Electron delocalization in homoconjugated 7,7-diarylnorbornane systems: a computational and experimental study

A joint computational–experimental study has been carried out to analyze the homoconjugative interactions in 7,7‐diarylnorbornane (DPN) derivatives. The experimentally observed new bands in their UV/Vis have been accurately assigned by means of TD‐DFT calculations. Both experimental data and computations show that aromatic homoconjugation in acyclic systems is an effective mechanism for electron delocalization that resembles the situation described for polyphenylenes and polyenes. The effective homoconjugation length in homoconjugated oligomers is in the range of 6–7 aryl rings. The effect of substituents directly attached to the para carbon atom of the DPN moiety have been also studied. We found that the HOMO→LUMO vertical transitions can indeed be modified by the nature of the aromatic substituents in order to provoke dramatic changes in the electronic properties (i.e., in the absorption spectra) of the studied species.     

Photoinduced intramolecular charge transfer and S2 fluorescence in thiophene-pi-conjugated donor-acceptor systems: experimental and TDDFT studies

Experimental and theoretical methods were used to study newly synthesized thiophene-pi-conjugated donor-acceptor compounds, which were found to exhibit efficient intramolecular charge-transfer emission in polar solvents with relatively large Stokes shifts and strong solvatochromism. To gain insight into the solvatochromic behavior of these compounds, the dependence of the spectra on solvent polarity was studied on the basis of Lippert-Mataga models. We found that intramolecular charge transfer in these donor-acceptor systems is significantly dependent on the electron-withdrawing substituents at the thienyl 2-position. The dependence of the absorption and emission spectra of these compounds in methanol on the concentration of trifluoroacetic acid was used to confirm intramolecular charge-transfer emission. Moreover, the calculated absorption and emission energies, which are in accordance with the experimental values, suggested that fluorescence can be emitted from different geometric conformations. In addition, a novel S(2) fluorescence phenomenon for some of these compounds was also be observed. The fluorescence excitation spectra were used to confirm the S(2) fluorescence. We demonstrate that S(2) fluorescence can be explained by the calculated energy gap between the S(2) and S(1) states of these molecules. Furthermore, nonlinear optical behavior of the thiophene-pi-conjugated compound with diethylcyanomethylphosphonate substituents was predicted in theory.     

A challenge to chemical intuition: donor-acceptor interactions in H3B-L and H2B+-L (L=CO; EC5H5, E=N-Bi)

The equilibrium geometries and bond energies of the complexes H(3)B-L and H(2)B(+)-L (L=CO; EC(5)H(5): E=N, P, As, Sb, Bi) have been calculated at the BP86/TZ2P level of theory. The nature of the donor-acceptor bonds was investigated by energy decomposition analysis (EDA). The bond strengths of H(3)B-L have the order CO>N>P>As>Sb>Bi. The calculated values are between D(e)=37.1 kcal mol(-1) for H(3)B-CO and D(e)=6.9 kcal mol(-1) for H(3)B-BiC(5)H(5). The bond dissociation energies of the cations H(2)B(+)-CO and H(2)B(+)-EC(5)H(5) are larger than for H(3)B--L, particularly for complexes of the heterobenzene ligands. The calculated values are between D(e)=51.9 kcal mol(-1) for H(2)B(+)-CO and D(e)=122.1 kcal mol(-1) for H(2)B(+)-NC(5)H(5). The trend of the BDE of H(2)B(+)-CO and H(2)B(+)-EC(5)H(5) is N>P>As>Sb>Bi>CO. A surprising result is found for H(2)B(+)-CO, which has a significantly stronger and yet substantially longer bond than H(3)B-CO. The reason for the longer but stronger bond in H(2)B(+)-CO compared with that in H(3)B-CO comes mainly from the change in electrostatic attraction and pi bonding at shorter distances, which increases more in the neutral system than in the cation, and to a lesser extent from the deformation energy of the fragments. The H(2)B(+)<--NC(5)H(5) pi( perpendicular) donation plays an important role for the stronger interactions at shorter distances compared with those in H(3)B-NC(5)H(5). The attractive interaction in H(2)B(+)--CO further increases at bond lengths that are shorter than the equilibrium value, but this is compensated by the energy which is necessary to deform BH(2) (+) from its linear equilibrium geometry to the bent form in the complex. The EDA shows that the contributions of the orbital interactions to the donor-acceptor bonds are always larger than the classical electrostatic contributions, but the latter term plays an important role for the trend in bond strength. The largest contributions to the orbital interactions come from the sigma orbitals. The EDA calculations suggest that heterobenzene ligands may become moderately strong pi donors in complexes with strong Lewis acids, while CO is only a weak pi donor. The much stronger interaction energies in H(2)B(+)-EC(5)H(5) compared with those in H(3)B-EC(5)H(5) are caused by the significantly larger contribution of the pi(perpendicular) orbitals in H(2)B(+)-EC(5)H(5) and by the increase of the binding interactions of the sigma+pi( parallel) orbitals.     

