Dipak K. Palit

Effect of Solvent on the Excited‐state Photophysical Properties of Curcumin¶

Abstract Photophysical properties of curcumin, 1,7-bis-(4-hydroxy-3-methoxy phenyl)-1,6-heptadiene-2,5-dione, a pigment found in the rhizomes of Curcuma longa (turmeric) have been studied in different kinds of organic solvent and also in Triton X-100 aqueous micellar media using time-resolved fluorescence and transient absorption techniques having pico and nanosecond time resolution, in addition to steady-state absorption and fluorescence spectroscopic techniques. Steady-state absorption and fluorescence characteristics of curcumin have been found to be sensitive to the solvent characteristics. Large change (Δμ = 6.1 Debye) in dipole moments due to photoexcitation to the excited singlet state (S1) indicates strong intramolecular charge transfer character of the latter. Curcumin is a weakly fluorescent molecule and the fluorescence decay properties in most of the solvents could be fitted well to a double-exponential decay function. The shorter component having lifetime in the range 50–350 ps and percent contribution of amplitude more than 90% in different solvents may be assigned to the enol form, whereas the longer component, having lifetime in the range 500–1180 ps with less than 10% contribution may be assigned to the di-keto form of curcumin. Our nuclear magnetic resonance study in CDCl3 and dimethyl sulfoxide-D6 also supports the fact that the enol form is present in the solution by more than about 95% in these solvents. Excited singlet (S1) and triplet (T1) absorption spectrum and decay kinetics have been characterized by pico and nanosecond laser flash photolysis. Quantum yield of the triplet is low (ϕT ≤ 0.12). Both the fluorescence and triplet quantum yields being low (ϕf + ϕT < 0.18), the photophysics of curcumin is dominated by the energy relaxation mechanism via the internal conversion process.

Photophysical properties of the fullerenes, C60 and C70

Photophysical properties of the singlet and triplet states of the fullerenes, C60 and C70, have been investigated in toluene, benzene, hexane and ethanol solutions by pico- and nano-second laser flash photolysis techniques. Lifetimes, extinction coefficients, quantum efficiencies of the formation of the excited states and such properties have been determined. True tripleti?½triplet absorption spectra of C60 and C70 are reported. C60, unlike C70, is found to undergo photoionization in ethanol solution giving solvated electrons.

Photodynamics of the S1 state of some hydroxy- and amino-substituted naphthoquinones and anthraquinones

Photodynamics of the S1 state of the 1,4- and 1,8-disubstituted hydroxyquinones and aminoquinones have been studied in different organic solvents by absorption and fluorescence spectroscopy. The internal conversion process is the major deactivation path for the S1 state. The solvent and temperature dependence and deuterium isotope effect on the fluorescence dynamics suggest that hydrogen stretching vibrations in different intra- and inter-molecular hydrogen bonds are responsible for very fast non-radiative decay processes. Among the intra-molecular hydrogen-bonded molecules studied only the S1 state of 1,8-dihydroxy-9,10-anthraquinone showed the evidence of excited-state intramolecular proton-transfer process. Preliminary observations on the interaction of the S1 state of these quinones with benzene and other aromatic hydrocarbon solvents via the formation of an exciplex have also been reported.

Preparation, characterization and surface modification of Cu metal nanoparticles

Copper metal nanoparticles have been formed by irradiation with 253.7 nm light from a low pressure Hg-arc lamp in the presence of a protective agent gelatin. The nanoparticles were characterized by their absorption maxima and transmission electron micrographs. The quantum yields of formation of the metal nanoparticles increase in the presence of the stabilizer as well as in the presence of a photosensitizer, benzophenone (BP). Damping of copper plasmon absorption band was not observed in the presence of benzotriazole.

Triplet excited states and semiquinone radicals of 1,4-disubstituted anthraquinones

Abstract The triplet state properties of 1,4-dihydroxy-9,10-anthraquinone (quinizarin, QNZ), 1-amino-4-hydroxy-9,10-anthraquinone (AHAQ) and 1,4-diamino-9,10-anthraquinone (DAAQ) were investigated in cyclohexane and isopropanol solutions using nanosecond laser flash photolysis. TT absorption maxima, extinction coefficients, triplet quantum yields and kinetic parameters were measured. The corresponding properties of the neutral semiquinone radicals were also determined in isopropanol. Pulse radiolysis data were used to supplement the measurements. The possibility of dimerization in solution and its effect on the photophysics of the triplet state are discussed for these quinones.

Ultrafast Dynamics of the Excited States of Curcumin in Solution

Dynamics of the excited singlet (S(1)) state of curcumin has been investigated in a wide varieties of solvents using subpicosecond time-resolved fluorescence and absorption spectroscopic techniques. As a consequence of extra stability of the cis-enol conformer due to the presence of an intramolecular hydrogen bond, it is the major form existing in the ground-state and the excited-state processes described here has been attributed to this form. Steady-state absorption and fluorescence spectra suggest significant perturbation of the intramolecular hydrogen bond and the possibility of formation of intermolecular hydrogen-bonded complex with the hydrogen-bonding solvents. Both the time-resolved techniques used here reveal that solvation is the major process contributing to the relaxation dynamics of the S(1) state. Solvation dynamics in protic solvents is multimodal, and the linear correlation between the longest component of the solvation process and the longitudinal relaxation time of the solvent suggests the specific hydrogen-bonding interaction between the solute and the solvent. However, a good correlation between the experimentally determined average solvation time and that predicted by the dielectric continuum model in all kinds of solvents also suggests that the dielectric relaxation of the solvent is also an important contributor to the solvation process. The lifetime of the S(1) state is very short in nonpolar solvents (∼44 ps in 1,4-dioxane) because of efficient nonradiative deactivation of the S(1) state, which is an important consequence of the ultrafast excited-state intramolecular hydrogen transfer (ESIHT) reaction in the six-membered hydrogen-bonded chelate ring of the cis-enol form. However, it has not been possible to monitor the ESIHT reaction in real time because of the symmetrical structure of the molecule with respect to the hydrogen-bonded chelate ring. In polar solvents, dipole-dipole interaction perturbs the intramolecular hydrogen bond leading to the reduced efficiency of the nonradiative deactivation process. However, stretching vibration in the intermolecular hydrogen bonds formed in the hydrogen-bonding (both donating and accepting) solvents induces another efficient channel for the nonradiative relaxation of the S(1) state of curcumin.

