First-principles molecular dynamics study of a photochromic molecular crystal

We present first-principles calculations for the thermal reaction in the ph otochromic molecular crystals of 2-(2',4'-dinitrobenzyl)-pyridine (DNBP). U nder irradiation this dye molecule undergoes a characteristic change of col or as a result of an intramolecular proton-transfer reaction. By simulating the thermal reaction with first-principles molecular dynamics, we show tha t during the thermal proton transfer a hydrogen bond from the o-nitro group to the migrating proton is formed intermediately, which serves as a "bridg e" between the educt and product forms. Furthermore, we present the experim entally not accessible structures of the crystalline photoisomers. On the b asis of our results we conclude that photoisomerization is possible in the crystalline phase. This property is of elementary interest for technical ap plications of this system.