Synthesis, Structure And Study Of Properties For Schiff-base Zinc Benzoic Acid Base Porphyrins

Porphyrin compounds are the main material of dye sensitized solar cells （DSSC）, theirsynthesis and application are an important research direction. In order to enlarge the lightabsorption range of the dyes, four donor-Ï€-spacer-acceptor porphyrin dyes, which have notbeen reported, were synthesized. By introducing Schiff base unit into the meso positions andreasonable structural modification of meso-substituted porphyrins, the photovoltaicperformance was improved greatly.In this paper, the dye was synthesized as following steps. For example P_Zn-BIA-COOH,first,5-（4-nitro） phenyl-10,15,20-tris （3,4,5-trimethoxylphenyl） porphyrin1was obtainedfrom3,4,5-trimethoxybenzaldehyde in the presence of4-nitrobenzaldehyde and pyrrole.Subsequently, compound1was converted into5-（4-amidogen） phenyl-10,15,20-tris （3,4,5-trimethoxylphenyl） porphyrin2by Tin（II） chloride dehydrate with36%HCl. Finally,compound2was treated with Zn（OAc）2, and then reacted with4-formyl benzoic acid bybonus-elimination reaction to give P_Zn-BIA-COOH. P_Zn-COOH were synthesized by the sameprocedure as that for P_Zn-BIA-COOH except4-carboxyl benzyl group was used instead ofp-（4-carboxyl benzyl idene amino） phenyl group at the meso position. To investigate how thestructure of Schiff base affects the cell performance of devices, the synthesis and the spectral,electrochemical, and photovoltaic properties of these porphyrin based sensitizers were studied.By introducing the Ï€-spacer consisting of a Schiff base unit, the light absorption ofP_Zn-BIA-COOH has been broadened and slightly red-shifted compared to that of P_Zn-COOH.Therefore, the cell performance of the device using porphyrin P_Zn-BIA-COOH as sensitizeroutperforms the better-reported porphyrin dye.The factors which effect reaction were discussed in this paper. The results showed the optimum reaction conditions, the mole ratio of paranitrobenzaldehyde,thiophene-2-formaldehyde, pyrrole were fixed, which was1:3:4, that the optimum reactiontime was45min, the suitable temperature was122℃, the optimum amount of propionic acidshould control the concentration of pyrrole was0.27mol/L. The results of infrared and1HNMR indicated that the four compounds are target product and their melting point were76～82℃.In1×10^-5mol/L concentration, the UV-vis absorption spectra of four kinds of zincmethoxy porphyrins in DMF solution show that the strong Soret band at429nm nearby,Q（Ⅰ）band at604nm nearby, Q（Ⅱ） band at563nm nearby. the Soret band ofP_Zn-BIA-COOH were red-shifted about8nm compared to P_Zn-COOH. Compare to othersimilar Schiff-base zinc porphyrin which don’t have peripheral substituents on porphyrin ring.Soret bands were red-shifted and absorbing intensity were stronger in some degree.The fluorescence spectra show that all zinc benzoic acid base porphyrins had twoemission peaks. The emission peaks around609nm were stronger and the emission peaksaround658nm were weaker. The emission maxima of P₁were red shifted compared with P₁.The maximum absorption edge of these compounds ranges from545to580nm. Thezerothezeroth energies （E0-0） were determined between2.17².88eV.From cyclic voltammetry, the electrochemical level of four kinds of compounds weredetermined between0.7¹.95eV. The electrochemical level of P₄is the least of all ofcompounds and minimal reached0.7eV. From the excited states oxidation potential, thedriving forces for electron injection from the excited porphyrin singlet state to the conductionband （CB） of TiO2（-0.74V vs SCE）（△Ginj） for the zinc benzoic acid base porphyrins dyeswere determined between-1.25^-0.83eV eV. The regeneration of porphyrin radical cation by（I/I3-）（+0.2V vs SCE）（△Greg） for the porphyrin dyes were determined between-0.99^-0.25eV. The experimental result show that the processes of electron injection and restore cationicdye are thermodynamically feasible.