Excited state properties of alpha,omega-diphenylpolyenes: photophysical and photochemical studies of donor-acceptor diarylbutadienes

alpha,omega-Diphenylpolyenes have attracted a great deal of attention as models of retinyl polyenes that are related to natural photoreceptors involved in energy and sensory phototransductions. Of particular interest have been the topics of their excited state electronic structure and spectroscopic properties. However, the exact nature of the lowest excited state in terms of their structure and energetics is not clearly known. Examination of the photophysics and photochemistry of donor-acceptor diphenylpolyenes can aid in understanding the excited states and photoprocesses of linear polyenes. In this paper are described the absorption, fluorescence and photoisomerization studies of donor-acceptor diarylbutadienes, namely: p-(N,N-dimethylamino)-p'-cyano-1,4-diphenylbuta-1E,3E-diene (1), p-(N,N-dimethylamino)-p'-nitro-1,4-diphenylbuta-1E,3E-diene (2), p-(N,N-dimethylamino)-m'-nitro-1,4-diphenylbuta-1E,3E-diene (3), p-(N,N-dimethylamino)-o'-nitro-1,4-diphenylbuta-1E,3E-diene (4). Absorption properties are affected as expected due to mesomeric stabilization by the substituent; however, solvent polarity does not significantly affect the absorption properties of these dienes. In contrast, a pronounced solvatochromic fluorescence behavior of these dienes in organic solvents is observed. Time-resolved fluorescence is characterized by a single exponential fluorescence decay with generally increasing lifetime in polar solvents. The fluorescence quantum yields are very low, particularly in polar solvents, but do not show any clear trend. Irradiation of 1E,3E- 1-4 in organic solvents yields the corresponding 1E,3Z-isomer due to one-photon-one-bond isomerization of the C=C double bond lying closer to the acceptor group. The photoisomerization also depended on the solvent polarity and on the concentration of diene. The photoisomerization efficiency of dienes 1 and 2 under direct irradiation condition is greater than dienes 3 and 4. In comparison to the efficiency of photoisomerization under direct irradiation condition, the photosensitized isomerization efficiency is much less, particularly for dienes 1 and 2. The results are discussed in terms of the involvement of excited-state intramolecular charge transfer and conformationally relaxed polar excited states in the photoprocesses of linear polyenes.     

Effect of axial ligands and macrocyclic structure on redox potentials and electron-transfer mechanisms of SnIV porphyrins

The electrochemical properties of dichloro- and dihydroxo-SnIV porphyrins with three different macrocycles were examined in CH2Cl2 containing 0.1 or 0.2 M tetra-n-butylammonium perchlorate as supporting electrolyte. The investigated compounds are represented as (TPP)SnX2, (P)Sn(X)2, and (PQ)Sn(X)2, where TPP = 5,10,15,20-tetraphenylporphyrin, P = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin, PQ = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b']porphyrin, and X = Cl or OH. Each porphyrin can be electroreduced in two one-electron-transfer steps with the half-wave potentials and stability of the eletroreduced compounds being dependent upon the type of coordinated axial ligand and specific macrocyclic structure. All reductions of (TPP)Sn(OH)2, (P)Sn(OH)2, and (PQ)Sn(OH)2 are reversible under the given experimental conditions and lead to the expected porphyrin pi-anion radicals and dianions, which were characterized by thin-layer UV-vis spectroelectrochemistry. This contrasts with what occurs upon the reduction of (PQ)SnCl2, which undergoes a chemical reaction with trace H2O in solution, leading to the formation of (PQ)Sn(OH)2 as well as to a protonated form of the quinoxalinoporphyrin, (PQH)Sn(OH)2, under the application of an applied potential. A protonation of the Q group breaks the conjugation between the fused quinoxaline unit and the porphyrin macrocycle, thus effectively giving a compound whose reduction properties resemble that of the metalloporphyrin in the absence of the fused ring. The electrooxidation of each neutral SnIV porphyrin was also investigated, and the effect of axial ligand and fused quinoxaline ring on the redox potentials and products of electron transfer are discussed.     

