Aleksander Filarowski

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.

STRENGTHENING OF THE INTRAMOLECULAR O...H...N HYDROGEN BONDS IN SCHIFF BASES AS A RESULT OF STERIC REPULSION

Abstract Three ortho hydroxy Schiff bases 2-( N- methyl- α -iminoethyl)-phenol ( I ), 2-( N- benzyl- α -iminoethyl)-phenol ( II ) and 2-( N- benzyl- α -iminopropyl)-phenol ( III ) were synthesised in which the hydrogen atoms in C–C(H)N group were replaced by an alkyl substituent. The crystal structures were determined and ab initio calculations at the B3LYP/6-31G** basis set were performed. One of the shortest known O–H⋯N hydrogen bonds were found with O⋯N distance equal to 2.459(3), 2.497(3) and 2.494(3) A, respectively in I , II and III . Steric repulsion of substituted alkyl results in this unusual strengthening of the hydrogen bonds. FT-IR spectra in solution and in the solid state in function of the temperature were determined. Strong influence of the temperature on the character of the hydrogen bond was observed.

Rational Design, Synthesis, and Spectroscopic and Photophysical Properties of a Visible‐Light‐Excitable, Ratiometric, Fluorescent Near‐Neutral pH Indicator Based on BODIPY

A visible-light-excitable, ratiometric, brightly fluorescent pH indicator for measurements in the pH range 5-7 has been designed and synthesized by conjugatively linking the BODIPY fluorophore at the 3-position to the pH-sensitive ligand imidazole through an ethenyl bridge. The probe is available as cell membrane permeable methyl ester 8-(4-carbomethoxyphenyl)-4,4-difluoro-3-[2-(1H-imidazol-4-yl)ethenyl]-1,5,7-trimethyl-3a,4a-diaza-4-bora-s-indacene (I) and corresponding water-soluble sodium carboxylate, sodium 8-(4-carboxylatophenyl)-4,4-difluoro-3-[2-(1H-imidazol-4-yl)ethenyl]-1,5,7-trimethyl-3a,4a-diaza-4-bora-s-indacene (II). The fluorescence quantum yield Φ(f) of ester I is very high (0.8-1.0) in the organic solvents tested. The fluorescence lifetime (ca. 4 ns) of I in organic solvents with varying polarity/polarizability (from cyclohexane to acetonitrile) is independent of the solvent with a fluorescence rate constant k(f) of 2.4×10(8) s(-1). Probe I is readily loaded in the cytosol of live cells, where its high fluorescence intensity remains nearly constant over an extended time period. Water-soluble indicator II exhibits two acid-base equilibria in aqueous solution, characterized by pK(a) values of 6.0 and 12.6. The Φ(f) value of II in aqueous solution is high: 0.6 for the cationic and anionic forms of the imidazole ligand, and 0.8 for neutral imidazole. On protonation-deprotonation in the near-neutral pH range, UV/Vis absorption and fluorescence spectral shifts along with isosbestic and pseudo-isoemissive points are observed. This dual-excitation and dual-emission pH indicator emits intense green-yellow fluorescence at lower pH and intense orange fluorescence at higher pH. The influence of ionic strength and buffer concentration on the absorbance and steady-state fluorescence of II has also been investigated. The apparent pK(a) of the near-neutral acid-base equilibrium determined by spectrophotometric and fluorometric titration is nearly independent of the added buffer and salt concentration. In aqueous solution in the absence of buffer and in the pH range 5.20-7.45, dual exponential fluorescence decays are obtained with decay time τ(1)=4.3 ns for the cationic and τ(2)=3.3 ns for the neutral form of II. The excited-state proton exchange of II at near-neutral pH becomes reversible on addition of phosphate (H(2)PO(4)(-)/HPO(4)(2-)) buffer, and a pH-dependent change of the fluorescence decay times is induced. Global compartmental analysis of fluorescence decay traces collected as a function of pH and phosphate buffer concentration was used to recover values of the deactivation rate constants of the excited cationic (k(01)=2.4×10(8) s(-1)) and neutral (k(02)=3.0×10(8) s(-1)) forms of II.

