The solvent effect on a styryl-bodipy derivative functioning as an AND molecular logic gate

We study via DFT and TDDFT calculations the photophysical processes of a styryl-bodipy derivative, ( 1 ), of its monometallic complexes 1-M ²⁺ (M = Ca, Zn, and Hg), and its trimetallic complex ( 2 ) unprotonated, protonated and complexed with water molecules in water solvent and in acidic conditions. The main targets of this study are to gain information regarding published reports on fluorophore species mentioning that fluorescent switching results from trace water, to study how 1 behaves in water solvent which is a common used solvent for molecular logic gates (MLG), and how it behaves in acidic conditions. We conclude that in water solvent, as in acetonitrile solvent (which was found before both theoretically and experimentally) there will be a quenching of emission spectra in 1 and 1-M ²⁺ and a retaining of emission in 2 . However, contrary to acetonitrile solvent, in water, a weak peak will be observed for 1 and 1-M ²⁺ , due to a small ratio of reversible protonation, showing that in acetonitrile 1 acts as a better MLG candidate than in water solvent. On the other hand, in acidic conditions all five species will emit and as a result, 1 will not be an AND MLG, showing that the selection of the solvent conditions is crucial for a species to act as an MLG candidate. Finally, we conclude that the retaining of emission is accomplished by the simultaneous tetrahedral geometry of all three aniline N atoms.     

Theoretical study of lanthanide-based in vivo luminescent probes for detecting hydrogen peroxide

The 4f-4f emissions from lanthanide trication (Ln³⁺) complexes are widely used in bioimaging probes. The emission intensity from Ln³⁺ depends on the surroundings, and thus, the design of appropriate photo-antenna ligands is indispensable. In this study, we focus on two probes for detecting hydrogen peroxide, for which emission intensities from Tb³⁺ are enhanced chemo-selectively by the H₂O₂-mediated oxidation of ligands. To understand the mechanism, the Gibbs free energy profiles of the ground and excited states related to emission and quenching are computed by combining our approximation—called the energy shift method—and density functional theory. The different emission intensities are mainly attributed to different activation barriers for excitation energy transfer from the ligand-centered triplet (T1) to the Tb³⁺-centered excited state. Additionally, quenching from T1 to the ground state via intersystem crossing was inhibited by intramolecular hydrogen bonds only in the highly emissive Tb³⁺ complexes. © 2018 Wiley Periodicals, Inc.     

A Cu protein cavity mimicking fluorescent chemosensor for selective Cu recognition: tuning of fluorescence quenching to enhancement through spatial placement of anthracene unit

Four new fluoroionophores possessing four ligating sites (2S+2N) and an essential hydrophobic environment, as prevailing in the plastocyanin and rusticyanin proteins, have been synthesized. In these PET fluoroionophores, the position of fluorophore anthracene moiety effectively modulates the Cu²⁺ induced emission properties (quenching vs enhancement) of the fluorophore. The addition of Cu²⁺ to solution of receptor with anthracene moiety in its center caused quenching in emission intensity through photoinduced fluorophore-to-metal electron transfer mechanism and in cases where anthracene is present at terminus nitrogen, the emission intensities increased by nearly 1000% due to inhibition of the photoinduced electron transfer from receptor-to-fluorophore in the presence of Cu²⁺ ions. The hydrophobic environment created by various aromatic rings clearly manifested the stability of fluorescence of these molecules above pH 2.0 and their Cu²⁺ complexes above pH 4. The application of such fluoroionophores has been elaborated for building OR and AND logic gates.     

Bulk solvent‐free melt ring‐opening polymerization of L‐lactide catalyzed by Cu(II) and Cu(II)–Nd(III) complexes of the Salen‐type Schiff‐base ligand

A monometallic (Cu²⁺, 1) and a bimetallic (Cu²⁺ Nd³⁺, 2) Salen‐type Schiff‐base complexes with different reactive species, could efficiently catalyze the bulk solvent‐free melt ring‐opening polymerization (ROP) of L ‐lactide. Especially for the bimetallic complex 2, the involvement of rare earth ion was important and influential to the catalytic behaviors. Copyright © 2011 John Wiley & Sons, Ltd.     

