Twisted Intramolecular Charge Transfer Band Research Articles

Deciphering the perturbation effect of urea on the supramolecular host-guest interaction of biologically active hydrophobic molecule inside the nanocavity of cyclodextrins

The present work articulates the supramolecular interaction and the formation of host-guest complex between the biologically active hydrophobic coumarin derivative and cyclodextrins by using several spectroscopic, calorimetric and microscopic techniques. All the studies clearly revealed that in presence of cyclodextrins (CDs), coumarin forms 1:1 stoichiometric complex. From all the study, we have found that with gradual increasing the cavity diameter of the hosts, the binding efficiency of the complexes gradually increases. The small population of the non emissive twisted intramolecular charge transfer (TICT) state of coumarin molecule turns into highly emissive in presence of γ-CD owing to its greater cavity diameter. The emissive TICT band is not found in β-CD complex due to its comparative small hydrophilic exterior and less polar environment. The present finding also interpret the perturbation effect of urea on host-guest complexes. In the presence of urea, the TICT emissive band of γ-CD is completely diminished. From, 1H NMR study it was observed that –NEt2 moiety of 7-DCCAE molecule is deeply buried inside the hydrophobic cavity of the CDs and forms host-guest complexes. Isothermal titration calorimetry measurement also indicates the formation of 1:1 host-guest complexes.

Fluorimetric determination of sulphathiazole in honey by means the formation of CDs inclusion complexes

The inclusion complex of sulphathiazole in β-cyclodextrin has been investigated. A 1:2 stoichiometry of the complex was established and formation constants K2 (42.83±3.27M−1) and K1 (4.98±0.36M−1) were calculated by using the changes produced on the native fluorescence of the drug, when included on the hydrophobic cyclodextrin cavity. An enhancement in the fluorescence emission of sulphathiazole and protection of the drug against photochemical reactions has been attained upon inclusion. In solutions of β-CD dual emission (458nm) was noticed in STZ. Formation of the inclusion complex of STZ should result in dual emission, which is due to a twisted intramolecular charge transfer band (TICT). A fluorimetric method for the determination of sulphathiazole has been proposed and applied in honey without sample treatment. The optimized fluorimetric method showed detection and quantitation limits of 9.74ng/g and 32.48ng/g, respectively. Selectivity is high, showing no cross-reactivity to other chemically related antibiotics. The results obtained for blind honey samples (mean recovery 97%), were in good agreement with those obtained by liquid chromatography separation and mass spectrometry detection (LC–MS) (mean recovery 102%), showing that the proposed method might be used for the determination of sulphathiazole residues without expensive equipment.

Spectral characteristics of sulphadiazine, sulphisomidine: effect of solvents, pH and β-cyclodextrin

The absorption and fluorescence spectra of sulphadiazine (SDA), sulphisomidine (SFM) and sulphanilamide (SAM) have been analysed in different solvents, pH and β-cyclodextin. The inclusion complexes of the above sulphonamides with β-CD were investigated by UV–Vis, fluorometry, DFT, FT–IR and 1H-NMR. The solvent study shows that the absorption and emission maxima of the SDA and SFM are more red-shifted than SAM molecule. In non-aqueous solvents, a single fluorescence band (340 nm) is observed, whereas in water and β-CD solutions, dual emission (at 340 and 430 nm) is noticed for SDA and SFM molecules. The dual emission is due to twisted intramolecular charge transfer band (TICT). Studies on β-CD solutions reveal that (1) sulphonamides form 1 : 1 inclusion complex with β-CD; and (2) the red-shift and the presence of TICT in the β-CD medium confirm that the heterocyclic ring is encapsulated in the hydrophobic part and aniline ring is present in the hydrophilic part of the β-CD cavity.

Spectral Characteristics of Sulfa Drugs: Effect of Solvents, pH and β-Cyclodextrin

The absorption and fluorescence spectra of sulfamethoxazole (SMO), sulfisoxazole (SFO), sulfathiazole (STO) and sulfanilamide (SAM) in different solvents, pH and β-cyclodextrin (β-CD) have been analyzed. The inclusion complexes of the above sulfa drugs with β-CD were investigated by UV-visible spectroscopy, fluorometry, DFT, SEM, FT-IR and 1H NMR. The solvent study indicates that the position of the substituent (oxazole or thiazole group) in the SAM molecule (R–SO2–NH-group) is not the key factor to change the absorption and emission behavior of these sulpha drug molecules. In aqueous solution, a single fluorescence band (340 nm) was observed whereas in solutions of β-CD dual emission (430 nm) was noticed in sulpha drug compounds. Formation of the inclusion complex in SMO, SFO and STO should result dual emission which is due to a Twisted Intramolecular Charge Transfer band (TICT). The β-CD study indicates that (i) sulpha drugs form 1:1 inclusion complexes with β-CD and (ii) the red shift and the presence of TICT in the β-CD medium confirms heterocyclic ring encapsulated in the β-CD cavity with the aniline ring present on the out side of the β-CD cavity.

