Water-Soluble Pyrene-Adorned Imidazolium Salts with Multicolor Solid-State Fluorescence: Synthesis, Structure, Photophysical Properties, and Application on the Detection of Latent Fingerprints.

New water-soluble acetylpyrene-bound imidazolium salts (1-N-methyl-3-(2-oxo-2-(pyren-1-yl)ethyl)-imidazolium bromide (1), 1-N-isopropyl-3-(2-oxo-2-(pyren-1-yl)ethyl)-imidazolium bromide (2), 1-N-allyl-3-(2-oxo-2-(pyren-1-yl)ethyl)-imidazolium bromide (3), and 1-N-isopropyl-3-(2-oxo-2-(pyren-1-yl)ethyl)-imidazolium hexafluorophosphate (4)) were synthesized from the reaction between 1-bromoacetylpyrene and N-substituted imidazoles in excellent yield. The new molecules were fully characterized by elemental analysis, FT-IR, multinuclear (1H, 13C, and 19F) NMR techniques, and single-crystal X-ray diffraction analysis. Investigations on the crystal packing of 1, 3, and 4 show the presence of inter/intramolecular weak interactions, including the π···π stacking interaction between the pairs of pyrene molecules. The photophysical properties were investigated in detail for the four imidazolium salts. Experiments show that the emissions observed for all the four compounds are due to the excited monomer and static excimer. Very interestingly, all the four compounds exhibit solid-state multicolor fluorescence depending on the excitation wavelength. The solid-state emissions were monitored using a fluorescence microscope. Finally, a fingerprint powder was formulated based on compound 4 and demonstrated as an efficient fluorescent fingerprint powder for forensic applications. The formulated powder revealed all the 3 level information along with peculiar individual characteristics of the fingerprints under investigation. The fingerprints were further viewed through a fluorescence microscope, and the results were discussed in detail.

Environmentally benign, facile and selective recovery of gold from aqueous media: synergic role of carbon dots as green reductant and sensor towards Au3+ ions.

Photoluminescent carbon dots (PL CDs) have drawn tremendous attention from researchers owing to their admirable properties and wide range of applications. Herein, highly PL nitrogen and sulfur doped carbon dots (N,S-CDs) were synthesized through a facile, green and rapid one-step microwave assisted method using goat hooves, a bio-waste and a green precursor. The structural and photophysical properties of as obtained N,S-CDs were thoroughly investigated. From the investigation, it is revealed that the N,S-CDs possess a spherical morphology with an average particle size of about 2 nm, highly amorphous nature, high functionality, negative zeta potential (-32 mV), good water-solubility, excitation dependant PL, high PL quantum yield (23.8%), nanosecond lifetime (τ avg = 3.38 ns) and excellent storage stability for 180 days without any agglomeration. In addition, the N,S-CDs exhibit high PL stability under diverse pH conditions, wide ionic strength and resistance towards photobleaching, which are very important properties for practical applications. The N,S-CDs selectively sense Au3+ ions and also reduce the Au3+ ions to metallic gold. Hence, the N,S-CDs were successfully applied as a potential candidate for sensing of Au3+ and simultaneous extraction of metallic gold in aqueous media without any further reducing agents. It is a significant green way for the recovery of gold in aqueous media.

Deciphering the incognito role of water in a light driven proton coupled electron transfer process.

