Molecular design and synthesis of fluorescence PET (photo-induced electron transfer) sensors for detection of water in organic solvents

A series of anthracene-boronic acid ester OM-1, OM-2, OU-1, and OU-2 have been designed and synthesized as a new class of fluorescence PET (photo-induced electron transfer) sensors for detection and quantification of a trace amount of water in organic solvents. OM-1 and OM-2 have a boronic acid ester and bisboronic acid ester, respectively, located in the proximity of a tertiary amino group as an electron donor via a methylene spacer at the 9-position on the anthracene fluorophore. In OU-1, on the other hand, a boronic acid ester is directly connected to the 10-position of the anthracene fluorophore. OU-2 has bisboronic acid esters, which are located at the same positions as those of OM-1 and OU-1. It was found that OM-1 and OM-2 show enhancement of fluorescence with the increase in water content for various solvents (polar, less polar, protic and aprotic solvents), which can be attributed to the suppression of PET due to the formation of fluorescent ionic structures OM-1a or OM-2a by hydrolysis. For OU-1 and OU-2, on the other hand, there is no appreciable change in fluorescence intensity upon addition of water to the solution. We propose that a key point for creating a highly-sensitive fluorescence PET sensor for detection of a trace amount of water is to design molecular structures capable of forming a stable fluorescent ionic structure between the protonated tertiary amino group and the hydroxylated boronic acid ester by hydrolysis.