Solvent dependence of the charge-transfer properties of a quaterthiophene–anthraquinone dyad

An electron donor–acceptor dyad (quaterthiophene–anthraquinone) mediates ultrafast intramolecular photoinduced charge separation and consequent charge recombination when in polar or moderately polar solvents. Alternatively, non-polar media completely impedes the initial photoinduced electron transfer by causing enough destabilization of the charge-transfer state and shifting its energy above the energy of the lowest locally excited singlet state. Furthermore, femtosecond transient-absorption spectroscopy reveals that for the solvents mediating the initial photoinduced electron-transfer process, the charge recombination rates were slower than the rates of charge separation. This behavior of donor–acceptor systems is essential for solar-energy-conversion applications. For the donor–acceptor dyad described in this study, the electron-transfer driving force and reorganization energy place the charge-recombination processes in the Marcus inverted region.     

Excited electronic states and photophysics of uracil–water complexes

The electronically excited singlet states of complexes of uracil with one water molecule have been studied theoretically using ab initio multireference configuration interaction methods. In agreement with previous theoretical and experimental results, four cyclic isomers of uracil forming hydrogen bonds with the water molecule have been located with energies within 0.2 eV from the lowest energy isomer. Focus has been given on the mechanism for radiationless decay to the ground state after initial UV absorption and on the effect of complexation with water on previously reported radiationless decay pathways. Features on the excited state potential energy surfaces, such as minima, transition states and conical intersections, have been located for all isomers and compared with those of free uracil. The hydrogen-bonded water molecule changes the relative energies of these features and may lead to different excited state dynamics and lifetimes, in agreement with experimental observations.     

Ground and excited state isomeric forms of the anthracene-diethylaniline molecular exciplex

The potential energy surfaces for the molecular complex formed between anthracene (the electron acceptor) andN ,N-diethylaniline (DEA) (the electron donor) were computed as the quasi-adiabatic states resulting from the configuration interaction between the ground (AD), locally excited (A^*D) and charge-transfer (A⁻D⁺) excited electronic configurations. The results clearly indicate the existence of three geometrically and energetically different isomeric forms of the complex in the ground state. In the excited state, the potential energy surfaces reveal the existence of five well-defined equilibrium configurations separated by energy barriers and characterized by different admixtures of the (A^*D) and (A⁻D⁺) electronic configurations. Such a variety of equilibrium configurations in the ground and excited states is, in part, accounted for by the existence of two different conformational forms of DEA that can form complexes with anthracene, and are characterized by different balances between steric effects and interactions of electronic charge distributions in the complex components. The energies of transitions between the relevant ground and excited state equilibrium configurations were calculated and compared with spectroscopic data of a jet-cooled complex obtained in supersonic beam experiments. These transitions were successfully assigned to the observed resonance-like and exciplex-like spectra, and this enabled interpretation of observed changes in the fluorescence excitation and fluorescence spectra of the complex upon excess excitation energy.     

Are donor–acceptor self organised aromatic systems NLO (non-linear optical) active?

This contribution to the understanding of non-linear optical (NLO) properties of organic systems is concentrated on donor–acceptor aromatic systems such as [2]catenanes. We report accurate ab initio quantum chemical calculations of the first static hyperpolarizability (β) of donor–acceptor aromatic systems containing naphthalene and anthracene as electron donors and pyromellitimide and naphthadiimide as electron acceptors. We investigate the NLO effect when the donor acceptor units are not connected through a bridge. In this kind of system the electronic communication between donor and acceptor is solely due to through space charge transfer mechanism. Geometries of all molecular systems were optimised at the Hartree–Fock level with STO-3G minimal basis set and with the 3-21G split valence basis and finally with 6-31G basis set using GAUSSIAN 98W. The first static hyperpolarizabilities of these molecular systems were calculated using Hartree–Fock level using 6-31G basis set using GAUSSIAN 98W. To understand the possibility of developing these systems as NLO materials we have also calculated the linear and nonlinear optical properties of bridged donor–acceptor systems. The study suggested that these unconnected donor–acceptor systems equivalent to some [2]catenanes reported in literature in general have little influence on the first static hyperpolarizability, however, the linked macro cyclic systems may have potential applications in the development of non-linear optical materials.     

