Marcin Ziółek

An ultrafast excited state intramolecular proton transfer (ESPIT) and photochromism of salicylideneaniline (SA) and its “double” analogue salicylaldehyde azine (SAA). A controversial case

Two simple, structurally related photochromic Schiff bases, salicylideneaniline (SA) and salicylaldehyde azine (SAA) were studied in femto– and picosecond time domains. In both systems an ultrafast excited state intramolecular proton transfer (ESIPT) reaction was stated with the characteristic time below 50 fs. For SA this result is in contrast to the recent data published by Mitra and Tamai (S. Mitra and N. Tamai, Chem. Phys. Lett., 1998, 282, 391; S. Mitra and N. Tamai, Chem. Phys., 1999, 246, 463; S. Mitra and N. Tamai, Phys. Chem. Chem. Phys., 2003, 5, 4647), reporting on the corresponding time as long as 200–300 fs. The kinetics of decay of keto-tautomers in S1 states was followed by the transient absorption (410 nm and 470 nm for SA and SAA, respectively) and stimulated emission bands. About 10–30% of excited molecules give birth to the long-lived ground states of photochromic forms.

Excited state proton transfer and photochromism of an aromatic Schiff base. Pico- and femtosecond kinetics of the N, N'-bis(salicylidene)-p-phenylenediamine (BSP)

Abstract The results of time-resolved pico- and femtosecond absorption and emission study performed for the title photochromic Schiff base, (BSP) are presented. Transient absorption spectra of intermediates, appearing in the excited state intramolecular proton transfer (ESIPT) were identified. A full scheme of deactivation of the excited BSP molecule, including the enol-, keto- and photochromic tautomers, was proposed. In particular, the characteristic time of the ESIPT process was determined as k K − PC =2.8×10 10 s −1 .

Comparison of TiO2 and ZnO Solar Cells Sensitized with an Indoline Dye: Time-Resolved Laser Spectroscopy Studies of Partial Charge Separation Processes

Time-resolved laser spectroscopy techniques in the time range from femtoseconds to seconds were applied to investigate the charge separation processes in complete dye-sensitized solar cells (DSC) made with iodide/iodine liquid electrolyte and indoline dye D149 interacting with TiO2 or ZnO nanoparticles. The aim of the studies was to explain the differences in the photocurrents of the cells (3-4 times higher for TiO2 than for ZnO ones). Electrochemical impedance spectroscopy and nanosecond flash photolysis studies revealed that the better performance of TiO2 samples is not due to the charge collection and dye regeneration processes. Femtosecond transient absorption results indicated that after first 100 ps the number of photoinduced electrons in the semiconductor is 3 times higher for TiO2 than for ZnO solar cells. Picosecond emission studies showed that the lifetime of the D149 excited state is about 3 times longer for ZnO than for TiO2 samples. Therefore, the results indicate that lower performance of ZnO solar cells is likely due to slower electron injection. The studies show how to correlate the laser spectroscopy methodology with global parameters of the solar cells and should help in better understanding of the behavior of alternative materials for porous electrodes for DSC and related devices.

Mechanism and deactivation kinetics of S2-xanthione in acetonitrile, a quenching solvent, and of S2-exciplex measured by pico- and femtosecond laser spectroscopy

Abstract The Letter discusses the mechanism and dynamics of intra- and intermolecular electronic relaxation of the S 2 state of a xanthione molecule in acetonitrile, a quenching solvent, as well as the most important properties of the S 2 -exciplex formed upon the quenching process. Despite very efficient quenching of S 2 -XT and other thiones it has not been possible so far to observe the S 2 -exciplex. Therefore transient absorption system with 120 fs resolution was successfully applied for this purpose: the S 2 -exciplex was identified for the first time, its lifetime (∼30 ps) and transient absorption spectrum were determined as well as the mechanism of its formation and decay. Moreover, the authors observed the presence of a very fast intramolecular electronic relaxation process (S 2 v >0 →S 0 ) during S 2 -state deactivation which competes with intramolecular vibrational redistribution in the S 2 -state.

Internal electrostatic control of the primary charge separation and recombination in reaction centers from Rhodobacter sphaeroides revealed by femtosecond transient absorption.

We report the observation of two conformational states of closed RCs from Rhodobacter sphaeroides characterized by different P(+)H(A)(-) --> PH(A) charge recombination lifetimes, one of which is of subnanosecond value (700 +/- 200 ps). These states are also characterized by different primary charge separation lifetimes. It is proposed that the distinct conformations are related to two protonation states either of reduced secondary electron acceptor, Q(A)(-), or of a titratable amino acid residue localized near Q(A). The reaction centers in the protonated state are characterized by faster charge separation and slower charge recombination when compared to those in the unprotonated state. Both effects are explained in terms of the model assuming modulation of the free energy level of the state P(+)H(A)(-) by the charges on or near Q(A) and decay of the P(+)H(A)(-) state via the thermally activated P(+)B(A)(-) state.

