Violet-to-Blue Tunable Emission of Aryl-Substituted Dispirofluorene-Indenofluorene Isomers by Conformationally-Controllable Intramolecular Excimer Formation

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.

Multifaceted Sulfone-Carbazole-Based D-A-D Materials: A Blue Fluorescent Emitter as a Host for Phosphorescent OLEDs and Triplet-Triplet Annihilation Up-Conversion Electroluminescence

Electroluminescence (EL) from the singlet-excited (S₁) state is the ideal choice for stable, high-performing deep-blue organic light-emitting diodes (OLEDs) owing to the advantages of an adequately short radiative lifetime, improved device durability, and low cost, which are the most important criteria for their commercialization. Herein, we present the design and synthesis of three donor-acceptor-donor (D-A-D)-configured deep-blue fluorescent materials (denoted as TC-1, TC-2, and TC-3) composed of a thioxanthone or diphenyl sulfonyl acceptor and phenyl carbazolyl donor. These systems exhibit strong deep-blue photoluminescence (422-432 nm) in solutions and redshifted emission (472-486 nm) in thin films. The solid-state photoluminescence quantum yield (PLQY) was estimated to be 78 and 94% for TC-2 and TC-3, respectively. TC-2 and TC-3 possess good molecular packing and large molecular cross-sectional areas, which not only improves the PLQY but enhances the triplet-triplet annihilation up-conversion (TTAUC) efficiency of fluorescent emitters. Furthermore, both compounds were applied as an acceptor for confirming their TTAUC property using bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)iridium(III) (Ir(MDQ)₂acac) as the sensitizer. Non-doped OLEDs based on TC-2 and TC-3 exhibit blue EL in the 461-476 nm range. In particular, TC-3 exhibits a maximum external quantum efficiency (EQE_max) of 5.1%, and its EL maximum is 476 nm. In addition, the three emitters were employed as hosts in red OLEDs using bis(1-phenylisoquinoline)(acetylacetonate)iridium(III) (Ir(piq)₂acac) as the phosphorescent dopant. The red phosphorescent OLEDs based on TC-1, TC-2, and TC-3 achieve excellent EQE_max values of 21.6, 22.9, and 21.9%, respectively, and peak luminance efficiencies of 12.0, 14.0, and 12.3 cd A^-1. These results highlight these fluorophores' versatility and promising prospects in practical OLED applications.

Rigidly Fused Spiro-Conjugated π-Systems

Spiro-fused π-systems have gained considerable attention for their application as semiconductors in molecular electronics. Here, a synopsis regarding recent breakthroughs in ladderized spirobifluorenes and indeno-spirobifluorenes, along with further spiro-condensed heteroatomic hydrocarbons with donor-acceptor moieties, is provided. Additionally, an extended range of rigid spirobifluorene polymers and specific doubly linked spiro-systems with partial chiral character is discussed. The diverse applications of the aforementioned structures are thoroughly evaluated.

Prediction of the Synthesis of Spiro Derivatives by Double Intramolecular Aromatic Electrophilic Substitution Using Reactivity Indices

The synthesis of heterocyclic spirobifluorene (SBF) analogs generally requires long and complicated synthetic pathways. Despite this synthetic effort, such structural modification allows the (opto)electronic properties of this remarkable three-dimensional node to be tuned especially for molecular electronic applications. For this reason, the development of a simple, robust, and efficient synthetic methodology to introduce various heterocycles in place of classical phenyl rings in the spirofluorene motif is highly and timely desirable. In this context, we describe herein our efforts to develop a straightforward and efficient synthesis leading to replacement of 2 phenyl rings by various heterocycles in spiro compounds from 2,2'-dibromobenzophenone. As the same procedure to form fully heterocyclic compounds failed, an original theoretical approach based mainly on the uncommon Fukui dual function was developed in order to determine clearly the limitation of this strategy and provide an efficient predictive tool. Indeed, such calculation allows prediction of the thermodynamic and kinetic aspects of the synthesis of spiro derivatives using a double aromatic electrophilic substitution. If this procedure reproduces well our experimental results focused on (heterocyclic) SBF compounds, it can be certainly adapted and generalized to other intramolecular substitutions.

Mechanical Stabilization of Helical Chirality in a Macrocyclic Oligothiophene

We introduce a design principle to stabilize helically chiral structures from an achiral tetrasubstituted [2.2]paracyclophane by integrating it into a macrocycle. The [2.2]paracyclophane introduces a three-dimensional perturbation into a nearly planar macrocyclic oligothiophene. The resulting helical structure is stabilized by two bulky substituents installed on the [2.2]paracyclophane unit. The increased enantiomerization barrier enabled the separation of both enantiomers. The synthesis of the target helical macrocycle 1 involves a sequence of halogenation and cross-coupling steps and a high-dilution strategy to close the macrocycle. Substituents tuning the energy of the enantiomerization process can be introduced in the last steps of the synthesis. The chiral target compound 1 was fully characterized by NMR spectroscopy and mass spectrometry. The absolute configurations of the isolated enantiomers were assigned by comparing the data of circular dichroism spectroscopy with TD-DFT calculations. The enantiomerization dynamics was studied by dynamic HPLC and variable-temperature 2D exchange spectroscopy and supported by quantum-chemical calculations.