A Photochemical Study of Uranyl Ion Interaction with the Triton X-100 Micellar System

This is a report on the spectroscopic characteristics of UO²⁺₂in the excited state in Triton X-100 micellar medium. It also indicates some important results of viscosity and surface tension measurements of the system which have direct relevance to the spectroscopic investigation in the excited state. The quenching of the UO²⁺₂fluorescence due to Triton X-100, upon micellization in the aqueous medium, reveals two kinds of microenvironments of the fluorophore from the Stern–Volmer plot. This has been verified by flash photolytic measurements. A blue shift of the quenched emission spectrum is ascribed to the collisional encounter of UO²⁺₂with the head groups of Triton X-100.     

Nitroanilines as Quenchers of Pyrene Fluorescence

The quenching of pyrene and 1‐methylpyrene fluorescence by nitroanilines (NAs), such as 2‐, 3‐, and 4‐nitroaniline (2‐NA, 3‐NA, and 4‐NA, respectively), 4‐methyl‐3‐nitroaniline (4‐M ‐3‐NA), 2‐methyl‐4‐nitroaniline (2‐M‐4‐NA), and 4‐methyl‐3,5‐dinitroaniline (4‐M‐3,5‐DNA), are studied in toluene and 1,4‐dioxane. Steady‐state fluorescence data show the higher efficiency of the 4‐NAs as quenchers and fit with a sphere‐of‐action model. This suggests a 4‐NA tendency of being in close proximity to the fluorophore, which could be connected with their high polarity/hyperpolarizability. In addition, emission and excitation spectra evidence the formation of emissive pyrene—NA ground‐state complexes in the case of the 4‐NAs and, in a minor degree, in the 2‐NA. Moreover, time‐resolved fluorescence experiments show that increasing amounts of NA decrease the pyrene fluorescence lifetime to a degree that depends on the NA nature and is larger in the less viscous solvent (toluene). Although the NA absorption and the pyrene (Py) emission overlap, we found no evidence of dipole–dipole energy transfer from the pyrene singlet excited state (¹Py) to the NAs; this could be due to the low NA concentration used in these experiments. Transient absorption spectra show that the formation of the pyrene triplet excited state (³Py) is barely affected by the presence of the NAs in spite of their efficiency in ¹Py quenching, suggesting the involvement of ¹Py—NA exciplexes which—after intersystem crossing—decay efficiently into ³Py.     

Tuning the Excited State of Photoactive Building Blocks for Metal‐Templated Self‐Assembly

The reaction of 2,2′:4,4′′:4′,4′′′‐quaterpyridyl (qtpy), with d⁶ ruthenium(II) (Ru^II), and rhenium(I) (Re^I) metal centers has been investigated. The pendant pyridyl groups on the products have also been methylated to produce a second series of complexes containing coordinated Meqtpy²⁺. The absorption spectra of the complexes are dominated by intraligand and charge‐transfer bands. The ruthenium(II) complexes display broad unstructured luminescence consistent with emission from a Ru(d)→diimine(π*) manifold in acetonitrile solutions. In aqueous solutions, their emissions are weaker and the lifetimes are shorter. This effect is particularly acute for complexes incorporating coordinated dipyridylpyrazine, dppz, ligands. Although the emission of the ruthenium(II) complexes containing Meqtpy²⁺ is generally shorter than their qtpy analogs, it is notable that solvent‐dependent effects are much less intense. The rhenium(I) complexes also display broad unstructured luminescence but, compared with the ruthenium(II) systems, they have a relatively short lifetime in acetonitrile. Electrochemical studies reveal that all of the Ru^II complexes display chemically reversible metal‐based oxidations. Re^I complexes only display irreversible metal‐based oxidations. In most cases, the reduction processes were not fully chemically reversible. The electrochemical and optical studies reveal that the nature of the lowest excited state of these complexes—particularly, the systems incorporating dppz—is highly dependent on the nature of the coordinated ligands. Calculations indicate that, although the excited state of most of the complexes is centered on the qtpy or Meqtpy²⁺ ligands, the excited state of the complexes containing dppz ligands is switched away from the dppz by qtpy methylation. A crystallographic study on one of the dicationic ruthenium(II) structures reveals that it forms an inclusion complex with benzene.     

