Exploiting the versatile alkyne-based chemistry for expanding the applications of a stable triphenylmethyl organic radical on surfaces

Abstract

The incorporation of terminal alkynes into the chemical structure of persistent organic perchlorotriphenylmethyl (PTM) radicals provides new chemical tools to expand their potential applications. In this work, this is demonstrated by the chemical functionalization of two types of substrates, hydrogenated SiO₂-free silicon (Si-H) and gold, and, by exploiting the click chemistry, scarcely used with organic radicals, to synthesise multifunctional systems. On one hand, the one-step functionalization of Si-H allows a light-triggered capacitance switch to be successfully achieved under electrochemical conditions. On the other hand, the click reaction between the alkyne-terminated PTM radical and a ferrocene azide derivative, used here as a model azide system, leads to a multistate electrochemical switch. The successful post-surface modification makes the self-assembled monolayers reported here an appealing platform to synthesise multifunctional systems grafted on surfaces.

Fig. 1. Chemical structure of the PTM radicals employed in this work.

Fig. 2. (a) Sketch of SAM-1-Rad-Si. (b) Cyclic voltammograms under red light illumination of SAM-1-Rad-Si at different scan rates (0.1, 0.2, 0.4, 0.6, 1, 2, 4, 6 and 10 V s^–1, from brown to purple) in CH₃CN + 0.1 M Bu₄NClO₄. The inset shows comparative CVs at 0.4 V s^–1 in the dark (dashed line) and under illumination (solid line).

Fig. 3. (a) Capacitance–potential curves of SAM-1-Rad-Si measured at 50 Hz in the dark and under red light illumination. (b) Capacitance–time profile at 50 Hz measured at 0.18 V during dark (OFF state, 30 s)/illumination (ON state, 30 s) switching cycles. Electrolytic solution: CH₃CN + 0.1 M Bu₄NClO₄.

Scheme 1. Synthetic route to the preparation of the target radical PTM–Fc dyad 3-Rad.

Fig. 4. CV curves of SAMs on gold: (a) SAM-3-H, (b) SAM-2-Rad, (c) SAM-3-Rad and (d) SAM-5-Rad in CH₂Cl₂ + 0.2 M Bu₄NPF₆ at different scan rates (0.05, 0.1, 0.25, 0.5, 1, 2, 3 and 4 V s^–1). The color code used for SAMs is the same as that used in Scheme 1. (Blue) PTM αH; (magenta) PTM radical; (yellow) triazole ring; (green) Fc and (violet) terminal alkyne moieties.

Fig. 5. (a) Cole–Cole plots recorded for the SAM-3-rad at different applied potentials to obtain three different redox states. The electrolytic medium (CH₂Cl₂ + 0.2 M Bu₄NPF₆) was used and the frequency was varied from 100 kHz to 0.1 Hz with a 20 mV amplitude. (b) Scheme of the three different interfaces at the specific applied voltage.

Fig. 6. Histograms and Gaussian fits of log|J| at 1 V for junctions Au^TS/SAM-2,3 and 5-Rad//GaO_x/EGaIn. Inset: Plots of mean log|J| versus E(V).

Fig. 7. SAM-5-Rad (a) Cl–L edge, (b) C–K edge (the arrows indicate the transitions as discussed in the text) and (c) N–K edge in grazing (30° to the substrate, red circles) and normal incidences (blue circles). (d) Fe–L edge NEXAFS spectrum at grazing incidence.