How To Reach Intense Luminescence for Compounds Capable of Excited-State Intramolecular Proton Transfer?

Photoinduced intramolecular direct arylation allows structurally unique compounds containing phenanthro[9',10':4,5]imidazo[1,2-f]phenanthridine and imidazo[1,2-f]phenanthridine skeletons, which mediate excited-state intramolecular proton transfer (ESIPT), to be efficiently synthesized. The developed polycyclic aromatics demonstrate that the combination of five-membered ring structures with a rigid arrangement between a proton donor and a proton acceptor provides a means for attaining large fluorescence quantum yields, exceeding 0.5, even in protic solvents. Steady-state and time-resolved UV/Vis spectroscopy reveals that, upon photoexcitation, the prepared protic heteroaromatics undergo ESIPT, converting them efficiently into their excited-state keto tautomers, which have lifetimes ranging from about 5 to 10 ns. The rigidity of their structures, which suppresses nonradiative decay pathways, is believed to be the underlying reason for the nanosecond lifetimes of these singlet excited states and the observed high fluorescence quantum yields. Hydrogen bonding with protic solvents does not interfere with the excited-state dynamics and, as a result, there is no difference between the occurrences of ESIPT processes in MeOH versus cyclohexane. Acidic media has a more dramatic effect on suppressing ESIPT by protonating the proton acceptor. As a result, in the presence of an acid, a larger proportion of the fluorescence of ESIPT-capable compounds originates from their enol excited states.

Calix[4]pyrrole-based heteroditopic ion-pair receptor that displays anion-modulated, cation-binding behavior.

A new ditopic ion-pair receptor 1 was designed, synthesized, and characterized. Detailed binding studies served to confirm that this receptor binds fluoride and chloride ions (studied as their tetraalkylammonium salts) and forms stable 1:1 complexes in CDCl(3). Treatment of the halide-ion complexes of 1 with Group I and II metal ions (Li(+), Na(+), K(+), Cs(+), Mg(2+), and Ca(2+); studied as their perchlorate salts in CD(3)CN) revealed unique interactions that were found to depend on both the choice of the added cation and the precomplexed anion. In the case of the fluoride complex [1⋅F](-) (preformed as the tetrabutylammonium (TBA(+)) complex), little evidence of interaction with the K(+) ion was seen. In contrast, when this same complex (i.e., [1⋅F](-) as the TBA(+) salt) was treated with the Li(+) or Na(+) ions, complete decomplexation of the receptor-bound fluoride ion was observed. In sharp contrast to what was seen with Li(+), Na(+), and K(+), treating complex [1⋅F](-) with the Cs(+) ion gave rise to a stable, receptor-bound ion-pair complex [Cs⋅1⋅F] that contains the Cs(+) ion complexed within the cup-like cavity of the calix[4]pyrrole, which in turn was stabilized in its cone conformation. Different complexation behavior was observed in the case of the chloride complex [1⋅Cl](-). In this case, no appreciable interaction was observed with Na(+) or K(+). In addition, treating [1⋅Cl](-) with Li(+) produces a tightly hydrated dimeric ion-pair complex [1⋅LiCl(H(2)O)](2) in which two Li(+) ions are bound to the crown moiety of the two receptors. In analogy to what was seen in the case of [1⋅F](-), exposure of [1⋅Cl](-) to the Cs(+) ion gives rise to an ion-pair complex [Cs⋅1⋅Cl] in which the cation is bound within the cup of the calix[4]pyrrole. Different complexation modes were also observed when the binding of the fluoride ion was studied by using the tetramethylammonium and tetraethylammonium salts.

Oligoether-strapped calix[4]pyrrole: an ion-pair receptor displaying cation-dependent chloride anion transport.

