Cation- and solvent-induced formation of supramolecular structures composed of crown-ether substituted double-decker phthalocyanine radicals

Geometric structures of three types of supramolecular aggregates composed of bis(phthalocyaninato)lutetium (Lu(Pc)(2)) radicals are investigated by means of ESR measurements and theoretical calculations. The tetra-crown-substituted complex, Lu(CR4Pc)(Pc) (CR4Pc = tetra-15-crown-5 substituted Pc), forms an aggregate showing an ESR signal of electronic triplet state on addition of cations such as K+ and NH4+, which have a radius large enough to form a 2:1 sandwich-type complex with 15-crown-5. Addition of Na+ cation, which forms a I:1 complex with 15-crown-5, did not give rise to such ESR signal. By comparison of the observed zero-field-splitting (zfs) constant D and theoretical calculation on the spin-spin interaction, the assumption that the aggregate has D-4h symmetry is confirmed to be reasonable. The center-to-center distance between the two Lu(Pc)(2) sites is estimated at 7.0 Angstrom. When no cation is added, Lu(CR4Pc)(Pc) gives another type of aggregate in methanol/chloroform mixed solvent. This also gives a triplet-state ESR signal with a zfs constant D which is 0.71 times that of the cation-induced ESR. From the theoretical calculation, we concluded that two Lu(Pc)(2) sites are placed parallel with a horizontal displacement in the Pc plane. The amount of the displacement is estimated at about 5 Angstrom. The mono-crown substituted complex, Lu(CR1Pc)(Pc) (CR1Pc = mono-15-crown-5 substituted Pc), forms a structure linked with one bridge on addition of K+. The bridge acts as a pivot around which two radical sites can rotate. The zfs constant D obtained from the triplet ESR spectrum is 0.66 times that of the cation-induced system of Lu(CR4Pc)(Pc). The rotation angle is estimated at about 30-40 degrees. It is suggested, by the calculation of the exchange coupling in the three cases, that spin multiplicity of the ground state can be switched between triplet and singlet depending on the amount of Slide or rotation of the Lu(Pc)(2) units in the xy plane, in which the Pc planes lie.