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Synthesis, fluorescence properties, and electrochemical behavior of highly substituted imidazole/benzofuran hybrid derivatives

This paper reports the synthesis of novel highly substituted imidazole-benzofuran hybrid derivatives via two distinct synthetic pathways and characterizes their structural, optical, and electrochemical properties using spectroscopic and cyclic voltammetry techniques.

Original authors: Erfan Jalilzadeh-Galeh¹, Zarrin Ghasemi, Somayeh Rashtabad Jadidi¹, Farima Shiri

Published 2026-07-13

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Erfan Jalilzadeh-Galeh¹, Zarrin Ghasemi, Somayeh Rashtabad Jadidi¹, Farima Shiri

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are a molecular architect trying to build a new kind of glowing brick. These aren't ordinary bricks; they are tiny, hybrid structures made by snapping together two very different Lego sets: an imidazole (a ring that loves to hold onto metals and do chemistry) and a benzofuran (a ring famous for glowing and acting like a biological superhero). The goal? To see if these new "hybrid bricks" can light up, conduct electricity, or perhaps do both.

The Two Construction Plans

The researchers didn't just throw ingredients into a pot and hope for the best. They tried two distinct construction blueprints to build these glowing hybrids.

Plan A: The "One-Pot" Sprint
Think of this as a high-speed relay race where the baton never leaves the runner's hand. The team started with a specific starting block called a salicylaldehyde (which has an imidazole ring attached) and mixed it with ethyl bromoacetate in a solution with a base called potassium carbonate ( $K_2CO_3$ ).

The Magic: In a single step, the molecules grabbed onto each other, swapped parts, and snapped into a new shape called a benzofuran.
The Result: They successfully built four different versions of these glowing bricks (labeled 2a through 2d). It was fast, efficient, and worked like a charm.

Plan B: The "Multi-Step" Journey
This route was more like a treasure hunt with four distinct checkpoints.

Checkpoint 1 (The Wittig Reaction): They took the starting blocks and used a special chemical tool (triphenylphosphine or tributylphosphine) to stretch them out into long, wavy chains called trans ethyl 2-hydroxycinnamates. They tested different tools and temperatures, eventually finding that using tributylphosphonium salt with 1.5 equivalents of potassium carbonate at room temperature for 2 hours was the perfect recipe, giving a 100% success rate for the first step.
Checkpoint 2 (The Snap-On): Next, they attached another piece (ethyl bromoacetate) to the chain, creating an "ether intermediate." This step was like adding a handle to a suitcase.
Checkpoint 3 (The Fold): Here, the molecule had to fold in on itself. They heated the mixture to 100°C and used 3.5 equivalents of potassium carbonate. This forced the molecule to curl up and form a new ring, creating a "dihydrobenzofuran." If they didn't use enough base or heat, the molecule just sat there; if they heated it too much (to 120°C), it fell apart. The sweet spot was 100°C for 4 hours, yielding about 78% of the folded product.
Checkpoint 4 (The Glow-Up): The folded molecule wasn't quite ready to shine yet; it needed to lose a few hydrogen atoms to become a perfect, flat, glowing benzofuran. They tried many different "oxidizing agents" (chemicals that steal electrons) to make this happen.
- They tried strong acids and air, but nothing happened.
- They tried a bromine-based chemical (NBS), which worked a little bit (31% yield).
- The Winner: They found that N-chlorosuccinimide (NCS) in carbon tetrachloride ( $CCl_4$ ) at room temperature was the best tool. It gave the highest yield of 43%. The paper suggests this worked better than the bromine version because the bond formed with chlorine is stronger, making the reaction more efficient.

The Light Show

Once the bricks were built, the team turned on the lights to see how they behaved. They dissolved the compounds in ethanol and shined a beam of light at them.

