Proton Conductivities of Stepwise Protonated Imidazole-Fused Tetraphenylene Derivatives

The control of crystal structure and proton conductivity was evaluated by stepwise protonation of tetraphenylene molecules (ImTP) containing a imidazole group. By controlling the degree of protonation from neutral ImTP to H₁ImTP⁺, H₂ImTP²⁺, and H₄ImTP⁴⁺, we demonstrated that the balance between intermolecular hydrogen bonding and electrostatic interactions affects structural stability and proton conductivity. X-ray diffraction analysis revealed that neutral ImTP forms a 2D hydrogen bond network, while H₁ImTP⁺ exhibits enhanced structural stability due to N─H⁺···Cl^- electrostatic interactions. H₂ImTP²⁺ forms a 3D network, and H₄ImTP⁴⁺ becomes amorphous. Under 40% relative humidity conditions, these compounds adsorb H₂O to form proton conduction pathways, exhibiting conductivities of 3.20 × 10^{- 7}, 3.38 × 10^{- 6}, 1.35 × 10^{- 6}, and 6.20 × 10^{- 6} S cm^{- 1} at ∼363 K for n = 0, 1, 2, and 4 of H_nImTPⁿ⁺Cl^- _n system, respectively. In particular, H₁ImTP⁺ exhibits an optimal balance between hydrogen bonding and electrostatic interactions, resulting in high structural stability, reversible H₂O adsorption, and proton conductivity.