Molecular-Engineered Biradicals Based on the Y-Phthalocyanine Platform.

A mixed-ligand phthalocyanine/porphyrin yttrium(III) radical double-decker complex () was synthesized using the custom-made 5,10,15-tris(4-methoxyphenyl)-20-(4-((trimethylsilyl)ethynyl)phenyl)porphyrin. The trimethylsilyl functionality was then used to couple two such complexes into biradicals through rigid tethers. Glaser coupling was used to synthesize a short-tethered biradical () and Sonogashira coupling to synthesize longer-tethered ones ( and ). Field-swept echo-detected (FSED), saturation recovery, and spin nutation-pulsed electron paramagnetic resonance experiments revealed marked similarities of the magnetic properties of with those of the parent [Y(pc)] complex, both in the solid state and in CDCl/CDCl 4:1 frozen glasses. FSED experiments on the biradicals and revealed a spectral broadening with respect to the spectra of and [Y(pc)] assigned to the effect of dipolar interactions in solution. Apart from the main resonance, satellite features were also observed, which were simulated with dipole-dipole pairs of shortest distances, suggesting spin delocalization on the organic tether. FSED experiments on yielded spectral line shapes that could not be simulated as the integration of the off-resonance echoes was complicated by field-dependent modulations. While, for all dimers, the on-resonance spin nutation experiments yielded Rabi oscillations of the same frequencies, off-resonance nutations on yielded Rabi oscillations that could be assigned to a = -1 to = 0 transition within a = 1 multiplet. The DFT calculations showed that the conformation of the complexes was significantly more stable than the cis one and that it induced a marked spin delocalization over the rigid organic tether. This "spin leakage" was most pronounced for the shortest biradical .