Impact of electronic asymmetry on photoexcited triplek-state spin distributions in conjugated porphyrin oligomers probed via EPR spectroscopy

The photophysics of triplet excitons in a series of electronically asymmetric "push-pull" pi-conjugated mesa-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 = I 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 T-conjugated organic oligomers and polymers.