Switching of photoinduced proton transfer from one six-membered hydrogen-bonded ring to other: a molecule of hydrazine and pH sensor.

The present study describes photophysical properties of 3-(benzo[d]oxazol-2-yl)-5-bromo-2-hydroxybenzaldehyde (HBOB) and (E)-2-(benzo[d]oxazol-2-yl)-4-bromo-6-(hydrazonomethyl)phenol (HBON) molecules with asymmetric two-way proton transfer sites. The purpose of this study is to know the direction of ESIPT out of the two-way proton transfer pathways in these molecules in both the solid and solution state. The steady state and time-resolved spectral behaviour of HBOB and HBON and a comparison of the spectral features with the two distinct control compounds 2-(benzo[d]oxazol-2-yl)-4-bromophenol (HBO) and (E)-4-bromo-2-(hydrazonomethyl)phenol (HBN) having single 6-membered hydrogen bonded network reveal that HBOB undergoes imine-amine photoisomerisation by proton transfer towards the oxazole side and HBON undergoes towards the hydrazone side with characteristic Stokes' shifted emission. Proton transfer forms with the red shifted emission of these molecules shows fast decay than the locally excited state. In the solid state, extremely high fluorescence intensity was observed, following a similar type of ESIPT pattern. Calculated ground (S0) and excited state (S1) energy barriers for the PT process obtained using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) corroborate the unidirectional excited state intramolecular proton transfer (ESIPT) process for HBOB and HBON, and the theoretical spectral features validate our experimental absorption and emission spectra well. Interestingly, the unique unidirectional ESIPT behaviour of HBOB was utilised to detect hydrazine both in solution and solid phases. On the other hand, HBON was found to be a good fluorescence pH sensor with a ratiometric color change from yellow to green in acidic and basic media.

Investigating the Photophysical Aspects of a Naphthalene-Based Excited-State Proton Transfer Dye 1-(1H-Benzo[d]imidazol-2-yl)naphthalen-2-ol: pH-Dependent Modulation of Photodynamics.

The steady state and time-resolved photophysical behavior of a proton transfer dye 1-(1H-benzo[d]imidazol-2-yl)naphthalen-2-ol (H-BINO) was investigated. The excited state intramolecular proton transfer (ESIPT) reaction in H-BINO is predominant in nonpolar solutions (toluene and DCM) with a lifetime of ∼1.0 ns. However, in polar media (DMF and MeOH), the excited state photodynamics is characterized by a complex equilibrium of emission from the locally excited state (0.1-2.3 ns), the phototautomer (0.5-1.2 ns), and the anionic emission (2.1-5.4 ns). In the solid state, emission from the various aggregated states dictates the photobehavior. Interestingly, the photodynamics in aqueous solution changes starkly as a function of pH with the anionic (2.1 ns) and phototautomeric (0.5-1.0 ns) emissions guiding the photodynamics as the pH of the medium increases. Optimized structural parameters at the proton donor and acceptor sites for the enol and keto forms and the calculated potential energy curve along the proton transfer coordinate at the density functional theory (DFT) level with the B3LYP/6-311G++(d,p) theory support a favorable and barrierless ESIPT process. The current results will surely boost the ongoing research on small molecule emissive materials.

Dual-Channel Imine-Amine Photoisomerization in a Benzoimidazole and Benzothiazole Coupled System: Photophysics and Applications.

