Fluorescent sensors for the detection of free-base illicit drugs – Effect of tuning the electronic properties

Fluorescence-based detection of chemical vapours is of interest for low power, portable devices that can be used in the field. Detection using fluorescence quenching via photoinduced electron transfer has been widely explored for the detection of chemicals that have high electron affinities. However, the method cannot be used for compounds that have low electron affinities such as illicit drugs. In this manuscript we describe a family of four compounds that are designed to have large ionisation potentials and are capable of detecting free-base illicit drugs via a photoinduced hole transfer process. The materials are composed of a 2-(9,9-di-n-propylfluoren-2-yl)benzothiadiazole (Fl-BT) base unit, with its properties tuned through addition of different donor (9,9-di-n-propylfluorene) (Fl-BT-Fl) and acceptor [aldehyde (Fl-BT-CHO) and dicyanovinyl (Fl-BT-DCV)] units. The compounds could be solution processed to form good quality, neat thin films with photoluminescence quantum yields of between 35 % and 87 %. The films of the compounds containing the aldehyde and dicyanovinyl electron withdrawing groups were able to detect the vapours of (+)-methamphetamine, (±)-3,4-methylenedioxyamphetamine, cocaine, and fentanyl, whereas the parent Fl-BT could not detect cocaine nor (±)-3,4-methylenedioxyamphetamine. In contrast, Fl-BT-Fl could only detect fentanyl. Combining the response of all four sensing materials provides a method for discriminating both fentanyl and (+)-methamphetamine. Finally, we found that while the ionisation potential was a key parameter for detecting free-base drugs, factors such as the physical interaction of the analytes with the sensing films were also important.