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Bulk Inclusions of Double Pyridazine Molecular Rotors in Hexagonal Tris(o‑phenylene)cyclotriphosphazene

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posted on 27.05.2019, 00:00 by Jiří Kaleta, Guillaume Bastien, Jin Wen, Martin Dračínský, Edward Tortorici, Ivana Císařová, Paul D. Beale, Charles T. Rogers, Josef Michl
A new generation of double pyridazine molecular rotors differing in intramolecular dipole–dipole spacing was synthesized. All rotor molecules formed bulk inclusions in a tris­(o-phenylenedioxy)­cyclotriphosphazene (TPP) host. Results of dielectric spectroscopy were fitted to a pair of nine-state models that accounted for interactions of neighboring dipoles at either an aligned or opposed possible orientation of the local threefold dipole rotation potentials within a channel of the TPP host. The results indicate dipole–dipole interaction strengths at the 100 to 200 K scale that lead dipoles to preferentially populate a subset of low-energy configurations. They also reveal that pyridazines with ethynyl substituents in 3- and 6-positions have slightly higher rotational barriers (3.2–3.5 kcal/mol) than those carrying one ethynyl and one tert-butyl group (1.9–3.0 kcal/mol). Upon cooling, these barriers reduce the rate of thermal transitions between the potential wells so much that the inclusions cannot achieve ordered dipolar ground states.