Blue LED and a Reusable Nanocomposite Light the Way to N-Alkylated Indazolones

Chemists at the Vellore Institute of Technology in India have developed a reusable photocatalyst that can forge carbon–nitrogen bonds in indazolone molecules using nothing more exotic than blue light from a light-emitting diode. The new work, published in Catalysis Letters, describes an iron tungstate and nitrogen-rich graphitic carbon nitride nanocomposite, abbreviated FeWO4/g-C3N5, that drives the N-alkylation of indol-yl-phenyl-indazolones with a range of benzyl chloride substrates in remarkable yields. Because the catalyst can be recovered and reused at least five times without any loss in activity, the method offers a practical and economical route to a family of compounds that matter greatly in medicinal chemistry.

Indazolones are bicyclic nitrogen-containing heterocycles that appear repeatedly in pharmaceutical research, and attaching an alkyl group to the ring nitrogen is one of the most common ways chemists tune their biological properties. The classic ways of doing this, however, often rely on precious transition metals such as palladium, ruthenium or gold, on stoichiometric oxidants, or on harsh conditions that generate substantial waste. The research team, Kumar Aravindraj and Selvaraj Mohana Roopan of the Department of Chemistry in the School of Advanced Sciences, set out to replace those expensive and environmentally burdensome approaches with a heterogeneous, light-driven process that operates under mild conditions and recycles its catalyst batch after batch.

The heart of the innovation lies in the pairing of two semiconducting materials with complementary properties. Iron tungstate, FeWO4, is an iron-based inorganic semiconductor that has attracted attention for electrocatalysis, water treatment and energy storage, but on its own it suffers from the limitations typical of narrow-band metal oxides, including rapid recombination of the charge carriers generated when light is absorbed. Graphitic carbon nitride, and in particular the nitrogen-rich variant g-C3N5, is a metal-free polymeric semiconductor with a smaller band gap than the widely studied g-C3N4, which allows it to harvest a larger share of the visible spectrum. When the two are combined into a single nanocomposite, the interface between them promotes the separation of photogenerated electrons and holes, extending the lifetime of the energetic charge carriers that ultimately do the chemical work.

Under blue LED irradiation, the nanocomposite absorbs photons and promotes electrons from valence to conduction bands, leaving behind positively charged holes. These electrons and holes participate in a photocatalytic cycle that activates the benzyl chloride partner and the nitrogen of the indazolone, enabling the formation of the new C–N bond that defines the N-alkylated product. The authors report that the process delivers benzyl-indol-yl-phenyl-indazolones in remarkable yields across a variety of substrate combinations, demonstrating that the method tolerates different benzyl chlorides and different indol-yl-phenyl-indazolone frameworks. The use of a simple blue LED rather than ultraviolet lamps or high-intensity light sources keeps the energy input low and the setup inexpensive, features that matter for any reaction intended to be scaled beyond a research laboratory.

A critical part of the study was the characterization of the catalyst itself. The team deployed an extensive battery of analytical techniques to confirm the structure, morphology and electronic properties of the nanocomposite. X-ray diffraction established the crystalline phases present, while field emission scanning electron microscopy and high-resolution transmission electron microscopy, together with selected area electron diffraction, revealed the nanoscale architecture and the intimate contact between the iron tungstate and the carbon nitride components. Energy dispersive X-ray spectroscopy confirmed the elemental composition, and X-ray photoelectron spectroscopy probed the chemical states of the constituent elements at the surface.

Optical measurements provided the mechanistic insight that underpins the catalyst’s performance. Diffuse reflectance spectroscopy was used to determine the light-absorption characteristics and band structure of the composite, while photoluminescence spectroscopy served as a window onto charge-carrier behavior: a quenched photoluminescence signal relative to the individual components indicates that electrons and holes are being separated efficiently at the heterojunction rather than recombining and releasing their energy as light. This suppression of recombination is precisely what allows the absorbed blue photons to be funneled into productive chemistry, and it explains why the composite outperforms what either semiconductor could achieve alone.

