Synthesis of Zinc Tetraphenylporphyrin Rigid Rods with a Built-In Dipole.

Three Zn(II) tetraphenylporphyrins (ZnTPP) were synthesized to study the influence of a molecular dipole on the energy level alignment of a chromophore bound to a metal oxide semiconductor: ZnTPP-PE(DA)-IpaOMe (1), ZnTPP-PE-IpaOMe (2), and ZnTPP-PE(AD)-IpaOMe (3). Each contained a rigid-rod linker made of a p-phenylene ethynylene (PE) moiety terminated with the methyl ester of an isophthalic acid unit (Ipa). Porphyrins 1 and 3 contained an intramolecular dipole in the central phenyl ring, which was built by introducing electron donor (D, NMe2) and acceptor (A, NO2) substituents in para position to each other. In 1 and 3, the relative position of the D and A substituents, and therefore the dipole direction, was reversed. Porphyrin 2, without substituents in the linker, was synthesized for a comparison. The structures of precursors to 1 and 3 and the structure of 1 were determined by single crystal X-ray analysis. Solution UV-vis and steady-state fluorescence spectra of 1-3 were identical to each other and exhibited the spectral features typical of the ZnTPP chromophore and their electrochemical properties were also very similar. Methyl esters 1-3 were hydrolyzed to the corresponding carboxylic acids for binding to metal oxide semiconductors.