Novel Discrete and Imprinted Fluoride-Selective Sensors: Bridging the Gap from DMSO to Aqueous Samples.

Fluoride in drinking water has beneficial or harmful health effects depending on its concentration. This highlights the need for new low-cost and portable sensors capable of in situ monitoring of F- ions. Unfortunately, achieving high levels of water compatibility and fluoride specificity remains a challenge. Here, four new urea-based discrete sensors are prepared and characterized. The sensors containing anthracenyl- (5) and 9H-fluorenyl- (7) signaling units exhibit intense luminescent emissions in dimethyl sulfoxide, the former being particularly sensitive and selective to fluoride. In water, 5 displays a superior sensitivity (871 M-1) and a detection limit (8 µm) below international guidelines, albeit with cross-sensitivity to H2PO4‾. To enhance the performance, 5 and 7 are embedded into a fluoride-imprinted polymeric matrix to give solid-state sensors (5P and 7P, respectively). 5P shows good sensitivity (360 M-1) and specificity in water. Besides, it has a low detection limit (35 µm) and a response linear range (118-6300 µm) encompassing the limit established by the Environmental Protection Agency (211 µm). Furthermore, 5P also displays good reusability and adequate recovery values in real-sample testing (102 ± 2%), constituting the first example of a low-cost anion-imprinted polymeric probe tailored for the selective sensing of fluoride in aqueous samples.

Pyrene-Containing Polyamines as Fluorescent Receptors for Recognition of PFOA in Aqueous Media.

The globally widespread perfluorooctanoic acid (PFOA) is a concerning environmental contaminant, with a possible toxic long-term effects on the environment and human health The development of sensible, rapid, and low-cost detection systems is a current change in modern environmental chemistry. In this context, two triamine-based chemosensors, L1 and L2, containing a fluorescent pyrene unit, and their Zn(II) complexes are proposed as fluorescent probes for the detection of PFOA in aqueous media. Binding studies carried out by means of fluorescence and NMR titrations highlight that protonated forms of the receptors can interact with the carboxylate group of PFOA, thanks to salt bridge formation with the ammonium groups of the aliphatic chain. This interaction induces a decrease in the fluorescence emission of pyrene at neutral and slightly acidic pH values. Similarly, emission quenching has also been observed upon coordination of PFOA by the Zn(II) complexes of the receptors. These results evidence that simple polyamine-based molecular receptors can be employed for the optical recognition of harmful pollutant molecules, such as PFOA, in aqueous media.

Probing Vibrational Symmetry Effects and Nuclear Spin Economy Principles in Molecular Spin Qubits.

The selection of molecular spin qubits with a long coherence time, Tm, is a central task for implementing molecule-based quantum technologies. Even if a sufficiently long Tm can be achieved through an efficient synthetic strategy and ad hoc experimental measurement procedures, many factors contributing to the loss of coherence still need to be thoroughly investigated and understood. Vibrational properties and nuclear spins of hydrogens are two of them. The former plays a paramount role, but a detailed theoretical investigation aimed at studying their effects on the spin dynamics of molecular complexes such as the benchmark phthalocyanine (Pc) is still missing, whereas the effect of the latter deserves to be examined in detail for such a class of compounds. In this work, we adopted a combined theoretical and experimental approach to investigate the relaxation properties of classical [Cu(Pc)] and a CuII complex based on the ligand tetrakis(thiadiazole)porphyrazine (H2TTDPz), characterized by a hydrogen-free molecular structure. Systematic calculations of molecular vibrations exemplify the effect of normal modes on the spin-lattice relaxation process, unveiling a different contribution to T1 depending on the symmetry of normal modes. Moreover, we observed that an appreciable Tm enhancement could be achieved by removing hydrogens from the ligand.

Exploring the Ability of Luminescent Metal Assemblies to Bind and Sense Anionic or Ionizable Analytes A Ru(phen)2bipy-Based Dizinc Complex for Bisphenol A (BPA) Recognition.

