A. Kathiravan

Photoinduced Charge Carrier Dynamics of Zn−Porphyrin−TiO2 Electrodes: The Key Role of Charge Recombination for Solar Cell Performance

Time resolved absorption spectroscopy has been used to study photoinduced electron injection and charge recombination in Zn-porphyrin sensitized nanostructured TiO(2) electrodes. The electron transfer dynamics is correlated to the performance of dye sensitized solar cells based on the same electrodes. We find that the dye/semiconductor binding can be described with a heterogeneous geometry where the Zn-porphyrin molecules are attached to the TiO(2) surface with a distribution of tilt angles. The binding angle determines the porphyrin-semiconductor electron transfer distance and charge transfer occurs through space, rather than through the bridge connecting the porphyrin to the surface. For short sensitization times (1 h), there is a direct correlation between solar cell efficiency and amplitude of the kinetic component due to long-lived conduction band electrons, once variations in light harvesting (surface coverage) have been taken into account. Long sensitization time (12 h) results in decreased solar cell efficiency because of decreased efficiency of electron injection.

Spectroscopic studies on the interaction of colloidal capped CdS nanoparticles with bovine serum albumin

Colloidal uncapped and starch capped CdS (SCdS) nanoparticles were prepared and interaction with bovine serum albumin (BSA) have been studied by UV-visible, FT-IR, steady state, time resolved and synchronous fluorescence spectroscopic measurements. BSA molecules adsorbed on the surface of colloidal CdS through the capping agent. The apparent association constant (K(app)=2.54x10(2) M(-1)) and degree of association has been calculated (alpha=1.12) from absorption studies. The binding constant from fluorescence quenching method (6.6x10(2) M(-1)) matches well with that determined from the absorption spectral changes. Static quenching mechanism and conformational changes on BSA molecules were confirmed by time resolved and synchronous fluorescence measurements respectively. The effect of starch capped CdS on the conformation of BSA has been analyzed by means of UV-visible absorption and synchronous fluorescence spectra.

Effect of anchoring group on the photosensitization of colloidal TiO2 nanoparticles with porphyrins

The interaction of porphyrins (TCPP, TSPP, TMeOPP and TPP) with colloidal TiO(2) nanoparticles was studied by infra-red, UV-visible absorption, steady state and time resolved fluorescence spectroscopy. The porphyrins (TCPP, TSPP and TMeOPP) adsorbed on the surface of colloidal TiO(2) nanoparticles through electrostatic interaction. According to absorption and fluorescence changes the apparent association constants (K(app)) were calculated. However, TPP did not interact with surface of colloidal TiO(2) nanoparticles due to the absence of anchoring group. The fluorescence quenching is attributable mainly to electron transfer from the excited state porphyrins to the conduction band of colloidal TiO(2). The rate of electron transfer process (k(et)) was calculated using lifetime measurement. The electron transfer mechanism has been proved by the calculation of free energy change (DeltaG(et)) by applying Rehm-Weller equation. Using all the spectroscopic measurements we confirmed that the presence of anchoring group plays major role in the adsorption as well as the electron transfer processes.

The interaction of sonochemically synthesized gold nanoparticles with serum albumins

Spherical gold nanoparticles of approximately 16nm were synthesized using a sonochemical reduction method and characterized using UV-vis spectroscopy, atomic force microscopy (AFM) and transmission electron microscopy (TEM). The binding of these gold nanoparticles with bovine serum albumin (BSA) and human serum albumin (HSA) was investigated using UV-vis absorption and fluorescence spectroscopic techniques. A strong quenching of the fluorescence from serum albumins was observed due to the formation of a ground state complex with gold nanoparticles (static quenching). The fluorescence quenching constants, number of binding sites and binding constants were determined using Stern-Volmer and Benesi-Hildebrand plots. Using Forster Resonance Energy Transfer (FRET) theory, the distance between the donor (serum albumins) and acceptor (gold nanoparticles) was obtained, which showed that HSA has more affinity towards sonochemically synthesized gold nanoparticles compared to gold nanoparticles synthesized using other methods.

Pyrene Schiff base: photophysics, aggregation induced emission, and antimicrobial properties.

