Anna Szemik-Hojniak

Anomalous strengthening of the intramolecular hydrogen bond by steric repulsion

Abstract The crystal structures of two ortho-aminomethylphenols, the 4,5-dimethyl- and 3,5,6-trimethyl-2( N , N -dimethylaminomethyl) phenols, were determined. The stronger intramolecular hydrogen bond was found in the compound with the higher number of methyl substituents. This result was supported by IR spectra. The atomic charge distribution calculated by the MNDO-PM3 method predicts higher strengths of the hydrogen bond in 4,5-dimethyl-2( N , N -dimethylaminomethyl) phenol molecules. Both theoretical calculations and crystallographic results explain the observed effects with reference to the steric interaction of the ortho methyl group which squeezes the OHtctdot;N hydrogen bridge.

Intramolecular excited state proton transfer in Mannich base — 3,5,6-trimethyl-2(N,N′-diethylaminomethyl) phenol

Abstract Steady state fluorescence spectra of 3,5,6-trimethyl-2( N,N ′-diethylaminomethyl) phenol ( I ) in various solvents at room temperature and in n -propanol solution at low temperature were investigated. Dual fluorescence was only observed in polar solvents. It was attributed to the existence of two tautomeric forms in the excited state: the molecular and ionic forms of the intramolecular hydrogen bond of I . Ground state semiempirical structure optimization was performed within the GRINDOL, MNDO, MINDO/3, AM1 and PM3 techniques. Only AM1 and PM3 gave a reasonable geometry of the hydrogen-bonded chelate ring. For these geometries calculation of the electronic excited states by the GRINDOL-CI method confirmed the 1 L b , state as the lowest emitting state in the hydrogen-bonded form of I . The existence of single and double fluorescence in non-polar and polar solvents respectively was correctly reproduced by the GRINDOL-CI procedure.

STRUCTURAL AND SPECTRAL ANALYSIS OF ORTHO- AND META-SUBSTITUTED NITRAMINOPYRIDINE N-OXIDES

Abstract The X-ray structure of three nitraminopyridine N-oxide (NAPNO) crystals have been determined out at room temperature. The following compounds have been studied: 2-nitraminopyridine N-oxide (2NAPNO), 3-nitraminopyridine N-oxide (3NAPNO) and a mixed crystal of 2NAPNO and the methylamine (2NAPNOMA). Quantum-chemical semi-empirical methods were used to optimize them (PM3) and to evaluate their excited-state properties in the gas phase (GRINDOL-CI) as well in polar environments (GRINDOL-CI-VCM). Solvent dependence of the intensity ratio of charge transfer to the 1Lb band in electronic absorption spectra of NAPNO compounds has been noticed. Contrary to 4-nitropyridine N-oxide derivatives, in the NAPNO series an increase in charge transfer connected with increasing displacement of the nitro group from the molecular plane has been observed.

X-ray crystal structure and magnetic and photophysical properties of novel copper(II) N-oxide adduct [(4-MPyO)2(CuCl2)2(H2O)(C2H5OH)] (4-MPyO = 4-(4-methoxystyryl)pyridine N-oxide).

A new mixed adduct, (4-MPyO)2(CuCl2)2(H2O)(C2H5OH) [where 4-MPyO is the 4-(4-methoxystyryl)pyridine N-oxide], was obtained for the first time. It has been characterized by X-ray studies, IR, electronic absorption, and emission spectra, lifetime measurements, and variable-temperature magnetic susceptibility measurements in the range 80-300 K. The single-crystal X-ray diffraction shows that the geometry around both of the copper(II) ions can be described as a tetragonal pyramid with a trapezoidal base at the corners of which are two oxygen atoms of N-oxide and two chlorine atoms. The oxygen atoms of either water or ethanol are at the apex of the pyramid. Besides that, two molecules of the adduct form a double-hydrogen-bonded superdimer in which they are connected to each other through hydrogen bonds of the O-H...Cl type as formed between the chlorine atoms and ethanol molecule (Cl...O 3.22 A). The copper(II) atoms are antiferromagnetically coupled within a dimeric unit, and a singlet-triplet separation of 2 J value (1100 cm(-1)) is greater than the value expected from Hatfields rule for the bridging angles Cu-O-Cu equal 108.9 degrees and 110.2 degrees . By means of the PM3-calculated values of vertical excitation energies, the ligand-to-metal charge-transfer (LMCT) and the metal-to-ligand charge-transfer transitions in the unresolved experimental absorption spectra of I have been revealed. From the large Stokes shift value of emission spectra in solvents of different polarity (more than 6500 cm(-1) in acetonitrile), the charge-transfer (CT) nature of the emissive (LMCT) state of I has been concluded. Biexponential decay of the excited complex in acetonitrile and frozen propanol suggests that the two different CT conformers (0.8, 4.12 ns and 1.99, 15.2 ns, respectively) are present in the excited state in solution while only one CT form is indicated by a monoexponential decay (9.0 micros) in the solid.

