On the role of intramolecular vibrations in the nonradiative decay of excited charge-transfer states of molecular complexes

An attempt for a theoretical treatment of radiationless transitions from excited charge-transfer states in molecular complexes is made within the framework of the statistical limit of radiationless transitions theory. This work deals with the S₁ → S₀ internal conversion in charge-transfer complexes of tetracyanoethylene (an electron acceptor) with benzene and toluene and their perdeuterated analogues. A dominant role of the high-frequency totally symmetric intramolecular vibrational modes in the nonradiative decay of excited charge-transfer states is assumed (this was inferred from the experimentally observed deuterium isotope effect on radiationless S₁ → S₀ transitions). Calculated absolute rate constants for internal conversion are found to be in good agreement with experimental ones. The results of our calculations reflect very well the observed moderate deuterium isotope effect.     

Vibronic coupling in excited charge-transfer states of solid-state charge-transfer complexes

Symmetry properties of CT excited states of some weak donor-acceptor complexes are discussed in the context of vibronic coupling with intermolecular vibrations. The results are applied to the analysis of electroabsorption spectra of anthracene-PMDA.     

Application of time-resolved infrared spectroscopy to electronic structure in metal-to-ligand charge-transfer excited states

Infrared data in the nu(CO) region (1800-2150 cm(-1), in acetonitrile at 298 K) are reported for the ground (nu(gs)) and polypyridyl-based, metal-to-ligand charge-transfer (MLCT) excited (nu(es)) states of cis-[Os(pp)2(CO)(L)](n)(+) (pp = 1,10-phenanthroline (phen) or 2,2'-bipyridine (bpy); L = PPh3, CH(3)CN, pyridine, Cl, or H) and fac-[Re(pp)(CO)3(4-Etpy)](+) (pp = phen, bpy, 4,4'-(CH3)2bpy, 4,4'-(CH3O)2bpy, or 4,4'-(CO2Et)2bpy; 4-Etpy = 4-ethylpyridine). Systematic variations in nu(gs), nu(es), and Delta(nu) (Delta(nu) = nu(es) - nu(gs)) are observed with the excited-to-ground-state energy gap (E(0)) derived by a Franck-Condon analysis of emission spectra. These variations can be explained qualitatively by invoking a series of electronic interactions. Variations in dpi(M)-pi(CO) back-bonding are important in the ground state. In the excited state, the important interactions are (1) loss of back-bonding and sigma(M-CO) bond polarization, (2) pi(pp*-)-pi(CO) mixing, which provides the orbital basis for mixing pi(CO)- and pi(4,4'-X(2)bpy)-based MLCT excited states, and (3) dpi(M)-pi(pp) mixing, which provides the orbital basis for mixing pipi- and pi(4,4'-X(2)bpy*-)-based MLCT states. The results of density functional theory (DFT) calculations on the ground and excited states of fac-[Re(I)(bpy)(CO)3(4-Etpy)](+) provide assignments for the nu(CO) modes in the MLCT excited state. They also support the importance of pi(4,4'-X2bpy*-)-pi(CO) mixing, provide an explanation for the relative intensities of the A'(2) and A' ' excited-state bands, and provide an explanation for the large excited-to-ground-state nu(CO) shift for the A'(2) mode and its relative insensitivity to variations in X.     

On the kinetics of formation of an excited charge-transfer state in 4-(9-anthryl)N,N-dimethylaniline

The kinetics of formation of an excited charge-transfer (CT) state in 4-(9-anthryl)-N,N-dimethylaniline have been studied by means of steady-state and nanosecond time-resolved fluorescence. It is concluded that the excited CT state is formed irreversibly at room temperature in ethyl acetate on a timescale shorter than 54 picoseconds.     

Nature and location of excited charge-transfer states in the porphine-magnesium porphine dimer

The ground and excited states of a cofacial porphine-magnesium porphine dimer with a ring separation of 5.35 Å are investigated by ab initio configuration interaction calculations, using a floating gaussian basis. A pair of charge-transfer states are found ≈23000 cm^?1 above the ground state, but are lowered by ≈7400 cm^?1 upon coordination of the Mg atom with chloride ion.     

