Laser Desorption
Mass Spectrometry of End Group-Protected
Linear Polyynes: Evidence of Laser-Induced Cross-Linking
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Abstract
End group-protected
linear polyynes of composition Tr*–(CC)<sub><i>n</i></sub>–Tr* (with Tr* representing the super
trityl group and <i>n</i> = 2, 4, 6, 8, 10) and <i>t</i>Bu–(CC)<sub>6</sub>–<i>t</i>Bu (with <i>t</i>Bu being the tertiary butyl group) have
been studied by laser desorption ionization (LDI) time-of-flight (ToF)
mass spectrometry. <i>t</i>Bu-terminated polyyne molecules
show considerably higher stability during laser activation than Tr*-end-capped
polyyne molecules. A key feature is the abundant formation of oligomeric
species upon laser activation. Tandem mass spectrometry reveals strong
bonding within the oligomers which indicates cross-linking of the
former polyynes within the oligomers. The process is more abundantly
occurring and less energy demanding than the laser-induced coalescence
of C<sub>60</sub>. Cross-linking is more efficient with the smaller
end group (<i>t</i>Bu), and larger oligomers are formed
when the chain length of the polyyne increases, both a result of enhanced
interaction of the triple bonds in neighboring chains. The presence
of the matrix molecules in matrix-assisted (MA)LDI hinders the polyyne
interaction, and oligomer formation is markedly reduced