Effect of electron withdrawing anchoring groups on the optoelectronic properties of pyrene sensitizers and their interaction with TiO2: A combined experimental and theoretical approach

Can a Small Change in the Heterocyclic Substituent Significantly Impact the Physicochemical and Biological Properties of (Z)-2-(5-Benzylidene-4-oxo-2-thioxothiazolidin-3-yl)acetic Acid Derivatives?

Rhodanine-3-acetic acid derivatives are attractive compounds with versatile effects. What is very important is that compounds of this type have many biological properties. They are tested, among others, as fluorescent probes for bioimaging and aldose reductase inhibitors. Rhodanine-3-acetic acid derivatives also have antibacterial, antifungal and anticancer activity. The presented work demonstrates that a slight change in the five-membered heterocyclic substituent significantly affects the properties of the compounds under consideration. Three rhodanine-3-acetic acid derivatives (A-1-A-3) were obtained in the Knoevenagel condensation reaction with good yields, ranging from 54% to 71%. High thermal stability of the tested compounds was also demonstrated above 240 °C. The absorption and emission maxima in polar and non-polar solvents were determined. Then, the possibility of using the considered derivatives for fluorescence bioimaging was checked. Compounds A-1 and A-2 were successfully used as fluorescent dyes of fixed cells of mammalian origin. In addition, biological activity tests against bacteria and fungi were carried out. Our results showed that A-1 and A-2 showed the most excellent antimicrobial activity among the newly synthesized compounds, especially against Gram-positive bacteria.

Impact of Electron-Donating Groups on Attaining Dual-Emitting Imidazole-Based Donor-Acceptor Materials

Imidazole-based donor-acceptor materials are well known for their polarity-controlled trade-off phenomenon between the localized excitation-based short-wavelength (SW) emission in nonpolar solvents and charge transfer dominated long-wavelength (LW) emission in polar solvents. To attain concurrent SW- and LW-based dual-emission characteristics, a series of imidazole-based donor-acceptor fluorophores (CBImDCN, TPImDCN, PZImDCN) possessing different electron-donating groups such as carbazole, triphenylamine, and phenothiazine linked via the N-position of the imidazole core unit were synthesized and verified by NMR and mass spectroscopic techniques. As a result, the strong donating TPImDCN and PZImDCN exhibited dual emission in different solvents of varying polarity, covering the blue (SW) and green/orange (LW) regions. On the other hand, in contrast, only an SW emission band is observed with the weak donating CBImDCN. Moreover, PZImDCN shows panchromatic emission under 365 nm illumination, while only orange color emission is observed under visible light excitation, revealing two different origins of SW and LW emissions, as also evidenced from DFT calculations. Overall, this work reveals a new approach for attaining concurrent SW and LW emission characteristics from imidazole-based D-A materials and sheds light on the design and development of novel panchromatic emitters with intriguing properties for lighting and display applications.

The influence of the shape and configuration of sensitizer molecules on the efficiency of DSSCs: a theoretical insight

The sensitizer is an active component of dye sensitized solar cell (DSSC) technology, which is highly influential for the performance of DSSCs. The present study attempts to investigate the relationship between the shape of the sensitizer molecule and efficiency of DSSCs. Specifically, 17 different structures were investigated, and classified into four categories based on the shape of the dye molecule, namely L-shaped (linear), V-shaped, X-shaped and Y-shaped, and into two different categories based on the donor moiety. The five of studied structures contained a triphenylamine (TPA) donor moiety, and twelve structures contained carbazole (CAR) donor moiety. Parameters related to the performance of DSSCs such as absorption spectra, intramolecular charge transfer indices, frontier molecular orbitals, light harvesting efficiency, excited-state lifetime, exciton binding energy, electrostatic potential, charge transfer and electron injection ability were studied using results obtained from electronic structure calculations. The analysis of these various parameters showed that the linear-shaped and V-shaped sensitizers possess better photon absorption ability, and the V-shaped structure is best suited shape for applications in high performance DSSCs.

Numerous studies have reported that the shape of a sensitizer matters for DSSC performance, while the present study investigates why and how it matters.

