Influence of redoxinert counterions on photoinduced intermolecular electron transfer rates in dichloromethane

Rate constants k_q of the fluorescence quenching of 9,10-dicyanoanthracene by bromide and chloride ions and of 9-cyanoanthracene by bromide ions were determined in the low polar solvent dichloromethane with a series of tetra-alkylammonium counterions. Cation size dependent electron transfer rate constants k_et were derived from k_q, and were fitted to a nonadiabatic electron transfer model equation. Because of ion pair formation in dichloromethane electrostatic interactions of the redoxinert countercations during electron transfer were considered. Accordingly, the position of the countercation within the precursor complex had to be described.

The electronic coupling matrix element derived by fitting of the experimental data is in satisfactory agreement with results of simple quantum chemical calculations.     

Control of proton transfer: intramolecular vs intermolecular

[reaction: see text] Proton transfer in ketonization of enolates is a critical step in a myriad of organic reactions. Its stereochemistry has been the object of our studies since we reported kinetic protonation from the less hindered face of the molecule under kinetic control some decades ago. Very recently, we have succeeded in reversing the stereochemistry using 2-pyridyl groups to deliver the proton. We now report intramolecular delivery by other moieties and control of intramolecular versus intermolecular proton delivery.     

Photochemical control of dielectric properties based on intermolecular proton transfer in a hydrogen-bonded diarylethene crystal

The photochromic reaction of a diarylethene derivative having imidazoline rings reversibly modulates the dielectric properties of the crystal, which is based on the intermolecular proton transfer in one-dimensional (1D) hydrogen-bonded chains.     

Ultrafast excited-state intermolecular proton transfer of cyanine fluorochrome dyes

Steady-state and time-resolved emission spectroscopy techniques were employed to study the excited-state proton transfer (ESPT) to water and D(2)O from QCy7, a recently synthesized near-infrared (NIR)-emissive dye with a fluorescence band maximum at 700 nm. We found that the ESPT rate constant, k(PT), of QCy7 excited from its protonated form, ROH, is ~1.5 × 10(12) s(-1). This is the highest ever reported value in the literature thus far, and it is comparable to the reciprocal of the longest solvation dynamics time component in water, τ(S) = 0.8 ps. We found a kinetic isotope effect (KIE) on the ESPT rate of ~1.7. This value is lower than that of weaker photoacids, which usually have KIE value of ~3, but comparable to the KIE on proton diffusion in water of ~1.45, for which the average time of proton transfer between adjacent water molecules is similar to that of QCy7.     

Intra- vs intermolecular photoinduced electron transfer reactions of a macrocyclic donor-acceptor dyad

The synthesis, structural characterization, and photophysical behavior of a 14-membered tetraazamacrocycle with pendant 4-dimethylaminobenzyl (DMAB) and 9-anthracenylmethyl groups is reported (L3, 6-((9-anthracenylmethyl)amino)-trans-6,13-dimethyl-13-((4-dimethylaminobenzyl)amino)-1,4,8,11-tetraazacyclotetradecane). In its free base form, this compound displays rapid intramolecular photoinduced electron transfer (PET) quenching of the anthracene emission, with both the secondary amines and the DMAB group capable of acting as electron donors. When complexed with Zn(II), the characteristic fluorescence of the anthracene chromophore is restored as the former of these pathways is deactivated by coordination. Importantly, it is shown that the DMAB group, which remains uncoordinated and PET active, acts only very weakly to quench emission, by comparison to the behavior of a model Zn complex lacking the pendant DMAB group, [ZnL2]2+ (Chart 1). By contrast, Stern-Volmer analysis of intermolecular quenching of [ZnL2]2+ by N,N-dimethylaniline (DMA) has shown that this reaction is diffusion limited. Hence, the pivotal role of the bridge in influencing intramolecular PET is highlighted.     

Colorimetric and ratiometric fluorescence sensing of fluoride ions based on competitive intra- and intermolecular proton transfer

Receptors 1 and 4 show fluoride ion selective changes in their absorbance and emission behaviours amongst F⁻, Cl⁻, Br⁻, I⁻, , CH₃COO⁻, , and anions. Fluoride ion mediated ‘ON-OFF-ON’ switching behaviour of 4 provides opportunities for ratiometric estimation of fluoride ions.     

