Manuel Mosquera

Dynamic Polar Solvation Is Reported by Fluorescing 4-Aminophthalimide Faithfully Despite H-Bonding

Solvation dynamics of 4-aminophthalimide (4AP) in methanol is measured by broadband upconversion of the fluorescence band. The peak emission frequency nu(t) is determined from 100 fs onward with 85 fs time resolution. Polar solvation based on simple continuum theory, including solute polarizability, describes the temporal shape of nu(t) quantitatively. Extrapolation nu(t-->0) points to an initial emission frequency which agrees with the result from stationary spectroscopy in a nonpolar solvent. The extent (4300 cm(-1)) of the dynamic Stokes shift is largely due (50%) to H-bonding, however. The observations imply that H-bonds with 4AP adiabatically follow the dielectric relaxation of the methanol network. The stimulated emission band is also used to measure solvation dynamics. The evolving band is monitored by transient absorption spectroscopy of supercontinuum probe pulses. But the excited-state absorption spectrum, its relative amplitude, and its evolution are needed to extract nu(t) from such measurements. These key data are obtained by comparison with the upconversion results. Thus calibrated photometrically, 4AP transient absorption can be used to monitor solvation dynamics in any solvent. The excited-state absorption spectrum is assigned with the help of time-dependent density-functional calculations. Fluorescence excitation and double-resonance spectroscopy of isolated 4AP, cooled in a supersonic jet, is used to determine optically active modes. An intramolecular reorganization energy is inferred which is consistent with the value in 2-methylbutane (2025 cm(-1)). The crystal structure is also provided.

Solvent control of molecular structure and excited-state proton-transfer processes of 1-methyl-2-(2′-hydroxyphenyl)- benzimidazole

The influence of the solvent on the structure and photoinduced proton-transfer processes of 1-methyl-2- (2′-hydroxyphenyl)benzimidazole (MeHBI) was studied by means of UV–VIS absorption and fluorescence spectroscopy. The steric hindrance of the methyl group determines the non-planar structure of the MeHBI anion and cation in their ground state, but they undergo a rotation towards planarity in their excited state. In hydroxylic solvents, the excited cation loses the hydroxylic proton to the solvent, affording the keto tautomer. The neutral form of MeHBI exhibits conformational equilibrium dependent on the solvent. The cis-enol conformer, with an intramolecular hydrogen bond and a planar structure, is the dominant species in non-hydroxylic solvents and undergoes an excited-state intramolecular proton-transfer (ESIPT) reaction, producing the keto tautomer. The non-planar enol conformer is the only species detected in the ground state in water; in the excited state it loses the hydroxyl proton, leaving the excited anion. Comparable proportions of both enol conformers exist in alcoholic solvents, their relative proportions having been determined. In these solvents the cis-enol form undergoes ESIPT, whereas the non-planar enol rapidly undergoes rotation towards planarity in the excited state, emitting fluorescence from this state. The solvent hydrogen-bond donor acidity determines the ratio of non-planar enol to cis-enol conformers.

Kinetic studies on the formation ofN-nitroso compounds VI. The reactivity of N2O3 as a nitrosating agent

Using a differential spectrophotometric technique in water at 25°C measurements were made of the reaction rate in the nitrosation of a number of secondary amines in conditions in which the effective nitrosating agent is thought to be dinitrogen trioxide. Analysis of the rate data leads to values ofk, the true rate coefficient for theN-nitrosation step, which, like the values recalculated here for other systems involving aliphatic and aromatic substrates ofpKa>5, exhibit the same unvarying order of magnitude, 108M−1s−1. This figure together with the invariance already mentioned indicates that the attack of the N2O3 upon free amines must be diffusion controlled; this hypothesis is supported by the values found for the enthalpies of activation (10–20 kJ/mol).ZusammenfassungEs wurde mittels einer differentiellen spektrophotometrischen Methode die Reaktionsgeschwindigkeit derN-Nitrosierung von sekundären Aminen unter Bedingungen untersucht, bei denen N2O3 als nitrosierendes Agens angesehen wird. Der wahre Reaktionsgeschwindigkeitskoeffizientk für denN-Nitrosierungsschritt — sowohl für die oben genannten als auch andere erneut berechnete Systeme mit aliphatischen und aromatischen Substraten mitpKa>5 — ergab immer die gleiche Größenordnung von 108M−1s−1. Dieser Befund zeigt an, daß der Angriff von N2O3 auf die freien Amine diffusionskontrolliert erfolgen muß, wobei diese Annahme auch von den experimentellen Aktivierungsenthalpien von 10–20 kJ/mol gestützt wird.

