small molecule applications in organic electronics of this intriguing and somewhat underexplored family of polyaryl hydrocarbons.