Phenanthrene-Extended Phenazine Dication: An Electrochromic Conformational Switch Presenting Dual Reactivity

Abstract

The synthesis and isolation of one of the few examples of a π-extended diamagnetic phenazine dication have been achieved by oxidizing a phenanthrene-based dihydrophenazine precursor. The resulting dication was isolated and fully characterized, highlighting an aromatic distorted structure, generated by the conformational change upon the oxidation of the dihydrophenazine precursor, which is also correlated with a marked electrochromic change in the UV–vis spectrum. The aromaticity of the dication has also been investigated theoretically, proving that the species is aromatic based on all major criteria (structural, magnetic, and energetic). Moreover, the material presents an intriguing dual reactivity, resulting in ring contraction to a π-extended triarylimidazolinium and reduction to the dihydrophenazine precursor, depending on the nature of the nucleophile involved. This result helps shed light on the yet largely unexplored reactivity and properties of extended dicationic polycyclic aromatic hydrocarbons (PAHs). In particular, the fact that the molecule can undergo a reversible change in conformation upon oxidation and reduction opens potential applications for this class of derivatives as molecular switches and actuators.

Redox Properties of N,N′-Disubstituted Dihydrophenazine and Dihydrodibenzo[a,c]phenazine: The First Isolation of Their Crystalline Radical Cations and Dications

“…It is also worth noting that some reports disclosed that solvent had remarkable influences on the cyclic voltammetry of N , N′ -disubstituted-dihydrophenazine as well as the stability of its corresponding dication species . Nevertheless, many attempts in the pursuit of the stable dication species of N , N′ -disubstituted-dihydrophenazine derivatives, especially obtaining their single crystals, have not succeeded so far …”

N,N‐Diphenyl Dihydrophenazines: Using π‐Extension to Access Dicationic Multifunctional Materials

“…The molecule presents an even easier second oxidation compared to the previously mentioned 5, giving further proof to the role of π-extension on the dication formation and stability. [12] Upon 2e À oxidation, the dicationic state was easily formed and isolated proving to be stable both in solution (Half-life > 12 h in CH 2 Cl 2 ) and solid state. This stability and the low second oxidation potential were explained by both the formation of a fully benzenoid armchair structure in the dicationic form and by an enhanced charge delocalization prompted by the extended π-surface.…”

“…[9,10] The reactive nature of the dicationic states can plausibly be addressed to high electrophilicity and coulombic repulsion, ultimately resulting in fast reduction or reaction with nucleophiles. [12] Considering this, the extension of the system can enhance the delocalization of the double charge of the aromatic dicationic state, thus reducing coulombic repulsion, possibly generating stable dications. This in turn, could open for new potential applications of dihydrophenazines as multifunctional materials and help shed light on the aromaticity and properties of the exotic doubly charged species.…”

“…While in the case of radical cations, a lot of studies have been performed due to the relative stability and catalytic importance of these intermediates, [1,11] the dicationic states are much less explored, with isolation attempts made difficult by the reactivity of these species [9,10] . The reactive nature of the dicationic states can plausibly be addressed to high electrophilicity and coulombic repulsion, ultimately resulting in fast reduction or reaction with nucleophiles [12] . Considering this, the extension of the system can enhance the delocalization of the double charge of the aromatic dicationic state, thus reducing coulombic repulsion, possibly generating stable dications.…”

Tailor-Made Dynamic Fluorophores: Precise Structures Controlling the Photophysical Properties

“…This dynamic luminescence mechanism, which we termed as vibration-induced emission (VIE), perfectly rationalized this unusual photophysical phenomena. In contrast to the A-L-type molecules, the A-A-type dihydrophenazines generally exhibited a normal Stokes shift, indicating the bent-to-planar processes were blocked in their S 1 states …”