Compound 2, which was designed to act as a fluorescent sensor for calcium according to the PET (Photoinduced Electron Transfer) principle, shows a relatively small Ca2+-induced fluorescence enhancement factor (FE) of 1.8 whereas its close relative 1 is known to display a far higher FE value of 16. Though designed as fluorescent PET sensors for solvent polarity, compounds 5 and 6 also show negligible fluorescence enhancement as their environments are made progressively less polar even though their relatives 3 and 4 show limiting FE values of 53 and 3, respectively. Indeed, 3 and 4 are useful since they are fluorescent sensors for solvent polarity without being affected by Bronsted acidity. The poor sensory performance of 2, 5, and 6 relative to their cousins is attributed to the presence of an oxygen proximal to the 9-position of an anthracene unit, which opens up a CT (charge transfer) channel. Normal PET sensing service is resumed when the offending oxygen is deleted.
Magri, DC., Callan, JF., de Silva, AP., Fox, DB., McClenaghan, ND., & Sandanayake, KRAS. (2005). The anthracen-9-ylmethyloxy unit: An underperforming motif within the fluorescent PET (photoinduced electron transfer) sensing framework. Journal of Fluorescence, 15(5), 769-775. https://doi.org/10.1007/s10895-005-2986-4