Positional isomerism and short hydrogen bonds govern photochromic behaviour in dipyridyl-NDI⋯formic acid co-crystals.

Positional isomerism in pyridyl naphthalene diimide formic acid co-crystals controls crystal symmetry, yielding centrosymmetric (I2/a) and polar (Pc) structures from 4- and 2-pyridyl isomers respectively. Both materials function as reversible photochromic switches upon UV irradiation via a proton-coupled electron transfer type mechanism, generating NDI radical anions, as confirmed by UV-vis and ATR-FTIR spectroscopy.