Javier Catalán

Basicity and Acidity of Azoles

Publisher Summary This chapter provides a large collection of solution data and a thoroughly updated discussion of thermodynamic, kinetic, and structural results. This chapter covers aromatic azoles exclusively—that is, aromatic five-membered rings containing only carbon and nitrogen atoms, and their benzo derivatives. This chapter also discusses parent structure on theoretical grounds. One of the oldest methods to get insight into molecular structure, the study of acid-base properties, is today in rapid development. This is particularly true for the azoles. Gas phase studies of the acidity and basicity of azoles provides information on lone pair interactions, transmission of substituent effects in five-membered rings, relationships between acidity and basicity, aromatic substituent effects of nitrogen-linked derivatives and thermodynamic and kinetic parameters for proton transfer between nitrogen atoms in polyazoles. Because of the biological significance of some azoles (pyrrole, indole, imidazole, benzimidazole) and the consequences of acid–base equilibria in their functions, a continuous interest in the behavior in water is to be expected. In the case of N-unsubstituted imidazoles and pyrazoles, complexation of the pyridine-like nitrogen causes a considerable increase in the acidity of the pyrrolelike nitrogen.

Pulsed liquid lasers from proton transfer in the excited state

Abstract The application of photoinduced intramolecular proton transfer to generate stimulated radiation is reported. Tunable laser pulses are produced with a 10% efficiency using sodium salicylate or 2-( o -hydroxyphenyl)benzimidazole as the active medium. In both compounds a large population inversion results from the proton transfer taking place in the electronically excited state.

Analysis of the solvent effect on the photophysics properties of 6-propionyl-2-(dimethylamino)naphthalene (PRODAN).

The absorption and emission spectroscopic properties of 6-propionyl-2-(dimethylamino)naphthalene (PRODAN) have been studied in a large number of protogenic, nonprotogenic, and amphiprotic solvents. The data obtained can be explained by the inclussion of a new term in the Lippert equation which takes into account the acidity of the solvent. This finding indicates that some precaution should be taken when using PRODAN as an indicator of the polarity of protein cavities if the environments involved include acid sites.

Photophysics of the 2-(2′-hydroxyphenyl)perimidine: On the fluorescence of the enol form

Abstract The photophysical behaviour of 2-(2′-hydroxyphenyl)perimidine (6), which possesses a strong intramolecular hydrogen bond is evidenced as due to the non-transferred enol form. This compound is highly stable to light, with a photodegradative quantum yield in degassed cyclohexane of 7 × 10 −6 , suggesting that the involvement of a lightinduced excited state intramolecular proton transfer (ESIPT) process in which the phenolic proton is transferred from the enol form to the neighbouring nitrogen atom is less important for conferring photostability than has hitherto been supposed for such type of systems.

Phenylbenzimidazole proton-transfer laser dyes: spectral and operational properties

Abstract The efficient emission of tunable stimulated light in the 420–550 nm range from solutions of a number of derivatives of 2-(2 hydroxyphenyl) benzimidazole is reported. The lasing mechanism is based on an intramolecular proton-transfer reaction taking place in the initially populated excited state. These dyes, when pumped with a XeCl or N 2 lasers, produce tunable radiation covering a range of more than 100 nm with efficiencies up to 15.5%.

On the Molecular Structure and UV/vis Spectroscopic Properties of the Solvatochromic and Thermochromic Pyridinium-N-Phenolate Betaine Dye B30

Quantum chemical calculations as well as vis absorption and fluorescence measurements of the pyridinium-N-phenolate betaine dye B30, dissolved in 1-chlorobutane at temperatures between 343 and 77 K, shed more light on the solvatochromism, thermosolvatochromism, and photophysical behavior of this probe dye, formerly used to establish an empirical scale of solvent polarity, called E(T)(30) or E(T)(N) scale. A new calculated gas-phase E(T)(30) value is reported. Complementary to recent work of Kharlanov and Rettig (J. Phys. Chem. A 2009, 113, 10693-10703), it is shown that fluorescence of B30 in 1-chlorobutane solution is observable already at temperatures just below the solvents melting point and not only at 77 K. Analogous to increasing solvent polarity, decreasing solvent temperature leads to a large hypsochromic shift of the vis absorption band of B30, dissolved in 1-chlorobutane (Deltalambda = -245 nm from 797 nm at 343 K to 552 nm at 77 K). This thermosolvatochromism can be easily seen: the solution color changes from greenish yellow (343 K) to magenta-violet (77 K).

