Internal and External Stereoisomers of Squaraine Rotaxane Endoperoxide: Synthesis, Chemical Differences, and Structural Revision

Photooxygenation
of permanently interlocked squaraine rotaxanes
with anthracene-containing macrocycles produces the corresponding
squaraine rotaxane endoperoxides (SREPs) quantitatively. SREPs are
stored at low temperature, and upon warming, they undergo clean cycloreversion,
releasing singlet oxygen and emitting light. The structural elucidation
in 2010 assigned the structure as the SREP-int stereoisomer,
with the endoperoxide unit directed inside the macrocycle cavity.
New experimental and computational evidence reported here proves that
the initial, kinetic photooxygenation product is the less stable SREP-ext stereoisomer with the endoperoxide unit directed
outside the macrocycle. The photophysical properties and subsequent
reactivity of mechanically strained SREP-ext depend on
the size of the end groups of the encapsulated squaraine dye. If the
end groups are sufficiently large to prevent dissociation of the interlocked
components, the strained SREP-ext stereoisomer undergoes
clean thermal cycloreversion. However, smaller squaraine end groups
allow transient dissociation, resulting in a pseudorotaxane dissociation/association
process that produces SREP-int as the thermodynamic stereoisomer
that does not cyclorevert. The large difference in endoperoxide reactivity
for the two SREP stereoisomers illustrates the power of the mechanical
bond to induce cross-component steric strain and selective enhancement
of a specific reaction pathway. The new insight enabled synthetic
development of triptycene-containing squaraine rotaxanes with high
fluorescence quantum yields and large Stokes shifts.