Fluorescent cyclodextrin carriers for a water soluble ZnII pyrazinoporphyrazine octacation with photosensitizer potential

A nitro-benzofurazan-triazolyl carboxymethylated b-cyclodextrin (NBFT-CMbCyD) and an oligomer of carboxymethyl b-cyclodextrin (sodium salt), crosslinked with epichlorohydrin and labeled with rhodaminyl groups (pbCyD-Rh), exhibit very high affinity in aqueous solution for octacationic photosensitizer [(CH3)8LZn]8+ neutralized by I ions (L ¼ tetrakis-2,3-[5,6-di(2-(pyridiniumyl)pyrazino] porphyrazinato dianion). The photosensitizer (PS) forms complexes with 1 : 2 and 2 : 2 CyD:PS stoichiometry, which were characterized as binding constants and UV-Vis absorption and fluorescence properties. The self-association tendency of [(CH3)8LZn]8+, leading to a monomer-dimer equilibrium shift towards the dimer even at very low concentrations (z106 M), is not contrasted by either NBFTCMbCyD or the pbCyD-Rh oligomer, both of which completely convert the [(CH3)8LZn]8+ monomer fraction to the dimer form in the bound state. Quenching of fluorescence observed in [(CH3)8LZn]8+ upon binding with either hosts is consistent with the conversion of the monomer to the negligibly fluorescent dimer. The complexes formed with the CMbCyD units of the pbCyD-Rh oligomer have average association constants, which are larger by 6-7 orders of magnitude than those with the CMbCyD monomer in the NBFT-labeled derivative.