A calculator program for the optimization of physico-chemical parameters by unidimensional search

Abstract A calculator program for the numerical optimization of physico-chemical parameters by the algorithm of Davies, Swann and Campey is described which includes estimation of the standard deviations of the optimized parameters.

Conformational effects on the photoinduced proton-transfer processes in 1-methyl-2-(3′-hydroxy-2′-pyridyl)benzimidazole

The ground- and excited-state behaviour of 1-methyl-2-(3′-hydroxy-2′-pyridyl)benzimidazole (MeHPyBI) in various solvents has been studied by UV–Vis absorption spectroscopy and by time-resolved and steady-state fluorescence spectroscopy. Three species were detected in the ground state in aqueous solutions of pH near to neutrality: a non-planar enol form, a planar cis-enol form and a planar keto tautomer. Electronic excitation of both the cis-enol form and the keto tautomer yields the excited keto form, the former through an ultrafast excited-state intramolecular proton-transfer (ESIPT). The non-planar enol form cannot undergo ESIPT and partly deprotonates at the hydroxy group to afford the anion. In non-aqueous solvents only the cis-enol form was detected in the ground state, yielding the keto tautomer in the excited state. The first protonation of MeHPyBI takes place at the benzimidazole N(3). Whereas the photoexcited cation shows fluorescence in acidified acetonitrile, it deprotonates at the hydroxy group in acidified ethanol, yielding the keto tautomer. In acidic aqueous solution, the excited cation undergoes a two-step tautomerization by two different routes to afford a keto cation, protonated at the pyridyl N. All the protonated and deprotonated forms of MeHPyBI are non-planar in the ground state both in water and ethanol, but they adopt a planar structure in the excited state. Comparison of the behaviour of MeHPyBI with related molecules reveals that the existence of planar forms and the neutral keto species in the ground state is favoured by the presence of the pyridyl nitrogen. The conformational equilibria in the ground state are crucial for the excited-state behaviour of these types of molecules.

Highly efficient and directional homo- and heterodimeric energy transfer materials based on fluorescently derivatized α,γ-cyclic octapeptides.

Cyclic octapeptides composed of α-amino acids alternated with cis-3-aminocycloalkanecarboxylic acids, self-assemble as drumlike dimers through β-sheet-like, backbone-to-backbone hydrogen bonding. Heterodimerization appears to be significantly more favored than homodimerization, and this represents a novel approach for the design and fabrication of highly stable heterodimeric assemblies. A multicomponent equilibrium network based on fluorescently derivatized self-assembling α,γ-cyclic octapeptides has been successfully used to form light-harvesting/light-converting ensembles with a distinctive organization of donor and acceptor units able to act as efficient artificial photosystems.

Dissociation of a Strong Acid in Neat Solvents: Diffusion Is Observed after Reversible Proton Ejection Inside the Solvent Shell

