The photophysics and solar cell efficiency of organic sensitizers comprising a cyanoacrylic acid group are greatly influenced by an equilibrium between the neutral (“non-deprotonated”, COOH) and ...anionic (“deprotonated”, COO–) forms, whose ratio depends on the solvent polarity and its H-bonding properties, dye concentration, and temperature used. Herein, we report a detailed investigation on the relationship between the portions of COOH and COO– dye forms and the photophysical and solar cell properties of an organic dipolar sensitizer, BTZA-II, bearing triphenylamine electron-donating and benzothiazole electron-withdrawing moieties. The photophysics has been studied by stationary and time-resolved fluorescence spectroscopy in apolar and polar solvents with a dye concentration ranging from 5 × 10–7 to 5 × 10–5 M, also upon addition of small amounts of an external acid or base to change the solvent acidity, allowing us to distinguish the contribution and lifetime of the neutral and anionic forms. The fluorescence of BTZA-II in apolar toluene originates from the neutral form, which has a lifetime of 1.9 ns. Addition of a strong base (1,8-diazabicyclo5.4.0undec-7-ene, DBU) shifts the equilibrium toward the less fluorescent anionic COO– form with a lifetime of 1.1 ns. The situation is different in the polar acetonitrile, where the fluorescence of the anionic form dominates (with a lifetime of 2.0 ns). Adding small amounts of acetic acid (AcOH) protonates the COO– form of the BTZA-II dye and reveals significant quenching of the fluorescence because of the increased contribution of the neutral species with a lifetime of 0.4–0.5 ns. This quenching of the neutral species in acetonitrile has been also observed in concentrated solutions and is due to excited-state proton transfer. By contrast, the photophysics of a dye Btz-NPh2, similar to BTZA-II but without the cyanoacrylic acid group, is not affected upon adding acetic acid (H-donor) or DBU base (H-acceptor), which rules out the Brønsted-base role of the benzothiazole scaffold in the aforementioned observations. Finally, dye-sensitized solar cells (DSSCs) with a solid-state electrolyte were prepared from toluene and toluene + acid solutions. A significant increase in the solar cell efficiency, η, by 58% (η reaching a value of 4.9%) has been achieved after addition of a small, appropriate amount of acetic acid into the initial BTZA-II dye solution.
A series of dipolar and octupolar triphenylamine-derived dyes containing a benzothiazole positioned in the matched or mismatched fashion have been designed and synthesized via palladium-catalyzed ...Sonogashira cross-coupling reactions. Linear and nonlinear optical properties of the designed molecules were tuned by an additional electron-withdrawing group (EWG) and by changing the relative positions of the donor and acceptor substituents on the heterocyclic ring. This allowed us to examine the effect of positional isomerism and extend the structure–property relationships useful in the engineering of novel heteroaromatic-based systems with enhanced two-photon absorption (TPA). The TPA cross-sections (δTPA) in the target compounds dramatically increased with the branching of the triphenylamine core and with the strength of the auxiliary acceptor. In addition, a change from the commonly used polarity in push–pull benzothiazoles to a reverse one has been revealed as a particularly useful strategy (regioisomeric control) for enhancing TPA cross-sections and shifting the absorption and emission maxima to longer wavelengths. The maximum TPA cross-sections of the star-shaped three-branched triphenylamines are ∼500–2300 GM in the near-infrared region (740–810 nm); thereby the molecular weight normalized δTPA/MW values of the best performing dyes within the series (2.0–2.4 GM·g–1·mol) are comparable to those of the most efficient TPA chromophores reported to date. The large TPA cross-sections combined with high emission quantum yields and large Stokes shifts make these compounds excellent candidates for various TPA applications, including two-photon fluorescence microscopy.
A series of push−pull chromophores comprising a dimethylamino or diphenylamino electron-donating functionality and a cationic benzothiazolium acceptor with an additional electron-withdrawing group ...(EWG = NO2 or CN) at various positions of the heterocyclic benzene ring have been synthesized and comprehensively investigated for their linear and quadratic nonlinear optical (NLO) properties by means of UV−visible spectroscopy and hyper-Rayleigh scattering, as well as by quantum-chemical calculations at different levels of theory (B3LYP, CAM-B3LYP, MP2, and RI-CC2). In general, all chromophores under study display large static quadratic hyperpolarizabilities β0, comparable to or in most cases even larger than their conventional stilbazolium-containing analogues, which makes these systems attractive for practical NLO applications. The introduction of an auxiliary EWG into the heterocyclic benzene ring causes a substantial red shift of the intramolecular charge-transfer band. Still, at the same time, this modification of the structure reduces the β0 values in systems with longer π-conjugated spacers. The unexpected negative impact of the EWG substitution pattern on the quadratic NLO activity is rationalized by quantum-chemical calculations as well as by experimentally determined one-photon absorption characteristics and is discussed in detail. Furthermore, computational studies revealed that push−pull benzothiazolium dyes with a “reverse” polarity with respect to the commonly used one would be a more worthwhile target for synthesis, because of their enhanced NLO response due to the positive effect of an auxiliary electron-withdrawing group.
