Characterization and structure of molecular aggregates of a tetracationic porphyrin in LB films with a lipid anchor

In this paper, the formation of the different aggregates of a water-soluble tetracationic porphyrin, TMPyP, anchored to an anionic phospholipid matrix , DMPA, and assembled in LB films has been investigated. The use of differe nt solid substrates such as hydrophilic glass and supports modified by the transfer of monolayers of several lipids has been employed to assemble the TMPyP molecules in the mixed monolayer with DMPA (in a molar ratio of 1:4) as monomer (TMPyP4+), dimer, diprotonated monomer (TMPyPH26+), and tetramer aggregates. The porphyrin is organized as a monomer (TMPyP4+) when the mix ed monolayer is directly transferred onto a hydrophilic glass plate. Also, if the glass and the porphyrin are separated by spacer layers such as a mon olayer sequence EA up arrow /AA down arrow /EA up arrow /AA down arrow, the porphyrin monomer is identified. Porphyrin dimers are formed at the air-wa ter interface in monolayers of 1:4 TMPyP/DMPA(20) and may be transferred to solid substrates by the transfer of this system on top of the DMPA monolay er, i.e., (DMPA down arrow /TD up arrow)(n). In these multilayers, a small fraction of porphyrin molecules is transferred as diprotonated monomer. Fin ally,the transfer on hydrophilic glass plates of 1:4 TMPyP/DMPA monolayers of the Y type assembled a packed tetramer of TMPyP. By applying the extende d dipole approximation to the spectra measured, we proposed a model where t he central planes of the two consecutive porphyrins are twisted by 22.5 deg rees, forming a helix.