Synthesis And Properties Of Linear Conjugated Oligocarbazoles

The design and optic-electronic properties of novel n-conjugated systems have attracted increasing attention,since functional molecular materials have been proved their high potential applications in solar cell,organic light-emitting diodes(OLEDs), sensors,field-effect transistors,frequency converters,etc.As a result of conjugated oligomers with the regular structure,easy access to high-purity materials and easy formation of supramolecular structure,the investigation on them has been changed from establishing the simple relationship between structure and properties into developing high performance organic optoelectronic materials and building supramolecular systems with a unique function,used in organic optoelectronic devices,nano-devices and bio-related devices and other fields.Therefore,rational design of synthetic routes and reaction conditions and simple,efficient synthesis of monodisperse conjugated oligomers are of great significance in the development of basic research and technological progress in the field of conjugated organic optoelectronic materials.In addition,carbazole is a high performance fluorescent chromophore,is easy to carry out chemical modification and is of excellent thermal stability,so it is widely used as a basic unit to build a fluorescent compound,as a variety of organic optoelectronic materials.Chemically,carbazole can be easily functionalized at its 3-,6-,or 9-positions and covalently linked to other molecular moieties,so it is a potential AB monomer for the construction of linear oligomers.Herein,we have designed,synthesized and characterized a series of linear carbazole-based oligomers,which were functionalized with anthracene and benzothiadiazole.We have investigated the thermal properties and photophysical properties of conjugated oligomers with different structure.The main results are outlined as following:Firstly,we have synthesized,three conjugated oligocarbazoles(An-Cz2, An-Cz4 and An-Cz6) containing anthracene moiety using Wittig reaction and Heck reaction alternately.The functional groups in these three oligomers link to each other through double bonds,so we used Heck reaction to ensure that the target molecules have all-trans conformations.By FT-IR spectroscopy,we found that all three molecules have strong absorption peaks in the vicinity of 960nm^-1,which locate in the characteristic absorption region of trans-double bond,so we can confirm that these three oligomers are all-trans conformations.By studying the thermal properties of three oligomers,we found that they have good thermal stability,upon which the length of oligomer chains have no significant impact. The good thermal properties provide a basis for the future design and synthesis of novel oligomers with good thermal stability.In addition,when we investigated the photophysical properties of the oligocarbazoles,we found that they have typical aggregation-induced emission(AIE) properties,that is,molecules exhibit no or faint luminescence in solution,but show intense fluorescence in the aggregation state (nanoparticles or neat film).In order to verify the existence of nanoparticles in the THF/H20 system,we made use of SEM to detect the THF/H₂O mixtures (THF:H₂O=1:9) of three oligomers,respectively.The SEM images show that An-Cz2 and An-Cz6 assemble into nanoparticles in the THF/H₂O system,while An-Cz4 assembles into nanorings.It means that An-Cz4 is of the nature of self-assembly.Further study on the photophysical properties of three oligomers in PMMA film and frozen state shows that they exhibit strong emission in both of states.Comparing the molecular structures and photophysical properties of three oligocarbazoles,we found that AIE property is not only from the aggregation between molecules,but from its own special structure,that is,the distyrylanthracene moiety in the molecular centre,which is consistent with the conclusions in our previous work. Finally,we have synthesized three conjugated oligocarbazoles(BCBT,TCBT and HCBT) containing 4,7-dibromobenzo[c][1,2,5]thiadiazole moiety using Wittig reaction and Heck reaction alternately.The functional groups in these three oligomers link to each other through double bonds,so we used Heck reaction to ensure that the target molecules have all-trans conformations.By FT-IR spectroscopy,we found that all three molecules have strong absorption peaks in the vicinity of 960nm^-1,which locate in the characteristic absorption region of trans-double bond,so we can confirm that these three oligomers are all-trans conformations.By studying the thermal properties of three oligomers,we found that they have good thermal stability,upon which the length of oligomer chains have no significant impact.The good thermal properties provide a basis for the future design and synthesis of novel oligomers with good thermal stability.In addition, when we investigated the photophysical properties of the oligocarbazoles,we found that we found that their absorption spectra have two absorption peaks,in the vicinity of 340nm and 506nm,respectively.The absorption located in the 304nm is attributed to the absorption of carbazole,andthe absorption located in the 304nm is attributed to the absorption of ICT.From the fluorescence spectra,we can see that the emission peaks of these three compounds are around 590nm,and there is no fine structure.This shows that these three compounds can serve as a red light in the material used in OLEDs.