Excited-state dynamics of cofacial pacman porphyrins

Cofacial palladium(II) bisporphyrins anchored by xanthene [Pd₂(DPX)] and dibenzofuran [Pd₂(DPD)] pillars were prepared, and the dynamics of their triplet excited states were investigated. The phosphorescence quantum yield [Φ_p = 4.59(121) × 10^-3 in CH₂Cl₂] and lifetime [τ_p = 18.2(2) μs in CH₂Cl₂] of Pd₂(DPD) are significantly attenuated compared to those of its Pd₂(DPX) congener [Φ_p = 29.4(5) × 10^-3 and τ_p = 102(3) μs in CH₂Cl₂]. Electronic absorption and emission spectroscopy and electrochemical measurements establish that the porphyrin rings of the DPX cofacial analogue are closer to each other than the porphyrin rings of the DPD analogue in fluid solution. These observations are supported by X-ray crystallographic analyses, which show that the metal-metal distance for the dibenzofuran-bridged bisporphyrin in the solid state is almost 3 Å greater than that for bisporphyrins linked by the xanthene spacer. The spectroscopic, photophysical, and structural results suggest that the decreased Φ_p and τ_p of Pd₂(DPD) are not a consequence of simple interplanar interactions but rather arise from the increased conformational flexibility of the porphyrin rings about the aryl ring of the DPD pillar. In support of this contention, the photophysical properties of monomeric palladium etioporphyrin I [Pd(Etio)] and meso-phenyl-substituted palladium etioporphyrin II [Pd(PhEtio)] were examined and found to be similar to those of Pd₂(DPX) and Pd₂(DPD), respectively. Density functional theory calculations reveal that torsional motion about the C(meso)-C(aryl bridge) bond induces nonplanar distortion of the porphyrin framework, causing a substantial decrease in the T₁-S₀ energy gap. This structural perturbation is consistent with the observed results of enhanced deactivation of Pd₂(DPD) and Pd(PhEtio) triplet excited states.