Extending the Solvent Acidiy Scale to Highly Acidic Organic Solvents: The Unique Photophysical Behaviour of 3,6-Diethyltetrazine

The SA scale, developed previously for weakly acidic solvents, was extended to include the most acidic solvents (haloalcohols and carboxylic acids) by using a unique solvatochromic probe (3,6-diethyl-1,2,4,5-tetrazine). The proposed new solvent scale has a number of advantages over widely used alternatives such as that based on Gutmanns acceptor number (AN). As shown in this paper, SA data provide an accurate description of the effect of solvent acidity on various chemical properties.

The concerted mechanism of photo-induced biprotonic transfer in 7-azaindole dimers: a model for the secondary evolution of the classic C2h dimer and comparison of four mechanisms.

A mechanism is proposed for the formation in gas phase, during a short time, of the delicately symmetrical coplanar C2h classic 7-azaindole (7AI) doubly hydrogen-bonded dimer. Of the five card-pack or otherwise random geometry structures most likely to be formed in the supersonic jet expansion molecular beam, none would be an obvious precursor to the C2h dimer. One unstable dimer with dipole–dipole, van der Waals, and plane-to-plane hydrogen bonding is shown to be capable of unhinging about the hydrogen-bond pair as an axis, from 0° to 90° to 180°, yielding a deep minimum for the C2h structure with its delicate geometry and symmetry. This relaxation mechanism is feasible in the 3-μs interval between the nozzle escape and the first laser pulse interception of the molecular beam. In the second part of the paper four published mechanisms are compared for concerted vs. two-step biprotonic phototransfer for the 7AI dimers. The dependence of the latter two models on H-atom instead of proton-transfer as an intermediate step negates the mechanism in a singlet (π,π*) electronic state by the valency repulsion, in the 3-electron orbital that would be generated. The concerted mechanism for biprotonic phototransfer is reaffirmed by the analysis of the quantum mechanical conditions set on the biprotonic transfer in the photo-excited molecular 7AI pair.

The role of the torsion of the phenyl moiety in the mechanism of stimulated ultraviolet light generation in 2‐phenylbenzazoles

The general difference observed between the photophysical behavior of 2‐phenylbenzazoles and parent unsubstituted benzazole is explained through an exchange of the energy order of the two lower excited singlet states of both compounds. The energy variation takes place because S1 ‐excited 2‐phenylbenzazoles suffer a subnanosecond conformational change towards planarity by torsion around the phenyl‐heterocycle bond. This torsion is in accordance with theoretical calculations, with ultraviolet (UV)‐visible and fluorescence data, and with results obtained on rotationally‐constrained 2‐phenylindoles. These changes, as well as the usually high fluorescence quantum yields and small Stokes shifts exhibited by unsubstituted benzazoles, make 2‐phenylbenzazoles suitable dye lasers for the UV‐zone of the spectrum.

The concerted mechanism of photo-induced biprotonic transfer in 7-azaindole dimers: Structure, quantum-theoretical analysis, and simultaneity principles

Six stable dimer models for 7-azaindole (including the classic C2h doubly hydrogen-bonded, coplanar, centrosymmetric dimer) are considered to be observable in adiabatic nozzle jet molecular beams. They are analyzed by hybrid density functional theory (DFT), the MP2 ab initio method for the ground electronic state, and the single-excitation configuration interaction (CIS) (over frozen ground state optimized geometries obtained from DFT) excited state calculations, for global potential minima and proton-transfer potential energy curves. Three simultaneity principles are stated: (i) intermolecular coherent excitation molecular exciton simultaneity, (ii) intramolecular acid–base change simultaneity at the pyrrolo-N-H and aza-N proton-donor, proton-acceptor sites, and (iii) intermolecular simultaneity of catalytic proton-donor, proton-acceptor action. It is suggested that the formation of the classic C2h dimer of 7-azaindole, which is considered exclusively by previous researchers, can be formed from at least one of the several card-pack hydrogen-bonded dimers in a secondary slower step approaching a microsecond scale, instead of the picosecond events at the supersonic nozzle. It is proposed that the complexity of dimerization modes is the basis of the postexcitation, postionization diverse kinetic isotope results.