Nile red probing for sphere-to-rod-to-wormlike micelle transition in aqueous surfactant solution

Nile red (phenoxazone-9) was used as a fluorescence probe to detect the sphere-to-rod-to-wormlike micellar transition in the aqueous mixtures of cetyltrimethylammonium bromide and sodium salicylate or potassium bromide, and also in the aqueous solution of a Gemini surfactant, ethanediyl-α,ω-bis(dimethyl dodecyl ammonium bromide). The results showed the I a/ I b of Nile red (the fluorescence intensity ratio of the twisted intramolecular charge transfer band to the planar locally excited band) was a more sensitive index than the I a (the fluorescence intensity of the twisted intramolecular charge transfer band) to represent the micellar transition. This was attributed to the special molecular structure of Nile red that has a large conjugated ring and the can form hydrogen bond with water molecules at the electron acceptor group. This led to sensitive response of the twisted intramolecular charge transfer state of Nile red to the change of microenvironment.

Photophysics of 4-dimethylamino cinnamic acid in different environments

4-dimethylamino cinnamic acid (DMACA) has been studied by steady state and time-resolved fluorescence spectroscopy in different environments in order to get the information about its photophysics and photochemistry. Dual fluorescence in polar medium has been assigned to be arising out of delocalized excited state (DE) and twisted intramolecular charge transfer (TICT) state from different experimental and theoretical considerations. The computed excited state dipole moments in different twisted geometries evince that a twist of 90° of N(CH 3) 2 produces minimum energy state and maximum dipole moment change. The experimentally measured value (9.23 D) for dipole moment change from ground to excited state tallies with the corresponding value computed theoretically (7.5 D) for twisted geometry. Polarization studies in polar, nonpolar solvents and in β-CD cavity suggest that the emission transition moment is perpendicular to the molecular axis confirming the presence of twisted conformer. Weak acid protonates the carbonyl group while strong acid favors protonation of amino nitrogen. The solvent dependent absorption and emission spectra were found to be due to intramolecular structure change rather than ‘solvent coordinate’ effect. TICT band intensity is enhanced in β-CD inclusion complex due to modulation of photophysics by it while the normal (DE) band failed to appear. The fluorescence dependence of DMACA inclusion complex on polarity and viscosity has been discussed in detail. Quantum chemical calculations with AM1 Hamiltonian have been performed to find the actual geometry producing twisted intramolecular charge transfer state by twisting N(CH 3) 2 group to different angles and to confirm experimental findings qualitatively.

Dual Fluorescence of p-Dimethylaminobenzoic Acid in Cetyltrimethylammonium Bromide (CTAB)/1-Heptanol/Water Reverse Micelle

Dual fluorescence of p-dimethylaminobenzoic acid (DMABOA), typical of excited-state twisted intramolecular charge transfer (TICT), in the cetyltrimethylammonium (CTAB)/1-heptanol/H2O reverse micelle has been monitored as a function of a number of factors such as the water pool size, the bulk concentrations of DMABOA and CTAB, and the pH and [KF]/[KBr] ratio in the water pool. Two fluorescence parameters, i.e., the wavelength of the TICT emission band and the intensity ratio of the TICT band to the LE (locally excited state) band, Ia/ Ib, are employed to measure the effects of these factors. The position of the DMABOA molecule in the reverse micelle is ascertained to be among the hydrocarbon chains close to the water pool with its (CH3) 2N group towards the bulk 1-heptanol while the carboxylic group interacts with the polar head of the surfactant electrostatically. The TICT of DMABOA at this position is shown to be similar to that in an organic solvent, and the reverse micelle exerts a restriction on the TICT of DMABOA. It seems that the water molecules in the water pool do not penetrate into the hydrocarbon region of the reverse micelle.

Time‐resolved fluorescence study of 4‐dimethylaminobenzonitrile in nonhydrogen‐bonding polymers, using picosecond dye laser pulses as excitation source

AbstractIt is shown that the time‐resolved fluorescence (TRF) spectra and the red edge effect (REE) phenomena are useful tools to observe the twisted intramolecular charge transfer (TICT) band, of 4‐dimethylaminobenzonitrile (DMABN) in nonhydrogen‐bonding polymers. The TRF results show that the relative intensity of the TICT band increases with the increase of time after excitation in all polymers used. The highest TICT relative intensity is observed in poly(butyl methacrylate) (PBMA) polymer matrix, which has a medium polarity and medium free volume compared with either poly(methylmethacrylate) (PHMA), which is more polar and more rigid, or poly(hexylmethacrylate) (PHMA), which is less polar and less rigid than PBMA polymer matrix. The lifetimes are rationalized to the opposite roles of local polarity and local free volume of polymer sites on TICT emission. © 1995 John Wiley & Sons, Inc.

