Effect of degree of β-chlorination on photocatalytic activity of meso-tetraphenylporphyrin under homogeneous and nanoscale heterogeneous conditions: Chlorination vs. bromination

Abstract

β-Chlorination of meso-tetraphenylporphyrin, H2TPP leads to the formation of H2TPPClx (X = 2, 4, 6 and 8) with absorption bands in the range of 421–451 and 650–715 nm, for the Soret and Q(0,0) bands, respectively. H2TPPClx were used as photosensitizers for the aerobic photooxidation of cyclooctene under mild conditions and compared with that of the non-chlorinated porphyrin. The lowest degree of catalyst degradation was observed in the case of H2TPPCl2. H2TPPCl2 and H2TPPCl4 showed the highest photocatalytic activity among the series. In spite of the electron-withdrawing effects of the chlorine atoms, with the exception of H2TPPCl2, the chlorinated porphyrins showed a comparable oxidative degradation to that of H2TPP towards oxidative degradation. The preference of blue LED lamp over the red one shows that the absorption at the Soret band region of the chlorinated porphyrins is mainly responsible for the photosensitizer ability of the catalyst. In this regard, H2TPPCl2 and H2TPPCl4 which have the highest overlap with the emission bands of the blue LED lamp showed the highest photocatalytic activity than the other H2TPPClx and H2TPP. Also, control reactions performed in the presence of 1,3-diphenylisobenzofuran and 1,4-benzoquinone provided evidence for the involvement of singlet oxygen as the main reactive oxygen species in this reaction. Singlet oxygen quantum yields (φF) of H2TPPClx were smaller than that of the non-chlorinated porphyrin and decreased as H2TPPCl8 > H2TPPCl4 > H2TPPCl6 ≥ H2TPPCl2. H2TPPClx showed much higher photocatalytic activities than their brominated counterparts. Also, the φF values and oxidative stability of the chlorinated porphyrins were usually higher than those of the brominated porphyrins, with the same degree of β-halogenation. Protonation of H2TPPClx with weak and strong acids (HCOOH, CF3COOH and HCl), also their immobilization on a nano-structured acidic polymer (Amberlyst 15) were used to study the influence of core protonation on photocatalytic activity and oxidative stability of the chlorinated porphyrins.