A molecule, namely 2-(1H-benzo[d]imidazol-2-yl)-6-(benzo[d]thiazol-2-yl)-4-bromophenol (BIBTB), having a two-way proton transfer unit of thiazole and imidazole moieties was synthesized and characterized by NMR, electrospray ionization mass spectrometry (ESI-MS), and single-crystal diffraction studies. Steady state and time-resolved spectral studies of BIBTB support excited state intramolecular proton transfer (ESIPT), causing imine-amine tautomerization through a two-way 6-membered H-bonded ring, where the N atoms of benzothiazole and the benzoimidazole unit are involved as proton acceptor sites. Interestingly, in a nonpolar and moderately polar solvent, photoisomerization in BIBTB is found to be favored toward the thiazole ring, whereas in a highly polar solvent, it is favored toward the imidazole ring. A spectral comparison of BIBTB with judicially designed molecules 2-(benzo[d]thiazol-2-yl)-4-bromophenol (HBT) and 2-(1H-benzo[d]imidazol-2-yl)-4-bromophenol (BIB) supports these inferences. Theoretical calculation using the Density Functional Theory (DFT) at CAM-B3LYP/6-311+G(d,p) level supports the existence of two low-energy 6-membered hydrogen-bonded planar conformers in the ground state in the gas phase and in solvents of different dielectrics. The potential energy curves (PECs) calculated along the proton transfer (PT) coordinate are found to have a high energy barrier in the ground state and to be barrierless or have a low energy barrier in the excited state for both the forms. The calculated vertical excitation and the emission energy from the relaxed excited and PT states show good correlation with the experimental spectral data. Aggregation of BIBTB in water with red shifted emission was established from X-ray single-crystal structure analysis, solid state emission, and Dynamic Light Scattering (DLS) measurement. The molecule BIBTB also acts as a fluorescence probe for sensing the explosive picric acid in the subnano scale and can be used to determine the proportion of water in dimethyl sulfoxide (DMSO) solvent.

Structural isomerism induces pH dependent AIE-coupled ESIPT: an in-depth spectroscopic exploration with application in amine vapor sensing.

A novel excited state intramolecular proton transfer probe 3-(benzo[d]thiazol-2-yl)-4-hydroxy-5-methoxybenzaldehyde (3BTHMB) was synthesized and characterized using NMR, ESIMS and single crystal diffraction studies. The steady state and time resolved studies using the time correlated single photon counting (TCSPC) technique revealed that 3BTHMB shows ESIPT reaction with a time constant of 0.10-0.15 ns and longer local emission and anion emissions of time constants 0.5-2.0 ns and 3.0-4.0 ns, respectively, that co-exist in polar solution. In a polar protic solution like methanol, the ESIPT process is damped. Interestingly, 3BTHMB shows aggregation induced emission (AIE) solely in an acidic environment, evidenced by the enhanced quantum yield of emission (∼35%) in acidic solution as well as a long lifetime component of 7.4 ns that corresponds to the lifetime in the solid state. The AIE phenomenon was explored using the DLS technique where enhancement of the hydrodynamic radius in an acidic medium was observed. The judicious positioning of the formyl group in 3BTHMB gives the mentioned probe to show AIE in an acidic medium, as opposed to its previously reported structural isomer BTHMB, which showed no AIE phenomenon as well as its parent molecule 2-(benzo[d]thiazol-2-yl)-6-methoxyphenol (TMP), which showed AIE in neutral pH. The AIE property of 3BTHMB was exploited by successful and rapid detection of amine vapor in the solid state, qualitative ratiometric detection of aqueous pH, by observing the colour change under UV light in acidic (yellow-green) and basic to neutral (blue) medium. The above results pave the way for multiutility fluorescent probes for environmental purposes along with a deep understanding of the underlying photophysical processes which bestow the probes with such outstanding utilities.

Interrogating the nature of aggregates formed in a model azine based ESIPT coupled AIE active probe: stark differences in photodynamics in the solid state and aggregates in water.