The electronic structure of low-lying excited states of two model systems of retinal

2,4-pentadienal and 2,4,6,8-nonatetraenal were studied as simple model systems of retinal. Four kinds of CI were performed on low-lying excited states of 2,4-pentadienal by using a split valence basis set. The results show that MR SD π CI is not adequate for the π–π* state and the single excitation σπ CI is a good compromise between cost and effectiveness as far as singly excited states are concerned. This CI was applied to the bigger model system. All-trans and 11-cis forms of aldehyde, Schiff base, and protonated Schiff base of the model system were studied. A fairly large energy lowering of about 1 eV of the first allowed excited state (π → π*) occurs when the Schiff base is protonated for both all-trans and 11-cis forms.     

Effect of pressure on the formation of the intramolecular excited complex state in 4-(9-anthrylmethyl)-N,N-dimethylaniline

The formation of an intramolecular excited complex state in 4-(9-anthrylmethyl)-N,N-dimethylaniline was investigated at high pressures. A change in the kinetics from thermodynamic control to kinetic control was observed in ethyl ether solvent. In toluene the process is kinetically controlled over the whole pressure range. Appearance of this process depends upon the solvent viscosity, suggesting a conformational change in the course of the formation of the excited complex state.     

Radiationless decay of red fluorescent protein chromophore models via twisted intramolecular charge-transfer states

We use CASSCF and MRPT2 calculations to characterize the bridge photoisomerization pathways of a model red fluorescent protein (RFP) chromophore model. RFPs are homologues of the green fluorescent protein (GFP). The RFP chromophore differs from the GFP chromophore via the addition of an N-acylimine substitution to a common hydroxybenzylidene-imidazolinone (HBI) motif. We examine the substituent effects on the manifold of twisted intramolecular charge-transfer (TICT) states which mediates radiationless decay via bridge isomerization in fluorescent protein chromophore anions. We find that the substitution destabilizes states associated with isomerization about the imidazolinone-bridge bond and stabilizes states associated with phenoxy-bridge bond isomerization. We discuss the results in the context of chromophore conformation and quantum yield trends in the RFP subfamily, as well as recent studies on synthetic models where the acylimine has been replaced with an olefin.     

Contrasts in the 77 K emission spectra, structures, and dynamics of metal-to-metal and metal-to-ligand charge-transfer excited states

The 77 K emission spectrum of trans-[(ms-Me6[14]aneN4)Cr(CNRu(NH3)5)2]5+ has components characteristic of ligand field (LF) and metal-to-metal charge transfer (MMCT) excited states (ms-Me6[14]aneN4=5,12-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). The LF component of the emission is best resolved for irradiations at appreciably higher energies than the MMCT absorption band, while only the MMCT emission is observed for irradiations on the low-energy side of the MMCT absorption band. The LF emission component from this complex has vibronic structure that is very similar to that of the trans-[(ms-Me6[14]aneN4)Cr(CN)2]+ parent, but it is red-shifted by 560 cm-1 and the bandwidths are much larger. The red shift and the larger bandwidths of the ruthenated complex are attributed to configurational mixing between the LF and MMCT excited states, and the inferred mixing parameters are shown to be consistent with the known electron-transfer properties of the Ru(NH3)5 moieties. The MMCT excited-state lifetime is about 1 micros at 77 K and am(m)ine perdeuteration of this complex leads to an isotope effect of kNH/kND approximately 15-20. However, the contribution of the N-H stretching vibration to the emission sideband is too weak for a single vibrational mode model to be consistent with the observed lifetimes or the isotope effect. These features are very similar to those reported previously (J. Phys. Chem. A 2004, 108, 5041) for the MMCT emission of trans-[([14]aneN4)Cr{CNRu(NH3)5}2]5+ ([14]aneN4=1,4,8,11-tetraazacyclotetradecane), with the exception that the higher energy LF emission was not well resolved in the earlier work. The energies of the charge transfer absorption and emission maxima of both of these Cr(CN)Ru complexes are very similar to those of [Ru(NH3)4bpy]2+, but the latter has a 50-fold shorter 77 K excited-state lifetime, a 10-fold smaller NH/ND isotope effect, and a very different structure of its vibronic sidebands. Thus, the vibronic sidebands imply that the dominant excited-state distortions are in the metal-ligand vibrational modes for the Cr(CN)Ru complexes and in the bipyridine vibrational modes for the [Ru(NH3)4bpy]2+ complex. While an "equivalent" single vibrational mode model based on the frequencies and amplitudes of the dominant distortion modes is not consistent the observed lifetimes, such models do appear to be a good basis for qualitatively distinguishing different classes of excited-state dynamic behavior. A multimode, multichannel model may be necessary to adequately describe the excited-state dynamics of these simple electron-transfer systems.     

