Active and passive control of zinc phthalocyanine photodynamics

In this work we report on the ultrafast photodynamics of the photosensitizer zinc phthalocyanine (ZnPc) and manipulation thereof. Two approaches are followed: active control via pulse shaping and passive control via strategic manipulation in the periphery of the molecular structure. The objective of both of these control experiments is the same: to enhance the yield of the functional pathway and to minimize loss channels. The aim of the active control experiments is to increase the intersystem crossing yield in ZnPc, which is important for application in photodynamic therapy (PDT). Pulse shaping allowed an improvement in triplet to singlet ratio of 15% as compared to a transform-limited pulse. This effect is ascribed to a control mechanism that utilizes multiphoton pathways to higher-lying states from where intersystem crossing is more likely to occur. The passive control experiments are performed on ZnPc derivatives deposited onto TiO2, serving as a model system of a dye-sensitized solar cell (DSSC). Modification of the anchoring ligand of the molecular structure resulted in an increased rate for electron injection into TiO2 and slower back electron transfer, improving the DSSC efficiency

Silver Nanocubes Coated in Ceria:Core/Shell Size Effects on Light-Induced Charge Transfer

Plasmonic sensitization of semiconductors is an attractive approach to increase light-induced photocatalytic performance; one method is to use plasmonic nanostructures in core@shell geometry. The occurrence and mechanism of synergetic effects in photocatalysis of such geometries are under intense debate and proposed to occur either through light-induced charge transfer (CT) or through thermal effects. This study focuses on the relation between the dimensions of Ag@CeO2 nanocubes, the wavelength-dependent efficiency, and the mechanism of light-induced direct CT. A 4-mercaptobenzoic acid (4-MBA) linker between core and shell acts as a Raman probe for CT. For all Ag@CeO2 nanocubes, CT increases with decreasing excitation wavelength, with notable increase at and below 514 nm. This is fully explainable by CT from silver to the 4-MBA LUMO, with the increase for excitation wavelengths that exceed the Ag/4-MBA LUMO gap of 2.28 eV (543 nm). A second general trend observed is an increase in CT yield with ceria shell thickness, which is assigned to relaxation of the excited electron further into the ceria conduction band, potentially producing defects

Functionality of epidermal melanin pigments: current knowledge on UV-dissipative mechanisms and research perspectives.

So far it is not known whether epidermal melanins are solely photoprotective or also phototoxic. Also largely unknown are underlying UV-induced mechanisms and impact of the chemical structure. This perspective will focus on the current insights into the UV-dissipative mechanisms in melanin model systems and the implications for the in vivo pigments. We will also identify future research perspectives towards a full understanding of the functionality of epidermal pigments

Photophysical Study on the Effect of the External Potential on NiO-Based Photocathodes

In present study we investigate the effect of the external potential on a dye-sensitized NiO photocathode on the light-induced charge carrier dynamics by time-resolved photoluminescence and femtosecond transient absorption spectroscopy under operating conditions. Instead of the anticipated acceleration of photoinduced hole injection from dye into NiO at more negative applied potential, we observe that both hole injection and charge recombination are slowed down. We assign this effect to a variation in OH- ion concentration in the inner Helmholtz plane (IHP) of the electrochemical double layer with applied potential, showing that ion adsorption and desorption onto the NiO surface play an essential role as a relay in light-induced charge transfer and recombination. Our work highlights the key role of ions at the electrode surface and in the electrolyte in the realization of efficient solar to fuel devices

Manipulating charge separation dynamics of zinc phthalocyanine based TiO2 films through asymmetrical push-pull structures

Zinc phthalocyanine (ZnPc) based dye-sensitized solar cells (DSSCs) that exhibit light absorption in the near IR and excellent chemical stability show great potential for efficient light harvesting and light to electrical energy conversion. However, they have exhibited poor device efficiencies (< 1%) for a long time [1]. Only recently efficiencies up to ∼5% have been realized [2,3]. An important tool in the development towards higher efficiencies involves replacing a symmetric phthalocyanine by an asymmetric equivalent [3] with one electron pulling group (anchoring onto the TiO2) and three electron pushing groups (Fig. 1(a)). This clearly suggests that such an asymmetrical push-pull structure supports light-induced forward electron transfer (from the ZnPc core into the TiO2) and prevents back electron transfer. However, ultrafast photodynamics studies have not been reported so far