Recent advances in phenothiazine-based dyes for dye-sensitized solar cells

Exploring the Theoretical Foundations of Thermally Activated Delayed Fluorescence (TADF) Emission: A Comprehensive TD-DFT Study on Phenothiazine Systems

This study conducts a thorough theoretical investigation of Thermally Activated Delayed Fluorescence (TADF) in phenothiazine-based systems, examining ten molecular configurations recognized experimentally as TADF-active. Employing Time-Dependent Density Functional Theory (TD-DFT), our analysis spans the investigation of singlet-triplet energy gaps (ΔEST), spin-orbit coupling, and excitation characteristics using Multiwfn. This approach not only validates the adherence to El Sayed's rule across these systems but also provides a detailed understanding of charge transfer dynamics, as visualized through heat maps. A significant aspect of our study is the exploration of different oxidation states of sulfur and site substitutions on phenothiazine. This systematic variation aims to identify additional TADF-active compounds, drawing parallels with properties characterizing other known TADF emitters. Our investigation into Reverse Intersystem Crossing (rISC) rates and the analysis of dihedral angles in relation to ΔEST values offer nuanced insights into the TADF behaviours of these molecules. By integrating rigorous computational analysis with practical implications, we provide a foundational understanding that enhances the design and optimization of phenothiazine-based materials for optoelectronic applications. This work not only advances our theoretical understanding of TADF in phenothiazine derivatives but also serves as a guide for experimentalists and industry professionals in the strategic design of new TADF materials.

A Photoelectrochemical Sensor for the Detection of Hypochlorous Acid with a Phenothiazine-Based Photosensitizer

This paper presents the development of a photoelectrochemical sensor for hypochlorous acid (HOCl) detection, employing a phenothiazine-based organic photosensitizer (Dye-PZ). The designed probe, Dye-PZ, follows a D-π-A structure with phenothiazine as the electron-donating group and a cyano-substituted pyridine unit as the electron-accepting group. A specific reaction of the phenothiazine sulfur atom with HOCl enables selective recognition. The covalent immobilization of Dye-PZ onto a titanium dioxide nanorod-coated fluorine-doped tin oxide electrode (FTO/TiO2) using bromo-silane coupling agent (BrPTMS) resulted in the fabrication of the photoanode FTO/TiO2/BrPTMS/Dye-PZ. The photoanode exhibited a significant photoresponse under visible-light irradiation, with a subsequent reduction in photocurrent upon reaction with HOCl. The oxidation of the phenothiazine sulfur atom to a sulfoxide diminished the internal charge transfer (ICT) effect. Leveraging this principle, the successful photoelectrochemical sensing of HOCl was achieved. The sensor showed high stability, excellent reproducibility, and selective sensitivity for HOCl detection. Our study provides a novel approach for the development of efficient photoelectrochemical sensors based on organic photosensitizers, with promising applications in water quality monitoring and biosensing.

Improving the Performance of Supported Ionic Liquid Phase Catalysts for the Ultra-Low-Temperature Water Gas Shift Reaction Using Organic Salt Additives

The water gas shift reaction (WGSR) is catalyzed by supported ionic liquid phase (SILP) systems containing homogeneous Ru complexes dissolved in ionic liquids (ILs). These systems work at very low temperatures, that is, between 120 and 160 °C, as compared to >200 °C in the conventional process. To improve the performance of this ultra-low-temperature catalysis, we investigated the influence of various additives on the catalytic activity of these SILP systems. In particular, the application of methylene blue (MB) as an additive doubled the activity. Infrared spectroscopy measurements combined with density functional theory (DFT) calculations excluded a coordinative interaction of MB with the Ru complex. In contrast, state-of-the-art theoretical calculations elucidated the catalytic effect of the additives by non-covalent interactions. In particular, the additives can significantly lower the barrier of the rate-determining step of the reaction mechanism via formation of hydrogen bonds. The theoretical predictions, thereby, showed excellent agreement with the increase of experimental activity upon variation of the hydrogen bonding moieties in the additives investigated.