Switching between ligand-to-ligand charge-transfer, intraligand charge-transfer, and metal-to-ligand charge-transfer excited states in platinum(II) terpyridyl acetylide complexes induced by pH change and metal ions

A series of platinum(II) terpyridyl alkynyl complexes, [Pt{4'-(4-R1-C6H4)terpy}(C[triple chemical bond]C-C6H4-R(2)-4)]ClO4 (terpy=2,2':6',2'-terpyridyl; R1=R2=N(CH3)2 (1); R1=N(CH3)2, R2=N-[15]monoazacrown-5 (2); R1=CH3, R2=N(CH3)2 (3); R1=N(CH3)2, R2=H (4); R1=CH3, R2=H (5)), has been synthesized and the photophysical properties of the complexes have been examined through measurement of their UV/Vis absorption spectra, photoluminescence spectra, and transient absorptions. Complex 3 shows a lowest-energy absorption corresponding to a ligand-to-ligand charge-transfer (LLCT) transition from the acetylide to the terpyridyl ligand, whereas 4 shows an intraligand charge-transfer (ILCT) transition from the pi orbital of the 4'-phenyl group to the pi* orbital of the terpyridyl. Upon protonation of the amino groups in 3 and 4, their lowest-energy excited states are switched to dpi(Pt)-->pi*(terpy) metal-to-ligand charge-transfer (MLCT) states. The lowest-energy absorption for 1 and 2 may be attributed to an LLCT transition from the acetylide to the terpyridyl. Upon addition of an acid to a solution of 1 or 2, the amino group on the acetylide is protonated first, followed by the amino group on the terpyridyl. Thus, the lowest excited state of 1 and 2 can be successively switched from the LLCT state to the ILCT state and then to the MLCT state by controlling the amount of the acid added. Such switches in the excited state are fully reversible upon subsequent addition of a base to the solution. Sequential addition of alkali metal or alkaline earth metal ions and then an acid to a solution of 2 also leads to switching of its lowest excited state from the LLCT state, first to the ILCT state and then to the MLCT state. All of the complexes exhibit a transient absorption of the terpyridyl anion radical, which is present in all of the LLCT, ILCT, and MLCT states. However, the shape of the transient absorption spectrum depends on both the substitution pattern on the terpyridyl moiety and the nature of the excited state.     

Influence of excited state aromaticity in the lowest excited singlet states of fulvene derivatives

The absorption spectra and excited state dipole moments of four differently substituted fulvenes have been investigated both experimentally and computationally. The results reveal that the excited state dipole moment of fulvenes reverses in the first excited singlet state when compared to the ground state. The oppositely polarized electron density distributions, which dominate the ground state and the first excited singlet state of fulvenes, respectively, reflect the reversed π-electron counting rules for aromaticity in the two states (4n + 2 vs. 4n, respectively). The results show that substituents indeed influence the polarity of fulvenes in the two states, however, cooperative interactions between the substituents and the fulvene moiety are most pronounced in the ground state.     

