New Type of Dual Solid-State Thermochromism: Modulation of Intramolecular Charge Transfer by Intermolecular π−π Interactions, Kinetic Trapping of the Aci-Nitro Group, and Reversible Molecular Locking

Abstract

When heated above room temperature, some crystalline polymorphs of the 1,3-bis(hydroxyalkylamino)-4,6-dinitrobenzenes (BDBn, n = 2-5), bis(hydroxyalkyl) analogues of the intramolecular charge-transfer molecule 1,3-diamino-4,6-dinitrobenzene, exhibit "dual" thermochromism: gradual color change from yellow to orange at lower temperatures, and sharp color change from orange to red at higher temperatures. These two thermochromic changes are related to different solid-state processes. When allowed to cool to room temperature, the yellow color of the thermochromic molecules with different alkyl length (n) is recovered with unexpectedly different kinetics, the order of the respective rate constants ranging from 10(-7)-10(-6) s(-1) for BDB2 to about 0.1 s(-1) in the case of BDB3. The thermochromic mechanism and the reasons behind the different kinetics were clarified on the basis of detailed crystallographic characterization, kinetic thermoanalysis, and spectroscopic study of eight crystalline forms (seven polymorphs and one solvate). It was found that the polymorphism is due to the possibility of "locking" and "unlocking" of the alkyl arms by formation of a strong intramolecular hydrogen bond between the hydroxyl groups at their hydroxyl termini. The locking of BDB2, with shortest alkyl arms, is reversible and it can be controlled thermally; either of the two conformations can be obtained in the solid state by proper thermal treatment. By use of high temperature in situ single crystal X-ray diffraction analysis of BDB3, direct evidence was obtained that the gradual thermochromic change is related to increased distance and weakened pi-pi interactions between the stacked benzene rings: the lattice expands preferably in the stacking direction, causing enhanced oscillator strength and red shift of the absorption edge of the intramolecular charge transfer transition. The second, sharp thermochromic change had been assigned previously to solid-solid phase transition triggered by intramolecular proton transfer of one amino proton to the nitro group, whereupon an aci-nitro form is thermally populated. Contrary to the numerous examples of solid thermochromic molecules based on either pericyclic reactions or keto-enol tautomerism, this system appears to be the first organic thermochromic family where the thermochromic change appears as an effect of intermolecular pi-pi interactions and thermal intramolecular proton transfer to aromatic nitro group.

Crystal structure, luminescent and thermochromic properties of bis-1,10-phenan-throlinium hexachlorotellurate(IV) dihydrate

“…One of these chains is a stack of phenanthrolinium cations involved in the π-π stacking interaction between themselves (Fig. 2), which can significantly alter the parameters of this interaction [14][15][16] is the phenanthrolinium cation (С 12 Н 9 N 2 ), F is the π-π stacking between the phenanthrolinium cations (Fig. 3).…”

“…3). The process of natural strengthening of the hydrogen bonding of compound I with lowering temperature can cause changes in both the complex as an acceptor and the (С 12 Н 9 N 2 ) + cation as a donor on the whole [12,13] and, as a consequence, to affect the π-π stacking interaction parameters [14][15][16]. A decrease in the temperature of the crystal leads to a decrease in the amplitude of thermal oscillations of carbon atoms in phenanthrolinium cations, a decrease in the distances between them in the stack, and, as a rule, to an increase in the π-π stacking interaction [14,15].…”

Why does mutation of Gln61 in Ras by the nitro analog NGln maintain activity of Ras‐GAP in hydrolysis of guanosine triphosphate?

“…The nitro form corresponds to the major isomer in the gas phase; however, in condense phases the aci ‐nitro form can be stabilized . Therefore, we attempted to simulate the reaction pathway, starting from ES, for both NGln isomers.…”

Luminescent Thermochromism in a Gold(I)–Copper(I) Phosphine–Pyridine Complex

“…[3][4][5][6] These thermochromic properties are often attributed to structural changes such as keto-enol tautomerism or modifications of π-π interactions. [7] Many transition-metal complexes also exhibit luminescent thermochromism often due to changes in phase or in molecular structure. [8] Additionally, it is possible to change the origin of the photophysical process at different temperatures.…”