The synthesis of a new RuII complex, in which the metal is coordinated by two 1,10-phenanthroline ligands and a 2,2'-bipyridyl unit linked, via methylene bridges in its 4 and 4' positions, to two 1,4,7,10-tetraazacyclododecane (cyclen) macrocycles ([Ru(phen)2 L]2+) is reported. Protonation and ZnII binding by [Ru(phen)2 L]2+ have been analyzed by potentiometric titration, evidencing the formation of mixed hetero-binuclear and hetero-trinuclear ZnII/RuII complexes. These complexes were tested as bis-phenol A (BPA) binders. Only the dizinc complex with [Ru(phen)2 L]2+ is able to bind BPA in aqueous solution, affording a remarkably stable {Zn2[Ru(phen)2 L]BPA(H-2)}4+ adduct at neutral pH, in which BPA is bound in its doubly deprotonated form to the two ZnII ions. BPA binding was found to quench the luminescence emission of the RuII(phen)2bipy core. Although the quenching effect is modest, this study demonstrates that appropriately designed dizinc complexes can be used for binding and optical sensing of BPA in water.

Highly Charged Ruthenium(II) Polypyridyl Complexes as Effective Photosensitizer in Photodynamic Therapy.

A comparative study between two novel, highly water soluble, ruthenium(II) polypyridyl complexes, [Ru(phen)2 L'] and [Ru(phen)2 Cu(II)L'] (L and L-CuII ), containing the polyaazamacrocyclic unit 4,4'-(2,5,8,11,14-pentaaza[15])-2,2'-bipyridilophane (L'), is herein reported. L and L-CuII interact with calf-thymus DNA and efficiently cleave DNA plasmid when light-activated. They also possess great penetration abilities and photo-induced biological activities, evaluated on an A375 human melanoma cell line, with L-CuII being the most effective. Our study highlights the key role of the Fenton active CuII center within the macrocycle framework, that would play a synergistic role with light activation in the formation of cytotoxic ROS species. Based on these results, an optimal design of RuII polypyridyl systems featuring specific CuII -chelating polyamine units could represent a suitable strategy for the development of novel and effective photosensitizers in photodynamic therapy.

Di- and Triphosphate Recognition and Sensing with Mono- and Dinuclear Fluorescent Zinc(II) Complexes: Clues for the Design of Selective Chemosensors for Anions in Aqueous Media.

The synthesis of a new ligand (L1) containing two 1,4,7-triazacyclononane ([9]aneN3 ) moieties linked by a 4,5-dimethylenacridine unit is reported. The binding and fluorescence sensing properties toward Cu2+ , Zn2+ , Cd2+ , and Pb2+ of L1 and receptor L2, composed of two [9]aneN3 macrocycles bridged by a 6,6''-dimethylen-2,2':6',2''-terpyridine unit, have been studied by coupling potentiometric, UV/Vis absorption, and emission measurements in aqueous media. Both receptors can selectively detect Zn2+ thanks to fluorescence emission enhancement upon metal binding. The analysis of the binding and sensing properties of the Zn2+ complexes toward inorganic anions revealed that the dinuclear Zn2+ complex of L1 selectively binds and senses the triphosphate anion (TP), whereas the mononuclear Zn2+ complex of L2 displays selective recognition of diphosphate (DP). Binding of TP or DP induces emission quenching of the Zn2+ complexes with L1 and L2, respectively. These results are exploited to discuss the role played by pH, number of coordinated metal cations, and binding ability of the bridging units in metal and/or anion coordination and sensing.

Catching anions with coloured assemblies: binding of pH indicators by a giant-size polyammonium macrocycle for anion naked-eye recognition.

A giant-size polyamine macrocycle L, composed of four 1,4,8,11-tetraazacyclotetradecane (cyclam) units linked by 1,3-dimethylenbenzyl spacers, strongly interacts in aqueous solution with four pH indicators (bromocresol purple (H2BCP), phenol red (H2PR), phenolphthalein (H2PP) and fluorescein (H2F)) in their anionic forms. Besides 1 : 1 complexes, L also forms assemblies with an unusual 1 : 2 receptor to dye stoichiometry, thanks to its large dimensions, which allow for the simultaneous interaction of the receptor protonated forms with two anionic dyes. The formation of the assemblies markedly affects the pKa values of the phenol groups of the dyes, which change colour upon complexation in well-defined pH ranges. This property can be effectively exploited for optical detection of anions. The L-H2BCP 1 : 2 assembly is able to selectively detect the triphosphate anion at slightly acidic pH values, thanks to the release, upon triphosphate coordination, of the dye from the ensemble, with a consequent colour change of the solution from purple-violet (complexed BCP(2-) dye) to yellow (free BCP(2-)). No effect is caused by other inorganic anions. The L-H2BCP 1 : 2 assembly represents a rare case of an optical chemosensor for the triphosphate anion.

