Aromatic guest inclusion by a tripodal ligand: Fluorescence and structural studies

The newly synthesized simple tripodal ligand tris-[2-(naphthalen-2-yloxy)-ethyl]-amine (L₁) act as a fluorescence signaling system for aromatic guest. It forms inclusion complexes with several electron deficient aromatic compounds. This inclusion phenomenon has been studied by steady-state fluorescence spectroscopy and solid-state structural analysis. Electron-rich L₁ shows dramatic color change and a concomitant quenching of luminescence in solution as well as solid phase when titrated with several other electron deficient aromatic guest molecules. Rather high selectivity towards the picric acid was observed. L₁ simultaneously forms inclusion complex and organic salt co-crystal with the composition [(L₁H⁺) (Pic⁻)]  PicH (PicH = picric acid) when crystallized in the presence of picric acid. In the solid state, it forms a strong π–π, C–Hπ and C–HO type interactions.     

Allosteric Control of Naphthalene Diimide Encapsulation and Electron Transfer in Porphyrin Containers: Photophysical Studies and Molecular Dynamics Simulation

The complexation processes of N,N’-dibutyl-1,4,5,8-naphthalene diimide ( NDI ) into two types of π-electron-rich molecular containers consisting of two Zn(II)-porphyrins connected by four flexible linkers of two different lengths, were characterized by means of absorption and emission spectroscopies and molecular dynamics simulation. Notably, the addition of NDI leads to a strong quenching of the fluorescence of both cages only when they are in an open conformation suitable for guest encapsulation, a situation triggered by silver(I) ions binding to the lateral triazoles. Molecular dynamics simulations confirm the fast binding of NDI , likely assisted by NDI -silver(I) interactions. Upon NDI complexation, the two porphyrin macrocycles get closer, with an optimized face to face orientation, suggesting an induced-fit mechanism through π–π interactions with the NDI aromatic cycle. Ultrafast transient absorption experiments allowed to identify the process of quenching of the Zn-porphyrin fluorescence as an efficient photoinduced electron transfer reaction between the cage porphyrin and the included NDI guest. The process occurs on fast and ultrafast time scales in the two complexes (1.5 ps and ≤300 fs) leading to a short-lived charge separated state (charge recombination lifetimes in the order of 30–40 ps). The combined computational and experimental approach used here is able to furnish a reliable model of the NDI -cage complexation mechanism and of the corresponding electron transfer reaction, attesting the allosteric control of both processes by the silver(I) ions.     

Phenol-containing bis(oxazolines): synthesis and fluorescence sensing of amines

The fluorescence sensing of primary amines as their neutral forms has been studied with bis(oxazolinyl)phenols (Me-BOP, Ph-BOP), which are efficiently synthesized starting from mesitylene in six steps and in overall 12-22% yields. The BOP sensors showed fluorescence enhancement toward butylamine and several arylethylamines, whereas they showed fluorescence quenching toward secondary and branched amines. The opposite fluorescence behavior is explained by an increased conformational restriction at the excited state, at which a proton transfer complex between the host and guest forms that is stabilized in a tripodal hydrogen bonding mode. This is the first example in which fluorescence enhancement is observed in amine sensing with phenolic fluorophores. Enantiomeric α-chiral organoamines were also sensed with different fluorescent intensity changes by Ph-BOP, complementing the previous tris(oxazolines) that sense enantiomeric α-chiral organoammonium ions.     

Recognition of polycyclic aromatic hydrocarbons and their derivatives by the 1,3-dinaphthalimide conjugate of calix[4]arene: emission, absorption, crystal structures, and computational studies

Polycyclic aromatic hydrocarbons (PAHs) are environmental pollutants as well as well-known carcinogens. Therefore, it is important to develop an effective receptor for the detection and quantification of such molecules in solution. In view of this, a 1,3-dinaphthalimide derivative of calix[4]arene (L) has been synthesized and characterized, and the structure has been established by single crystal XRD. In the crystal lattice, intermolecular arm-to-arm π···π overlap dominates and thus L becomes a promising receptor for providing interactions with the aromatic species in solution, which can be monitored by following the changes that occur in its fluorescence and absorption spectra. On the basis of the solution studies carried out with about 17 derivatives of the aromatic guest molecular systems, it may be concluded that the changes that occur in the fluorescence intensity seem to be proportional to the number of aromatic rings present and thus proportional to the extent of π···π interaction present between the naphthalimide moieties and the aromatic portion of the guest molecule. Though the nonaromatic portion of the guest species affects the fluorescence quenching, the trend is still based on the number of rings present in these. Four guest aldehydes are bound to L with K(ass) of 2000-6000 M(-1) and their minimum detection limit is in the range of 8-35 μM. The crystal structure of a naphthaldehyde complex, L.2b, exhibits intermolecular arm-to-arm as well as arm-to-naphthaldehyde π···π interactions. Molecular dynamics studies of L carried out in the presence of aromatic aldehydes under vacuum as well as in acetonitrile resulted in exhibiting interactions observed in the solid state and hence the changes observed in the fluorescence and absorption spectra are attributable for such interactions. Complex formation has also been delineated through ESI MS studies. Thus L is a promising receptor that can recognize PAHs by providing spectral changes proportional to the aromatic conjugation of the guest and the extent of aromatic π···π interactions present between L and the guest.     

