Synthesis And Photovoltaic Performance Of Phenothiazine Sensitizing Dyes

Dye Sensitized Solar Cells(DSSCs)have drawn more and more attention because of the advantages of high photo-electricity conversion efficiency,low cost,simple process and environmentally friendly.As the key material of the DSSCs,sensitizers affect the performance and stability of DSSCs through light harvesting,electron transfer,reduction regeneration processes.Phenothiazine is a non-planar heterocyclic compound with a butterfly conformation.It has been widely employed in organic sensitizers due to the strong electron-donating ability,good hole-transporting property and flexible modification in recent years.In the dissertation,eight sensitizers were designed and synthesized with phenothiazine and its derivatives as the electron donors.Their structures were characterized by ~1H NMR and HRMS and the key reaction mechanisms were studied.Their spectral properties,photoelectric performance,and electrochemical properties were measured.Density functional theory(DFT)method was used to perform the theoretical calculation and the structure-property relationship was analyzed.A series of three“D-?-A”sensitizers DPA-1?DPA-3 were synthesized with N-phenyl phenothiazine as the electron donor,cyanoacetic acid as the electron acceptor,fluorenone-benzene,fluorenone-furan or fluorenone-thiophene as?-bridge respectively.A phenyl group was introduced to the phenothiazine on the N atom by nucleophilic substitution,and then the bromide obtained through bromination with NBS was transformed to N-phenyl phenothiazine borate.2,7-dibromo-9-fluorenone was coupled successively with N-phenyl phenothiazine borate and 4-formylphenylboronic acid to obtain the aldehyde.Finally,the aldehyde reacted with cyanoacetic acid by Knoevenagel condensation to get compound DPA-1.2,7-dibromo-9-fluorenone was coupled with 2-furylboronic acid or 2-thiopheneboronic acid and N-phenyl phenothiazine borate in sequence,then an aldehyde group was introduced on the furan or thiophene ring by Vilsmeier reaction,finally,the aldehydes reacted with cyanoacetic acid by Knoevenagel condensation to get compounds DPA-2 and DPA-3.Thespectral properties,photoelectric performance,and electrochemical properties of DPA-1?DPA-3were measured,and the results show DPA-2 bearing fluorenone-furan bridge exhibits better molecular planarity,the bathochromical shift of the UV-vis absorption spectra,and shows higher IPCE values and photoelectric conversion efficiency of 4.35%(Jsc=12.76 m A/cm~2,Voc=0.69 V,ff=0.66).A series of three“D-A-?-A”phenothiazine dye sensitizers DAPA-1?DAPA-3 were designed and synthesized with N-phenyl phenothiazine as the electron donor,cyanoacetic acid as the electron acceptor,benzene,furan or thiophene as?-bridge respectively and benzothiadiazole as auxiliary acceptor embedded between the electron donor and?-bridge.N-phenyl phenothiazine borate reacted with the 4,7-dibromo-2,1,3-benzothiadiazole through Suzuki coupling to get the“D-A”unit.Three aldehydes were obtained when the“D-A”unit coupled with 4-formylphenylboronic acid,5-formylfuran-2-ylboronic acid or 5-formylthiophen-2-ylboronic acid,respectively.Knoevenagel condensation between the aldehydes and cyanoacetic acid gave the target compounds DAPA-1?DAPA-3.The spectral properties,photoelectric performance,and electrochemical properties of DAPA-1?DAPA-3 were measured,and the results show the introduction of benzothiadiazole can adjust the HOMO-LUMO energy gap effectively,and optimize the absorption spectral correspondingly.Compared with DAPA-2 or DAPA-3,DAPA-1 bearing benzene bridge shows larger open circuit voltage and short circuit current due to its higher electron injection efficiency and IPCE values,and it exhibits a higher photoelectric conversion efficiency of 6.76%(Jsc=14.25m A/cm~2,Voc=0.69 V,ff=0.69).Two T-shape sensitizers T-1 and T-2 were designed and synthesized with phenothiazine as the electron donor,cyanoacetic acid as the electron acceptor.Phenothiazine was substituted with 4-(4-bromophenyl)-2-(thiophen-2-yl)thiazole,and then the aldehyde group was introduced by Vilsmeier reaction to prepare simultaneously monoaldehyde compound with one aldehyde group on thiazole ring and dialdehyde compound with one aldehyde group on thiazole ring and the other on phenothiazine unit.Finally,the intermediate aldehydes reacted with cyanoacetic acid by Knoevenagel condensation to get compound T-1 with single acceptor and T-2 with double acceptors.The spectral properties,photoelectric performance,and electrochemical properties of T-1 and T-2 were measured,and the results show T-2 bearing double acceptors exhibits extended absorption spectral with stronger absorption intensity when compared with T-1,which benefits the photocurrent and shows a higher photoelectric conversion efficiency of 4.35%(Jsc=10.29 m A/cm~2,Voc=0.65 V,ff=0.65).