2-[2-[2-Chloro-3-[2-(1,3-dihydro-1,1,3-trimethyl-2H-benz[e]indol-2-ylidene)ethylidene]-1-cyclohexen-1-yl]ethenyl]-1,1,3-trimethyl-1H-benz[e]indolium salt with 4-methylbenzenesulfonate (1:1) is a synthetic organic dye belonging to the family of polymethine or cyanine dyes. Compounds of this structural class are characterized by two nitrogen-containing heterocyclic systems connected by a conjugated polymethine chain. The extended system of alternating double and single bonds is responsible for strong absorption in the visible or near-infrared region of the electromagnetic spectrum. The specific structure indicated by this name consists of two benz[e]indolium-derived moieties linked through a substituted cyclohexenyl and ethenyl bridge, forming a delocalized cation that is paired with a 4-methylbenzenesulfonate counterion in a 1:1 ratio.
The development of cyanine dyes dates back to the 19th century, when early members of the class were synthesized and found to exhibit intense coloration and photographic sensitizing properties. Systematic research in the late 19th and early 20th centuries established that linking quaternized heterocyclic nuclei through methine chains produced compounds with strong, tunable absorption bands. As structural variations were introduced, including modifications of the heterocyclic rings and substitution along the polymethine chain, chemists were able to shift absorption maxima across the visible spectrum and into the near-infrared region. Benz[e]indolium derivatives in particular became important components in the design of longer-wavelength-absorbing dyes.
The compound described by this name contains two 1,1,3-trimethylbenz[e]indolium units. Quaternization of the indole nitrogen generates a positively charged heterocycle, which stabilizes the conjugated cation formed along the polymethine chain. The presence of a substituted cyclohexenyl group and a chloro substituent within the bridging framework further modifies the electronic distribution. Structural features of this type have been examined using spectroscopic techniques such as ultraviolet-visible absorption spectroscopy and nuclear magnetic resonance, confirming the delocalized nature of the cation and the contribution of the entire conjugated system to light absorption.
Cyanine dyes with benz[e]indolium nuclei have been widely applied in optical and imaging technologies. Their strong and relatively narrow absorption bands, combined with high molar absorptivity, make them useful as sensitizers in photographic emulsions and as functional dyes in optical recording materials. During the 20th century, extensive empirical studies established correlations between polymethine chain length and absorption wavelength, providing a rational basis for the design of dyes tailored to specific spectral regions. Compounds incorporating extended aromatic systems such as benz[e]indole were found to shift absorption toward longer wavelengths compared with simpler indole analogues.
In addition to photographic applications, polymethine dyes have been investigated for use in fluorescence-based detection and labeling. The conjugated cationic framework can exhibit fluorescence under appropriate conditions, and derivatives have been incorporated into analytical reagents and probes. The counterion, in this case 4-methylbenzenesulfonate, serves to balance the positive charge of the dye cation and can influence properties such as solubility and crystallinity. Sulfonate counterions are commonly employed because they provide good stability and compatibility with organic media.
The synthesis of dyes of this type typically involves condensation reactions between quaternized heterocyclic salts and suitable aldehyde or activated methylene intermediates to form the polymethine linkage. Such methods were developed and refined throughout the 20th century, allowing reproducible preparation of structurally defined cyanine salts. Analytical characterization, including elemental analysis and spectroscopic measurement, has been used to confirm composition and purity.
Compounds with extended polymethine systems are also of interest in materials science because of their nonlinear optical properties and their responsiveness to changes in environment. Experimental studies have examined their absorption behavior in different solvents and solid matrices, demonstrating that electronic transitions are sensitive to molecular structure and surrounding conditions.
Overall, this benz[e]indolium-based polymethine salt represents a member of a well-established class of synthetic dyes whose discovery and development were driven by advances in heterocyclic chemistry and photophysical research. Its structure reflects principles that have been experimentally validated over more than a century of investigation into cyanine dye chemistry and its practical applications in imaging, optics, and analytical science.
References
2010. Near-Infrared Organic Compounds and Emerging Applications. Chemistry � An Asian Journal.
DOI: 10.1002/asia.200900596
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![CAS # 134127-48-3, 2-[2-[2-Chloro-3-[2-(1,3-dihydro-1,1,3-trimethyl-2H-benz[e]indol-2-ylidene)ethylidene]-1-cyclohexen-1-yl]ethenyl]-1,1,3-trimethyl-1H-benz[e]indolium salt with 4-methylbenzenesulfonate (1:1), (2E)-2-[(2E)-2-[2-chloro-3-[(E)-2-(1,1,3-trimethylbenzo[e]indol-3-ium-2-yl)ethenyl]cyclohex-2-en-1-ylidene]ethylidene]-1,1,3-trimethylbenzo[e]indole,4-methylbenzenesulfonate](/structures/134127-48-3.gif)
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