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.