Electrochemical Switching of First-Generation Donor-Acceptor Stenhouse Adducts (DASAs): An Alternative Stimulus for Triene Cyclisation

Abstract

Donor-acceptor Stenhouse adducts (DASAs) are a photo-switch class that undergoes triene cyclisation in response to visible light. Herein, electrochemical oxidation is demonstrated as an effective alternative stimulus for the triene cyclisation commonly associated with photo-switching.

Organic photo-switches have gained significant attention due to the contrasting properties of their initial and metastable states [1]. Accessing these states via controlled light irradiation allows efficient modulation of their properties which are relevant to applications including chemical sensing [2,3], drug delivery [2,3], heterogenous catalysis [4], molecular machines [5,6], and data storage [7].

Donor-acceptor Stenhouse adducts (DASAs) [8,9,10,11,12] are a class of photo-switches that are visible light-responsive, thus minimising photochemical degradation [13]. DASAs are comprised of donor (provided by the nucleophile used during synthesis) and acceptor (provided by a carbon acid, usually Meldrum’s or 1,3-dimethyl barbituric acids) components linked by a central triene chain (Figure 1c–h) [8,9,10]. Typically, excitation with white light results in triene chain’s cyclisation to yield a cyclopentenone zwitterionic species (Figure 2d) [8,9,10]. Further appeal for this photo-switch class is based on the relatively simple two-step synthetic procedure [10]. Syntheses detailed in the literature involve the preparation of an activated furan precursor through Knoevenagel condensation between a carbon acid and furan [8,9,10]. The respective DASA is completed through nucleophilic addition of the respective donor component [8,9,10]. In addition to the aforementioned applications [14,15,16], DASAs have also been used in preparing orthogonal photo-switches [17] and photolithography [18,19].

While most DASA studies have focused on photo-switching behaviour, acidic and basic conditions have been shown to stabilise the metastable state [15,20]. Furthermore, other organic photo-switch classes have responded to alternative stimuli such as heating [21,22], pH [21,22], and redox potential [21,22,23,24,25,26]. In particular, select azobenzene [25,26], chromene [23,24,27,28], and diarylethene [29,30] photo-switches can undergo their photo-switching behaviour in response to an applied redox potential.

Herein, a series of first-generation DASA compounds (denoted 1–6 in Figure 1) were subjected to voltammetric and UV-vis spectroelectrochemical (UV-vis SEC) studies to elucidate their redox activity and establish the potential for electrochemical switching. These inquiries were expected to demonstrate a link with photo-controlled DASA cyclisation. DASAs included structures prepared from P1 and P2 precursors, as shown in Figure 1.

DASA compounds and their precursors were prepared in accordance with procedures from the literature [8,9]. Successful syntheses were confirmed using ¹H NMR (ESI, Figures S1–S8), ESI-MS (ESI, Figures S9–S16), and ATR-FTIR (ESI, Figures S17–S24) spectroscopies. UV-vis spectra (ESI, Figures S25–S30) in MeCN indicated a transition in the visible region for 1–6. This signal was consistent with the S₀–S₁ transition associated with the respective DASAs’ triene chain [8,9,10]. λ_max was considerably higher for compounds prepared from 1,3-dimethyl barbituric acid instead of Meldrum’s acid (1: 524 nm, 2: 526 nm, and 3: 523 nm, compared to 4: 549 nm, 5: 552 nm, and 6: 548 nm). As expected, the peak intensity decreased following irradiation with white LEDs (4 Watts) as a result of expected photo-switching behaviour [8,9,10]. Compound 2 was the only DASA that exhibited near quantitative conversion in MeCN, based on the S₀–S₁ transition intensity reducing by ca. 98%. Other DASA species underwent incomplete photo-switching in MeCN (ca. 68–87%) based on the partial disappearance of the S₀–S₁ band. Spectral changes following photo-irradiation in MeCN were irreversible.

Cyclic and square wave voltammetry (CV and SQW, respectively) for 1–6 (Figure 2a and ESI, Figures S31–S36) indicated two quasi-reversible oxidation processes of interest. These were designated as Ox1 and Ox2 with average E_1/2 values of ca. 0.117 and 0.680 V (ESI, Table S1), respectively. Oxidation processes were consistent with the 4π-eletrocyclisation step where the alcohol functional group was oxidised as part of the photo-switching mechanism [31]. It is expected that the electrolyte would stabilise the zwitterionic state as observed in redox active spiropyran functional groups [24]. E_1/2 values for Ox1 and Ox2 were higher in DASAs 1–3 compared to 4–6. This was attributed to the carbon acid (either Meldrum’s or 1,3-dimethylbarbituric acid), where a reduced electron withdrawing effect in 4–6 promoted electron transfer [8,10,32]. The exceptions were compounds 3 and 6, where the latter had a higher E_1/2 compared to the former. Additionally, E_1/2 values for Ox1 varied based on the amine derivative used. Potentials increased for DASAs derived from piperidine (3: 0.118, 6: 0.0355 V), pyrrolidine (2: 0.166 V, 5: 0.0840 V), and Et₂NH (1: 0.190 V, 4: 0.111 V) nucleophiles. The electron-donating effects of the donor component of 1–6 was expected to reduce E_1/2 based on documented nucleophile effects on DASA photo-switching wavelengths [8,10,32]. Scan rate dependence plots for Ox1 and Ox2 indicated that these processes were affected by diffusion (ESI, Figures S39–S44). Red2 in 1–6 was consistent with redox processes associated with P1 and P2 precursors (ESI, Figures S37–S38 and Table S1) and were not attributed to the triene cyclisation.

