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

Abstract

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.

Photoelectrochemical properties of porphyrin dyes with a molecular dipole in the linker

“…All of the dyes exhibit standard features of a ZnTPP chromophore with a strongly absorbing Soret band (S 0 →S 2 ) centered at 422 nm (ε ~ 5 x 10 5 M -1 cm -1 ) and weaker Q-bands (S 0 →S 1 ) centred at 556 nm and 597 nm (ε ~ 1 -2 x 10 4 M -1 cm -1 ). [21][22][23] Absorption spectra for the chemisorbed dyes 1a, 2a and 3a recorded on transparent cover glass/TiO 2 thin films are, for the most part, consistent with the solution phase spectra reported earlier. However, a slight broadening on the high energy side of the Soret bands is observed for all dyes (including a 1:1 mixture of dyes 1a+3a) most likely due to H-aggregation upon self-assembly at the semiconductor/dye interface.…”

“…Because the porphyrin macrocycle is almost perpendicular to the rigid-rod bridge, the electronic properties (HOMO-LUMO gap) of the ZnTPP porphyrin chromophore head group are decoupled from the bridge dipole, as determined by UV-Vis absorption, fluorescence emission and redox potentials. 21 This molecular design allows us to associate any difference in energy level alignment at the semiconductor/molecule interface with the dipole orientation in the bridging unit and its influence on the IID effect. In a proof-of concept experiment we recently reported the UV photoemission spectroscopy (UPS) study of 1a, 2a and 3a bound to a ZnO (11−20) single crystal surface to probe the dipole effect on the alignment of the frontier orbital energies with respect to the band edges of the semiconductor.…”

Design of robust 2,2′-bipyridine ligand linkers for the stable immobilization of molecular catalysts on silicon(111) surfaces

“…72 Such sp-hybridized groups have been used in rigid-rod linker constructions for dye-sensitized solar cells (DSSCs). [73][74][75] To assess these groups as linkers for redox catalytic applications, 4-ethynyl-2,2 0 -bipyridine moieties were reduced both molecularly and on the cluster (Fig. 9).…”

Heterogeneous Electron-Transfer Dynamics through Dipole-Bridge Groups

“…The three dyes are terminated by an isophthalic acid group (Ipa) as anchor. Recent publications have discussed the synthesis, 10 as well as the electrochemical and steady state spectral properties of these dyes. 15 Our experiments in solution were performed in spectroscopy grade solvents: diethyl ether (ether) and tetrahydrofuran (THF) from Fisher Scientific.…”