Light induced multisite electron proton transfer in two different phenol (simple and phenol carrying an intramolecularly hydrogen bonded base) pendants on acridinedione dye (ADD) and an NADH analogue was studied by following fluorescence quenching dynamics in an ultrafast timescale. In a simple phenol derivative (ADDOH), photo-excited acridinedione acquires an electron from phenol intramolecularly, coupled with the transfer of a proton to solvent water. But in a phenol carrying hydrogen bonded base (ADDDP), both electron and proton transfer occur completely intramolecularly. The sequence of this electron and proton transfer process was validated by discerning the pH dependency of the reaction kinetics. Since photo-excited ADDs are stronger oxidants, the sequential electron first proton transfer mechanism (ETPT) was observed in ADDOH and hence there is no change in the PCET reaction kinetics kETPT ∼ 6.57 × 109 s-1 in the entire pH range (pH 2-12). But the phenol carrying hydrogen bonded base (ADDDP) unleashes concerted electron proton transfer where the PCET reaction rate decreases upon decreasing the pH below its pKa. Noticeably, the concerted EPT process in ADDDP mimics the donor side of photosystem II and it occurs by two distinct pathways: (i) through direct intramolecular hydrogen bonding between the phenol and amine, kDEPT ∼ 12.5 × 1010 s-1 and (ii) through the bidirectional hydrogen bond extended by the water molecule trapped in between the proton donor and acceptor, which mediates the proton transfer and serves as a proton wire, kWMEPT ∼ 2.85 × 1010 s-1. These results unravel the incognito role played by water in mediating the proton transfer process when the structural elements do not favor direct hydrogen bonding between the proton donor and acceptor in a concerted PCET reaction.

Encumbrance in desilylation triggered fluorogenic detection of the fluoride ion - a kinetic approach.

Highly selective fluorogenic detection of the fluoride ion becomes viable due to its propensity towards cleaving Si-O and Si-C bonds, the key reactive elements in fluoride selective chemodosimeters. Herein, acridinedione derived, two novel fluorescent probes bearing tertiarybutyldiphenylsilyloxy (TBDPS) and tertiarybutyldimethylsilyloxy (TBDMS) groups were synthesized and their fluoride selective dosimetric action in organic solvents and in mixed aqueous medium was established through steady state and time resolved fluorescence techniques. Unusually, these molecular probes maintain their sensitivity down to 10 ppb in both organic and mixed aqueous medium; hence they can be considered as highly selective and sensitive fluorescent probes for the fluoride anion. By following the kinetics of the desilylation process it is established that the reaction follows second order kinetics with respect to fluoride ion concentration in acetonitrile whereas it becomes first order in mixed aqueous medium owing to its high degree of hydration. Also, the hydrophobic and sterically crowded substitution on the silyl receptor hampers the reaction kinetics only in organic solvents whereas its influence in mixed aqueous medium is relatively very less. However, common inorganic cations (Na+) effectively hinder the reaction kinetics through strong ion pair interaction and prolong the response time. Therefore, the indigenous influences of three different factors which encumber the desilylation process were quantitatively enumerated and the prospective application of these fluorescent probes in detecting and validating fluoride ions in various environmental samples is demonstrated.

Fluorescent carbon nano dots from lignite: unveiling the impeccable evidence for quantum confinement.

Synthesizing nano carbon from its bulk precursors is of recent research interest. In this report, luminescent carbon nanoparticles (CNPs) with tunable particle size and surface functionality are fabricated from lignite using ethylenediamine as the reactive solvent and surface passivating agent via different experimental methods. From the steady-state and time-resolved photophysical studies of these differently sized CNPs, it is unveiled that the energy of the excitons generated after photoexcitation is quantum confined, and it influences the observed photophysical behaviour significantly only when the particle size is less than 10 nm. A larger size of the CNPs and less surface functionalization lead to aggregation, and quenching of the fluorescence. But by dispersing smaller size CNPs in sodium sulfate matrix exhibits fluorescence in the solid state with an absolute fluorescence quantum yield of ∼34%. The prospective application of this hybrid material in sensing and removal of moisture in the atmosphere is illustrated.

Visible-light activation of the bimetallic chromophore-catalyst dyad: analysis of transient intermediates and reactivity toward organic sulfides.