Presence and absence of excited state intramolecular charge transfer with the six isomers of dicyano-N,N-dimethylaniline and dicyano-(N-methyl-N-isopropyl)aniline

The excited state behavior of the six m,n-dicyano-N,N-dimethylanilines (mnDCDMA) and m,n-dicyano-(N-methyl-N-isopropyl)anilines (mnDCMIA) is discussed as a function of solvent polarity and temperature. The dicyano moiety in these electron donor (D)/acceptor (A) molecules has a considerably larger electron affinity than the benzonitrile subgroup in 4-(dimethylamino)benzonitrile (DMABN). Nevertheless, the fluorescence spectra of the mnDCDMAs and mnDCMIAs in n-hexane all consist of a single emission originating from the locally excited (LE) state, indicating that a reaction from LE to an intramolecular charge transfer (ICT) state does not take place. The calculated energies E(ICT), obtained by employing the reduction potential of the dicyanobenzene subgroups and the oxidation potential of the amino substituents trimethylamine (N(Me)(3)) and isopropyldimethylamine (iPrNMe(2)), are lower than E(LE). The absence of an LE → ICT reaction therefore makes clear that the D and A units in the dicyanoanilines are not electronically decoupled. In the polar solvent acetonitrile (MeCN), dual (LE + ICT) fluorescence is found with 24DCDMA and 34DCDMA, as well as with 24DCMIA, 25DCMIA, and 34DCMIA. For all other mnDCDMAs and mnDCMIAs, only LE emission is observed in MeCN. The ICT/LE fluorescence quantum yield ratio Φ'(ICT)/Φ(LE) in MeCN at 25 °C is larger for 24DCDMA (1.2) than for 34DCDMA (0.35). The replacement of methyl by isopropyl in the amino substituent leads to a considerable increase of Φ'(ICT)/Φ(LE), 8.8 for 24DCMIA and 1.4 for 34DCMIA, showing that the LE ? ICT equilibrium has shifted further toward ICT. The appearance of an ICT reaction with the 2,4- and 3,4-dicyanoanilines is caused by a relatively small energy gap ΔE(S(1),S(2)) between the two lowest excited singlet states as compared with the other m,n-dicyanoanilines, in accordance with the PICT model. The observation that the ICT reaction is more efficient for 24DCMIA and 34DCMIA than for their mnDCDMA counterparts is mainly caused by the fact that iPrNMe(2) is a better electron donor than N(Me)(3): E(D/D(+)) = 0.84 against 1.05 V vs SCE. That ICT also occurs with 25DCMIA, notwithstanding its large ΔE(S(1),S(2)), is due to the substantial amino twist angle θ = 42.6°, which leads to partial electronic decoupling of the D and A subgroups. The dipole moments μ(e)(ICT) range between 18 D for 34DCMIA and 12 D for 25DCMIA, larger than the corresponding μ(e)(LE) of 16 and 11 D. The difference between μ(e)(ICT) and μ(e)(LE) is smaller than with DMABN (17 and 10 D) because of the noncollinear arrangement of the amino and cyano substituents (different dipole moment directions). The dicyanoanilines that do not undergo ICT, have LE dipole moments between 9 and 16 D. From plots of ln(Φ'(ICT)/Φ(LE)) vs 1000/T, the (rather small) ICT reaction enthalpies ΔH could be measured in MeCN: 5.4 kJ/mol (24DCDMA), 4.7 kJ/mol (24DCMIA), and 3.9 kJ/mol (34DCMIA). With the mnDCDMAs and mnDCMIAs only showing LE emission, the fluorescence decays are single exponential, whereas for those undergoing an LE → ICT reaction the LE and ICT picosecond fluorescence decays are double exponential. In MeCN at 25 °C, the decay times τ(2) have values between 1.8 ps for 24DCMIA and 4.6 ps for 34DCMIA at 25 °C. Longer times are observed at lower temperatures. Arrhenius plots of the forward and backward ICT rate constants k(a) and k(d) of 25DCMIA in tetrahydrofuran, obtained from the LE and ICT fluorescence decays, give the activation energies E(a) = 4.5 kJ/mol and E(d) = 11.9 kJ/mol, i.e., ΔH = -7.4 kJ/mol. From femtosecond transient absorption spectra of 24DCDMA and 34DCDMA at 22 °C, ICT reaction times τ(2) = 1/(k(a) + k(d)) of 1.8 and 3.1 ps are determined. By combining these results with the data for the fluorescence decays and Φ'(ICT)/Φ(LE), the values k(a) = 49 × 10(10) s(-1) (24DCDMA) and k(a) = 23 × 10(10) s(-1) (34DCDMA) are calculated. An LE and ICT excited state absorption is present even at a pump/probe delay time of 100 ps, showing that an LE ? ICT equilibrium is established.     

