Manuel Galán

Ground and singlet excited state hydrogen bonding interactions of betacarbolines

To study the ground and singlet excited state hydrogen bonding donor/acceptor properties of the betacarboline ring, 9H-pyrido[3,4-b]indole, we have carried out a spectroscopic study of the interactions of harmane, 1-methylbetacarboline, HN, and its N9-methyl derivative, MHN, with different hydrogen bonding acceptor/donor molecules in the non-polar solvent cyclohexane. UV–visible, steady-state and time-resolved fluorescence measurements show that HN and MHN form fluorescent 1:1 ground state hydrogen bonded pyridinic complexes with the hydrogen bond donors tert-butanol, 2-chloroethanol and hexafluoropropan-2-ol. At high concentrations, the strongest hydrogen bond donors chloroethanol and hexafluoropropan-2-ol form additional proton transfer ground-state 1:2 hydrogen-bonded complexes which, upon photoexcitation, give phototautomers of zwitterionic structures. The aromatic donor phenol also forms hydrogen bonded pyridinic complexes with HN, but zwitterionic species are not observed. Furthermore, the hydrogen bonding HN–phenol interaction quenches the HN fluorescence. On the other hand, the interactions of HN with the proton acceptors tetrahydrofuran, N,N-dimethylformamide and hexamethylphosphoramide also give fluorescent 1:1 hydrogen bonded pyrrolic complexes which do not form phototautomeric zwitterions. These results conclusively show that the formation of zwitterionic phototautomers involves the initial attack of a hydrogen bonding donor molecule on the pyridinic nitrogen atom of the betacarboline and the formation of a 1:2 proton transfer complex.

Dynamic Study of Excited State Hydrogen‐bonded Complexes of Harmane in Cyclohexane–Toluene Mixtures¶

Abstract Photoinduced proton transfer reactions of harmane or 1-methyl-9H-pyrido[3,4-b]indole (HN) in the presence of the proton donor hexafluoroisopropanol (HFIP) in cyclohexane–toluene mixtures (CY–TL; 10% vol/vol of TL) have been studied. Three excited state species have been identified: a 1:2 hydrogen-bonded proton transfer complex (PTC), between the pyridinic nitrogen of the substrate and the proton donor, a hydrogen-bonded cationlike exciplex (CL*) with a stoichiometry of at least 1:3 and a zwitterionic exciplex (Z*). Time-resolved fluorescence measurements evidence that upon excitation of ground state PTC, an excited state equilibrium is established between PTC* and the cationlike exciplex, CL*, λem ≈ 390 nm. This excited state reaction is assisted by another proton donor molecule. Further reaction of CL* with an additional HFIP molecule produces the zwitterionic species, Z*, λem ≈ 500 nm. From the analysis of the multiexponential decays, measured at different emission wavelengths and as a function of HFIP concentration, the mechanism of these excited state reactions has been established. Thus, three rate constants and three reciprocal lifetimes have been determined. The simultaneous study of 1,9-dimethyl-9H-pyrido[3,4-b]indole (MHN) under the same experimental conditions has helped to understand the excited state kinetics of these processes.

The pyrrole ring as hydrogen-bonding π-donor base: an experimental and theoretical study of the interactions of N-methylpyrrole with alcohols

Abstract The interactions of N -methylpyrrole, MPY, with hexafluoroisopropanol, HFIP, trifluoroethanol, TFE, 2-chloroethanol, CLE, and 1-butanol, BU, have been studied by FTIR measurements and ab initio calculations. The experiments, carried out on the OH stretching band of the alcohols, proved the formation of 1:1 stoichiometric hydrogen-bonded complexes in which the OH group acts as H-donor and the aromatic ring as acceptor. DFT calculations with the B3LYP functional and the 6-31++ G** basis sets found minima, for all the complexes, with T-shape geometries, where the OH group of the alcohol points to the C-3 in the aromatic ring of MPY. The experimental association constants and the shifts of the OH bands showed a decrease as the H-donor ability of the alcohol diminishes. These parameters are in qualitative agreement with the stabilization energies and frequency-shifts theoretically calculated. Measurements carried out in different solvents showed that the solvent polarity affects the association constant values, but the shifts of the associated bands keep unchanged.

A fluorescence study of the molecular interactions of harmane with the nucleobases, their nucleosides and mononucleotides.

Fluorescence binding studies of harmane to the elemental components of the nucleic acids were undertaken to investigate the origin of the interaction between the drug and DNA. Most of the tested substrates have been found to induce hypochromism in the absorption spectrum of harmane and to quench its fluorescence. The quenching process induced by the nucleobases and their nucleosides is mainly due to the formation of ground state 1:1 complexes. However, in the case of the mononucleotides a dynamic quenching component is also observed. This quenching component is likely due to the excited state interaction of harmane with the phosphate group of the nucleotides. UV-vis spectral changes and quenching measurements have been used to quantify the ground state association constants of the complexes and the quenching rate constants.