Thermodynamics of the decomposition processes of donor-acceptor complexes MX3 x en x MX3 and MX3 x en.

The structural and thermodynamic properties of the donor-acceptor (DA) complexes of Group 13 metal halides (MX3) with ethylenediamine and their decomposition products have been studied theoretically at the B3LYP/LANL2DZ(d,p) level of theory. Gas-phase dissociation into various components and HX elimination reactions are considered. Both processes are endothermic but favored by entropy. Complexes of 2:1 composition are predicted to be stable in the gas phase up to 640-1000 K. It is found that complexation with the second acceptor molecule lowers the HX elimination enthalpy; in turn, HX elimination increases DA bonding with a second MX3 molecule. Exceptionally high values of the dissociation enthalpies (310-390 kJ mol(-1)) and HX elimination reactions (360-420 kJ mol(-1)) of the amido compounds MX2NHC2H4NH2 and MX2NHC2H4NHMX2 make them important intermediates in the decomposition processes. Dissociation reactions of the complexes are more favorable than HX elimination reactions; however, the subsequent oligomerization and cyclization processes of coordinationally unsaturated amido and imido compounds may facilitate HX elimination. Since HI elimination reactions are predicted to be the least endothermic, and aluminum-containing compounds have the strongest M-N dissociation enthalpies, it is expected that compounds based on aluminum iodide are promising objects for experimental studies.     

Divalent carbon(0) chemistry, part 2: Protonation and complexes with main group and transition metal Lewis acids.

Quantum-chemical calculations with DFT (BP86) and ab initio methods (MP2, SCS-MP2) were carried out for protonated and diprotonated compounds N-H(+) and N-(H(+))(2) and for the complexes N-BH(3), N-(BH(3))(2), N-CO(2), N-(CO(2))(2), N-W(CO)(5), N-Ni(CO)(3) and N-Ni(CO)(2) where N=C(PH(3))(2) (1), C(PMe(3))(2) (2), C(PPh(3))(2) (3), C(PPh(3))(CO) (4), C(CO)(2) (5), C(NHC(H))(2) (6), C(NHC(Me))(2) (7) (Me(2)N)(2)C==C==C(NMe(2))(2) (8) and NHC (9) (NHC(H)=N-heterocyclic carbene, NHC(Me)=N-substituted N-heterocyclic carbene). Compounds 1-4 and 6-9 are very strong electron donors, and this is manifested in calculated protonation energies that reach values of up to 300 kcal mol(-1) for 7 and in very high bond strengths of the donor-acceptor complexes. The electronic structure of the compounds was analyzed with charge- and energy-partitioning methods. The calculations show that the experimentally known compounds 2-5 and 8 chemically behave like molecules L(2)C which have two L-->C donor-acceptor bonds and a carbon atom with two electron lone pairs. The behavior is not directly obvious when the linear structures of carbon suboxide and tetraaminoallenes are considered. They only come to the fore on reaction with strong electron-pair acceptors. The calculations predict that single and double protonation of 5 and 8 take place at the central carbon atom, where the negative charge increases upon subsequent protonation. The hitherto experimentally unknown carbodicarbenes 6 and 7 are predicted to be even stronger Lewis bases than the carbodiphosphoranes 1-3.     

