Design,Syntheses And Properties Of Triphenodioxazine-based Photoelectric Materials

Conjugated polymers have received a great deal of attention from both academic and industrial laboratories since they combine the best features of metals or inorganic semiconducting materials(excellent electrical and optical properties),with those of synthetic polymers(mechanical flexibility,simple processing,low cost).This synergy not only makes these functional materials useful in existing optoelectronic devices,but also creates completely new opportunities.The development of organic solar cells and supercapacitors as alternatives to the conventional electronic devices in the pursuit for generating and storing electrical energy has attracted a lot of attention.Thiophene and its derivatives are the dominant structure units of photovoltaic polymers in the current literatures about organic solar cells;and only a few types of conducting polymers,including polyaniline,polypyrrole,and derivatives of polythiophene,are applicable as organic electrode materials for supercapacitors.Although these materials exhibit relatively high performances,the lack of the diversity of polymer species hinders the corresponding photoelectric devices from becoming more efficient.On the other hand,triphenodioxazine is a fused heterocyclic structure,whose micromolecular derivatives have been developed as pigments and dyes for nearly a century.There are few reports on triphenodioxazine-based polymers,and even less on applications of these polymers for organic electronics,although triphenodioxazine have the potential to constitute a high-performance conjugated polymer.In view of this,several works are studied in this paper:(1)As a novel class of organic semiconductors,conjugated polymer should have appropriate band-gaps to cater to specific photoelectric applications.In order to reveal the relationship between band-gaps and polymer structures,a simple theoretical approach is developed in this paper:the repeating unit of a conjugated polymer is reduced to an elementary structure according to the characteristics of the atoms and the bonds,and several trends of the band-gaps of simple homopolymers,their substituted derivatives and alternating copolymers are given by using Hiickel molecular orbital method and graph theory with the parameters for the electron-withdrawing powers and the strengths of the bonding interactions of atoms.The approximation for complex structures and the trends of simple structures make it very convenient to analyze and predict the band-gap of a conjugated polymer qualitatively.(2)Two series of triphenodioxazine-based conjugated polymers,including the photovoltaic linear polymers--poly(2-viny 1-3,10-dialkoxytriphenodioxazine)(PVDATPDO)and poly(2-ethynyl-3,10-dialkoxytriphenodioxazine)(PEDATPDO),and pseudo-capacitive ladder polymers--poly(7H-phenoxazine-2,3-diyl:7-ylidene-8-yl-7-nitrilo-8-oxy)(H-LPP)and poly(6,9-dichloro-7H-phenoxazine-2,3-diyl:7-ylidene-8-yl-7-nitrilo-8-oxy)(Cl-LPP)are synthesized in this study.Since specific groups should be introduced into the polymers to meet the requirement of solution process and being photoelectric-activated materials,the synthesis of PVDATPDO and PEDATPDO is relatively complicated.Eight photovoltaic polymers are prepared via an optimized route,and the fluorescence behavior of the monomer in acid solution is investigated and confirmed as a dynamic quenching process.LPPs(H-LPP and Cl-LPP)are prepared via a facile route containing condensation and ring closure reactions.(3)The relationships among polymer structures,optical and electrochemistry properties of PVDATPDO and PEDATPDO are given by the experimental data and theoretical simulation.The results show that the polymer molecules are highly planar,which ensures good ?-conjugation along their backbones;both polymers are thermostable,and PVDATPDO is more rigid than PEDATPDO;their optical absorption bands are very broad,which generally match the solar spectrum;the band-gaps of PVDATPDO and PEDATPDO films are 1.4 eV and 1.5 eV,respectively,the length of the alkyl side chains has little effect on the optical and electrochemical properties of the polymers.Although the ideal planar structures,absorption spectra,band-gaps and frontier molecular orbitals endow the polymers with the potential of being powerful photovoltaic materials,the high rigidity of the polymer planes produces excessively strong interchain ?-? interaction,which hinders the materials from forming high-quality films and goes against the compatibility of the polymers with other constituents in bulk heterojunction solar cells,resulting unsatisfactory power conversion efficiencies of the photovoltaic devices,whose values are about 1%.Lengthening the alkyl side chains can improve the performances to some extent.(4)The microparticles of LPPs appear as micron-sized robs.LPPs,especially Cl-LPP,have excellent thermal stability,owing to their tight molecular structures.The intrinsic LPPs are insoluble,but Cl-LPP can be doped with and dissolve/swell in a strong acid.H2SO4-doping improves the hydrophilicity and backbone conjugation of Cl-LPP,and extends the optical absorption band of the polymer solution from the visible to the near-infrared.As electrode materials for supercapacitor,LPPs are suitable for both acid and neutral electrolytes.The hydrophobicity of intrinsic LPPs greatly limits the performances of corresponding supercapacitors,while H2SO4-doped Cl-LPP exhibits a much higher specific capacitance in a high-concentrated H2SO4 electrolyte,which is closed to the capacitance of polyaniline under the same test condition,and the energy density is as large as 2.2 times of that of a polyaniline supercapacitor due to the perfect ?-conjugation along Cl-LPP backbone.The performance capacitates doped Cl-LPP to be a potential competitor to the classical polymer electrode materials for powerful supercapacitors.A type of sheet-like quasi-solid-state supercapacitor based on doped Cl-LPP and a gel electrolyte is also developed in this work to exhibit the practicability of the novel material.The device is convenient to be fabricated and in-series connected,although the performance is not as good as an aqueous supercapacitor.