The riddle of α-resorcinol revisited-New perspectives on "pocket-like" structure.

The riddle of α-resorcinol was initially studied extensively as an abnormal growth phenomenon, which significantly contributed to our understanding of asymmetric growth mechanisms in organic polar crystals, yet the crystalline surface structure and solvent behavior have not received adequate attention. To address this and meet the demand for advanced organic nonlinear optical (NLO) materials, 2,7-dihydroxynaphthalene (2,7-DHN) was studied as a model substance to revisit the riddle of α-resorcinol. Surprisingly, 2,7-DHN crystals expose naphthalene-rich and hydroxyl-rich surfaces along polar c-axis, similar to the packing of α-resorcinol. Single-crystal growth experiments and simulations show that solvent type significantly influences the asymmetric growth and morphology of 2,7-DHN. Compared to α-resorcinol, the "pocket-like" structure of 2,7-DHN plays a more decisive role, thanks to the strong hydrogen bonding and deeper matching between the rougher crystalline surface and solvent molecules. This work aims to provide a new perspective for revealing the riddle of α-resorcinol and investigates the defect control and morphology prediction in organic NLO crystals.