In recent years, dye sensitized solar cells have attracted considerable attention due to their low cost and potential for getting higher light harvesting efficiency.¹ Porphyrin derivatives have immerged as a promising candidate for DSSC due their photo stability, broad absorption within solar spectrum with high molar extinction coefficients and tunability of electrochemical properties toward synthetic modifications. Extensive studies have been done on meso-substituted porphyrins as sensitizers in DSSC.² In 2014, highest efficiency of 13% has been archived by installing donor–Π-bridge–acceptor at the porphyrin meso-position.³Due to limited synthetic methods, there are very few β functionalized porphyrin used in DSSC.

                In this work, we report the synthesis, properties and photovoltaic performance of novel series of benzoporphyrins bearing two carboxyl anchoring groups through different П-linkers at their 2³ and 2⁴ positions. Palladium catalyzed double Heck reaction followed by electrocyclization/dehydrogenation strategy was used to obtain various 2³, 2⁴-functionalized benzoporphyrin from 2,3-dibromoporphyrin.⁴ The dyes bearing vinylene linkage and phenyl acetylene linkage at 2³ and 2⁴ position of benzoporphyrin displayed much broader and red shifted B and Q bands in UV-Vis absorption compared to its phenyl linker substituted derivative. Cyclic voltammetry results of all studied compounds confirms the thermodynamic feasiblity of electron injection from their excited states to the CB of TiO₂ and regeneration of the resulting porphyrin radical cation by redox mediators. However, the photovoltaic performance of the cells sensitized with vinylene linker substituted benzoporphyrin showed highest power conversion efficiency with highest J_sc and V_oc values. On the other hand the photovoltaic performance of the cells sensitized with phenyl linker substituted benzoporphyrin showed lowest power conversion efficiency in this series. Our results reflect light harvesting efficiency (LHE), length and flexibility of the linker and dye loading mainly determined the difference in power conversion efficiency in this series.  DFT calculation provided support that the electron density distribution in frontier molecular orbitals of vinylene linker substituted benzoporphyrin and phenyl acetylene substituted benzoporphyrin could facilitate the electron injection to semiconductor compared to phenyl linker substituted benzoporphyrin. 

                (1)          Clifford, J. N.; Martinez-Ferrero, E.; Viterisi, A.; Palomares, E. Chem. Soc. Rev. 2011, 40, 1635.

                (2)          Higashino, T.; Imahori, H. Dalton Trans. 2015, 44, 448.

                (3)          Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. Nat. Chem. 2014, 6, 242.

                (4)          Jinadasa, R. G. W.; Fang, Y.; Kumar, S.; Osinski, A. J.; Jiang, X.; Ziegler, C. J.; Kadish, K. M.; Wang, H. J. Org. Chem.[just accepted], DOI: 10.1021/acs.joc.5b01906 Nov., 18, 2015.