Absorption: All the new molecules loved to soak up light in the range of 263–288 nm. One specific molecule, 2d, was the champion of soaking up light, with a massive absorption coefficient of 6.00 × 10⁴ M⁻¹cm⁻¹. In contrast, the derivative 5g exhibited the lowest absorption coefficient (0.65 × 10⁴ M⁻¹cm⁻¹), highlighting the significant variation in optical properties across the different structures.
Emission: When they glowed back, they emitted light at longer wavelengths. The "Stokes shift" (the difference between the light they ate and the light they spit out) ranged from 93 nm to 147 nm.
The Glow Factor: To measure how bright they were, the team compared them to a standard glowing stone called naphthalene. The best performers were the final benzofuran products 6b, 6f, and 6g, which had quantum yields of 0.49, 0.30, and 0.45 respectively. This means they were quite efficient at turning absorbed light into emitted light.

The Electric Pulse

Finally, the team wanted to see if these glowing bricks could also conduct electricity. They mixed the molecules with activated carbon (a sponge-like material) and tested them in a solution of potassium hydroxide.

The Test: They used a machine called a cyclic voltammetry (CV) to push electricity back and forth through the material.
The Discovery: The machine showed clear "peaks" where the molecules were gaining and losing electrons. This proved that the electricity storage wasn't just a simple battery effect; it was a Faradaic process, meaning the molecules were chemically reacting to store the charge.
The Speed Limit: When they ran the test faster (increasing the scan rate from 10 mV/s to 100 mV/s), the current went up. However, the paper suggests this might be because the electrolyte (the liquid carrying the charge) couldn't penetrate the sponge fast enough at high speeds, causing a buildup of current.

What Didn't Work?

It's important to note what the researchers tried and discarded.

They explicitly ruled out using DDQ, nitric acid, or just air to turn the folded molecules into the final glowing benzofurans; those methods resulted in no reaction.
They found that using NBS (bromine-based) was less effective than NCS (chlorine-based).
They discovered that heating the folding step to 120°C actually hurt the yield, dropping it from 78% down to 53%.
One specific molecule, 6e (the one with a 3-thienyl group), simply refused to form during the final oxidation step, yielding 0%.

The Bottom Line

The paper successfully demonstrates that you can build these complex, glowing hybrid molecules using two different paths. The "One-Pot" method is quick, while the "Multi-Step" method allows for more variety in the final design. The resulting molecules are confirmed to glow with decent brightness and can participate in chemical reactions to store electricity. The authors have measured these properties directly in the lab, confirming that these new structures are real, stable, and optically active, paving the way for potential uses in things like sensors or light-emitting devices, though the paper stops short of building those devices itself.

Technical Summary: Synthesis, Fluorescence Properties, and Electrochemical Behavior of Highly Substituted Imidazole/Benzofuran Hybrid Derivatives

Problem Statement
Oxygen heterocycles, particularly benzofuran and dihydrobenzofuran derivatives, are recognized for their significant biological activities (anti-cancer, anti-oxidant, anti-inflammatory) and applications in optoelectronic materials, including organic fluorescent brighteners, OLEDs, and chemosensors. While various methods exist for synthesizing benzofurans, such as reactions between salicylaldehydes and $\alpha$ -haloketones or $\alpha$ -haloesters, there is a continued need for efficient synthetic routes to novel hybrid structures. Specifically, the integration of highly substituted imidazoles with benzofuran cores offers potential for diverse applications in biological and material science fields. This work addresses the need for the synthesis and characterization of new imidazole-benzofuran hybrid derivatives to evaluate their optical and electrochemical properties.

Methodology
The authors employed two distinct synthetic strategies to generate a series of monosubstituted benzofuran derivatives linked to triarylimidazole moieties.