QSPR modeling of absorption maxima of dyes used in dye sensitized solar cells (DSSCs)

Dye-sensitized solar cells (DSSCs) have recently received a significant attention as possible sources of renewable energy. As a result, a significant effort is being made to develop organic dyes for highly power conversion efficient DSSCs, in order to overcome the disadvantages of previous solar cell systems, such as cost reduction, weight reduction, and production methods that minimize environmental pollution. As shown by multiple recent research publications, computational techniques such as quantitative structure-property relationship (QSPR) modeling may aid in the development of suitable dyes for DSSCs satisfying many fundamental desired characteristics. The current report provides robust, externally verified QSPR models for five chemical classes of organic dyes (Triphenylamines, Phenothiazines, Indolines, Porphyrins and Coumarins) based on experimentally determined absorption maxima values. The size of the dye data points utilized to develop the models is the largest known to date. The QSPR models were constructed using only two-dimensional descriptors with clear physicochemical meaning. Using the best subset selection approach, we built 5, 3, 4, 3 and 2 descriptor models for the Triphenylamine, Phenothiazine, Indoline, Porphyrin and Coumarin classes, respectively. The models were validated both internally and externally, and then consensus predictions were made for specific categories of dyes using the developed partial least squares (PLS) models, and the "Intelligent consensus predictor" tool (http://teqip.jdvu.ac.in/QSAR_Tools/) was used to determine whether the quality of test set compound predictions can be improved through the "intelligent" selection of multiple PLS models. We identified from the insights gained from the developed models several chemical attributes that are important in enhancing the absorption maxima. Thus, our study may be utilized to predict the λ_max values of novel or untested organic dyes and to give insights that will aid in the development of new dyes for use in solar cells with increased λ_max values and enhanced power conversion efficiency.

Synthesis and characterization of a new phenothiazine-based sensitizer/co-sensitizer for efficient dye-sensitized solar cell performance using a gel polymer electrolyte and Ni–TiO2 as a photoanode

Efficiency enhancement of a DSSC using a metal-free co-sensitizer, Ni–TiO2 photoanode, and blend gel polymer electrolyte.

Extended phenothiazines: synthesis, photophysical and redox properties, and efficient photocatalytic oxidative coupling of amines

Herein, we successfully developed a ring-fusion approach to extend the conjugation length of phenothiazines that were demonstrated to be efficient photocatalysts for visible-light-driven oxidative coupling reactions of amines under an air atmosphere.

Photophysical Properties and Electronic Structure of Symmetrical Curcumin Analogues and Their BF2 Complexes, Including a Phenothiazine Substituted Derivative

Symmetrically substituted curcumin analogue compounds possess electron donor moieties at both ends of the conjugated systems; their difluoroboron complexes were synthesized, and their structures were fully characterized. A novel compound with enhanced photophysical properties bearing phenothiazine moieties is reported. The introduction of BF2 into the molecular structures resulted in bathochromic shifts both in the absorption and emission spectra, indicating that the π-conjugation was more extended than the one in the initial compounds. The solvatochromic effects were studied, which in case of the phenothiazinyl-curcumin BF2 complex was the most notable. Theoretical study of the investigated compounds was carried out using DFT and TD-DFT methods to evaluate the ground state geometries and vertical excitation energies.

Prediction of power conversion efficiency of phenothiazine-based dye-sensitized solar cells using Monte Carlo method with index of ideality of correlation

Simplified molecular-input line-entry system (SMILES) notation and inbuilt Monte Carlo algorithm of CORAL software were employed to construct generative and prediction QSPR models for the analysis of the power conversion efficiency (PCE) of 215 phenothiazine derivatives. The dataset was divided into four splits and each split was further divided into four sets. A hybrid descriptor, a combination of SMILES and hydrogen suppressed graph (HSG), was employed to build reliable and robust QSPR models. The role of the index of ideality of correlation (IIC) was also studied in depth. We performed a comparative study to predict PCE using two target functions (TF₁ without IIC and TF₂ with IIC). Eight QSPR models were developed and the models developed with TF₂ was shown robust and reliable. The QSPR model generated from split 4 was considered a leading model. The different statistical benchmarks were computed for the lead model and these were rtraining set2=0.7784; rinvisible training set2=0.7955; rcalibration set2=0.7738; rvalidation set2=0.7506; Qtraining set2=0.7691; Qinvisible training set2=0.7850; Qcalibration set2=0.7501;  Qvalidation set2=0.7085; IIC_{training set} = 0.8590; IIC_{invisible training set} = 0.8297; IIC_{calibration set} = 0.8796; IIC_{validation set} = 0.8293, etc. The promoters of increase and decrease of endpoint PCE were also extracted.