Noncovalent one-to-one donor-acceptor assembled systems based on porphyrin molecular gels for unusually high electron-transfer efficiency

A new approach for fabricating donor-acceptor assembled systems is demonstrated, based on J-type ordered aggregation of a low-molecular zinc porphyrin derivative and subsequent integration of a pyridylated fullerene derivative with coordination and orientation onto the porphyrin aggregates. This system achieves unusually high efficiencies in fluorescence quenching during one-to-one mixing of the donor and acceptor. Moreover, the Stern-Volmer constant (K(SV)) and association constant (K) of this system are 2520 and 56 times higher, respectively, than those of the corresponding nonassembled system. The quenching efficiency is thermotropically switchable, since ordered-to-disordered transitions are essential characteristics of noncovalent low molecular assemblies.     

Excited states in electron-transfer reaction products: ultrafast relaxation dynamics of an isolated acceptor radical anion

The spectroscopy and ultrafast relaxation dynamics of excited states of the radical anion of a representative charge-transfer acceptor molecule, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, have been studied in the gas phase using time-resolved photoelectron spectroscopy. The photoelectron spectra reveal that at least two anion excited states are bound. Time-resolved studies show that both excited states are very short-lived and internally convert to the anion ground state, with the lower energy state relaxing within 200 fs and a near-threshold valence-excited state relaxing on a 60 fs time scale. These excited states, and in particular the valence-excited state, present efficient pathways for electron-transfer reactions in the highly exergonic inverted region which commonly displays rates exceeding predictions from electron-transfer theory.     

Integrating ultrafast and stochastic dynamics studies of Brownian motion in molecular systems and colloidal particles

Ultrafast spectroscopy and stochastic dynamics studies of chemical dynamics in solution with high resolution in both space and time have been undertaken for many years, but it is still challenging to connect fundamental knowledge obtained from stroboscopic approaches at ultrashort timescales and small length scales with that obtained by directly measuring individual particle motion at longer timescales. Therefore, it is interesting, conceptually and experimentally, to understand the similarities and differences between these two approaches to the study of chemical dynamics in condensed phase systems. We discuss recent advances in the understanding of the transition from ballistic to diffusive motion and chemical reaction rate theories and describe the significance of the findings in relation to the study of thermally activated processes at multiple time and length scales.     

Electrical conductivity studies on discotic liquid crystal-ferrocenium donor-acceptor systems

The dispersion of electron-deficient ferrocenium ions was studied in the electron-rich media of two different triphenylene-based columnar hexagonal liquid-crystalline phases. These composites were characterized using polarizing optical micrography (POM), differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), visible absorption spectroscopy, and dc and ac conductivity measurements. It was found that these composites form donor-acceptor systems that enhance the quasi-one-dimensional conductivity of the discotic system without altering the hexagonal columnar mesophase. The absorbance spectra confirm the formation of a charge-transfer complex between the electron-rich discotic molecules and the electron-deficient ferrocenium ions.     

Theoretical study of vibrational wave-packet dynamics in electron-transfer systems

The vibrational wave-packet dynamics associated with ultrafast electron-transfer reactions in a condensed phase environment is studied. Applying the recently proposed self-consistent hybrid method to simulate the quantum dynamics of these reactions, we consider various electron-transfer systems including the spin-boson model with Debye spectral density, a reaction-coordinate model, as well as photoinduced electron-transfer reactions in a mixed valence compound. The results of the study demonstrate the interplay between electron-transfer dynamics and coherent vibrational motion of inner-sphere modes. Furthermore, the occurrence and quenching of electronic and vibrational coherence effects in electron-transfer reactions is discussed in some detail.     

