Aminomethyl-anthracene derivatives (1) and (2) form a series of fluorescent pH indicators with spectral properties independent of substituents and pH except for the fluorescent quantum yield, which shows that their PKa" values are identical to the corresponding PKa values.Photo-induced electron transfer has become established as an important principle in photochemistry.1 Now we show that this principle can be profitably applied to solve several problems concerning fluorescent pH indicators. The majority of these are phenols, anilines, or heterocyclic derivatives where the acid-base site is an integral part of the chromophore.2The results in Table 1 show that, with the appropriate statistical corrections, the relation pKa* = pKa is obeyed for (1) and (2). 7 We also find that all the cases possess virtually

N-(aminoalkyl)-4-chloronaphthalene-1,8-dicarboximides 1, N-(aminoalkyl)-4-acetamidonaphthalene-1,8-dicarboximides 3 and N,N'-bis(aminoalkyl)-perylene-3,4:9,10-tetracarboxydiimides 4 show good fluorescent off ± on switching in aqueous alcoholic solution with protons as required for fluorescent PET sensor design. The excitation wavelengths lie in the ultraviolet (l max 345 and 351 nm) for 1 and 3 and in the blue-green (l max 528, 492 and 461 nm) for 4; the emission wavelengths lie in the violet (l max 408 nm) for 1, in the blue (l max 474 nm) for 3 and in the yellow-orange (l max 543 and 583 nm) for 4. Compound 4 b shows substantial fluorescence enhancement with protons when immobilized in a poly(vinylchloride) matrix, provided that 2-nitrophenyloctyl ether plasticizer and potassium tetrakis(4-chlorophenyl)borate additive are present to prevent dye crystallization and to facilitate proton diffusion into the membrane, respectively.

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