Isotopic and steric perturbations of 2E-chromium(III) excited state behavior in fluid solution: Evidence for a solvent mediated,strongly coupled quenching channel

The lifetimes of the lowest-energy excited state designated ²E, of chromium(III) complexes in fluid solution decrease markedly with increasing temperatures near ambient conditions, but become nearly temperature independent at lower temperatures. The details of the temperature dependence are dependent on the medium and the solvent is implicated as a participant in the quenching process. Perdeuteration of coordinated amine (or ammine) ligands in Cr([14]ane-N₄)(CN)₂⁺ai] ([14]aneN₄ = 1,4,8,11-tetraazacyclodecane) and Cr(NH₃)₆³⁺ results in increased lifetimes in all the temperature regimes investigated. This implicates a substantial nuclear reorganizational barrier for the quenching process in fluid solutions. The most likely quenching channel in fluid solutions involves an adiabatic crossing from the excited state potential energy surface of a ground state reaction intermediate.     

Selective fluorescent PET sensing of fluoride (F) using naphthalimide-thiourea and -urea conjugates

The thiourea and urea functionalised 4-amino-1,8-naphthalimide sensors 1-3, based on the fluorophore-spacer-receptor principle, were synthesised in high yield in three steps. The sensors were shown to signal selectively the detection of fluoride in the fluorescence emission spectrum in DMSO. On all occasions the emission was quenched due to enhanced photoinduced electron transfer quenching (PET) from the receptor to the excited state of the fluorophore upon recognition of F⁻, particularly for the thiourea sensors 1 and 2. In comparison, the changes in the absorption spectra were minor for all three, even after the addition of 80-100 equiv of F⁻. The sensing of acetate or dihydrogenphosphate gave rise to only ∼5-20% quenching.     

The exquisite integration of ESIPT,PET and AIE for constructing fluorescent probe for Hg(II) detection and poisoning

Excessive mercury ions (Hg²⁺) in the environment can accumulate in human body along with the food chain to cause serious physiological reactions. The fluorescence probes were considered as convenient tool with great potential for Hg²⁺ detection. Most existing probes suffer from aggregation-induced quenching (ACQ) effects and insufficient sensitivity. Herein, a novel type of fluorophore was developed by combining the aggregation-induced emission (AIE) and excited state intramolecular proton transfer (ESIPT) characteristics. Subsequently, a phenyl thioformate group with photoinduced electron transfer (PET) effect was connected to give an efficient "turn-on" probe (HTM), which exhibited good selectivity toward Hg²⁺, short response time (30 min), coupled with extremely low detection limit (LOD = 1.68 nmol/L). In addition, HTM was used successfully in real samples, cells and drug evaluation, underlying the superiority of HTM to detect Hg²⁺ in practical applications.     

Hg sensing in aqueous solutions: an intramolecular charge transfer emission quenching fluorescent chemosensors

Compounds 4a and 4b, comprising an anthracene moiety as the fluorophore and a pair of dithiocarbamate functionalities as ligating groups, were designed as fluorescent chemosensors for Hg(II). In aqueous solvent systems, upon excitation, in addition to the normal emission bands of locally excited (LE) state of anthracene, both compounds show a prominent pH-independent intramolecular charge transfer (ICT) emissive band, which can be modulated by Hg²⁺ binding. The systems can be exploited to develop a fluorescent sensitive probe for Hg²⁺.     

Quenching of the fluorescence of riboflavin by the histidinates and alaninates of nickel,copper, and zinc

The formation of complexes of histidinates and alaninates of Ni(II), Cu(II), and Zn(II) with riboflavin (RF) in the ground state and the quenching of the fluorescence of RF by these compounds has been investigated. It has been found that the quenching of the fluorescence of RF by the Cu²⁺ and Ni²⁺ complexes is caused mainly by the nonemissive energy transfer from the donor (RF) to the metal ions. In the case of the Cu²⁺, Ni²⁺, and Zn²⁺ histidinates the formation of nonfluorescing unstable complexes (K_stab — 3–10) of the metal histidinates with RF in the ground state also contribute to the quenching. Free histidine and zinc histidinate quench the fluorescence of RF by the transfer of an electron to the excited molecule of the flavin with the formation of nonfluorescing reduced RF.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 4, pp. 488–493, July–August, 1985.     