Theoretical investigation of charge transfer excitation and charge recombination in acenaphthylene–tetracyanoethylene complex

Ab initio calculations were performed to investigate the charge separation and charge recombination processes in the photoinduced electron transfer reaction between tetracyanoethylene and acenaphthylene. The excited states of the charge‐balanced electron donor–acceptor complex and the singlet state of ion pair complex were studied by employing configuration interaction singles method. The equilibrium geometry of electron donor–acceptor complex was obtained by the second‐order Møller–Plesset method, with the interaction energy corrected by the counterpoise method. The theoretical study of ground state and excited states of electron donor–acceptor complex in this work reveals that the S₁ and S₂ states of the electron donor–acceptor complexes are excited charge transfer states, and charge transfer absorptions that corresponds to the S₀ → S₁ and S₀ → S₂ transitions arise from π–π* excitations. The charge recombination in the ion pair complex will produce the charge‐balanced ground state or excited triplet state. According to the generalized Mulliken–Hush model, the electron coupling matrix elements of the charge separation process and the charge recombination process were obtained. Based on the continuum model, charge transfer absorption and charge transfer emission in the polar solvent of 1,2‐dichloroethane were investigated. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 23–35, 2003     

Vectorial Electron Transfer in Donor–Photosensitizer–Acceptor Triads Based on Novel Bis‐tridentate Ruthenium Polypyridyl Complexes

The first examples of rodlike donor–photosensitizer–acceptor arrays based on bis‐2,6‐di(quinolin‐8‐yl)pyridine Ru^II complexes 1 a and 3 a for photoinduced electron transfer have been synthesized and investigated. The complexes are synthesized in a convergent manner and are isolated as linear, single isomers. Time‐resolved absorption spectroscopy reveals long‐lived, photoinduced charge‐separated states (τ_CSS ( 1 a )=140 ns, τ_CSS ( 3 a )=200 ns) formed by stepwise electron transfer. The overall yields of charge separation (≥50 % for complex 1 a and ≥95 % for complex 3 a ) are unprecedented for bis‐tridentate Ru^II polypyridyl complexes. This is attributed to the long‐lived excited state of the [Ru(dqp)₂]²⁺ complex combined with fast electron transfer from the donor moiety following the initial charge separation. The rodlike arrangement of donor and acceptor gives controlled, vectorial electron transfer, free from the complications of stereoisomeric diversity. Thus, such arrays provide an excellent system for the study of photoinduced electron transfer and, ultimately, the harvesting of solar energy.     

Highly accurate relativistic universal Gaussian basis set for Dirac–Fock–Breit calculations

Ion mobilities of H₂O⁺ drifting in helium are calculated and compared with experiment. These calculations employ global potential energy surfaces of the H₂O⁺–He complex, which in the present case were calculated ab initio at the unrestricted MP2 level of theory using a basis set of aug‐cc‐pVTZ quality, and treating the ion as a rigid body. Details are presented of the general characteristics of both the ground and first‐excited electronic states of the complex. Although only the ground‐state surface was used for the mobility calculations, the ab initio determination of the ground state necessitated the inclusion of the first‐excited state owing to the presence of a crossing between the two. This crossing is also described. Mobilities calculated from the global surfaces are in good agreement with experiment. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

An Argon–Oxygen Covalent Bond in the ArOH+ Molecular Ion

The OH⁺ cation is a well‐known diatomic for which the triplet (³Σ^?) ground state is 50.5 kcal mol^?1 more stable than its corresponding singlet (¹Δ) excited state. However, the singlet forms a strong donor–acceptor bond to argon with a bond energy of 66.4 kcal mol^?1 at the CCSDT(Q)/CBS level, making the singlet ArOH⁺ cation 3.9 kcal mol^?1 more stable than the lowest energy triplet complex. Both singlet and triplet isomers of this molecular ion were prepared in a cold molecular beam using different ion sources. Infrared photodissociation spectroscopy in combination with messenger atom tagging shows that the two spin isomers exhibit completely different spectral signatures. The ground state of ArOH⁺ is the predicted singlet with a covalent Ar?O bond.     