Theoretical analysis of the absorption spectrum, electronic structure, excitation, and intramolecular electron transfer of D-A'-π-A porphyrin dyes for dye-sensitized solar cells

A series of porphyrin dyes with D-A'-π-A structure were designed and theoretically investigated by DFT and TD-DFT methods. Different electron-withdrawing auxiliary acceptor units were introduced into the dye molecule skeleton to shed light on how the type and position of auxiliary acceptors influence the photoelectric performance of the dyes. It was found that the introduction of electron-withdrawing units, BTD, TPZ, BTZ, PP and DPPZ, between the Zn-porphyrin core and the anchoring group had a significantly positive influence on the performance of the dye molecules. Also, more appropriate electron distribution in the molecular orbital and the lower HOMO-LUMO energy gap, more extensive absorption coverage, higher light-harvesting efficiency, lower orbital overlap, and more effective long-range intramolecular electron transfer (IET) process can be achieved as compared to the reference dye. Among these five electron-withdrawing units, BTD and TPZ had the effect leading to the greatest improvement and therefore, are the best candidates for auxiliary acceptors. The calculated results indicated that the longitudinal π-conjugation of the electron-withdrawing unit also had an obvious effect on the performance of the dye molecule, and NTD is expected to be a more effective auxiliary acceptor than BTD. The effects of the relative positions of the auxiliary acceptors in the dye molecular skeleton were also investigated. A comparative study of AX1-3 and AA1-3 showed that the introduction of BTD, TPZ and BTZ units between the donor part and the Zn-porphyrin core may have a negative impact on the performance of the dyes. Our studies are expected to provide new insight for the design and screening of high-efficiency porphyrin dyes for DSSCs applications.

Effect of Auxiliary Donors on 3,8-Phenothiazine Dyes for Dye-Sensitized Solar Cells

Phenothiazines are one of the more common dye scaffolds for dye-sensitized solar cells. However, these sensitizers are exclusively based on a 3,7-substitution pattern. Herein, we have synthesized and characterized novel 3,8-substituted phenothiazine dyes in order to evaluate the effect of auxiliary donor groups on the performance of this new dye class. The power conversion efficiency increased by 7%–10% upon insertion of an auxiliary donor in position 8 of the phenothiazine, but the structure of the auxiliary donor (phenyl, naphthyl, pyrene) had a low impact when electrodes were stained with chenodeoxycholic acid (CDCA) additive. In the absence of CDCA, the highest power conversion efficiency was seen for the phenyl-based sensitizer attributed to a higher quality dye-monolayer. By comparing the novel dyes to their previously reported 3,7- analogues, only subtle differences were seen in photophysical, electrochemical, and performance measurements. The most notable difference between the two geometries is a lowering of the oxidation potentials of the 3,8-dyes by 40–50 mV compared to the 3,7-analogues. The best auxiliary donor for the 3,8-phenothiazine dyes was found to be pyrenyl, with the best device delivering a power conversion efficiency of 6.23% (99 mW cm⁻², 10 eq. CDCA, J_SC = 10.20 mA cm⁻², V_OC = 791 mV, and FF = 0.765).

Tuning optoelectronic properties of triphenylamine based dyes through variation of pi-conjugated units and anchoring groups: A DFT/TD-DFT investigation

Dye-sensitized solar cells (DSSCs) have attracted widespread attention due to their unique features. In the present work, molecular engineered triphenylamine based dyes featuring donor-bridge-acceptor architecture have been considered and investigated for suitable properties for DSSCs applications. Hydantoin anchoring group has been introduced replacing the commonly used cyanoacrylic acid to improve the long-term stability of the device. Results on the effects of varied anchoring groups and pi-spacers have been interpreted from the viewpoint of DFT/TD-DFT calculations. Designed sensitizers exhibit suitable light-harvesting efficiencies, excited-state lifetimes, electron injection and regeneration abilities. Red-shifted electronic spectra are observed for three hydantoin dyes compared to others in the same family. Further analysis of chemical descriptors and observation from full-electron donor-acceptor map reveal that the three dyes among nine are potential materials with promising properties towards improving DSSCs performance.