A novel Schiff base 4-bromo-2-((E)-((E)-(1-(naphthalen-2-yl)ethylidene)hydrazono)methyl)phenol (BNHMP) was synthesized and characterized by NMR, ESI-MS, FTIR and single crystal X-ray diffraction studies. In the solution phase, BNHMP shows prominent emission from the keto-form, a consequence of excited state intramolecular proton transfer (ESIPT). The quantum yield and excited state lifetime decrease in polar solvent THF compared to relatively non-polar solvent DCM. Interestingly, in aqueous solution (pH 7.0), the quantum yield along with the excited state lifetime undergoes tremendous increment. Dynamic light scattering experiments and FESEM reveal the formation of aggregates in water as reflected by the increased hydrodynamic radius of BNHMP in water. Hence, aqueous phase studies revealed BNHMP to be an AIE active probe. On the other hand, BNHMP shows huge emission intensity in the solid state. Interestingly, the emission decay behavior of BNHMP changes upon excitation, as BNHMP shows very broad absorption in the solid state. Upon excitation at 360 nm, a triexponential decay pattern is found, which changes to a biexponential one upon excitation at 450 nm. Meticulous analysis of the fluorescence lifetimes led to the assignment of J and H aggregates coexisting in the solid state with the former dominating the photodynamics. A judicious comparison of the lifetime behavior in the solid state to that in water leads to the conclusion that BNHMP undergoes AIE by the formation of J and H aggregates to an equal extent, a phenomenon starkly different from the solid-state scenario. The current results hold significance as this is among a few reports where such comprehensive spectrodynamic dissection has been performed for an ESIPT-AIE active Schiff base in solution as well as in the solid phase, thereby giving a holistic vision of the nature and fate of aggregation occurring in such azine based systems and subsequently advancing the understanding of such systems in terms of their photo behavior.

Evaluating the merit of a diethylamino coumarin-derived thiosemicarbazone as an intramolecular charge transfer probe: efficient Zn(ii) mediated emission swing from green to yellow.

We report the synthesis and photophysical properties of a coumarin based probe (1E)-1-(1-(7-(diethylamino)-2-oxo-2H-chromen-3-yl) ethylidene) thiosemicarbazide (DIDOT). DIDOT shows a polarity dependent change in the emission maxima in the solution phase. This is explained by the increased dipole moment in the excited state by an intramolecular charge transfer (ICT) process. DIDOT can successfully detect Zn(ii) in aqueous methanol by a shift in the charge transfer emission maxima from approximately 506 to approximately 535 nm. This shift led to a change in the color of the emission from green to yellow under UV-light. The mechanism of Zn(ii) detection has been delineated using electrospray ionization-mass spectrometry (ESI-MS), Fourier-transform infrared spectroscopy (FTIR) and fluorescence time resolved studies coupled with theoretical calculations. The increment in the charge transfer in the Zn(ii) complex of DIDOT over the bare receptor as a consequence of conformational locking was determined to be the underlying cause of the cation detection phenomenon. The limit of detection and binding constant values of DIDOT towards Zn(ii) were approximately 3 × 10-8 M and 2.35 × 105 M-1 respectively. Finally, the practical utility of DIDOT has been demonstrated by successful detection and quantification of Zn(ii) in spiked water samples.

Comparative Photophysical Study of Differently Substituted Cinnamaldehyde-Based Chalcones: From Intramolecular Charge Transfer to Fluorogenic Solvent Selectivity.

We synthesized three cinnamaldehyde-based chalcone derivatives, (2E,4E)-1-(2-hydroxyphenyl)-5-phenylpenta-2,4-dien-1-one (HPD), (2E,4E)-5-(4-(dimethylamino)phenyl)-1-phenylpenta-2,4-dien-1-one (DPPD), and (2E,4E)-5-(4-(dimethylamino)phenyl)-1-(2-hydroxyphenyl)penta-2,4-dien-1-one (DPHPD). The molecule HPD was totally nonfluorescent. Exclusion of a phenolic -OH moiety from HPD along with the introduction of a dimethylamino moiety in DPPD resulted in excellent excited-state charge-transfer properties in the solution phase. Interestingly, introduction of phenolic -OH and dimethylamino moieties in DPHPD resulted in solvent selectivity in the excited state. DPHDP behaved as a strong fluorophore only in carbonyl- or thiocarbonyl-containing solvents, such as dimethylsulfoxide (DMSO), dimethylformamide (DMF) and dimethylacetamide (DMAC) and showed a prominent red color under UV light. However, no emission was observed in similar carbonylated solvents, such as acetone or formamide, or in viscous medium, such as glycerol. The difference in solvent response was probed by various spectroscopic techniques and analyzed using the crystal structure of the three chalcones along with theoretical calculations. The practical utility of DPHPD was explored by detecting the percentage of water in DMSO solvent.

Imine-Amine Tautomerism vs Keto-Enol Tautomerism: Acceptor Basicity Dominates Over Acceptor Electronegativity in the ESIPT Process through a Six-Membered Intramolecular H-Bonded Network.

