Laboratory of Molecular Physics, Dept. of Biomolecular Sciences,
Wageningen Agricultural University
Dreijenlaan 3, 6703 HA, Wageningen, The Netherlands
(1)On leave from Chemistry Dept. & Solid State Institute, Israel Institute of
Technology
Technion City, Haifa 32000, Israel
Energy transport and -trapping has been studied by various spectroscopic techniques in organised porphyrin layers with thickness in the nm-mm range and used as energy-transporting antennae for e.g. dye-sensitised TiO2 solar cells. Such assemblies are inspired by the molecular organisation of the photosynthetic apparatus of plants and bacteria. Transport and trapping of singlet excitation, limiting the free exciton path length, is demonstrated by the fluorescence quenching of self-organising films of a metallo-porphyrin doped with its free base. Doping with 0.1-1% tetra-octyl substituted free base porphyrin (H2TOPP) of spin coated films of the Zn-substituted analogue (ZnTOPP) results in trapping of singlet excitation in the free base traps and subsequent fluorescence from these traps, competing with emission from defect traps in the Zn-porphyrin host, demonstrating singlet energy transfer from host to trap. By heating to 150 øC the films partially self-organise to an ordered array, resulting in experimentally measured higher fluorescence yield of the free base traps than before heating. This indicates transport of singlet excitons to the free base traps over larger distances than in disordered films. Alterrnatively, the ZnTOPP host has been doped with varying concentrations of the non-emissive CuTOPP analogue, which quenches the host fluorescence. Modelling the energy transport in these doped films allows an estimate of the effective singlet exciton path length. Optically Detected Magnetic Resonance (ODMR) both at zero and high magnetic field and optical spectroscopy both at 1.4 K have been applied to photo-excited triplet states in partially ordered ZnTOPP films, spin coated from chloroform or toluene. ODMR spectra show the resonances of two main triplet species in these films, i.e. (i) resonances at 854 and 1046 MHz of a Jahn-Teller distorted, non-ligated ZnTOPP triplet with zero field splitting parameters D = 316.9 x10-4 cm-1 and a distribution of E values centred around E = 32.0 x10-4 cm-1 (Fig. 1) and (ii) a resonance at 1248 MHz of a second triplet species assigned to a chloroform- ligated ZnTOPP trap with D = 300x10-4 cm-1, E = 100x10-4 cm-1 (Fig.2). The results indicate that by exposure of the films to chloroform non-ligated porphyrins are converted to ligated ones. In films of Zn-tetraphenylporphyrin, which do not contain ordered domains, only a single triplet species triplet is found, similar to that of the ligated ZnTOPP. A tentative model of the ordered ZnTOPP lattice is presented to explain the results obtained by optical and ODMR spectroscopy. The main conclusions from this work are: (i) transport of singlet excitation energy in "antennae" of functionalized dyes to photochemically active traps is feasible; (ii) the photophysical properties of these "antennae" are extremely sensitive to adsorption of small molecules. The assignment of the ZFODMR spectra to the two triplet species is supported by microwave induced fluorescence spectra (Figs. 3,4).
Acknowledgement: This work has been financially supported by the Netherlands Organisation for Energy and Environment (NOVEM).