Mechanistic Analysis of Solid-State Colorimetric Switching: Monoalkoxynaphthalene-Naphthalimide Donor–Acceptor Dyads

Abstract

There is growing interest in creating solids that are responsive to various stimuli. Herein we report the first molecular-level mechanistic picture of the thermochromic polymorphic transition in a series of MAN-NI dyad crystals that turn from orange to yellow upon heating with minimal changes to the microscopic morphology following the transition. Detailed structural analyses revealed that the dyads assemble to create an alternating bilayer type structure, with horizontal alternating alkyl and stacked aromatic layers in both the orange and yellow forms. The observed dynamic behavior in the solid state moves as a yellow wavefront through the orange crystal. The overall process is critically dependent on a complex interplay between the layered structure of the starting crystal, the thermodynamics of the two differently colored forms, and similar densities of the two polymorphs. Upon heating, the orange form alkyl chain layers become disordered, allowing for some lateral diffusion of dyads within their own layer. Moving to either adjacent stack in the same layer allows a dyad to exchange a head-to-head stacking geometry (orange) for a head-to-tail stacking geometry (yellow). This transition is unique in that it involves a nucleation and growth mechanism that converts to a faster cooperative wavefront mechanism during the transition. The fastest moving of the wavefronts have an approximately 38° angle with respect to the long axis of the crystal, corresponding to a nonconventional C–H···O hydrogen bond network of dyad molecules in adjacent stacks that enables a transition with cooperative character to proceed within layers of orange crystals. The orange-to-yellow transition is triggered at a temperature that is very close to the temperature at which the orange and yellow forms exchange as the more stable, while being lower than the melting temperature of the original orange, or final yellow, solids.

Free Charge Carriers in Homo-Sorted π-Stacks of Donor–Acceptor Conjugates

“…316,317 Recently, the group reported a series of connected D−A dyads consisting of naphthalimides (acceptor) and monoalkoxynaphthalene (donor) units 26 that demonstrate a thermochromic polymorphic transition (Figure 28a). 318 The orange polymorphs, which are the more stable form at room temperature, have a highly ordered segregated arrangement of D−A moieties alongside ideally packed alkyl side-chain units. At higher temperatures, the crystal systems undergo polymorphic transformation into yellow forms that present electrostatically favored alternating or mixed packing and reduced-order in packing of alkyl side chains (Figure 28b−d).…”

“…While continuous crystal lattices of segregated D–A stacks having great ordered arrangement and lower defects are greatly desirable due to the highly ordered, defect-free pathways for excitons and carriers, the factors that drive the processes of crystallization into specific arrangements are yet to be understood entirely. Iverson and co-workers’ work on polymorphism in single crystals of a series of D–A dyads gives insights into the interlaced role of crystalline structure, external conditions, and their thermodynamics on the stability of segregated and mixed crystalline arrangements. , Recently, the group reported a series of connected D–A dyads consisting of naphthalimides (acceptor) and monoalkoxynaphthalene (donor) units 26 that demonstrate a thermochromic polymorphic transition (Figure a) . The orange polymorphs, which are the more stable form at room temperature, have a highly ordered segregated arrangement of D–A moieties alongside ideally packed alkyl side-chain units.…”

“…(c) Segregated arrangement of the dyads in the orange crystal that reversibly transitions into (d) alternating packing within the yellow form. Reproduced with permission from ref . Copyright 2020 American Chemical Society.…”

Bridging the Void: Halogen Bonding and Aromatic Interactions to Program Luminescence and Electronic Properties of π-Conjugated Materials in the Solid State

“…A subsequent mechanistic study in 2020 examined the crystalto-crystal thermochromism in MAN−NI dyads, elucidating the ordered conversion from head-to-head to head-to-tail stacking. 157 In a related system of NDI−coronene cocrystals, Wang et al discovered that cocrystallization with a volatile guest (THF) attenuated close packing of NDI−coronene, rendering the structure susceptible to disruption (Figure 10b). 158 In contrast to a binary cocrystal prepared from dichloromethane, ternary cocrystals exhibited reversible green-to-orange MFC, and the authors posited donor−acceptor cocrystals with volatile guest incorporation as general strategy for accessing MFC behavior.…”

Sequence-complementarity dependent co-assembly of phosphodiester-linked aromatic donor-acceptor trimers

“…S10). Thermochromism in aromatic CT complexes [30][31][32] has been observed to occur via structural reorganisation [33] and phase separation. [34] Since phase separation would result in two CT-active compounds, it must be a degree of conformational reorganisation upon interaction of the two compounds which is occurring.…”