Architecture Of Ordered Polyimide Organic Semiconductors And Application In Photocatalysis

Photocatalytic solar-to-chemical energy conversion presents an alternative promising way to address energy crisis and environmental issues due to its great potentials in exploring hydrogen based clean energy,pollutant degradation and green chemical synthesis.During the past decade,Polymer semiconductors have attracted extensive attention because of their merit feature of abundant sources,low-cost,accessible synthesis protocols and designable structures.Polyimide,used to be a well-known engineering material,has been reported as a promising polymer photocatalyst by our group in the past few years.However,preparation of polyimide photocatalyst is very limited and thus efficient structure tailoring is far from available.Consequently,the photocatalytic performance of the material remains undesirable and further deep insight into the structure-property relation is also unclear.Therefore,exploring facile strategy towards modulation of polymer microstructure and a better understanding of structure-property relation is much desired to boost the improvement of imide-based semiconductor in solar-to-chemical energy conversion.In this context,aimed to constructing polyimide with novel microstructure and chemical backbone,we carried out the following research works:1.Preparation of a novel(001)dominated polyimide supermolecule framework with facet by solvent induced crystallization.We developed a novel method for synthesis of polyimide with highly ordered ?-? stacking structure by employing DMF/EG as the reaction solvent.The DMF molecule can served as a template guiding the polymer chain self-organized along the stacking direction into ?-conjugated delocalization framework.This unique ?-stacking structure largely promoted the electron couplings between the adjacent molecular layers and thus created an efficient transport channel for photo-induced charge carriers in the vertical stacking direction.Compared with the conventional PI-TC from thermal annealing,higher charge mobility together with enhanced carrier separation efficiency can be obtained with this newly designed ?-? stacking supermolecule system.As expected,this material exhibited consecutive photocatalytic hydrogen production with an average rate of 1640?mol h-1g-1 which is 3 times as that of PI-TC and the representative polymer semiconductor-graphitic carbon nitride(g-C3N4).In addition,the effect of solvent and reaction temperature on the crystal structure and photoelectrical property are discussed systematically,paving the way of designing new efficient polymer photocatalysts in the future.2.Polyimides with different conformation were synthesized using solvents with different polarity and just by this means,the mechanism of solvent induced crystallization and the structure-property relationship were thoroughly investigated.Five organic solvents like Toluene,m-Cresol?NMP,DMF and DMAc together with EG are selected as binary solvent mixture for preparing polyimide and resulted in polymers with dendritic(PI-Toluene),spherulitic(PI-m-Cresol,PI-NMP),and laminar(PI-DMF,PI-DMAc)conformation,respectively.Hansen solubility parameter(HSP)combined with quantum calculation disclosed that the dipole and hydrogen bond interaction between solvents and polymer chains played a critical role in controlling the organization and rearrangement of crystalline polymer chains.With the increasing polarity,the solvent-polymer interactions gradually increased,and the polymer conformation exhibited evolutionary transformation from randomly dendritic to ordered ?-? stacking.This solvent mediated conformation transfer process provides a platform for structure-property relation study.The photoelectrical properties of polyimide were sensitively influenced by the crystal structure and polymer morphology.In laminar conformation with strong intermolecular ?-? interactions,the light absorption as well as the mobility and separation of photoinduced carriers were greatly improved compared with those of dendritic and spherulitic ones.For photocatalytic hydrogen evolution from water splitting,polyimide with laminar conformation(PI-DMF/DMAc)exhibited 16 times enhancement in activity than the dendritic PI-Toluene.At last,the general applicability of this protocol is confirmed by treating the as-prepared PI-TC under the same condition.After treatment,the pristine dendritic PI-TC shifts to more ordered ?-? stacking structure,and the photoelectrical properties as well as photocatalytic hydrogen production are also much enhanced.3.By introducing building blocks with extended ?-conjugated core,polyimides with new chemical structure were synthesized using the same method developed above.Perylene based imide(PTCDI-T,PTCDI-H)ultrathin nanosheet were prepared by linking perylenetetracarboxylic dianhydride PTCDA with amines with triazine(T-)and heptazine(H-)core.The integration of more extended ?-conjugated core not only improved the light harvest and charge separation efficiency,at the same time,the valence band level shifted to much lower(positive)position,providing stronger driving force for photocatalytic oxidation reactions.PTCDIs exhibited superior oxygen activation capability in transforming oxygen to superoxide radicals and singlet oxygen species at room temperature.When used in photocatalytic aerobic oxidation couplings of benzylamine,PTCDI-T and PTCDI-H showed outstanding conversion yield with high selectivity.The turnover frequency reached up to 230 ?mol g-1 h-1(PTCDI-T)and 250 ?mol g-1 h-1(PTCDI-H),respectively.This value was much higher than that reported photocatalysts ranging from metal oxide to conventional polymer semiconductors(up to 2 order of magnitude enhancement for poly(Benzooxadizaole)and mpg-C3N4).Besides,these newly designed PTCDIs exhibited excellent durability and recyclability as more than 95%conversion yield can be obtained in five consecutive catalytic runs.