Readily Observable Morphological Changes in Monoalkoxynaphthalene-Naphthaleneimide Donor–Acceptor Dyad Crystals Induced by Specific H/D Substitution

Organic
materials that can change their photophysical properties
upon exposure to external stimuli have a wide-ranging list of potential
applications. We have been exploring a series of monoalkoxynaphthalene-naphthaleneimide
(MAN-NI) donor–acceptor dyads with alkyne linkers and appended
alkyl side chains that undergo a dramatic solid-state orange-to-yellow
polymorphic transition when exposed to stimuli such as heat or pressure.
In the present study, the more significant changes in dyad design
involving extended or saturated linkers as well as fluorinated side
chains directed major crystal structure changes explaining why these
derivatives did not display solid-state polymorphic transitions. Dyads
with specifically deuterated side chains were also investigated in
order to probe a previously identified nonconventional C–H···O
hydrogen bond thought to play a critical role during the solid-state
polymorphic transition. All of the deuterated MAN-NI derivatives crystallized
with unit cells analogous to the nondeuterated version, and thus exhibited
the expected solid-state orange-to-yellow polymorphic transitions.
The surprising conclusion from the present study is that changes as
subtle as specific H/D substitution can lead to a small but measurable
geometric isotope effect (GIE), that is apparently amplified by the
large number of such interactions involved in a solid structure, leading
to significant and predictable changes of crystal morphology that
are readily apparent to the naked eye. The bottom line is that for
the MAN-NI dyad system, it appears possible to extrapolate from extraordinarily
subtle changes in crystal packing geometry, not only the energetics
of supramolecular assembly, but also the dynamics of crystal growth,
and thus macroscopic morphology.