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: <b>ZnTPP-PE­(DA)-IpaOMe</b> (<b>1</b>), <b>ZnTPP-PE-IpaOMe</b> (<b>2</b>), and <b>ZnTPP-PE­(AD)-IpaOMe</b> (<b>3</b>). Each contained a rigid-rod linker made of a <i>p</i>-phenylene ethynylene (PE) moiety terminated with the methyl ester of an isophthalic acid unit (Ipa). Porphyrins <b>1</b> and <b>3</b> 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 <b>1</b> and <b>3</b>, the relative position of the D and A substituents, and therefore the dipole direction, was reversed. Porphyrin <b>2</b>, without substituents in the linker, was synthesized for a comparison. The structures of precursors to <b>1</b> and <b>3</b> and the structure of <b>1</b> were determined by single crystal X-ray analysis. Solution UV–vis and steady-state fluorescence spectra of <b>1</b>–<b>3</b> 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 <b>1</b>–<b>3</b> were hydrolyzed to the corresponding carboxylic acids for binding to metal oxide semiconductors