Effect of thienyl units in cyanoacrylic acid derivatives toward dye-sensitized solar cells

A series of heterocyclic donor-acceptor systems were synthesized and well characterized by using ¹H, ¹³C NMR, FT-IR, and elemental analysis. They were designed to investigate the effect of thiophene and cyanoacrylic acid number units on the thermal, optical, electrochemical and finally photovoltaic properties of dye-sensitized solar cells prepared with the selected compounds. The effect of chemical structure on their properties was demonstrated. They showed the beginning of thermal decomposition between 230 and 270 °C. The compounds absorbed the radiation in the range of 300-500 nm or 200-400 nm. They were electrochemically active and varied in energy band gap from 3.40 to 1.58 eV. Additionally, their optimized geometry, HOMO-LUMO levels, ionization potential, and electron affinity were evaluated using density functional theory. The photovoltaic devices based on TiO₂ sensitized with the obtained molecules exhibited low power conversion efficiency, which was the highest for the device containing the symmetrical molecule with bithiophene structure. Under co-sensitization, the cell made of the same compound gave significant enhancement of efficiency of 6.3% being higher to that of the individual device prepared from dye N719 (5.75%). Moreover, the effects of immersion time of TiO₂ electrode in the dye solution and co-sensitization methods were tested. The surface morphology of photoanode was investigated using atomic force microscopy.

Impact of tunable 2-(1H-indol-3-yl)acetonitrile based fluorophores towards optical, thermal and electroluminescence properties

Herein, we have synthesized 4,5-diphenyl-1H-imidazole and 2-(1H-indol-3-yl)acetonitrile based donor–π–acceptor fluorophores and studied their optical, thermal, electroluminescence properties. Both the fluorophores exhibit high fluorescence quantum yield (Φ_f = <0.6) and good thermal stability (T_d10 = <300 °C), and could be excellent candidates for OLED applications. Moreover, the ground and excited state properties of the compounds were analysed in various solvents with different polarities. The geometric and electronic structures of the fluorophores in the ground and excited states have been studied using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The absorption of BIPIAN and BITIAN in various solvents corresponds to S₀ → S₁ transitions and the most intense bands with respect to the higher oscillator strengths are mainly contributed by HOMO → LUMO transition. Significantly, the vacuum deposited non-doped OLED device was fabricated using BITIAN as an emitter, and the device shows electroluminescence (EL) at 564 nm, maximum current efficiency (CE) 0.687 cd A⁻¹ and a maximum external quantum efficiency (EQE) of 0.24%.

Herein, we have synthesized 4,5-diphenyl-1H-imidazole and 2-(1H-indol-3-yl)acetonitrile based donor–π–acceptor fluorophores and studied their optical, thermal, electroluminescence properties.

Charge-transfer excited state in pyrene-1-carboxylic acids adsorbed on titanium dioxide nanoparticles

The electronic structure of excited photosensitizer adsorbed at the surface of a solid is the key factor in the electron transfer processes that underlie the efficiency of dye-sensitized solar cells and photocatalysts. In this work, Stark effect (electroabsorption) spectroscopy has been used to measure the polarizability and dipole moment changes in electronic transitions of pyrene-1-carboxylic (PCA), -acetic (PAA) and -butyric (PBA) acids in ethanol, both free and adsorbed on colloidal TiO₂, in glassy ethanol at low temperature. The lack of appreciable increase of dipole moment in the excited state of free and adsorbed PAA and PBA points that two or more single bonds completely prevent the expansion of π-electrons from the aromatic ring towards the carboxylic group, thus excluding the possibility of direct electron injection into TiO₂. In free PCA, the pyrene's forbidden S₀→S₁ transition has increased intensity, exhibits a long progression in 1400cm^-1 A_g mode and is associated with |∆μ| of 2 D. Adsorption of PCA on TiO₂ causes a broadening and red shift of the S₀→S₁ absorption band and an increase in dipole moment change on electronic excitation to |∆μ|=6.5 D. This value increased further to about 15 D when the content of acetic acid in the colloid was changed from 0.2% to 2%, and this effect is ascribed to the surface electric field. The large increase of |∆μ| points that the electric field effect can not only change the energetics of electron transfer from the excited sensitizer into the solid, but can also shift the molecular electronic density, thus directly influencing the electronic coupling factor relevant for electron transfer at the molecule-solid interface.

