Polyamines containing naphthyl groups as pH-regulated molecular machines driven by light

Abstract

A series of compounds made up by linking methylnaphthalene fragments at both ends of different polyamine chains have shown to behave as pH-regulated molecular machines driven by light and fluorescence emission studies have proved the formation of an excimer between the two naphthalene units whose appearance, fluorescence intensity and decay times depend on the pH value of the media.Many biological systems can be considered as more or less complex molecular machines operated by chemical or physical stimuli. Examples of this behaviour are found in the triggering effect of many calcium binding proteins or in the astonishing ATP synthase molecular rotor. 1,2 Therefore, in the last few years a lot of research effort has been devoted to identifying systems able to perform molecular motions following chemical or physical inputs. 1-9 Herewith, we communicate on a family of very simple compounds whose molecular movements driven by light can be controlled and even modulated by inputs like the concentration of hydrogen ions and/or metal ions. Compounds L1-L5 have been prepared in good yields by reaction of the elected polyamine with naphthalene-1-carbaldehyde in ethanol followed by reduction with sodium borohydride. 10 While the absorption spectra of these compounds do not change significantly with pH, the fluorescence emission intensity dramatically depends on their protonation state (see Fig. 1A for L1). As described for related compounds, 10 the fully protonated forms of L1-L5 exhibit the most intense fluorescence emission. Unprotonated amines are efficient electron transfer quenchers of the aromatic excited state and depending on the distance to the fluorophore can produce a partial or complete quenching. This trend is illustrated in Fig. 1B, where the fluorescence emission intensity monitored at 334 nm is plotted together with the mole fraction distribution of the different protonated species calculated from the protonation constants determined potentiometrically. 11 In order to have a full picture of the situation, the protonation sequence established for L1 by means of the 1 H and 13 C NMR data has to be taken into account. As shown in Fig. 1B, the first deprotonation that occurs on the central nitrogen atom leads to a partial quenching, ca. 80% of the emission of the fully protonated form. Total quenching takes place only upon removing the second proton from one of the side nitrogens. However, the most remarkable feature in the emission spectra of these compounds is the presence of a red-shifted and non-structured band attributable to excimer formation (Fig. 1A). This red shitted band does not appear in the case of the compound containing a single terminal naphthalene (L5), or in the case of an analogue receptor possessing a reinforcing piperazine ring (L6, see ESI). This absence in L5 excludes the possibility of a charge transfer (CT) state involving the deprotonated amine and the fluorophore. 12 Excimer formation is only observed for the H 2 L1 2+ species. Neither the fully protonated species H 3 L1 3+ nor the sp...

Long Range Electron Transfer Quenching in Polyamine Chains Bearing a Terminal Naphthalene Unit

“…[9][10][11] In particular, we have observed from NMR studies, taken with the compounds here reported and also with similar systems, containing anthracene (or benzene) as the emissive chromophore instead of naphthalene, that the first proton release always takes place with the central nitrogens. [8][9][10][11] On the other hand, the protonation sequence is mainly ruled by the existence of a minimum of repulsion between equally charged species. The obtained exponential dependence for the quenching rate constant, shown in Figure 2, is of general formula, k q ) k q (0) exp(-d), leading to a factor of 0.45 Å -1 , 1-7 suggesting that electron transfer would occur at longer distances than expected for an aliphatic system.…”

“…The most intense emission occurs for the fully protonated form, and the subsequent proton removal (from the chain) leads to a decrease in the emission intensity. [9][10][11] This quenching effect is due to an intramolecular electron-transfer process involving the lone pair of a deprotonated amine.…”

“…[5][6][7] In previous work, carried out in systems containing a polyamine chain bearing a terminal aromatic fluorophore, we have shown that the most intense emission occurs for the fully protonated form and that proton abstraction from the polyaminic chain leads to a decrease in the emission intensity. [8][9][10][11] This quenching effect is due to the intramolecular electron transfer process involving the lone pair of a deprotonated amine and the excited fluorophore. Moreover, it was also observed that the degree of quenching shows a crucial dependence, not only with the chain length but also with the stage of protonation.…”

Cd2+-sensiting bichromophore: Excimer emission from an EDTA-methylnaphthalene derivative

“…1b, the spectrum of ligand 2 exhibited a weak structureless band at 391 nm together with an intense band having a structure at 335 nm at pH 9.6, at which the ligand molecule was completely deprotonated to exist as L 2À . The 335 nm band is attributable to monomeric (or isolated) naphthyl group, and the 391 nm band to naphthyl excimer by comparison with the emission spectra reported for naphthalene derivatives [1][2][3][4][5][6][7][8][13][14][15]22]. The excimer is formed as a result of an intramolecular interaction because an intermolecular interaction is not operative for naphthalene at concentration as low as 10 À6 M [13].…”

A turn-on fluorescent anion receptor based on N,N′-di-β-naphthyl-1,10-phenanthroline-2,9-diamide

“…Coordination of acetate with the amide proton would bring together the naphthalene subunits and induce the appearance of the excimer emission. In other words, the new band at 450 nm could be assigned to the formation of an excimer between the two naphthalene subunits [18].…”