Game of Isomers: Bifurcation in the Catalytic Formation of Bis[1]benzothieno[1,4]thiazines with Conformation-Dependent Electronic Properties

Two regioisomers of bis[1]benzothieno[1,4]thiazine are unexpectedly obtained by tuning the catalytic conditions of the intermolecular-intramolecular Buchwald-Hartwig amination. Mechanistic insights and evidence of intermediates support a conclusive mechanistic rationale. Furthermore, a computationally based study on the influence of conformational aspects on the HOMO energy level of anellated 1,4-thiazine paves the way to enhance the electronic properties, thus successfully achieving higher luminescent and easier oxidizable syn- syn bis[1]benzothieno[1,4]thiazines.

DFT/TD-DFT study of novel T shaped phenothiazine-based organic dyes for dye-sensitized solar cells applications

Five novel T shaped phenothiazine-based organic dyes DTTP1~5 with different spacers at N (10) position were designed. The geometries, electronic structures, absorption spectra, electron transfer and injection properties of these isolated dyes and dye/(TiO₂)₉ systems were investigated via density functional theory (DFT) and time dependent density functional theory (TD-DFT) calculation. The optimized geometries indicate that these T shaped dyes show non-planar conformations, which are helpful in suppressing the close intermolecular π-π aggregation in device and enhancing thermal stability. The calculated results indicate that type of π-conjugated spacers can affect the molecular absorption spectra. Introduction of thiophene-benzothiadizole-thiophene unit as π-conjugated spacer can most effectively shift the light absorption to near infrared region and enhance the light harvesting efficiency (LHE). Moreover, it is found that these dyes show a good performance of electron injection and dye regeneration owing to the proper electron injection driving force (ΔG_inject) and dye regeneration driving force (ΔG_reg). The theoretical results reveal that these dyes could be used as potential sensitizers for DSSCs, and DTTP4 would be the most plausible sensitizer for high-efficiency DSSCs due to the narrow HOMO-LUMO energy gap (Δ_H-L), broad absorption spectrum, high LHE value, and large dipole moment (μ_normal).

Numbers of cyanovinyl substitutes and their effect on phenothiazine based organic dyes for dye-sensitized solar cells

A series of phenothiazine based dyes (OMS1–3), comprising different conjugation lengths and numbers of electron deficient (cyanovinyl) moieties with cyanoacrylic acid as an anchor, have been synthesized. The dyes display broad UV-visible absorption, from 389 nm to 484 nm. The higher molar extinction coefficient and longer absorption peak are achieved as the conjugation length and numbers of electron deficient units increase. The cell performance based on these dyes exhibits efficiencies ranging from 0.68–4.00%, compared to a standard N719-based device (PCE = 7.49%) fabricated under similar conditions. Although the OMS3 dye has two electron deficient units between phenothiazine units, an insignificant electron trapping effect is observed. From the results, the OMS3 based cell exhibits the highest short circuit current (J_SC) at 8.72 mA cm⁻² and the highest open-circuit voltage (V_OC) at 0.66 V, together with the best cell performance at 4.00%.

Phenothiazine based dyes (OMS1–3), comprising different conjugation lengths, numbers of electron deficient (cyanovinyl) moieties have been synthesized. OMS3 dye has two cyanovinyl moieties between phenothiazine core exhibits the best cell performance at 4.00%.

On how electron density affects the redox stability of phenothiazine sensitizers on semiconducting surfaces

The stabilities of three organic dyes that differ only by two substituents (-OMe, -H and -Br) about the phenothiazine donor unit were evaluated when immobilized on a semiconductor surface. All three dyes delivered modest power conversion efficiencies (PCEs) in the dye-sensitized solar cell (DSSC), but maintained 75% of their initial PCE over 300 h of sustained simulated sunlight. Electron-donating substituents increased the stability of the phenothiazine radical unit created after light-induced charge injection into the semiconductor; however, this did not translate to higher DSSC stability, which appears to be more sensitive to the basicity of the anchoring group for this series.

Comparative analysis of triarylamine and phenothiazine sensitizer donor units in dye-sensitized solar cells

A homologous set of dyes that differ only in the donor fragments, namely phenothiazine (PTZ) and triarylamine (TPA) units, were evaluated in dye-sensitized solar cells (DSSCs). The novel PTZ-based dye differs from the TPA-based dye in that it contains a sulfur bridge that planarizes two aromatic rings and enables higher dye loading and higher stability in the oxidized form. These positive features notwithstanding, the superior absorptivity of devices sensitized by TPA-based dyes resulted in significantly higher power conversion efficiencies (PCEs) than those sensitized by PTZ-based dyes.