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

Abstract

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.

Photoinduced electron transfer inN,N-bis(pyridylmethyl)naphthalenediimides: study of their potential as pH chemosensors

“…The compound 1B shows the best result and potential as a pH chemosensor because it has a linear pH range between 1.7 and 4.1 and a FE H + of 107 compared with 1C and 1D which present a linear range of 1.3 to 3.3 and a FE H + of 86 and 47, respectively. However, the FE H + for 1B-1D are considerably superior compared with other pH sensors based on napthalimide or NDI fluorophores reported in literature (24,36,51). The linear response interval presented by these BIPy-NDIs is associated to the pK a2 value usually, the electron deficient protonated pyridine causes a PET process acting as an acceptor from an excited state of a fluorophore, and an ON-OFF fluorescent response is observed (53).…”

“…Such low pH-sensitive sensor may be useful for determining the pH in strongly acidic media as in human stomach (pH = 1.0 to 3.0) or wastewaters derived from specific processes (pH = 0.8) where the use of pH glass electrode is not appropriated (34,35). Previously, has been reported the effect of pH over the fluorescence intensity of NDIs bearing alkyl amino and morphine groups as substituents in the imide (36). There is an important enhancement of NDI fluorescence as the pH decreases from 8.9 to 4.6, due to the suppression of PET occurring from the alkyl amino to the NDI core.…”

Building pH Sensors into Paper-Based Small-Molecular Logic Systems for Very Simple Detection of Edges of Objects

“…43 We have generalized the principle of fluorescent PET sensing previously. 44,45 Sensor 4 38 behaves similarly whereas sensor 5 is a straightforward PET sensor (see supporting information, section 1). The protons required for such switching 'on' of 1 are provided in the present case by 254 nm photolysis of 2 which is an established photo acid generator, with 2-(phenylthio)biphenyl (3) as another major photoproduct.…”

Information gathering and processing with fluorescent molecules

“…However, the lesson concerning 4 suggests that some caution is in order. Either detailed structural variation studies 40 or direct laser photolysis experiments 42 could form part of this cautionary approach. …”

“…+ -driven YES logic gate from our laboratory, 40 where the fluorescence switching 'on' is not due to a simple H + -induced arrest of PET. Rather the cause is an electrostatic destabilization of a non-emissive twisted internal charge transfer (ICT) excited state.…”