Photophysical properties of a synthesized asymmetric two-way proton transfer molecule 3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde (BTHMB) were studied in detail. BTHMB could undergo excited-state intramolecular proton transfer (ESIPT) involving a 6-membered H-bonded network toward either the N or the O atom present in the molecule as proton acceptors. From tedious spectroscopic dissection, however, it was established that ESIPT was driven exclusively toward the N center over the O center in the solid state as well as in the solution phase. The aforementioned deduction was based on spectral comparison with judicially designed molecules 2-hydroxy-5-methoxybenzaldehyde (HMB) and 2-(benzo[d]thiazol-2-yl)-4-methoxyphenol (BTMP). In solution, the coexistence of the anionic and neutral forms of BTHMB in basic solvents dimethylsulfoxide (DMSO) and dimethylformamide (DMF) enables BTHMB to behave as a white light emitter. Thus, apart from establishment of directionality of ESIPT in double ESIPT probes, the current work deserves special merit as BTHMB can be considered as a standard in future designs involving red light and solvent-specific white light emitters.

Anthraimidazoledione Based Reversible and Reusable Selective Chemosensors for Fluoride Ion: Naked-Eye, Colorimetric and Fluorescence "ON-OFF".

Novel anthraimidazoledione-based compounds (1-3) are synthesized as selective colorimetric and fluorescent sensors for fluoride ion. The binding properties of the probes (1-3) are studied with different anions in acetonitrile solvent. Spectral red shifts in the absorption spectra and 'turn-off' emission are observed when fluoride is added to 1-3. The striking green to orange color change in the ambient light is thought to be due to the deprotonation of the N-H proton of the imidazole moiety of the probes by the basic F- ion. Interestingly, in all three cases the nonfluorescent probe-F- solutions, on treatment with copper perchlorate, show distinct color change from orange to golden yellow with resumption of fluorescence intensity. Furthermore, the reversibility of sensors (1-3) for the detection of F- ion is tested for four cycles indicating that "ON-OFF-ON" mechanism is operative. Test strip based on sensor 2 acts as a reusable cost-effective F- sensor.

Modulated photophysics of an anthracene-based fluorophore within bile-salt aggregates: the effect of the ionic strength of the medium on the aggregation behavior.

In this article, the binding interactions of a promising chloride channel blocker 9-methyl anthroate (9-MA) with a series of bile-salt aggregates of varying hydrophobicity have been thoroughly demonstrated. The altered photophysical properties of the fluorescent probe within the concerned microheterogeneous environments have been exploited spectroscopically to assess the communication between the aggregates and the guest. The contrived hydrophobic environment provided by the aggregates appreciably diminishes the water-assisted non-radiative decay channels and thus extends the fluorescence lifetime and the rotational relaxation time of the probe. NaDC aggregates, being more rigid and hydrophobic, provide a better protection to the bound guest from the external influence which is apparent from a much longer fluorescence lifetime and rotational correlation time for the encapsulated probe in NaDC aggregates compared to those in NaC and NaTC aggregates, as is further validated by fluorescence quenching experiments. Salt induced alterations of the binding behavior of the probe with the bile-salt aggregates have also been evaluated via fluorimetric studies, which conclude larger and tighter aggregate formation resulting in a superior degree of rigidity imposed on the aggregate-bound probe at high ionic strength of the medium.

A Novel Cr(3+) Fluorescence Turn-On Probe Based on Rhodamine and Isatin Framework.

A novel turn-on fluorescent dye (E)-3',6'-bis(diethylamino)-2-((1-(naphthalen-2-ylmethyl)-2-oxoindolin-3-ylidene)amino)spiro[isoindoline-1,9'-xanthen]-3-one (RBNI) based on a rhodamine-isatin hybrid molecular architecture was synthesized by condensation of isatin derivative with rhodamine hydrazide. The dye RBNI is selective and sensitive for recognition of Cr(3+) ion in aqueous CH3CN media over other tested metal ions. The sensor shows large fluorescence enhancement upon complexation with Cr(3+) and simultaneous color change occurs from colorless to pink-red. Spectroscopic study predicted 1:1 binding stoichiometry between RBNI and Cr(3+) ion and this was again verified through ESI-MS (Electrospray Ionisation Mass Spectrometry). Detection limit of Cr(3+) ion by this dye was calculated to be 2.4 µM. Furthermore, the potential application of this dye for the monitoring of Cr(3+) ions in pond water and tap water samples was demonstrated.