A ditopic ion-pair receptor (1), which has tunable cation- and anion-binding sites, has been synthesized and characterized. Spectroscopic analyses provide support for the conclusion that receptor 1 binds fluoride and chloride anions strongly and forms stable 1:1 complexes ([1·F](-) and [1·Cl](-)) with appropriately chosen salts of these anions in acetonitrile. When the anion complexes of 1 were treated with alkali metal ions (Li(+), Na(+), K(+), Cs(+), as their perchlorate salts), ion-dependent interactions were observed that were found to depend on both the choice of added cation and the initially complexed anion. In the case of [1·F](-), no appreciable interaction with the K(+) ion was seen. On the other hand, when this complex was treated with Li(+) or Na(+) ions, decomplexation of the bound fluoride anion was observed. In contrast to what was seen with Li(+), Na(+), K(+), treating [1·F](-) with Cs(+) ions gave rise to a stable, host-separated ion-pair complex, [F·1·Cs], which contains the Cs(+) ion bound in the cup-like portion of the calix[4]pyrrole. Different complexation behavior was seen in the case of the chloride complex, [1·Cl](-). Here, no appreciable interaction was observed with Na(+) or K(+). In contrast, treating with Li(+) produces a tight ion-pair complex, [1·Li·Cl], in which the cation is bound to the crown moiety. In analogy to what was seen for [1·F](-), treatment of [1·Cl](-) with Cs(+) ions gives rise to a host-separated ion-pair complex, [Cl·1·Cs], in which the cation is bound to the cup of the calix[4]pyrrole. As inferred from liposomal model membrane transport studies, system 1 can act as an effective carrier for several chloride anion salts of Group 1 cations, operating through both symport (chloride+cation co-transport) and antiport (nitrate-for-chloride exchange) mechanisms. This transport behavior stands in contrast to what is seen for simple octamethylcalix[4]pyrrole, which acts as an effective carrier for cesium chloride but does not operates through a nitrate-for-chloride anion exchange mechanism.

Control of on-off or off-on fluorescent and optical [Cu²âº] and [Hg²âº] responses via formal Me/H substitution in fully characterized thienyl "scorpionate"-like BODIPY systems.

One 8-phenyl and two 8-mesityl-substituted "scorpionate"-like BODIPY-type species of the formula [3,4,4-tris(5-R-(2-thienyl))-8-(2,4,6-R'-phenyl)-4-bora-3a,4a-diaza-s-indacene (R = H, R' = H, 3a; R, = H, R' = Me, 2a; R, = Me, R' = Me, 2b)] have been synthesized and fully characterized. Importantly, differences in their solution (MeCN) optical Cu(2+) and Hg(2+) probing capacity via SSS-chelation were investigated. Compounds 2a-3a were prepared from the requisite 8-substituted BODIPY complexes. They were characterized first by complete (1)H, (11)B and (13)C NMR spectroscopic assignments (CD(3)Cl or CD(3)C(O)CD(3)); the molecular structures of 2a and 3a were determined by X-ray crystallography. Compounds 2a-3a were studied by UV-vis and fluorescence spectroscopy [Φ(F) = 0.27 ± 0.013 (2a); 0.024 ± 0.0016 (2b); 0.0034 ± 0.00047 (3a)]. Importantly, low [Cu(2+)] with 3a (<3.0 × 10(-5) M) gave rise to an increase of fluorescence intensity (off-on; 6.3-fold), whereas with 2a it decreased (on-off). When [Hg(2+)] (<3.0 × 10(-5) M) was added to 2b, the λ(em,max) value increased (off-on; 3.2-fold), and for 2a, it decreased (on-off). The association constant (K(a)) for Hg(2+)·2a was determined to be 3120 ± 307 M(-1). An approximate stoichiometric 1:1 binding determined by Job plot analysis is in line with successful DFT modeling of SSS-Cu(2+) binding for this system type. (1)H NMR spectroscopy also revealed tentative sets of product complex peaks. These simple differences caused by formal ligand Me-group incorporation are the first for any related fluorophores, to the best of our knowledge.

Beta-vinyl substituted calix[4]pyrrole as a selective ratiometric sensor for cyanide anion.

A new calix[4]pyrrole-based, dual functional, chemodosimetric sensor was developed and studied as a cyanide selective indicator; complete color bleaching was observed even in the co-presence of an excess of other anions.