Photochemistry and Photophysics in Silica-Based Materials: Ultrafast and Single Molecule Spectroscopy Observation

Silica-based materials (SBMs) are widely used in catalysis, photonics, and drug delivery. Their pores and cavities act as hosts of diverse guests ranging from classical dyes to drugs and quantum dots, allowing changes in the photochemical behavior of the confined guests. The heterogeneity of the guest populations as well as the confinement provided by these hosts affect the behavior of the formed hybrid materials. As a consequence, the observed reaction dynamics becomes significantly different and complex. Studying their photobehavior requires advanced laser-based spectroscopy and microscopy techniques as well as computational methods. Thanks to the development of ultrafast (spectroscopy and imaging) tools, we are witnessing an increasing interest of the scientific community to explore the intimate photobehavior of these composites. Here, we review the recent theoretical and ultrafast experimental studies of their photodynamics and discuss the results in comparison to those in homogeneous media. The discussion of the confined dynamics includes solvation and intra- and intermolecular proton-, electron-, and energy transfer events of the guest within the SBMs. Several examples of applications in photocatalysis, (photo)sensors, photonics, photovoltaics, and drug delivery demonstrate the vast potential of the SBMs in modern science and technology.

Carbazole Dye-Sensitized Solar Cells Studied from Femtoseconds to Seconds—Effect of Additives in Cobalt- and Iodide-Based Electrolytes

Comprehensive studies of all charge-separation processes in efficient carbazole dye-sensitized solar cells are correlated with their photovoltaic parameters. An important role of partial, fast electron recombination from the semiconductor nanoparticles to the oxidized dye is revealed; this takes place on the picosecond and sub-nanosecond timescales. The charge-transfer dynamics in cobalt tris(bipyridyl) based electrolytes and iodide-based electrolyte is observed to depend on potential-determining additives in a similar way. Upon addition of 0.5 M 4-tert-butylpiridine to both types of electrolytes, the stability of the cells is greatly improved; the cell photovoltage increases by 150-200 mV, the electron injection rate decreases about five times (from 5 to 1 ps(-1) ), and fast recombination slows down about two to three times. Dye regeneration proceeds at a rate of about 1 μs(-1) in all electrolytes. Electron recombination from titania to cobalt electrolytes is much faster than that to iodide ones.

Optimization of absorption bands of dye-sensitized and perovskite tandem solar cells based on loss-in-potential values

A numerical study of optimal bandgaps of light absorbers in tandem solar cell configurations is presented with the main focus on dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs). The limits in efficiency and the expected improvements of tandem structures are investigated as a function of total loss-in-potential (V(L)), incident photon to current efficiency (IPCE) and fill factor (FF) of individual components. It is shown that the optimal absorption onsets are significantly smaller than those derived for multi-junction devices. For example, for double-cell devices the onsets are at around 660 nm and 930 nm for DSSCs with iodide based electrolytes and at around 720 nm and 1100 nm for both DSSCs with cobalt based electrolytes and PSCs. Such configurations can increase the total sunlight conversion efficiency by about 35% in comparison to single-cell devices of the same VL, IPCE and FF. The relevance of such studies for tandem n-p DSSCs and for a proposed new configuration for PSCs is discussed. In particular, it is shown that maximum total losses of 1.7 V for DSSCs and 1.4 V for tandem PSCs are necessary to give any efficiency improvement with respect to the single bandgap device. This means, for example, a tandem n-p DSSC with TiO2 and NiO porous electrodes will hardly work better than the champion single DSSC. A source code of the program used for calculations is also provided.

Factors Affecting the Performance of Champion Silyl-Anchor Carbazole Dye Revealed in the Femtosecond to Second Studies of Complete ADEKA-1 Sensitized Solar Cells

Record laboratory efficiencies of dye-sensitized solar cells have been recently reported using an alkoxysilyl-anchor dye, ADEKA-1 (over 14 %). In this work we use time-resolved techniques to study the impact of key preparation factors (dye synthesis route, addition of co-adsorbent, use of cobalt-based electrolytes of different redox potential, creation of insulating Al2 O3 layers and molecule capping passivation of the electrode) on the partial charge separation efficiencies in ADEKA-1 solar cells. We have observed that unwanted fast recombination of electrons from titania to the dye, probably associated with the orientation of the dyes on the titania surface, plays a crucial role in the performance of the cells. This recombination, taking place on the sub-ns and ns time scales, is suppressed in the optimized dye synthesis methods and upon addition of the co-adsorbent. Capping treatment significantly reduces the charge recombination between titania and electrolyte, improving the electron lifetime from tens of ms to hundreds of ms, or even to single seconds. Similar increase in electron lifetime is observed for homogenous Al2 O3 over-layers on titania nanoparticles, however, in this case the total solar cells photocurrent is decreased due to smaller electron injection yield from the dye. Our studies should be important for a broader use of very promising silyl-anchor dyes and the further optimization and development of dye-sensitized solar cells.

Temperature influence on deactivation paths and tautomeric equilibrium of some photochromic Schiff bases studied by time-resolved and stationary spectroscopy

The effect of temperature on spectroscopic properties (stationary and time-resolved) of selected compounds belonging to the salicylideneaniline and hydroquinone families of photochromic Schiff bases is studied. The nonradiative decay of the excited keto tautomer consists of two components: the temperature-independent one (internal conversion, dominant below 200 K) and the temperature-dependent one (assigned to structural changes). The parameters of these components are remarkably different in the two families of the photochromic compounds. The influence of the hydrogen bonding ability of the solvent on the tautomeric equilibrium between the enol and keto forms indicates changes in the order of the ground state energy levels when going from methanol to trifluoroethanol.