Hydrogen bonding NH/π interactions between betacarboline and methyl benzene derivatives

In the presence of benzene, toluene, m-xylene, mesitylene and durene, the pyrrolic NH stretching band of betacarboline, 9H-pyrido[3,4-b]indole, and its 1-methyl derivative, harmane, in tetrachloroethane diminishes in intensity while a new red-shifted band grows up. The shifts of the associated bands increase linearly with the pi-electron density of the substrates. These spectral changes are attributed to the formation of 1:1 molecular association complexes between the betacarbolines and the benzenoid substrates. The complexes are stabilized by the hydrogen-bonding interaction between the pyrrolic NH group of betacarboline and the pi-delocalized electrons of the benzene derivatives. The influence of these NH/pi hydrogen-bonding interactions in the fluorescence spectra of betacarboline is discussed.

A spectroscopic study of the molecular interactions of harmane with pyrimidine and other diazines

FTIR, UV-vis, steady state and time-resolved fluorescence measurements show that harmane (1-methyl-9H-pyrido/3,4-b/indole) interacts with pyrimidine and its isomers pyrazine and pyridazine in its ground and lowest singlet states. The mechanisms of interaction are dependent on both the structure of the diazine and the nature of the solvent. Thus, in a low polar solvent such as toluene, harmane forms ground state 1:1 hydrogen-bonded complexes with all the diazines. These complexes quench the fluorescence of harmane and diminish its fluorescence lifetime. Conversely, in buffered (pH 8.7) aqueous solutions, pyrimidine behaves differently from the other diazines. Thus, whereas pyrimidine only interacts with harmane in its ground state, pyrazine and pyridazine also interact in the excited state. The harmane-pyrimidine ground state interaction is an entropic controlled process. Therefore, we propose the formation of pi-pi stacked 1:1 complexes between these substrates. Association constants for the different types of complexes and quenching parameters are reported.

Electronic spectra and photophysics of δ-carboline (5H-pyrido[3,2-b]indole)

Abstract The absorption, excitation, fluorescence and phosphorescence spectra of δ-carboline, 5H-pyrido[3,2-b]indole, have been investigated in non-aqueous solvents of various polarity at room temperature and 77 K. The δ-carboline fluorescence shows time-resolved red shifts, solvatochromic effects, non-exponential decays and anomalous temperature dependence. These results suggest that the fluorescence emission of δ-carboline originates from two close low lying excited states which do not equilibrate on the time scale of fluorescence emission: a non-polar locally excited state (LE) and a polar intramolecular charge transfer state (CT). The contributions of the LE and CT states to the total fluorescence spectrum of δ-carboline progressively change as the solvent polarity increases. The possibility of a vibronically induced coupling of these excited states could also reasonably explain the reduction of the fluorescence quantum yield and the increase of the phosphorescence quantum yield of δ-carboline as compared with carbazole.

Kinetic salt effects in intramolecular electron transfer

A study is made of the kinetic salt effects upon intramolecular electron transfer. These effects are analysed in terms of the solvent and ionic-cloud reorganizations and the influence of the electrolyte on the thermodynamics of the process. The latter seems to be the main cause of the negative effect (decrease in reaction rate) observed in the presence of a supporting electrolyte.

Kinetic salt effects in the bromide oxidation by bromate

The kinetic salt effect on the oxidation of bromide ion by bromate have been studied. It is concluded that the salt effects are the result of ion-solvent interactions and seem to be connected not only to the strength of these interactions, as measured by the lowering of solvent activity, but also by the structural characteristics of ion-solvent interactions. This conclusion is based on the use of Pitzers treatment to estimate the activity coefficients.

CALCULATION OF RATE CONSTANTS FROM UV-VIS SPECTROSCOPIC DATA : AN APPLICATION OF THE MARCUS-HUSH MODEL

Abstract The MMCT band in the binuclear complex μ -cyano pentacyano ruthenium(II) pentaamminecobalt(III) was obtained in different salt solutions. A correlation of optical data with rate constants of the thermal electron transfer in a related binuclear compound was found. Combination of the spectroscopic data with the results of quantum yield measurements allows an estimation of the backward (Co → Ru) electron transfer rate constant, k b . The value of k found is about 10 12 s −1 , and is shown to be equal to the preexponential factor of other rate constants corresponding to other electron transfer reactions in the binuclear complex. This analysis allows determination of all magnitudes characterizing the different optical and thermal electron transfer processes involved. Determination of these parameters is usually not possible from experiments. The present work shows how to overcome difficulties that arise when not all the required experimental information is available.