Structural and spectral analysis of 3- and 4-methylated nitraminopyridine N-oxides

Abstract The X-ray structures of 3-methyl-2-nitraminopyridine N -oxide (3M) and 4-methyl-2-nitraminopyridine N -oxide (4M) has been determined and experimental electronic absorption spectra investigated. The X-ray results show that both compounds occur as dimers either in the molecular form (3M) or in the proton transferred (PT) form (4M). A ‘driving force’ for protonation depends on the relative position of the two monomers in the crystal lattice (parallel for 3M and perpendicular for 4M) and on the degree of electronic conjugation of –NH–NO 2 substituent with the π-system of the pyridine ring. Position of the methyl group does not affect the direction of the CT process. Quantum-chemical semi-empirical PM3-CI calculations allowed an interpretation of experimental electronic absorption spectra and suggested the prototropic amino–imino equilibria which possible to occur in the solution.

The impact of solvent polarity on intramolecular proton and electron transfer in 2-alkylamino-4-nitro-5-methyl pyridine N-oxides

The crystal structure of 2-butylamino-4-nitro-5-methyl pyridine N-oxide (2B5M) and solution studies of both 2B5M and 2-methylamino-4-nitro-5-methyl pyridine (2M5M) N-oxide are presented. Steady-state absorption and emission measurements were employed for both molecules while a picosecond fluorescence up-conversion technique was used to follow the dynamic behavior of the 2M5M system. The experimental methods were complemented by DFT and TD DFT B3LYP/6-31G(d,p) calculations involving ground and excited-state optimization which in the case of the smaller 2M5M molecule were extended to the CAM-B3LYP/6-31G(d,p) method. The solvent effect is incorporated by applying the polarizable continuum (PCM) model. The data reveal that the 2B5M molecule crystallizes in the monoclinic space group P2(1)/c and its crystal lattice is composed of monomers with intramolecular N-H···O [2.572(3) Å] hydrogen bonds, connected into a polymer network by weak intermolecular C-H…O [3.2-3.4 Å]-type interactions. Quantum-chemical calculations show that the aminoalkyl substitutent in aminoalkyl-pyridine N-oxides is a specific determinant of the CT nature of the lowest-lying excited electronic ππ* state, distinguishing them from other nitroaromatic compounds. The results of both picosecond fluorescence up-conversion experiments in different solvents and quantum-chemical calculations suggest that in nonpolar media the ESIPT process in 2M5M is favored, while in polar acetonitrile, the N* → PT* transition demands barrier-crossing and thus unfavorable thermodynamic conditions do not allow the ESIPT to occur. The signals of picosecond fluorescence up-conversion of 2M5M are solvent- and emission-wavelength dependent. The three time components found in a weakly polar isooctane-dioxane mixture have been attributed to solvation dynamics (∼500 fs), and to relaxation of N* and PT* forms while in acetonitrile, a very rapid fluorescence decay with a time constant (2.3-4.0 ps) indicative of the presence of the normal (N*) form was observed. Much shorter fluorescence lifetimes in alcohols (a few picoseconds) and in D(2)O (less than 200 fs) than in aprotic solvents suggest that in protic media, the solvent molecules participate in the ESIPT, bridging between the methylamine group and the N-oxide group of 2M5M.