Efficient sensitization of europium, ytterbium, and neodymium functionalized tris-dipicolinate lanthanide complexes through tunable charge-transfer excited states

A series of push-pull donor-pi-conjugated dipicolinic acid ligands and related tris-dipicolinate europium and lutetium complexes have been prepared. The ligands present broad absorption and emission transitions in the visible spectral range unambiguously assigned to charge-transfer transitions (CT) by means of time-dependent density functional theory calculations. The photophysical properties (absorption, emission, luminescence quantum yield, and lifetime) of the corresponding europium complexes were thoroughly investigated. Solvatochromism and temperature effects clearly confirm that Eu(III) sensitization directly occurs from the ligand CT state. In addition, modulation of the energy of the CT donating state by changing the nature of the donor fragment allows the optimal energy of the antennae for europium sensitization to be determined, and this optimal energy was found to be close to the (5)D 1 accepting state. Finally, this CT sensitization process has been successfully extended to near-infrared emitters (neodymium and ytterbium).     

The structure and energetics of TPD ground and excited states

We analyse the optimized geometric structure and energies of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) neutral molecule in ground and excited states. Ab initio calculations are performed at the Hartree–Fock (HF) or single-excitation (CIS) level with 6-31G basis set using Gaussian 98 program. We got a good qualitative agreement with the results for the neutral molecule in the ground state obtained by density functional theory methods, which were previously reported in the literature. TPD in the ground state has a pronounced twisted geometry, with both central and peripheral dihedral angles of about 40°. On the other hand, the structure of excited state is more planar, as the central dihedral angle becomes smaller. We estimated the reorganization energies associated with the optical transitions, corresponding transition dipole moments, as well as HOMO and LUMO. On the basis of ab initio calculations, we developed a semi-empirical model for the observed absorption and photoluminescence and interpreted the large Stokes shift of about 0.4 eV.     

Character of the electronic ground state and of charge-transfer excited states in ionic solids: An ab initio cluster model approach

The electronic structure of the ground electronic state and of some special charge-transfer excited states in ionic solids is examined from the ab initio cluster model approach. Different ab initio wave functions, including a frozen orbital approach, the Hartree–Fock self-consistent field, and multireference configuration interaction wave functions, are considered and analyzed using different theoretical techniques. We explicitly consider some alkaline–earth oxides such as CaO, a more difficult case such as A1₂O₃, a transition-metal oxide such as NiO, and a system with a more complicated structure such as KNiF₃. Analysis of ab initio wave functions in terms of valence bond components shows that all these compounds are largely ionic, thus supporting the simple picture arising from the ionic model. However, the nature of the excited states is more complex. Alkaline–earth oxides lowest excited states are essentially described as charge-transfer excitations dominated by a single resonant valence bond structure and the calculated energy difference is comparable to the experimental optical gap. In the case of A1₂O₃, the electronic spectra presents excitonic features and the local charge-transfer excitation excited states provide a reasonable representation of these phenomena. Finally, several different valence bond structures are present in the lowest electronic states of KNiF₃. © 1994 John Wiley & Sons, Inc.     

