Abstract
The design and synthesis of mechanically responsive materials is interesting because they are potential candidates to convert thermal energy into mechanical work. Reported in this paper are thermosalient effects in a series of halogen derivatives of salinazids. The chloro derivative, with higher electronegativity and a weaker inter-halogen bond strength (Cl⋯Cl) exhibits an excellent thermal response, whereas the response is weaker in the iodo derivative with stronger I⋯I halogen bonding. 3,5-Di-chloro-salinazid (Compound-A) exists in three polymorphic forms, two room-temperature polymorphs (Forms I and II) and one high-temperature modification (Form III). The transformation of Form I to Form III upon heating at 328-333 K is a reversible thermosalient transition, whereas the transformation of Form II to Form III is irreversible and non-thermosalient. 3,5-Di-bromo- (Compound-B) and 3-bromo-5-chloro- (Compound-C) salinazid are both dimorphic: the Form I to Form II transition in Compound-B is irreversible, whereas Compound-C shows a reversible thermosalient effect (362-365 K). In the case of 3,5-di-iodo-salinazid (Compound-D) and 3,5-di-fluoro-salinazid (Compound-E), no phase transitions or thermal effects were observed. The thermosalient behaviour of these halosalinazid molecular crystals is understood from the anisotropy in the cell parameters (an increase in the a axis and a decrease in the b and c axes upon heating) and the sudden release of accumulated strain during the phase transition. The di-halogen salinazid derivatives (chlorine to iodine) show a decrease in thermal effects with an increase in halogen-bond strength. Interestingly, Compound-B shows solid-state photochromism in its polymorphs along with the thermosalient effect, wherein Form I is cyan and Form II is light orange.
Highlights
The development of mechanically responsive materials which respond to external stimuli such as heat, light and pressure is important because these materials are known to be selfactuating and energy-harvesting (Kim et al, 2013; Morimoto & Irie, 2010; McCrone, 1965; Panda et al, 2015)
In order to understand the events observed on the hot-stage microscope, both Differential scanning calorimetry (DSC) and variable-temperature Powder X-ray diffraction (PXRD) were performed on all the materials
In the present investigation we have explored the mechanical response of organic crystals towards thermal stress and the role of type II halogen bonding in tuning the properties of materials
Summary
The development of mechanically responsive materials which respond to external stimuli such as heat, light and pressure is important because these materials are known to be selfactuating and energy-harvesting (Kim et al, 2013; Morimoto & Irie, 2010; McCrone, 1965; Panda et al, 2015). They have wide-ranging applications, such as artificial muscles and biomimetic and technomimetic materials (Cabane et al, 2012; Ikeda et al, 2007; Lehn, 2002; Mather, 2007; Rowan, 2009; Sagara & Kato, 2009; Sato, 2016; Takashima et al, 2012). Chemical diagrams are shown in Scheme 1, and the 2-hydroxybenzylidene isonicotinohydrazide skeleton is referred to as salinazid
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