Color polymorphism in organic crystals

Abstract

Color polymorphism is an interesting property of chemical systems which present crystal polymorphs of different colors. It is a rare phenomenon, with only a few examples reported in the literature hitherto. Nevertheless, systems exhibiting color polymorphism have many potential applications in different domains, such as pigment, sensor, and technology industries. Here, known representative chemical systems showing color polymorphism are reviewed, and the reasons for them to present such property discussed. Also, since some of the concepts related to color polymorphism have been frequently used imprecisely in the scientific literature, this article provides concise, systematic definitions for these concepts. T his paper focuses on color polymorphism, a fascinating property exhibited by chemical systems that present polymorphs showing different colors. In the first few sections, unambiguous definitions of the structural properties and of the relevant physical concepts related with this subject will be provided, since some of them have been frequently used in an imprecise way in the scientific literature. Then, the physicochemical reasons leading to color polymorphism will be described briefly using an essentially phenomenological approach. The last few sections of this paper will survey representative chemical systems showing color polymorphism. Terminology Polymorphism. In the crystallographic context, polymorphism, from the Greek poly (many) and morphe (form), also known as crystal polymorphism, refers to the ability of a certain compound to exist in different crystallographic structures, resulting from different packing arrangements of its molecules in the crystal structure 1. It is worth to differentiate between polymorphism and allotrophism. While the latter term describes the existence of different crystal structures of the same element, polymorphism is used regarding different crystalline structures of compounds. It is also important to mention that in this review, one will not consider solvates, hydrates, or dynamic isomers (including geometric isomers and tautomers) as polymorphic phases, but instead, as pseudopolymorphic states. Hence, a safe criterion for the classification of a system as polymorphic would be if the crystal structures are different but give rise to the same liquid and vapor states. Why is polymorphism so important? The main reason is that although being composed of the same compound, different polymorphic structures can behave as different materials 2. Indeed, in

Conformational Landscape and Polymorphism in 5-Acetic Acid Hydantoin

“…The selection upon crystallization of a conformer that for the isolated molecule is not the absolute minimum energy structure is not a rare phenomenon, and it is on the basis, for example, of conformational polymorphism. 52 However, this is an extreme case that, to be best of The hydrogen-bonding pattern in the crystal is similar to that reported in the low temperature study 36 and consists of (0 1 -2) planes of 5AAH molecules bound by O16-H17 … O7, N1-H16 … O16 and N3-H8 … O9 hydrogen bonds. Further N1-H16 … O16 and O16-H17 … O7 hydrogen bonds interconnect these planes establishing a three-dimensional network.…”

Direct evidence for distinct colour origins in ROY polymorphs

“…Our study concerns just ve of the thirteen known ROY polymorphs (and an additional three were also considered by Feng et al), 22 which leaves open the possibility that the colours of some of the other forms could also be a result of intermolecular excitations. The ROY polymorphs have been loosely categorised based on distinct regions they occupy on their conformational potential energy surface, 5,6 however if additional polymorphs were revealed to show similar electronic behaviour to the ON form, then perhaps grouping the forms by intra/intermolecular colour origins might be an appropriate, alternative, classication system.…”

“…1); the molecules show markedly different conformations and packing across all the known crystal forms. 5,6 The propensity for ROY to form so many crystal structures spontaneously, from the melt or solution, 7 has drawn signicant attention from the crystal growth community, in attempts to control the polymorphic outcome. [8][9][10][11][12][13] More recent work on crystal growth has exploited the cross-nucleating ability of ROY where synthetic analogues can be used to seed supercooled melts of structurally 'normal' material, leading to yet more polymorphs.…”

Stabilization of non‐native polymorphs for electrocatalysis and energy storage systems

“…Discussions on thermodynamic principles behind phase‐controlled synthetic methods, accompanying crystal growth and its mechanistic pathways, phase transition, and phase‐dependent electrochemical properties of the transition metal and transition‐metal oxides and chalcogenides are included. However, these principles are not restricted to only inorganic compounds but can also be extended to the organic compounds, a topic discussed in detail elsewhere (Brog, Chanez, Crochet, & Fromm, 2013; Dunitz & Bernstein, 1995; Gavezzotti & Filippini, 1995; Nogueira, Castiglioni, & Fausto, 2020; Sato, 1993). In general, polymorphism in organic compounds is due to relatively weak interactions where structures can easily rearrange by small relative shift or slight conformational rotations involving low activation energies.…”