In order to develop a new photocatalytic system, we designed a new redox-active module (5) to hold both a photosensitizer part, [Ru(II)(terpy)(bpy)X](n+) (where terpy = 2,2':6',2''-terpyridine and bpy = 2,2'-bipyridine), and a popular Jacobsen catalytic part, salen-Mn(III), covalently linked through a pyridine-based electron-relay moiety. On the basis of nanosecond laser flash photolysis studies, an intramolecular electron transfer mechanism from salen-Mn(III) to photooxidized Ru(III) chromophore yielding the catalytically active high-valent salen-Mn(IV) species was proposed. To examine the reactivity of such photogenerated salen-Mn(IV), we employed organic sulfide as substrate. Detection of the formation of a Mn(III)-phenoxyl radical and a sulfur radical cation during the course of reaction using time-resolved transient absorption spectroscopy confirms the electron transfer nature of the reaction. This is the first report for the electron transfer reaction of organic sulfide with the photochemically generated salen-Mn(IV) catalytic center.

Photoionization and time-dependent stokes shift of coumarin 307 in soft matter: solvation and radical-ion pair recombination dynamics.

Photoionization, fluorescence time-dependent Stokes shift (TDSS), and rotational dynamics of coumarin 307 (C307) have been investigated in soft matter system such as micelles using time-resolved transient absorption and fluorescence spectroscopy. Photoionization of C307 leads to the formation of coumarin radical cation and hydrated electron, which were characterized by their respective transient absorption. The photoionization yields are significantly higher in anionic sodium dodecyl sulfate (SDS) micelle than in cationic cetyltrimethylammonium bromide (CTAB) and neutral Triton X-100 (TX-100) micelles, indicating the influence of micellar surface charge on the efficient separation of radical cation-hydrated electron pair. The CTAB micelle favors the recombination of radical cation and hydrated electron leading to the formation of triplet state of C307, which causes a decrease in the photoionization yield. C307 exhibits TDSS in all micelles; the time evolution and the magnitude of the TDSS depend on nature of the micelle. In TX-100 micelles, the decay of the TDSS exhibits ultraslow component (165 ns) and is affected by the presence of electron scavengers. The ultraslow component in TX-100 micelle originates from the recombination of radical cation-hydrated electron, which results in the formation of twisted intramolecular charge transfer (TICT) state; such formation of TICT state was not observed in SDS and CTAB micelles. To the best of our knowledge, this is the first report where the radical-ion pair recombination dynamics is probed using TDSS in combination with time-resolved transient absorption studies. The activation energy for the solvent relaxation and radical-ion pair (solvent separated) recombination process was found to be 6.1 and 3.0 kcal mol(-1), respectively. Temperature effect on TDSS in TX-100 micelles confirmed the increase in the water hydration, and size of the micelle influences the relative contribution of the solvation and radical-ion pair recombination dynamics toward the total TDSS. We propose that TDSS observed in neutral micelles and reported in other biomolecules such as proteins by the 7-amino coumarin probe is not only due to the solvation dynamics alone but also due to the radical-ion pair recombination dynamics.

Highly selective, sensitive and quantitative detection of Hg2+ in aqueous medium under broad pH range.

Amidothiourea linked acridinedione derivatives selectively detect Hg(2+) in unbuffered aqueous solution under broad pH range with both single- and two-photon excitation. The observed linear fluorescence intensity change allows the quantitative detection of Hg(2+) in the concentration range of 22 nM-0.33 µM with the lower detection limit of 2 nM.

Head-to-tail intermolecular hydrogen bonding of OH and NH groups with fluoride.

To explore the anion-recognition ability of the phenolic hydroxyl group and the amino hydrogen, we synthesized three different acridinedione (ADD) based anion receptors, 1, 2 and 3, having OH, NH, and combination of OH and NH groups, respectively. Absorption, emission and (1)H NMR spectral studies revealed that receptor 1, having only a phenolic OH group, shows selective deprotonation of the hydroxyl proton towards F(-), which results in an "ON-OFF"-type signal in the fluorescence spectral studies. Receptor 2, which only has an amino hydrogen, also shows deprotonation of the amino hydrogen with F(-), whereas receptor 3 (having both OH and NH groups) shows head-to-tail intermolecular hydrogen bonding of OH and NH groups with F(-) prior to deprotonation. The observation of hydrogen bonding of the OH and NH groups in a combined solution of 1 and 2 with F(-) in a head-to-tail hetero-intermolecular fashion, and the absence of head-to-head and tail-to-tail intermolecular hydrogen bonding in 1 and 2 with F(-), prove that the difference in the acidity of the OH and NH protons leads to the formation of an intermolecular hydrogen-bonding complex with F(-) prior to deprotonation. The presence of this hydrogen-bonding complex was confirmed by absorption spectroscopy, 3D emission contour studies, and (1)H NMR titration.

