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

A novel porphyrin - cyclodextrin conjugate, 5[ 4-( 6- O- beta- cyclodextrin)- phenyl], 10,15,20- tri( 4hydroxyphenyl)- porphyrin ( CD- THPP), has been synthesized in good yield ( 30%) starting from 6- O- tosyl- b- 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,. uorescence and laserash 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 con. firmed 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. RI Sortino, Salvatore/E-4684-2011

A novel porphyrin - cyclodextrin conjugate, 5[ 4-( 6- O- beta- cyclodextrin)- phenyl], 10,15,20- tri( 4hydroxyphenyl)- porphyrin ( CD- THPP), has been synthesized in good yield ( 30%) starting from 6- O- tosyl- b- 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,. uorescence and laserash 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 con. firmed 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.