Strong-acid dissociation was studied in alcohols. Optical excitation of the cationic photoacid N-methyl-6-hydroxyquinolinium triggers proton transfer to the solvent, which was probed by spectral reconstruction of picosecond fluorescence traces. The process fulfills the classical Eigen-Weller mechanism in two stages: (a) solvent-controlled reversible dissociation inside the solvent shell and (b) barrierless splitting of the encounter complex. This can be appreciated only when fluorescence band integrals are used to monitor the time evolution of the reactant and product concentrations. Band integrals are insensitive to solvent dynamics and report relative concentrations directly. This was demonstrated by first measuring the fluorescence decay of the conjugate base across the full emission band, independently of the proton-transfer reaction. Multiexponential decay curves at single wavelengths result from a dynamic red shift of fluorescence in the course of solvent relaxation, whereas clean single exponential decays are obtained if the band integral is monitored instead. The extent of the shift is consistent with previously reported femtosecond transient absorption measurements, continuum theory of solvatochromism, and molecular properties derived from quantum chemical calculations. In turn, band integrals show clean biexponential decay of the photoacid and triexponential evolution of the conjugate base in the course of the proton transfer to solvent reaction. The dissociation step follows the slowest stage of solvation, which was measured here independently by picosecond fluorescence spectroscopy in five aliphatic alcohols. Also, the rate constant of the encounter-complex splitting stage is compatible with proton diffusion. Thus, for this photoacid, both stages reach the highest possible rates: solvation and diffusion control. Under these conditions, the concentration of the encounter complex is substantial during the earliest nanosecond.

Toward the rational design of molecular rotors ion sensors based on α,γ-cyclic peptide dimers

A dimer-forming self-assembling cyclic hexapeptide with a control register and a large association constant in water is described. The self-assembly process is followed by pyrene-excimer emission and the main diastereomeric dimer present in solution is switched by controlled addition of divalent cations (e.g., Ca, Mg) or oxalic acid.

Ground-state tautomerism and rotational isomerization in 4,5-dimethyl-2-(2-hydroxyphenyl)imidazole in the gas phase and in polar solvents: a theoretical study of the aromaticity, intramolecular hydrogen-bond strength and differential solute–solvent interactions

Ground-state tautomerism and rotational isomerization in 4,5-dimethyl-2-(2-hydroxyphenyl)imidazole in the gas phase and in solution have been investigated by means of quantum mechanical calculations, NMR and steady-state fluorescence spectroscopy. In the gas phase, the cis-enol form is the most stable species, followed by the trans-enol and the keto forms. Several theoretical approaches were employed to characterize the electronic structure of the different isomers in the gas phase at the RB3LYP/6-31 + G* level of theory. The observed behavior could be rationalized according to the different weights of the resonance and the hydrogen-bond energies in the overall energy of each isomer. The hydroxyphenyl rings of the trans-enol and the cis-enol forms exhibit a nearly aromatic structure, whereas the electronic structure of the keto-form shows a higher degree of localization. In turn, it was found that the intramolecular hydrogen bond shows similar strength in the cis-enol and keto-forms, whereas this interaction is very weak in the trans-enol form. Polar solute–solvent non-specific interactions were modeled through the Onsager, SCI-PCM and COSMO methods based on ab initio and semiempirical Hamiltonians. In solution, the keto-form is stabilized by its much greater solute–solvent electrostatic interaction through its dipolar term and the aromatization of its phenyl ring. The trans-enol form is mainly stabilized by electrostatic interactions through higher multipolar terms than dipolar and specific solute–solvent interactions through the lone pair of the imidazole N3.

Influence of acidity on the fluorescence spectra of 2-pyridylbenzimidazoles in aqueous solution

The fluorescence spectra of 2-(3-pyridyl)benzimidazole (3PBI) and the quaternary salt 2-(1-methyl-4-pyridinio)-benzimidazole iodide (4PBIQS) in aqueous solutions covering a wide range of acidities have been studied to investigate their photophysical behaviour and the acid–base processes occurring in the first excited singlet states of these species. Depending on the acidity, 3PBI exists in the excited state as the dication, the monocation protonated at the benzimidazole N(3) atom (C*), the neutral species and the anion. Of all the possible processes of interconversion among these species, only the protonation of C* is fast enough to compete with its deactivation. 4PBIQS too has the same species in excited and ground states: the dication D*, the monocation with a positive charge on the pyridyl nitrogen (T*) and the zwitterion Z*. Deprotonation of D*, protonation of T* and deprotonation of T* by the dihydrogen phosphate anion were all detected. 3PBI and 4PBIQS thus allow separate study of the photophysical properties of the two 2-pyridylbenzimidazole monocations, C* and T*, with no interference between the two.