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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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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. 8;4,, 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. † Dedicated to Jean-Pierre Desvergne and to Hubert Le Bozec. ‡ This article is published as part of a themed issue in honour of Jean-Pierre Desvergne on the occasion of his 65th birthday.