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Traditional luminescent polymers typically contain extended π–π conjugated structures that suffer from associated aggregation-caused quenching (ACQ) and high cytotoxicity issues. Therefore, there is a huge demand for stand-alone luminescent polymers that can circumvent these drawbacks and can be employed in diverse applications. In view of that, nonconventional luminescent polymers (NLPs) are being developed and studied extensively for their distinctive characteristics and potential applications in wide-ranging areas, and for an in-depth understanding of photophysical processes in science. These polymers do not contain classical chromophores, but often involve isolated benzene rings or only electron-rich moieties (e.g. amine, CO, –OH, ether, imide, and heteroatoms N, O, P, S, etc.). This review article highlights the recent progress in the field of NLPs, including their synthesis, underlying emission mechanisms, and applications in materials and biological sciences. Besides, it is identified to be the foremost review that extensively emphasizes on both the fluorescence and phosphorescence features of NLPs. Finally, some prospects have been suggested for further exploration in this flourishing field of research.

Herein, a leucine-based perfectly alternating polyampholyte was synthesized by placing positively charged primary ammonium groups and negatively charged carboxylate groups alternatively onto the side chain of the poly(styrene-alt-maleimide) skeleton. The well-defined non-ionic macromolecular precursor was first prepared by sequence-controlled reversible addition–fragmentation chain transfer (RAFT) copolymerization of an N-maleoyl-L-leucine tert-butyl ester (M1) with a tert-butyl carbamate (Boc)-protected leucine appended styrenic monomer (M2), which could serve as an anionic and a cationic unit of the co-zwitterion monomers respectively, as a result of exclusion of the protecting groups. The aqueous solution behavior of the polyampholyte was studied by a combination of techniques, including pH, turbidity and zeta potential measurements, revealing net zero charge (isoelectric point (IEP)) with concomitant phase separation at pH = 7.0. The protected version of the as-synthesized copolymer displayed intense blue luminescence only in organic solvents, albeit it does not contain any conjugated fluorophore moiety. Interestingly, the alternating polyampholyte exhibited fluorescence activity in water and was used for the specific detection of carbon disulfide (CS2) among various volatile organic compounds (VOCs) in a 100% aqueous environment. Moreover, the polyampholyte could accomplish a colorimetric “naked-eye” sensing of CS2 vapour with high selectivity and sensitivity under ambient conditions.

Natural biopolymers such as proteins and nucleic acids in living organisms possess an inherent ability to respond to local environmental stimuli, which motivated researchers to make biomolecule-derived non-biological macromolecules with a biomimetic structure having stimuli-responsive properties. This review mainly focuses on stimuli-responsive polymers having natural amino acid units either in the main-chain or in the side-chain, their self-assembled nanostructures and hydrogel networks. Recent advances in the design and synthesis of amino acid-derived polymers that are responsive to various physical, chemical, or biochemical stimuli such as temperature, light, pH, redox-, metal ions, gas, glucose, enzyme, proteins, DNA or a combination of these are illustrated. Their potential for use as stimuli-responsive “smart” nanomaterials in biomedical and biotechnological applications such as in controlled drug delivery, gene delivery, non-fouling materials, etc. is also highlighted. The primary aim of this review article is to motivate researchers towards the design and synthesis of novel stimuli-responsive biohybrid materials for making next generation smart materials.

Development of a speedy, selective, and sensitive detection method for picric acid (PA) in an aqueous medium using a simple, cost-effective, and portable sensory technique is in high demand not only because of PA's explosive nature but also because of its superior water solubility. This study reports a nonconjugated macromolecular luminogen for prompt sensing of PA in 100% aqueous environments. A novel water-soluble unconventional fluorescent alternating copolymer based on a poly(styrene-alt-maleimide) skeleton has been found to be selective for nanomolar detection of PA in pure water over other aromatic compounds, without any interference from other nitroaromatics and metal ions or common anions that may coexist. The polymeric sensor molecule also has the capability to detect PA in the solid state and vapor phase. The fluorimetric sensing mechanism of PA in the present fluorescent probe is attributed to combined ground-state electrostatic complex formation and ground-state electron transfer from picrate anion to the probe molecule, as evidenced by UV–vis and 1H NMR studies, time-resolved fluorescence measurement, cyclic voltammetry, and density functional theory (DFT) calculations.

Herein, we report the synthesis of a new class of traditional fluorophore-free dual pH- and thermo-responsive fluorescent copolymer through sequence-controlled copolymerization of rationally designed monomers. The N-substituted maleimide monomer bearing a diethylene oxide side-chain, (N-(methoxy diethylene glycol) maleimide, M1), was copolymerized with a tert-butyl carbamate (Boc)-protected leucine appended styrenic monomer (M2) to obtain well-defined copolymers with perfectly alternating sequences of M1 and M2. The as-synthesized copolymers displayed bright-blue fluorescence in organic solvents. After Boc-group expulsion, the copolymers showed dual pH- and thermo-responsiveness, they retained their luminescence properties in organic solvents, and also showed pH/thermo-tunable fluorescence activity in water. The origin of the fluorescence in the copolymers was ascertained using density functional theory (DFT), where we observed that the “through-space” π–π interaction between the benzene ring and the neighbouring carbonyl group of the maleimide unit is responsible for the unexpected fluorescence in the alternating copolymer.