Divalent carbon(0) chemistry, part 1: Parent compounds

Quantum-chemical calculations with DFT (BP86) and ab initio methods [MP2, SCS-MP2, CCSD(T)] have been carried out for the molecules C(PH(3))(2) (1), C(PMe(3))(2) (2), C(PPh(3))(2) (3), C(PPh(3))(CO) (4), C(CO)(2) (5), C(NHC(H))(2) (6), C(NHC(Me))(2) (7) (Me(2)N)(2)C=C=C(NMe(2))(2) (8), and NHC (9), where NHC=N-heterocyclic carbene and NHC(Me)=N-methyl-substituted NHC. The electronic structure in 1-9 was analyzed with charge- and energy-partitioning methods. The results show that the bonding situations in L(2)C compounds 1-8 can be interpreted in terms of donor-acceptor interactions between closed-shell ligands L and a carbon atom which has two lone-pair orbitals L-->C<--L. This holds particularly for the carbodiphosphoranes 1-3 where L=PR(3), which therefore are classified as divalent carbon(0) compounds. The NBO analysis suggests that the best Lewis structures for the carbodicarbenes 6 and 7 where L is a NHC ligand have C==C==C double bonds as in the tetraaminoallene 8. However, the Lewis structures of 6-8, in which two lone-pair orbitals at the central carbon atom are enforced, have only a slightly higher residual density. Visual inspection of the frontier orbitals of the latter species reveals their pronounced lone-pair character, which suggests that even the quasi-linear tetraaminoallene 8 is a "masked" divalent carbon(0) compound. This explains the very shallow bending potential of 8. The same conclusion is drawn for phosphoranylketene 4 and for carbon suboxide (5), which according to the bonding analysis have hidden double-lone-pair character. The AIM analysis and the EDA calculations support the assignment of carbodiphosphoranes as divalent carbon(0) compounds, while NHC 9 is characterized as a divalent carbon(II) compound. The L-->C((1)D) donor-acceptor bonds are roughly twice as strong as the respective L-->BH(3) bond.     

A density functional study to choose the best fluorophore for photon-induced electron-transfer (PET) sensors

Anthracenes bearing aliphatic or aromatic amino substituents, which behave as molecular sensors, have shown their potential to act as photon-induced electron-transfer (PET) systems. In this PET, the fluorophore moieties are responsible for electron release during protonation and deprotonation. The principle of hard and soft acids and bases (HSAB) deals with both intra- and intermolecular electron migration. It is possible to calculate the localized properties in terms of Fukui functions in the realm of density functional theory (DFT) and thus calculate and establish a numerical matchmaking procedure that will generate an a priori rule for choosing the fluorophore in terms of its activity. We calculated the localized properties for neutral, anionic, and cationic systems to trace the course of the efficiency. A qualitative scale is proposed in terms of the feasibility of intramolecular hydrogen bonding. To investigate the effect of the environment of the nitrogen atom on protonation going from mono- to diprotonated systems, we calculated the partial density of states and compared the activity sequence with reactivity indices. The results show that location of the nitrogen atom in an aromatic ring does not influence the PET, but for aliphatic chains it plays a role. Furthermore, the protonation/deprotonation scenario has been explained. The results show that the reactivity indices can be used as a suitable property for scaling the activity of fluorophore molecules for the PET process.     

The influence of terminal push-pull substitution on the electronic structure and optical properties of pentacenes

The synthesis of 2,3-R(2)-9,10-(OMe)(2)-substituted pentacenes (R=OMe, F, Br, CN; 1-4) from 2,3-R(2)-9,10-dimethoxy-6,13-dihydro-6,13-ethanopentacene-15,16-diones (α-diketone-bridged precursors) by photochemically induced bis-decarbonylation (Strating-Zwanenburg reaction) is described. Under matrix-isolation conditions (solid Ar, 10 K) the S(1) transitions of 1 and 2 undergo hypsochromic and those of 3 and 4 bathochromic shifts compared to parent pentacene. The S(1) transition wavelengths correlate well with the difference of substituent parameters σ(p). A computational analysis of the excited states at the CAM-B3LYP/6-311+G** level of theory provides an assignment of the electronic transitions. Photolysis in solution at room temperature yields red [R=OMe (1)], blue [R=Br (3), F (2)], and green [R=CN (4)] pentacenes. The compounds are oxygen-sensitive and have low solubility, but their formation can be monitored by UV/Vis and, in the case of R=CN, also by (1)H NMR spectroscopy. The S(1) transition in 4 does not show the typical pentacene fine structure in the electronic absorption spectrum. Photogeneration in the presence of oxygen leads to a number of photoproducts that could be identified by monitoring the reaction by (1)H NMR spectroscopy for R=OMe.     

