Designing diverse coordination modes for the covalent attachment of Wells-Dawson type polyoxometalate onto porphyrins

Five covalently bonded polyoxometalate (POM)-porphyrin hybrids were synthesized by reacting the Wells-Dawson type polyoxometalate [N(C₄H₉)₄]₅H₄P₂W₁₅V₃O₆₂ with five tris-functionalized porphyrins containing different numbers of tris groups at different peripheral positions. These hybrids were thoroughly characterized using elemental analysis, NMR (¹H, ³¹P, and ⁵¹V), mass spectrometry (ESI-MS, MALID-TOF-MS), FT-IR, UV-Vis, and fluorescence spectroscopies. The results proved that different quantities (one, two, and three) of the vanadium-capped Wells-Dawson type metal-oxide cluster P₂W₁₅V₃O₆₂^9- can be grafted onto a porphyrin moiety via covalent bonding with different orientations, depending on the number and position of peripheral functional groups on the porphyrin. Interestingly, remarkable fluorescence quenching (60% in 3Py-P@1POM, 75% in trans-2PyP@2POM, 80% in cis-2PyP@2POM, 85% in cis-2PhP@2POM, and 55% in 1Py-P@3POM, as compared to the fluorescence intensity of their corresponding porphyrin precursor) was observed under excitation (λ_exc = 328 nm), indicating electron transfer from the porphyrin moiety to the POM moiety through covalent linkage.