Negative thermal expansion in molecular materials

Abstract

Negative thermal expansion (NTE), whereby lattices contract upon heating, is of considerable interest for its wide applications in many fields. Molecular materials have been widely investigated as catalysts, sensors, etc., which usually endure temperature vibration. NTE can become a substantial means for controlling the coefficients of thermal expansion. Molecular materials possess plentiful structures and can be easily decorated, making them ideal platforms for thermal expansion modification. In this feature article, we provide an overview of the recent developments in utilizing NTE in molecular materials and summarize some mechanisms leading to NTE. The discussion of NTE in molecular materials concerns many factors, including transverse vibration, geometric flexibility, host-guest interactions, spin crossover, molecular packing rearrangement and molecular conformational changes.

A positive to negative uniaxial thermal expansion crossover in an organic benzothienobenzothiophene structure

“…through a phase transition. Some mechanisms responsible for NTE in molecular materials which are bonded by supramolecular interactions have been described by Liu et al (2018). They distinguish for this type of materials NTE caused by conformational changes of the molecules with temperature and NTE caused by rearrangement of molecules.…”

Controlling Thermal Expansion Behaviors of Fence-Like Metal-Organic Frameworks by Varying/Mixing Metal Ions

“…Most solids expand slightly as temperature increases (0 < α < 20 × 10 −6 K −1 , α for axial thermal expansion coefficients, α = ∂ l /∂ T × 1/ l ), which is known as thermal expansion or positive thermal expansion (PTE). Though the structure changes of PTE materials are very small, thermal expansion can often affect other material properties, for example, lead to the loss of precision and function for optical instruments, microelectronic devices so on (Evans, 1999 ; Liu et al, 2018 ). In contrast, materials with abnormal thermal expansion behaviors, such as zero thermal expansion (ZTE, |α| ≈ 0 × 10 −6 K −1 ), negative thermal expansion (NTE, α < 0 × 10 −6 K −1 ), or very large thermal expansion (|α| > 100 × 10 −6 K −1 ), are scarce (Mary et al, 1996 ; Chapman et al, 2005 ; Goodwin et al, 2008 ; Das et al, 2010 ; Zhou et al, 2015 ).…”

Giant thermal expansion of a two-dimensional supramolecular network triggered by alkyl chain motion

“…T he expansion of a material upon heating, known as thermal expansion is a fundamental property of solid systems that governs many of their mechanical applications [1][2][3] . Typically, thermal expansion coefficients (α ¼ Δl l 0 ΔT with Δl is the length variation, l 0 the initial length and ΔT the change of temperature) of inorganic materials are in the range of 0-20 × 10 −6 K −1 , whereas molecular counterparts vary from 50-60 × 10 −6 K −1 (poly(styrene)) 2 up to~420 × 10 −6 K −1 for porous coordination polymers 1 .…”

“…T he expansion of a material upon heating, known as thermal expansion is a fundamental property of solid systems that governs many of their mechanical applications [1][2][3] . Typically, thermal expansion coefficients (α ¼ Δl l 0 ΔT with Δl is the length variation, l 0 the initial length and ΔT the change of temperature) of inorganic materials are in the range of 0-20 × 10 −6 K −1 , whereas molecular counterparts vary from 50-60 × 10 −6 K −1 (poly(styrene)) 2 up to~420 × 10 −6 K −1 for porous coordination polymers 1 . Compared to inorganic materials, molecular architectures enable larger positive (or even negative 4,5 ) expansions due to various mechanisms involving the intrinsic molecules flexibility, their propensity to conformational changes or the weak intermolecular interactions 2 .…”

“…Typically, thermal expansion coefficients (α ¼ Δl l 0 ΔT with Δl is the length variation, l 0 the initial length and ΔT the change of temperature) of inorganic materials are in the range of 0-20 × 10 −6 K −1 , whereas molecular counterparts vary from 50-60 × 10 −6 K −1 (poly(styrene)) 2 up to~420 × 10 −6 K −1 for porous coordination polymers 1 . Compared to inorganic materials, molecular architectures enable larger positive (or even negative 4,5 ) expansions due to various mechanisms involving the intrinsic molecules flexibility, their propensity to conformational changes or the weak intermolecular interactions 2 . To date, however, understanding of the underlying mechanisms governing such large expansion coefficients at the molecular scale remains elusive.…”