Polystyrene Cyclization Using Pyrene Excimer Formation. Effect of Geminate Pairs in Good Solvents

Fluorescence Quenching of Dipyrenylalkanes by an Electron/Charge Acceptor

Compounds possessing two fluorophoric moieties may exhibit dual fluorescence, one characterizing the monomeric fluorophore unit and the other characterizing the intramolecular aggregate. Fluorescence quenching of two dipyrenylalkanes, 1,3-bis(1-pyrenyl)propane [1Py(3)1Py] and 1,10-bis(1-pyrenyl)decane [1Py(10)1Py] having different alkyl chains separating the two termini pyrenyl groups that are capable of forming an intramolecular excimer, by an electron/charge-accepting quencher, nitromethane, is investigated in four different solvents-nonpolar (cyclohexane (CH)), polar-aprotic (acetonitrile (ACN)), polar-protic (ethanol (EtOH)), and chlorinated (dichloromethane (DCM))-under ambient conditions. For a given probe in a solvent, fluorescence from the monomer and the intramolecular excimer are quenched with similar efficiencies; the efficiency of quenching is higher for the probe with the longer alkyl chain separating the two fluorophores. Quenching efficiency is significantly higher in chlorinated solvent DCM. The bimolecular quenching rate constants for intramolecular excimer, however, are either comparable or lower for the longer alkyl chain compound. It is suggested that, while the donor electronic excited-state energetics is more favorable for the long alkyl chain compound, the approach of the quencher to the intramolecular excimer appears to be hindered by the presence of a longer alkyl chain.

Excimer Formation Dynamics of Dipyrenyldecane in Structurally Different Ionic Liquids

Ionic liquids, being composed of ions alone, may offer alternative pathways for molecular aggregation. These pathways could be controlled by the chemical structure of the cation and the anion of the ionic liquids. Intramolecular excimer formation dynamics of a bifluorophoric probe, 1,3-bis(1-pyrenyl)decane [1Py(10)1Py], where the fluorophoric pyrene moieties are separated by a long decyl chain, is investigated in seven different ionic liquids in 10-90 °C temperature range. The long alkyl separator allows for ample interaction with the solubilizing milieu prior to the formation of the excimer. The ionic liquids are composed of two sets, one having four ionic liquids of 1-butyl-3-methylimidazolium cation ([bmim⁺]) with different anions and the other having four ionic liquids of bis(trifluoromethylsulfonyl)imide anion ([Tf₂N^-]) with different cations. The excimer-to-monomer emission intensity ratio (I_E/I_M) is found to increase with increasing temperature in sigmoidal fashion. Chemical structure of the ionic liquid controls the excimer formation efficiency, as I_E/I_M values within ionic liquids with the same viscosities are found to be significantly different. The excited-state intensity decay kinetics of 1Py(10)1Py in ionic liquids do not adhere to a simplistic Birk's scheme, where only one excimer conformer forms after excitation. The apparent rate constants of excimer formation (k_a) in highly viscous ionic liquids are an order of magnitude lower than those reported in organic solvents. In general, the higher the viscosity of the ionic liquid, the more sensitive is the k_a to the temperature with higher activation energy, E_a. The trend in E_a is found to be similar to that for activation energy of the viscous flow (E_a,η). Stokes-Einstein relationship is not followed in [bmim⁺] ionic liquids; however, with the exception of [choline][Tf₂N], it is found to be followed in [Tf₂N^-] ionic liquids suggesting the cyclization dynamics of 1Py(10)1Py to be diffusion-controlled and to depend on the viscosity of the ionic liquid irrespective of the identity of the cation. The dependence of ionic liquid structure on cyclization dynamics to form intramolecular excimer is amply highlighted.

Two-step FRET mediated metal ion induced signalling responses in a probe appended with three fluorophores

A tri-fluorophore appended Tren receptor based probe exhibited chelation induced ratiometric fluorescence signalling through a two-step FRET process; enhancement of F_An→ F_Rhenergy transfer efficiency through an F_NBDintermediate was observed.

