Gonzalo Angulo

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.

Bimolecular Photoinduced Electron Transfer Beyond the Diffusion Limit: The Rehm–Weller Experiment Revisited with Femtosecond Time Resolution

To access the intrinsic, diffusion free, rate constant of bimolecular photoinduced electron transfer reactions, fluorescence quenching experiments have been performed with 14 donor/acceptor pairs, covering a driving-force range going from 0.6 to 2.4 eV, using steady-state and femtosecond time-resolved emission, and applying a diffusion-reaction model that accounts for the static and transient stages of the quenching for the analysis. The intrinsic electron transfer rate constants are up to 2 orders of magnitude larger than the diffusion rate constant in acetonitrile. Above ∼1.5 eV, a slight decrease of the rate constant is observed, pointing to a much weaker Marcus inverted region than those reported for other types of electron transfer reactions, such as charge recombination. Despite this, the driving force dependence can be rationalized in terms of Marcus theory.

Bimolecular Photoinduced Electron Transfer in Imidazolium-Based Room-Temperature Ionic Liquids Is Not Faster than in Conventional Solvents

The fluorescence quenching of 3-cyanoperylene upon electron transfer from N,N-dimethylaniline in three room-temperature ionic liquids (RTILs) and in binary solvent mixtures of identical viscosity has been investigated using steady-state and time-resolved fluorescence spectroscopy. This study was stimulated by previous reports of bimolecular electron transfer reactions faster by one or several orders of magnitude in RTILs than in conventional polar solvents. These conclusions were usually based on a comparison with data obtained in low-viscous organic solvents and extrapolated to higher viscosities and not by performing experiments at similar viscosities as those of the RTILs, which we show to be essential. Our results reveal that (i) the diffusive motion of solutes in both types of solvents is comparable, (ii) the intrinsic electron transfer step is controlled by the solvent dynamics in both cases, being slower in the RTILs than in the conventional organic solvent of similar viscosity, and (iii) the previously reported reaction rates much larger than the diffusion limit at low quencher concentration in RTILs originate from a neglect of the static and transient stages of the quenching, which are dominant in solvents as viscous as RTILs.

Intramolecular Charge-Transfer Dynamics in Covalently Linked Perylene−Dimethylaniline and Cyanoperylene−Dimethylaniline

The excited-state dynamics of covalently linked electron donor-acceptor systems consisting of N, N-dimethylaniline (DMA) as electron donor and either perylene (Pe) or cyanoperylene (CNPe) as acceptor has been investigated in a large variety of solvents, including a room-temperature ionic liquid, by using femtosecond time-resolved fluorescence and absorption spectroscopy. The negligibly small solvent dependence of the absorption spectrum of both compounds and the strong solvatochromism of the fluorescence are interpreted by a model where optical excitation results in the population of a locally excited state (LES) and emission takes place from a charge-separated state (CSS). This interpretation is supported by the fluorescence up-conversion and the transient absorption measurements that reveal substantial spectral dynamics in polar solvents only, occurring on time scales going from a few hundreds of femtoseconds in acetonitrile to several tens of picoseconds in the ionic liquid. The early transient absorption spectra are similar to those found in nonpolar solvents and are ascribed to the LES absorption. The late spectra due to CSS absorption show bands that are red-shifted relative to those of the radical anion of the acceptor moiety by an amount that depends on solvent polarity, pointing to partial charge separation. Global analysis of the time-resolved data indicates that the charge separation dynamics in PeDMA is essentially solvent controlled, whereas that in CNPeDMA is faster than diffusive solvation, this difference being accounted for by a larger driving force for charge separation in the latter. On the other hand, the CSS lifetime of PeDMA is of the order of a few nanoseconds independently of the solvent, whereas that of CNPeDMA decreases with increasing solvent polarity from a few nanoseconds to a few hundreds of picoseconds. Comparison of these results with previously published data on the fluorescence quenching of Pe and CNPe in pure DMA shows that the charge separation and the ensuing charge recombination occur on similar time scales independently of whether these processes are intra- or intermolecular.

Spurious observation of the Marcus inverted region in bimolecular photoinduced electron transfer.

The effect of viscosity on the bimolecular electron transfer quenching of a series of coumarins by N,N-dimethylaniline was investigated using steady-state and time-resolved fluorescence spectroscopy. The data reveal that the static and transient stages of the quenching become dominant as viscosity increases. When extracting the quenching rate constants using a simple Stern-Volmer analysis, a decrease of the rate constant with increasing driving force is observed above ~2 cP. However, this apparent Marcus inverted region, already reported several times with the same system in micelles and room temperature ionic liquids, totally vanishes when analyzing the data with a model accounting for the static and transient stages of the quenching. It appears that the apparent Marcus inverted region arises from the neglect of these quenching regimes together with the use of fluorophores with different excited-state lifetimes.