One-Pot Synthesis (Method 1):
- Precursors: 5-(triarylimidazole-1-yl) salicylaldehyde derivatives (1a–d) were prepared via Duff formylation.
- Reaction: These salicylaldehydes were reacted with ethyl bromoacetate in the presence of potassium carbonate ( $K_2CO_3$ ) in DMF at 80°C.
- Mechanism: The process involves a nucleophilic substitution followed by an intramolecular Knoevenagel condensation to directly yield benzofuran derivatives (2a–d).
Multi-Step Synthesis (Method 2):
- Step 1 (Wittig Reaction): Salicylaldehyde derivatives (1a–h) were reacted with tributylphosphonium bromide and $K_2CO_3$ at room temperature to synthesize trans-ethyl 2-hydroxycinnamates (3a–h) in high yields (94–100%).
- Step 2 (Ether Formation): The hydroxyl group of compounds 3a–h underwent nucleophilic substitution with ethyl bromoacetate in the presence of $K_2CO_3$ in DMF at 60°C to form ether intermediates (4a–h).
- Step 3 (Intramolecular Michael Addition): Intermediates 4a–h were treated with 3.5 equivalents of $K_2CO_3$ in DMF at 100°C to induce intramolecular Michael cycloaddition, yielding dihydrobenzofuran derivatives (5a–h).
- Step 4 (Aromatization): The dihydrobenzofurans were oxidized to benzofuran derivatives (6a–h) using N-chlorosuccinimide (NCS) in carbon tetrachloride ( $CCl_4$ ) at room temperature. Optimization studies indicated NCS was superior to NBS, DDQ, or nitric acid for this transformation.

Characterization and Evaluation

Structural Confirmation: All intermediates and final products were characterized using FT-IR, $^1$ H NMR, $^{13}$ C NMR, and elemental analysis. FT-IR analysis tracked the evolution of carbonyl stretching frequencies (e.g., aldehyde at 1655 cm $^{-1}$ shifting to ester carbonyls around 1700–1740 cm $^{-1}$ ) to confirm structural transformations.
Optical Properties: UV-Vis absorption and fluorescence emission spectroscopies were conducted in ethanol. Quantum yields were calculated using naphthalene as a standard ( $\Phi = 0.23$ ).
Electrochemical Behavior: Cyclic voltammetry (CV) was performed on activated carbon-based electrodes coated with the synthesized compounds in a 6 M KOH electrolyte. Measurements were taken at scan rates ranging from 10 to 100 mV/s.

Key Results

Synthesis Efficiency: The multi-step route proved highly effective for diversifying the benzofuran core. The Wittig reaction yielded 3a–h with 94–100% efficiency. The subsequent ether formation and Michael addition steps proceeded with yields ranging from 71% to 79% for the dihydrobenzofurans. The final oxidation step using NCS provided benzofurans 6a–h with yields between 25% and 43%, with the exception of the 3-thienyl derivative (6e), which was not successfully isolated.
Optical Data:
- Absorption: All compounds exhibited maximum absorption wavelengths ( $\lambda_{max}$ ) between 263 and 288 nm, indicating strong $\pi$ – $\pi^*$ transitions. Compound 2d showed the highest molar absorption coefficient ( $6.00 \times 10^4$ M $^{-1}$ cm $^{-1}$ ).
- Fluorescence: Emission spectra were recorded with Stokes shifts ranging from 93 to 147 nm. The highest quantum yields were observed for benzofuran derivatives 6b (0.49), 6f (0.30), and 6g (0.45).
Electrochemical Findings: CV plots for compounds 1a, 2a, 3a, 4a, 5a, and 6a displayed distinct anodic and cathodic peaks, confirming a Faradaic charge storage mechanism. The current response increased with scan rates, suggesting that the KOH electrolyte did not fully penetrate the electrode surface at higher rates, leading to increased current response.

Significance and Claims
The paper claims the successful synthesis of a novel series of highly substituted imidazole-benzofuran hybrids via two distinct methodologies. The authors highlight that the multi-step approach allows for the diversification of the benzofuran core through specific intermediates (cinnamates and ethers) that are not accessible via the one-pot method. The study establishes a correlation between the structural modifications and the resulting optical properties, identifying specific derivatives (notably 6b, 6f, and 6g) with promising fluorescence quantum yields. Furthermore, the electrochemical investigation confirms the redox activity of these compounds when supported on activated carbon, suggesting their potential utility in electrochemical applications, though the paper focuses primarily on characterizing the Faradaic nature of the charge storage rather than proposing specific device applications. The work contributes to the library of functionalized heterocycles available for further study in biological and material science contexts.