Organic Fluorescent Thermometers Based on Borylated Arylisoquinoline Dyes

Borylated arylisoquinolines with redshifted internal charge-transfer (ICT) emission were prepared and characterized. Upon heating, significant fluorescence quenching was observed, which forms the basis for a molecular thermometer. In the investigated temperature range (283-323 K) an average sensitivity of -1.2 to -1.8% K-1 was found for the variations in fluorescence quantum yield and lifetime. In the physiological temperature window (298-318 K) the average sensitivity even reaches values of up to -2.4% K-1. The thermometer function is interpreted as the interplay between excited ICT states of different geometry. In addition, the formation of an intramolecular Lewis pair can be followed by B-11 NMR spectroscopy. This provides a handle to monitor temperature-dependent ground-state geometry changes of the dyes. The role of steric hindrance is addressed by the inclusion of a derivative that lacks the Lewis pair formation.