A survey of methods to study zinc porphyrin aggregates in various media

Abstract

Metalloporphyrin aggregation is critical for triplet-triplet annihilation (TTA) to occur. In order to maximize the efficiency of TTA, to use as a mechanism of photon upconversion in dye-sensitized solar cells, it is important to understand the phenomenon of absorber aggregation. The aggregation of ZnTPP in polymer films was investigated by fluorescence anisotropy and total internal reflection fluorescence microscopy (TIRFM). Single molecule spectroscopy (SMS) and spectromicroscopy were the techniques used to study single molecules and multimolecular aggregates of ZnTPP/ZnTPPS in polymer films/adsorbed on glass substrates.

Fluorescence anisotropy measurements consistently showed depolarized emission from films most concentrated with ZnTPP. This observation was likely a result of energy transfer in and among porphyrin aggregates. Fluorescence intensity decays were also obtained and they illustrated a pattern of decreased fluorescence lifetime (i.e. faster decays) as the concentration of porphyrin in the film increased. These results are consistent with the formation of aggregates, and their increased presence in more concentrated films. The formation of these aggregates quenches the fluorophore’s fluorescence, resulting in the observed shorter fluorescence lifetimes.

TIRFM was performed to study the structure of these polymer films doped with ZnTPP. It was determined that these films consisted of discrete domains and thus lacked homogeneity, and the presence of aggregates was clearly visible. Time-resolved TIRFM measurements were also performed but no interesting results were collected.

SMS and spectromicroscopy were the final techniques employed to study porphyrin aggregation. Preliminary measurements were performed with polymer films doped with ZnTPP, and the single step decay time trajectories collected indicated that single molecules were being studied. Furthermore, emission spectra of these molecules were collected and they were similar to those obtained for a bulk measurement, but the bands were slightly shifted in comparison. These measurements were repeated with ZnTPPS adsorbed to glass substrates. Two different patterns of decay trajectories were measured: (i) single step decays corresponding to single ZnTPPS molecules and (ii) multi step/complex decays representative of multimolecular aggregates. Emission spectra were also collected for the multimolecular aggregates, and they were consistent with those of an ensemble measurement but slightly blue-shifted. Such a shift is common when studying aggregates on such a highly polar surface. Thus, these results demonstrate that ZnTPPS aggregates form even at concentrations as low as 10-8 M, and can be studied using SMS despite their weak fluorescence emission.