2-[2-[2-Chloro-3-[(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)ethylidene]-1-cyclohexen-1-yl]ethenyl]-1,3,3-trimethyl-3H-indolium salt with 4-methylbenzenesulfonic acid
[CAS# 205744-92-9]

Identification

• Classification: Organic raw materials >> Heterocyclic compound >> Indoles
• Name: 2-[2-[2-Chloro-3-[(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)ethylidene]-1-cyclohexen-1-yl]ethenyl]-1,3,3-trimethyl-3H-indolium salt with 4-methylbenzenesulfonic acid
• Synonyms: 2-[2-[2-Chloro-3-[2-(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)ethylidene]-1-cyclohexen-1-yl]ethnyl]-1,3,3-trimethyl-3H-indolium 4-methylbenzenesulfonate
• Molecular Formula: C₃₂H₃₆ClN₂^.C₇H₇O₃S
• Molecular Weight: 655.29
• CAS Registry Number: 205744-92-9
• EC Number: 812-296-6
• SMILES: CC1=CC=C(C=C1)S(=O)(=O)[O-].CC1(C2=CC=CC=C2[N+](=C1C=CC3=C(C(=CC=C4C(C5=CC=CC=C5N4C)(C)C)CCC3)Cl)C)C

Safety Data

• Hazard Symbols: GHS07 Warning
• Hazard Statements: H317-H319
• Precautionary Statements: P280-P305+351+338

Discovory and Applicatios

2-[2-[2-Chloro-3-[(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)ethylidene]-1-cyclohexen-1-yl]ethenyl]-1,3,3-trimethyl-3H-indolium salt with 4-methylbenzenesulfonic acid is a member of the heptamethine cyanine family of dyes, a class of polymethine compounds that have been extensively studied for their intense absorption in the visible and near-infrared regions. The scientific foundations for this compound lie in the nineteenth-century discovery of cyanine dyes, which were first observed as colored byproducts in reactions of heterocyclic quaternary salts. Systematic investigation of cyanine structures in the late nineteenth and early twentieth centuries established that their color arises from a conjugated polymethine chain linking two nitrogen-containing heterocycles. This understanding led to the rational design of dyes with tunable spectral properties.

The compound described here contains two indolium-based heterocyclic units connected through an extended conjugated chain incorporating a cyclohexenyl moiety. The positive charge on the indolium system is balanced by 4-methylbenzenesulfonate, the conjugate base of 4-methylbenzenesulfonic acid. The use of sulfonate counterions in cyanine dyes became common because such salts often display improved crystallinity and handling properties compared with halide salts. The introduction of bulky and lipophilic substituents, such as trimethyl groups on the indolium rings and a chloro substituent on the polymethine framework, reflects well-established strategies for modulating solubility, stability, and photophysical behavior.

Heptamethine cyanine dyes, including indolium derivatives of this type, were developed in the twentieth century as photographic sensitizers. Research demonstrated that extending the polymethine chain length shifts the absorption maximum toward longer wavelengths. Compounds with seven methine units were found to absorb in the near-infrared region, enabling their use in infrared-sensitive photographic plates and films. The ability of cyanine dyes to transfer excitation energy to silver halide grains underpinned their commercial importance in photographic technology for many decades.

Beyond photography, indolium-based heptamethine cyanines have been widely applied as fluorescent probes. Their strong absorption coefficients, high molar absorptivity, and characteristic fluorescence emission in the red to near-infrared region have made them valuable in analytical chemistry and biomedical research. Derivatives containing sulfonate or other ionic groups have been used to enhance water solubility, facilitating applications in aqueous media. The fundamental photophysical behavior of these dyes, including their absorption and emission properties, has been investigated in numerous spectroscopic studies that correlate molecular structure with electronic transitions along the conjugated chain.

The presence of a cyclohexenyl bridge within the polymethine system is consistent with structural variations explored to improve photostability and adjust spectral characteristics. Substituents such as chlorine atoms on the conjugated framework have also been reported to influence electronic distribution and chemical reactivity. These modifications are grounded in experimental work demonstrating that substitution patterns along the polymethine chain can affect both the wavelength of maximum absorption and the resistance of the dye to photodegradation.

Salts formed with 4-methylbenzenesulfonic acid have been described in the literature as stable forms of cationic dyes. The tosylate counterion is known to provide good compatibility with organic solvents and to support crystallization for characterization. Such properties are relevant in both research and industrial contexts, where reproducibility and purity are essential.

Overall, this indolium heptamethine salt exemplifies the evolution of cyanine dye chemistry from early empirical discoveries to structurally defined compounds with predictable optical properties. Its development is part of a broader body of experimentally verified work demonstrating how controlled modification of heterocyclic end groups, conjugated chain length, and counterions yields dyes with specific and reproducible applications in imaging, spectroscopy, and photonic technologies.

References

2014. Near-infrared sensitive, positive-working, image forming composition and photographic element containing a 1,1-di[(alkylphenoxy)ethoxy]cyclohexane. US Patent (granted).
URL: US-8632943-B2

2013. Near-infrared sensitive, positive-working, image forming composition and photographic element containing a 1,1-di[(alkylphenoxy)ethoxy]cyclohexane. US Patent Application.
URL: US-2013156710-A1