Experimental and theoretical studies on organic D-π-A systems containing three-coordinate boron moieties as both π-donor and π-acceptor

Four linear π-conjugated systems with 1,3-diethyl-1,3,2-benzodiazaborolyl [C(6)H(4)(NEt)(2)B] as a π-donor at one end and dimesitylboryl (BMes(2)) as a π-acceptor at the other end were synthesized. These unusual push-pull systems contain phenylene (-1,4-C(6)H(4)-; 1), biphenylene (-4,4'-(1,1'-C(6)H(4))(2)-; 2), thiophene (-2,5-C(4)H(2)S-; 3), and dithiophene (-5,5'-(2,2'-C(4)H(2)S)(2)-; 4) as π-conjugated bridges and different types of three-coordinate boron moieties serving as both π-donor and π-acceptor. Molecular structures of 2, 3, and 4 were determined by single-crystal X-ray diffraction. Photophysical studies on these systems reveal blue-green fluorescence in all compounds. The Stokes shifts for 1, 2, and 3 are notably large at 7820-9760 cm(-1) in THF and 5430-6210 cm(-1) in cyclohexane, whereas the Stokes shift for 4 is significantly smaller at 5510 cm(-1) in THF and 2450 cm(-1) in cyclohexane. Calculations on model systems 1'-4' show the HOMO to be mainly diazaborolyl in character and the LUMO to be dominated by the empty p orbital at the boron atom of the BMes(2) group. However, there are considerable dithiophene bridge contributions to both orbitals in 4'. From the experimental data and MO calculations, the π-electron-donating strength of the 1,3-diethyl-1,3,2-benzodiazaborolyl group was found to lie between that of methoxy and dimethylamino groups. TD-DFT calculations on 1'-4', using B3LYP and CAM-B3LYP functionals, provide insight into the absorption and emission processes. B3LYP predicts that both the absorption and emission processes have strong charge-transfer character. CAM-B3LYP which, unlike B3LYP, contains the physics necessary to describe charge-transfer excitations, predicts only a limited amount of charge transfer upon absorption, but somewhat more upon emission. The excited-state (S(1)) geometries show the borolyl group to be significantly altered compared to the ground-state (S(0)) geometries. This borolyl group reorganization in the excited state is believed to be responsible for the large Stokes shifts in organic systems containing benzodiazaborolyl groups in these and related compounds.     

Temperature dependence of electronic coupling through oligo-p-phenyleneethynylene bridges

A series of donor-bridge-acceptor (D-B-A) systems with varying donor-acceptor distances has been studied with respect to the temperature dependence of the triplet excitation energy transfer (TEET) rates. The donor and acceptor, zinc(II) and free-base porphyrin, respectively, were separated by oligo-p-phenyleneethynylene (OPE) bridges, where the number of phenyleneethynylene groups was varied between two and five, giving rise to edge-to-edge separations ranging between 12.7 and 33.4 A. The study was performed in 2-MTHF between room temperature and 80 K. It was found that the distance dependence was exponential, in line with the McConnell model, and the attenuation factor, beta, was temperature dependent. The experimentally determined temperature dependence of beta was evaluated by using a previously derived model for the conformational dependence of the electronic coupling based on results from extensive quantum chemical, DFT and time-dependent DFT (TD-DFT), calculations. Two regimes in the temperature interval could be identified: one high-temperature, low-viscosity regime, and one low-temperature, high-viscosity regime. In the first regime, the temperature dependence of beta was, according to the model, well described by a Boltzmann conformational distribution. In the latter, the molecular motions that govern the electronic coupling are slowed down to the same order of magnitude as the TEET rates. This, in effect, leads to a distortion of the conformational distribution. In the high-temperature regime the model could reproduce the temperature dependence of beta, and the extracted rotational barrier between two neighboring phenyl units of the bridge structure, E(i)=1.1 kJ mol(-1), was in line with previous experimental and theoretical studies. After inclusion of parameters that take the viscosity of the medium into account, successful modeling of the experimentally observed temperature dependence of the distance dependence was achieved over the whole temperature interval.     

Structures and stabilities of group 13 adducts [(NHC)(EX3)] and [(NHC)2(E2X(n))] (E=B to In; X=H, Cl; n=4, 2, 0; NHC=N-heterocyclic carbene) and the search for hydrogen storage systems: a theoretical study