Quantum-chemical study of photoisomerization and photoinduced proton transfer in hydroxystyrylquinolines

The structures of 2-(4-hydroxystyryl)quinoline 1 and 2-(2-hydroxystyryl)quinoline 2 have been optimized by the semiempirical methods PM3 and PM3-CI(8 × 8) with configuration interaction for the ground (S₀) and the excited singlet (S₁) states, respectively. The relative stability of the E- and Z-isomers, the quinoid tautomers, and the spiropyran form for compound 2 was calculated. It was found that hydroxyl-containing tautomers were more stable in the S₀ state, and the quinoid tautomers are more stable in the S₁ state. The calculations predict the possibility of photoisomerization and photoinduced proton transfer in hydroxystyrylquinolines.     

Light-induced intermolecular proton transfer from gas-phase ions to neutral molecules

It is shown by use of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry that photoexcitation of protonated naphthalene by visible laser light of 488 nm can effect a proton transfer from this ion to acetonitrile. The reaction of the ground state reaction partners is endoergic by 31 kJ/mol.     

Salt effects in photoinduced electron transfer reactions

Metal salts and oxygen react synergistically to inhibit back-electron-transfer in photoinduced reactions.     

Photochemistry and spectroscopy of "stable organic radicals": steric and electronic effects in intermolecular photoinduced electron transfer

The intermolecular reactivities of amino-substituted perchlorotriphenylmethyl radicals 1-3 were studied, with particular emphasis on electron transfer (ET) reactions. The natural fluorescence lifetimes and the rates of the electron-transfer quenching were studied with several electron donors and acceptors. Fluorescence quenching studies demonstrate the importance of the redox potentials of the ET pair on the observed steric and electronic properties.     

Features of photoinduced proton transfer in the presence of a polyelectrolyte

Using the example of a series of sulfo derivatives of 2-naphthol, it was shown that the charge field formed by the polyelectrolyte coil significantly changes the constants k₁ and k₋₁ of the photoinduced proton transfer reaction, but no noticeable shift in the equilibrium constant K * was found. This observation is fundamentally different from the behavior of these substances in micellar media, where K * increases by an order of magnitude. The binding constants of the dyes with the cationic polyelectrolyte were also determined.     

Solvent effects on intermolecular proton transfer: the rates of nitrene protonation and their correlation with Swain acity

The rate of protonation of singlet 2-fluorenylnitrene was studied in eight protic solvents. The protonation rates vary between 10(11) and 10(9) s(-1) and are well-correlated with Swain's acity parameters. This demonstrates that Swain acity parameters and nitrene protonation rates can be used to evaluate bulk solvent acidity.     

Excited-state intermolecular proton transfer of firefly luciferin V. Direct proton transfer to fluoride and other mild bases

We studied the direct proton transfer (PT) from electronically excited D-luciferin to several mild bases. The fluorescence up-conversion technique is used to measure the rise and decay of the fluorescence signals of the protonated and deprotonated species of D-luciferin. From a base concentration of 0.25 M or higher the proton transfer rates to the fluoride, dihdyrogen phosphate or acetate bases are fast and comparable. The fluorescence signals are nonexponential and complex. We suggest that the fastest decay component arises from a direct proton transfer process from the hydroxyl group of D-luciferin to the mild base. The proton donor and acceptor molecules form an ion pair prior to photoexcitation. Upon photoexcitation solvent rearrangement occurs on a 1 ps time-scale. The PT reaction time constant is ～2 ps for all three bases. A second decay component of about 10 ps is attributed to the proton transfer in a contact pair bridged by one water molecule. The longest decay component is due to both the excited-state proton transfer (ESPT) to the solvent and the diffusion-assisted PT process between a photoacid and a base pair positioned remotely from each other prior to photoexcitation.     

Influence of intermolecular orientation on the photoinduced charge transfer kinetics in self-assembled aggregates of donor-acceptor arrays

