Deprotonation-Induced Color Modulation in N,N'-Dihydroxynaphthalenediimide-Based Organic Crystals

N,N'-dihydroxy-1,4,5,8-naphthalenetetracarboxdiimide (NDI-(OH)₂ ) has attracted much attention in recent years, because its doubly deprotonated state, (O-NDI-O)^2- , has metal-coordination ability and characteristic electronic transition useful for designing electronic and optical functions. In contrast, a molecular crystal with the mono-deprotonated (HO-NDI-O)^- ion remains unknown. We herein report an organic crystal containing non-disproportionated (HO-NDI-O)^- ions, which are connected by very strong O-H-O hydrogen bonds. Its lowest energy absorption band (450 to 650 nm) is observed in between that of NDI-(OH)₂ (380 nm) and isolated (O-NDI-O)^2- (500 to 850 nm) species, consistent with the molecular orbital calculations. This absorption originates from the electronic transition from deprotonated imide-based orbitals to NDI-core orbitals, which can be influenced by the hydrogen bonds around imide group. Consequently, the optical properties of NDI-(OH)₂ can be modulated by the stepwise deprotonation and hydrogen-bonding interactions.