Magnetic Antiaromaticity─Paratropicity─Does Not Necessarily Imply Instability

Abstract

Magnetically induced ring currents are a conventional tool for the characterization of aromaticity. Dia- and paratropic currents are thought to be associated with stabilization (aromaticity) and destabilization (antiaromaticity), respectively. In the present work, I have questioned the validity of the paratropic currents as a measure of antiaromaticity among monocyclic hydrocarbons. I have shown that while reduced/oxidized radical ions of hydrocarbons sustain strong paratropic currents, they often gain extra stabilization via cyclic conjugation compared to their acyclic counterparts.

Unraveling the effect of aromaticity for the dynamics of excited states of single benzene fluorophores

“…These observations are in line with the previously obtained results that negative/positive NICS values are not necessarily signatures of aromatic stabilization/ antiaromatic destabilization. [19][20][21][22] The investigation undertaken in this section demonstrates that the AI values obtained from LFCs and RBSOs calculated for the ground and excited electronic states of cyclic π-conjugated molecules display a sufficiently good correlation with the relative stability of the respective species. Previously, it was established that AI furnishes a reliable tool to characterize the degree of π-delocalization and accompanying aromatization or antiaromatization in ground and excited stationary states of cyclic molecules.…”

“…Furthermore, characterization of aromaticity by criteria, such as NICS, based on magnetically induced current is prone to ambiguities, where the ability to sustain diamagnetic current (i.e., large negative NICS) can be mistaken for a signature of energetic stability (i.e., aromatic stabilization) 19–22 …”

“…[16][17][18] Furthermore, characterization of aromaticity by criteria, such as NICS, based on magnetically induced current is prone to ambiguities, where the ability to sustain diamagnetic current (i.e., large negative NICS) can be mistaken for a signature of energetic stability (i.e., aromatic stabilization). [19][20][21][22] Here, we reopen a computational study of the photophysical properties of the DAPA chromophores with the purpose of understanding: (i) to what extent their luminescence properties are rooted in π-delocalization and aromaticity attributes of the excited states, and (ii) what is the difference between the luminescent (o-and p-DAPA) and nonluminescent (m-DAPA) chromophores and what renders the latter isomer nonfluorescent. To this end, we employed a computational methodology capable of accurately describing the electronic structure and the noadiabatic dynamics of the excited states of molecules, which is based on the emerging concept of ensemble DFT (eDFT).…”

Tetraquinolines; four linked quinoline units or porphyrinoids

“…35–56 Apart from the relevance of the magnetic response properties to the phenomenon of aromaticity, i.e. , stability, 57 some patterns are universal among porphyrinoids. Even sterically distorted porphyrinoids, 38,45,48–50,52,53,55 or systems in which conjugation is interrupted by an sp 3 carbon, 35 sustain strong diatropic or paratropic global ring currents.…”

“…The inter-quinoline current intensity in TEQ 2+ increases to 8.1 nA T −1 which is consistent with the current in aromatic molecules (the current intensity of benzene is 12.0 nA T −1 at the same computational level). 57 Nevertheless, the current intensity remains relatively low because the molecule is not fully planar. Other counterparts of TEQ show similar global diatropic ring currents (TEQZ = 10.4 nA T −1 , TEN = 9.8 nA T −1 , and TEPP = 10.0 nA T −1 ).…”