Crystal structures and thermochromic and fluorescence properties of N-salicylideneaniline derivatives having a naphthyl-isoquinoline group

N-Salicylideneaniline (SA) is a classical chromic organic compound exhibiting both photochromism upon photoirradiation and thermochromism upon cooling. In this paper, I report the synthesis of novel SA derivatives, N-salicylidene-1-isoquinoline-2-naphthylamine (SN1-4), and their crystal structures and photochromic, thermochromic, and luminescence properties. The molecular conformations of SN1-4 in the crystals were very similar despite the different substituent groups. The naphthalene and isoquinoline rings were largely twisted with each other due to the H…H steric repulsion. In contrast, the phenol and naphthyl rings were co-planar, with a dihedral angle less than 10°. The planar molecular conformation induced thermochromic properties in these derivatives and also rendered them non-photochromic. Moreover, crystals of all the SA derivatives exhibited excited-state intramolecular proton-transfer fluorescence with a large Stokes shift, and the emission wavelength could be tuned by the substituent group.