Novel phenomena for aggregation induced emission enhancement: highly fluorescent hydrophobic TPE-BODIPY couples in both organic and aqueous media

Abstract

A series of easily prepared BODIPY (BOD) derivatives bearing tetraphenylethene (TPE) groups at the 8-position (TPE-BOD), 2,6-positions ((TPE)₂-BOD), and 2,6,8-positions ((TPE)₃-BOD) are presented. Although these fluorophores are highly emissive in THF, they are weakly fluorescent in the 60% water–THF solution. (TPE)₂-BOD and (TPE)₃-BOD compounds are able to become highly emissive in solutions with higher water content due to the amorphous aggregate formation, which is associated with dynamic crystallization. In contrast, highly intense emission of (TPE)BOD in THF is smoothly quenched on decreasing the solvation. The role of TPE units on AIEE activity has been investigated with fluorescence spectroscopic studies by means of comparison experiments performed on a BODIPY derivative carrying only phenyl substituents at the 2 and 6 positions. Aggregate formation in these derivatives was also demonstrated by LB-film studies and theoretical calculations supported the AIEE effect in longer wavelengths due to an increased conjugation. SEM was used to examine the morphological structure of organized microcrystals in THF–water mixtures of variable composition. Aggregate dimensions, determined from DLS analyses, are also consistent with results reported for many well-known AIEE active chromophores. To imagine the nature of the amorphous aggregates in solutions confocal fluorescence microscopy was used. In the light of these results, (TPE)₂BOD and (TPE)₃BOD are presented as novel long-wavelength AIEE active compounds being highly emissive in both polar and nonpolar solvents, functioning according to transition from their highly soluble states to amorphous aggregate states.