Preferential recognition of zinc ions through a new anthraquinonoidal calix[4]arene

A novel anthraquinonoidal calix[4]arene derivative was designed and synthesized for the preferential recognition of biologically important zinc in preference to prominently similar cadmium ions and other metal ions via quenching of fluorescence intensity. The stoichiometry of host guest complexation has been determined to be 1:1. The fluorescence changes associated with the recognition event may be attributed to the interaction of zinc ions with the nitrogenous functionality attached at the lower rim of calix[4]arene cavity which allows spatial disposition of the anthraquinonoid segments.     

Crystal structures and spectroscopic properties of polycyano-polycadmate host clathrates including a CT complex guest of methylviologen dication and aromatic donor

A series of polycyano-polycadmate (PCPC) host clathrates including a CT complex of methylviologen dication (MV2+) and an aromatic donor as a guest were synthesized, and their crystal structures and spectroscopic properties were investigated. The PCPC host has a framework structure built with Cd2+ ions as coordination centres and cyanides as bridging ligands. This framework host has negative charge and includes a cationic guest together with an ordinary neutral guest. MV2+, which is a strong acceptor, was included as a cationic guest and an aromatic compound, which works as a donor, was included as a neutral guest. Crystal structures of seven clathrates, whose neutral guests were o-cresol, m-cresol, p-cresol, 1-methylnaphthalene, 1,2,4-trimethoxybenzene, pyrrole and aniline, were determined by single crystal X-ray diffraction methods. In all cases MV2+ and the neutral guest formed a CT complex with a face to face stacking structure and were included as a CT complex guest. However, depending on each clathrate the ratio of aromatic donor to MV2+ was different and several variations were found in their PCPC host structures. The clathrates had their own colour depending on their neutral guest. The plot of the CT transition energies estimated from optical CT bands against the ionization potentials of the neutral guests satisfied a linear relationship predicted by Mulliken theory. However, the CT transition energies observed in the clathrates showed a shift to lower energy by ca. 0.6 eV compared with those observed in corresponding acetonitrile solutions.     

Diarylethylene guest anchored into a cyclodextrin molecular host: optical,quantum chemical studies and biological activity

The host–guest complexation between a novel guest namely; 2-(4-pyridinylbenzothiazolyl) ethane, PBE and β-cyclodextrin was studied using steady-state absorption and emission techniques. The fluorescence maximum is strongly blue-shifted with a great enhancement in the fluorescence intensity upon addition of β-CD, confirming the formation of inclusion complexes. The solid inclusion complex between PBE and β-CD has been prepared, characterised using FT-IR, X-ray diffraction and scanning electron microscope techniques. PBE is encapsulated with β-CD nanocavity and 1:1 PBE–β-CD host–guest interaction is identified. This is confirmed using semi-empirical quantum chemical calculations. PBE guest entered into the less polar cavity through the benzothiazole moiety. The negative values of enthalpy and free energy changes suggest that the encapsulation process is thermodynamically favourable. Additionally, the fluorescence is more sensitive to the micellar medium, whether it was cationic, anionic or neutral as well as metal ions like, Li⁺, Cu²⁺ and Fe³⁺. Finally, the antimicrobial activities of PBE guest and its inclusion complex with β-CD host are studied.     

The mechanism of fluorescence quenching of aromatic compounds by acids: ZDO calculations

The fluorescence quenching of cyano, hydroxy, methoxy, and amino derivatives of naphthalene, anthracene and pyrene by acids in polar solvents has been studied by quantum-chemistry methods in semi-empirical approximation. Quenching mechanisms, including protonation of the aromatic nucleus and electron transfer have been considered. It is shown that the mechanism of radiationless deactivation in encounter complexes of reagents consists in the specific interaction of the solvated proton with certain carbon atoms of the aromatic nucleus, not in the electron transfer. It is found that the rate constant of radiationless deactivation correlates with the charge at the corresponding carbon atom of the excited molecule. It is shown that the fluorescence quenching is determined mainly by the nature of the fluorescent state and electron donor-acceptor properties of the aromatic nucleus and the substituent. The present model makes it possible to predict the fluorescent properties of some aromatic compounds in the presence of acids.     

