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.