Crown ether model systems for the study of photoexcited-state response to oriented perturbers. How does a naphthalene derivative respond to an alkali metal cation in its .pi. face?
“…Compound 6 switches “on” its fluorescence upon rigidification via macrocyclization with disaccharide guests. The situation was less clear-cut in pathfinding cases such as 7 − 9 when rigidification and conformational effects were invoked to explain the small fluorescence changes observed upon ion binding near ππ* fluorophores. , The clearest effect in these experiments was due to “heavy” atom-induced spin−orbit coupling which can even happen in a remote fashion. − …”
Section: ππ* Excited Statesmentioning
confidence: 97%
“…We begin this discussion of the small handful of examples with some cryogenic experiments. Sousa and Larson's pioneering investigations into macrocyclic systems 7 − 9 with cation-responsive emission included phosphorescence measurements on 77 K glasses. , The changes observed can be largely rationalized by “heavy” atom induced spin−orbit coupling. Shirai and Tanaka's study focused on the aryl ketone phosphor within 105 …”
“…Compound 6 switches “on” its fluorescence upon rigidification via macrocyclization with disaccharide guests. The situation was less clear-cut in pathfinding cases such as 7 − 9 when rigidification and conformational effects were invoked to explain the small fluorescence changes observed upon ion binding near ππ* fluorophores. , The clearest effect in these experiments was due to “heavy” atom-induced spin−orbit coupling which can even happen in a remote fashion. − …”
Section: ππ* Excited Statesmentioning
confidence: 97%
“…We begin this discussion of the small handful of examples with some cryogenic experiments. Sousa and Larson's pioneering investigations into macrocyclic systems 7 − 9 with cation-responsive emission included phosphorescence measurements on 77 K glasses. , The changes observed can be largely rationalized by “heavy” atom induced spin−orbit coupling. Shirai and Tanaka's study focused on the aryl ketone phosphor within 105 …”
“…Chemosensors are molecules capable of binding a substrate and at the same time signaling its presence. One attractive method of signaling is the use of the fluorescence emission technique, because of its great sensitivity. − In order to obtain a quantification of a fluorescence emission signal upon the binding of a receptor it is necessary to observe one of these two cases: chelation enhancement of the fluorescence (CHEF) or chelation enhancement of quenching (CHEQ) effects. The first generation of chemosensors were constituted by aromatic heterocycles, for example those containing nitrogen or oxygen, in which both functions (binding and signaling) were attributed to the same part of the molecule, and for this reason they are called intrinsic chemosensors (Scheme ).…”
Ligand 2,5,8-triaza[9]-10,23-phenanthrolinophane (L) contains a triamine chain connecting the 2,9 positions of
a phenanthroline unit. Protonation of L has been studied by means of potentiometric and 1H and 13C NMR
techniques, allowing the determination of the basicity constants and of the stepwise protonation sites. Protonation
strongly affects the fluorescence emission properties of the chemosensor L. The two benzylic amine groups,
namely, the two aliphatic amine groups adjacent to phenanthroline, are the most efficient nitrogens in fluorescence
emission quenching. In the diprotonated receptor [H2
L]2+ both of these nitrogens are protonated, and therefore
this species is the most emissive. In the [H3
L]3+ species the three acidic protons are located on the amine groups
of the polyamine chain. This species is still emissive, but less so than [H2
L]2+, due to formation of a hydrogen
bond network involving the phenanthroline nitrogens, as shown by the crystal structure of the [H3
L]Br3·H2O salt.
A potentiometric investigation of Zn(II) binding in aqueous solution suggests that some nitrogen donors are not
involved, or weakly involved in metal coordination. Actually, the crystal structure of the [ZnL(H2O)](ClO4)2
complex shows that both of the benzylic amine groups are weakly bound to the metal. This Zn(II) complex does
not show any fluorescence emission. This rather unusual feature can be explained considering an electron transfer
process involving the benzylic nitrogens.
“…This quenching effect was attributed to the photoinduced electron transfer (PET) that can occur between the lone pair of the deprotonated amines in the polyamine chain, and the excited benzene moiety. Therefore, those receptors incorporating a polyamine chain and an aromatic subunit can be used as fluorescent chemosensors because their coordination to substrates, like protons, modifies the fluorescence emission of the benzene unit, signaling the receptor−substrate interaction. − Metal cations are another kind of substrate that can change the fluorescence pattern of the receptors. Coordination of these substrates can either enhance the fluorescence emission intensity (CHEF, chelation-enhanced fluorescence) or, alternatively, produce the opposite effect (CHEQ, chelation-enhanced quenching), whenever the substrate chelation leads to a decrease of the fluorescence emission intensity of the chemosensor…”
The thermodynamic properties of the Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+), and Pb(2+) complexes of a family of N,N'-dibenzylated open-chain polyamines are described. For comparison, similar studies are reported for polyazacyclophane macrocyclic receptors containing an aromatic subunit linking the ends of a polyamine bridge. The metal complexes of the dibenzylated ligands show lower stability constants than those reported for related nonbenzylated open-chain polyamines. On the other hand, the stability constants of these complexes are clearly higher than those found for complexes of polyazacyclophane macrocycles containing a single para-substituted benzene spacer interrupting saturated polyamine bridges. All the studied complexes follow the Irving-Williams stability order. The crystal structure of [Cu(L7)(H(2)O)](ClO(4))(2)( )()(L7 = 1-benzyl-1,5,8,12-tetrazadodecane) shows a very strongly axially distorted square planar coordination geometry for Cu(2+). Crystals of [Cu(L7)(H(2)O)](ClO(4))(2)()()(C(15)H(24)Cl(2)CuN(4)O(9)) are orthorhombic, space group P2(1)2(1)2(1), with a = 7.586(1) Å, b = 10.715(3) Å, and c = 28.13(2) Å, Z = 4, R(1) = 0.0572, and wR(2) = 0.1570. Steady-state fluorescence emission studies performed on the Cu(2+) and Zn(2+) complexes show that, while none of the Cu(2+) complexes is emissive (CHEQ effect), fluorescence emission is observed for those Zn(2+) complexes with all the nitrogen donors either protonated or coordinated to the metal ions (CHEF effect). The composition of the frontier molecular orbitals of the free-ligands and of the Cu(2+) and Zn(2+) complexes supports this behavior. The use of these water-soluble ligands as chemosensors by means of enhancement or quenching of the fluorescence emission is also discussed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