Proton transfer and self-association of sterically modified Schiff bases

Abstract The intramolecular proton transfer process has been studied by measuring of the dipole moment, an average molecular weight, and UV/VIS spectra as a function of temperature and concentration in a few solvents of low polarity. Ten Schiff bases were selected for such studies, where steric repulsion of alkyl- and aryl-substituents in azomethine moiety enhances the strength of hydrogen bond and the proton transfer process. The self-association of species with intramolecular proton transfer has been investigated as a function of solvent polarity. Behaviour of these systems has been compared with those of Mannich bases, where intramolecular electron coupling between acid and base centres is considerably prevented by –CH 2 – bridge. A substantially different behaviour of these two types of compounds has been found out.

Specificity of the intramolecular hydrogen bond. The differences in spectroscopic characteristics of the intermolecular and intramolecular H-bonds

Abstract IR spectroscopic properties—positions of ν (OH) bands and their integrated intensities for series of nine Schiff bases, 16 complexes of phenols with N , N -dimethylbenzylamine and 11 complexes of phenols with N -benzylidenemethylamine—were determined in CCl 4 solutions and compared with their Δp K a values. Characteristics of particular groups of compounds with the hydrogen bond were determined. It was shown that the strong decrease of the intensity of the ν (OH) bands is a specific feature of the intramolecular hydrogen bonding in comparison to related intermolecular complexes. It results from bending of the intramolecular hydrogen bond and is not limited to pseudoaromatic chelate rings only. Such conclusion results also from semiempirical and ab initio calculations.

Proton transfer reaction of a new orthohydroxy Schiff base in protic solvents at room temperature.

Ground and excited state inter- and intramolecular proton transfer reactions of a new o-hydroxy Schiff base, 7-ethylsalicylidenebenzylamine (ESBA) have been investigated by means of absorption, emission and nanosecond spectroscopy in different protic solvents at room temperature and 77 K. The excited state intramolecular proton transfer (ESIPT) is evidenced by a large Stokes shifted emission (approximately 11000 cm(-1)) at a selected excited energy in alcoholic solvents. Spectral characteristics obtained reveal that ESBA exists in more than one structural form in most of the protic solvents, both in the ground and excited states. From the nanosecond measurements and quantum yield of fluorescence we have estimated the decay rate constants, which are mainly represented by nonradiative decay rates. At 77 K the fluorescence spectra are found to be contaminated with phosphorescence spectra in glycerol and ethylene glycol. It is shown that the fluorescence intensity and nature of the species present are dependent upon the excitation energy.

H/D Isotope Effects in Hydrogen Bonded Systems

An extremely strong H/D isotope effect observed in hydrogen bonded A-H…B systems is connected with a reach diversity of the potential shape for the proton/deuteron motion. It is connected with the anharmonicity of the proton/deuteron vibrations and of the tunneling effect, particularly in cases of short bridges with low barrier for protonic and deuteronic jumping. Six extreme shapes of the proton motion are presented starting from the state without possibility of the proton transfer up to the state with a full ionization. The manifestations of the H/D isotope effect are best reflected in the infra-red absorption spectra. A most characteristic is the run of the relationship between the isotopic ratio νH/νD and position of the absorption band shown by using the example of NHN hydrogen bonds. One can distinguish a critical range of correlation when the isotopic ratio reaches the value of ca. 1 and then increases up to unusual values higher than . The critical range of the isotope effect is also visible in NQR and NMR spectra. In the critical region one observes a stepwise change of the NQR frequency reaching 1.1 MHz. In the case of NMR, the maximal isotope effect is reflected on the curve presenting the dependence of Δδ (1H,2H) on δ (1H). This effect corresponds to the range of maximum on the correlation curve between δH and ΔpKa that is observed in various systems. There is a lack in the literature of quantitative information about the influence of isotopic substitution on the dielectric properties of hydrogen bond except the isotope effect on the ferroelectric phase transition in some hydrogen bonded crystals.

Density functional theory study of intramolecular hydrogen bonding and proton transfer in o-hydroxyaryl ketimines.