Phosphate binding by a novel Zn(II) complex featuring a trans-1,2-diaminocyclohexane ligand. Effective anion recognition in water.

In this work we have investigated the binding properties of a new synthetic receptor for phosphate anions that combines metal ion coordination with electrostatic and H-bonding interactions. The described receptor is obtained by assembling an iminodiacetic (IDA) fragment, as a Zn(II) binding site, with a polyamine macrocyclic portion containing two trans-1,2-diaminocyclohexane (DAC) units and a pyrrole ring, as a cationic binding site, into an adaptive structure appropriately spanning the length of di- and tridentate phosphates. Potentiometric measurements together with (1)H and (31)P NMR investigation showed that, in a wide pH range including values of physiological interest, the Zn(II) complex of the receptor binds di- and triphosphates, such as ADP, ATP, pyrophosphate (PP) and triphosphate (TP), far better than monophosphate (MP), and that TP is poorly bound by methyliminodiacetate (MIDA) as a model for the Zn(II) binding site. Besides the excellent selectivity over other phosphates, the affinity for TP is the largest reported to date for Zn(II) complexes in water.

A fluorescent receptor for halide recognition: clues for the design of anion chemosensors.

The chemosensing properties of the polyaza-macrocycle 1-(6,7)-acridine-3,6,9,12-tetraaza-tridecaphane have been investigated by means of emission fluorescence spectroscopy, considering halide ions as substrates. As in the case of the free ligand, the fluorescence emission of the complexes is due to the acridinium species which are formed after photoinduced proton transfer reaction. The complexation constants have been obtained for the bi- and tri-protonated ligands in deoxygenated aqueous solutions. Two different emission behaviours have been observed varying the anion. Fluoride and chloride give rise to fluorescence enhancement whereas bromide and iodide strongly quench the emission. The macrocycle shows an unusual higher selectivity towards the chloride anion rather than fluoride. The fluorescence emission has been modelled considering a modified Stern-Volmer equation, taking into account the quenching effects of the largest anions, which can be considered negligible for fluoride and chloride anions. Ab initio calculations allow us to interpret the fluorescence emission of the complexes in terms of activation energy related to the proton transfer reaction responsible for the emission process.

An OFF-ON chemosensor for biological and environmental applications: sensing Cd2+ in water using catanionic vesicles and in living cells.

A new OFF-ON fluorescent chemosensor (L(1)) for Cd(2+) recognition based on a 5-chloro-8-hydroxyquinoline pendant arm derivative of 1,4,7-triazacyclononane ([9]aneN3) will be presented and its photochemical features in an MeCN-H2O 1 : 1 (v/v) mixture, in pure water, after inclusion within catanionic vesicles, and in living cells will be discussed. The coordination properties of L(1) both in solution and in the solid state were preliminarily studied and its selectivity towards Cd(2+)versus a set of different metal ions (Cu(2+), Zn(2+), Cd(2+), Pb(2+), Al(3+), Hg(2+), Co(2+), Ni(2+), Mn(2+), Mg(2+), K(+), Ca(2+), Ag(+), and Na(+)) was verified in MeCN-H2O 1 : 1 (v/v). In water, upon addition of increasing amounts of Cd(2+) to L(1) an enhancement of the fluorescence emission was detected. To overcome this serious drawback, L(1) was dissolved in an innovative catanionic vesicular solution based on sodium bis(2-ethylhexyl) sulfosuccinate, a traditional surfactant, and 1-dodecyl-3-methylimidazolium bromide, an ionic liquid. When enclosed within the vesicle bilayers in water, L(1) restored its fluorescence emission property upon addition of Cd(2+). Remarkably, L(1) enters the cellular membrane of living cells thus allowing the detection of intracellular Cd(2+). These findings encourage the application of this new fluorescent chemosensor in real samples for histological and environmental analyses.

Tuning the emission properties of fluorescent ligands by changing pH: the unusual case of an acridine-containing polyamine macrocycle.