Photoinduced intramolecular charge transfer (ICT) reaction in trans-methyl p-(dimethylamino) cinnamate: A combined fluorescence measurement and quantum chemical calculations

The photophysical behaviour of trans-methyl p-(dimethylamino) cinnamate (t-MDMAC) donor–acceptor system has been investigated by steady-state absorption and emission spectroscopy and quantum chemical calculations. The molecule t-MDMAC shows an emission from the locally excited state in non-polar solvents. In addition to weak local emission, a strong solvent dependent red shifted fluorescence in polar aprotic solvents is attributed to highly polar intramolecular charge transfer state. However, the formation of hydrogen-bonded clusters with polar protic solvents has been suggested from a linear correlation between the observed red shifted fluorescence band maxima with hydrogen bonding parameters (). Calculations by ab initio and density functional theory show that the lone pair electron at nitrogen center is out of plane of the benzene ring in the global minimum ground state structure. In the gas phase, a potential energy surface along the twist coordinate at the donor (–NMe₂) and acceptor (–CH = CHCOOMe) sites shows stabilization of S₁ state and destabilization S₂ and S₀ states. A similar potential energy calculation along the twist coordinate in acetonitrile solvent using non-equilibrium polarized continuum model also shows more stabilization of S₁ state relative to other states and supports solvent dependent red shifted emission properties. In all types of calculations it is found that the nitrogen lone pair is delocalized over the benzene ring in the global minimum ground state and is localized on the nitrogen centre at the 90° twisted configuration. The S₁ energy state stabilization along the twist coordinate at the donor site and localized nitrogen lone pair at the perpendicular configuration support well the observed dual fluorescence in terms of proposed twisted intramolecular charge transfer (TICT) model.     

Long‐Lived,Directional Photoinduced Charge Separation in RuII Complexes Bearing Laminate Polypyridyl Ligands

Ru^II complexes incorporating both amide‐linked bithiophene donor ancillary ligands and laminate acceptor ligands; dipyrido[3,2‐a:2′,3′‐c]phenazine (dppz), tetrapyrido[3,2‐a:2′,3′‐c:3′′,2′′‐h:2′′′,3′′′‐j]phenazine (tpphz), and 9,11,20,22‐tetraazatetrapyrido[3,2‐a:2′,3′‐c:3′′,2′′‐l:2′′′,3′′′]‐pentacene (tatpp) exhibit long‐lived charge separated (CS) states, which have been analyzed using time‐resolved transient absorption (TA), fluorescence, and electronic absorption spectroscopy in addition to ground state electrochemical and spectroelectrochemical measurements. These complexes possess two electronically relevant ³MLCT states related to electron occupation of MOs localized predominantly on the proximal “bpy‐like” portion and central (or distal) “phenazine‐like” portion of the acceptor ligand as well as energetically similar ³LC and ³ILCT states. The unusually long excited state lifetimes (τ up to 7 μs) observed in these complexes reflect an equilibration of the ³MLCT_prox or ³MLCT_dist states with additional triplet states, including a ³LC state and a ³ILCT state that formally localizes a hole on the bithiophene moiety and an electron on the laminate acceptor ligand. Coordination of a Zn^II ion to the open coordination site of the laminate acceptor ligand is observed to significantly lower the energy of the ³MLCT_dist state by decreasing the magnitude of the excited state dipole and resulting in much shorter excited state lifetimes. The presence of the bithiophene donor group is reported to substantially extend the lifetime of these Zn adducts via formation of a ³ILCT state that can equilibrate with the ³MLCT_dist state. In tpphz complexes, Zn^II coordination can reorder the energy of the ³MLCT_prox and ³MLCT_dist states such that there is a distinct switch from one state to the other. The net result is a series of complexes that are capable of forming CS states with electron–hole spatial separation of up to 14 Å and possess exceptionally long lifetimes by equilibration with other triplet states.     

New study of the stability and of the spectroscopy of the molecular anions NCO- and CNO-

Using highly correlated wave functions, the ground and the low lying excited states of the molecular NCO(-) and CNO(-) anions have been reinvestigated. The stability of the electronic ground state of the two isomers with respect to dissociation and to electron detachment has been checked along the isomerization pathway. The regions of stability of the excited electronic states have been analyzed and identified and it is shown that only the ground state is stable and the corresponding potential energy surface presents three equilibrium positions. The rovibronic spectroscopy of the X (1)Σ(+) state of both NCO(-) and CNO(-) isomers has been determined by a variational approach leading to remarkable agreement with experimental data.     

Excited-state potential surfaces of ruthenium nitrosyl complexes: conical intersections and the Jahn-Teller effect

The Jahn-Teller effect in excited states of nitrosyl complexes was analyzed in terms of the density functional theory at the B3LYP level. This effect was shown to play a role in transitions between metastable isomers and their photoactivated relaxation to the ground state. In the compounds under study, the Jahn-Teller effect can be interpreted as the Renner-Teller effect. The characteristics of the Jahn-Teller potential energy surfaces were determined.     

