Dipoles Inside of Dipoles: Insertion Complexes of Polar versus Nonpolar Molecules in Ion Pairs

Highly polar molecular systems are in demand as a means of enabling many important practical applications based on light-matter interactions. In the present work, the insertion complexes of recently synthesized polar molecules trapped between alkali halide counterions are studied. For specific selected compositions, the M-molecule-X systems are predicted to be stable to dissociation into molecule + alkali halide. It is found that unlike their nonpolar molecule-based counterparts, the polar molecule complexes can be even more stable than their common dipole-dipole MX-molecule isomers. This makes them thermodynamically stable, highly polar species, with very large dipoles of about 20 D, and they could be used, for example, to develop efficient light sensors. Furthermore, due to the neutralization of the M-X charge transfer in the excited triplet state, such complexes represent unique spin-controlled dipole-switch molecular systems with the large dipole turned off and even inverted by the spin state for the nonpolar and polar molecule complexes, respectively. This potentially could allow various spintronic and optoelectronic applications. In addition, the IR intensity spectra are predicted to sensitively indicate the formation of both the M-molecule-X and MX-molecule isomers, thus facilitating their reliable detection and differentiation in experiments.