UV-vis SEC measurements linked electrochemical oxidation processes to the triene cyclisation. Applying an anodic potential resulted in a decrease in the S₀–S₁ band intensity in all DASA compounds, with representative data for 1 provided in Figure 2b (see also ESI, Figures S45a–S49a). The trends in E_1/2 values calculated for 1–6 from voltammetric experiments were consistent with the potentials required for electrochemical oxidation during SEC experiments (ESI, Table S2).

The spectral behaviour noted during UV-vis SEC experiments indicated complete conversion to the cyclised state following application of a potential, compared to typically incomplete switching under photo-irradiation. The reduction in S₀–S₁ band intensity in response to an anodic potential also indicated that it was a more effective stimulus than photo-irradiation. Most DASA compounds studied herein showed a more substantial decrease in band intensity during SEC experiments. Photo-switching was likely, in part, hindered by the DASA concentration, as outlined by Lui et al. [33] for DASAs in chloroform and toluene solvents. This would be compounded by MeCN being a polar solvent that has inhibited switching behaviour in past studies [11]. In this respect, an anodic potential was able to overcome these impediments, as both switching experiments in the current work were conducted at approximately the same concentration. Additionally, the charge carrier effect of the electrolyte likely further enhances the efficiency of DASA cyclisation via electrochemical oxidation. The main exception was compound 6, where photo-irradiation (ca. 87% conversion) was more effective than electrochemical oxidation (ca. 77% conversion) at inducing cyclisation. Additionally, for compound 2, both stimuli resulted in nearly quantitative yield of the cyclised state. It is likely that this is in part attributable to the electron donating nature of the respective nucleophile [34], although this will require further investigation for confirmation.

Removing the potential resulted in a partial return of the S₀–S₁ band (Figure 2c and ESI, Figure S45b–Figure S49b), in keeping with quasi-reversible behaviour observed in CV data. The reappearance of the S₀-S₁ band also suggests that the Red1 peak detected in CV data for 1–6 (Figure 1a and ESI, Figures S31–S36) was associated with a reduction in the linear isomer. Note the discrepancy between voltammetry and SEC voltages is due to the Fc/Fc⁺ internal reference in CV experiments. Recovery ranged from 4–47% recovery (ESI, Table S2) across the series, with 4 and 3 representing the lower and upper limits, respectively. It was further noted that the nucleophile appeared to affect recovery, where Et₂NH < pyrrolidine < piperidine (ESI, Table S2). Trends in recovery were also attributed to the electron withdrawing effect of the carbon acid as noted previously [8,10,32]. It was expected that the electron withdrawing behaviour of the acceptor component would adversely affect reversibility. Additionally, the diminished fatigue resistance compared to that observed in photo-switching studies [9] is also attributable to the harsh nature associated with an electrochemical potential.

The SEC data confirmed that application of an anodic potential resulted in the conversion of the linear isomer to the cyclised zwitterion (Figure 2d). Similar behaviour has also been observed in spiropyran-based materials where the electrolyte stabilises the zwitterionic merocyanine species [24]. The immediate decrease in S₀–S₁ band intensity confirmed that the Ox1 processes in 1–6 were associated with the 4π-electrocyclisation step, precluding a multistep process as observed during photo-switching [31,35]. The initial conversion demonstrated an alternative stimulus for DASA switching. The poor recovery of the linear configuration following electrochemical oxidation indicates exposure to this stimulus would impede long-term switching function during practical applications. Therefore, compounds and materials with improved fatigue resistance are necessary to exploit this switching behaviour in a functional context.

In summary, selected first-generation DASAs exhibited the cyclisation traditionally associated with their photo-switching behaviour in response to an anodic potential. Furthermore, electrochemical oxidation was demonstrated to be more effective than photo-irradiation for cyclising most of the DASAs studied in the current work. Despite this, the alternative stimulus was only partially effective, with poor recovery of the linear triene isomer. This work points to the potential for electrochemical switching of DASA-incorporated systems of interest for electrochemical sensing, electrocatalysis, and electrochemically driven molecular machines. These studies are currently underway in our laboratory.