Charge Transfer Mechanism Of Triphenylamine-substituted Zinc Porphyrin-sensitized TiO₂ At Photodegradation Interfaces

In contrast with single chromophore dyes,triphenylamine（TPA）-containing non-planar multi-chromophoric sensitizers can improve the three-dimensional light capture ability and prolong the photoelectron lifetime.The advantages of this kind of porphyrin have been deeply explored in dye-sensitized solar cells,while the application of these dyes,particularly as dye-sensitized semiconductor particles,in photodegradation of organic pollutants has been rarely reported.Therefore,this work aims to explore the efficacy of non-planar porphyrins in photocatalysis by designing seven zinc porphyrins.Triphenylamine derivatives were selected as the main building blocks to construct the molecules,with detailed structural characterization.Those zinc porphyrins are used for sensitizing titanium dioxide（TiO₂）particles to get composite catalysts,which were applied in photodegradation of organic pollutants.Firstly,ZP1 and TPA-containing porphyrins XZP1-3 were designed and synthesized to explore the application of TPA-containing non-planar multi-chromophoric sensitizers in photocatalytic degradation.The electronic structures of the porphyrins were investigated by density functional theory（DFT）calculations,and the dye-sensitized composites were investigated using spectroscopic and electrochemical characterizations.XZP2/TiO₂exhibited the smallest charge transfer resistance and longest electron lifetime among the studied composites and the the highest photodegradation performance for acid black（AB1）and tetracycline（TC）contaminated water samples.TPA,as the second chromophore of XZP2,was linked with a freely rotating benzene ring with the porphyrin,while XZP1 was designed with directly linkage of TPA with the porphyrin.It is found that XZP2/TiO₂had better photocatalytic performance than XZP1/TiO₂.Compared with the XZP3/TiO₂composite,the XZP2/TiO₂catalyst with the linear substituent group had better performance.This work demonstrates that photodegradation using triphenylamine-substituted non-planar porphyrin-sensitized semiconductor particles can effectively improve the performance of semiconductors and promote catalytic interfacial charge separation.Based on the aforementioned findings,we concluded that the configuration of the substituents affects the separation and recombination of charge carriers at the catalytic interface and the photoelectron lifetime.In order to achieve better carrier separation and to further reduce charge recombination,whilst keeping the linear configuration,the structure of porphyrin XZP2was expanded by inserting an extra phenyl unit inbetween the TPA derivative and the porphyrin cycle,giving zinc porphyrin XZP4.We found that the performance of XZP4/TiO₂was improved,revealing that the steric distance between triphenylamine and zinc porphyrin has an important effect on photodegradation performance.This work indicates that by optimizing the spatial distance between the two chromophores,the charge separation efficiency can be effectively improved and the photodegradation can be accelerated.In order to further verify the effect of substituent structure,especially the effect of their electron donating ability on charge separation and recombination efficiency at photodegradation interface,we designed two more zinc porphyrins XZP5 and XZP6 which were substituted by carbazole and thiophene derivatives,respectively.The photodegradation properties of XZP2/TiO₂was compared and the interfacial charge transfer mechanism was discussed.We found that XZP5/TiO₂had excellent photodegradation performance,even better than XZP2/TiO₂.This work suggests that the effect of substituent structure could affect the ability of catalytic interface charge separation and migration.The above results demonstrate that the modification of TiO₂with triphenylamine-substituted non-planar multi-chromophoric chromophore zinc porphyrins not only broadens the photoresponse range of the catalytic particles,but also promotes the separation of photogenerated carriers and prolongs the photoelectron lifetime.This work provides a new method for improving the performance of dye-sensitized semiconductor photocatalysts.