Molecular Vibrations Induced Potential Diradical Character in Hexazapentacene

While the photoelectrochemical behavior of azapentacene has been investigated successfully, insight into the dynamic electronic properties of azapentacene triggered by different energy pulses is very scarce. The present work reports a fascinating phenomenon about potential diradical character governed by structural vibrations in hexazapentacene. In complete contrast to the static equilibrium configuration of hexazapentacene without diradical character, due to the vibration-based structural perturbation, DFT calculations show that some of the transient configurations possess diradical character and thus magnetism, which exhibit different periodic pulse behavior in time evolution. Since each vibrational mode refers to two distortion ways (positive/negative distortions from equilibrium configuration), 7 different possibilities are observed for the vibration-induced diradical character for all vibrational modes (e.g., a combination of nonradical, singlet diradical, or triplet diradical for positive distortion and those of for negative distortion for each vibrational mode). This intriguing diradical character is rationalized by structural distortions with considerable changes of some energy quantities. The structural distortions cause the HOMO energy raising and LUMO energy lowering and thus an efficient reduction of the HOMO–LUMO energy gap and singlet–triplet gap of the system, which are favorable to the formation of the broken-symmetry open-shell singlet or triplet states. The periodic pulsing behavior is attributed to persistent molecular vibrations and is thus vibrational mode controlled. Compared with pentacene, the remarked effects of nitrogen substitution on the diradical properties and their pulsing behaviors are mainly due to the decreases of both the HOMO and the LUMO energies and considerable narrowing of their gaps in the vibrations-distorted configurations. This intriguing potential diradical character and its different dynamic behavior suggest hexazapentacene potential applications as promising building blocks in the rational design of novel electromagnetic materials because of its controllable magnetism through energy pulses. This work provides comprehensive understanding of the nature of dynamic variations of the electronic structures and properties of the nitrogen-rich acene derivatives and other materials molecules