The local anaesthetic tetracaine as a quencher of perylene fluorescence in micelles

At neutral pH the local anaesthetic tetracaine hydrochloride quenches the fluorescence of the lipophilic dye perylene incorporated into non-ionic micelles. The process follows the Stern-Volmer equation, suggesting that quenching occurs through encounter of fluorophore and quencher. As the pH is lowered from 5 to 1, the apparent quenching constant decreases sigmoidally, the midpoint of the curve being at pH 2.3, close to the pK value characterizing the ionization of the anaesthetic aromatic butylamino group. Quenching is completely reversed below pH 1. These results show that the ability of tetracaine to quench the fluorescence of perylene incorporated into micelles depends on the absence of charge on its aromatic amine. Quenching was also studied in homogeneous dioxane-water solution. In this system the quenching constant also decreases sigmoidally as the pH is lowered. The infection point of the curve is nearly coincident with the pK of tetracaine butylamino group in the same partially non-aqueous medium. Protonation of this group induces 60% reversal of the quenching, suggesting that the main mechanism of fluorescence extinction could be the electron transfer from unprotonated tetracaine aromatic amine to perylene in the excited state. However, an additional process which remains operative even when such an amino group is positively charged must also be involved. It can be concluded that the complete reversal of tetracaine quenching of perylene fluorescence in micelles induced by low pH is due to the inability of the anaesthetic to become partitioned into micelles upon protonation of its aromatic amine. In contrast, at neutral pH the local anaesthetic is able to reach the micelle non-polar core where perylene is located. This is consistent with the models, suggesting that the membrane-bound tetracaine assumes a rod-like configuration parallel to the surface normal with the aromatic butylamino group located into a highly hydrophobic region.     

Triplet state formation of aromatic hydrocarbons quenched with silver ion in ethanol

Fluorescence quenching rate constants of aromatic hydrocarbons by silver ions are determined. The values obtained are not well interpreted with thermodynamical considerations based on an electron transfer mechanism, but related to the intersystem crossing rate constant of the fluorescer. Triplet state formation in the course of quenching has been confirmed by laser photolysis studies.     

Transient Radicals Formed by Electron Transfer between inorganic ions and excited aromatic molecules in polar solvents

Fluorescence quenching of aromatic molecules by inorganic anions has been the subject of many investigations, yet the nature of the quenching mechanism is not fully understood. The fluorescence-quenching rate constants correlate with electrochemical data, but the radicals expected to form upon transfer of an electron to the excited aromatic molecules have escaped observation. We report the first observation of radical-ion species formed by electron-transfer quenching with inorganic anions in acetonitrile. A decisive step leading to formation of separated radical ions is the trapping of the primary charge-transfer complex by a second inorganic ion.     

Fluorescence quenching of naphthols by Cu2+ in micelles

Effect of the micelles of anionic, cationic and non-ionic surfactants on the fluorescence quenching of 1- and 2-naphthols has been studied in the presence of copper ion. The excited state lifetime, dynamic and static quenching constants for these systems have been determined. Fluorescence quenching in water and SDS micelle is due to the collision of the fluorophore with the quencher with a small static component. The negatively charged naphtholate ions in the excited state are quenched with significantly higher rates than the neutral naphthol molecules, which are located further inside the mesophase. CTAB micelle is less effective than the SDS micelle for fluorescence quenching. The effect of CTAB on water-assisted excited-state deprotonation has been investigated in the presence of ZnSO₄. For TX-100 micelle there is negligible quenching even at higher concentration of the quencher.     

Colorimetric and Fluorescent Chemosensors for the Detection of 2,4,6‐Trinitrophenol and Investigation of their Co‐Crystal Structures

A full account of our studies of 2,4,6‐trinitrophenol (TNP) sensing is provided. A series of chemosensors 2 , 3 , 4 , 5 with a variety of aromatic chromophores for specific recognition of TNP has been designed and then realized through the fluorescence “on/off” mechanism. These chemosensors demonstrated highly selective, sensitive, and fluorescent quenching of TNP with remarkable visual changes through the intramolecular charge‐transfer (ICT) process. Their host–guest interactions were investigated by ¹H NMR spectroscopic titrations and their corresponding co‐crystal structures, which showed that the 1:1 host–guest complexes were formed by multiple hydrogen‐bond interactions in solution or in the solid state. The origins of the significant affinity demonstrated during the fluorescence recognition process were further disclosed through DFT calculations of corresponding compounds.     

