Fine tuning of the photophysical properties of cofacial diporphyrins via the use of different spacers

The crystal and molecular structures of two unmetallated diporphyrin species using the biphenylene and dibenzofuran spacers, H4(DPB) and H4(DPO), respectively (DPB4−=1,8-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]biphenylene; DPO4−=4,6-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]dibenzofuran), are reported. These data are compared to their literature metallated analogs, stressing on the properties related to the flexibility of the ligands, π⋯π and M⋯M interactions. In addition, the lowest energy fluorescence properties of these non-phosphorescent diporphyrin compounds as well as three other related species, H4(DPA), H4(DPX), and H4(DPS) (DPA4−=1,8-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]anthracene; DPX4−=4,5-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]-9,9-dimethylxanthene; DPS4−=4,6-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]dibenzothiophene), have been examined both at room temperature in 2-MeTHF in the presence of Ar, air and O2, and at 77 K. In all cases, the fluorescence arises from the 1Q(ππ*), and the photophysical data at 77 and 298 K under Ar atmosphere correlate readily with the molecular geometry of these pincer ligands, where the non-radiative rate constants increase as the interplanar distances decrease. In the presence of dioxygen in solution, both the fluorescence lifetimes and quantum yields decrease as expected for quenching, with the second-order rate constants for bimolecular deactivation (kQ) ranging from 0.9×1010 to 1.7×1010 s−1 M−1. The H4(DPB) compound exhibits the lowest kQ indicating lesser ability for O2 to interact with the interior of the diporphyrin cavity.