An investigation into the role of polarity of diamines in controlling the electron migration mechanism of photoluminescent poly(amide–imide)s

Several diamines with remarkable different polarities were used to produce photoactive poly(amide–imide)s (PAI)s in a quantitative yield. The absorption, fluorescence and photophysical properties of series of poly(amide–imide)s containing fused aromatic systems as energy donor and energy acceptor with different diamines cores are described. Poly(amide–imide)s exhibit broad fluorescent characteristic, and its fluorescent intensity is related to the intermolecular chain–chain or chain–solvent interaction. The fluorescence spectra confirmed an efficient singlet–singlet energy transfer between fused aromatic systems. The self-quenching mechanism was studied according to the specific behavior of these polymers in different solvents. The self-quenching rate constant for the association reaction in the excited state (Kq) could be measured from the Stern–Volmer equation. The kind of fused system and diamines show different electron migration mechanisms and photoluminescent properties in the singlet-excited states. By using the exothermic energy transfer as a function of diamine polarity, the electron transfer mechanism was evaluated for aromatic poly(amide–imide)s. In principle, the fluorescence energy is absorbed by different (PAI)s and raises the molecules to one of its excited states. Afterwards this excitation energy transfers through the different relaxation channels, i.e. columbic or exchange energy transfer.     

Comparison of the protein adsorption selectivity of salt-promoted agarose-based adsorbents: Hydrophobic, thiophilic and electron donor–acceptor adsorbents

Protein adsorption of human serum onto six different agarose-based chromatographic gels that were representative of the salt-promoted adsorbent family [octyl- and phenyl-Sepharose, mercaptoethanol–divinyl sulfone agarose (T gel), mercaptomethylene pyridine-derivatized agarose gel (MP gel), tricyanoaminopropene–divinyl sulfone agarose (DVS–TCP gel), tricyanoamino-propene–bisoxirane agarose (bisoxirane–TCP gel)] was studied in the presence of moderate or high concentrations of the water structuring salt, sodium sulfate. Study of the protein adsorption selectivity by two-dimensional gel electrophoresis revealed an opposed selectivity for hydrophobic interaction adsorbents and electron donor–acceptor adsorbents. The T gel, MP gel and TCP gels belonged to the electron donor–acceptor adsorbents, displaying a main selectivity for immunoglobulins, whereas octyl-Sepharose belonged to the hydrophobic adsorbents, displaying a main selectivity for ‘hydrophobic' proteins. Phenyl-Sepharose for its part was described as an example of a composite selectivity of both families. The conclusion of this work is two-fold: (1) hydrophobic interaction chromatography (HIC) and electron donor–acceptor chromatography (EDAC) have opposed protein selectivities and are both salt-promoted. As a main consequence, it means that high concentrations of a water-structuring salt can promote different types of weak molecular interactions, resulting in different protein adsorption selectivities: (2) thiophilic adsorption chromatography (TAC) should be renamed EDAC as similar protein selectivity is demonstrated for electron donor–acceptor ligand devoid of sulfur atoms.     

Charge‐Recombination Fluorescence from Push–Pull Electronic Systems Constructed around Amino‐Substituted Styryl–BODIPY Dyes

A small series of donor–acceptor molecular dyads has been synthesized and fully characterized. In each case, the acceptor is a dicyanovinyl unit and the donor is a boron dipyrromethene (BODIPY) dye equipped with a single styryl arm bearing a terminal amino group. In the absence of the acceptor, the BODIPY‐based dyes are strongly fluorescent in the far‐red region and the relaxed excited‐singlet states possess significant charge‐transfer character. As such, the emission maxima depend on both the solvent polarity and temperature. With the corresponding push–pull molecules, there is a low‐energy charge‐transfer state that can be observed by both absorption and emission spectroscopy. Here, charge‐recombination fluorescence is weak and decays over a few hundred picoseconds or so to recover the ground state. Overall, these results permit evaluation of the factors affecting the probability of charge‐recombination fluorescence in push–pull dyes. The photophysical studies are supported by cyclic voltammetry and DFT calculations.     

