Systematic photophysical, thermal and electrochemical analysis of a series of phenothiazine cored conjugated aromatic unit appended D–π–A based high-solid state luminescent materials: their applications in reversible mechanofluorochromic and volatile acid sensing

Abstract

The advancement of unique, organic materials possessing exclusive solid-state photoluminescence properties is in high demand due to their noteworthy contribution to materials chemistry and technology. Considering this, a class of novel scissor shaped phenothiazine (PT) derivatives, named PT–Cn–(Ar)₂ where n = 2, 12 and Ar = phenyl, naphthyl, anthracenyl, phenanthryl, and pyrenyl units, have been synthesized, which show divergent fluorescence emission characteristics in a variety of solvents by virtue of their twisted intramolecular charge transfer (TICT) state. Among the studied PT derivatives, naphthyl and pyrenyl derivatives generate an unusual blue-shifted aggregation-induced emission (AIE) in the THF–water binary mixture due to the suppression of the TICT state. Interestingly, among all the products, only the phenyl derivative shows reversible mechanofluorochromic (MFC) behavior, where it offers cyan to yellowish-green emission upon mechanical agitation and fumigation concurrently. Mechanistic inspection derived from P-XRD indicates loss in crystallinity, generating redshifted emission upon mechanical pressure. The SEM study shows two distinct morphologies of the MFC material before and after applying the mechanical force. Furthermore, the MFC characteristic has been verified by single-point energy calculations using DFT. Also, a narrow bandgap (E_g) value and lower excited state singlet (S₁) and triplet (T₁) energy gaps (ΔE_STs) derived from DFT calculations for anthracene and pyrene derivatives suggest their potential in organic photovoltaic cells. Furthermore, reversible acid-sensing behavior has been demonstrated by a model PT derivative PT–C2–(Pn)₂. Additionally, the high electrochemical stability of the derivatives up to 200 cycles suggests a feasible structural engineering approach for fabricating stable redox-active organic materials for redox flow batteries and OLEDs.