Quantum chemical calculations using density functional theory at the BP86/TZVPP level and ab initio calculations at the SCS-MP2/TZVPP level have been carried out for the group 13 complexes [(NHC)(EX(3))] and [(NHC)(2)(E(2)X(n))] (E=B to In; X=H, Cl; n=4, 2, 0; NHC=N-heterocyclic carbene). The monodentate Lewis acids EX(3) and the bidentate Lewis acids E(2) X(n) bind N-heterocyclic carbenes rather strongly in donor-acceptor complexes [(NHC)(EX(3))] and [(NHC)(2)(E(2)X(n))]. The equilibrium structures of the bidentate complexes depend on the electronic reference state of E(2)X(n), which may vary for different atoms E and X. All complexes [(NHC)(2)(E(2)X(4))] possess C(s) symmetry in which the NHC ligands bind in a trans conformation to the group 13 atoms E. The complexes [(NHC)(2)(E(2)H(2))] with E=B, Al, Ga have also C(s) symmetry with a trans arrangement of the NHC ligands and a planar CE(H)E(H)C moiety that has a E=E π bond. In contrast, the indium complex [(NHC)(2)(In(2) H(2))] has C(i) symmetry with pyramidal-coordinated In atoms in which the hydrogen atoms are twisted above and below the CInInC plane. The latter C(i) form is calculated for all chloride systems [(NHC)(2)(E(2)Cl(2))], but the boron complex [(NHC)(2)(B(2)Cl(2))] deviates only slightly from C(s) symmetry. The B(2) fragment in the linear coordinated complex [(NHC)(2)(B(2))] has a highly excited (3)(1)Σ(g)(-) reference state, which gives an effective B≡B triple bond with a very short interatomic distance. The heavier homologues [(NHC)(2)(E(2))] (E=Al to In) exhibit a anti-periplanar arrangement of the NHC ligands in which the E(2) fragments have a (1)(1) Δ(g) reference state and an E=E double bond. The calculated energies suggest that the dihydrogen release from the complexes [(NHC)(EH(3))] and [(NHC)(2)(E(2)H(n))] becomes energetically more favourable when atom E becomes heavier. The indium complexes should therefore be the best candidates of the investigated series for hydrogen-storage systems that could potentially deliver dihydrogen at close to ambient temperature. The hydrogenation reaction of the dimeric magnesium(I) compound [LMgMgL] (L=β-diketiminate) with [(NHC)(EH(3))] becomes increasingly exothermic with the trend B相似文献   

The role of the π linker in donor-π-acceptor organic dyes for high-performance sensitized solar cells

The key elements arising from different linkers between donor (D) and acceptor (A) fragments in D-π-A organic dyes are computationally studied. Taking triarylamine and the cyanoacrylic acid fragments as donor and acceptor units, respectively, the role of the different separators is computationally explored by means of optimized geometries, frontier molecular orbitals, static polarizabilities and hyperpolarizabilities, excitation energies to the lowest excited singlet, the charge-transfer character of the transition, and simulated absorption spectra. The results are compared to two closely related sets of linkers. Electronic-structure calculations on the studied organic dyes are performed with the CIS(D) wave function based method and time-dependent density functional theory (ωB97, ωB97X, and ωPBEh functionals). Solvation effects are introduced with the polarizable continuum model (PCM).     

Lewis acid stabilized OPI3: implications for the nature of free OPI3

While reinvestigating the published synthesis of OPI(3), it became evident from the experiments that phosphoryl triodide may only be formed as an intermediate and that the end products of the reaction of OPCl(3) with LiI are P(V) oxides, PI(3), I(2), and LiCl. This is also in agreement with MP2/TZVPP calculations, which assign Delta(r)H degrees (Delta(r)G degrees ) [Delta(r)G degrees in CHCl(3)] for the disproportionation of OPI(3) as -7 (-18) [-17 kJ mol(-1)] (assuming P(4)O(10) as the P(V) oxide). The first products of this reaction visible in a low-temperature in situ (31)P NMR experiment are P(2)I(4) and PI(3), as well as traces of a compound that may be OPCl(2)I. By contrast, it was possible to prepare and structurally characterize Lewis acid [A] stabilized [A]<--OPX(3) adducts, where [A] is Al(OR(F))(3) for X=Br and Al(OR(F))(2)(mu-F)Al(OR(F))(3) for X=I (R(F)=C(CF(3))(3)). These adducts are formed on decomposition of PX(4) (+)[Al(OR(F))(4)](-); high yields of Br(3)PO-->Al(OR(F))(3) (delta((31)P)=-65) were obtained, while I(3)PO-->Al(OR(F))(3) (delta((31)P)=-337) and I(3)PO-->Al(OR(F))(2)(mu-F)Al(OR(F))(3) (delta((31)P)=-332) are only formed as by-products. The main product of the room-temperature decomposition of PI(4) (+)[Al(OR(F))(4)](-) is PI(4) (+)[(R(F)O)(3)Al(mu-F)Al(OR(F))(3)](-), which was also characterized by X-ray crystallography and was independently prepared from Ag(+)[(R(F)O)(3)Al(mu-F)Al(OR(F))(3)](-), PI(3), and I(2).     