An intermolecular pyrene excimer in the pyrene-labeled N-thiophosphorylated thiourea and its nickel(ii) complex

(1-pyrene)NHC(S)NHP(S)(OiPr)₂ (HL) and its Ni^II complex ([NiL2]) have been synthesized. Both HL and [NiL2] were found to be emissive in solution of CH₂Cl₂, which is due to the concentration dependent emission of the pyrene monomer and excimer.

Ratiometric sensing of lysine through the formation of the pyrene excimer: experimental and computational studies

Lysine assisted ratiometric monomer to excimer conversion of pyrene derivatives allows intracellular lysine detections.

Switching selectivity between Pb2+ and Hg2+ ions through variation of substituents at xanthene end; ‘turn-on’ signalling responses by FRET modulation

Substitution at amino groups attached to the xanthene core modulates the stereo-electronic situation to switch selectivity; a FRET mediated signalling with a distinctive coupling of fluorophores.

An easy method to monitor lactide polymerization with a boron fluorescent probe

Fluorescence spectroscopy has been widely used to monitor different polymer processes such as polymerization kinetics, chain entanglements, and thermal transitions. The solvent-free controlled ring-opening polymerization (ROP) of lactide is significant both commercially and for research; thus, monitoring this process with a simple fluorescence method can be very useful. Here, a fluorescent dye, difluoroboron 4-methoxydibenzoylmethane (BF(2)dbmOMe) is employed to probe lactide bulk ROP by measuring the emission from solidified reaction aliquots at room temperature. It was found that, through the course of polymerization, the fluorescence of BF(2)dbmOMe in the solid-state aliquots exhibited a systematic shift from yellow to green and then to blue, accompanied by a gradual reduction in the decay lifetime. The fluorescence color change is sensitive to the monomer percent conversion, not the polymer molecular weight. On the basis of these observations and experimental data, we propose that the long-wavelength emission with perceivably longer lifetimes arises from BF(2)dbmOMe dye aggregates (ground and/or excited states), while the dissolved individual dye molecules are responsible for the blue fluorescence with a shorter lifetime. This demonstration of the utility of BF(2)dbmOMe as a fluorescent probe for lactide polymerization could have important practical implications.

Ratiometric Determination of Hg2+ Ions Based on Simple Molecular Motifs of Pyrene and Dioxaoctanediamide

A new podand-type dioxaoctanediamide-based chemosensor having two pyrene moieties was prepared, and its fluoroionophoric behaviors toward transition-metal ions were investigated. Pyrene-appended dioxaoctanediamide 1 showed a selective fluorescence quenching toward Hg2+ ions over other transition-metal ions in an aqueous methanol solution. Unique responses in pyrene monomer and excimer emissions allowed selective ratiometric determination of Hg2+ ions in aqueous environments, and the detection limit was found to be 1.6 x 10(-6) M. [structure: see text]

Fluorescent Ratiometry of Tetrahomodioxacalix[4]arene Pyrenylamides upon Cation Complexation

[structure: see text] C-1,2-alternate tetrahomodioxacalix[4]arene pyreneamides were synthesized. Pb(2+) coordination gave a quenched monomer and excimer fluorescence emission, while upon Ca(2+) ion binding, the receptor provides an enhanced excimer and declined monomer emission with ratiometric response. The excimer emission spectra changes are rationalized by frontier molecular orbitals that the effective Py-Py interaction induces emission intensity increases upon Ca(2+) ion complexation, whereas there is no such interaction observed upon Pb(2+) binding.

Fluorescence Ratiometry of Monomer/Excimer Emissions in a Space-Through PET System

[Reaction: see text]. Fluorogenic calix[4]arenes (1 and 2) bearing a pendent ethyleneamine on their triazacrown rings, respectively, were synthesized in the cone conformation. Compared with 4, free 1 and 2 display a relatively weak emission, reflecting that a PET process from the pendent amine group (-CH2CH2NH2) to the fluorogenic pyrenes is mainly operated. Addition of various metal ions or anions to the solution of 1 or 2 reduces the PET because the pendent alkylamine takes part in the complexation, causing their fluorescence spectra to be changed. When Pb2+, a quenching metal ion, is added to 1 or 2, their pyrene monomer emission is enhanced with their excimer emission quenched, which is due to conformational changes of the facing carbonyl groups as well as to the participation of the ethyleneamine into the three-dimensional Pb2+ ion encapsulation. In contrast, upon addition of alkali metal ions to the 1 and 2, both monomer and excimer emissions are observed to increase, which is attributable to the CHEF effect and the retained conformations. For anion sensing, both 1 and 2 show a high selectivity for F- ions over other anions tested. When the F- ion is bound to 1 or 2 by hydrogen bonding between the amide NH of the triazacrown ring and F-, both their monomer and excimer emissions are weakened due to PET from the bound F- to the pyrene units.

