Spectroscopic and Crystal Structural Analyses of Zinc (II) Tetraphenylporphyrin J-aggregates

Abstract

The self-assembly behavior of non-aqueous zinc (Ⅱ) tetraphenylporphyrin (ZnTPP) was studied in dry acetonitrile. A red shift in the absorption and fluorescence spectra implies that a part of ZnTPP molecules self-associate into J-aggregates in a head-to-head arrangement. The formation of ZnTPP J-aggregates is dependent on the solvents. The spectroscopic and excited state lifetime measurements indicate that the radioactive decay rate of the aggregates is two times faster than that of the monomer, which is indicative of superradiance from the aggregates. Optical microscopy of a ZnTPP microcrystal shows a rod-like molecular arrangement during crystal growth. An X-ray structural analysis and an illustration of crystal packing in ZnTPP (CH3CN) also show that one of the peripheral phenyl groups of a ZnTPP molecule interacts perpendicularly with the pyrrole ring of an adjacent porphyrin molecule. These π-π interactions are actually an edge-to-face C-H…π interaction. We, therefore, propose a structural model for the ZnTPP J-aggregates. The effects of the Zn-N coordination bond distance in a single crystal are discussed based on different packing arrangements.