Fluorescence sensors for detection of water based on tetraphenylethene–anthracene possessing both solvatofluorochromic properties and aggregation-induced emission (AIE) characteristics

Abstract

We have designed and synthesized an SFC (solvatofluorochromism)/AIEE (aggregation-induced emission enhancement)-based fluorescence sensor TPE-(An-CHO)₄ composed of an electron-donating tetraphenylethene (TPE) core and four electron-accepting anthraldehydes as peripheral units. TPE-(An-CHO)₄ possesses both solvatofluorochromic properties and AIEE characteristics for detection of water over a wide range from low water content to high water content region in organic solvents. TPE-(An-CHO)₄ showed a feeble fluorescence band at around 500 nm originating from the anthraldehyde units in absolute THF. The addition of water to the THF solution containing TPE-(An-CHO)₄ caused red-shifts of the fluorescence band and changes in the fluorescence intensity. In the low water content region below 30 wt%, TPE-(An-CHO)₄ exhibited a linear increase in the fluorescence intensity with a red-shift of the fluorescence band with the increase in the water content, that is, the positive solvatofluorochromic properties based on the intramolecular charge transfer (ICT) characteristics due to increasing the solvent polarity. A broad fluorescence band at around 540 nm originating from the AIEE characteristics due to the restricted intramolecular rotation (RIR) of the TPE core associated with the aggregate formation was enhanced and dominated the fluorescence spectra from the relatively low water content of 40 wt%. Indeed, the plots against the water fraction in the THF solutions for TPE-(An-CHO)₄ indicated that the fluorescence intensity gradually and almost linearly increased with the increase in the water content over a wide range from 0 wt% to 90 wt%. In contrast, TPE-(An-CHO) composed of a TPE and an anthraldehyde unit exhibited AIEE characteristics in the high water content region over 80 wt%, although in the low water content region below 20 wt%, TPE-(An-CHO) showed positive solvatofluorochromic properties. The AIEE characteristics of TPE-(An-CHO)₄ observed in the low water content region are attributed to the fact that the four anthraldehyde units lowered the solubility in organic solvents and water. This work reveals the solvatofluorochromic properties and the AIEE characteristics of donor–acceptor-type TPE–anthracene-based fluorescence sensors for water.