Spin Changes Accompany Ultrafast Structural Interconversion in the Ground State of a Cobalt Nitrosyl Complex

Ultrafast, reversible intersystem crossing (ISC) is reported under ambient conditions for the electronic ground state of the pentacoordinate cobalt nitrosyl complexes, [CoX₂(NO)(PMePh₂)₂] (X=Cl, Br), in solution. ISCs on such short timescales are more typically observed in electronically excited states reached by absorption of ultraviolet or visible light. Singlet and triplet electron spin states of the complex, corresponding to two different isomers, are populated at room temperature, and the two isomers exchange on a timescale of a few picoseconds. Ultrafast two‐dimensional infrared spectroscopy observes the change in wavenumber of the NO ligand band accompanying the isomerization and associated ISC on the (spin) adiabatic ground potential energy surface. Comparison of the dynamics of the chloro‐ and bromo‐complexes shows that inertial effects of the ligand motion have a greater effect than spin–orbit coupling on determining the forward and reverse isomerization and ISC rates.     

Application of transform theory to the resonance Raman spectra of the 1:1 hexamethylbenzene: TCNE electron donor—acceptor complex

The resonance Raman excitation profiles of the 1:1 electron donor—acceptor complex between hexamethylbenzene and tetra-cyanoethylene (TCNE) have been gathered and analyzed using transform theory. Scaling factors and non-Condon factors have been obtained for the excitation profiles of the donor—acceptor stretch at 168 cm⁻¹, the C=C stretch at 1550 cm⁻¹, and the CN stretch at 2223 cm⁻¹, and used to estimate the change in donor—acceptor distance and in TCNE bond lengths in the charge trasfer excited state.     

Theoretical investigation of electron transfer transition in tetracyanoethylene-contained organic complexes

In this work, the authors use complete active space self-consistent field method to investigate the photoinduced charge-separated states and the electron transfer transition in complexes ethylene-tetracyanoethylene and tetramethylethylene-tetracyanoethylene. Geometries of isolated tetracyanoethylene, ethylene, and tetramethylethylene have been optimized. The ground state and the low-lying excited states of ethylene and tetracyanoethylene have been optimized. The state energies in the gas phase have been obtained and compared with the experimentally observed values. The torsion barrier of tetracyanoethylene has been investigated through the state energy calculation at different conformations. Attention has been particularly paid to the charge-separated states and the electron transfer transition of complexes. The stacked conformations of the donor-acceptor complexes have been chosen for the optimization of the ground and low-lying excited states. Equilibrium solvation has been considered by means of conductor-like screening model both in water and in dichloromethane. It has been found that the donor and tetracyanoethylene remain neutral in complexes in ground state (1)A(1) and in lowest triplet state (3)B(1), but charge separation appears in excited singlet state (1)B(1). Through the correction of nonequilibrium solvation energy based on the spherical cavity approximation, pi-->pi* electron transfer transition energies have been obtained. Compared with the experimental measurements in dichloromethane, the theoretical results in the same solvent are found higher by about 0.5 eV.     

Quantum-chemical study of the role of conical intersections in photoisomerization processes of the [RuCl<Subscript>5</Subscript>NO]<Subscript>2−</Subscript> complex

The potential surfaces of the ground and lowest excited states of the [RuCl₅NO]_2? complex ion were studied by density functional theory. The conical intersections between the potential surfaces of the ground and lowest excited states were found and characterized. The possible routes from the conical intersection points to the ground state and metastable bond isomers were traced. A preliminary scheme, describing photoisomerizations in the complex, was suggested.     

Long-lived photoinduced charge separation in carbazole–pyrene–viologen system incorporated in Langmuir–Blodgett films of substituted diazacrown ethers

Diaza-18-crown-6 ethers appending two pyrenyl (Py) or two carbazolyl (Cz) groups were synthesized. These macrocyclic compounds form 1:1 host–guest complexes with methyl viologen chloride (MV²⁺), and these complexes were assembled into monolayers by Langmuir–Blodgett technique. The generated assembly involves the general structure of donor–sensitizer–acceptor (Cz–Py–MV²⁺) in space, although any of the photo- and redox-active components are not covalently bonded. Photoirradiation of the pyrenyl group resulted in the charge-separated pair Cz√⁺–Py–MV√⁺ which survived up to hours in a well anaerobic atmosphere. An electrode was fabricated by transferring the L–B film on an ITO glass. The photoinduced voltage of this electrode was measured with a saturated calomel reference electrode in hydroquinone (H₂Q) solution to be ca. 168 mV when the light intensity was 218 mW/cm². This electrode was also used as the light electrode to construct a photogalvanic cell with a platinum electrode as the dark electrode. Irradiation of the light electrode with visible light results in anodic photocurrent, and there is no net chemical change associated with the function of the cell which converts light to electricity.