Naphthalenic compounds are a rich resource for designers of fluorescent sensing/switching/logic systems. The degree of internal charge transfer (ICT) character in the fluorophore excited states can vary from negligible to substantial. Naphthalene-1,8;4,5-diimides (11–13), 1,8-naphthalimides (16) and 4-chloro-1,8-naphthalimides (15) are of the former type. The latter type is represented by the 4-alkylamino-1,8-naphthalimides (1). Whether ICT-based or not, these serve as the fluorophore in ‘fluorophore-spacerreceptor’switching systems where PET holds sway until the receptor is bound to H+. On the other hand, 4-dialkylamino-1,8-naphthalimides (3–4) show modest H+-induced fluorescence switching unless the 4-dialkylamino group is a part of a small ring (5). Electrostatic destabilization of a non-emissive twisted internal charge transfer (ICT) excited state is the origin of this behaviour. An evolution to the nonemissive twisted ICT excited state is responsible for the weak emission of the model compound 6 (and related structures 7 and 8) across the pH range. Twisted ICT excited states are also implicated in the switch 9 and its model compound 10, which are based on the 6-dialkylamino-3H-benzimidazo[2,1-a]-benz[d,e]isoquinolin-3-one fluorophore.

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https://pureadmin.qub.ac.uk/ws/files/47024037/structural_effects.pdf

Fluorescent PET (Photoinduced Electron Transfer) Sensors

Structural effects on the pH-dependent fluorescence of naphthalenic derivatives and consequences for sensing/switching

Sensor systems have long been needed for detecting the presence in solution of certain chemically or biologically important species. Sensors are used in a wide range of applications from simple litmus paper that shows a single color change in acidic or basic environments to complex biological assays that use enzymes, antibodies and antigens to display binding events. With this work the use of boronic acids in the design and synthesis of sensors for saccharides (diols) will be presented. The fluorescent sensory systems rely on photoinduced electron transfer (PET) to modulate the observed fluorescence. When saccharides form cyclic boronate esters with boronic acids, the Lewis acidity of the boronic acid is enhanced and therefore the Lewis acid-base interaction between the boronic acid and a neighboring amine is strengthened. The strength of this acid-base interaction modulates the PET from the amine (acting as a quencher) to anthracene (acting as a fluorophore). These compounds show increased fluorescence at neutral pH through suppression of the PET from nitrogen to anthracene on saccharide binding. The general strategy for the development of saccharide selective systems will be discussed. The potential of the boronic acid based systems will be illustrated using the development of glucose and glucosamine selective fluorescent sensors as examples.

Fluorescent sensors based on boronic acids

Chemistry indexing and abstracting service

Fluorescent PET (photoinduced electron transfer) sensors are considered to be those molecular systems where the binding of ions and other species leads to the perturbation of the competition between the de-excitation pathways of fluorescence and electron transfer. The early developments in this field are traced and the design logic of these sensors is detailed. A variety of examples drawn from different areas of chemistry are classified according to the ‘fluorophore-spacer-receptor’ format and their photophysical behaviour is rationalized in terms of fluorescent PET sensor principles. Cases are pointed out where such experimental data are unavailable but desirable. During these discussions, the relevance of twisted fluorophore-receptor systems and the contrast with integrated fluorophore-receptor systems is noted. The utility of the fluorescence ‘on-off’ phenomenon in these PET sensors for the area of molecular photoionic devices is pointed out.

Fluorescent PET (Photoinduced Electron Transfer) Sensors

Citations

Structural effects on the pH-dependent fluorescence of naphthalenic derivatives and consequences for sensing/switching

Fluorescent sensors based on boronic acids

References

Fluorescent PET (photoinduced electron transfer) sensors

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