Effect of the excitation wavelength on the ultrafast charge recombination dynamics of donor-acceptor complexes in polar solvents

The effect of the excitation wavelength on the charge recombination (CR) dynamics of several donor-acceptor complexes (DACs) composed of benzene derivatives as donors and of tetracyanoethylene or pyromellitic dianhydride as acceptors has been investigated in polar solvents using ultrafast time-resolved spectroscopy. Three different wavelength effects have been observed. (1) With complexes exhibiting two well-separated charge-transfer bands, the CR dynamics was found to be slower by a factor of about 1.5 upon excitation in the high-energy band. This effect was measured in both fast and slow relaxing solvents and was discussed in terms of different DAC geometries. (2) When the CR is faster than diffusive solvation, a slowing down of the CR with increasing excitation wavelength accompanied by an increase of the nonexponential character of the dynamics was measured. This effect appears only when exciting on the red edge of the charge-transfer absorption band. (3) When the driving force for CR is small, both nonequilibrium (hot) and thermally activated CR pathways can be operative. The results obtained with such a complex indicate that the relative contribution of these two paths depends on the excitation wavelength.     

Charge transfer dynamics of donor-acceptor systems in solutions and sol-gel matrices

Charge transfer reactions are photoinduced by exciting with UV and visible light strongly coupled donor acceptor pairs. The systems investigated are composed either of porphyrin and phthalocyanine mixed dimers. In this case, the proton transfer competes with the electron transfer process.The results obtained in sol-gel matrices are compared to the ones in solutions and organized media, and discussed in terms of the effect of the confinement of the reactants and degree of organization of the environment on the efficiency of charge transfer reactions.     

Structural dynamics in floppy systems: ultrafast conformeric motions in Rydberg-excited triethylamine

Rotations about its three carbon-nitrogen bonds give triethylamine a complex, 3-dimensional potential energy landscape of conformeric structures. Electronic excitation to Rydberg states prepares the molecule in a high-energy, nonequilibrium distribution of such conformers, initiating ultrafast transitions between them. Time-resolved Rydberg electron binding energy spectra, observed using photoionization-photoelectron spectroscopy with ultrashort laser pulses, reveal these time-evolving structures. The time-dependent structural fingerprint spectra are assigned with the aid of a computational analysis of the potential energy landscape. Upon 209 nm electronic excitation to the 3p Rydberg state, triethylamine decays to 3s with a 200 fs time constant. The initially prepared conformer reacts to a mixture of structures with a time constant of 232 fs and settles into a final geometry distribution on a further subpicosecond time scale. The binding energy of the Rydberg electron is found to be an important determinant of the conformeric energy landscape.     

Water dynamics at protein interfaces: ultrafast optical Kerr effect study

The behavior of water molecules surrounding a protein can have an important bearing on its structure and function. Consequently, a great deal of attention has been focused on changes in the relaxation dynamics of water when it is located at the protein surface. Here we use the ultrafast optical Kerr effect to study the H-bond structure and dynamics of aqueous solutions of proteins. Measurements are made for three proteins as a function of concentration. We find that the water dynamics in the first solvation layer of the proteins are slowed by up to a factor of 8 in comparison to those in bulk water. The most marked slowdown was observed for the most hydrophilic protein studied, bovine serum albumin, whereas the most hydrophobic protein, trypsin, had a slightly smaller effect. The terahertz Raman spectra of these protein solutions resemble those of pure water up to 5 wt % of protein, above which a new feature appears at ~80 cm(-1), which is assigned to a bending of the protein amide chain.     

Experimental and theoretical studies of the redox potentials of cyclic nitroxides

The redox potentials of 25 cyclic nitroxides from four different structural classes (pyrrolidine, piperidine, isoindoline, and azaphenalene) were determined experimentally by cyclic voltammetry in acetonitrile, and also via high-level ab initio molecular orbital calculations. It is shown that the potentials are influenced by the type of ring system, ring substituents and/or groups surrounding the radical moiety. For the pyrrolidine, piperidine, and isoindolines there is excellent agreement (mean absolute deviation of 0.05 V) between the calculated and experimental oxidation potentials; for the azaphenalenes, however, there is an extraordinary discrepancy (mean absolute deviation of 0.60 V), implying that their one-electron oxidation might involve additional processes not considered in the theoretical calculations. This recently developed azaphenalene class of nitroxide represents a new variant of a nitroxide ring fused to an aromatic system and details of the synthesis of five derivatives involving differing aryl substitution are also presented.     

