Synthesis Of Boron-containing Conjugated Polymers And Study On Their Photocatalytic Hydrogen Production Performances

In today’s world,the demand for new renewable energy is increasingly urgent.With the ambition of solving the challenges of the shortage of fossil fuels such as petroleum,the use of semiconductor photocatalysts to split water into hydrogen under the drive of visible light is considered to be the hope of the energy crisis.Among the currently developed photocatalysts,organic conjugated polymers have become one of the research hotspots in the energy and environmental fields due to their unique advantages such as excellent light absorption efficiency,tunable electronic properties,easy synthesis and high stability.The unique properties of boron-containing conjugated polymers due to the electron-deficient properties of boron have also attracted much attention in recent years.The introduction of boron into the conjugated backbone can effectively reduce the LUMO energy level,change the electron density distribution of the structure,accelerate the charge separation and transport,and thus promote the photocatalytic hydrogen evolution performance,but boron-containing conjugated polymers have rarely been studied in the field of photocatalysis.In this paper,we introduced boron into various organic conjugated skeletons using lithiation reactions,re-aromatization reactions,and coupling reactions,and were the first to investigate their hydrogen evolution performance for photocatalytic water splitting.The effects of triarylboron system and boron-nitrogen system on the electronic properties,molecular structure,physical and photophysical properties of the materials are systematically investigated to reveal the relationship between the structure and hydrogen evolution performance.Meanwhile,the mechanism and transition state forms of boron-containing materials for photocatalytic hydrogen evolution are investigated to achieve a deeper exploration of the photocatalytic hydrogen evolution process of boron-containing organic conjugated polymers,which can further guide and optimize the design and synthesis of boron-containing photocatalysts.1.A series of D-A linear conjugated oligomers（PBn S）with the thiophene unit as the donor and the borofluorene unit as the acceptor were successfully synthesized and their photophysical properties were investigated.The experimental results show that the energy band structure changes regularly with the increase of thiophene chains.In addition,we have attempted to apply these boron-containing materials to the photocatalytic water splitting for hydrogen production for the first time,and the experimental results show that PB2S exhibits a high hydrogen evolution rate（HER）of 223μmol g^-1 h^-1 under visible light（λ>420 nm）,which is better than many reported linear polymers.The enhanced photocatalytic hydrogen evolution performance may be attributed to the combination of strong electron-absorbing property of the organoboron moiety and good electron-donating ability of the thiophene moiety promoting the charge separation ability of the material,thus allowing more photogenerated charge carriers to participate in the water reduction reaction.The results indicate that our strategy is successful and the rational design of the molecules is instructive for the development of conjugated（oligomeric）polymers for high-performance photocatalytic hydrogen evolution.2.A series of conjugated polymers based on dibenzothiophene-S,S-dioxide,C6-SO ofπ-A type,N-SO of D-π-A type,and B-SO and C3N3-SO of A₁-π-A₂ type,were synthesized and applied to the study of photocatalytic hydrogen production performance.The photocatalytic experiments revealed that the A₁-π-A₂ type B-SO and C3N3-SO photocatalysts exhibited competitive hydrogen evolution rates under visible light irradiation,reaching 778 and 1603μmol g^-1 h^-1,respectively,which were significantly better than the hydrogen evolution rates of C6-SO and N-SO.This indicates that the introduction of secondary acceptor units（organoboron or triazine groups）in the repeating units of the conjugated polymer backbone is beneficial to enhance the photocatalytic hydrogen production efficiency.The underlying reason is that the multi-level acceptor unit can promote the separation efficiency of photogenerated carriers,which increases the chance of photogenerated carriers reaching the material surface and thus promotes the photocatalytic hydrogen production performance of the material.3.Three conjugated polymers were successfully prepared,including all-carbon NCC,nitrogen-doped NNC,and boron-nitrogen-doped NBN.The structural characteristics,electronic properties,and photophysical properties of polymers can be tuned by introducing heteroatoms nitrogen and boron.The experimental results show that the presence of N and B had a positive effect on the hydrogen precipitation rate of photolysis,and the HERs of the three polymers were 0μmol g^-1 h^-1（NCC）,65μmol g^-1 h^-1（NNC）and 21000μmol g^-1 h^-1（NBN）.The mechanism study confirms that NBN has a suitable energy band structure and electron density distribution,which ensures the strong redox ability of the photocatalyst while taking into account the broad visible light absorption.In addition,the large dipole moment of NBN ensures the construction of a strong built-in electric field,which enhances the separation and migration efficiency of photogenerated carriers.4.Three linear isoelectronic conjugated polymers PCC,PBC,and PBN were synthesised by self-polymerization or co-polymerization of fluorene monomer and/or the B←N fused monomer for photocatalytic hydrogen production from water.PBN presented an excellent photocatalytic hydrogen evolution rate（HER）of 22350μmol g^-1 h^-1(AQY₄₂₀=23.3%)under visible light irradiation（λ>420 nm）,which is 7times that of PBC(HER=3120μmol g^-1 h^-1)and 31 times that of PCC(HER=720μmol g^-1 h^-1).The enhanced photocatalytic activity of PBN is due to the improved charge separation and transport of photoinduced electrons/holes originating from the lower exciton binding energy（E_b）,longer effective fluorescence lifetime,and stronger built-in electric field,which is caused by the introduction of the polar B←N unit into the polymer backbone.Moreover,the extension of the visible light absorption region and the enhancement of surface catalytic ability further increase the photocatalytic hydrogen production activity and apparent quantum efficiency of PBN.This work reveals the potential of B←N fused structures as building blocks as well as proposes a rational design strategy for achieving high photocatalytic performance.