Intercalation of Aminophenyl- and Pyridinium-Substituted Porphyrins into Zirconium Hydrogen Phosphate: Evidence for Substituent-Derived Orientational Selectivity

Aminophenyl- and pyridinium-substituted porphyrins were intercalated into the α phase of zirconium hydrogen phosphate (α-ZrP) by exchanging the porphyrins into the p-methoxyaniline (PMA) preintercalated compound Zr(O₃POH)₂·2PMA (α-ZrP·2PMA). Porphyrin exchange reactions with α-ZrP and its ethanol intercalate were unsuccessful. Powder X-ray diffraction patterns of p-H₂TAPP- and H₂TMPyP-exchanged α-ZrP·2PMA revealed complete conversion of the starting material to phases with interlayer spacings near 17 Å, corresponding to a guest layer thickness of ca. 11 Å. The p-H₂TAPP derivative was found to partially convert over several weeks to α-ZrP and a 24-Å phase material. The large interlayer spacing cannot be accounted for by a porphyrin monolayer and is tentatively assigned to a porphyrin bilayer in which the heme planes are tilted relative to the host lamellae. Conversely, intercalation of α-ZrP·2PMA with the α,α,α,α, α,α,α,β, and α,β,α,β atropisomers of o-H₂TAPP afforded multiphasic products with interlayer spacings of 13, 17, 19–20, and 25 Å. The relative intensities of the phases were highly dependent on the atropisomer, with the α,α,α,α isomer forming mainly a 19-Å phase and o-α,β,α,β-H₂TAPP forming predominantly a 17-Å phase. In order to determine the orientations of porphyrins comprising the 17- and 19–20-Å phases relative to the host lamellae, p-CuTAPP and o-α,α,α,α-CuTAPP intercalation complexes were prepared, and anisotropic EPR spectra of uniaxially ordered thin films oriented at 0 and 90° relative to the applied magnetic field were recorded. The EPR spectra of the p-CuTAPP-exchanged host displayed little orientational dependence, consistent with tilting of the heme macrocycles near 45° relative to the host lamellae. Conversely, the EPR spectra of uniaxially ordered films of the o-α,α,α,α-CuTAPP-exchanged host exhibited a strong orientation dependence, with the perpendicular component of the Cu(II) signal dominating the spectrum in samples oriented at 90° relative to B₀ and the parallel component predominating in films oriented at 0° relative to B₀, indicating that the porphyrins were highly oriented parallel to the host layers. Together, the XRD and EPR data suggest that the guest morphology of the p-TAPP derivatives consisted of a monomolecular porphyrin layer in which the heme planes were tilted nearly 45° relative to the host lamellae, whereas o-α,α,α,α-TAPP derivatives predominantly assembled into a porphyrin bilayer in which the heme macrocycles lay parallel with the host sheets. The preferred porphyrin orientations are discussed in terms of maximization of electrostatic and hydrogen-bonding interactions between the host and guest.