Fluorescence spectroscopic evidence for hydrogen bonding and deprotonation equilibrium between fluoride and a thiourea derivative.

Interaction of anions with thiourea-linked acridinedione fluorophore was studied by absorption, (1)H NMR, steady-state and time-resolved fluorescence techniques. Addition of AcO(-) and H(2)PO(4)(-) shows a genuine H-bonded complex with thiourea receptor; whereas, F(-) shows stepwise H-bonding and deprotonation of thiourea NH as confirmed by (1)H NMR titration. Free receptor 1 shows emission maximum at 418 nm; whereas, H-bonded complex of 1·F(-) shows a new redshifted emission maximum at 473 nm and the deprotonated 1 exhibits an emission peak at 502 nm. Presence of these three different emitting species was probed by 3D emission spectroscopic studies. Equilibrium between the free receptor 1, 1·F(-) H-bonded complex and deprotonated 1 was confirmed by time-resolved fluorescence studies. Time-resolved area normalised emission spectra (TRANES) of 1 in the presence of F(-) shows two isoemissive points at 456 and 479 nm between time delays of 0-0.5 ns and 1-20 ns, respectively, due to the existence of three emitting species in equilibrium. Observation of such an equilibrium based on fluorescence spectroscopic studies further proves the earlier reported absorption and (1)H NMR spectroscopic studies of H-bonding and deprotonation processes and also illustrates the dynamics of anion-receptor interactions.

Unprecedented formation of an N-benzamidobisthiourea derivative and its role in the formation of a new CT state specific towards fluoride ion.

A novel N-benzamidobisthiourea derivative (1) was synthesized. Tuning of its emission from the locally excited (LE) state to the charge transfer (CT) state was observed specifically for fluoride ion.

Fluorescent sensing of anions with acridinedione based neutral PET chemosensor.

Newly synthesised fluorescent chemosensor ADDTU contains the thiourea receptor connected to the acridinedione (ADD) fluorophore via a covalent bond, giving rise to a fluorophore-receptor motif. In this fluorescent chemosensor, the anion recognition takes place at the receptor site which result in the concomitant changes in the photophysical properties of a ADD fluorophore by modulation of photoinduced electron transfer (PET) process. The binding ability of these sensor with the anions F(-), Cl(-), Br(-), I(-), HSO(4)(-), ClO(4)(-), AcO(-), H(2)PO(4)(-) and BF(4)(-) (as their tetrabutylammounium salts) in acetonitrile were investigated using UV-vis, steady state and time-resolved emission techniques. ADDTU system allows for the selective fluorescent sensing of AcO(-), H(2)PO(4)(-) and F(-) over other anions in acetonitrile.

Antioxidant activity measured in different solvent fractions obtained from Mentha spicata Linn.: an analysis by ABTS*+ decolorization assay.

Antioxidant compounds are abundantly available in plants and play an important role in scavenging free radicals, thus providing protection to humans against oxidative DNA damage. Mentha spicata Linn., commonly called spearmint, belongs to the family lamiaceae. It was selected in the present study because Mentha extracts have antioxidant properties due to the presence of eugenol, caffeic acid, rosmarinic acid and alpha-tocopherol. Four solvent fractions (hexane, chloroform, ethyl acetate and water) of ethanolic extract of dried leaves powder of M. spicata were analyzed for total antioxidant activity (TAA) and relative antioxidant activity (RAA) and compared with standard antioxidants such as Quercetin, beta-carotene, L-ascorbic acid and glutathione using ABTS*+ decolorization assay (ABTS/Potassium persulphate). The antioxidant activity was assumed to be from the total phenolic content of the ethanolic extract. Total phenolics are found to be highest in ethyl acetate fraction (54 mg/g) and least in hexane fraction (13 mg/g) and more or less similar in water and chloroform fractions (30-32 mg/g). TAA is found to be less in hexane and chloroform fractions (<53% at 50 microg/ml) and highest in ethyl acetate (95% at 20 microg/ml) and water (84% at 30 microg/ml) fractions. The RAA of ethyl acetate fraction is 1.1 compared to quercetin (at 5 microM/ml), but greater when compared to beta-carotene (15 microM/ml), L-ascorbic acid (15 microM/ml) and glutathione (15 microM/ml). The RAAs with these antioxidants are in the range of 1.31 -1.6. The values of RAAs for water fraction also show similar trend and are in the range of 1.0-1.4. The antioxidant activities of the solvent factions are closely related to the content of total phenolics present in them.

