Redox/pH Dual Stimuli‐Responsive Acridine Spiropyran

Abstract

A spiropyran derivative containing an acridinium and a phenoxide moieties was found to exhibit both halochromism and electrochromism in response to acid stimulation and oxidative stimulation, respectively. Variable scan-rate voltammetric studies revealed that the acridine spiropyran undergoes a reversible conversion between a closed-ring and an open-ring form during the redox cycle. The presence of a ring-opened radical cation species was demonstrated by simultaneous electrochemical ESR measurements.

Electrochemical Ring-Opening and -Closing of a Spiropyran

“…The redox driven ring-opening of a photochromic spiropyran is facilitated by adding electron-donating tert-butyl substituents ortho- and para- to the phenolic oxygen of the benzopyran. The importance of the tert-butyl groups in achieving electrocatalytic ring-opening was evident in the work of Kubo et al also, 38 and thus, we expect the formation of a thermodynamically stable phenoxy radical to drive the merocyanine formation. Additionally, next to preventing dimerization, the chloro substituent on the indoline unit is expected to destabilize the indoline-based radical cation that is formed initially also.…”

“…In contrast, the EPR spectrum of the oxidized open form of the acridine spiropyran by Kubo et al did exhibit hyperfine splitting. 38 This suggests that the radical in tbMC +• is delocalized over an extensive part of the molecule including the indoline unit. The DFT-computed spin density supports this analysis (see the Supporting Information ).…”

“…Indeed, direct oxidative ring-opening of a spiropyran was recently shown by Kubo et al, where the stabilization of the phenoxyl radical was driven by the aromatization of the acridine substituent. 38 Subsequent reduction, however, recovered the spiro compound rather than the open zwitterionic form.…”

N‐Substituted Acridinium as a Multi‐Responsive Recognition Unit in Supramolecular Chemistry

“…Responsive acridinium systems upon chemical and electrochemical stimuli a) from T. Suzuki and co-workers (13 2 + ) [25] and b) from T. Kubo and co-workers (15 + ). [26] Reproduced from reference [26] with permission from Wiley-VCH. base (N(CH CH ) ) leading to the formation of a dioxopyrene motif (14).…”

“…A system involving a spiro-acridane motif was also reported in 2019 by Y. Hirao, T. Kubo and co-workers (16, Scheme 6b). [26] Under acidic conditions, the ring opening was monitored by UV-Vis spectroscopy thus leading to the typical spectrum of the acridinium moiety (15 + ). The presence of isosbestic points suggests the reversibility of the ring opening process.…”