Twisted intramolecular charge transfer of methyl p-dimethylaminobenzoate in aqueous β-cyclodextrin solution

This paper reports the investigation of the twisted intramolecular charge transfer (TICT) of methyl p-dimethylaminobenzoate (MDMAB) in aqueous β-cyclodextrin (β-CD) solution by the TICT-typical dual fluorescence. In pure water, MDMAB emits only LE fluorescence, and the TICT fluorescence band is developed with the addition of β-CD. Both the LE and TICT fluorescence bands are continuously enhanced upon the increase of β-CD concentration. The intensity ratio of the TICT band to the LE band shows a hillshaped dependence on β-CD concentration, and a blue shift in both TICT and LE bands is observed with the increase of β-CD concentration, of which the blue shift in TICT band is more appreciable. Formation of a 1:1 MDMAB-β-CD inclusion complex, with an association constant of 580±80 l mol −1, is evaluated. The effect of β-CD on TICT of MDMAB is discussed, with consideration of the fact that aqueous β-CD solution is pseudoaquaorganic binary mixture and that TICT in aqueous solution acts differently than in organic solvents. A comparison is made between the TICT of MDMAB and of DMABN in aqueous β-CD solution.

Fluorescence Spectroscopic Investigation of the Effect of α-Cyclodextrin on the Twisted Intramolecular Charge Transfer of p-Dimethylaminobenzoic Acid in Aqueous Media

The effect of α-cyclodextrin (α-CD) on the twisted intramolecular charge transfer (TICT) of p-dimethylaminobenzoic acid (DMABOA) in aqueous media is investigated with the use of TICT-typical dual fluorescence. With the increase of α-CD concentration in aqueous DMABOA solution, the LE fluorescence is slightly quenched and is not shifted in position, while the TICT fluorescence is strongly enhanced and shifted to blue. The correlation of the intensity ratio of the TICT band to the normal band, Ia/ Ib, of DMABOA with medium polarity in aqueous α-CD media is opposite to that in pure organic solvent, which is explained in terms of the different TICT behavior in aqueous solution. The p Ka of DMABOA is not affected by the presence of α-CD, and the orientation of the DMABOA molecule in the DMABOA/α-CD inclusion complex is determined to be with the dimethylamino group in the cavity. The stoichiometry and association constant of the DMABOA/α-CD inclusion complex are investigated by monitoring the Ib/ Ia of DMABOA as a function of α-CD concentration. The Ia/ Ib value of DMABOA in α-CD media is generally much lower than that of DMABN in the same media, which is interpreted by the difference in the energy barrier from the locally excited (LE) to the TICT state and in the orientation of the guest molecules in the α-CD cavity. The effect of the cyclodextrin cavity is clearly demonstrated to be a micro-environmental effect.

The role of the angle of pretwist of some molecules forming twisted intramolecular charge transfer (TICT) states in polymer matrices

It is shown that the relative intensity of the twisted intramolecular charge transfer band of ester and benzonitrile derivatives of dialkylaniline in non-hydrogen-bonding polymers increases with increasing size of the donor (dialkylamino) group and is sensitive to both the local polarity and local free volume of polymer sites.

Fluorescence decay characteristics of 4-dimethylaminobenzonitrile in different polymer matrices: A time-resolved study in the nanosecond range

The fluorescence decay curves of 4-dimethylaminobenzonitrile (DMABN) in different polar protic and aprotic polymer matrices are nonexponential. The lifetimes of the twisted intramolecular charge transfer (TICT) band are in the nanosecond range. With nanosecond time-resolved fluorescence spectroscopy dual fluorescence was observed for DMABN in a poly (vinyl alcohol) (PVA) polymer matrix at room temperature. These observations contradict results obtained by Cazeau-Dubroca and coworkers who report lifetimes of the TICT band of DMABN in PVA polymer matrices of the order of 1 s and claim the emission is an α-delayed fluorescence. Free volume effects are shown with 1,1,4,4-tetraphenylbutadiene (TPB) in PVA matrices.

The role of free volume in the twisted intramolecular charge transfer (TICT) emission of dimethylaminobenzonitrile and related compounds in rigid polymer matrices

The amount of free volume present in a polymer matrix represents a driving force toward twisted intramolecular charge transfer (TICT) emission of p-dimethylaminobenzonitrile (DMABN) and related compounds. The intensity of the TICT band for these compounds in PVC (poly(vinyl chloride), PVA (poly(vinyl alcohol)), PHEMA (poly(hydroxyethyl methacrylate)) and PMMA (poly(methyl methacrylate)) was found to increase in this order, which is consistent with the increased free volume present in the same sequence. This rules out specific solute-solvent interactions as being responsible for the TICT emission in PVA polymer matrices.

Fluorescence studies on solvent-induced intramolecular charge separation in symmetric biaryls

Fluorescence studies on 5,5′-bi-benz[a]-pyrenyl reveal that the emitting state in polar solvents possesses a high dipole moment. Even in non-polar solvents such as n-hexane, dual fluorescence is emitted, with a structured twisted intramolecular charge transfer band. This puts an upper limit to the dipolar solvent fluctuations necessary to induce symmetry reduction and charge separation in the excited state. Theoretical guidelines to predict further cases of biaryls that exhibit charge separation in the excited state are derived.