Synthesis and Photophysical Properties of Meso-Substituted Bisporphyrins:  Comparative Study of Phosphorescence Quenching for Dioxygen Sensing

The 4,6-bis(10-mesityl-5,15-di-p-tolylporpyrinyl)dibenzothiophene (H₄DPSN) free base was obtained in five steps from commercially available materials. The metalation of DPSN^2- with zinc(II), copper(II), and palladium(II) led to three new homobimetallic systems, (Zn)₂DPSN, (Cu)₂DPSN, and (Pd)₂DPSN, respectively. The cofacial structures of these molecules offer the possibility of having dioxygen molecules inside the cavity for a period of time, allowing dynamic (collisional) phosphorescence quenching to be more efficient. The bimolecular excited-state deactivation rate constant for deactivation by dioxygen (k_Q:  (Pd)₂DPB, 2.98 × 10⁹; (Pd)₂DPSN, 3.99 × 10⁹; (Pd)₂DPX, 6.94 × 10⁹; (Pd)TPP, 8.95 × 10⁹; (Pd)₂DPS, 8.95 × 10⁹ M^-1 s^-1) of (Pd)₂DPSN, which exhibits an intense phosphorescence at 699 nm, was compared to those observed for (Pd)TPP, (Pd)₂DPS, (Pd)₂DPX, and (Pd)₂DPB (TPP^2- = tetraphenylporphyrin dianion, DPS^4- = 4,6-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]dibenzothiophene tetraanion, DPX^4- = 4,5-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]-9,9-dimethylxanthene tetraanion, and DPB^4- = 1,8-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]biphenylene tetraanion). These collision-induced deactivation data were interpreted by estimating a series of physical parameters such as the surface area and bisporphyrin radii, the diffusion coefficient of the bismacrocycles, and the theoretical deactivation efficiency for the five compounds addressing the role of steric hindrance of the macrocycles on each other and the aryl groups at the meso positions. For sensing purposes, (Pd)₂DPX is characterized by a Stern−Volmer constant k_SV of 2.91 × 10⁶ M^-1, placing the lower detection limit for [O₂] in solution at 0.58 ppm, which is better than that for (Pd)TPP (k_SV = 2.31 × 10⁶ M^-1; lower detection limit of 0.73 ppm), the classically used monoporphyrin complex.