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The ionization of a molecule into a molecular ion inside the mass spectrometer causes instability in the molecule's structure due to the loss of an electron. This eventually leads to the fragmentation or breaking of some bonds in the molecule. The fragmentation occurs predominantly at specific bonds to yield relatively stable fragments.
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A bond is formed between two atoms by sharing two electrons. When this bond is broken by supplying sufficient energy, either two electrons can be taken up by one atom forming ions by the cleavage called heterolysis, or the two electrons are shared by two atoms, with one each creating radicals by the cleavage called homolysis.
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Alcohols (R-OH) ionize to lose one non-bonded electron from the oxygen atom, forming molecular ions. Due to their tendency to fragment rapidly, the intensity of the molecular ion peak in the mass spectrum is weak or sometimes absent. The fragmentation patterns for alcohols occur in two ways, i.e. ⍺-cleavage and dehydration. During ⍺-cleavage, the bond at the ⍺-position adjacent to the hydroxyl group cleaves to give a resonance-stabilized cation and a radical. However,...
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Upon ionization, aromatic compounds generate a molecular ion that is observed as a prominent peak in their mass spectra. For example, the molecular ion peak for benzene appears at a mass-to-charge ratio of 78, while toluene is observed at a mass-to-charge ratio of 92. The molecular ion benzene is highly stable and does not readily undergo further fragmentation due to the significant amount of energy required to disrupt the aromatic stability of the benzene ring. In contrast, the molecular ion...
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Breaking Bonds by Light: The Absorbance-Fragmentation Paradox.

Petra Dunkel1, Christine Tran1, Delphine Rigault1

  • 1Pharmacological and Toxicological Chemistry and Biochemistry Laboratory, Université Paris Cité, 45 rue des Saints-Pères, Paris, 75270 cedex 05, France.

Chemistry (Weinheim an Der Bergstrasse, Germany)
|July 22, 2025
PubMed
Summary

A new multi-branched probe effectively releases organic substrates under UV and two-photon (2P) activation. Compound 1 shows superior performance for uncaging applications using accessible laser wavelengths.

Keywords:
caged substrateslight‐sensitive probesoctupolar probesquinoline photocagestwo‐photon activation

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Area of Science:

  • Photochemistry
  • Organic Synthesis
  • Molecular Probes

Background:

  • Development of efficient photoactivatable probes is crucial for controlled release applications.
  • Triphenylamine (TPA) and 8-dimethylaminoquinoline (8-DMAQ) are common building blocks for photoresponsive molecules.

Purpose of the Study:

  • To synthesize and compare multi-branched probes based on 8-DMAQ and TPA for UV and two-photon (2P) activation.
  • To evaluate the relationship between absorption properties and fragmentation efficiency.
  • To identify the most effective probe for uncaging organic substrates.

Main Methods:

  • Synthesis of dipolar (1), quadrupolar (2), and octupolar (3) 8-DMAQ-TPA probes.
  • Characterization of UV and 2P absorption cross-sections (σ₂).
  • Assessment of probe fragmentation efficiency and uncaging quantum yield (Q).

Main Results:

  • Octupolar (3) and quadrupolar (2) probes exhibited higher ε and σ₂ values than the dipolar (1) probe.
  • Fragmentation efficiency showed an inverse correlation with enhanced absorption.
  • Compound 1 demonstrated the highest uncaging efficiency (Q = 12%) with a σ₂ of 86 GM and uncaging cross-section δu of 10.2 GM at 735 nm.

Conclusions:

  • The dipolar probe (1) is the most effective for uncaging organic substrates under both UV and 2P activation.
  • The findings provide valuable insights into structure-property relationships for photoactivatable probes.
  • Compound 1 offers a promising tool for applications requiring precise spatiotemporal control of substrate release.