Interaction of triblock co-polymer micelles with phospholipid-bilayer: a spectroscopic investigation using a potential chloride channel blocker.

Interaction of a potential chloride channel blocker, 9-methyl anthroate (9-MA), has been studied with zwitterionic l-α-phosphatidylcholine (egg-PC) lipid vesicles, which ascertains the utility of the drug as an efficient molecular reporter for probing the microheterogeneous environment of lipid-bilayers. The effect of a non-ionic triblock co-polymer P123 on the stability of these drug-bound lipid-bilayers has also been investigated by means of steady state and time-resolved spectroscopic techniques exploiting the fluorescence properties of the drug. Experimental results reveal that the addition of P123 to the drug-bound lipid results in a preferential complexation of the drug with the Pluronic leaving the lipid vesicles aside, which has been attributed to a substantially stronger binding interaction of the drug with P123 than that with egg-PC. The result is of potential interest from a medical perspective owing to the context of excess drug desorption from bio-membranes.

Spectroscopic and viscometric elucidation of the interaction between a potential chloride channel blocker and calf-thymus DNA: the effect of medium ionic strength on the binding mode.

The present study demonstrates a detailed characterization of the binding interaction of a potential chloride channel blocker 9-methyl anthroate (9-MA) with calf-thymus DNA. The modulated photophysical properties of the emissive molecule within the microheterogeneous bio-assembly have been spectroscopically exploited to monitor the drug-DNA binding interaction. Experimental results based on fluorescence and absorption spectroscopy aided with DNA-melting, viscometric and circular dichroism studies unambiguously establish the binding mode between the drug and DNA to be principally intercalative. Concomitantly, a discernible dependence of the mode of binding between the concerned moieties on the ionic strength of the medium is noteworthy. A dip-and-rise characteristic of the rotational relaxation profile of the drug within the DNA environment has been argued to be originating from a substantial difference in the lifetime as well as amplitude of the free and DNA bound drug molecule. In view of the prospective biological applications of the drug, the issue of facile dissociation of the intercalated drug from the DNA helix via a simple detergent-sequestration technique has also been unveiled. The utility of the present work resides in exploring the potential applicability of the fluorescence properties of 9-MA for studying its interactions with other relevant biological or biomimicking targets.

Interaction of a potential chloride channel blocker with a model transport protein: a spectroscopic and molecular docking investigation.

The present work demonstrates a detailed characterization of the interaction of a potential chloride channel blocker, 9-methyl anthroate (9-MA), with a model transport protein, Bovine Serum Albumin (BSA). The modulated photophysical properties of the emissive drug molecule within the microheterogeneous bio-environment of the protein have been exploited spectroscopically to monitor the probe-protein binding interaction. Apart from evaluating the binding constant, the probable location of the neutral molecule within the protein cavity (subdomain IB) is explored by an AutoDock-based blind docking simulation. The absence of the Red-Edge Effect has been corroborated by the enhanced lifetime of the probe, being substantially greater than the solvent reorientation time. A dip-and-rise characteristic of the rotational relaxation profile of the drug within the protein has been argued to originate from a significant difference in the lifetime as well as amplitude of the free and protein-bound drug molecule. Unfolding of the protein in the presence of the drug molecule has been probed by the decrease of the α-helical content, obtained via circular dichroism (CD) spectroscopy, which is also supported by the gradual loss of the esterase activity of the protein in the presence of the drug molecule.

Functional group induced excited state intramolecular proton transfer process in 4-amino-2-methylsulfanyl-pyrimidine-5-carboxylic acid ethyl ester: a combined spectroscopic and density functional theory study.