New generations of spirobifluorene regioisomers for organic electronics: tuning electronic properties with the substitution pattern

In the present feature article, we present the new generations of spirobifluorenes for organic electronics and we detail the impact of positional isomerism on the electronic properties and device performance.

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.

Gold-Catalyzed Divergent Ring-Closing Modes of Indole-Tethered Amino Allenynes

Indole-tethered amino allenynes were chemodivergently cyclized for the controlled preparation of fused polycyclic indoles using gold catalysis. Double cyclization of terminal allenynes afforded hexacyclic 15 H-indolo[1,2,3-de]quinolino[3,2,1-ij]quinoxalines, in which allenynes bearing a substituted alkyne at the terminal end generated 12,13-dihydro-7 H-indolo[3,2-c]acridines, which are 5-membered cyclized adducts. Density functional theory calculations were performed to shed light on this difference in reactivity.

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.

Switching Selectivity of α-Enolic Dithioesters: One Pot Access to Functionalized 1,2- and 1,3-Dithioles

An operationally simple cascade protocol has been developed for the construction of 1,2- and 1,3-dithiole derivatives from α-enolic dithioesters. 1,2-Dithioles are achieved by the reaction of dithioesters with elemental sulfur in the presence of InCl₃ under solvent-free conditions. 1,3-Dithioles have been constructed via DABCO mediated self-coupling of dithioesters in open air enabling the formation of two new C-S bonds and one ring in a single operation in contiguous fashion. The reactions proceeded smoothly affording the desired sulfur-rich heterocycles in good to excellent yields, exhibiting gram-scale ability and broad functional group tolerance utilizing easy to handle cheap and easily available reagents. The probable mechanisms for the formation of 1,2- and 1,3-dithioles from α-enolic dithioesters have been suggested.

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.

FeCl3-Mediated Oxidative Spirocyclization of Difluorenylidene Diarylethanes Leading to Dispiro[fluorene-9,5'-indeno[2,1-a]indene-10',9''-fluorene]s

A novel FeCl3-mediated oxidative spirocyclization for construction of a new class of di-spirolinked π-conjugated molecules, dispiro[fluorene-9,5'-indeno[2,1-a]indene-10',9''-fluorene]s (DSFIIFs), has been reported. The combination of FeCl3 with FeO(OH) triggered an unprecedented double one-electron oxidation of difluorenylidene diarylethanes to afford the corresponding dispirocycles in high yields. The highest fluorescence quantum yield was up to 0.94 in solution. This protocol is also applicable to the synthesis of the non-spirolinked dihydroindenoindenes.

ortho-, meta-, and para-dihydroindenofluorene derivatives as host materials for phosphorescent OLEDs

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.

C-shaped diastereomers containing cofacial thiophene-substituted quinoxaline rings: synthesis, photophysical properties, and X-ray crystallography

Synthesis and characterization of two diastereomeric C-shaped molecules containing cofacial thiophene-substituted quinoxaline rings are described. A previously known bis-α-diketone was condensed with an excess of 4-bromo-1,2-diaminobenzene in the presence of zinc acetate to give a mixture of two C-shaped diastereomers with cofacial bromine-substituted quinoxaline rings. After chromatographic separation, thiophene rings were installed by a microwave-assisted Suzuki coupling reaction, resulting in highly emissive diastereomeric compounds that were studied by UV-vis, fluorescence, and NMR spectroscopy, as well as X-ray crystallography. The unique symmetry of each diastereomer was confirmed by NMR spectroscopy. NMR data indicated that the syn isomer has restricted rotation about the bond connecting the thiophene and quinoxaline rings, which was also observed in the solid state. The spectroscopic properties of the C-shaped diastereomers were compared to a model compound containing only a single thiophene-substituted quinoxaline ring. Ground state intramolecular π-π interactions in solution were detected by NMR and UV-vis spectroscopy. Red-shifted emission bands, band broadening, and large Stokes shifts were observed, which collectively suggest excited state π-π interactions that produce excimer-like emissions, as well as a remarkable positive emission solvatochromism, indicating charge-transfer character in the excited state.

Synthesis and properties of annulated 2-(Azaar-2-yl)- and 2,2'-Di(azaar-2-yl)-9,9'-spirobifluorenes

A series of 9,9'-spirobifluorene-derived N-heterocycles were prepared by the reactions of 8,9-dihydrospiro(benzo[b]fluorene-11,9'-fluoren)-6(7H)-one and 8,8',9,9'-tetrahydro-11,11'-spirobi(benzo[b]fluorene)-6,6'(7H,7'H)-dione with a series of 2-amino-arenecarbaldehydes such as 2-aminobenzaldehyde, 2-aminonicotinealdehyde, 1-amino-2-naphthaldehyde, and 8-aminoquinoline-7-carbaldehyde. In addition to the absorption maxima based on the parent 9,9'-spirobifluorene skeleton in the 225–234, 239–280, 296–298, and 308–328 nm regions, the absorptions due to the π-π* transitions of the heterocycles were observed in the 351–375 nm region in the UV absorption spectra. All the compounds showed strong photoluminescences in the 390–430 nm region.