Emission band shape probes of the mixed-valence excited state properties of polypyridyl-bridged bis-ruthenium(II) complexes

The absorption spectra and emission spectral band shapes of several polypyridine-ligand (PP) bridged bis-ruthenium(II) complexes imply that the Ru(II)/Ru(III) electronic coupling is weak in their lowest energy metal to ligand charge transfer (MLCT) excited states. Many of these PP-bridging ligands contain pyrazine moieties and the weak electronic coupling of the excited states contrasts to the strong electronic coupling inferred for the correlated mixed-valence ground states. Although the bimetallic complexes emit at significantly lower energy than their monometallic analogs, the vibronic contributions to their 77 K emission spectra are much stronger than expected based on comparison to the monometallic analogs (around twofold in some complexes) and this feature is characteristic of bimetallic complexes in which the mixed-valence excited states are electronically localized. The weaker excited state than ground state donor/acceptor electronic coupling in this class of complexes is attributed to PP-mediated super-exchange coupling in which the mediating orbital of the bridging ligand (PP-LUMO) is partly occupied in the MLCT excited states, but is unoccupied in the ground states; therefore, the vertical Ru(III)-PP⁻ (MLCT) energy is larger and the mixing coefficient smaller in these excited states than is found for Ru(II)-PP in the corresponding ground states.     

Ratiometric fluorescent detection of Cu(II) in semi-aqueous solution using a two-fluorophore approach

Schiff base sensor 1, containing naphthalene and naphthalimide fluorophores with separate and distinct emission wavelengths, showed good selectivity for Cu(II) over other tested physiological and environmentally important cations through changes in its fluorescence spectra in THF/H₂O (9:1) HEPES buffered solution. By taking the ratiometric change of the emissions at 435 nm (naphthalene-Schiff base) and 510 nm (naphthalimide) good linearity was observed in the 0-10 μM range. The enhancement of the 435 nm emission upon binding Cu²⁺ was attributed to a prevention of the rapid CN isomerisation that otherwise leads to non-radiative decay, while the quenching of the naphthalimide emission was attributed to electron transfer between the excited naphthalimide fluorophore and the redox active Cu²⁺.     

Pseudo-Octahedral Iron(II) Complexes with Near-Degenerate Charge Transfer and Ligand Field States at the Franck-Condon Geometry

Increasing the metal-to-ligand charge transfer (MLCT) excited state lifetime of polypyridine iron(II) complexes can be achieved by lowering the ligand's π* orbital energy and by increasing the ligand field splitting. In the homo- and heteroleptic complexes [Fe(cpmp)₂]²⁺ ( 1²⁺ ) and [Fe(cpmp)(ddpd)]²⁺ ( 2²⁺ ) with the tridentate ligands 6,2’’-carboxypyridyl-2,2’-methylamine-pyridyl-pyridine (cpmp) and N,N’-dimethyl-N,N’-di-pyridin-2-ylpyridine-2,6-diamine (ddpd) two or one dipyridyl ketone moieties provide low energy π* acceptor orbitals. A good metal-ligand orbital overlap to increase the ligand field splitting is achieved by optimizing the octahedricity through CO and NMe units between the coordinating pyridines which enable the formation of six-membered chelate rings. The push-pull ligand cpmp provides intra-ligand and ligand-to-ligand charge transfer (ILCT, LL'CT) excited states in addition to MLCT excited states. Ground and excited state properties of 1²⁺ and 2²⁺ were accessed by X-ray diffraction analyses, resonance Raman spectroscopy, (spectro)electrochemistry, EPR spectroscopy, X-ray emission spectroscopy, static and time-resolved IR and UV/Vis/NIR absorption spectroscopy as well as quantum chemical calculations.     

Radiative decay channel assessment to understand the sensing mechanism of a fluorescent turn-on Al3+ chemosensor

The turn-on luminescent chemosensor [2-Hydroxy-1-naphthaldehyde-(2-pyridyl) hydrazone] (L), selective to Al³⁺ ions, was studied by means of density functional theory (DFT) and time-dependent-DFT quantum mechanics calculations. The UV-Vis absorption and the radiative channel from the adiabatic S₁ excited state were assessed in order to elucidate the selective sensing mechanism of L to Al³⁺ ions. We found that twisted intramolecular charge transfer (TICT) and photoelectron transfer (PET), which alter the emissive state, are responsible for the luminescence quenching in L. After coordination with Al³⁺, the TICT is blocked, and PET is no longer possible. So, the emission of the coordination complex is activated, and a fluorescence effect enhanced by chelation is observed. For compounds with Zn²⁺ and Cd²⁺, the luminescence quenching is caused by PET, while for Ni²⁺, ligand to metal charge transfer is the prominent mechanism. To go into more detail, the metal-ligand interaction was analyzed via the Morokuma-Ziegler energy decomposition scheme and the natural orbital of chemical valence.     

