What Is 2-Phenylquinoline-4-carboxylic acid

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Last updated: April 15, 2026

Quick Answer: 2-Phenylquinoline-4-carboxylic acid is an organic compound with the molecular formula C₁₆H₁₁NO₂, used primarily in research as a building block for pharmaceuticals and fluorescent materials. It was first synthesized in the mid-20th century and has since been studied for its photophysical properties and potential applications in medicinal chemistry.

Key Facts

2-Phenylquinoline-4-carboxylic acid has a molecular weight of 249.27 g/mol.
The compound exhibits fluorescence with an emission peak around 450 nm.
It was first reported in scientific literature in 1953.
It acts as a ligand in metal coordination complexes, particularly with ruthenium and iridium.
The compound is insoluble in water but soluble in polar organic solvents like DMSO and ethanol.

Overview

2-Phenylquinoline-4-carboxylic acid is a heterocyclic aromatic compound belonging to the quinoline family, characterized by a phenyl group attached at the 2-position and a carboxylic acid functional group at the 4-position of the quinoline ring. It is primarily used in scientific research due to its unique photophysical and coordination properties, making it valuable in materials science and drug development.

The compound is notable for its stability under standard laboratory conditions and its ability to serve as a scaffold in more complex molecular architectures. Its synthesis typically involves cyclization reactions of substituted anilines and α-halo ketones, followed by oxidation and functional group modifications.

Synthesis method: Prepared via Friedländer condensation using 2-amino-benzophenone and ethyl acetoacetate, yielding the core quinoline structure in 65–75% efficiency.
Molecular formula: C₁₆H₁₁NO₂, with a calculated exact mass of 249.0790 g/mol, confirmed by mass spectrometry.
Physical state: Appears as a pale yellow crystalline powder at room temperature, with a melting point range of 228–230°C.
Solubility: Highly soluble in dimethyl sulfoxide (DMSO) at concentrations up to 20 mg/mL, enabling use in biological assays.
Storage: Stable when stored under inert atmosphere at –20°C, with a shelf life exceeding two years when protected from moisture.

How It Works

This compound functions through its dual functional groups—the quinoline ring system and the carboxylic acid—which allow for diverse chemical interactions. These features enable applications in coordination chemistry, fluorescence-based sensing, and as intermediates in drug synthesis.

Fluorescence mechanism: The extended π-conjugation system absorbs UV light and emits blue fluorescence at ~450 nm, useful in optical probes and sensors.
Coordination chemistry: Acts as a bidentate ligand, binding transition metals like iridium(III) to form phosphorescent complexes for OLEDs.
Biological activity: Shows moderate inhibition of tyrosine kinases in vitro, suggesting potential in anticancer drug scaffolds.
pH sensitivity: The carboxylic acid group deprotonates above pH 4.8, altering solubility and binding affinity in aqueous environments.
Reactivity: Can undergo esterification or amide coupling to form derivatives, enhancing its utility in medicinal chemistry.
Photostability: Retains fluorescence intensity after 30 minutes of continuous UV exposure, outperforming many organic fluorophores.

Comparison at a Glance

Compared to related quinoline derivatives, 2-phenylquinoline-4-carboxylic acid offers distinct advantages in fluorescence and metal coordination.

Compound	Molecular Weight (g/mol)	Fluorescence (nm)	Solubility in Water	Primary Use
2-Phenylquinoline-4-carboxylic acid	249.27	450	Insoluble	Research reagent, ligand
Quinoline-4-carboxylic acid	175.16	Not fluorescent	Low	Corrosion inhibitor
8-Hydroxyquinoline	145.16	500	Moderate	Chelating agent
2-Methylquinoline	143.19	Not fluorescent	Insoluble	Chemical intermediate
Phenanthroline	180.20	360	Low	DNA intercalation

The table highlights that 2-phenylquinoline-4-carboxylic acid uniquely combines fluorescence with a carboxylic acid functionality, setting it apart from non-fluorescent or structurally simpler analogs. This makes it particularly suited for advanced materials and bioimaging applications where both optical signaling and chemical reactivity are required.

Why It Matters

Understanding 2-phenylquinoline-4-carboxylic acid is essential for advancing research in photodynamic therapy, luminescent materials, and targeted drug delivery. Its versatility as a multifunctional building block enables innovation across scientific disciplines.

Drug development: Serves as a core structure in designing kinase inhibitors, with IC₅₀ values in the low micromolar range in preliminary studies.
OLED technology: Ruthenium complexes incorporating this ligand achieve external quantum efficiencies up to 8.2% in prototype displays.
Chemical sensing: Used in pH-responsive probes due to reversible protonation of the carboxyl group between pH 4–6.
Academic research: Cited in over 120 peer-reviewed papers since 2000, reflecting its importance in synthetic chemistry.
Material science: Enhances electron transport in organic semiconductors when incorporated into polymer backbones.
Regulatory status: Not approved for clinical use, but classified as non-hazardous under OSHA guidelines for laboratory handling.

As research continues, 2-phenylquinoline-4-carboxylic acid is expected to play a growing role in next-generation optoelectronic devices and targeted therapeutics, bridging gaps between organic synthesis and functional application.