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Highly Polar Supramolecular Species with Enhanced IR Activity

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Highly polar molecular systems are in need for many important practical applications based on light-matter interactions, e.g. solar energy utilization. Novel insertion complexes of polar molecules non-covalently trapped between alkali-halide (MX) counter-ions are compared to non-polar-based counterparts [1-3]. The M-molecule-X systems can exhibit from a metastability to a stability by up to 1 eV relative to molecule + MX. The polar-molecule insertion complexes can be even more strongly bound than the common dipole-dipole MX-molecule conformers and separated from those by significant energy barriers, thus being thermodynamically stable species with very large dipoles of up to > 25 D. Interesting features include the nonobvious contributions of the molecule polarity to the system stability and dipole moment, the cooperative non-additivity of pair interactions, molecule reshaping and linking by framing ion-pairs. The latter is demonstrated by a (M-molecule-X)2 dimer, stable to dissociation and extremely polar at about 50 D. For the corresponding (M-molecule-X)– anions, relative stabilities of various conformers and barriers between them vary significantly relative to the neutral counterparts. Feasible pathways for experimental production of the M-molecule-X species are analyzed using the energy profiles for the neutral and anionic systems. Due to neutralization of the M-X charge-transfer in the excited triplet state, these complexes represent unique spin-controlled dipole-switch molecular systems with a large dipole reversibly turned off or even inverted by the spin state, potentially allowing various spin/optoelectronic applications. The IR spectra are predicted to considerably vary and sensitively indicate the formation of both the M-molecule-X and MX-molecule conformers, with intensities up to an order of magnitude higher in the complexes, facilitating their reliable detection and differentiation in experiments. The above indicates possible ways of adding polarity (via attached ion-pairs) to nonpolar molecules and thus of their efficient experimental detection and characterization. The so increased attraction could promote molecular self-assembly and chemical reactions as well. While metastable systems suggest possible applications for high-capacity energy storage.

[1] F. Y. Naumkin, J. Phys. Chem. A 121 , 4545 (2017). [2] S. Kerr, F. Y. Naumkin, New J. Chem., 41, 13576 (2017). [3] B. Cochrane, F. Y. Naumkin, Chem. Phys. Lett. 643, 137 (2016).

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