Synthesis and characterization of a new macrocyclic compound, composed by a triethylentetraamine chain linking the 4 and 5 positions of an acridine moiety, are reported. The molecule, devised as a fluorescent chemosensor for anions, has revealed an intriguing pH-dependent spectroscopic behavior, whose features are the specific object of this article. Ligand protonation in aqueous solution has been analyzed by means of potentiometric, (1)H NMR, UV-vis, and fluorescence emission measurements. The molecule binds up to four protons in the pH range 2-11. Protonation takes place on the aliphatic tetraamine chain, while the acridine nitrogen does not participate to proton binding even at very low pH. Differently from acridine, the UV-vis spectra are almost unaffected by the pH. On the opposite, the emission spectra are strongly pH-dependent. In fact, at low pH values, the spectra show a blue-shifted emission, resembling that of unprotonated acridine, while at slightly acidic and alkaline pH the fluorescence features a red-shifted band similar to that of acridinium cation. This unusual behavior occurs in the mono-, bi-, and triprotonated forms of the compound and is interpreted as due to an excited state proton transfer from an aliphatic ammonium group adjacent to the acridine moiety to the acridine nitrogen. In the fully protonated state, this process is prevented owing to unfavorable molecular arrangements mainly determined by electrostatic repulsions. This interpretation is supported by quantum mechanical calculations as well as molecular dynamics simulations.

Therapeutic effects of the superoxide dismutase mimetic compound MnIIMe2DO2A on experimental articular pain in rats.

Superoxide anion (O(2) (•-)) is overproduced in joint inflammation, rheumatoid arthritis, and osteoarthritis. Increased O(2) (•-) production leads to tissue damage, articular degeneration, and pain. In these conditions, the physiological defense against O(2) (•-), superoxide dismutases (SOD) are decreased. The Mn(II) complex MnL4 is a potent SOD mimetic, and in this study it was tested in inflammatory and osteoarticular rat pain models. In vivo protocols were approved by the animal Ethical Committee of the University of Florence. Pain was measured by paw pressure and hind limb weight bearing alterations tests. MnL4 (15 mg kg(-1)) acutely administered, significantly reduced pain induced by carrageenan, complete Freund's adjuvant (CFA), and sodium monoiodoacetate (MIA). In CFA and MIA protocols, it ameliorated the alteration of postural equilibrium. When administered by osmotic pump in the MIA osteoarthritis, MnL4 reduced pain, articular derangement, plasma TNF alpha levels, and protein carbonylation. The scaffold ring was ineffective. MnL4 (10(-7) M) prevented the lipid peroxidation of isolated human chondrocytes when O(2) (•-) was produced by RAW 264.7. MnL4 behaves as a potent pain reliever in acute inflammatory and chronic articular pain, being its efficacy related to antioxidant property. Therefore MnL4 appears as a novel protective compound potentially suitable for the treatment of joint diseases.

Inorganic anion recognition in aqueous solution by coupling nearby highly hydrophilic and hydrophobic moieties in a macrocyclic receptor.

Receptor L, composed of a tripropylenetetramine chain linking the 2 and 7 positions of an acridine unit via methylene bridges, behaves as a pentaprotic base in aqueous solution. The first four protonation steps occur on the tetra-amine chain, while the acridine nitrogen protonates only below pH 4. The penta-protonated receptor assumes a folded conformation, resulting in a cleft delimited by the aliphatic tetramine and acridine moieties, in which anions of appropriate size can be hosted. Potentiometric titrations reveal that F- forms the most stable complexes, although the stability constants of the Cl- and Br- adducts are unusually only slightly lower than those observed for F- complexes. A remarkable drop in stability is observed in the case of I- adducts. Oxo-anions, including H2PO4-, NO3- and SO42-, are not bound or weakly bound by the protonated receptor, despite the known ability of charged oxygens to form stable O-⋯HN+ salt bridges. This unexpected stability pattern is explained in the light of the X-ray crystal structures of H5LCl5·4H2O, H5LBr5·4H2O, H5L(NO3)5·3H2O and H5L(H2PO4)5·(H3PO4)2·4H2O complexes, coupled with MD simulations performed in the presence of explicit water molecules, which reveal that Cl- and, overall, Br- possess the optimal size to fit the receptor cleft, simultaneously forming strong salt bridging interactions with the ammonium groups and anion⋯π contacts with protonated acridine. I- and oxo-anions are too large to conveniently fit the cavity and are only partially enclosed in the receptor pocket, remaining exposed to solvent, with a lower entropic stabilization of their complexes. Although F- could be enclosed in the cavity, its smaller size favours the F-⋯HN+ salt bridging interaction from outside the receptor pocket. The fluorescence emission of the acridinium unit is quenched by anion binding. The quenching ability parallels the stability of the complexes and is related to the relevance of the anion⋯π contacts in the overall host-guest interaction.

