Design and synthesis of multi-stimuli responsive π-extended phenothiazine aldehydes: Solvatochromism, acidochromism, moisture detection and fluorochromic sensing of amine vapors

Abstract

A series of phenothiazine-based A-π-D-A type ‘push-pull’ molecules containing sites sensitive to external stimuli was designed and synthesized by phase transfer Heck coupling reaction. A positive solvatochromism was observed with an increase in solvent polarity resulting in an enhanced intramolecular charge transfer (ICT). The existence of TICT was prominent in highly polar media such as methanol and water. Single crystal X-ray diffraction revealed the existence of two structures of VPTZ, with torsional angles of 145.92⁰ and −161.04⁰, respectively along C–N–S–C plane of the phenothiazine rings. These compounds were used as probes for quantitative measurement of moisture present in three organic solvents, namely CH3CN, THF, and DMSO. The probes could detect moisture as low as 0.0034% (v/v) in CH3CN. Further, these compounds displayed reversible acidochromism in both solution and solid-state. VPTZ, containing pyridine ring as the receptor, displayed excellent sensing of trifluoroacetic acid (TFA) with a detection limit of 91.4 nM. A detailed mechanism involved in reversible acid sensing was studied by 1H NMR. Finally, silica-coated TLC plates were used to fabricate test strips by coating them with VPTZ and then were used as a portable device for selective turn-on sensing of triethyl amine (TEA) and n-dibutyl amine (DBA). TEA and DBA exhibited exceptional fluorochromic turn-on response from purplish-pink to yellow-green and mustard yellow, respectively.