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 (delta mu = 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 (phi T < or = 0.12). Both the fluorescence and triplet quantum yields being low (phi f + phi T < 0.18), the photophysics of curcumin is dominated by the energy relaxation mechanism via the internal conversion process.

Dynamics of interfacial electron transfer (ET) in the ruthenium-polypyridyl complex [{bis(2,2'-bpy)-(4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol)} ruthenium(II) hexafluorophosphate] (Ru-cat)-sensitized TiO(2) nanoparticles has been investigated using femtosecond transient absorption spectroscopy detecting in the visible and near-infrared region. It has been observed that Ru-cat is coupled strongly with the TiO(2) nanoparticles through its pendant catechol moiety. Electron injection has been confirmed by direct detection of electrons in the conduction band, cation radical of the adsorbed dye, and a bleach of the dye in real time as monitored by transient absorption spectroscopy. A single-exponential and pulse width limited (<100 fs) electron injection has been observed, and the origin of it might have been from the nonthermalized excited states of the Ru-cat molecule. The result gave a strong indication that the electron injection competes with the thermalization of the photoexcited states due to large coupling elements for the forward ET reaction. Back-ET dynamics has been determined by monitoring the decay kinetics of the cation radical and injected electron and also from recovery kinetics of the bleach of the adsorbed dye. It has been fit with a multiexponential function, where approximately 30% of the injected electrons are recombined with a time constant of <2 ps, again indicating large coupling elements for the charge recombination reaction. However, our results have shown relatively long-lived charge separation in the Ru-cat/TiO(2) system as compared to other organic dye-sensitized TiO(2) nanoparticles with similar interactions.

Steady-state fluorescence and time-resolved absorption measurements in pico- and femtosecond time domain have been used to investigate the dynamics of hydrogen bond in the excited singlet (S(1)) state of fluorenone in alcoholic solvents. A comparison of the features of the steady-state fluorescence spectra of fluorenone in various kinds of media demonstrates that two spectroscopically distinct forms of fluorenone in the S(1) state, namely the non-hydrogen-bonded (or free) molecule as well as the hydrogen-bonded complex, are responsible for the dual-fluorescence behavior of fluorenone in solutions of normal alcoholic solvents at room temperature (298 K). However, in 2,2,2-trifluoroethanol (TFE), a strong hydrogen bond donating solvent, emission from only the hydrogen-bonded complex is observed. Significant differences have also been observed in the temporal evolution of the absorption spectroscopic properties of the S(1) state of fluorenone in protic and aprotic solvents following photoexcitation using 400 nm laser pulses. An ultrafast component representing the solvent-induced vibrational energy relaxation (VER) process has been associated with the dynamics of the S(1) state of fluorenone in all kinds of solvents. However, in protic solvents, in addition to the VER process, further evolution of the spectroscopic and dynamical properties of the S(1) state have been observed because of repositioning of the hydrogen bonds around the carbonyl group. In normal alcohols, two different kinds of hydrogen-bonded complex of the fluorenone-alcohol system with different orientations of the hydrogen bond with respect to the carbonyl group and the molecular plane of fluorenone have been predicted. On the other hand, in TFE, formation of only one kind of hydrogen-bonded complex has been observed. These observations have been supported by theoretical calculations of the geometries of the hydrogen-bonded complexes in the ground and the excited states of fluorenone. Linear correlation between the lifetimes of the equilibration process occurring because of repositioning of the hydrogen bonds and Debye or longitudinal relaxation times of the normal alcoholic solvents establish the fact that, in weakly hydrogen bond donating solvents, the hydrogen bond dynamics can be described as merely a solvation process. Whereas, in TFE, hydrogen bond dynamics is better described by a process of conversion between two distinct excited states, namely, the non-hydrogen-bonded form and the hydrogen-bonded complex.

Electron injection and back-electron transfer (BET) dynamics of triphenylmethane (TPM) dyes adsorbed on TiO 2 nanoparticles have been studied by fast and ultrafast pump-probe spectroscopy in the subpico-to microsecond time domain. TPM dyes form charge-transfer complex with TiO 2 nanoparticles as they get adsorbed on the surface. Among the three TPM dyes, pyrogallol red (PGR) and bromo-pyrogallol red (Br-PGR) have higher electronic coupling to or interaction with TiO 2 nanoparticles compared to aurin tricarboxylic acid (ATC). Excitation of the dyes adsorbed on the TiO 2 nanoparticle surface leads to electron injection into the conduction band of TiO 2 . Electron injection has been confirmed by direct detection of an electron in the conduction band, a cation radical of the adsorbed dye, and a bleach of the dye in real time as monitored by transient absorption spectroscopy in the visible and near-IR regions. The dynamics of BET from TiO 2 to the parent cation have been measured by monitoring the recovery kinetics of the bleach of the adsorbed dye. BET dynamics have been found to be multiexponential, and it is extremely fast for the strong coupling dyes (PGR and Br-PGR). The majority of the injected electrons are found to come back to the parent cation with a time constant of ∼2 ps. BET dynamics have been compared for the strong (PGR) and moderate (ATC) coupling dyes in the microsecond time domain. Intensity dependence experiments show BET reactions for PGR-and Br-PGR-sensitized TiO 2 nanoparticles follow a first-order ET rate, where as ATC-sensitized TiO 2 nanoparticles follow a second-order ET rate. It has been observed that for strongly coupled dyes, BET reaction occurs much faster than that for weakly coupled dyes. Coupling elements for the BET processes from TiO 2 nanoparticles to the parent cation have been determined for shallow and deep trap electrons in the above systems.

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.