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Aryldiazonium Salts to Azo Dyes: Diazo Coupling01:11

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The reaction of weakly electrophilic aryldiazonium (also called arenediazonium) salts with highly activated aromatic compounds leads to the formation of products with an —N=N— link, called an azo linkage. This reaction, presented in Figure 1, is known as diazo coupling and occurs without the loss of the nitrogen atoms of the aryldiazonium salt. Highly activated aromatic compounds such as phenols or arylamines favor the diazo coupling reaction. The coupling generally occurs at the...
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Arenediazonium substitution reactions occur when the diazonium group is substituted by various functional groups such as halides, hydroxyl, nitrile, etc. For instance, arenediazonium salts react with copper(I) salts of chloride, bromide, or cyanide to form corresponding aryl chlorides, bromides, and nitriles. These reactions are named Sandmeyer reactions. Although the mechanism of this reaction is complicated, as illustrated in Figure 1, they are believed to progress via an aryl copper...
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Diazonium Group Substitution: –OH and –H01:19

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Nitrous acid, a weak acid, is prepared in situ via the reaction of sodium nitrite with a strong acid under cold conditions. This nitrous acid prepared in situ reacts with primary arylamines to form arenediazonium salts. Such reactions are known as diazotization reactions. As shown in Figure 1, the formation of arenediazonium salts begins with the decomposition of nitrous acid in an acidic solution to give nitrosonium ions.
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Halogenation of Alkenes02:46

Halogenation of Alkenes

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Halogenation is the addition of chlorine or bromine across the double bond in an alkene to yield a vicinal dihalide. The reaction occurs in the presence of inert and non-nucleophilic solvents, such as methylene chloride, chloroform, or carbon tetrachloride.
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Bromination and chlorination of aromatic rings by electrophilic aromatic substitution reactions are easily achieved, but fluorination and iodination are difficult to achieve. Fluorine is so reactive that its reaction with benzene is difficult to control, resulting in poor yields of monofluoroaromatic products. To address this, Selectfluor reagent is used as a fluorine source in which a fluorine atom is bonded to a positively charged nitrogen.
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Chlorination and bromination are important classes of electrophilic aromatic substitutions, where benzene reacts with chlorine or bromine in the presence of a Lewis acid catalyst to give halogenated substitution products. A Lewis acid such as aluminium chloride or ferric chloride catalyzes the chlorination, and ferric bromide catalyzes the bromination reactions. During the bromination of alkenes, bromine polarizes and becomes electrophilic. However, in the bromination of benzene, the bromine...
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Utilizing Selective Chlorination to Synthesize New Triangulenium Dyes.

Jesper Dahl Jensen1, Niels Bisballe1, Laura Kacenauskaite1

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The Journal of Organic Chemistry
|November 18, 2021
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Summary

New chlorination methods provide novel triangulenium dyes with redshifted properties. This research also isolated a key intermediate in [4]helicene formation, advancing understanding of related molecular systems.

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Area of Science:

  • Organic Chemistry
  • Materials Science
  • Photochemistry

Background:

  • Triangulenium dyes are versatile molecular platforms with tunable photophysical properties.
  • Previous functionalization strategies for triangulenium structures had limitations in accessing specific substitution patterns.

Purpose of the Study:

  • To develop new synthetic routes for functionalized triangulenium dyes.
  • To investigate the mechanism of [4]helicene formation and isolate key intermediates.
  • To characterize the photophysical and redox properties of novel chlorinated triangulenium dyes.

Main Methods:

  • Early-stage chlorination of acridinium precursors using N-chlorosuccinimide (NCS).
  • Selective ring-closing reactions to form [4]helicene products.
  • Isolation and characterization of reaction intermediates.
  • Spectroscopic (absorption, emission) and electrochemical (cyclic voltammetry) analysis of new dyes.

Main Results:

  • Synthesis of two new triangulenium dyes (1 and 3) via early-stage chlorination.
  • Selective synthesis of a single [4]helicene regioisomer.
  • First isolation of a proposed intermediate in the SNAr reaction mechanism.
  • Late-stage chlorination yielded a different dichlorinated dye (2).
  • Chlorinated dyes exhibited redshifted absorption/emission, high fluorescence quantum yields (36-41%), and long lifetimes (12.5-16 ns).
  • Chlorination lowered reduction potentials, enabling tunable redox properties.

Conclusions:

  • Early-stage chlorination offers a complementary approach to functionalize triangulenium dyes and access novel structures.
  • The mechanistic study provides crucial insights into the formation of aza-bridged helicenium systems.
  • The synthesized chlorinated triangulenium dyes demonstrate potential for applications requiring tailored photoredox and optical properties.