Photophysical properties of borondipyrromethene analogues in solution

The photophysical properties of seven new 8-(p-substituted)phenyl analogues of 4,4-difluoro-3,5-dimethyl-8-(aryl)-4-bora-3a,4a-diaza-s-indacene (derivatives of the well-known fluorophore BODIPY) in several solvents have been studied by means of absorption and steady-state and time-resolved fluorimetry. For each compound, the fluorescence quantum yield and lifetime are lower in solvents with higher polarity owing to an increase in the rate of nonradiative deactivation. Increasing the electron withdrawing strength of the p-substituent on the phenyl group in position 8 also leads to lower fluorescence quantum yields and lifetimes. When the p-substituent on the phenyl group in position 8 is a tertiary amine [8-(4-piperidinophenyl), 8-(4-N,N-dimethylaminophenyl), and 8-(4-morpholinophenyl)], the low quantum yields of these compounds in more polar solvents can be rationalized by the inversion of the energy levels of an apolar, highly fluorescent and a polar, nonfluorescent excited state, where charge transfer from the tertiary amine to the BODIPY unit occurs. These amine analogues can be protonated at low pH in aqueous solution. Fluorescence titrations yielded pK(a) values of their conjugate ammonium salts which are in agreement with the electron donating tendency of the amine group: piperidino (4.15) > dimethylamino (2.37) > morpholino (1.47), with the pK(a) values in parentheses. The rate constant of radiative deactivation (k(f)) is the same for all compounds in all solvents studied (k(f) = 1.4 x 10(8) s(-1)).     

Laser pulse fluorometry as a tool in polymer research

A time-resolved fluorescence spectrometer with laser excitation and picosecond time-correlated single photon counting detection is described. The parameter recovery from experimental data is discussed in detail. Correction methods for errors in the time-correlated single photon counting technique are indicated. Applications of this photophysical tool to polymer research are given.     

Synthesis,Spectroscopy, Crystal Structure Determination,and Quantum Chemical Calculations of BODIPY Dyes with Increasing Conformational Restriction and Concomitant Red‐Shifted Visible Absorption and Fluorescence Spectra

Starting from the conformationally unconstrained compound 3,5‐di‐(2‐bromophenoxy)‐4,4‐difluoro‐8‐(4‐methylphenyl)‐4‐bora‐3a,4a‐diaza‐s‐indacene ( 1 ), two BODIPY dyes ( 2 and 3 ) with increasingly rigid conformations were synthesized in outstanding total yields through palladium catalyzed intramolecular benzofuran formation. Restricted bond rotation of the phenoxy fragments leads to dyes 2 and 3 , which absorb and fluoresce more intensely at longer wavelengths relative to the unconstrained dye 1 . Reduction of the conformational flexibility in 2 and 3 leads to significantly higher fluorescence quantum yields compared to those of 1 . X‐ray diffraction analysis shows the progressively more extended planarity of the chromophore in line with the increasing conformational restriction in the series 1 → 2 → 3 , which explains the larger red shifts of the absorption and emission spectra. These conclusions are confirmed by quantum chemical calculations of the lowest electronic excitations in 1 , 1a , 2 , 2a , 3 and dyes of related chemical structures. The effect of the molecular structure on the visible absorption and fluorescence emission properties of 1 , 1a , 2 , 2a , 3 has been examined as a function of solvent by means of the new, generalized treatment of the solvent effect (J. Phys. Chem. B 2009 , 113, 5951–5960). Solvent polarizability is the primary factor responsible for the small solvent‐dependent shifts of the visible absorption and fluorescence emission bands of these dyes.     

Compartmental modeling of reversible intermolecular two-state excited-state processes coupled with rotational diffusion or with added quencher

Analysis of related time-resolved fluorescence measurements can possibly lead to the determination of the kinetic parameters of excited-state processes. A deterministic identifiability analysis on an error-free fluorescence decay data surface has to be executed to verify whether the parameters of a particular model can be determined and may point to the minimal experimental conditions under which this will become possible. In this work, similarity transformation is chosen as an identifiability analysis approach because it also gives the explicit relationships between the true and alternative model parameters. Results are presented for two kinetic models of a reversible intermolecular two-state excited-state process in isotropic environments: (a) with coupled species-dependent rotational diffusion described by Brownian reorientation and (b) with added quencher. For model a, both spherically and cylindrically symmetric rotors, with no change in the principal axes of rotation in the latter, are considered. The fluorescence delta-response functions I(parallel)(t) and I(perpendicular)(t), for fluorescence polarized respectively parallel and perpendicular to the electric vector of linearly polarized excitation, are used to define the sum S(t) = I( parallel)(t) + 2 I( perpendicular)(t) and the difference D(t) = I(parallel)(t) - I(perpendicular)(t) function. The identifiability analysis is carried out using the S(t) and D(t) functions. The analysis involving S(t) shows that two physically acceptable possible solutions for the overall rate constants of the excited-state process exist. Inclusion of information from polarized fluorescence measurements on the rotational kinetic behavior contained in D(t) results in the unique set of rate constants and rotational diffusion coefficients when the rotational diffusion coefficients are different. For model b, it is shown that addition of quencher plays formally the same role as rotational diffusion as far as the identification is concerned. When the quenching rate constants are different, the rate constants of a reversible intermolecular two-state excited-state process with added quencher can be uniquely determined.     