Singlet quenching of tetraphenylporphyrin and its metal derivatives by iron(III) coordination compounds

The quenching of fluorescence of the free-base tetraphenylporphyrin, H₂TPP, and its metal derivatives, MgTPP and ZnTPP by diverse iron(III) complexes, [Fe(CN)₆]³⁻, Fe(acac)₃, [Fe(mnt)₂]⁻, Fe(Salen)Cl, [Fe₄S₄(SPh)₄]²⁻·, FeTPPCl and [Fe(Cp)₂]⁺ has been studied both in homogeneous medium (CH₃CN) and micellar media, SDS., CTAB and Triton X-100. The quenching efficiencies are analysed in terms of diffusional encounters and it has been possible to separate static quenching components. The quenching constants are dependent on the nature of the ligating atoms around iron(III) and also on the extent of π-conjugation of the ligands. The quenching mechanism has been investigated using steady-state irradiation experiments. Evidence for oxidative quenching by iron(III) complexes was obtained, though the spin multiplicities of the excited electronic states of iron(III) complexes permit both energy and electron transfer mechanisms for quenching of the singlet excited state of the porphyrins.     

Crystallization-Induced Reversal from Dark to Bright Excited States for Construction of Solid-Emission-Tunable Squaraines

Squaraines (SQs) with tunable emission in the solid state is of great importance for various demands; however a remaining challenge is emission quenching upon aggregation. Herein, a unique SQ, named as CIEE-SQ, is designed to exhibit strong emission in crystal, undergoing crystallization-induced reverse from dark ¹(n+σ,π*) to bright ¹(π,π*) excited states. Such an excited state of CIEE-SQ can be subtly tuned by molecular conformation changes during the unexpected temperature-triggered single-crystal to single-crystal (SCSC) reversible transformation. Furthermore, co-crystallization between CIEE-SQ and chloroform largely stabilize the ¹(π,π*) state, enhancing the transition dipole moment and decreasing the reorganization energy to boost the fluorescence, which is promising in data encryption and decryption.     

Synthesis and Photochemical Properties of Covalently-Linked Dimers of Ruthenium(II) Tris (Bipyridine) Complex: Comparison with Ru(II)(bpy)2 (Poly-4-Methyl-4′-Vinylbipyridine)

Covalently linked dimers of Ru(bpy)₃ ²⁺ (3 and 4) connected by two or three carbon atoms were synthesized as models for Ru(bpy)₃ ²⁺-containing vinyl polymer (1). The luminescence properties of 3 and 4 were compared with those of its component monomer, 4,4′-dimethyl-2,2′-bipyridine-bis(2,2′-bipyridine)ruthenium(II) complex (2). In excited 3 and 4, intramolecular interaction leading to enhanced quenching was not observed. Electron-transfer quenching of the excited dimer with methylviologen (MV²⁺) and the zwitterionic viologen, 1,1′-bis(3-sulfopropyl)4,4′-bipyridinium (SPV), was studied and compared with that of 1 and 2. The low quenching efficiency in 1, 3, and 4 systems can be ascribed to the steric hindrance of unexcited ruthenium complex. Energy migration between ruthenium complexes in the excited 1, 3, and 4 can be ruled out from the kinetic evidence.     

The effect of heavy metals on the anthracene–Me-β-cyclodextrin host–guest inclusion complexes

Anthracene was used to form an inclusion complex with methylated-β-cyclodextrin (Me-β-CD) in water. In aqueous Me-β-CD solution, typical fluorescence emission of anthracene was observed. Benesi–Hildebrand's method was used to obtain the stoichiometry of the anthracene–Me-β-CD complex. The Stern–Volmer quenching constants, K_sv, and fluorescence quantum yields were calculated according to changes in the fluorescence emission intensity of anthracene–Me-β-CD inclusion complexes by adding various amounts of Pb²⁺ and Cd²⁺ salts in water. The K_sv values and fluorescence quantum yields indicate that Pb²⁺ salts quench the anthracene–Me-β-CD inclusion complexes more efficiently than Cd²⁺ salts.