Symmetry-Broken Intermolecular Charge Separation of Cationic Radicals

A quadrupolar compound Pyr-BA with two pyrrole-type nitrogen atoms doped externally was prepared in this work. In high contrast with other pi ionic radicals, its cationic radical Pyr-BA center dot(+) undergoes unusual symmetry-broken charge separation (SB-CS), generating the mixed valence complex of Pyr-BA(+1-q)center dot center dot center dot Pyr-BA(+1+q), where q is the degree of charge transfer. Variable-temperature (VT) single-crystal analysis, absorption and EPR experiments all confirmed that aggregation and lower temperature would help to facilitate this SB-CS process. Gibbs energy calculations and gauge-including magnetically induced current simulation both validate that, for Pyr-BA center dot(+), SB-CS behavior is more favorable than the conventional dimerization mode. To the best of our knowledge, this is the first study that shows solid single-crystal evidence for spontaneous SB-CS between identical ionic radicals. Such a unique phenomenon is of great significance both in terms of fundamental aspects and uncharted material science.

Automated summary

A quadrupolar compound Pyr-BA with external doping of two pyrrole-type nitrogen atoms was prepared. Unlike other pi ionic radicals, its cationic radical Pyr-BA·(+) undergoes unusual symmetry-broken charge separation (SB-CS), resulting in the formation of the mixed valence complex Pyr-BA(+1-q)···Pyr-BA(+1+q), where q represents the degree of charge transfer. Experimental analysis and calculations demonstrate the favorable SB-CS behavior of Pyr-BA·(+), which is of significant importance in both fundamental aspects and unexplored material science.