Photochromism Dependent on Crystal Packing: Photoinduced and Thermal Proton-Transfer Processes in Single Crystals of 6-(2,4-Dinitrobenzyl)-2,2′-Bipyridine

Despite structural similarities, different photoreactivities are displayed by the two crystalline phases of 6‐(2,4‐dinitrobenzyl)‐2,2′‐bipyridine. Whereas one phase (the arrangement of the molecules in the unit cell of this phase is shown on the right) undergoes a photoinduced proton transfer process, the other is photoinert. Deuteration causes a marked increase in the lifetime of the colored phototautomer.