Impact of Electronic Asymmetry on Photoexcited Triplet-State Spin Distributions in Conjugated Porphyrin Oligomers Probed via EPR Spectroscopy

The photophysics of triplet excitons in a series of electronically asymmetric “push−pull” π-conjugated meso -to- meso ethyne-bridged (porphinato)metal oligomers, along with electronically symmetric analogues, were studied by X-band electron paramagnetic resonance (EPR) spectroscopy under continuous-wave (CW) optical pumping conditions in the 4−100 K temperature range. In all of the systems studied, the spatial extent of the triplet wave function, as inferred from the | D | zero-field splitting (ZFS) parameter, never exceeds the dimensions of a single porphyryl moiety and its meso -pendant ethynyl groups. The | D | values determined for an oligomeric series of these electronically asymmetric species that span one through four porphyryl units are respectively 0.0301, 0.0303, 0.0300, and 0.0301 cm -1, indicating a common triplet wave function spatial delocalization of approximately 0.4−0.45 nm. Electron spin−spin and spin−lattice relaxation times were determined over the 4−30 K temperature range using progressive microwave power saturation for benchmark, structurally related electronically symmetric conjugated porphyrin species which possessed either terminal electron-rich [4-dimethylamino(phenyl)]ethynyl or electron-poor [4-nitro(phenyl)]ethynyl substituents. The spin−lattice relaxation times obtained from these experiments reveal no significant scaling of this parameter with conjugation length, consistent with a S = 1 spin system that is confined to a single monomeric porphyrin unit and its pendent ethynyl substituents. These results are discussed within the global context of a broader body of experiments that have probed the extent of triplet exciton delocalization within a number of families of highly π-conjugated organic oligomers and polymers.