Examples of PCT fluorescent probes for polarity

2) Hagan T., Pilloud D. and Suppan P. (1987) Chem. Phys. Lett. 139, 499. 

The assumptions made in theories of solvatochromic shifts, together with the uncertainty over the size and shape of the cavity radius, explain why the determination of excited-state dipole moments is not accurate. Examples of values of excited state dipole moments are given in Table 7.3. 

One of the most well known polarity probes is ANS (l-anilino-8-naphthalene sulfonate), discovered by Weber and Lawrence in 1954. It exhibits the interesting feature of being non-fluorescent in aqueous solutions and highly fluorescent in solvents of low polarity. This feature allows us to visualize only hydrophobic regions of biological systems without interference from non-fluorescent ANS molecules remaining in the surrounding aqueous environment. 

An ideal polarity probe based on photoinduced charge transfer and solvent relaxation should (i) undergo a large change in dipole moment upon excitation but without change in direction, (ii) bear no permanent charge in order to avoid contributions from ionic interactions, (iii) be soluble in solvents of various polarity, from the apolar solvents to the most polar ones. 

Another chemical variant of PRODAN is ACRYLODAN [6-acryloyl-2-(dimethyl-amino)naphthalene], which covalently binds to protein-SH groups. 

I 7 Effect of polarity on fluorescence emission. Polarity probes 

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