Indium(III)-Induced Fluorescent Excimer Formation and Extinction in Calix[4]arene−Fluoroionophores

New fluorogenic or/and chromogenic calix[4]arenes 1-3 with two facing amide groups linked to fluorescent pyrene units are synthesized. Orientations of the pyrene units are remote from each other in 1 and face-to-face pi-stacked in 2, which produces different photophysical properties. In the excited state, the two pyrene units of 2 form a strong intramolecular excimer displaying an emission at 472 nm with a relatively weak monomer emission at 395 nm. In contrast, 1 exhibits only a monomer emission at 398 nm because intramolecular hydrogen bonding between the phenolic OH oxygens and the amide hydrogens prevents pi-stacking of the two pyrene groups. Fluorescence changes upon addition of various metal ions show that 1 has a remarkably high selectivity for In(3+) over the other metal ions tested. Compound 1 forms 2:1 (metal:ligand), as well as 1:1 complexes, with In(3+), with fluorescence varying uniquely with the complex stoichiometry. Compound 3, which possess two pyrene units and two chromogenic azo groups, shows almost the same binding behavior toward metal ions as does 1, together with additional bathochromic shifts of the absorption maximum. Compared with 1, compound 3 emits a considerably weaker fluorescence, which is attributed to electron transfer from the pyrene units to the nitro groups of the phenylazo moieties.

Temperature-Dependent Tail−Tail Dynamics of Pyrene-Labeled Poly(dimethylsiloxane) Oligomers Dissolved in Ethyl Acetate

We have determined the tail-tail dynamics and energetics of linear poly(dimethylsiloxane) (PDMS) polymers (M(n) = 3100; M(w)/M(n) = 1.07) that have been terminally labeled with pyrene (Py-PDMS-Py) in ethyl acetate between 255 and 323 K. The upper critical solution temperature (theta(u)) for the PDMS/ethyl acetate system is 278 K. The Py-PDMS-Py photophysics follow the Birks model at all of the temperatures studied. However, there is evidence for an increase in the local composition of PDMS surrounding the Py- residues in Py-PDMS-Py below the theta temperature. The recovered activation energy for Py-PDMS-Py tail-tail cyclization is the same as the activation energy for diffusion in ethyl acetate. The activation energy for cyclization is about fourfold larger in comparison to the barrier for PDMS internal backbone rotations. The estimated internal activation barrier for Py-PDMS-Py in ethyl acetate is in line with the internal barrier rotation for PDMS backbone rotation. The intramolecular excimer binding energy for Py-PDMS-Py in ethyl acetate is similar to the intermolecular pyrene excimer. In the ground state, the mean distance between the Py-PDMS-Py tail segments does not change over the temperature range studied. A comparison of these temperature-dependent Py-PDMS-Py/ethyl acetate results with previous results on Py-PDMS-Py in pure supercritical CO2 and CO2-dilated liquids shows that the addition of CO2 to the liquid phase offers the way to tune to Py-PDMS-Py tail-tail dynamics over the greatest range.

An Excimer-Based, Binuclear, On−Off Switchable Calix[4]crown Chemosensor

A new fluorescent chemosensor with two different types of cation binding sites on the lower rims of a 1,3-alternate calix[4]arene (1) is synthesized. Two pyrene moieties linked to a cation recognition unit composed of two amide groups form a strong excimer in solution. For 1, the excimer fluorescence is quenched by Pb2+, but revived by addition of K+ to the Pb2+ ligand complex. Thus, metal ion exchange produces an on-off switchable, fluorescent chemosensor. Computational results show that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbitals (LUMO) of the two pyrene moieties interact under UV irradiation of 1 and its K+ complex, while such HOMO-LUMO interactions are absent in the Pb2+ complex.