Molecular organization in self-assembled binary porphyrin nanotubes revealed by resonance Raman spectroscopy

Porphyrin nanotubes were formed by the ionic self-assembly of tetrakis(4-sulfonatophenyl) porphyrin diacid (H4TPPS42-) and Sn(IV) tetra(4-pyridyl) porphyrin (Sn(OH )(X)TPyP4+/5+ [X = OH- or H2O]) at pH 2.0. As reported previously, the tubes are hollow as revealed by transmission electron microscopy, approximately 60 nm in diameter, and can be up to several micrometres long. The absorption spectrum of the porphyrin nanotubes presents monomer-like Soret bands, as well as two additional red-shifted bands characteristic of porphyrin J-aggregates (offset face-to-face stacks). To elucidate the origin of the J-aggregate bands and the internal interactions of the porphyrins, the resonance Raman spectra have been obtained for the porphyrin nanotubes with excitations near resonance with the Soret J-aggregate band and the monomer-like bands. The resonance Raman data reveal that the Sn porphyrins are not electronically coupled to the J-aggregates within the tubes, which are formed exclusively by H4TPPS42-. This suggests that the internal structure of the nanotubes has H4TPPS42- in aggregates that are similar to the widely studied H4TPPS42- self-aggregates and that are segregated from the Sn porphyrins. Possible internal structures of the nanotubes and mechanisms for their formation are discussed.