Alternating Donor Acceptor Research Articles

2,5-di(thiophen-2-yl)thiazolo[5,4-d]thiazole-based donor–acceptor type copolymers for photovoltaic cells

Alternating donor–acceptor type copolymers, poly[{5,11-di(9′-heptadecanyl)indolo[3,2-b]carbazole}-alt-{2,5-di(thiophen-2-yl)thiazolo[5,4-d]thiazole-5,5′-diyl}] ( PIC-TZ) and poly[{6,6′,12,12′-tetraoctylindeno[1,2-b]fluorene}-alt-{2,5-di(thiophen-2-yl)thiazolo[5,4-d]thiazole-5,5′-diyl}] ( PIF-TZ), were synthesized and examined for applications in polymeric photovoltaic cells. The polymers have a fused coplanar main backbone with good planarity for intermolecular packing and high charge mobility. The indolocarbazole and indenofluorene units contain two or four binding sites for alkyl substituents that have pronounced solution processiblity compared to the carbazole and fluorene moieties. The number-average molecular weights ( M n) of the synthesized polymers were determined to be 11,000 g/mol (PDI = 2.27) for PIC-TZ, and 17,000 g/mol (PDI = 1.77) for PIF-TZ. The optical band gap of PIC-TZ and PIF-TZ in film was determined to be 2.14 eV and 2.21 eV, respectively, and an electrochemical study confirmed the desirable HOMO/LUMO levels of the copolymers, which enabled efficient electron transfer and a high open circuit voltage ( V OC) when blending them with fullerene derivatives. The space charge limited current mobility measurements showed a hole mobility of 10 −3 cm 2 V −1 s −1 for the copolymers. When the polymers were blended with [6,6]phenyl-C 61-butyric acid methyl ester (PCBM), PIC-TZ showed the best performance with V OC, short-circuit current and power conversion efficiency of 0.86 V, 4.16 mA/cm 2 and 1.64%, respectively, under AM 1.5G illumination conditions (100 mW cm −2).

The role of the double peaked absorption spectrum in the efficiency of solar cells based on donor–acceptor–donor copolymers

An interesting procedure to optimize organic solar cells is to lower the band gap of the polymer layer by introducing alternating donor–acceptor (D–A) units in the backbone. However, many aspects of the fundamental physics involved in this low band gap design and their relation with the solar cell performance are still unknown. In this work we address those aspects by studying electro-optical properties of the poly(9,9′-dioctyl-2,7-fluorenediylvinylene- co-2,5-thiophene) (PFTBT) copolymer, which consists of a fluorene linked by a vinylene to a donor–acceptor–donor segment (benzothiadiazole unit between 2 thiophenes groups). Using density functional theory we have performed a detailed analysis of the geometric structure, ground state electronic structure and excited state vertical transitions of the PFTBT. By comparing the results of this theoretical analysis with the solution and thin film UV–vis absorption spectra we were able to identify the main chemical units involved in the two strong transitions responsible for the PFTBT dual absorption. We have found that the absence of alkyl substitutions on the thiophene units increases the planarity of the polymer backbone leading to a thin film with a higher absorption at low wavelengths and larger hole mobility compared to an analogous copolymer. Improved photovoltaic properties of PFTBT were demonstrated by fabricating bilayer devices using C 60 as acceptor. The power conversion efficiency reached 1% which is comparable to those obtained using optimized alternating copolymers in the bulk heterojunction architecture. Finally we found that the light absorption around the “blue” band has a higher efficiency on photocarrier generation because it involves the transition from a delocalized occupied state to a more localized excited state in the polymer backbone. This lower initial electron–hole spatial overlap facilitates the exciton dissociation which increases the photocurrent yield.

Tuning the electronic coupling in a low-bandgap donor–acceptor copolymer via the placement of side-chains

We present a spectroscopic and theoretical investigation of the effect of the presence and position of hexyl side-chains in the novel low-bandgap alternating donor-acceptor copolymer poly[bis-N,N-(4-octylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine-alt-5,5'-4',7',-di-2-thienyl-2',1',3'-benzothiadiazole] (T8TBT). We use electronic absorption and Raman spectroscopic measurements supported by calculations of chain conformation, electronic transitions, and Raman modes. Using these tools, we find that sterically demanding side-chain configurations induce twisting in the electronic acceptor unit and reduce the electronic interaction with the donor. This leads to a blue-shifted and weakened (partial) charge-transfer absorption band together with a higher photoluminescence efficiency. On the other hand, sterically relaxed side-chain configurations promote coupling between donor and acceptor units and exhibit enhanced absorption at the expense of luminescence efficiency. The possibility of tuning the donor-acceptor character of conjugated polymers by varying the placement of side-chains has very important ramifications for light emitting diode, Laser, display, and photovoltaic device optimization.

