Synthesis, Self-assembly, Fluorescence Sensory And Electroluminescent Properties Of Indole-fused Heterocyclic Compounds

Organic π-conjugated molecules have received much attention on account their promising applications in organic solar cells, organic light-emitting diode(OLED), organic field-effect transistors and fluorescent sensors. Therefore, the design and synthesis of organic molecules with novel structures and expected optoelectronic functional have been widely concerned. It is well known that the electron-rich N-fused heterocyclic compounds(such as indole, indolocarbazole, phenothiazine, etc) have been widely applied in the fields of hole-transport materials, luminescent materials, organic semiconductors. Meanwhile, they could become the building blocks of functional supramolecular assemblies to fabricate novel organic nanostructures. The conjugated molecules arrange orderly in the nanofibers, which could be beneficial to the intermolecular electro-optical signal transportation(such as the amplification of fluorescent sensor signals), so the organogelation of π-gelators has become an important strategy to construct 1D nanostructures. Herein, we intended to design and synthesize indole-fused heterocyclic compounds with excellent self-assembling abilities. The effect of the molecular structures on the self-assembling and luminescent properties has been discussed. Some creative results have been obtained and outlined as below:We have synthesized new indolocarbazole derivatives with extended π-conjugated systems 2-4- 2-9. It was found that the indolocarbazole derivatives could emit strong blue light in solutions and in gel states. The indolocarbazoles without long carbon chains 2-4- 2-6 tended to form crystal or crystal-like aggregates due to strong π-π interactions. It should be noted that the strength of π–π interactions could be reduced to some extent via the introduction of long alkyl groups into N-positions of indolocarbazoles. As a result, the organogels based on indolocarbazoles were gained in alcohols(such as tert-amyl alcohol, n-octyl alcohol and the mixed solvents containing methanol), acetone and DMSO, directed by balanced π–π interactions. The molecular packing models of indolocarbazoles were proposed, and the layered structureswere involved in the nanostructures. Notably, the xerogels-based film of compound 2-9 could sense gaseous TNT with high sensitivity via fluorescence quenching. We suggested that the amplified fluorescence quenching could be induced by the enhanced intermolecular exciton diffusion along the long axis of the 1D nanofibers and the high surface-to-volume ratio and large interspace in the 3D networks favored the adsorption, accumulation and diffusion of gaseous molecules. Therefore, the gelation of polycyclic aromatic heterocyclic compounds was useful for the construction of novel functional soft materials, exhibiting unique photophysical and optoelectronic properties.We have synthesized new triphenylvinyl substituted idolocarbazole and carbazole derivatives with extended π-conjugated systems. They exhibited AIE behaviors during the organogelation processes, so that highly luminescent nanofibers could be generated. It was interesting that gaseous DNT and TNT could lead to obvious fluorescence quenching of TPEICE and TPEC in the xerogel-based films, meaning that they could be used as fluorescence sensory materials for detecting nitro compounds-based explosives. TPECBr could not sense DNT and TNT since its electron donating ability was poor. The decrease of the fluorescence lifetimes of TPEICE and TPEC in the xerogel-based films upon exposed to DNT suggested the occurrence of photo-induced electron transfer. Moreover, the charge-transfer complex between TPEICE and DNT was formed on account of the strong electron donating ability of indolocarbazole derivative. Therefore, the gelation of polycyclic aromatic heterocyclic compounds with AIE was useful for the construction of novel functional soft materials.We have synthesized four different N-alkylate di(carbazol-3-yl)methanone compounds DC2, DC4, DC8 and DC16. We found that DC4, DC8 and DC16 could form stable organogel in the organic solvents of ethanol, n-hexane, etc. Due to the absences of long alkyl chain and hydrogen bonding units in DC4, it could be regarded as non-traditional organogelator. In addition, the single crystal structure of DC2 could demonstrate that the two carbazole units showed a non-planar configuration which could reduce the intermolecular π-π interaction during the aggregation. We could deduced that J-aggregates were formed during the organogelation of DC4, while DC8 and DC16 formed H-aggregates in gel states.We have synthesized two new isomers of phospholes 2-DIPO and 3-DIPO, in which indoles were fused with phospholes in different manners. It was found that the OLED using 2-DIPO as the emitter exhibited moderate performance, but the performance of OLED based on 3-DIPO was poor due to its weak emission in solid state. Notably, the filter paper strip containing 2-DIPO could sense gaseous strong acid by naked eyes, and the emitting color of the test paper changed from green to yellow upon exposed to HCl vapor. This was the first report on the fluorescence probe based on phosphole for detecting acid vapors. 3-DIPO could not probe acid on account of the steric effect of neighboring groups of O=P to prevent the formation of H-bonding. Therefore, the optoelectronic properties of phosphole derivatives could be tuned by the fused manner of heterocyclic rings to phospholes. It provided a strategy for design novel phospholes with desired functionalities.We have synthesized three new salicylaldimine difluoroboron complexes with tert-butyl groups 1B-3B and their photophysical, self-assembling and piezofluorochromic properties were investigated. It was found that only salicylaldehydehydrazone difluoroboron complex 1B could form organogels in n-hexane and petroleum ether/CH2Cl2(v/v = 25/7) under ultrasound stimulus. We deemed that the nonplanar aromatic unit and the large steric effect tert-butyl groups in 1B would lead to balanced π-π interaction and induce the gel formation. The other two salicylaldimine difluoroboron complexes 2B and 3B could not gelate the selected solvents due to the strong π-π interaction. It was worth noting that as a nontraditional organogelator, 1B could self-assemble into 1D nanoribbons with intense blue emission. On the other hand, 2B and 3B showed reversible piezofluorochromism. The as-prepared crystals of 2B and 3B emitted strong blue-green light and could be transformed into the powders emitting yellow light after grinding, and the fluorescence could recover when the ground powders were heated or fumed with organic solvent. It was found that the reversible changes of the emitting colors for 2B and 3B induced by grinding and heating/fuming was resulted from the transformation between crystalline and amorphous states. It provided the strategy for designing new nontraditional ?-gelators and piezofluorochromic materials via tuning π-π interaction.