The forbidden emission of protonated proton sponge

Abstract The forbidden and strongly redshifted emission of monoprotonated proton sponge (PS·H + ) in water and in acidified acetonitrile is reported. An explanation consistent with both the strongly forbidden nature of the emission as well as its highly redshifted character is proposed to involve an overlap forbidden transition connected with two approximately orthogonal orbitals localized in different parts of the molecule. The corresponding relaxation coordinate leading to the fluorescing charge transfer state is linked with pyramidal (p) to nonpyramidal (np) interconversion of the dimethylamino group of PS·H + and results in a significant energetic lowering of the CT state.

Inversion of states in two conformers of 1,8-bis(dimethylamino) naphthalene—the proton sponge

Abstract The S 1 ↔S 0 transitions of two conformers of 1,8-bis(dimethylamino) naphthalene, the “proton sponge”, have been studied by semiempirical AM1 calculations. They reveal that “inversion of states” occurs in the asymmetric conformer DMAN-2, which in the gas phase may be emitted from the 1 L a state in comparison to the 1 L b state in symmetric DMAN-1. It was also concluded that because of the mixed character of the HOMO-1 orbital in both conformers, a certain CT contribution to the S 0 ↔S 1 transition has to be taken into account. The calculated maxima of absorption and emission have been compared to those experimentally obtained in supersonic expansion.

Impact of micelle ionic electrical double layer structure on the excited state protolytic reaction in the fluorescent probe bound to the colloidal nanoparticles.

The fluorescent probe, 2-hydroxynaphthalene(dodecylo)-6-sulfonamide (NSDA) bound selectively to shear plane of various electrostatic charges was synthesized and its photophysical properties have been investigated by means of steady state fluorescence and nanosecond time-resolved spectroscopy. Our experimental data allowed us to determine the excited state proton transfer (ESPT) rate and equilibrium constants of NSDA bound to micelles and to estimate the electric potential value (Ψ) at the particle surface. The spatial dependence of proton movement velocity through electric double layer (EDL) of micelles has been thoroughly analyzed. In this article, a new approach of estimating the values of the micelle potential (Ψ(R)) from the excited state proton transfer rate constant of the fluorescent probe bound at a certain distance (R) to a micellar surface has been proposed. The Ψ(R) values, obtained in this way, are compared with electrophoretic values of the particle potential (ζ). Our results on electrophoretic potentials and the reaction course of the ESPT in colloidal environment may contribute to a deeper understanding of micellar interactions and behavior of the living cells in contact with various diluted substances such as pharmacological drugs, hormones, proteins, and other colloidal particles.

Cryogenic fluorescence and absorption spectroscopy studies on monomeric and dimeric species of 2-butylamino-6-methyl-4-nitropyridine N-oxide.

2-Butylamino-6-methyl-4-nitropyridine-N-oxide (2B6M) belongs to a group of compounds that can undergo not only excited-state intra-, but also intermolecular proton transfer. The latter of course requires the presence of dimeric species. Previously, we have shown that for 2B6M in aprotic non-polar solvents in the liquid state such dimers play no role. Under these conditions, only one single monomeric species exists, exhibiting anomalous fluorescence behavior, i.e. proton transfer not only starting from the lowest excited electronic singlet state, but also from higher excited states. However, we also noted that under frozen, crystalline matrix conditions more species show up in the spectra. In order to study this multi-species system in more detail, we present absorption and fluorescence experiments on 2B6M, recorded in n-octane at various temperatures between 293 and 5 K. High-resolution spectra are included, not only in fluorescence but also in absorption. We demonstrate that under cryogenic conditions three species can be discerned, two of these providing high-resolution spectra with their main 0-0 lines around 452 and 465 nm, respectively. A detailed vibrational analysis of their emission spectra is included. The third species gives broad-banded spectra, in absorption extending to about 520 nm with its long-wavelength maximum around 460 nm, in emission with a maximum around 535 nm. We tentatively assign the three species to a monomer, a H-bonded dimer and a strongly interacting (pi-pi-stacked) dimer, respectively. We conclude from the excitation spectra that (anomalous) intramolecular proton transfer at higher excited states is still operative under cryogenic conditions. Indications for excited-state intermolecular proton transfer in the stacked dimeric species were not found.