Dynamics of charge transfer in the excited amine complexes of the fullerenes C60 and C70.: A picosecond laser flash photolysis study

In benzene solution, C60 and C70 interact weakly in the ground state with amines having favourable oxidation potentials. Picosecond time-resolved absorption measurements show that on photoexcilation, the weak complexes undergo charge separation to produce ion pairs which in turn undergo fast geminate recombination either to produce the triplet state of the fullerenes or give back the ground slate of the complex, depending on the oxidation potential of the amine. Free-ion yield is generally negligible.

Some aspects of steady state and time-resolved fluorescence of tyrosine and related compounds

Abstract The fluorescence characteristics decay kinetics of para -tyrosine and related compounds, e.g. ortho -tyrosine, meta -tyrosine, N -acetyl tyrosine, tyrosine methyl ester, N -acetyl tyrosine ethyl ester, tyramine and phenyl alanine, were studied in aqueous solution at various pH values. A dynamic analysis of the protonation—deprotonation equilibria provides values of p K a * of the COOH group. The fluorescence decay is monoexponential except at pH ≈ p K a (COOH). Fluorescence from tyrosine methyl ester and N -acetyl tyrosine ethyl ester decays biexponentially, and is attributed to the existence of rotamers at room temperature. The difference in the fluorescence decay kinetics of the isomeric tyrosines suggests a possible method for detecting ortho -tyrosine in a mixture with para -tyrosine and phenyl alanine. Phosphate ions quench the fluorescence of all the compounds except phenyl alanine. The quenching rate constants obtained using steady state and time-resolved fluorescence methods compare well and confirm the presence of excited state interaction through the OH group. From studies on the effect of temperature on the fluorescence characteristics, values of τ O F , k O NR and Δ E are obtained.

Excited states and electron transfer reactions of C60(OH)18in aqueous solution

Dynamic light scattering of fullerenol solutions [C60(OH)18] reveals evidence for the formation of fullerene aggregates at high solute concentration (up to 3.85×10-2 mol dm-3). This hydrophilic fullerene derivative emits very weak fluorescence regardless of its concentration. Photolysis (35 ps; λex=355 nm) of C60(OH)18 in aqueous solution yields the immediate formation of a transient singlet excited state with broad absorption in the 550–800 nm region with e670nm=2130 d mol-1 cm-1. The energetically higher-lying singlet excited state transforms via intersystem crossing (i.e., with τ1/2=500 ps) to the also broadly absorbing (550–800 nm), triplet excited state. In contrast, at low solute concentration, the features of the (*T1→*Tn) absorption differ significantly exhibiting an absorption maximum at 650 nm concomitant to a shoulder at 570 nm. The π-radical anion of fullerenol, [C60(OH)18]-, generated by electron transfer from hydrated electrons and (CH3)2C(OH) radicals, absorbs with λmax at 870, 980 and 1050 nm. Based on electron transfer studies with suitable electron donor/acceptor substrates, the reduction potential of the C60(OH)18/[C60(OH)18]-couple was estimated to be in the range between -0.358 and -0.465 V vs. NHE.

Ultrafast twisting dynamics in the excited state of auramine.

Relaxation dynamics of the excited state of bis-[4-(dimethylamino)-phenyl] methaniminium chloride (Auramine) has been investigated using subpicosecond time-resolved absorption spectroscopic technique in both aprotic and alcoholic solvents. The locally excited (LE) state, formed following photoexcitation of Auramine using 400 nm light, undergoes intramolecular charge transfer (ICT) process, which is accompanied by the twisting of the dimethylanilino groups. Time evolution of the transient absorption-stimulated emission spectra as well as the wavelength dependence of the temporal dynamics investigated in each kind of solvents suggest that the relaxation process proceeds via the formation of at least two transient states (TS I and TS II), which are geometrical conformers and consecutively formed following the decay of the LE state. Twisting of the dimethylaniline groups are nearly barrierless processes, the rates of which show linear correlation both with the macroscopic or shear viscosities as well as the solvation times of the solvents. Time-dependent and fractional viscosity dependence of the relaxation rates of the LE and the TS I states in aprotic solvents suggest the multidimensionality of the reaction coordinate as well as reveal the viscoelastic property of the solvents. However, in normal alcohols, in addition to these two factors, activation energy of the solvent viscosity may be another important factor for the slower twisting dynamics of Auramine in alcohols. To explain the viscosity dependence of the decay time of the TS II state, which undergoes an efficient internal conversion process to the ground state, the possibility of occurrence of different mechanisms, such as, energy gap law, involvement of intramolecular high frequency modes, as well as the phenyl group twisting motion on a potential energy surface having a photochemical funnel, have been discussed. TDDFT method has been applied to obtain the optimized electronic structures of the transient states but it has been possible to obtain only that for the TS II state.