A novel colorimetric and fluorescent chemosensor for anions involving PET and ICT pathways.

A novel colorimetric and fluorescent chemosensor ADDTU-1 bearing dual receptor sites, which shows specific optical signaling for AcO-, H2PO4-, and F- over other anions and dual response toward AcO- and F- via PET and ICT mechanisms, is described. [structure: see text]

A novel fluorophore with dual fluorescence: local excited state and photoinduced electron-transfer-promoted charge-transfer state.

Absorption and emission spectra of 9-N,N-dimethylaniline decahydroacridinedione (DMAADD) have been studied in different solvents. The fluorescence spectra of DMAADD are found to exhibit dual emission in aprotic solvents and single emission in protic solvents. The effect of solvent polarity and viscosity on the absorption and emission spectra has also been studied. The fluorescence excitation spectra of DMAADD monitored at both the emission bands are different. The presence of two different conformation of the same molecule in the ground state has lead to two close lying excited states, local excited (LE) and charge transfer (CT), and thereby results in the dual fluorescence of the dye. A CTstate involving the N,N-dimethylaniline group and the decahy droacridinedione chromophore as donor and acceptor, respectively, has been identified as the source of the long wavelength anomalous fluorescence. The experimental studies were supported by ab initio time dependent-density functional theory (TDDFT) calculations performed at the B3LYP/6-31G* level. The molecule possesses photoinduced electron transfer (PET) quenching in the LE state, which is confirmed by the fluorescence lifetime and fluorescent intensity enhancement in the presence of transition metal ions.

Excited-state behavior and photoionization of 1,8-acridinedione dyes in micelles--comparison with NADH oxidation.

The photophysics and photochemistry of 1,8-acridinedione dyes, which are analogues of reduced nicotinamide adenine dinucleotide (NADH), are studied in anionic and cationic micelles. Acridinedione dyes (ADDs) are solubilized in micelles at the micelle-water interface and are in equilibrium between the aqueous and micellar phase. The binding of the ADDs with micelles is attributed to hydrophobic interactions and the binding constants are determined with steady-state and time-resolved techniques. Nanosecond laser flash photolysis studies are carried out in aqueous, anionic, and cationic micellar solutions. The ADD undergoes photoionization in the excited state to give a solvated electron. The solvated electron reacts with the ADD to give an anion radical. In anionic micelles, the yield of the solvated electron increases because of the efficient separation of the cation radical and the electron. Cation radicals derived from the photooxidation of ADDs are involved in keto-enol tautomerization. Under acidic conditions, an enol radical cation of the acridinedione dye is formed from the keto form of the cation radical by intramolecular hydrogen atom transfer. In cationic micelles, due to electrostatic attraction, the electron cannot escape from the micelle and recombination of the cation radical and the electron results in the formation of a triplet state. For the first time, a solvated electron is observed in the laser flash photolysis of ADDs in anionic micelles. The photoionization of ADDs depends on the excitation wavelength and is biphotonic at 355 nm and monophotonic at 248 nm. From the results with this NADH model compound, the sequential electron-proton-electron transfer oxidation of NADH is confirmed and the nature of the intermediates involved in the oxidation is unraveled; these intermediates are found to depend on the pH value of the medium.