tBuP(NH2)2--a reactive synthon for the synthesis of molecular imidophosphinates of group 13 metals

Reactions of tBuP(NH(2))(2) with Group 13 trialkyls MR(3) (M=Al, Ga, In; R=Me, tBu) were investigated in detail. According to variable-temperature (VT) NMR investigations, the reaction proceeds stepwise with the initial formation of aminophosphane adducts, which subsequently react to give iminophosphorane adducts and finally the heterocyclic metallonitridophosphinates. BP86/TZVPP (DFT) calculations were performed to verify this reaction pathway, to elucidate the influence of the central Group 13 element on the stability of the reaction intermediates and the heterocycles, as well as to assess the thermodynamics of their formation. The relative stability of free and complexed aminophosphane RP(NH(2))(2) and iminophosphorane R(H(2)N)(H)P=NH (adducts) with P(III) and P(V) centers was studied in more detail with DFT and MP2 methods. In addition, the influence of the substituent R was investigated by variation of R from H to Me, tBu, F, and NH(2). In general, the aminophosphane form was found to be favored for the free ligand, however, upon complexation with MR(3) (M=Al, Ga; R=alkyl) both forms are almost equal in energy.     

Paradoxical solvent effects on the absorption and emission spectra of amino-substituted perylene monoimides.

In N-(2,5-di-tert-butylphenyl)-9-pyrrolidinoperylene-3,4-dicarboximide (5PI) the absorption and emission spectra display large solvatochromic shifts, but, remarkably, the Stokes shift is practically independent of solvent polarity. This unique behavior is caused by the extraordinarily large ground-state dipole moment of 5PI, which further increases upon increasing the solvent polarity, whereas the excited-state dipole moment is less solvent dependent. In the corresponding piperidine compound, 6PI, this effect is much less important owing to the weaker coupling between the amino group and the aromatic imide moiety, and in the corresponding naphthalimide, 5NI, it is absent. The latter shows the conventional solvatochromic behavior of a push-pull substituted conjugated system, that is, minor shifts in absorption and a larger change in the emission energy with solvent polarity.     

Donor-substituted 1,1,4,4-tetracyanobutadienes (TCBDS): new chromophores with efficient intramolecular charge-transfer interactions by atom-economic synthesis

A wide variety of monomeric and oligomeric, donor-substituted 1,1,4,4-tetracyanobutadienes (TCBDs) have been synthesized by [2+2] cycloaddition between tetracyanoethylene (TNCE) and donor-substituted alkynes, followed by electrocyclic ring opening of the initially formed cyclobutenes. Reaction yields are often nearly quantitative but can be affected by the electron-donating power and steric demands of the alkyne substituents. The intramolecular charge-transfer (CT) interactions between the donor and TCBD acceptor moieties were comprehensively investigated by X-ray crystallography, electrochemistry, UV-visible spectroscopy, and theoretical calculations. Despite the nonplanarity of the new chromophores, which have a substantial twist between the two dicyanovinyl planes, efficient intramolecular CT interactions are observed, and the crystal structures demonstrate a high quinoid character in strong donor substituents, such as N,N-dimethylanilino (DMA) rings. The maxima of the CT bands shift bathochromically upon reduction of the amount of conjugative coupling between strong donor and acceptor moieties. Each TCBD moiety undergoes two reversible, one-electron reduction steps. Thus, a tri-TCBD derivative with a 1,3,5-trisubstituted benzene core shows six reversible reduction steps within an exceptionally narrow potential range of 1.0 V. The first reduction potential E(red,1) is strongly influenced by the donor substitution: introduction of more donor moieties causes an increasingly twisted TCBD structure, a fact that results in the elevation of the LUMO level and, consequently, a more difficult first reduction. The potentials are also strongly influenced by the nature of the donor residues and the extent of donor-acceptor coupling. A careful comparison of electrochemical data and the correlation with UV-visible spectra made it possible to estimate unknown physical parameters such as the E(red,1) of unsubstituted TCBD (-0.31 V vs Fc+/Fc) as well as the maxima of highly broadened CT bands. Donor-substituted TCBDs are stable molecules and can be sublimed without decomposition. With their high third-order optical nonlinearities, as revealed in preliminary measurements, they should become interesting chromophores for ultra-thin film formation by vapor deposition techniques and have applications in opto-electronic devices.