The kinetics of photoinduced charge transfer reactions in covalently linked donor-acceptor molecules often undergoes dramatic changes when these molecules self-assemble from a molecular dissolved state into a nanoaggregate. Frequently, the origin of these changes is only partially understood. In this paper, we describe the intermolecular spatial organization of three homologous arrays, consisting of a central perylene bisimide (PERY) acceptor moiety and two oligo(p-phenylene vinylene) (OPV) donor units, in nanoaggregates and identify both face-to-face (H-type) and slipped (J-type) stacking of the OPV and PERY chromophores. For the J-type aggregates, short intermolecular OPV-PERY distances are created that give rise to a charge-transfer absorption band. The proximity of the donor and acceptor groups in the J-type aggregates enables a highly efficient photoinduced charge separation with a rate (k(cs) > 10(12) s(-1)) that significantly exceeds the rate of the intramolecular charge transfer of the same compounds when molecularly dissolved, even in the most polar media. In the H-type aggregates, on the other hand, the intermolecular OPV-PERY distance is not reduced compared to the intramolecular separation, and hence, the rates of the electron transfer reactions are not significantly affected compared to the molecular dissolved state. Similar to the forward electron transfer, the kinetics of the charge recombination in the aggregated state can be understood by considering the different interchromophoric distances that occur in the H- and J-type aggregates. These results provide the first consistent rationalization of the remarkable differences that are observed for photoinduced charge-transfer reactions of donor-acceptor compounds in molecularly dissolved versus aggregated states.     

Exploring the extent of magnetic field effect on intermolecular photoinduced electron transfer in different organized assemblies

Magnetic field effect (MFE) on the photoinduced electron transfer (PET) between phenazine (PZ) and the amines, N,N-dimethylaniline , N,N-diethylaniline, 4,4'-bis(dimethylamino)diphenylmethane (DMDPM), and triethylamine, has been studied in micelles, reverse micelles, and small unilamellar vesicles (SUVs) with a view to understand the effect of spatial location of the donor and acceptor moieties on the magnetic field behavior. The structure of the assembly is found to influence greatly the PET dynamics and hence the MFE of all the systems studied. The magnetic field behavior in micelles is consistent with the hyperfine mechanism, but high B(1/2) values have been obtained which have been ascribed to hopping and lifetime broadening. The variation of MFE with W(0), in reverse micelles, proves yet again that the MFE maximizes at an optimum separation distance between the acceptor and donor. This is the first example of such behavior for intermolecular PET in heterogeneous medium. We have also reported for the first time MFE on intermolecular PET in SUVs. In this case, the PZ-DMDPM system responds most appreciably to an external field compared to the other acceptor-donor systems because it is appropriately positioned in the bilayer. The differential behavior of the amines has been discussed in terms of their confinement in different zones of the organized assemblies depending on their bulk, hydrophobic, and electrostatic effects.     

Intra- and intermolecular proton transfer reactions in 2-phenyl substituted benzazoles

The present review describes the salient features of inter- and intramolecular proton transfer reactions of 2-(2′-aminophenyl)-, 2-(3′-aminophenyl)-, 2-(4′-aminophenyl)-, 2-(2′-hydroxyphenyl)-, 2-(3′-hydroxyphenyl)- and 2-(4′-hydroxyphenyl)-benzimidazoles, benzoxazoles and benzothiazoles. Fluorescence quantum yield of the phototautomer produced by the intramolecular hydrogen bonding decreases on going from benzimidazole to benzoxazole to benzothiazole. This indicates that the rate of internal conversion increases in the order of compounds as mentioned above. The biprotonic phototautomerism or the presence of intermolecular proton transfer has led to the formation of (i) nonfluorescent zwitterions in case of hydroxyphenyl derivatives and the ground state precursor of this species in neutral molecules, (ii) nonfluorescent monoanions from fluorescent monoanions and (iii) nonfluorescent monocations from monocations in case of aminophenyl derivatives. In the case of 2-(4′-aminophenyl)-substituted compounds, the first protonation has always led to the formation of two types of monocations; one by protonating the amino group and the other by protonating the tertiary nitrogen atom. The former is more stable in aqueous media and the latter in non-polar media.     

Specifics of photoinduced excited-state intramolecular proton transfer in o-tosylaminobenzoic and o-acetylaminobenzoic acids

In the ground state, o-tosylaminobenzoic and o-acetylaminobenzoic acids exist in the form of two rotamers with intramolecular hydrogen bonds N-H...O=C (cis) and N-H...O(OH)-C (trans). In nonpolar solvents, the formation of dimers with hydrogen bonding between carboxyl groups takes place. Efficient barrierless excited state intramolecular proton transfer (ESIPT) occurs along the N-H...O=C hydrogen bond upon excitation of o-tosylaminobenzoic acid. The efficiency of ESIPT in o-acetylaminobenzoic acid is lower because of the low acidity of the substituted amino group.