Carbazole/fluorene based conjugated small molecules: synthesis and comparative studies on the optical, thermal and electrochemical properties

Abstract

A series of new molecular materials with varying carbazole and fluorene contents were prepared employing Suzuki coupling and their properties were compared. A variation of the core nucleus from fluorene to carbazole was a key point to study its effect on thermal, optical and electrochemical properties. The materials were characterized by FTIR, 1 H-, 13 C-NMR, APCI-MS and elemental microanalyses. The synthesized materials exhibit the slight bathochromic shifted UV/Vis spectra in dilute solution and emission maxima in the blue region along with good thermal stability. The dyes with fluorene core are electrochemically and spectrally stable with high lying HOMO levels, in contrast to the carbazole only materials, having the potential as light emitting and hole transporting materials in organic light emitting diodes. chemical stabilities, exceptional electronic and optical properties. 1-3 Moreover, the advantages of oligomers includes the tuning of their properties by varying the molecular design by selecting the suitable side chain and terminal groups for excellent processing, changing the end-capping groups, or by insertion of certain groups. Thus, by alteration of the conjugated cores charge transporting and emissive properties can be controlled. Consequently, the optoelectronic properties can be optimized by divergent molecular design along with the processing. [4][5][6] Molecular materials can serve as model systems for polymeric systems due to their well-defined structure, monodispersity as well as high purity since the purification of small molecules is much easier, as compared to the polymers. 7The excellent hole-transporting capacity, high charge carrier mobility, the formation of stable radical cations coupled with efficient thermal, morphological and photophysical properties 8 of carbazole makes possible its proficient use in organic semiconductor devices like OLED 9 , solar cells, 10 and non-linear optic (NLO) materials 11 either as a main core/or as a substituent. Zhu et al have reported the synthesis of carbazole only oligomers with improved properties for OLEDs. 12 A series of phosphine oxide hosts with different hole transporting carbazolyl units were designed for blue phosphorescent OLEDs with improved efficiency. 13-16 Recently, aryl amine modified phosphine oxide hosts have also been developed. 17 Conversely, on account of the synthetic versatility of fluorene moiety at aromatic 2,7 and C-9 positions alongwith thermal and chemical stability 18 its use in optoelectronic devices has extensively been investigated. Fluorene derivatives have been used as blue-emitting materials in (OLEDs). 19 Solubility and photoluminescent quantum yield of a number of fluorene based compounds are appropriate for use in OLEDs different electron donor and acceptor substituents in their structure in order to adjust the HOMO and LUMO energy levels. For this purpose, carbazole-fluorene based dendrimers are being synthesized with improved hole transporting properties for multilayer OLEDs applications. 22 Nobile et al reported...

Indole‐Based Aza[n]helicenes (n=5, 6) with Violet‐Blue Fluorescence and Two‐Photon Absorption (TPA)

“…In addition, energy levels of HOMOs and LUMOs for these [6]helicenes were in the order HTPI < the carbazole-based monoaza[6]helicene [23] < HBTNI, indicating that HTPI had excellent electrochemical stablity. [25] Ultraviolet-visible (UV-vis) absorption and single-photon excited fluorescence (SPEF). HTPI, HBTNI, HIPD and HPI were soluble in the common solvents such as anhydrous hexane, dichloromethane, ethyl acetate, trichloromethane and methanol.…”

“…The organic layer was washed with water for three times (3 × 100 mL) and dried over anhydrous Na 2 SO 4 . The solvent was removed in vacuo, the residue was purified by flash chromatography on silica gel (petroleum ether/EtOAc, 25…”

Fluorene Derivatives Bearing Halogenomethyl Groups: Synthesis, Molecular Structures, and Halogen/Hydrogen Bonding Patterns in the Crystalline State

“…Fluorene‐based compounds were shown to be suitable for the construction of systems with electrically conductive and electroluminescent properties, which have, for example, been investigated as luminophore in organic light‐emitting diodes (OLEDs) . Furthermore, some fluorene derivatives have been used in the development of dyes,, insecticides[12a] and pharmaceuticals. [12b], A great variety of fluorene‐containing compounds, from single molecules to oligomeric, and polymeric structures is also known to be suitable for a selective detection of biologically, environmentally and economically relevant species.…”

“…In contrast to fully conjugated fluorene derivatives, required by some of the above mentioned applications to achieve a planar configuration and thus a more efficient π–π interaction surface,, , , , , compounds of the type shown in Figure c, showing a higher degree of flexibility, have been rarely reported. The dialkylation in the 9‐position is necessary to achieve higher quantum yields and enhances the thermo‐ and photostability by preventing an oxidation to fluorenone …”

Synthesis and device performance of an oxygen‐interrupted bipolar polymer host with carbazole and triphenylphosphine oxide in the main chain

“…Therefore, the development of host materials is of great importance for polymer light emitting diodes (PLEDs) with high efficiencies. In contrast to small molecular hosts, polymer hosts attracted our attention due to their low‐cost solution processing and large‐area fabrication …”