The nature of the rapid relaxation of excited charge-transfer complexes

Examples of contact radical-ion-pair (CRIP) formation from excited charge-transfer (CT) complexes are described. The reduced absorption and emission spectra of the CT complexes formed between hexamethylbenzene, pentamethylbenzene, and durene donors and 1,2,4,5-tetracyanobenzene (TCNB) in 1,2-dichloroethane (DCLE) exhibit a mirror image relationship, suggesting that each set of spectra describes transitions between the same two states. It was concluded that a CRIP is produced immediately upon excitation of the CT complex and that relaxation of the CRIP includes only minor geometry changes and changes in solvent polarization. In contrast to these results, the reduced absorption and emission spectra of the mesitylene (MES)/TCNB CT complex in DCLE are distinctly different and do not display a mirror image relationship. Time-resolved emission decay traces reveal the presence of an initial intermediate species that contributes approximately 10% of the total steady-state emission. The emission spectrum of this initial species mirrors the absorption spectrum of the MES/TCNB complex. In the MES/TCNB complex, excitation does not lead directly to the CRIP, and the relaxation of the excited complex must include an electronic component in addition to changes in geometry and solvation. The implications of these results on the applicability of golden-rule expressions of electron transfer are discussed.     

Vibrational spectra and intramolecular dynamics of six-membered azacycles in the ground and excited states

The geometrical structure, force fields, and parameters of vibrational-rotational interaction of six-membered azacyclic compounds in the ground and excited states are estimated using the structural dynamic model of a polyatomic molecule. Translated fromZhurnal Struktumoi Khimii, Vol. 38, No. 2, pp. 345–349, March–April, 1997.     

Pentacyano-N,N-dimethylaniline in the excited state. Only locally excited state emission, in spite of the large electron affinity of the pentacyanobenzene subgroup

Pentacyano-N,N-dimethylaniline (PCDMA) does not undergo an intramolecular charge transfer (ICT) reaction, even in the strongly polar solvent acetonitrile (MeCN), in clear contrast to 4-(dimethylamino)benzonitrile (DMABN). Within the twisted ICT (TICT) model, this is unexpected, as the electron affinity of the pentacyanobenzene moiety of PCDMA is much larger than that of the benzonitrile subgroup in DMABN. According to the TICT model, the energy of the ICT state of PCDMA would be 2.05 eV (～16550 cm(-1)) lower than that of DMABN, on the basis of the reduction potentials E(A(-)/A) of pentacyanobenzene (-0.29 V vs saturated calomel electrode (SCE)) and benzonitrile (-2.36 V vs SCE), more than enough to compensate for the decrease in energy of the locally excited (LE) state of PCDMA (E(S(1)) = 19990 cm(-1)) relative to that of DMABN (E(S(1)) = 29990 cm(-1)). This absence of a LE → ICT reaction shows that the TICT hypothesis does not hold for PCDMA in the singlet excited state, similar to what was found for DMABN, N-phenylpyrrole, and their derivatives. In this connection, the six dicyano-substituted dimethylanilines are also discussed. The energy gap ΔE(S(1),S(2)) between the two lowest singlet excited states is, at 7170 cm(-1) for PCDMA in MeCN, considerably larger than that for DMABN (2700 cm(-1) in n-hexane, smaller in MeCN). The absence of ICT is therefore in accord with the planar ICT (PICT) model, which considers a sufficiently small ΔE(S(1),S(2)) to be an important condition determining whether an ICT reaction will take place. The fluorescence quantum yield of PCDMA is very small: Φ(LE) = 0.0006 in MeCN at 25 °C, predominantly due to LE → S(0) internal conversion (IC), as the intersystem crossing yield Φ(ISC) is practically zero (<0.01). From the LE fluorescence decay time of 27 ps for PCDMA in MeCN at 25 °C, a radiative rate constant k(f)(LE) = 2 × 10(7) s(-1) results, comparable to the k(f)(LE) of DMABN (6.5 × 10(7) s(-1)) and 2,4,6-tricyano-N,N-dimethylaniline (TCDMA) (1.2 × 10(7) s(-1)) in this solvent, but clearly larger than the k'(f)(ICT) = 0.79 × 10(7) s(-1) of DMABN in MeCN. The IC reaction with PCDMA in MeCN at room temperature, with a rate constant k(IC) of 3.6 × 10(10) s(-1), is much faster than with TCDMA (25 × 10(7) s(-1)) and DMABN (1.3 × 10(7) s(-1), in n-hexane). This is connected with the nonzero (37°) amino twist angle of PCDMA, which leads to a decrease of the effective LE-S(0) energy gap. The femtosecond excited state absorption (ESA) spectra of PCDMA in MeCN at 22 °C are similar to the LE ESA spectra of TCDMA and DMABN and are therefore attributed to the LE state, confirming that an ICT reaction does not occur. The decay of the LE ESA spectra of PCDMA is single exponential, with a decay time of 22 ps, in reasonable agreement with the LE fluorescence decay time of 27 ps at 25 °C. The spectra decay to zero, showing that there is no triplet or other intermediate.     