The potential energy surface and the reaction pathway for the intramolecular hydrogen transfer in o-hydroxyaryl ketimines are characterized using DFT methods. Structural changes in the proton-transfer process in quasi-aromatic hydrogen bonding are described. A transition state and a state with a low proton-transfer barrier were studied in sterically compressed o-hydroxyaryl ketimines (2(N-methyl-alpha-iminoethyl)phenols) possessing two potential minimums. The potentials for proton vibration in the OH and HN tautomers of o-hydroxyaryl ketimines were investigated and anharmonic frequencies were determined. Solvent and substituent effects were analyzed. The energies of the various conformers of the OH and HN tautomers were compared with the related forms of o-hydroxyaryl aldimine.

Steric Modification of the Intramolecular Hydrogen Bond in 2-(Methylimino-phenyl-methyl)-phenols

Summary. Three ortho-hydroxy Schiff bases (2-(methylimino-phenyl-methyl)-phenol (1), 4-methyl-2-(methylimino-phenyl-methyl)-phenol (2), 2-(benzylimino-phenyl-methyl)-phenol (3)) were synthesized in which the hydrogen atom in the C*C(H)*N group was substituted by a phenyl ring. Their crystal structures were determined. Strong O*H⋯N type intramolecular hydrogen bonds were found (dON=2.496(2) Å, dOH=1.11(3) Å, dHN=1.45(4) Å and dON=2.488(2) Å, dOH= 1.20(4) Å, dHN=1.37(4) Å in 1; dON=2.505(2) Å, dOH=1.16(3) Å, dHN=1.39(3) Å in 2; dON=2.528(2) Å, dOH=1.08(3) Å, dHN=1.54(4) Å in 3 together with a large proton delocalization, especially in 1 and 2. It was demonstrated that the strengthening of the hydrogen bond in comparison to related non substituted compounds results from the steric repulsion exerted by the phenyl ring.Zusammenfassung. Drei ortho-hydroxy-substituierte Schiffsche Basen (2-(Methylimino-phenyl-methyl)-phenol (1), 4-Methyl-2-(methylimino-phenyl-methyl)-phenol (2), 2-(Benzylimino-phenyl-methyl)-phenol (3)) wurden synthesiert, in welchen das Wasserstoffatom der C*C(H)*N-Gruppe durch einen Phenylring substituiert ist. Es wurden starke Wasserstoffbrückenbindungen vom Typ O*H⋯N (dON=2.496(2) Å, dOH=1.11(3) Å, dHN=1.45(4) Å und dON=2.488(2) Å, dOH=1.20(4) Å, dHN=1.37(4) Å in 1; dON=2.505(2) Å, dOH=1.16(3) Å, dHN=1.39(3) Å in 2; dON=2.528(2) Å, dOH=1.08(3) Å, dHN=1.54(4) Å in 3) gefunden, wobei vor allem in 1 und 2 eine beträchtliche Delokalisierung des Protons auftritt. Es konnte gezeigt werden, daß die Stärkung der Wasserstoffbrückenbindung im Vergleich zu nichtsubstituierten Verbindungen aus der sterischen Abstoßung des Phenylrings resultiert.

The photoinduced isomerization and its implication in the photo-dynamical processes in two simple Schiff bases isolated in solid argon

Two Schiff bases: 2-(1-(methylimino)methyl)-phenol (SMA) and its chlorosubstituted derivative 2-(1-(methylimino)methyl)-6-chlorophenol (SMAC), and SMA complexes with water were studied by infrared matrix isolation spectroscopy and DFT/B3LYP/6-311G++(2d,2p) quantum chemical calculations. SMA and SMAC bases trapped in an argon matrix from the vapor above the liquid and solid samples have the most stable enol conformation with intramolecular O-H···N bonding. Irradiation (λ > 320 nm) leads in both bases to a rotational isomerization reaction in which the scission of the O-H···N bond occurs and the C(H)NCH(3) and OH groups are turned by 180° around the C-C and C-O bonds, respectively. In SMAC a competitive photoreaction channel yields the trans-keto tautomer. The identification of the two SMAC photoproducts evidences that in the excited enol form of this compound two processes compete with each other: the rotational isomerization and intramolecular proton transfer (ESIPT). In the argon matrices doped with SMA and H(2)O the SMA-water complexes were identified and characterized spectroscopically. Interaction of SMA with one or two water molecules does not affect the photochemistry of SMA.