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

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-spacer-receptor' 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 non-emissive 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-3 H -benzimidazo[2,1- a ]benz[ d , e ]isoquinolin-3-one fluorophore. Naphthalenic derivatives display a range of H + -induced fluorescence switching actions.