Probing the Highly Efficient Electron Transfer Dynamics between Zinc Protoporphyrin IX and Sodium Titanate Nanosheets

Sodium titanate nanosheets (NaTiO₂ NS) have been prepared by a new method and completely characterized by TEM, SEM, XRD, EDX, and XPS techniques. The sensitization of nanosheets is carried out with Zn protoporphyrin IX (ZnPPIX). The emission intensity of ZnPPIX is quenched by NaTiO₂ NS, and the dominant process for this quenching has been attributed to the process of photoinduced electron injection from excited ZnPPIX to the nanosheets. Time resolved fluorescence measurement was used to elucidate the process of electron injection from the singlet state of ZnPPIX to the conduction band of NaTiO₂ NS. Electron injection from the dye to the semiconductor is very fast (<i>k</i>_et ≈ 10¹¹ s^–1), much faster than previously reported rates. The large two-dimensional surface offered by the NaTiO₂ NS for interaction with the dye and the favorable driving force for electron injection from ZnPPIX to NaTiO₂ NS (Δ<i>G</i>_inj = −0.66 V) are the two important factors responsible for such efficient electron injection. Thus, NaTiO₂ NS can serve as an effective alternative to the use of TiO₂ nanoparticles in dye sensitized solar cells (DSSCs)

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 ¹H, ¹³C 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 (<i>Rhodococcus rhodochrous</i> and <i>Staphylococcus aureus</i>) and Gram-negative (<i>Escherichia coli</i> and <i>Pseudomonas aeruginosa</i>) bacterial species and also fungal species (<i>Candida albicans</i>). The result shows KB-1 can act as an excellent antimicrobial agent and as a photolabeling agent. <i>S. aureus</i>, <i>P. aeruginosa</i>, and <i>C. albicans</i> were found to be the most susceptible microorganisms at 1 mM concentration among the bacteria used in the present investigation

Role of Adsorption Structures of Zn-Porphyrin on TiO₂ in Dye-Sensitized Solar Cells Studied by Sum Frequency Generation Vibrational Spectroscopy and Ultrafast Spectroscopy

Several Zn-porphyrin (ZnP) derivatives were designed to build highly efficient dye-sensitized solar cells (DSC). It was found that solar cell efficiencies normalized for surface coverage (η_rel) are affected by the molecular spacer connecting the porphyrin core to the TiO₂ surface, the sensitization conditions (solvent and time), and, to a lesser extent, the nature of the terminal group of the ZnP. Ultrafast transient absorption spectroscopy shows that electron transfer rates are strongly dependent on spacer and sensitization conditions. To understand this behavior at a molecular level, surface-sensitive vibrational spectroscopy, sum frequency generation (SFG), has been employed to investigate the adsorption geometries of these ZnP derivatives on the TiO₂ surface for the first time. The average tilt angles and adsorption ordering of the ZnP molecules on the TiO₂ surface were measured. A simple linear correlation between adsorption geometry of the adsorbed ZnP molecules, η_rel, and the concentration of long-lived electrons in the conduction band of TiO₂ was shown to exist. The more perpendicular the orientation of the adsorbed ZnP (relative to the TiO₂ surface), the higher the concentration of long-lived electrons in the conduction band, which contributes to the increase of photocurrent and solar cell efficiency. This result indicates that the electron transfer between ZnP and TiO₂ occurs “through-space” rather than “through the molecular spacer”. It is also revealed that the sensitization solvent (methanol) may affect adsorption geometry and adsorption ordering through coadsorption and modify the electron transfer dynamics and consequently solar cell efficiency. Aggregation effects, which were observed for the longer sensitization times, are also discussed in relation to adsorption geometry and radiationless quenching processes. With the work reported here we demonstrate a novel strategy for DSC material characterization that can lead to design and manufacturing of photoactive materials with predictable and controlled properties