Palladium catalyzed synthesis of Ca indicators with aryl bithiophene and terthiophene fluorophores

Five new fluorescent indicators for Ca²⁺ were synthesized using the Stille reaction. They all consist of the tricarboxylate chelator APTRA (o-aminophenol-N,N,O-triacetic acid) linked to a (substituted) bithiophene or terthiophene fluorophore. The dissociation constants K_d measured via fluorimetric titrations at 21 °C in 100 mM KCl buffered solution, pH 7.05, for the Ca²⁺ complexes with the new probes are in the range between 10 and 40 μM.     

Solvent and pH dependent fluorescent properties of a dimethylaminostyryl borondipyrromethene dye in solution

Steady-state and time-resolved fluorescence techniques have been used to study the photophysical properties of the fluorescent BODIPY-derived dye 3-{2-[4-(dimethylamino)phenyl]ethenyl}-4,4-difluoro-8-(4-methoxyphenyl)-1,5,7-trimethyl-3a,4a-diaza-4-bora-s-indacene. This compound has been synthesized via a microwave-assisted condensation of p-N,N-dimethylaminobenzaldehyde with the appropriate 1,3,5,7-tetramethyl substituted borondipyrromethene unit. The fluorescence properties of the dye are strongly solvent dependent: increasing the solvent polarity leads to lower fluorescence quantum yields and lifetimes, and the wavelength of maximum fluorescence emission shifts to the red. The Catalán solvent scales are found to be the most suitable for describing the solvatochromic shifts of the fluorescence emission. These are dominated by polarity/polarizability effects, as confirmed by quantum-chemical calculations performed in the dielectric continuum approximation. Fluorescence decay profiles of the dye can be described by a single-exponential fit in most solvents investigated, while two decay times are found in alcohols. The dye undergoes a reversible protonation-deprotonation reaction in the acidic pH range with a pK(a) of 2.25 in acetonitrile solution. Fluorimetric titrations as a function of pH produce fluorescence emission enhancements at lower pH. The fluorescence excitation spectra show a hypsochromic shift from 600 nm for the neutral amine to 553 nm for the ammonium form, so that ratiometric measurements can be used to determine pK(a).     

BODIPY-based hydroxyaryl derivatives as fluorescent pH probes

[structures: see text] Seven new 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes with phenolic or naphtholic subunits on position 8 and with substituents having different electron driving forces on positions 3 and 5 were synthesized. Their absorption and steady-state fluorescence properties were investigated as a function of solvent. The novel compounds, with the exception of 4,4-difluoro-8-(4-hydroxyphenyl)-3,5-bis-(4-methoxyphenyl)-4-bora-3a,4a-diaza-s-indacene, are characterized by absorption maxima in the range 493-515 nm and small (400-600 cm(-1)) Stokes shifts. The exceptional dye has absorption maxima in the 570-580 nm region and fluorescence emission maxima around 610-620 nm, depending on the solvent. In aqueous solution, the dyes show a large fluorescent enhancement upon increasing the acidity of the solution. They can be used in aqueous solution as fluorescent pH probes excitable with visible light, with pKa values ranging from 7.5 to 9.3, depending on the substitution pattern on positions 3, 5, and 8.     

Compartmental analysis in photophysics: fluorescence decay kinetics and identifiability analysis of a model for successive complexation

The triple-exponential fluorescence delta-response function is derived for the photophysical model of successive complexation between ligand and analyte. Initially, a complex with 1:1 stoichiometry between ligand and analyte is formed. Further binding leads to a complex with two analyte molecules per ligand molecule. We show that this model is uniquely identifiable. This means that all deactivation and exchange rate constants in the excited state and all spectral parameters associated with photoexcitation and fluorescence emission can be uniquely determined. The issues of controllability and observability are discussed for this photophysical system. The conditions, under which a non-controllable or non-observable system is obtained, are described.     

1,7-Disubstituted boron dipyrromethene (BODIPY) dyes: synthesis and spectroscopic properties

1,7-Dihalogenated boron dipyrromethene dyes were successfully synthesized and substituted, thus providing an entry to the final, elusive reactivity pattern. The spectroscopic properties of 1,7-disubstituted BODIPY dyes were studied and are discussed as a function of their structure.