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Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

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The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
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Simple aryl halides do not react with nucleophiles. However, nucleophilic aromatic substitutions can be forced under certain conditions, such as high temperatures or strong bases. The mechanism of substitution under such conditions involves the highly unstable and reactive benzyne intermediate. Benzyne contains equivalent carbon centers at both ends of the triple bond, each of which is equally susceptible to nucleophilic attack. This 50–50 distribution of products is...
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Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
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Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

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Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.
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The addition of a hydrogen halide to 1,3-butadiene gives a mixture of 1,2- and 1,4-adducts. Since more substituted alkenes are more stable, the 1,4-adduct is expected to be the major product. However, the product distribution is strongly influenced by temperature; low temperature favors the 1,2-adduct, whereas the 1,4-adduct is predominant at high temperature.
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Thermal Electrocyclic Reactions: Stereochemistry01:17

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The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
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Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
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Isomerization-Induced Excimer Formation of Pyrene-Based Acylhydrazone Controlled by Light- and Solvent-Sensing

Sanjoy Mondal1, Aditi Panja1, Debabrata Halder2

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This study developed a fluorescent sensor, Pyrene conjugate (Py@B), for detecting aromatic analytes. The sensor utilizes reversible E-Z isomerization and fluorescence quenching in aggregated states for efficient detection.

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Area of Science:

  • Supramolecular Chemistry
  • Fluorescent Sensing
  • Organic Synthesis

Background:

  • Polycyclic aromatic hydrocarbons (PAHs) like pyrene are fluorescent compounds with potential sensing applications.
  • Developing supramolecular systems for analyte detection requires careful molecular design and understanding of aggregation behavior.
  • C═N-based conjugates offer tunable photophysical properties for sensing.

Purpose of the Study:

  • To synthesize and characterize a novel pyrene-based Schiff base conjugate, (E)-3,4,5-tris(dodecyloxy)-N'-(pyren-1-ylmethylene)benzohydrazide (Py@B).
  • To investigate the potential of Py@B in its supramolecular aggregated state for sensing aromatic analytes.
  • To explore the photo-induced reversible E-Z isomerization of Py@B and its effect on fluorescence.

Main Methods:

  • Synthesis of Py@B via alkylation, substitution, and Schiff base reactions.
  • Spectroscopic analysis (UV-vis absorption and fluorescence) to study excimer formation, isomerization, and analyte interactions.
  • Photochemical studies using UV irradiation (λ = 254 nm and 365 nm) to induce and reverse E-Z isomerization.
  • Morphological studies of aggregated states using microscopy.
  • Time-dependent density functional theory (TDDFT) calculations for theoretical validation.

Main Results:

  • The E-isomer of Py@B (E-Py@B) exhibits bright fluorescence in nonaromatic solvents due to excimer formation at optimal concentrations (2 × 10⁻⁴ M).
  • Photoirradiation induces reversible E-Z isomerization of Py@B, leading to fluorescence quenching and changes in aggregation morphology.
  • Aromatic analytes efficiently quench the fluorescence of E-Py@B in solution and in the solid state via static and dynamic pathways.
  • Experimental and theoretical UV-vis spectra confirm adduct formation between E-Py@B and aromatic analytes.

Conclusions:

  • Py@B serves as an effective fluorescent sensor for aromatic analytes in both solution and solid states.
  • The reversible E-Z isomerization and excimer formation are key mechanisms for the sensing capabilities of Py@B.
  • Nonaromatic solvents facilitate excimer formation, crucial for sensing, while aromatic solvents hinder it.