Abstract:The present works report the first structure-properties relationship study of a key class of organics semiconductors, ie the four spirobifluorene positional isomers possessing a para, meta or ortho linkage. The remarkable and surprising impact of the ring bridging and of the linkages on the electronic properties of the regioisomers has been particularly highlighted and rationalized. The impact of the ring bridging on the photophysical properties has been stressed with notably the different influence of the linkages and the bridge on the singlet and triplet excited states. The first member of a new family of spirobifluorenes substituted in position 1, which presents better performances in blue Phosphorescent OLEDs than those of its regioisomers is reported. These features highlight not only the great potential of 1-substituted spirobifluorenes but also the remarkable impact of regioisomerism on electronic properties.

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.

Reported here are C1‐linked spiro‐bifluorene dimers. A comprehensive study is carried out to analyze the electronic properties of these highly twisted structures. This work shows that the C1‐position enables the design of pure hydrocarbon materials, with a high triplet energy, for hosting blue phosphors in efficient phosphorescent OLEDs (PhOLEDs). To date, this work describes the highest performance of blue PhOLEDs ever reported for pure hydrocarbons (external quantum efficiency of ca. 23 %), thus highlighting the potential of the C1‐spirobifluorene scaffold in organic electronics.

Presented here is the study of a new example of [4]cyclofluorene, with ethyl chains on the bridgeheads. Its molecular structure was established by solution NMR spectroscopy and single‐crystal X‐ray diffraction. Three successive oxidation processes and one reversible reduction were observed through cyclic voltammetry. The optical properties were characterized both in solution and thin film by UV/visible spectroscopy as well as stationary and time‐resolved fluorescence. It was found that this [4]cyclofluorene displays different characteristics compared with the other [4]cyclofluorenes substituted by methyl or propyl chains: a simple modification of the chain length induces a non‐negligible effect on the emission properties, which must be linked to the specific arrangement of the fluorene units. Furthermore, single‐crystal X‐ray diffraction reveals the formation of a pseudo‐tubular solid‐state arrangement of fully symmetrical ring structures, which was not observed for the other members of the [4]cyclofluorenes family. This finding could open the way to modulation of properties of cyclofluorenes through alkyl chain engineering.

Herein, a high‐efficiency host material for single‐layer phosphorescent organic light‐emitting diodes (SL‐PhOLEDs) is reported. This host material is synthesized via an efficient approach and is constructed on the association of an electron‐rich phenylacridine unit connected by a spiro carbon atom to an electron‐deficient 2,7‐bis(diphenylphosphineoxide)‐fluorene. In addition to a high ET value and adequate highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels, the key point in this molecular design is the suitable balance between hole and electron mobilities, which leads to a high‐performance blue SL‐PhOLED with an external quantum efficiency of 17.6% (current efficiency = 37.8 cd A−1 and power efficiency = 37.1 lm W−1) and a low Von of 2.5 V. This performance shows that the molecular design of the present host fulfills the criteria required for high‐efficiency SL‐PhOLEDs. The present performance is one of the highest reported to date for blue SL‐PhOLEDs and more importantly shows the potential of such a molecular design to reach very high‐performance single‐layer devices.

An electroactive polyazomethine is prepared from a solution processable 2,5-diaminothiophene derivative and 4,4 ′ -triphenylamine dialdehyde by spraycoating the monomers on substrates, including indium tin oxide (ITO) coated glass and native glass slides. The conjugated polymer was rapidly formed in situ by heating the substrates at 120 ° C for 30 min in an acid saturated atmosphere. The resulting immobilized polymer is easily purifi ed by rinsing the substrate with dichloromethane. The on-substrate polymerization is tolerant towards large stoichiometry imbalances of the comonomers, unlike solution step-growth polymerization. The resulting polyazomethine is electroactive and it can be switched reversibly between its neutral and oxidized states both electrochemically and chemically without degradation. A transmissive electrochromic device is fabricated from the immobilized polyazomethine on an ITO electrode. The resulting device is successfully cycled between its oxidized (dark blue) and neutral (cyan/light green) states with applied biases of + 3.2 and − 1.5 V under ambient conditions without signifi cant color fatigue or polymer degradation. The coloration effi ciency of the oxidized state at 690 nm is 102 cm 2 C − 1 .2 was chosen as a complementary monomer for condensing with 1 . This was because the resulting polyazomethine ( 3 ) was expected to absorb more in the yellow region of the spectrum than its all-thiophene polyazomethine counterpart. [ 17 ] This is a result of large twist angles between the homoaryl moieties and the azomethines of 3 that limit its degree of conjugation. However, its charged state was expected to be signifi cantly colored because of intramolecular charge transfer courtesy of the electron rich triarylamine. [ 18 ] A bold color transition between the neutral and charged states was therefore expected for the electroactive 3 , resulting in a electrochromic material whose switching could easily be seen by the eye. This is a desired property for use in low-resolution and large area electrochromic display applications such as signs and billboards.The on-substrate polymerization of 3 was initially done using stoichiometric amounts of the complementary comonomers. These conditions were used because step-growth polymerization is known to be intolerant towards comonomer stoichiometric imbalances. [ 19 ] Only trimers and lower molecular weight oligomers are obtained when the molar ratio of the comonomers is varied by more than 1 mol%. The 1:1 mole ratio of 1 to 2 was further used to test the polymerization conditions required to produce 3 . Since this ratio was expected to yield 3 , the effect of the monomer concentration on the resulting comonomer and polymer fi lms deposited on the substrate could also be examined.The polymerization of 3 was done by spray-coating the premixed comonomer solutions onto glass substrates. The substrates were then heated between 60 and 150 ° C in a TFA saturated environment. TFA was chosen as the catalyst because of its high vapor pressure. A saturated acid atmosphe...

Nanorings, which are macrocyclesp ossessing radially directed p-orbitalsh ave shown fantastic developmenti n the last ten years. Unravelling their unusuale lectronicp roperties has been one of the driving forces of this research field. However,a nd despite promising properties, their incorporationi no rganic electronic devices remains very scarce. In this work, we aim to contribute to bridge the gap between organice lectronics and nanorings by reporting the synthesis, the structurala nd electronic properties and the incorporation in an organic field-effect transistor (OFET) of ac yclic tetracarbazole, namely[ 4]cyclo-N-ethyl-2,7-carbazole( [4]C-Et-Cbz). The structural, photophysicala nd electrochemical properties have been compared to those of structurally related analogues [4]cyclo-9,9-diethyl-2,7-fluorene [4]C-diEt-F (with carbon bridges) and [8]-cycloparaphenylene [8]CPP (withouta ny bridge) in ordert os hed light on the impact of the bridging in nanorings. This work shows that nanorings can be used as an active layer in an OFET and provides a first benchmark in term of OFET characteristicsf or this type of molecules.

For the last ten years, ring‐shaped π‐conjugated macrocycles possessing radially directed π‐orbitals have been subject to intense research. The electronic properties of these rings are deeply dependent on their size. However, most studies involve the flagship family of nanorings: the cyclo‐para‐phenylenes. We report herein the synthesis and study of the first examples of cyclofluorenes possessing five constituting fluorene units. The structural, optical and electrochemical properties were elucidated by X‐ray crystallography, UV‐vis absorption and fluorescence spectroscopy, and cyclic voltammetry. By comparison with a shorter analogue, we show how the electronic properties of [5]‐cyclofluorenes are drastically different from those of [4]‐cyclofluorenes, highlighting the key role played by the ring size in the cyclofluorene family.