Variation in solvato-, AIE- and mechano-fluorochromic behavior for furanyl and thiophenyl-substituted anthranyl π-conjugates: the role of tiny flanking donor groups

Abstract

Only a few reports exist on thiophene or furan-linked typical tetraphenylethene or difluoroboron systems known as aggregation-induced emission-active fluorogens (AIEgens) with a substantial difference in solvato and mechano-fluorochromic (MFC) features. With a pressing demand for finding a smart strategy to achieve MFC-active materials, we herein report unsymmetrically substituted anthracene-vinyl-phenothiazine π-conjugates (TAPTZ and FAPTZ) linked with furan or thiophene as a tiny flanking donor group. Both the compounds are easily synthesized using an economic path by avoiding metal catalysts and harsh reaction conditions. In solvatofluorochromic studies, by varying polarity of solvents from hexane to acetonitrile, the bathochromic shifts of 118 nm (3082 cm⁻¹ Stokes shift) for TAPTZ and 112 nm (2840 cm⁻¹ Stokes shift) for FAPTZ are noticed. Such electron-rich but conformationally twisted molecules create a twisted intramolecular charge transfer (TICT) state responsible for the solvatofluorochromic feature. A relatively more significant change in the excited state dipole moment for TAPTZ causes the difference. Both these molecules exhibit blue-shifted AIE features (faint orange to intense yellow), where TAPTZ has appeared to be a relatively better AIEgen with a 65 nm average particle size. Interestingly, FAPTZ has emerged as a stronger emitter than TAPTZ in the solid state. By grinding in a mortar and pestle or by quick pressing (in an infrared pellet maker: 20 MPa), TAPTZ and FAPTZ display reversible MFC features with a 15 nm and 22 nm redshift, respectively. Single crystal study discloses the difference in the molecular twisting and packing in the solid-state for these analogs. A relatively large number of intramolecular interactions (dominated by S⋯S and C⋯S) make the crystal packing stronger for TAPTZ and make such a difference in the sensitivity. Thus, we found a decent impact of the flanking donor groups thiophene/furan on the emission behavior under different environments. The observed fact is further supported by powder X-ray diffraction and lifetime measurement studies. Also, the intermolecular interactions are quantified by Hirshfeld-surface analyses to validate the empirical facts further. Finally, FAPTZ is documented as a favorable platform for rewritable optical-recording/security-based applications.