Bioinspired Heterocyclic Partnership in a Cyanine-Type Acidichromic Chromophore

Abstract

A new red hair-inspired 1,4-benzothiazine-based scaffold is disclosed herein, built upon a modular D-π-A architecture via condensation of the easily accessible 3-phenyl-2H-1,4-benzothiazine with indole-3-carboxaldehyde. The compound was obtained in around 50% yields and was characterized by complete spectroscopic analysis. The new benzothiazine-based cyanine displayed a characteristic reversible acidichromic behavior with a marked bathochromic shift upon acidification. The chromophore resisted at least fifteen hydrochloric acid/sodium hydroxide cycles without appreciable alterations. The expedient and scalable synthetic procedure together with the pH sensitive chromophoric properties would make the new compound an attractive prototype for novel modular chromophore for pH-sensing and other applications.

Nature‐Inspired Functional Chromophores from Biomimetic o‐Quinone Chemistry

“…This position experiences the acid-activatable electron withdrawing effect of the imine moiety, sustaining either facile H-atom abstraction leading to captodatively stabilized free radical intermediates, [26,27] or condensation with various aldehyde derivatives to produce a range of cyanine-type chomophores. Noticeable applications of this chemistry include colorimetric peroxide and/or redox biometal detection, [26] pH sensing via reversible chromophoric changes that can be repeated over several cycles, [28] or the development of a modular dye platform for one-time-pad molecular cryptography. [29] The manifold synthetic opportunities offered by the BTZ system are schematically illustrated in Figure 2.…”

The Multifaceted Opportunities Provided by the Pheomelanin-Inspired 1,4-Benzothiazine Chromophore: A Still-Undervalued Issue

“…As a proof of concept, a paper filter dipped into a methanolic solution of the (1,2-ethanediylidene)bis(3-phenyl-2H-1,4-benzothiazine) described above and left to dry (red in color) was found to be impermeable in water but not acid solutions, which rapidly passes through the filter with the associated color change to blue, allowing one, therefore, to monitor the hydrophobic/hydrophilic state of the device (Figure 6) over several cycles given the high reversibility and robustness of the system compared to other cyanines [52]. Similarly, the condensation products of the same 3-phenyl-2H-1,4-benzothiazine with indole-3-carboxaldehyde showed a marked and reversible (up to 15 cycles) acidichromic behavior, with a shift from a yellow (pH > 4) to a purple (pH < 3) color, even when applied as a coating or adsorbed on different materials (glass, nylon, cotton, paper) (Figure 5) [53].…”

“…Similarly, the condensation products of the same 3-phenyl-2H-1,4-benzothiazine with indole-3-carboxaldehyde showed a marked and reversible (up to 15 cycles) acidichromic behavior, with a shift from a yellow (pH > 4) to a purple (pH < 3) color, even when applied as a coating or adsorbed on different materials (glass, nylon, cotton, paper) (Figure 5) [53]. Similarly, the condensation products of the same 3-phenyl-2H-1,4-benzothiazine with indole-3-carboxaldehyde showed a marked and reversible (up to 15 cycles) acidichromic behavior, with a shift from a yellow (pH > 4) to a purple (pH < 3) color, even when applied as a coating or adsorbed on different materials (glass, nylon, cotton, paper) (Figure 5) [53].…”

Development of Polymethine Dyes for NIR‐II Fluorescence Imaging and Therapy

“…[61] Researchers have found that using a larger conjugated benzindole group as the heterocyclic structure can lead to absorption at ≈1080 nm in synthesized heptamethyl polymethine cyanine dye (FD-1080). [62,63] Through this approach, longer-wavelength polymethine fluorophores can be obtained by rationally modifying the heterocyclic ring, without needing to extend the length of the polymethine chain. This efficient approach enables the creation of polymethine fluorophores that are more stable compared to those featuring longer polymethine silane chains.…”