Promoting the thermal back reaction of vinylheptafulvene to dihydroazulene by physisorbtion on nanoparticles

Abstract

We investigate the effects of nanoparticles on molecular solar thermal energy storage systems and how one can tune chemical reactivities of a molecular photo- and thermoswitch by changing the nanoparticles. We have selected the dihydroazulene/vinylheptafulvene system to illustrate the effects of the nanoparticles on the chemical reactivities of the molecular photo- and thermoswitch. We have utilized the following nanoparticles: a TiO₂ nanoparticle along with nanoparticles of gold, silver and copper. We calculate the rate constants for the release of the thermal energy utilizing a QM/MM method coupled to a transition state method. The molecular systems are described by density functional theory whereas the nanoparticles are given by molecular mechanics including electrostatic and polarization dynamics. In order to investigate whether the significant stabilization of the transitions state provided by the nanoparticles is general to the DHA/VHF system, we calculated the transition state rate constant of the parent- and 3-amino-substituted-DHA/VHF systems at 298.15 K in the four different orientations and at the three different separations. We observe that the transition state rate constant of the parent system is only increased as the cyano groups are oriented towards the nanoparticle while the presence of the nanoparticle actually impedes the reactions using the three other orientations. On the other hand, for the substituted system the nanoparticle generally leads to a significant increase in the rate of the reaction. We find that the nanoparticles can have a substantial effect on the calculated rate constants. We observe, depending on the nanoparticle and the molecular orientation, increases of the rate constants by a factor of 10⁶. This illustrates the prospects of utilizing nanoparticles for controlling the release of the stored thermal energy.