The effect of solvent polarity on the photophysical properties of 4-cyano-(4'-methylthio)diphenylacetylene: a prototypic donor-acceptor system

The photophysical properties of the target compound are extremely sensitive to changes in solvent polarity since the lowest-energy excited states possess considerable charge-transfer character. Excitation results in a greatly increased dipole moment, with the resultant excited singlet state retaining a lifetime of ca. 1 ns in all solvents. Radiative decay involves coupling between the lowest-energy excited singlet state and both the ground state and an upper excited singlet state. The level of coupling to the upper singlet decreases in non-polar solvents, presumably due to symmetry factors. The radiative rate constant decreases smoothly with increasing solvent polarity function as the molecule acquires an ever increasing dipolar character. Non-radiative decay includes both intersystem crossing and internal conversion, but the former process dominates in polar solvents. The excited singlet state lifetime is very weakly dependent upon temperature in the solid state. However, in polar solutions where the Stokes' shift decreases with decreasing temperature, there is clear evidence for an activated process. This is believed to involve coupling to the upper-lying singlet excited state.     

Solvent effect on the redox potential of quinone-semiquinone systems

Polarographic reduction of 1,4-benzoquinone, 1,4-naphthoquinone, 9,10-phenanthrenequinone and 9,10-anthraquinone was studied in pyridine, acetone, hexamethylphosphoramide, dimethylformamide, dimethylacetamide, acetonitrile, dimethylsulfoxide and propylene carbonate. The variation of the E_1/2-values proper for the first electron transfer step (measured against oxidation potential of ferrocene) with the properties of solvent is analyzed in terms of the donor-acceptor concept. Linear correlations of E_1/2 vs. acceptor number are observed; the slopes of corresponding lines correlate with the spin densities at oxygen atoms of semiquinones.     

ESR studies of methallyl monomers with redox systems

The effects of three types of free radical initiators (HO·, H₂N·, and H₃C·) from redox systems, have been studied for four types of methallyl monomers, by use of ESR with a flow system. The structure, the relative concentrations, and the steric conformations of the monomer radical intermediates have been derived from the ESR spectra. In the case of H₂N · and HO · addition to methallyl alcohol (MAA), methallyl amine (MAAm), and sodium methallyl sulfonate (SMAS), the ESR spectra of the reacting species are interpreted as monomer head radicals only (H₂N · and HO · are added to the monomer tail). Methallyl acetate (MAAc) with HO ·, is an exception, giving hydrogen abstraction to form an allyl type radical. This reaction may influence the polymerization behavior of MAAc. The methallyl monomers behave differently from the allyl monomers, where appreciable amounts of monomer tail radicals were found in addition to the head radicals which were the main species. For methallyl monomers, this may be due to steric hindrance caused by the two substituents on the α carbon. The CH₃ radicals add only to positively polarized reactive double bonds, i.e., in SMAS in this study, and allyl alcohol in a previous study. The coupling constants of β CH₂ protons vary considerably with the substituents. For β₁ protons, the coupling constants decrease in the order OH > CH₃ > NH₂. For β₂ protons (allyl hydrogen), the coupling constants decrease in the order CH₂OH > CH₂NH₂ > CH₂OCOCH₃Na, i.e., the constants decrease in the order of increased bulkiness of the groups. Some exceptions are interpreted as due to complex formation with Ti⁴⁺. The effects of pH of the reaction medium are largely those expected.     

Sonoelectrochemical studies on mass transport in some standard redox systems

A systematic study pertaining to the effects of ultrasound on four redox processes at a glassy carbon electrode has been undertaken. Ru(NH₃)₆Cl₃, K₄Fe(CN)₆, methylviologen, and benzoquinone are the redox probes investigated. Ultrasound intensity, distance between the electrode and sonoprobe, the type of sonoprobe and its geometrical arrangement with respect to the working electrode are the variables invoked. Rotating disc electrode and impedance measurements have been made to obtain information on the mass transport processes.