Stimuli-responsive aggregation-induced fluorescence in a series of biphenyl-based Knoevenagel products: effects of substituent active methylene groups on π–π interactions

Abstract

A series of three biphenyl‐based Knoevenagel products (denoted 1a, 1b, 1c) with active methylene groups has been synthesized. Compounds 1a and 1b show strong solid‐state fluorescence, whereas 1c displays low emission. Effects of substituent groups in condensed phase packing of the molecules have been investigated and correlated with their photophysical properties. Interestingly, compound 1a exhibits mechanofluorochromism with emission color changes from yellow to green (wavelength shift of 40 nm) after mechanical grinding. Furthermore, fluorescence of 1a and 1b is turned off under alkaline conditions, making them potential candidates for aggregation‐enhanced emission‐based pH sensors.

Exceptional Thermo-Mechano-Fluorochromism and Nanomechanical Analysis of Mechanically Responsive J and H Type Polymorphic Systems

“…Solid-state luminescence, pH-dependent fluorescence sensing, and mechano-fluorochromic behavior of the 1a-N needle form were discussed in our previous report . We measured the solid-state absolute photoluminescence quantum yield of the crystal and ground powder of both polymorphs.…”

“…Out of them, the biphenyl-dicyano derivative labeled as 1a (Figure a) exhibits solid-state fluorescence, mechano-fluoro-chromism, as well as pH-dependent fluorescence quenching. Herein, we have explored the nanomechanical properties of the reported needle-shaped polymorph of 1a (labeled 1a-N ) and a newly discovered block-shaped polymorphic form ( 1a-B ). The two polymorphic forms exhibit distinct emission colors in the solid state under UV light at 365 nm.…”

“…In a very recent report, Thakuria and co-workers developed 46 a series of biphenyl-based Knoevenagel products with active methylene groups. Out of them, the biphenyl-dicyano derivative labeled as 1a (Figure 1a) exhibits solid-state fluorescence, mechano-fluoro-chromism, as well as pHdependent fluorescence quenching.…”

“…Solid-state luminescence, pH-dependent fluores- cence sensing, and mechano-fluorochromic behavior of the 1a-N needle form were discussed in our previous report. 46 We measured the solid-state absolute photoluminescence quantum yield of the crystal and ground powder of both polymorphs. Absolute photoluminescence quantum yields (PLQY) were measured using an integrating sphere accessory attached to EDINBURGH-FLS1000 spectrometers.…”

Solid-state fluorescence of a quasi-isostructural polymorphic biphenyl based Michael addition product

“…52,53 Subtle modifications in intermolecular interactions such as hydrogen bonding, halogen bonding, π⋯π interactions, and C-H⋯π interactions produce a different molecular arrangement and a drastic change in their optoelectronic properties. In a recent report, Sarma et al 54 synthesized a few biphenyl based donor-π-acceptor organic fluorophores that showed AIE along with mechanofluorochromism. As an extension to that, a dimorphic biphenyl based Michael addition product was synthesized.…”