Evaluation of coumarin-tagged deferoxamine as a Zr(IV)-based PET/fluorescence dual imaging probe.

Desferoxamine (DFO) is currently the golden standard chelator for 89Zr4+, a promising nuclide for positron emission tomography imaging (PET). The natural siderophore DFO had previously been conjugated with fluorophores to obtain Fe(III) sensing molecules. In this study, a fluorescent coumarin derivative of DFO (DFOC) has been prepared and characterized (potentiometry, UV-Vis spectroscopy) for what concerns its protonation and metal coordination properties towards PET-relevant ions (Cu(II), Zr(IV)), evidencing strong similarity with pristine DFO. Retention of DFOC fluorescence emission upon metal binding has been checked (fluorescence spectrophotometry), as it would - and does - allow for optical (fluorescent) imaging, thus unlocking bimodal (PET/fluorescence) imaging for 89Zr(IV) tracers. Crystal violet and MTT assays on NIH-3 T3 fibroblasts and MDA-MB 231 mammary adenocarcinoma cell lines demonstrated, respectively, no cytotoxicity nor metabolic impairment at usual radiodiagnostic concentrations of ZrDFOC. Clonogenic colony-forming assay performed on X-irradiated MDA-MB 231 cells showed no interference of ZrDFOC with radiosensitivity. Morphological biodistribution (confocal fluorescence, transmission electron microscopy) assays on the same cells suggested internalization of the complex through endocytosis. Overall, these results support fluorophore-tagged DFO as a suitable option to achieve dual imaging (PET/fluorescence) probes based on 89Zr.

Polyamine receptors containing anthracene as fluorescent probes for ketoprofen in H2O/EtOH solution.

Triamine receptors containing anthracene units are able to bind and sense ketoprofen via fluorescence enhancement in a H2O/EtOH 50 : 50 (Vol : Vol) mixture exploiting their protonation features, which are tuned by the interaction with the analyte.

Glyphosate sensing in aqueous solutions by fluorescent zinc(II) complexes of [9]aneN3-based receptors.

Herein we describe the binding abilities of Zn(II) complexes of [12]aneN4- (L1) and [9]aneN3-based receptors (L2, L3) towards the herbicides N-(phosphonomethyl)glycine (glyphosate, H3PMG) and 2-amino-4-[hydroxy(methyl)phosphoryl]butanoic acid (glufosinate, H2GLU), and also aminomethylphosphonic acid (H2AMPA), the main metabolite of H3PMG, and phosphate. All ligands form stable Zn(II) complexes, whose coordination geometries allow a possible interaction of the metal center with exogenous anionic substrates. Potentiometric studies evidenced the marked coordination ability of the L2/Zn(II) system for the analytes considered, with a preferential binding affinity for H3PMG over the other substrates, in a wide range of pH values. 1H and 31P NMR experiments supported the effective coordination of such substrates by the Zn(II) complex of L2, while fluorescence titrations and a test strip experiment were performed to evaluate whether the H3PMG recognition processes could be detected by fluorescence signaling.

Exploring the binding ability of polyammonium hosts for anionic substrates: selective size-dependent recognition of different phosphate anions by bis-macrocyclic receptors.