The molecule methyl-2-aminonicotinate (2-MAN) does not exhibit excited state intramolecular proton transfer (ESIPT), but its derivative 4-amino-2-methylsulfanyl-pyrimidine-5-carboxylic acid ethyl ester (AMPCE), widely used in the preparation of pyrimidopyrimidines as a protein kinase inhibitor, does exhibit ESIPT. Increasing acidic and basic character at the proton donor and proton acceptor sites by adding functional groups is found to be responsible for the large Stokes shifted ESIPT emission (Δν = 12,706 cm(-1)) in AMPCE. The photophysics of AMPCE have been explored on the basis of steady state and time resolved spectral measurements, quantum yield calculation with variation of polarity, as well as hydrogen bonding ability of solvents. Experimental findings have been correlated with the calculated structure and potential energy surfaces based on the intramolecular proton transfer model obtained by density functional theory (DFT). Properties based on the calculated excited state surfaces generated in vacuo and methanol solvent using time dependent density functional theory (TDDFT) and time dependent density functional theory polarized continuum model (TDDFT-PCM), respectively, show good agreement with the experimental findings. HOMO and LUMO diagrams also support the favorable ESIPT process in the first excited state potential energy surface.

Selective fluorescence sensing of Cu(II) and Zn(II) using a new Schiff base-derived model compound: naked eye detection and spectral deciphering of the mechanism of sensory action.

A new Schiff base compound 2-((benzylimino)-methyl)-naphthalen-1-ol (2BIMN1O) has been synthesized and characterized by (1)H NMR, (13)C NMR, DEPT, FT-IR and mass spectroscopic techniques. The significantly low fluorescence yield of the compound has been rationalized in connection with photo-induced electron transfer (PET) from the imine receptor moiety to the naphthalene fluorophore unit. Subsequently, an evaluation of the transition metal ion-induced modification of the fluorophore-receptor communication reveals a promising prospect for the title compound to function as a fluorosensor for Cu(2+) and Zn(2+) ions selectively, through remarkable fluorescence enhancement. While perturbation of the PET process in 2BIMN1O has been argued to be the responsible mechanism behind the fluorescence enhancement, the selectivity for these two metal ions has been interpreted on the grounds of an appreciably strong binding interaction. Particularly notable aspects regarding the chemosensory activity of the compound are its ability to detect the aforesaid transition metal ions down to the level of micromolar concentration (detection limit being 0.82 and 0.35 µM respectively), along with a simple and efficient synthetic procedure. Also the spectral modulation of 2BIMN1O in the presence of the transition metal ions paves the way for the construction of a calibration curve in the context of its fluorescence signaling potential.

Unraveling the binding interaction and kinetics of a prospective anti-HIV drug with a model transport protein: results and challenges.

The present contribution reports a detailed characterization of the binding interaction of a potential anticancer, anti-HIV drug 1-phenylisatin (1-PI) with a model transport protein Bovine Serum Albumin (BSA) using fluorescence spectroscopic techniques. The thermodynamic parameters e.g., ΔH, ΔS and ΔG for the binding phenomenon have been evaluated on the basis of the van't Hoff equation to reveal that the binding process is principally driven by ionic interactions mediated by charge transfer interaction. This line of argument has been substantiated by frontier molecular orbital analysis of 1-PI. However, the drug-induced quenching of the intrinsic tryptophanyl fluorescence of the protein is found not to abide by a linear Stern-Volmer regression (displaying an upward curvature) when an extensive time-resolved fluorescence spectroscopic characterization of the quenching process has been undertaken to unveil the actuating quenching mechanism. Based on the constancy of the fluorescence lifetime of the protein as a function of drug concentration the observed quenching is inferred to proceed through a static mechanism between the quenching partners. Constant wavelength synchronous fluorescence, excitation-emission matrix fluorescence and circular dichroic (CD) spectroscopic techniques have been exploited to unravel the tertiary and secondary conformational changes in the protein (BSA) induced by drug (1-PI)-binding. The probable binding location of the drug molecule within the protein cavity (hydrophilic subdomain I) has been explored by AutoDock-based blind docking simulation and the inference is further substantiated by site-competitive replacement experiments with specific site-markers. Light is also cast on the drug-protein binding kinetics using the stopped-flow fluorescence technique which reveals an association rate constant of k(a) (± 5%) = 1.471 × 10(-3) s(-1) for the interaction of 1-PI with BSA.

