Abstract
The spin delocalization in the radical cations of a series of ethyne-linked oligoporphyrins was investigated using EPR spectroscopy. The room-temperature spectral envelope for these oligomers deviates significantly from the benchmark N–0.5 trend in line width expected for a completely delocalized spin density, in contrast to the butadiyne-linked analogues measured previously. Here, we show, using DFT calculations and complementary low-temperature ENDOR measurements, that this deviation is primarily driven by a more pronounced inequivalence of the 14N spins within individual subunits for the ethyne-linked oligoporphyrins. Once this 14N inequivalence is taken into consideration, the room-temperature and ENDOR spectra for both butadiyne-linked and ethyne-linked oligomers, up to N = 5, can be simulated by similar static delocalization patterns. This work highlights the importance of EPR in exploring such spin delocalization phenomena while also demonstrating that the N–0.5 trend should not be interpreted in isolation but only in combination with careful simulation and theoretical modeling.
Highlights
Molecular wires consisting of porphyrins linked by meso− meso ethyne bridges display long-range electronic communication resulting in strong near-infrared absorption and fluorescence,[1] nonlinear optical behavior,[2] and efficient charge transport.[3]
Following methodology originally introduced by Norris et al.,[13] these conclusions were based on narrowing of the line width of the continuous wave Electron paramagnetic resonance (EPR) signature of the radical cations with increasing oligomer length
In a recent study,[4] room-temperature cw-EPR was combined with 1HENDOR of the frozen sample to fully characterize the spin delocalization in the radical cations of butadiyne-linked porphyrin oligomers
Summary
The room-temperature X-band cw-EPR spectra of the oligomers l-PN+ and c-P6·T6+ are depicted in Figures 2a (left) and 3 (left). 1H-ENDOR further confirmed these distortions in spin density Once this inhomogeneity in the 14N couplings is accounted for, the ethyne-linked oligomers exhibit a similar delocalization trend across the wire series as compared to the butadiyne analogues. This result highlights that any conclusion on trends in spin density cannot be solely based on an interpretation of cw-EPR using eq 1 but must be accompanied by thorough computational analysis and, if possible, further experimental data, such as ENDOR spectra.
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