Excited state properties and acid–base equilibria of trans-2-styrylbenzoxazoles

Synthesis of Styrylbenzazole Photoswitches and Evaluation of their Photochemical Properties

Styrylbenzazoles form a promising yet under-represented class of photoswitches that can perform a light-driven E-Z isomerization of the central alkene double bond without undergoing irreversible photocyclization, typical of the parent stilbene. In this work, we report the synthesis and photochemical study of 23 styrylbenzazole photoswitches. Their thermal stabilities, quantum yields, maximum absorption wavelengths and photostationary state (PSS) distributions can be tuned by changing the benzazole heterocycle and the substitution pattern on the aryl ring. In particular, we found that push-pull systems show large redshifts of the maximum absorption wavelengths and the highest quantum yields, whereas ortho-substituted styrylbenzazole photoswitches exhibit the most favorable PSS ratios. Taking advantage of both design principles, we produced 2,6-dimethyl-4-(dimethylamino)-styrylbenzothiazole, a thermally stable and efficient P-type photoswitch which displays negative photochromism upon irradiation with visible light up to 470 nm to obtain a near-quantitative isomerization with a very high quantum yield of 59 %. Furthermore, 4-hydroxystyrylbenzoxazole was demonstrated to be a pH-sensitive switch which exhibits a 100 nm redshift upon deprotonation. Ortho-methylation of its benzothiazole analogue improved the obtained PSS ratio in its deprotonated state from E : Z=53 : 47 to E : Z=18 : 82. We anticipate that this relatively unexplored class of photoswitches will form a valuable expansion of the current family of photoswitches.

Intense White Molecular Fluorescence from Naphthoxazole-Quinoline Derivatives

Naphthoxazole derivatives are small heterocyclic compounds endowed with outstanding fluorescence properties. In this work, we report a detailed study of the intense white light fluorescence observed in naphthoxazole-quinoline dyads in solvent mixtures including at least a strong hydrogen bonding solvent. The same phenomenon was also studied in inclusion complexes naphthoxazole derivatives-sulfonated-βCD either in aqueous solution as well as in solid phase. A novel mechanism of white molecular fluorescence generation based on solvent-to-fluorophore proton transfer facilitated by ground state hydrogen bonding was characterized. The emission combines both, a blue charge transfer fluorescence emitted by the locally excited singlet state along with a red-shifted emission from a proton transfer complex.

Photophysical and thermal properties of novel solid state fluorescent benzoxazole based styryl dyes from a DFT study

Solid state photoluminescent benzoxazole styryl dyes – synthesis and study of the photophysical properties from a DFT study.

Synthesis and properties of fluorescence poly(benzoxazole–imide)s containing naphthalene

Novel aromatic poly(benzoxazole–imide)s containing naphthalene were prepared from synthesized 1,4-di(5-aminobenzoxazol-2-yl)naphthalene and commercial dianhydrides by conventional two-step polymerization. The polymers showed high levels of tensile strength of up to 294 MPa and modulus of up to 6.5 GPa. The glass transition temperatures of the polymers were observed between 267°C and 345°C. The 5% weight loss temperatures of the polymers were tested in the range of 517–562°C in nitrogen atmosphere. The excellent properties of polymers were attributed to their rigid-rod-like molecular structure. The polymers emitted different fluorescence with maximum emission wavelengths in the range of 470–560 nm. Increasing the dianhydride electron affinity, the emission spectra peak value of polyimides (PIs) except PI6 increased gradually, but the fluorescent intensity of the PIs decreased. The 1,4-bis(3,4-dicarboxyphenoxy)benzene dianhydride-derived PI film exhibit highly fluorescent characteristics; however, the PI film from pyromellitic dianhydride was nonfluorescent.

Design and electronic structure of new styryl dye bases: steady-state and time-resolved spectroscopic studies

A comprehensive investigation of the electronic structure and fast relaxation processes in the excited states of new styryl base-type derivatives was performed using steady-state, pico-, and femtosecond time-resolved spectroscopic techniques. Linear photophysical parameters of new compounds, including steady-state absorption, fluorescence, and excitation anisotropy spectra, were obtained in a number of organic solvents at room temperature. A detailed analysis of the fluorescence lifetimes and ultrafast relaxation processes in the electronically excited state of the styryl bases revealed an important role of solvate dynamics and donor-acceptor strength of the molecular structures in the formation of their excited state absorption spectra. Experimental data were in good agreement with quantum chemical calculations at the time dependent density functional theory level, combined with a polarizable continuum model.

Size- and solvent-dependent kinetics for cis-trans isomerization in donor-acceptor systems

We have investigated, using time-resolved and steady-state optical spectroscopy, the cis-trans isomerization dynamics in a series of charge transfer, donor-acceptor compounds. The number of donor (dithiafulvene) and acceptor (p-nitrophenyl) moieties as well as their spatial arrangement around a central ethynylethene core has been varied in a systematic way. All compounds in the series are weakly fluorescent. We show that the fluorescence spectrum red-shifts within a few picoseconds, a shift which occurs concurrently with a blue-shift of the transient absorption spectrum. The kinetics following the initial relaxation are in all cases multi-exponential, and the time constants correlate with molecular size and solvent viscosity. We interpret the data as a result of conformational change where the conjugation through the central double bond is broken upon excitation into the charge-transfer transition, and the time for rotation around this bond is dependent on the molecular interactions between solute and solvent.

Stilbene-Like Molecules as Fluorescent Probes Applied for Monitoring of Polymerization Process

2-(p-N,N-dimethylaminostyryl)benzoxazole (OS), 2-(p-N,N-dimethylaminostyryl)-benzothiazole (SS) and 2-(p-N,N-dimethylaminostyryl)naphtiazole (PS) were prepared and their absorption and fluorescence spectra were measured in various solvents at room temperature. On the basis of the solvatochromic behavior the ground state (mu(g)) and excited state (mu(e)) dipole moments of these pN,N-dimethylaminostyryl derivatives were evaluated. The dipole moments (mu(g) and mu(e)) were estimated from solvatochromic shifts of absorption and fluorescence spectra as function of the dielectric constant (epsilon) and refractive index (n) of applied solvents. The absorption spectra only slightly are affected by the solvent polarity in contrast to the fluorescence spectra that are highly solvatochromic and display a large Stokes shift. The analysis of the solvatochromic behavior of the fluorescence spectra as function of Deltaf (epsilon, n) revealed that the emission occurs from a high polarity excited state. The large dipole moment change along with the strongly red-shifted fluorescence, as the solvent polarity is increased, demonstrate the formation of an intramolecular charge transfer state (ICT). Compounds under the study were used as fluorescence probes for monitoring the kinetics of polymerization. The study on the changes in fluorescence intensity and spectroscopic shifts of studied compounds were carried out during thermally initiated polymerization of methyl methacrylate (MMA) and during photoinitiated polymerization of 2-ethyl-2-(hydroxymethyl)propane-1,3-diol triacrylate (TMPTA).