Excited state intramolecular charge transfer suppressed proton transfer process in 4-(diethylamino)-2-hydroxybenzaldehyde.

In this work, we report intramolecular charge transfer (ICT) suppressed excited state intramolecular proton transfer (ESIPT) process in 4-(diethylamino)-2-hydroxybenzaldehyde (DEAHB). Photophysical properties of DEAHB have been extensively studied in different solvents with varying pH, polarity, and hydrogen bonding capability of the solvent using steady state and time-resolved spectroscopy. To establish the competition between the ICT and ESIPT processes in DEAHB, we have synthesized and studied the photophysical properties of 4-(diethylamino)-2-methoxybenzaldehyde (DEAMB) molecule where only the charge transfer process has been observed. Recently, we have reported simple Schiff base molecules (J. Phys. Chem. A 2012, 116, 10948) formed by condensation of DEAHB and hydrazine (5-(diethylamino)-2-[(4-(diethylamino)benzylidene)hydrazonomethyl]phenol (DDBHP) and N,N'-bis(4-N,N-(diethylamino)salisalidene)hydrazine (DEASH)), where charge transfer is assisted by the proton transfer process. In the present case, the DEAHB molecule shows the reverse phenomenon; i.e., charge transfer is suppressed by the proton transfer process. Comparing the photophysical properties of DEAHB with DEAMB it is also found that ICT process in DEAHB is suppressed by the ESIPT process.

Fluorescence detection of Al3+ ion in aqueous medium and live cell imaging by ESIPT probe (E)-N'-(5-bromo-2-hydroxybenzylidene)-4-hydroxybenzohydrazide.

The molecule (E)-N'-(5-bromo-2-hydroxybenzylidene)-4-hydroxybenzohydrazide (BHHB) has been synthesized and its photophysical properties have been investigated by using steady state absorption, emission and time resolved emission spectroscopy. The molecule shows excited state intramolecular proton transfer (ESIPT) process with characteristics large Stoke shifted emission. Fluorescence enhancement of BHHB only in presence of Al3+ ion is used as selective aluminium ion sensor in the sub-nano molar scale in aqueous solution. BHHB-Al3+ ion complex can penetrate through live Hepatocellular Carcinoma (HepG2) cell membranes and is capable for imaging of nucleus of live cells by fluorescence confocal microscopy.

Differential interactions of a biological photosensitizer with liposome membranes having varying surface charges.

The present work demonstrates the interaction of promising cancer cell photosensitizer, harmane (HM), with liposome membranes of varying surface charges, dimyristoyl-l-α-phosphatidylcholine (DMPC) and dimyristoyl-l-α-phosphatidylglycerol (DMPG). Electrostatic interaction of the cationic probe (HM) with the surface charges of the lipids is responsible for differential modulation of the spectral properties of the drug in different lipid environments. Estimation of partition coefficient (K(p) (±10%) = 5.58 × 10(4) in DMPC and 3.28 × 10(5) in DMPG) of HM between aqueous buffer and lipid phases reflect strong binding interaction of the drug with both the lipids. Evidence for greater degree of partitioning of HM into DMPG membrane compared to DMPC membrane has been deduced and further substantiated from experimental studies such as steady-state fluorescence anisotropy, micropolarity determination. The molecular modeling investigation by docking simulation coupled with fluorescence quenching experiment has been exploited to substantiate the location of drug at the lipid head-group region. Modulation of the dynamical properties of the drug within the lipid environments has also been addressed. Rotational relaxation dynamics studies unravel the impartation of a significant degree of motional restriction on the probe molecule within the lipids and reinforce the differential interactions of HM with the two lipid systems along the lines of other findings. Fluorescence kinetics studies reveal a faster association (in terms of apparent rate constants describing the process of interaction) of the drug with DMPG membrane compared to DMPC. This result is argued in connection with the electrostatic interaction between the drug and the liposome surface charges.