Chain-dependent emission color codes of extended tetraphenylethylene derivatives: discrimination between water and methanol

Abstract

In this paper, we report the emission properties of two luminogens, which possess an identical extended tetraphenylethylene (TPE) aromatic core, but different hydrophobic dodecyl (1) and hydrophilic di(ethylene oxide) (2) peripheral chains. In comparison to the dilute chloroform solutions (0.1 μM), the emission of the chloroform solutions (10 μM) was red-shifted, which indicates an intermolecular interaction with increasing concentration. To investigate the chain-dependent emission behavior of 1 and 2, polar methanol and nonpolar n-hexane were employed as two poor solvents. As a consequence, upon adding each poor solvent, the emission properties were predominantly dependent upon the solubility between the peripheral chain and the mixture solvent (chloroform/poor solvent). Despite the identical aromatic chromophore, the emission could be guided by the nature of the attached peripheral chains. Upon increasing the fraction of the poor solvent for a peripheral chain, the crystallization of the TPE-based aromatic cores occurred, which produced a bright bluish emission. Scanning electron microscopy observation could confirm the ordered morphologies of the bluish aggregates. By comparing the integral values of the aromatic segments in the ¹H-NMR spectra with increasing the poor solvents, it can be said that the crystallization is initiated by the retardation of the conformational motions of the TPE aromatic ring, not the whole aromatic segment. In addition, by replacing chloroform with water-mixable tetrahydrofuran, two polar solvents, i.e., water and methanol, could be distinguished using the two emission color codes of 1 and 2. The molecular approach (the simple variation of the peripheral chain) in this study proved an alternative way to tune the solution emission of the identical chromophores.