Selective sensing of anions with calix[4]pyrroles strapped with chromogenic dipyrrolylquinoxalines.

New calix[4]pyrroles bearing dipyrrolylquinoxaline as strapping elements have been synthesized and characterized by spectroscopic means. The binding behavior of these receptors at 25 degrees C was investigated first by proton NMR spectroscopy in CD(3)CN/DMSO-d(6) (9:1 v/v), as well as by UV-vis spectroscopy and isothermal titration calorimetry (ITC) in CH(3)CN/DMSO (97:3 v/v). The receptors displayed a selective colorimetric response when exposed to the fluoride, dihydrogen phosphate, and acetate anions (studied in the form of the corresponding tetrabutylammonium salts) and an enhanced affinity as compared to a comparable calix[4]pyrrole system lacking the dipyrrolylquinoxaline-containing strap.

Unique prototropy of meso-alkylidenyl carbaporphyrinoid possessing one meso-exocyclic double bond.

Generic synthesis, identification of structural identity, unique prototropy and spectroscopic properties of meso-alkylidenyl-thia(m-benzi)porphyrinoid containing one exocyclic double bond at the meso-position were presented.

The development of a nucleus staining fluorescent probe for dynamic mitosis imaging in live cells.

A low-toxicity nucleus staining fluorescent probe, , was developed for real time mitosis imaging in live cells. was identified by unbiased high-throughput imaging-based screening of a new xanthone library (AX). Unlike the conventional Hoechst dye, the low toxicity of allows long term monitoring of cell division over more than one cell cycle.

Discovery of a structural-element specific G-quadruplex "light-up" probe.

The development of a fluorescent probe capable of detecting and distinguishing the wide diversity of G-quadruplex structures is particularly challenging. Herein, we report a novel BODIPY-based fluorescent sensor (GQR) that shows unprecedented selectivity to parallel-stranded G-quadruplexes with exposed ends and four medium grooves. Mechanistic studies suggest that GQR associates with G-quadruplex grooves close to the end of the tetrad core, which may explain the dye's specificity to only a subset of parallel structures. This specific recognition favours the disaggregation of GQR in aqueous solutions thereby recovering the inherent fluorescence of the dye. Due to its unique features, GQR represents a valuable tool for basic biological research and the rapid discovery of novel, specific ligands that target similar structural features of G-quadruplexes.

Conformational and spectroscopic properties of π-extended, bipyrrole-fused rubyrin and sapphyrin derivatives.

Two new expanded porphyrins, naphthorubyrin and naphthosapphyrin, were synthesized. The π-extended rubyrin was isolated and structurally characterized in its monoprotonated form. The sapphyrin congener undergoes pyrrole inversion as a function of the protonation state. These conformational effects are reflected in the spectroscopic features, including the excited singlet state lifetimes.

Anion-modulated, highly sensitive supramolecular fluorescence chemosensor for C(70).

A new readily synthesized fluorescent chemosensor, calix[1]pyreno[3]pyrrole, displays excellent affinity with C(70). Anion-induced facile conformational changes and the dynamic nature of the receptor could be an excellent scaffold for sensing electron-deficient guests.

Strapped calix[4]pyrroles bearing a 1,3-indanedione at a beta-pyrrolic position: chemodosimeters for the cyanide anion.

A strapped calix[4]pyrrole bearing a 1,3-indanedione group at a beta-pyrrolic position has been synthesized and studied as a ratiometric cyanide-selective chemosensor. A concentration-dependent bleaching of the initial yellow color was observed upon addition of the cyanide anion. The bleaching, which was observed exclusively with the cyanide anion, occurred even in the presence of other anions. Spectroscopic studies provide support for a mechanistic interpretation wherein the cyanide anion forms a complex with the receptor (K = 2.78 x 10(4) M(-1)) through a fast equilibrium, which is followed by slow nucleophilic addition to the beta-position of the 1,3-indanedione group. A minimum inhibitory effect from other anions was observed, a feature that could be beneficial in the selective sensing of the cyanide anion.