Perylene Bisimide Cyclophanes: Structure–Property Relationships upon Variation of the Cavity Size

Five cyclophanes composed of two perylene bisimide (PBI) dyes and various CH2–arylene–CH2 linker units were synthesized. PM6-D3H4 geometry-optimized structures and a single crystal for one of these cyclophanes reveal well-defined distances between the two coplanar PBI units in these cyclophanes, spanning the range from 5.0 to 12.5 Å. UV/vis absorption spectra reveal a redistribution of oscillator strength of the vibronic bands due to a H-type exciton coupling even for the cyclophane with the largest interchromophoric distance. A quantitative evaluation according to the Kasha–Spano theory affords exciton coupling strengths ranging from 64 cm−1 for the largest cyclophane up to 333 cm−1 for the smallest one and a surprisingly good fit to the cubic interchromophoric distance in the framework of the point-dipole approximation. Interchromophoric interaction is also noticed in fluorescence lifetimes that are significantly increased for all five cyclophanes as expected for H-coupled chromophores due to a decrease of the radiative rate. For the three largest cyclophanes with interchromophoric distances of >9 Å, fluorescence quantum yields remain high in chloroform (>88%), whilst for the smaller ones with interchromophoric distances <6 Å, additional nonradiative pathways lead to a pronounced fluorescence quenching.

Zero-Field and Field-Induced Interactions between Multicore Magnetic Nanoparticles

In this paper, the Langevin dynamics simulation method is used to study magnetic interactions between a pair of multicore magnetic nanoparticles subjected to a uniform magnetic field. Multicore nanoparticles are modelled as spherical rigid clusters of single-domain superparamagnetic cores coupled via dipole-dipole interactions. It is shown that the magnetic force between two well-separated clusters in a strong applied field can be accurately described within the induced point-dipole approximation. However, this approximation also assumes that there are no interactions between clusters in the zero-field limit. On the contrary, simulations indicate the existence of a relatively small attractive magnetic force between clusters, even in the absence of an applied field. It is shown that this force is a direct superparamagnetic analog of the van der Waals interaction between a pair of dielectric spheres.

Ab initio paramagnetic NMR shifts via point-dipole approximation in a large magnetic-anisotropy Co(ii) complex

Point-dipole approximation provides accurate reproduction of full quantum-chemical results for paramagnetic NMR chemical shifts and allows extensive configurational sampling at low computational cost.