Spectroscopic and self-association behavior of a porphyrin-β-cyclodextrin conjugate

Abstract

A novel porphyrin–cyclodextrin conjugate, 5[4-(6-O-β-cyclodextrin)-phenyl],10,15,20-tri(4-hydroxyphenyl)-porphyrin (CD-THPP), has been synthesized in good yield (30%) starting from 6-O-tosyl-β-cyclodextrin using 5,10,15,20-tetra(4-hydroxyphenyl)porphyrin (p-THPP) as a nucleophile. The spectroscopic properties of CD-THPP have been investigated by combining NMR, circular dichroism, UV-vis absorption, fluorescence and laser flash photolysis techniques, allowing us to gaining insights into the self-association mode of the conjugate. In neat methanol, CD-THPP is present in a monomeric form and exhibits spectroscopic features virtually identical to those of the model compound p-THPP. The role of the CD macrocycle is essential in differentiating the behavior of the conjugate with respect to the non-modified porphyrin in the mixture water : methanol, 9 : 1 (v/v). Under these conditions, CD-THPP shows a marked tendency to self-arrange as a dimer, whose formation is strongly encouraged by stacking interactions between the aromatic rings, in addition to intermolecular hydrophobic porphyrin–cyclodextrin interactions. This supramolecular species exhibits a strong exciton coupling and, contrary to the unmodified p-THPP, shows a good response to light excitation, as confirmed by the considerable fluorescence emission and triplet–triplet transient absorption. It is also demonstrated that incorporation of suitable guests into the CD cavity triggers a new rearrangement of the dimer with a higher degree of overlap between the porphyrin chromophores, facilitating the occurrence of intramolecular photoinduced electron transfer between the guest and the porphyrin unit.