Pyrene containing Schiff base molecule, namely 4-[(pyren-1-ylmethylene)amino]phenol (KB-1), was successfully synthesized and well characterized by using (1)H, (13)C NMR, FT-IR, and EI-MS spectrometry. UV-visible absorption, steady-state fluorescence, time-resolved fluorescence, and transient absorption spectroscopic techniques have been employed to elucidate the photophysical processes of KB-1. It has been demonstrated that the absorption characteristics of KB-1 have been bathochromatically tuned to the visible region by extending the π-conjugation. The extended π-conjugation is evidently confirmed by DFT calculations and reveals that π→π* transition is the major factor responsible for electronic absorption of KB-1. The photophysical property of KB-1 was carefully examined in different organic solvents at different concentrations and the results show that the fluorescence of this molecule is completely quenched due to photoinduced electron transfer. Intriguingly, the fluorescence intensity of KB-1 increases enormously by the gradual addition of water up to 90% with concomitant increase in fluorescence lifetime. This clearly signifies that this molecule has aggregation-induced emission (AIE) property. The mechanism of AIE of this molecule is suppression of photoinduced electron transfer (PET) due to hydrogen bonding interaction of imine donor with water. A direct evidence of PET process has been presented by using nanosecond transient absorption measurements. Further, KB-1 was successfully used for antimicrobial and bioimaging studies. The antimicrobial studies were carried out through disc diffusion method. KB-1 is used against both Gram-positive (Rhodococcus rhodochrous and Staphylococcus aureus) and Gram-negative (Escherichia coli and Pseudomonas aeruginosa) bacterial species and also fungal species (Candida albicans). The result shows KB-1 can act as an excellent antimicrobial agent and as a photolabeling agent. S. aureus, P. aeruginosa, and C. albicans were found to be the most susceptible microorganisms at 1 mM concentration among the bacteria used in the present investigation.

Photosensitization of colloidal TiO2 nanoparticles with phycocyanin pigment.

Bluish phycocyanin is a water soluble pigment having strong visible light absorption at 615 nm. The dynamics of photoinduced electron injection from phycocyanin to colloidal TiO(2) nanoparticles has been studied by absorption, FT-IR, steady state and time-resolved fluorescence spectroscopic methods. The phycocyanin adsorbed on the surface of colloidal TiO(2) nanoparticles, the apparent association constant (K(app)) for the association between colloidal TiO(2) nanoparticles and phycocyanin was measured from both absorption changes (K(app)=4.01x10(2) M(-1)) and fluorescence quenching data (K(app)=5.20x10(2) M(-1)). The value of K(app) obtained from the data of fluorescence quenching matches with that determined from the absorption spectral changes. The good agreement between these values of K(app) highlighted the validity of assumption proposed for the association between phycocyanin and colloidal TiO(2) nanoparticles. The free energy change (DeltaG(et)) for electron injection has been calculated by applying Rehm-Weller equation. Electron injection from excited state phycocyanin into the conduction band of TiO(2) is suggested.

An investigation on electron transfer quenching of zinc(II) meso-tetraphenylporphyrin (ZnTPP) by colloidal TiO2

The interaction of zinc(II) meso-tetraphenylporphyrin (ZnTPP) with colloidal TiO(2) was studied by absorption, steady state and time-resolved fluorescence spectroscopy. The quenching was found to obey the Stern-Volmer equation and the corresponding Stern-Volmer plots were linear in the range of quencher concentration used 0-5 x 10(-4)M. The bimolecular quenching rate constants (k(q)) were 20.5 x 10(10)M(-1)s(-1) (steady-state) and 2.85 x 10(10)M(-1)s(-1) (time resolved). The quenching process is suggested to involve electron transfer from the ZnTPP to TiO(2) considering the experimental evidences obtained.

Photoinduced electron transfer from phycoerythrin to colloidal metal semiconductor nanoparticles

Phycoerythrin is a water soluble pigment which absorbs in the visible region at 563 nm. The interaction of phycoerythrin with colloidal metal semiconductors was studied by absorption, FT-IR and fluorescence spectroscopy. Phycoerythrin adsorbed strongly on the surface of TiO(2) nanoparticles, the apparent association constant for the association between colloidal metal-TiO(2) nanoparticles and phycoerythrin was determined from fluorescence quenching data. The free energy change (DeltaG(et)) for electron transfer process has been calculated by applying Rehm-Weller equation.

Photoinduced interaction between riboflavin and TiO2 colloid

The adsorption of riboflavin on the surface of TiO(2) colloidal particles and the electron transfer process from its singlet excited state to the conduction band of TiO(2) were examined by absorption and fluorescence quenching measurements. The apparent association constants (K(app)) were determined. The quenching mechanism is discussed involving electron transfer from riboflavin to TiO(2).

Control of the size and shape of TiO2 nanoparticles in restricted media

Template-capped TiO2 nanostructures have been synthesized. In certain template conditions, TiO2 hexagons are found to form. These hexagonal structures can be effectively sensitized by fluorescein dye without any change in the protonation state of the dye. Bare TiO2 nanoparticles are not so useful for sensitization with dyes like fluorescein as they alter the dye protonation state. The novelty of this work is twofold-the hitherto elusive hexagonal phase of TiO2 nanoparticles has been stabilized and the synthesis of TiO2 in the rutile phase has been achieved under mild conditions.