The Development Of Environmentally Sensitive Fluorescent Viscosity Sensors Based On The Principle Of Photoinduced Electron Transfer (PET)

Microviscosity has profound effects on the functions of biological systems. Many diseases have been linked to the changes in the microviscosity of biological systems. Benefited from their high spacial and temporal resolutions, environmentally sensitive fluorescent viscosity sensors have become one of the most important methods in the determination of the microviscosity of biological systems.Most of the reported fluorescent viscosity sensors are developed based on the mechanism of internal charge transfer (ICT) and are surfered from a few drawbacks, such as relatively low sensitivity, etc. Photoinduced electron transfer (PET) is one of the most employed principles in the design of fluorescence sensors. PET fluorescence sensors usually have high sensitivity. This research is aimed at building a rational strategy for the development of environmentally sensitive fluorescent viscosity sensors based on the principle of photoinduced electron transfer (PET).The mechanism of the intramolecular PET process between4-aminonaphthalimides and anilines was investigated by examining the absorption and fluorescence spectra of this system in solvents with different polarity and viscosity. It was found that the intramolecular PET process of this system is through two different pathways and that intramolecular hydrogen bond and conformational motions of the linkage play important roles. Two different mechanisms of the intramolecular PET process were proposed:1, Proton-coupled electron transfer through the formation of intramolecular hydrogen bonds;2, Intramolecular PET through the formation of folded exciplexies.Based on this understanding, a new strategy for the development of fluorescent viscosity sensors was developed. The effects of the length of the substituent and its hydrophobic and hydrophilic properties, the length of the linkage on the sensitivity and the suitable working ranges of the PET fluorescent viscosity sensors were carefully examined. A clear relationship between the molecular structure and viscosity sensitivities were established.By this strategy, tewelve fluorescent viscosity sensors (B1-B8. C2-C3, D2-D3) were designed and synthesized. Among these three series.B3-B8show higher sensitivity than most of the reported ICT fluorescent viscosity sensors. For example, the fluorescence intensity of B6increased more than40folds when solvent viscosity changes from5-15mPa.s with a sensitivity reaches at4.31. This sensitivity is more than6folds sensitive than ICT fluorescent sensors.Six PET polarity sensors (A1-A3, C1-C3) were also designed and synthesized. An ICT fluorescence sensor (B0) was designed and synthesized, which shows both polarity and viscosity sensitive fluorescence properties.