Exploration of Molecular Shape-Dependent Luminescence Behavior: Fluorogenic Organic Nanoparticles Based on Bent Shaped Excited-State Intramolecular Proton-Transfer Dyes

Abstract

Fluorogenic materials that emit both in solution and in solid states have received special attention for their potential applicability to optoelectronic devices and biochemical sensing systems. To develop such materials, we have explored the molecular shape-dependent luminescence behavior for the bent-shaped molecules. We have synthesized several bent-shaped 1-(benzo[d]thiazol-2-yl)-6-substituted-naphthalen-2-ol compounds, which also have an excited-state intramolecular proton-transfer (ESIPT) feature, which is expected to modulate their optical properties. All the compounds and their 2-methoxy derivatives showed luminescence both in solution as well as in solid states, plausibly due to unfavorable orbital interactions between stacked molecules nearby as suggested by single crystal structure packing pattern analysis. The naphthol compounds also showed large Stokes shifts due to the ESIPT feature, along with good optical brightness and tunable emission color by changing the 6-substituent. Fluorescent organic nanoparticles were then prepared from selected compounds, and their size distribution and polydispersity were analyzed by dynamic light scattering and transmission electron microscope. Photophysical properties of the new dyes and their nanoparticles in solution as well as in solid states (in powder and crystalline forms) were characterized by spectral analysis and fluorescence lifetime measurements. The new organic nanoparticles were shown to stain cells by both confocal and two-photon microscopic imaging. The approach of molecular shape control demonstrated here thus opens a door toward solid-state luminescent organic compounds and their nanoparticles.