Daniel Pelczarski

Detailed characterization of the lowest energy electronic excited states for dimethyl-acridine diphenyl-sulfone derivative (DMAC-DPS) in solid neat films has been performed by electroabsorption (EA) spectroscopy. The EA spectra were parametrized in the framework of Liptay theory, providing changes in the dipole moment and electronic polarizability. The extracted values of the corresponding parameters in the lowest energy optical absorption bands were assigned to molecular dipoles upon optical excitation associated with the intramolecular charge transfer (CT) from the electron-donating part (DMAC) to the electron-accepting part (DPS) of the molecule. The standard treatment of EA spectra based on Liptay theory was successfully reinterpreted applying the time-dependent density functional (TDDFT) theory, without additional assumptions regarding the shape of the EA signal, characteristic of the Liptay formalism. To correctly describe the EA spectrum, the TDDFT calculations have to be performed not for a single molecule, but for a dimer structure, established by the Monte Carlo (MC) calculations. Although the main EA features are ascribed to the excited states of intramolecular CT origin, the lowest energy excited states gain oscillator strengths due to the geometry distortion of the ground state or excited state interaction introduced by the dimer structure. The implications of these results for the first singlet excited state (S1) are discussed in the context of the thermally activated delayed fluorescence (TADF) emitters employed currently in organic light emitting diodes (OLEDs).

Adsorption of mononuclear tris(bipyridine) ruthenium(II) complexes and binuclear tris(bipyridine) ruthenium(II) complexes equipped with carboxyl groups (-COOH) on the (111) surface of TiO₂ crystal in anatase form was modeled using Monte Carlo simulations, applying the Universal force field. It was shown that the adsorption efficiency of the ruthenium-based dyes on the TiO₂ surface depends on the position of the anchoring -COOH group in the molecular structure. The increase in the number of possible anchor groups in the dyes increases their ability to deposit on the surface of semiconductors. The chemisorbed molecules, such as mononuclear tris(bipyridine) ruthenium(II) complexes with the -COOH group in para position (RuLp) and binuclear tris(bipyridine) ruthenium(II) complexes called B3 with two anchoring -COOH groups and phenyl in the spacer, interact with the adsorber and other neighboring dyes, changing their electron and optical properties. The obtained computational results help to explain the behavior of the dyes on the TiO₂ surface, giving impact on their DSSC applications.

Comprehensive characterization of the lowest energy electronic excited states for mono- and binuclear Ru(II) complexes containing bipyridine ligands has been performed by electroabsorption (EA) spectroscopy. The EA spectra of Ru complexes sensitizing a TiO₂ semiconductor were compared with the spectra of these complexes in the form of solid neat films, both of which parametrized within the Liptay theory. The extracted values of relevant parameters, relating to molecular dipoles after optical MLCT (metal-to-ligand-charge-transfer) excitation, exhibit a clearly noticeable increase for Ru complexes adsorbed on TiO₂, but they are too small to be attributed to excitation associated with the direct transfer of an electron from the dye adsorbate to the TiO₂ semiconductor. Due to the difficulties arising from standard analysis based on Liptay formalism, we have for the first time successfully reproduced the EA spectra of the Ru/TiO₂ systems using the time-dependent density functional (TDDFT) calculations, incorporating into the Hamiltonian a term describing the interaction of a molecule with the local electric field it experiences from the TiO₂ structure and the neighboring Ru complex molecules, without additional assumptions about the lineshape of the EA signal. The implications of these results are briefly discussed in the context of dye-sensitized solar cells.

We present the electric field-induced absorption (electroabsorption, EA) spectra of the solid neat films of tris(bipyridine) Ru(II) complexes, which were recently functionalized in our group as photosensitizers in dye-sensitized solar cells, and we compare them with the results obtained for an archetypal [Ru(bpy)₃]²⁺ ion (RBY). We argue that it is difficult to establish a unique set of molecular parameter values by discrete parametrization of the EA spectra under the Liptay formalism for non-degenerate excited states. Therefore, the experimental EA spectra are compared with the spectra computed by the TDDFT (time-dependent density-functional theory) method, which for the first time explains the mechanism of electroabsorption in tris(bipyridine) Ru complexes without any additional assumptions about the spectral lineshape of the EA signal. We have shown that the main EA feature, in a form close to the absorption second derivative observed in the spectral range of the first MLCT (metal-to-ligand charge transfer) absorption band in Ru(bpy)₃(PF₆)₂, can be attributed to a delocalized and orbitally degenerate excited state. This result may have key implications for the EA mechanism in RBY-based systems that exhibit similar EA spectra due to the robust nature of MLCT electronic states in such systems.

