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Formation and properties of donor-acceptor dyads of Zn porphyrins and C60-imidazole adduct in the Langmuir and Langmuir-Blodgett Films

Renata Marczak 1Robert Nowakowski 1Wlodzimierz Kutner 1Krzysztof R. Noworyta 2Gadde Suresh 3Melvin E. Zandler 3Francis D'Souza 3Bernard Desbat 4

1. Polish Academy of Sciences, Institute of Physical Chemistry, Kasprzaka 44/52, Warszawa 01-224, Poland
2. University of Geneva, 24 quai Ernest-Ansermet, Geneva 1211, Switzerland
3. Wichita State University, Department of Chemistry, 1845, Fairmount, Wichita, KS 67260-0051, United States
4. Laboratoire de Pysicochimie Moléculaire - Université Bordeaux I, 351 Cours de la Libération, Talence 33405, France


In search for donor-acceptor dyads capable of photoinduced charge separation, being thus suitable for constructing organic photovoltaic elements, we examined self assembly of C60 adduct of imidazole via complex formation with Zn porphyrins in the Langmuir and Langmuir-Blodgett, LB, films. The Langmuir films of the C60-imidazole adduct, C60im, were prepared at the interfaces of air and aqueous subphase solution of water-soluble porphyrins, viz. Zn tetrakis (N-methylpiridyl)porphyrin cation, Zn(TMPyP), or Zn tetrakis (4-sulfonatophenyl)porphyrin anion, Zn(TPPS). In effect, relatively stable donor-acceptor dyads of C60im-Zn(TMPyP) and C60im-Zn(TPPS) were formed in the interfacial films. Comparison of the determined and calculated values of area per molecule, being dependent on the composition of the subphase solution, indicated that dyads were oriented with their porphyrin macrocycles in plane of the air-solution interface. Two control experiments indirectly confirmed that dyads were formed through axial coordination at the air-water interface. In the first experiment, spreading properties of the ligand-containing adduct floating on the surface of water or aqueous solution of Zn(TMPyP) were compared with those floating on solutions of free-base porphyrin, TMPyP. In the second one, spreading behavior of adducts containing imidazole ligand, C60im, was compared with that of a blank adduct, viz. that containing a phenyl substitutent, C60ph, floating on the surface of water or aqueous solution of Zn(TMPyP). Calculated by molecular modeling thickness of the Langmuir films was in accord with that determined by ellipsometry. The Langmuir films were transferred, by using the LB technique, onto different solid substrates for spectroscopic, microscopic, electroanalytical and photochemical characterization. It appeared that the Zn porphyrins were transferred together with the adduct onto solid substrates. Orientation of the dyad molecules in the LB films was detemined by IR spectroscopy. Molecularly modeled hexagonal packing and thickness of the LB films was in accord with that measured by the STM imaging and determined by ellipsometry, respectively. The electrochemical redox states of the dyads were established by performing simultaneous cyclic voltammetry and piezoelectric microgravimetry measurements on the LB films deposited on the Au-quartz electrodes. Time-resolved emission studies of the fullerene-(Zn porphyrin) LB films revealed efficient quenching of the singlet-excited Zn porphyrin. Based on the free-energy calculations and dyad orientation in the film, this quenching was attributed to vectorial electron transfer within the dyad.


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Presentation: Poster at SMCBS'2005 Workshop, by Renata Marczak
See On-line Journal of SMCBS'2005 Workshop

Submitted: 2005-10-27 07:45
Revised:   2009-06-07 00:44