Photophysical behavior of lipophilic xanthene dyes without the involvement of photoinduced electron transfer mechanism

Oxyethylated Fluoresceine-(thia)calix[4]arene Conjugates: Synthesis and Visible-Light Photoredox Catalysis in Water-Organic Media

Fluorescent derivatives attract the attention of researchers for their use as sensors, photocatalysts and for the creation of functional materials. In order to create amphiphilic fluorescent derivatives of calixarenes, a fluorescein derivative containing oligoethylene glycol and propargyl groups was obtained. The resulting fluorescein derivative was introduced into three different (thia)calix[4]arene azide derivatives. For all synthesized compounds, the luminescence quantum yields have been established in different solvents. Using UV-visible spectroscopy, dynamic light scattering, as well as transmission and confocal microscopy, aggregation of macrocycles was studied. It was evaluated that calixarene derivatives with alkyl substituents form spherical aggregates, while symmetrical tetrafluorescein-containing thiacalix[4]arene forms extended worm-like aggregates. The macrocycle containing tetradecyl fragments was found to be the most efficient in photoredox ipso-oxidation of phenylboronic acid. In addition, it was shown that in a number of different electron donors (NEt₃, DABCO and iPr₂EtN), the photoredox ipso-oxidation proceeds best with triethylamine. It has been shown that a low molecular weight surfactant Triton-X100 can also improve the photocatalytic abilities of an oligoethylene glycol fluorescein derivative, thus showing the importance of a combination of micellar and photoredox catalysis.

Controlled Synthesis of Luminescent Xanthene Dyes and Use of Ionic Liquid in Thermochromic Reaction

In this study, we demonstrate six novel xanthene derivatives and their spectroscopic and chemical properties. The presented synthesis examination allowed us to obtain two different compounds during one step, with open and closed lactone rings substituted with different length alkyl chains. Increasing the reaction efficiency to 77% was obtained using the microwave-assisted method. Moreover, the modification of O-alkylation synthesis in an ecofriendly way using a ball mill led to achieving exclusively one opened ring product. All of the synthesized compounds showed different spectroscopic behaviors in comparison with the different organic dyes; the typical concentration quenching of luminescence was not observed. The relationship between the length of the alkyl chain and the time of luminescence decay is presented. Synthetized closed forms of dyes turned out to be promising leuco dyes. For the first time, an ionic liquid was used as a developer of synthesized xanthene derivatives (as leuco dyes), which led to obtaining an irreversible thermochromic marker.

Aminofluoresceins Versus Fluorescein: Ascertained New Unusual Features of Tautomerism and Dissociation of Hydroxyxanthene Dyes in Solution

Within the course of this spectroscopic research, we revealed novel features of the protolytic behavior, which extend the knowledge of the chemistry of xanthene dyes and rationalize the utilization of these compounds. In addition to the well-known tautomerism of the molecular form, H₂R, of fluorescein dyes, new aspects of tautomeric transformation of anions are disclosed. First, for the dyes bearing the substituents in the phthalic acid residue, 4'- and 5'-aminofluoresceins and 4'-fluorescein isothiocyanate, the monoanion HR^- exists in non-hydrogen-bond donor solvents not only as a tautomer with the ionized carboxylic and nonionized OH group but also as a "phenolate" ion with a nonionized COOH group. Such state of HR^- ions is typical for dyes bearing halogen atoms or NO₂ groups in the xanthene moiety but was not observed until now in the case of substitution in the phthalic residue. Second, the possibility of the existence of the HR^- species in DMSO in the form of colorless lactone is deduced for the 5'-aminofluorescein using the visible and infrared spectra. This results in a dramatic difference in medium effects. For instance, whereas for fluorescein in DMSO, the inversion of the stepwise ionization constants takes place and the K_a1/K_a2 value equals 0.08, the same ratio for 5'-aminofluorescein is as high as ∼800. In addition, the pK_a values of sulfonefluorescein, erythrosin, methyl ether of fluorescein, and phenol red were obtained to verify the acidity scale in DMSO and to support the detailed scheme of protolytic equilibria of fluorescein dyes.

Ionization and tautomerism of methyl fluorescein and related dyes

The protolytic equilibrium of methyl ether of fluorescein is studied in water, aqueous ethanol, and in other solvents. The constants of the two-step dissociation are determined by spectrophotometry. In water, the fractions of the zwitterionic, quinonoid, and lactonic tautomes are correspondingly 11%, 6%, and 83%, as deduced from the UV-visible spectra. Corresponding study of the ionization of the methyl ether ester of fluorescein, fluorescein ethyl ester, and sulfonefluorescein allows testing the correction of the attribution of the microscopic dissociation constants of methoxy fluorescein. The results of nuclear magnetic resonance and infrared spectroscopy, as well as the X-ray analysis confirm the predomination of the lactonic structure of the molecular species in solid state and in DMSO. Contrary to it, the spectroscopic studies in both hydrogen-donor bond (HDB) and non-HBD solvents confirm that the presence of lactonic monoanion is atypical for the dye under study and, with high probability, also for the mother compound fluorescein.

Fluorescence properties of twenty fluorescein derivatives: lifetime, quantum yield, absorption and emission spectra

The fluorescence lifetime (τf), emission quantum yield (Φf), absorption and emission spectral data of 20 fluorescein derivatives were measured under the same conditions by using time-correlated single photon counting, steady state fluorescence and absorption methods to get comparable data. Based on the results, the factors and mechanism that control the fluorescence properties of the fluorescein dyes are discussed. Both Φf and τf are remarkably dependent on the substitution on either xanthene or phenyl rings, but their ratio (Φf/τf), i.e. rate constant of radiation process, is a constant value (0.20 × 10(9) s(-1)). The rate constant of nonradiation process, on the other hand, is varied with both the structure and the solvent used.