Among various methods of improving visible light activity of titanium(IV) oxide, the formation of defects and vacancies (both oxygen and titanium) in the crystal structure of TiO₂ is an easy and relatively cheap alternative to improve the photocatalytic activity. In the presented work, visible light active defective TiO₂ was obtained by the hydrothermal reaction in the presence of three different oxidizing agents: HIO₃, H₂O₂, and HNO₃. Further study on the effect of used oxidant and calcination temperature on the physicochemical and photocatalytic properties of defective TiO₂ was performed. Obtained nanostructures were characterized by X-ray diffractometry (XRD), specific surface area (BET) measurements, UV-Vis diffuse reflectance spectroscopy (DR-UV/Vis), photoluminescence spectroscopy (PL), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) spectroscopy. Degradation of phenol as a model pollutant was measured in the range of UV-Vis and Vis irradiation, demonstrating a significant increase of photocatalytic activity of defective TiO₂ samples above 420 nm, comparing to non-defected TiO₂. Correlation of EPR, UV-Vis, PL, and photodegradation results revealed that the optimum concentration of HIO₃ to achieve high photocatalytic activity was in the range of 20–50 mol%. Above that dosage, titanium vacancies amount is too high, and the obtained materials’ photoactivity was significantly decreased. Studies on the photocatalytic mechanism using defective TiO₂ have also shown that ^•O₂⁻ radical is mainly responsible for pollutant degradation.

Hybrid materials of conjugated polymer and titanium(IV) oxide have attracted considerable attention concerning their potential benefits, including (i) efficient exploitation of visible light, (ii) a high adsorption capacity for organic contaminants, (iii) and effective charge carriers separation. The new class of the photocatalysts is promising for the removal of environmental pollutants in both aqueous and gaseous phases. For the first time, in this study, the polyaniline (PANI)–TiO₂ hybrid composite was used for the degradation of phenol in water and toluene in the gas phase. Polyaniline–TiO₂ was prepared by the in situ polymerization of aniline on the TiO₂ surface. The obtained hybrid material was characterized by diffuse reflectance spectroscopy (DR/UV-Vis), X-ray diffraction (XRD), fast-Fourier transformation spectroscopy (FTIR), photoluminescence (PL) spectroscopy, microscopy analysis (SEM/TEM), and thermogravimetric analysis (TGA). An insight into the mechanism was shown based on the photodegradation analysis of charge carrier scavengers. Polyaniline is an efficient TiO₂ photosensitizer for photodegradation in visible light (λ > 420 nm). The trapping experiments revealed that mainly h+ and ˙OH were the reactive oxygen species that were responsible for phenol degradation. Furthermore, the PANI–TiO₂ hybrid nanocomposite was used in gypsum plaster to study the self-cleaning properties of the obtained building material. The effect of PANI–TiO₂ content on the hydrophilic/hydrophobic properties and crystallographic structure of gypsum was studied. The obtained PANI–TiO₂-modified gypsum plaster had improved photocatalytic activity in the reaction of toluene degradation under Vis light.

The singlet exciton diffusion lengths are determined by the photoconductivity as well as the luminescence surface quenching technique, in vacuum‐evaporated layers of (4,4′,4″‐tris(N‐(3‐methylphenyl)‐N‐phenylylamino) triphenylamine) (m‐MTDATA), 4,4′,4″‐tris[2‐naphthyl(phenyl)amino] triphenylamine (2TNATA), and N,N′‐diphenyl‐N,N′‐bis(3‐methylphenyl)‐[1,1′‐biphenyl]‐4,4′‐diamine (TPD) which are frequently used for fabrication of electroluminescent and photovoltaic devices. The values are found to be as high as (30 ± 10) nm in 2TNATA and m‐MTDATA, as well as (46 ± 9) nm in TPD films. The corresponding singlet diffusion coefficients of the range between 1 × 10⁻³and 1 × 10⁻² cm² s⁻¹, according to a comprehensive study undertaken, are assigned to a sizable electronic coupling induced by strong interactions of intra‐ and inter‐molecular origin in the investigated materials.