Failure of time-dependent density functional theory for long-range charge-transfer excited states: the zincbacteriochlorin-bacteriochlorin and bacteriochlorophyll-spheroidene complexes

It is well-known that time-dependent density functional theory (TDDFT) yields substantial errors for the excitation energies of charge-transfer (CT) excited states, when approximate standard exchange-correlation (xc) functionals are used, for example, SVWN, BLYP, or B3LYP. Also, the correct 1/R asymptotic behavior of CT states with respect to a distance coordinate R between the separated charges of the CT state is not reproduced by TDDFT employing these xc-functionals. Here, we demonstrate by analysis of the TDDFT equations that the first failure is due to the self-interaction error in the orbital energies from the ground-state DFT calculation, while the latter is a similar self-interaction error in TDDFT arising through the electron transfer in the CT state. Possible correction schemes, such as inclusion of exact Hartree-Fock or exact Kohn-Sham exchange, as well as aspects of the exact xc-functional are discussed in this context. Furthermore, a practical approach is proposed which combines the benefits of TDDFT and configuration interaction singles (CIS) and which does not suffer from electron-transfer self-interaction. The latter approach is applied to a (1,4)-phenylene-linked zincbacteriochlorin-bacteriochlorin complex and to a bacteriochlorophyll-spheroidene complex, in which CT states may play important roles in energy and electron-transfer processes. The errors of TDDFT alone for the CT states are demonstrated, and reasonable estimates for the true excitation energies of these states are given.     

Trialkyltetrathiafulvalene-sigma-tetracyanoanthraquinodimethane [R(3)TTF-sigma-TCNAQ] diads: synthesis, intramolecular charge-transfer properties, and X-ray crystal structure.

We report the use of the electron-donating 4,5-dipentyl-4'-methyl-TTF (TTF = tetrathiafulvalene) moiety in combination with the electron acceptor 11,11,12,12-tetracyanoanthraquinodimethane (TCNAQ) unit in the novel D-sigma-A diad molecules 11, 17, and 18. These compounds display a weak, broad, low-energy intramolecular charge-transfer (ICT) band in the UV-vis spectra (lambda(max) 430-450 nm). Cyclic voltammetric studies show two reversible one-electron oxidation processes for the R(3)TTF moiety, and a reversible two-electron reduction process for the TCNAQ moiety. The electron affinity of TCNAQ is significantly enhanced by the electron-withdrawing sulfonamide and sulfonic ester groups (compounds 17 and 18, respectively). Simultaneous electrochemistry and EPR (SEEPR) experiments show no significant intramolecular interaction between the R(3)TTF and TCNAQ moieties in compounds 11 and 18. X-ray crystallographic data are presented for 5, 11, and 20. The structure of 5 reveals hydrogen-bonded dimers. In molecule 11 the bond lengths and conformations of both donor and acceptor moieties are typical for neutral species. Compound 20 is an unusual calcium complex of TCNAQ derivative obtained by dicyanomethylation of anthraquinone-2-carboxylic acid.