Triarylcarbenium ions, or trityl cations (Tr⁺, Figure 1) are brightly colored, resonance-stabilized carbocations with absorption spectra tunable by chemical design. These characteristics have led to their establishment as one of the most powerful class of colorants, the triarylmethane dyes. Thus far, the trityl cations have been produced from the corresponding colorless triarylmethane compound via C-X heterolytic cleavage by treatment with acids, irradiation with light, electrochemistry, and radiolysis (Figure 1, X = OH, CN, …). All these methods are solution-based and require the presence of a (relatively polar) fluid to dissolve both the neutral and cationic species, which limits applicability to solid-state systems.

Here, we will discuss our recently developed approach to generate trityl cations in solid-state materials via polymer mechanochemistry. We initially screened potential triarylmethane mechanophore designs guided by the constrained geometry simulate external force (COGEF) computational method. The virtual compounds were subsequently converted into actual small molecule mechanophores, and covalently incorporated as crosslinks in either glassy (e.g., poly(N,N-dimethylacrylamide)), or elastic (e.g., poly(methyl acrylate-co-2-hydroxyethyl acrylate)) polymer networks. Mechanical stimulation of the materials by either compression or tensile deformation produced a strong mechanochromic response, transforming the originally colorless networks into materials displaying the color of the mechanically-generated trityl cations (Figure 1), as confirmed by combining mechanical experiments with in situ transmittance measurements. We will discuss the novelty associated to our triarylmethane mechanophores also in the context of polymer mechanochemistry.

<b>Figure 1. </b>Stimuli-induced heterolytic dissociation of the C-X bond in triarylmethane systems reported (mostly) in solutions thus far (left). The here discussed approach for the heterolytic dissociation of triarylmethane mechanophores in solid-state materials via polymer mechanochemistry (right).

Figure 1. Stimuli-induced heterolytic dissociation of the C-X bond in triarylmethane systems reported (mostly) in solutions thus far (left). The here discussed approach for the heterolytic dissociation of triarylmethane mechanophores in solid-state materials via polymer mechanochemistry (right).

Jose Augusto Berrocal

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