Binding of mono-, di-, and triphosphate, adenosine diphosphate (ADP), and adenosine triphosphatase (ATP) with receptors L1-L3, composed of two [9]aneN(3) units separated by a 2,9-dimethylene-1,10-phenanthroline (L1), a 2,6-dimethylenepyridine (L2), or a 2,3-dimethylenequinoxaline (L3) spacer, has been studied by means of potentiometric titrations, (1)H and (31)P NMR measurements in aqueous solutions, and molecular modeling calculations. In the case of inorganic phosphates, the binding properties of the receptors appear to be determined by their geometrical features, in particular the distance between the two [9]aneN(3) units imposed by the spacer separating the two macrocyclic units. While L1 is able to selectively bind triphosphate over di- and monophosphate, L3 selectively coordinates the smaller monophosphate anion. Finally, L2 shows preferential binding of diphosphate. (1)H and (31)P NMR measurements show that the complexes are essentially stabilized by charge-charge and hydrogen-bonding interactions between the anion and the protonated amine groups of the macrocyclic subunits of the receptors. Molecular dynamics simulations suggest that the larger distance between the two macrocyclic units of L1 allows this receptor to form a larger number of hydrogen-bonding contacts with triphosphate, justifying its selectivity toward this anion. Conversely, in the case of L3, the two facing [9]aneN3 units give rise to a cleft of appropriate dimensions where the small monophosphate anion can be conveniently hosted. Considering nucleotide coordination, L1 is a better receptor for ATP and ADP than L2, thanks to the higher ability of phenanthroline to establish stabilizing π stacking and hydrophobic interactions with the adenine units of the guests.

Tailoring cyclic polyamines for inorganic/organic phosphate binding.

In this tutorial review, we will discuss the most significant achievements in the structure-based design of cyclic receptors containing mainly aliphatic amine groups as metal-free hosts for phosphate-based guests in aqueous environment; in particular, inorganic phosphates and mononucleotide anions. The bulk of our attention will be devoted to show how this class of receptors has structurally evolved following the attempts by chemists to optimize all factors indicated by Nature as necessary to achieve the best possible binding of phosphates via non-covalent interactions.

Switching on the Fluorescence Emission of Polypyridine Ligands by Simultaneous Zinc(II) Binding and Protonation.

The synthesis and characterization of the two new open-chain ligands 1,15-bis-[6-(2,2'-bipyridyl)]-2,5,8,11,14-pentaaza-octadecane (L1) and 1,15-bis-[2-(1,10-phenanthroline)-9-methyl]-2,5,8,11,14-pentaazaoctadecane (L2), both featuring a tetraethylenpentaamine chain linking via methylene bridges the 6 and 2 positions of two identical 2,2'-bipyridyl (bpy) and 9-methyl-1,10-phenanthroline (9-methyl-phen) moieties respectively, are reported. Their protonation and binding ability for Cu2+ , Zn2+ , Cd2+ and Pb2+ have been studied by coupling potentiometric titrations with UV-vis absorption and fluorescence emission measurements in water. L1 and L2 afford stable mono- and dinuclear complexes, in which the metal ion is bound by a single bpy or 9-methyl-phen unit and the amine groups on the aliphatic chain. However, L1 displays a greater binding ability for Cu2+ and Zn2+ with respect to L2, the stability constants of the [ML1]2+ complexes being 21.8 (Cu2+ ) and 19.4 (Zn2+ ) log units vs 20.34 and 16.8 log. units for the corresponding L2 species. Among all the metal ions tested, only the Zn2+ complex with L2 features an enhanced fluorescence emission at neutral pH, thanks to the simultaneous binding of one Zn2+ ion and H+ ion(s), that inhibits any possible photoinduced electron transfer (PET) process from the amine donors to the excited phen moiety. Binding of a second metal switches off the emission again.

Aza- and Mixed Thia/Aza-Macrocyclic Receptors with Quinoline-Bearing Pendant Arms for Optical Discrimination of Zinc(II) or Cadmium(II) Ions.

The synthesis and coordination properties of two fluorescent chemosensors, featuring [9]aneN3 (1,4,7-triazacyclononane; L1) and [12]aneNS3 (1-aza-4,7,10-trithiacyclododecane; L2) as receptor units, and a quinoline pendant arm with an amide group as a functional group spacer are described. The optical responses of L1 and L2 in the presence of several metal ions were analysed in MeCN/H2 O (1 : 4 v/v) solutions. A selective chelation enhancement of fluorescence (CHEF) effect was observed in the presence of Zn2+ in the case of L1, and in the presence of Cd2+ in the case of L2, following the formation of a 1 : 1 and a 1 : 2 metal/ligand complex, respectively, which was also confirmed by potentiometric measurements. 1 H and 13 C NMR measurements in CD3 CN/CDCl3 in combination with molecular mechanics calculations show that for both complexes of L1 and L2 with Zn2+ and Cd2+ , respectively, the coordination of the carbonyl group from the pendant arm could be the origin of the observed optical selectivity.