4,4'-Bis(2-benzoxazolyl)stilbene

A 4,4'-bis(2-benzoxazolyl)stilbene luminescent probe: assessment of aggregate formation through photophysics experiments and quantum-chemical calculations.[Pubmed:30324198]

Phys Chem Chem Phys. 2018 Nov 7;20(41):26249-26258.

A combination of experimental and quantum mechanical investigations is applied to the study of the optical features of 4,4'-bis(2-benzoxazolyl)stilbene (BBS) dissolved in solution or in a poly(l-lactic acid) (PLA) thermoplastic matrix at different concentrations. The experimental analyses allow the characterization of BBS solutions and dispersions in terms of absorption and emission features, along with the collection of some key parameters such as fluorescence quantum yield, anisotropy and lifetime, while the computational approach gives a detailed description of the photophysical behavior of BBS in the different environments. For the 10(-5) M BBS solution, the fluorescence spectra show the expected peaks at 425 and 455 nm of the non-interacting BBS molecules with a single fluorescence lifetime of 0.85 ns without revealing any aggregation phenomena, prevented by the short lifetime and fast diffusion rate of the monomer. Moreover, the calculated spectra are in excellent agreement with the experiments, thus showing the reliability of the computational approach. In time-resolved emission experiments (TRES) on more concentrated solutions (10(-4) M) and on BBS crystals, the presence of an excimer is revealed by the appearance of a broad peak around 540 nm, followed by the disappearance of the two main peaks at 460 nm on a time scale of about 10 ns. The computational analysis attributes this behavior to the formation of aggregates of different geometries. The BBS dispersions in PLA reveal the presence of different BBS architectures depending on the fluorophore content. Even at low concentrations, BBS is mainly dispersed as a monomer in the matrix, spheroid aggregates of about 800-900 nm in diameter are also present and the relevant fluorescence spectra arise from the combination of monomer and aggregate contributions. At higher concentrations, BBS starts forming crystals of a peculiar helicoidal shape, with a diameter of about 2 mum, variable length up to several hundreds of mum and emission spectra similar to those of isolated BBS crystals.

Photophysical and electrochemical investigations of the fluorescent probe, 4,4'-bis(2-benzoxazolyl)stilbene.[Pubmed:23305534]

J Phys Chem A. 2013 Feb 7;117(5):836-44.

In solution, 4,4'-bis(2-benzoxazolyl)stilbene (BBS) was found to exhibit consistently high absolute fluorescence quantum yields (Phi(fl) >/= 0.88) and a monoexponential lifetime, both independent of BBS concentration. The BBS steady-state and time-resolved photophysics were investigated by different techniques to understand the various deactivation pathways. Nonradiative deactivation of BBS singlet excited state by intersystem crossing was found to be negligible. Other than fluorescence, the excited state of BBS was found to be deactivated by trans-cis photoisomerization. At low concentrations ( approximately 5 mug/mL), UV spectroscopy and laser flash photolysis showed concordant results that the photoinduced cis isomer gradually replaced the original absorption spectrum of the pure trans isomer. However, at high concentrations ( approximately 0.2 mg/mL), (1)H NMR and DOSY measurements confirmed that irradiating BBS at 350 nm induced a conversion from the trans-BBS into its cis isomer by photoisomerization. It was further found that the stilbene moiety of both isomers was photocleaved. The resulting photoproduct was an aldehyde that was oxidized under ambient conditions to its corresponding carboxylic acid, i.e., 4-(1,3-benzoxazol-2-yl)benzoic acid. The structure of the photoproduct was unequivocally confirmed by X-ray diffraction. Spectroscopic investigation of BBS showed a limited photoisomerization after irradiation at 350 nm of a trans solution. The BBS electrochemistry showed irreversible oxidation, resulting in an unstable and highly reactive radical cation. Similarly, the cathodic process was also found to be irreversible, giving rise to a radical anion and showing its n-doping character.

Dichroic properties of bis(benzoxazolyl)stilbene and bis(benzoxazolyl)thiophene dispersed into oriented polyethylene films: a combined experimental and density functional theory approach.[Pubmed:16494319]

J Phys Chem B. 2006 Feb 23;110(7):3127-34.

In this work, a combination of experimental and quantum mechanical investigations is performed for the study of dichroic absorption properties of melt-processed linear low-density polyethylene (LLDPE) oriented films containing < or =0.5 wt % of either 4,4'-bis(2-benzoxazolyl)stilbene (BBS) or 2,5-bis(5-tert-butyl-2-benzoxazolyl)thiophene (BTBBT). The data acquired reveal that the film optical anisotropy and the performances as linear polarizer are strongly dependent on the molecular structure of the chromophore. In particular, the rodlike structure of BBS favors the alignment of the dye along the drawing direction of the PE film, providing dichroic ratios as high as 100 and optical performances as linear polarizer close to the pseudo-affine deformation scheme. On the contrary BTBBT, although characterized by huge anisotropic potentialities, confers the oriented film very poor dichroism and is unsuitable for linear polarizer applications. This behavior is attributed to the more complex banana-shaped structure of BTBBT dye caused by the thiophene 2,5-functionalization that limits the molecule parallel orientation to the drawing direction.

Development of combinatorial chemistry methods for coatings: high-throughput optimization of curing parameters of coatings libraries.[Pubmed:12433104]

Anal Chem. 2002 Nov 1;74(21):5676-80.

An automated analytical system has been implemented for the high-throughput optimization of processing conditions such as curing parameters in fabrication of UV-cured automotive organic protective coatings. Selection of optimum process conditions of combinatorial arrays of coatings is essential to correlate the high-throughput screening and conventional processes and to achieve the desired physical properties of coatings. For monitoring of curing conditions of each coating in the array, a viscosity-sensitive fluorophore 4,4'-bis(2-benzoxazolyl)stilbene was incorporated into coating formulations. This fluorescence tagging approach permitted us to combine a gradient temperature heater and a UV curing system with the full capabilities of our high-throughput screening system, including generation of spectroscopic data and its analysis. This investigation demonstrated the possibility of rapid decoupling of temperature and radiation effects in curing of UV-curable coating formulations by using multiple coatings and process conditions at once. While the system described here was implemented for high-throughput optimization of temperature conditions of radiation curing of arrays of organic protective coatings for automotive applications, this system can be further applied for a variety of other applications where optimization of process parameters can be studied in situ or off-line using optical spectroscopic tools.