The formation mechanism of ZnTPyP fibers fabricated by a surfactant-assisted method

Fibers composed of 5,10,15,20-tetrakis(4-pyridyl)porphyrinatozinc(ii) (ZnTPyP) were synthesized by a surfactant-assisted method using cetyltrimethylammonium bromide (CTAB) and chloroform. The presence of CTAB was essential to make and to maintain the fibers and their formation rate became slower with increasing the molar ratio of CTAB to ZnTPyP. Measurements of absorption spectra of the synthesized fibers showed splitting of the Soret band at 426 nm into two peaks at 416 and 454 nm in accordance with the formation of the ZnTPyP fibers as revealed by transmission electron microscopy. The aging process at higher temperature made the fibers longer and the apparent activation energy for the formation of the fibers was estimated to be 74.8 kJ mol(-1). When 5,10,15,20-tetrakis(4-pyridyl)porphyrin (TPyP) or 5,10,15,20-tetrakis(phenyl)porphyrinatozinc(ii) (ZnTPP) was used instead of ZnTPyP, no fiber formation was observed. On the other hand, when a chloroform solution of ZnTPP was mixed with pyrazine or 4,4 '-bipyridine, the fiber formation was observed. Proton nuclear magnetic resonance spectra indicated upfield shifts of the pyridinic proton in the presence of ZnTPP and 4,4 '-bipyridine, suggesting the coordination of nitrogen to zinc(ii) (Zn-N) in ZnTPP. These findings indicate that the Zn-N coordination is crucial for the formation of the fibers and that nitrogen in the pyridyl moiety of ZnTPyP is coordinated to the central zinc(ii) ion of another ZnTPyP molecule to make the ZnTPyP fibers. Theoretical calculations were performed using the DFT/B97D functional to estimate the stability of the pi-pi stacking and the coordination of Zn-N. The presence of the CTAB micelles suppresses the aggregation of ZnTPyP molecules due to the pi-pi stacking, which is thermodynamically more favorable. The Zn-N coordination proceeds moderately during the aging process for 10 days by inducing the transition from spherical CTAB micelles to rod-like micelles by fusion.