Ultrafast Decay of the Excited Singlet States of Thioxanthone by Internal Conversion and Intersystem Crossing

The experimental ultrafast photophysics of thioxanthone in several aprotic organic solvents at room temperature is presented, measured using femtosecond transient absorption together with high-level ab initio CASPT2 calculations of the singlet- and triplet-state manifolds in the gas phase, including computed state minima and conical intersections, transition energies, oscillator strengths, and spin-orbit coupling terms. The initially populated singlet pi pi* state is shown to decay through internal conversion and intersystem crossing processes via intermediate n pi* singlet and triplet states, respectively. Two easily accessible conical intersections explain the favorable internal conversion rates and low fluorescence quantum yields in nonpolar media. The presence of a singlet-triplet crossing near the singlet pi pi* minimum and the large spin-orbit coupling terms also rationalize the high intersystem crossing rates. A phenomenological kinetic scheme is proposed that accounts for the decrease in internal conversion and intersystem crossing (i.e. the very large experimental crescendo of the fluorescence quantum yield) with the increase of solvent polarity.

Extremely efficient electrochemiluminescence systems based on tris(2-phenylpyridine)iridium(III)

Electron transfer generation of the excited tris(2-phenylpyridine)iridium(III) 3*Ir(ppy)3 has been studied in acetonitrile–dioxane (1 ∶ 1) solutions containing 0.1 M (n-C4H9)4NPF6 as the supporting electrolyte. A triple-potential-step technique was used to create electrochemiluminescence (ECL) emission by annihilation of the electrochemically generated ions Ir(ppy)3+ + Ir(ppy)3− (single ECL system) as well as Ir(ppy)3+ + A− (mixed ECL systems with the radical anions of aromatic nitriles and ketones). Very high ECL emission efficiencies (up to 0.67, close to the excited 3*Ir(ppy)3 luminescence yield of 0.75) have been found.

Photophysics of two prototypical molecular-wire building blocks: solvent-induced conformational dynamics?

The photophysics of two molecular wire building blocks of different lengths based on p-phenyleneethylene, namely, 1,4-bis[p-(N,N-dimethylamino)phenyl]-1,2-ethyne and 1,4-bis[p-(N,N-dihexylamino)phenylethynyl]benzene, are studied experimentally in a wide range of organic solvents. The band shape and position of the electronic absorption and fluorescence emission of both compounds are discussed in terms of the empirical Catalán linear solvent energy relationship and the analytical solvation model of Liptay. It is found that solute polarizability plays an important part in the description of the pronounced solvatochromism for these highly symmetric molecules. In addition, the dependence of the emission quantum yield and the excited-state lifetime on the solvent are measured. The experimental findings can only be partially rationalized by the common theoretical models. They indicate that not only torsion about the triple bonds but also solvent-solute reorganization must be taken into account.

Experimental Evidence of the Relevance of Orientational Correlations in Photoinduced Bimolecular Reactions in Solution

A major problem in the extraction of the reaction probability in bimolecular processes is the disentanglement from the influence of molecular diffusion. One of the strategies to overcome it makes use of reactive solvents in which the reactants do not need to diffuse to encounter each other. However, most of our quantitative understanding of chemical reactions in solution between free partners is based on the assumption that they can be approximated by spheres because rotation averages their mutual orientations. This condition may not be fulfilled when the reaction takes place on time scales faster than that of molecular reorientation. In this work, the fluorescence quenching of two very similar polyaromatic hydrocarbons with different electric dipole moments is measured. The concentration of a liquid electron-donating quencher is varied from very dilute solutions to pure quencher solutions. In both cases, the thermodynamics of the reactions are very similar and, according to the Marcus expression, the kinetics are expected to proceed at similar rates. However, one of them is 10 times faster in the pure quencher solution. This difference starts at relatively low quencher concentrations. An explanation based on the fluorophore-solvent dipole-dipole interaction and the consequent orientational solvent structure is provided. The orientational correlation between fluorophore and quencher is calculated by means of computer simulations. Important differences depending on the fluorophore dipole moment are found. The kinetics can be explained quantitatively with a reaction-diffusion model that incorporates the effects of the presence of the dipole moment and the rotational diffusion, only in the highest quencher concentration case, but not in dilute solutions, most likely due to fundamental limitations of the kinetic theory.

Characterization of dimethylsulfoxide/glycerol mixtures: a binary solvent system for the study of "friction-dependent" chemical reactivity.

The properties of binary mixtures of dimethylsulfoxide and glycerol, measured using several techniques, are reported. Special attention is given to those properties contributing or affecting chemical reactions. In this respect the investigated mixture behaves as a relatively simple solvent and it is especially well suited for studies on the influence of viscosity on chemical reactivity. This is due to the relative invariance of the dielectric properties of the mixture. However, special caution must be taken with specific solvation, as the hydrogen-bonding properties of the solvent change with the molar fraction of glycerol.