Phenothiazine-Based Mechano-Fluorochromic Gels and Solids: Superhydrophobic Surface Formation and Crystal-to-Crystal Phase Transition

Abstract

Organic materials exhibiting mechano-fluorochromic behavior in their gel state are extremely rare in the literature. Similarly, examples of mechano-fluorochromic organic materials with crystal-to-crystal phase transitions have not been reported until date. Herein, we achieved these two remarkable properties in phenothiazine derivatives S1 and S2[(E)-N′-((10-ethyl-10H-phenothiazin-3-yl)­methylene)-3,4,5-tris­(hexadecyloxy)­benzohydrazide and (E)-N′-((10-ethyl-10H-phenothiazin-3-yl)­methylene)­isonicotinohydrazide)], where S1 formed mechano-luminescent xerogel and S2 exhibited the rare feature of crystal-to-crystal phase transition with mechano-luminescence behavior. Study of the photophysical properties of S1 and S2 indicated the suppression of twisted intramolecular charge transfer (TICT) upon self-assembly, leading to blue-shifted emission from S1 and S2. While both S1 and S2 exhibited the propensity to aggregate and disclosed the aggregation induced emission (AIE) behavior in a binary solvent (THF/H2O) mixture, only S1 resulted in a robust gel, which was further utilized for developing a superhydrophobic surface with a contact angle of 157.56°. Upon mechanical grinding, both pristine sample and xerogel of S1 showed an emission color change from light green (505 nm) to yellowish green (530 nm), whereas S2 exhibited an emission color change from cyan blue (480 nm) to green (535 nm). Furthermore, both fluorophores exhibited red-shifted emission under application of increased pressure. Mechanistic studies indicated that grinding resulted in crystal to amorphous phase transition in S1, whereas grinding in S2 demonstrated the rare feature of crystal-to-crystal phase transition. The mechano-fluorochromic (MFC) natures of S1 and S2 were supported by single point energy calculations using density functional theory (DFT).

[1]Benzothieno[3,2-b][1]benzothiophene-based dyes: effect of the ancillary moiety on mechanochromism and aggregation-induced emission

“…According to the above-mentioned statement, the aggregation-induced redshift in emission is attributed to the unrestricted rotation in the aggregation state of the BTBT derivatives. 30 Closer inspection of the emission spectra revealed that the compound in DMSO covered most of the visible region, which may guide single molecular white light emission.…”

“…According to the above-mentioned statement, the aggregationinduced redshift in emission is attributed to the unrestricted rotation in the aggregation state of the BTBT derivatives. 30 Closer inspection of the emission spectra revealed that the compound in DMSO covered most of the visible region, which may guide single molecular white light emission. The solid-state PL emission was investigated using different percentage loadings of the compounds, namely 25%, 50%, 75% and 100%, and it was observed that an increase in the percentage loading caused an increase in emission intensity (Fig.…”

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

“…57 The observation also shows an aggregation-induced unusual blue-shift in PT-C2-(Nap) 2 and PT-C12-(Nap) 2 , which might be due to the inhibition TICT in the aggregated state of the naphthyl derivatives. 58 In general, the AIE effect leading to a blue shift are relatively less compared to those which exhibit a red shift upon aggregation. Among PT-C2-(An) 2 , PT-C2-(Pn) 2 and PT-C2-(Pr) 2 (Fig.…”

“…Furthermore, it has been seen that upon grinding, the fluorescence lifetime of the ground sample has been enhanced. This can be attributed to the improved interaction between phenothiazine (PT) phenyl (Ph) and PT-long alkyl chain noncovalent interactions 58 (viz. p-p, C-HÁ Á Áp and C-HÁ Á ÁH-C) (Fig.…”

“…10). According to Lu et al, the increased dihedral angle results in the highly twisted conformation of the molecule 58,77,78.…”