Large change of thermal conductivity induced by temperature and light in azobenzene-based mesophases

Achieving an efficient management of heat is of paramount importance in a wide range of applications. Thus, one of the biggest challenges in materials science is to develop functional materials allowing an active control over thermal conductivity.

‘Soft matter’ thermal devices are promising candidates in this direction. Shin et al. reported a reversible threefold change in the thermal conductivity of an azobenzene polymer induced by light (Shin et al., PNAS, 116, 8629, 2019).

Here we report the synthesis and thermal conductivity of 4,4’-dialkyloxy-3-methylazobenzene derivatives, with different alkyl chain lengths. These systems show a rich thermal phase diagram, with different crystalline, anisotropic mesophases, and isotropic liquid configurations, as a function of temperature. The reversible changes in the alignment of the alkyl chains along these phases results in reversible variations of the thermal conductivity up to 30%. Moreover, the conformational transition between trans-cis azobenzene groups under UV illumination leads to reversible crystal-to-liquid transition, changing the thermal conductivity up to 40%.

The large variety of light-responsive chemical groups suggests that this could be a successful approach to develop effective thermal switches.