Unravelling the effect of anchoring groups on the ground and excited state properties of pyrene using computational and spectroscopic methods

Anchoring groups play an important role in dye sensitized solar cells (DSCs). In order to acquire a suitable anchoring group for DSCs, a deeper understanding of the effect of anchoring groups on the ground and excited state properties of the dye is significant. In this context, various anchoring group connected pyrene derivatives are successfully synthesized and well characterized by using (1)H, (13)C-NMR, FT-IR and EI-MS spectrometry. The anchoring groups employed are carboxylic acid, malonic acid, acrylic acid, malononitrile, cyanoacrylic acid, rhodanine and rhodanine-3-acetic acid. The optimized geometries, HOMO-LUMO energy gap, light harvesting efficiency (LHE) and electronic absorption spectra of these dyes are studied by using density functional theory (DFT) calculations. The results show that pyrene connected with anchoring groups with weak electron pulling strength (PC, PAC and PMC) has a larger HOMO-LUMO energy gap, whereas that connected with anchoring groups with strong electron pulling strength (PCC, PMN, PR and PRA) has a reduced HOMO-LUMO energy gap. These molecules with a reduced energy gap are primarily preferred for DSC applications. Moreover, P, PC, PAC and PMC molecules undergo π→π* transition, whereas PCC, PMN, PR and PRA molecules show significant charge transfer along with π→π* transition. UV-visible absorption spectral studies on these dyes reveal that connecting various anchoring groups with different electron pulling abilities enables the pyrene chromophore to absorb in the longer wavelength region. Notably, an efficient bathochromic shift is observed for PCC, PMN, PR and PRA molecules in both electronic absorption and fluorescence spectral measurements, which suggests that the excitation is delocalized throughout the entire π-system of the molecules. Both theoretical and spectral studies reveal that dyes with an ICT character (PCC, PMN, PR and PRA) are suitable for dye sensitized solar cell applications.

Effect of π-spacers and anchoring groups on the photovoltaic performances of ullazine-based dyes

Three ullazine-based organic sensitizers were designed, synthesized and applied in dye-sensitized solar cells (DSSCs). The tuning of π-spacers and anchoring groups led to a variation of the photovoltaic performances.

Tunable anchoring groups@acridone-linked triphenylamine based pendant chromophores and their effects on the photovoltaic performance as sensitizers for dye-sensitized solar cells

In this study, a series of pendant chromophores bearing methacrylate polymers with different anchoring/acceptor groups were synthesized for use as sensitizers in the dye-sensitized solar cells.

Impact of strength and size of donors on the optoelectronic properties of D–π–A sensitizers

A series of carbazole based sensitizers have been synthesized to understand the influence of the strength of the donor moiety on the optical, electrochemical and photovoltaic properties.

Synthesis, characterization and binding interactions of amino acids coupled perylene diimides with colloidal doped and undoped TiO₂

Two sensitizers based on amino acids coupled with perylene moiety having absorption in the visible region have been designed and their interaction with doped and undoped TiO2 for the application of dye sensitized solar cells (DSSCs) has been studied. The synthesized compounds PDI-PA and PDI-AA were characterized using (1)H and (13)C NMR, Mass and FT-IR spectroscopic techniques. The optical properties and lifetime measurements of the sensitizers were analyzed using various solvents with different polarity. The solvatochromism effect was studied using Lippert-Mataga plot. The electrochemical studies of both dyes were investigated in DMF with various scan rate ranging from 200 to 1000 mV s(-1). Colloidal doped and undoped TiO2 was prepared and characterized by using absorption measurements. Binding ability of the sensitizers with the nanoparticles was studied through absorption, fluorescence quenching, cyclic voltammetry and FT-IR measurements. Results obtained from all the above analysis suggest the mode of quenching may be static. The binding constant values were calculated using Kamat-Fox equation indicates the binding behavior of the sensitizers with the nanoparticles. The fluorescence quenching was mainly attributed to electron transfer from the excited state of PDI's to the conduction band of colloidal semiconductors. The electron transfer mechanism was explained based on the Rehm-Weller equation as well as the energy level diagram.

Dithiafulvalene functionalized diketopyrrolopyrrole based sensitizers for efficient hydrogen production

Dithiafulvalene functionalized diketopyrrolopyrrole sensitizers were decorated with TiO₂ and Pt nanoparticles as co-catalysts for photocatalytic H₂ production.

Influence of thermal treatment and Au-loading on the growth of versatile crystal phase composition and photocatalytic activity of sodium titanate nanotubes

A coalescence influence of Au-loading followed by calcination at 800 °C led to a notable change in crystal-structure, morphology, phase composition and photocatalytic activity of titanate-nanostructures.