The products themselves were rigorously verified. The researchers used nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry to confirm the identity and purity of the N-alkylated indazolones, and single crystal X-ray diffraction, reported with an ORTEP thermal ellipsoid plot, provided unambiguous structural proof for at least one representative product. Such crystallographic confirmation is particularly valuable in heterocycle synthesis, where regioisomeric products can complicate assignment, and it demonstrates the analytical thoroughness of the study. Control experiments with radical traps such as TEMPO, 2,2,6,6-tetramethyl-1-piperidinyloxy, were consistent with the mechanistic picture of a light-driven process involving reactive intermediates generated at the catalyst surface.

Perhaps the most practically significant result is the recyclability of the catalyst. Heterogeneous photocatalysts often degrade, leach metal into solution, or lose activity through fouling, which undermines their green credentials. In this work, the FeWO4/g-C3N5 nanocomposite was recovered after each reaction and redeployed for five consecutive cycles of N-alkylation without any loss in catalytic activity. That durability means the cost and environmental footprint of catalyst preparation are amortized over multiple batches, and it simplifies product purification because the solid catalyst can simply be filtered off at the end of the reaction. The authors also note that no specific funding was received for the study and that no datasets were generated or analysed beyond those reported in the paper.

The new report builds on the same group’s earlier demonstration of a blue LED assisted indazolone preparation using a reusable CuWO4/g-C3N5 nanocomposite, published in the Journal of Photochemistry and Photobiology A: Chemistry in 2024. By swapping copper tungstate for its iron analogue, the team has moved to an even more earth-abundant and inexpensive metal while retaining the recyclability and efficiency of the previous system. Iron is among the most benign and plentiful elements in the periodic table, and iron-based heterogeneous catalysts have become a major focus of sustainable chemistry research precisely because they avoid the toxicity and supply concerns associated with palladium, ruthenium, gold and platinum systems.

The broader context makes the advance timely. Visible-light photocatalysis has transformed synthetic organic chemistry over the past decade by allowing reactions to be triggered under exceptionally mild conditions, and carbon nitride materials have emerged as robust, metal-free light absorbers for applications ranging from hydrogen peroxide production to pollutant degradation and fine chemical synthesis. Meanwhile, N-alkylation remains a workhorse transformation in medicinal chemistry, and methods that couple it to abundant catalysts and low-energy light sources are exactly what industrial and academic laboratories are seeking. By demonstrating that a FeWO4/g-C3N5 heterojunction can deliver N-alkylated indazolones in remarkable yields under a simple blue LED, and that the catalyst survives five reuse cycles intact, Aravindraj and Mohana Roopan have added a genuinely reusable, iron-based option to the photocatalysis toolbox. The work suggests that carefully engineered interfaces between metal tungstates and nitrogen-rich carbon nitrides could serve as a general platform for light-driven C–N bond formation, and it brings the vision of sustainable, low-cost photochemical manufacturing of pharmacologically relevant heterocycles a step closer to routine practice.

Subject of Research: A reusable FeWO4/g-C3N5 nanocomposite photocatalyst for blue LED-driven N-alkylation of indazolones via C–N bond formation

Article Title: FeWO4/g-C3N5 Nanocomposite: A Reusable Photocatalyst for N-Alkylation of Indazolones Under Blue LED

Article References: FeWO4/g-C3N5 Nanocomposite: A Reusable Photocatalyst for N-Alkylation of Indazolones Under Blue LED. (n.d.). https://doi.org/10.1007/s10562-026-05523-0

Image Credits: AI Generated

DOI: 10.1007/s10562-026-05523-0

Keywords: photocatalysis, FeWO4, g-C3N5, nanocomposite, indazolone, N-alkylation, blue LED, C–N bond formation, heterogeneous catalysis, iron tungstate, graphitic carbon nitride, green chemistry