ARTICLES Characterization and Structure of Molecular Aggregates of a Tetracationic Porphyrin in LB Films with a Lipid Anchor I. Prieto, † J. M. Pedrosa, ‡ M. T. Martı ´n-Romero, ‡ D. Mo 1 bius, § and L. Camacho* ,‡ Departamento de Quı ´mica Fı ´sica y Quı ´mica Orga ´ nica, Facultad de Ciencias, Campus Lagoas-Marcosende, UniVersidad de Vigo, E-36200 Vigo, Spain, Departamento de Quı ´mica Fı ´sica y Termodina ´ mica Aplicada, Facultad de Ciencias, UniVersidad de Co ´ rdoba, Campus de Rabanales, Edificio C-3, E-14014 Co ´ rdoba, Spain, and Max-Planck-Institut fu ¨ r biophysikalische Chemie, Am Fassberg 11, D-37077 Go ¨ ttingen, Germany ReceiVed: June 21, 2000 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 different 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 (TMPyP 4+ ), dimer, diprotonated monomer (TMPyPH 2 6+ ), and tetramer aggregates. The porphyrin is organized as a monomer (TMPyP 4+ ) when the mixed monolayer is directly transferred onto a hydrophilic glass plate. Also, if the glass and the porphyrin are separated by spacer layers such as a monolayer sequence EAv/AAV/EAv/AAV, the porphyrin monomer is identified. Porphyrin dimers are formed at the air-water 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 monolayer, i.e., (DMPAV/TDv) n . In these multilayers, a small fraction of porphyrin molecules is transferred as diprotonated monomer. Finally, 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 extended dipole approximation to the spectra measured, we proposed a model where the central planes of the two consecutive porphyrins are twisted by 22.5°, forming a helix. Introduction The study on the organic ultrathin films has recently attracted great interest due to the potential technological applications of these materials, e.g., in molecular photoelectronic devices, medical applications, and other appliances. 1-5 One of the main problems related with the fabrication of these organic films is controlling the orientation and aggregation of the molecules. This problem is particularly important in the case of porphyrins due to the high tendency of these molecules to form different types of aggregates. In recent years, the formation of aggregates of ionic porphyrins in aqueous solutions has been controlled by using several surfactants in premicellar conditions. 6-8 One of the successful methods for controlling porphyrin aggregation is the preparation of mixed Langmuir-Blodgett (LB) films containing a water-soluble porphyrin and a lipid anchor. 9-19 In the present 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 TMPyP/DMPA system with a molar ratio of 1:4 has been previously studied at the air- water interface where two different forms for the organization of the porphyrin depending on the surface pressure have been found. Thus, a monomeric form was found for low surface pressure (π e 8 mN/m) and a particular dimer at high surface pressure (π > 35 mN/m), both forms coexisting for intermediate surface pressures. 20 A particular structure has been proposed for the porphyrin dimer in which the parallel rings of the molecules are twisted by 45° with respect to each other (see Scheme 1). 20 The mixed 1:4 TMPyP/DMPA system, which corresponds to the stoichiometry of the molecular charges of the components, was transferred on hydrophilic glass at a surface pressure of 35 mN/m. The absorption spectrum obtained for * Corresponding author. E-mail: qf1cadel@uco.es. † Universidad de Vigo. ‡ Universidad de Co ´rdoba. § Max-Planck-Institut fu ¨r biophysikalische Chemie. SCHEME 1: Structures of DMPA (vertical section) and TMPyP (in flat orientation) and the in-plane Configuration of Two Stacked Molecules of Porphyrin (dimer) 9966 J. Phys. Chem. B 2000, 104, 9966-9972 10.1021/jp0022306 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/05/2000