Photo-degradation and emission characteristics of benzidine in halomethane solvents

Benzidine is an aromatic base of importance in industry. It represents a serious pollutant in many industrial effluents and its photodegradation is of great interest. The mechanism of photo-chemical decomposition of benzidine in different halomethane solvents in addition to the corresponding UV absorption spectra and fluorescence emission spectra are discussed. The photochemical quantum yields (phiC lambda ex = 254 nm) of benzidine in halomethanes is dependent upon the halogen content in the solvent. This effect may be explained by the assumption that free radicals are formed during photolysis of these solvents followed by the abstraction of electrons from a benzidine molecule forming macroradicals of the latter. Both mono- and di-radical benzidine cations have been detected by different techniques. The well known electron absorption peak at 283 nm is characteristic of neutral benzidine while those observed at ca. 370 and 565 nm are assigned to the blue-monoradical cation and yellow-diradical cation, respectively. The blue-monoradical cation has been isolated after irradiation as a blue precipitate. A comparison between neutral benzidine and the blue monoradical cation are made using both IR and differential thermal analysis (DTA) techniques. The fluorescence quenching of solutions of benzidine in inert solvents using halomethanes have been studied. The results obtained are interpreted in terms of a diffusional quenching mechanism.     

Photo-induced intermolecular electron transfer from electron donating solvents to Coumarin dyes in bile salt aggregates: role of diffusion in electron transfer reaction

The photo-induced electron transfer between Coumarin dyes and aromatic amines has been investigated using steady state and time-resolved fluorescence quenching studies. We have observed a Marcus type inversion in the electron transfer rate in correlation of quenching constant to the free energy change occurred during reaction. To justify the "inverted region" obtained in the correlation of quenching constant versus free energy curve, we have performed anisotropy measurement and estimated the several diffusional parameters. The translational diffusion coefficients exhibit a similar picture like electron transfer rate constant when it is plotted against free energy. Thus we argued that the diffusion has played an important role in the electron transfer kinetics.     

Fluorescence quenching by charge transfer interaction. Exciplexes of fluorene and hetero-analogues with electron donors and acceptors

The fluorescence quenching of fluorene, dibenzofuran and dibenzothiophen by aliphatic amines (electron donors) and aromatic nitriles (electron acceptors) has been studied in different solvents. Emission spectra, solvent effects and the observed relationship between the quenching rate constants and ionization potentials of the donors or reduction potentials of the acceptors support an electron-transfer mechanism with exciplex formation. Carbazole does not enter in this scheme and is probably quenched via a different mechanism.     

金属锌卟啉与咪唑类客体形成配合物的光谱性质研究

利用紫外-可见光谱法和荧光光谱法,考察了三种金属锌卟啉与三种咪唑类客体小分子形成配合物的光谱性质。实验结果表明:(1)咪唑类客体通过配位键将电子转移给锌卟啉,配合物的形成能力与主体中含有不同性质的基团和客体的结构均有关;(2)该类反应是焓驱动、自发进行的配位反应,温度升高不利于配合物的形成;(3)随客体浓度的不断增加,三种主体与咪唑类客体所形成配合物的体系均具有荧光猝灭的效应。     

Photophysical studies of some dyes in aqueous solution of triton X-100

The spectral (both absorption and fluorescence) and photoelectrochemical studies of a few selective dyes, namely, anionic erythrosin B, neutral riboflavin and cationic safranin O have been carried out in aqueous solution of triton X-100, a neutral surfactant. The results show that the ionic dyes, erythrosin B and safranin O form 1:1 electron donor-acceptor (EDA) or charge-transfer (CT) complexes with triton X-100 both in the ground and excited states, whereas neutral dye riboflavin in its excited state forms 1:1 complex with triton X-100. In these complexes, the dyes act as electron acceptors whereas triton X-100 acts as an electron donor. The fluorescence spectra of erythrosin B and safranin O in presence of triton X-100 show enhancement of fluorescence intensity with red and blue shifts respectively while riboflavin shows normal quenching of fluorescence. A good correlation has been found among photovoltage generation of the systems consisting of these dyes and triton X-100, spectral shift due to complex formation and thermodynamic properties of these complexes.     

Inclusion behavior of water-soluble thiacalix- and calix[4]arenes towards substituted benzenes in aqueous solution

Inclusion abilities of thiacalix- and calix[4]arenetetrasulfonate (3 and 4) towards mono-substituted benzenes were investigated in neutral aqueous solution. In general, the hosts regioselectively encapsulated the guests from the aromatic moiety except the complexation of toluene by 4, in which the guest penetrated from either the aromatic or the methyl group. Stabilities of the inclusion complexes increased with the electron-withdrawing ability of the substituent on the guest, suggesting pi-pi electronic interaction between the host and guest. In spite of the lower electron density of the aromatic ring, thiacalix[4]arene 3 showed higher inclusion ability than calix[4]arene 4, suggesting that the size rather than the electron density of the calix framework is a more important factor in determining the inclusion ability.     

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.