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•Novel donor-π-acceptor structures containing benzothiazol were measured.•The largest TPA cross section of 4600 GM was obtained at a wavelength of 890 nm.•The large TPA cross sections ...in the NIR region and the high QYs were determined.
Two-photon absorption (TPA) cross sections of conjugated donor-π-acceptor dipolar structures containing benzothiazole or benzobisthiazolium moieties are determined in a broad spectral range from 700 nm to 1000 nm using two-photon induced fluorescence technique. The TPA cross section values range from 150 GM to 4600 GM. The largest values are observed in near-infrared region. The dipolar derivative of benzothiazole has the largest TPA cross section of 4600 GM at wavelength of 890 nm. A combination of the large TPA in the near-infrared region and the high emission quantum yield makes these compounds excellent candidates for two-photon fluorescence microscopy.
The bromate–aniline oscillatory reaction was discovered 4 decades ago, but neither the detailed mechanism nor the key products or intermediates of the reaction were described. We report herein a ...detailed study of this reaction, which yielded new insights. We found that oscillatory oxidation of aniline by acidic bromate proceeds, to a significant extent, via a novel reaction pathway with the periodic release of carbon dioxide. Several products were isolated, and their structures, not described so far, were justified on the basis of MS and NMR. One of the main products of the reaction associated with the CO2 release route can be assigned to 2,2-dibromo-5-(phenylimino)cyclopent-3-en-1-one. A number of known compounds produced in the studied reaction, including unexpected brominated 1-phenylpyrroles and 1-phenylmaleimides, were identified by comparison with standards. A mechanism is suggested to explain the appearance of the detected compounds, based on coupling of the anilino radical with the produced 1,4-benzoquinone. We assume that the radical adduct reacts with bromine to form a cyclopropanone intermediate that undergoes a Favorskii-type rearrangement. Further oxidation and bromination steps including decarboxylation lead to the found brominated phenyliminocyclopentenones. The detected derivatives of 1-phenylpyrrole could be produced by a one-electron oxidation of a proposed intermediate 2-phenylamino-5-bromocyclopenta-1,3-dien-1-ol followed by β-scission with the abstraction of carbon monoxide. Such a mechanism is known from the combustion chemistry of cyclopentadiene. The proposed mechanism of this reaction provides a framework for understanding the observed oscillatory kinetics.
Herein, we investigate the structure-property relationships in a new series of benzothiazole based unsymmetrical hexafluorocyclopentene dithienylethenes (DTEs) and compare the results with the known ...facts for symmetric diarylethenes (DAEs). We reveal high photocyclization efficiency resulting from a significant shift of ground state equilibrium to the antiparallel conformation and a barrierless excited state pathway to conical intersection, which remains unperturbed even in polar solvents for most of the prepared DTEs. Furthermore, we uncover that the rate of back thermal cycloreversion correlates clearly more with the central C-C bond-length in the transition state than with the central C-C bond-length in the ground state of the cyclic form. Finally, our detailed vibrational spectral analysis of studied DTEs points out significant changes in Raman and infrared spectra during photoswitching cycles which pave the way for a non-destructive readout of stored information.
Unsymmetrical benzothiazole based dithienylethenes represent P-type photoswitches with high thermal stability, efficient photocyclization, good resistance to photodegradation and non-destructive readout possibility of coded information using vibrational spectroscopy.
The excited state and electron injection dynamics of three new organic sensitizers, comprising a triphenylamine moiety connected by an ethenylene (C−C double-bond) or ethynylene (C−C triple-bond) ...π-spacer to an electron-withdrawing benzothiazole bearing a cyanoacrylic acid anchoring group, have been studied using a combination of steady-state and femtosecond-resolved spectroscopies. The measurements were carried out for the three dyes in predominantly neutral and completely deprotonated forms in liquid solutions and bound on nanocrystalline TiO2 and Al2O3 thin films. In addition, quantum-chemical calculations were performed to predict absorption spectra of the sensitizers and their corresponding cation radicals. Time-resolved fluorescence (TRF) measurements on TiO2 indicate that electron injection takes place on a <0.2 ps time scale. Transient electronic absorption (TA) measurements provide evidence for the formation of radical cations not only in dye-sensitized TiO2 films but also in Al2O3 ones. The cation lifetime in Al2O3 is significantly shorter compared to TiO2, indicating a faster recombination of injected electrons with the dye cations. In addition, the ground-state bleach band in dye-sensitized TiO2 films experiences a gradual red-shift, which is indicative of a transient Stark effect. Finally, femtosecond transient absorption measurements in the IR region point to an ultrafast generation of injected electrons for all dyes. A faster recombination of the injected electrons with the dye cations is observed for the sensitizer decorated with auxiliary electron-donating methoxy groups on the triphenylamine moiety.
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