ATROPISOMER-SPECIFIC FORMATION OF PREMICELLAR PORPHYRIN J-AGGREGATES IN AQUEOUS SURFACTANT SOLUTIONS

The 4,0-atropisomer of intermediate (C6 or C7) to long (C16) chain length picket fence porphyrins (PFPs) exhibit red-shifted absorption and reduced singlet and triplet lifetimes in dilute aqueous surfactant solutions (i.e. below the critical micelle concentration). The spectral and photophysical behavior of these species is consistent with formation of premicellar aggregates containing two or more porphyrin chromophores lying in an offset, face-to-face arrangement (lambda-max approximately 436 nm, v sharp, for C6 or C7 compounds), which differ from normal aqueous porphyrin aggregates (lambda-max approximately 426 nm, broad) or monomeric porphyrin in homogeneous or micellar solution (lambda-max = 418-420 nm, sharp). The most characteristic behavior is noted at intermediate chain lengths for the free base or Pd(II) complex of 4,0-meso-tetrakis(2-hexanamidophenyl)porphyrin (4,0-THex) and for the free base of 4,0-THept, suggesting an optimal molecular hydrophobicity, side-chain length, and molecular topology for complex formation. The premicellar surfactant-porphyrin complexes are formed selectively from the 4,0-atropisomer and exhibit an SDS/porphyrin ratio of 3.2 for 4,0-THex. Acid-base titration behavior of the 4,0-THex J-aggregates suggests a structure in which the porphyrin core is isolated from the anionic surfactant head groups in a relatively hydrophobic microenvironment. Addition of alcohols or surfactant to disorganized aqueous aggregates of short- and long-chain 4,0-PFPs, respectively, apparently results in a modest reorganization of porphyrin chromophores (lambda-max approximately 436-438 nm) indicative of an inherent tendency toward J-aggregation for the 4,0-isomer.