To prepare chiral nanostructures coated with bioactive molecules, a side-chain amino acid containing macromolecular chain transfer agent (macroCTA), poly(Boc-L-alanine methacryloyloxyethyl ester) (PBLAEMA), has been used as the steric stabilizer for the reversible addition–fragmentation chain transfer (RAFT) mediated dispersion polymerization of benzyl methacrylate (BzMA) in methanol at 65 °C. Gel permeation chromatography (GPC) analysis confirmed an efficient and well-controlled block copolymerization. A full spectrum of morphologies spanning spherical micelles, worm like micelles, fibres and vesicles could be attained by tuning (i) the length of the solvophobic block and (ii) the total solid content at which the block copolymerization is performed. Interestingly, a purely fibre phase morphology formed a thermoresponsive gel at room temperature above a critical fibre entanglement concentration, which underwent degelation upon heating because of the morphological transformation from anisotropic fibre to isotropic sphere. In actual fact, twisted nano-fibres have been formed through the hierarchical self-assembling of polymerization induced self-assembly (PISA) generated macromolecules in the gel state. Circular dichroism (CD) spectroscopy was used to elucidate chiroptical properties. Additionally, a high demanding wrinkle surface has been constructed preliminarily from this copolymer dispersion solution. Successful Boc-group expulsion facilitates the disassembly of vesicles to either worms or spheres with an appreciable cationic character below pH 7.0 as revealed by aqueous electrophoresis studies. Though the creation of nano-objects through PISA is well-known, fabricating chiral nanostructures with reactive handles and their hierarchical self-organization to have functional architectures is a less explored area. In this present work we were able to hybridize PISA, chirality and hierarchical self-assembling.

pH-reversible covalently cross-linked polymeric gels from a side-chain polymer with multi-amino groups and a difunctional aldehyde cross-linker have been prepared by employing dynamic covalent chemistry in the form of reversible imine (CN) bond formation. Two different series of dual pH- and thermo-responsive amino acid based copolymers with primary amine groups located at different positions and various chemical compositions are synthesized by reversible addition fragmentation chain transfer (RAFT) copolymerization of tert-butyl carbamate (Boc)-tryptophan methacryloyloxyethyl ester (Boc-Trp-HEMA) with 2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA) and N,N-(dimethylamino)ethyl methacrylate (DMAEMA) respectively, followed by Boc-group expulsion. The pH-induced thermoresponsive properties of the copolymers have been observed with the high content of thermoresponsive MEO2MA/DMAEMA segments (∼80%) in the copolymer. The tryptophanyl moiety present in the copolymer chain provided florescence characteristics, which were studied by florescence spectroscopy. Copolymers with free amino groups were employed for cross-linking with a difunctionalized aldehyde cross-linker, 4-formylphenyl 4′-formylbenzoate (FPFB), to form dynamic covalent gels through the formation of CN bonds in the network. Rheological measurements have been conducted with various ratios of CHO/NH2 from 1 to 0.33 for homopolymer gels and copolymer gels with different amino acid contents (20–80%), which showed a plateau of storage modulus (G′) over a wide range of angular frequencies indicating the independent nature of G′ with respect to angular frequency, and the mechanical properties of dynamic gels are a function of cross-linking density. The pH-dependent reversible sol–gel transition has also been investigated.

Reversible addition-fragmentation chain transfer (RAFT) polymerization was employed to afford two different chiral copolymer series having opposite chiroptical properties based on 2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA) and Boc-L/D-leucine methacryloyloxyethyl ester (Boc-L/D-Leu-HEMA) with varying co-monomer proportions. The random nature of the copolymers was indicated by the reactivity ratios of the two monomers for the two systems (MEO2MA with Boc-L-Leu-HEMA and Boc-D-Leu-HEMA), and further supported by smooth variation of the single glass transition temperature (Tg) determined from differential scanning calorimetry (DSC). With an increasing amino acid content in the copolymer, the specific rotation and molar ellipticity values increased, as examined by polarimeter and circular dichroism (CD) spectroscopy, respectively. After Boc-group cleavage under acidic conditions, all the copolymers exhibited cationic nature, as confirmed by dynamic light scattering (DLS), as well as the dual thermo and pH-dependent solubility behaviour in aqueous solution. The temperature induced phase transition of these copolymers was investigated as a function of the copolymer compositions over the pH range from 5.5 to 6.5 using a UV-Vis spectrophotometer. Furthermore, the chiral recognition ability of the synthesized monomers and homopolymers towards 1,1′-bi-2-naphthol (rac-BINOL) was studied by 1H NMR spectroscopy.