Thermal and spectral stability of fluorescent copolymers containing tetraphenylthiophene‐quinoline unit

AbstractFluorescent alternative copolymers containing tetraphenylthiophene‐quinoline (TP‐Qu) moiety were synthesized from Friedländer reaction between o‐aminoketone and acetyl groups. With the wholly aromatic‐quinoline chain structure, the alternative donor–acceptor copolymers of PTPQu and PTPTPQu are rigid materials with high glass transitions (Tg = 245 and 295 °C, respectively) and with high thermal stability (with ≤2% weight loss at 500 °C). The multiple phenyl pendant rings inherited PTPQu and PTPTPQu copolymers with good solubility in common organic solvents, which facilitate the easy fabrications of light‐emitting diode (LED) devices. CV study resulted in the reversible Eox/onset values of 0.78 and 0.75 V for the PTPQu and PTPTPQu copolymers, respectively. With high quantum yields (ϕFs, = 0.59 for PTPQu and 0.74 for PTPTPQu) in the film states, the two copolymers and their devices exhibit stable photoluminescence (PL) and electroluminescence (EL) spectra, respectively, on change of annealing temperatures to at least higher than 250 °C. After annealing at 250 °C (or 300 °C), no change was observed in the respective PL and EL spectra of PTPQu (or PTPTPQu) copolymers. The results indicate that alternative copolymers containing donor–acceptor TP‐Qu unit are promising candidates for LEDs with high efficiency and spectral stability. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Linker effects on optoelectronic properties of alternate donor–acceptor conjugated polymers

In this study, three conjugated polymers (DA1, DA2 and DA3) with different linkers between an electron-rich unit (3,4-didodecylthiophene as electron donor, D) and an electron-deficient unit (benzothiadiazole as electron acceptor, A) were synthesized via Suzuki, Heck and Sonogashira polycondensation, respectively. The D and Agroups are connected in three different ways: without any linker (DA1), by a vinylene linker (DA2), and by an ethynylene linker (DA3). Among these three polymers, DA2 exhibited the lowest bandgap (Eg) at 1.65 eV and the highest planarity evidenced by the smallest dihedral angle (1°) between the donor and acceptor units. The femtosecond up-conversion fluorescence measurement showed that DA1 and DA3 exhibited a rise process for the energetically downhill excitonic energy transfer (EET) in a timescale of sub-ps to several ps, while DA2 exhibited a decay process in such downhill EET. In addition, DA2 showed the shortest time constant of ∼4.6 ps for torsional relaxation with the highest amplitude at ∼43.1% among all the three polymers. This suggested that the vinylene linker has the fastest torsional relaxation from a flexible ground-state structure to a more rigid planar geometry, which may reduce conformational defects and improve exciton migration. This work studied the linker effects between the D and A units on the alternate D–Apolymers and may shed light on the design of new D–Apolymers with improved properties.

Molecular design and density functional theory investigation of novel low‐band‐gap benzobisthiadiazole‐based systems: from monomer to polymer

AbstractThe electronic structure and properties of benzobisthiadiazole‐based alternating donor–acceptor conjugated oligomers and their periodic copolymers of donor and acceptor units with ratios of 1:1 and 2:1 were investigated systematically using the density functional theory method. The donors include thiophene, thieno[3,2‐b]thiophene and pyrrole. The ratio of donor to acceptor units (D:A ratio) plays a very important role in the geometric and electronic properties. The intramolecular charge transfer increases and the bond length alternation decreases with an increase in the D:A ratio for these oligomers and polymers. Moreover, an increase in D:A ratio can greatly reduce the band gap and effective mass of holes and electrons for these alternating donor‐acceptor conjugated copolymers. The unusually large intramolecular charge transfer caused by intramolecular hydrogen bonds reveals that pyrrole is not only a strong electron donor but also a potential hydrogen bond donor. The theoretical results suggest those copolymers possessing a D:A ratio of 2:1 are better candidates for conducting materials compared to those with a D:A ratio of 1:1. The almost zero band gap, large bandwidth and small effective mass of holes and electrons of poly(4,8‐bis(thieno[3,2‐b]thiophene‐2‐yl)benzo[1,2‐c:4,5‐c′]bis[1,2,5]thiadiazole) indicate that it is a very good candidate for an electrically conductive material. Copyright © 2010 Society of Chemical Industry

ChemInform Abstract: Charge-Transfer Absorption in Doped Columnar Liquid Crystals.

The optical properties of doped columnar phases formed either by a 1 : 1 mixture of an electron donor [2,3,6,7,10,11-hexa-(n-pentyloxy)triphenylene] and an electron acceptor (2,4,7-trinitrofluoren-9-one) or by a dimer in which the donor (triphenylene moiety) and the acceptor (trifluorenone moiety) are covalently linked are reported and compared with those of the solutions. In concentrated solutions a charge transfer (CT) absorption is observed for both donor–aceptor pairs corresponding to 2.51 and 2.43 eV, respectively. For the 1 : 1 mixture, CT complex formation in solution is greatly favoured by the presence of molecular aggregates. The polarized absorption spectra obtained with oriented thin films reveal that the charge transfer occurs along the alternate donor–acceptor stacking.

Synthesis and characterization of alternative donor–acceptor arranged poly(arylene ethynylene)s derived from 1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP)

A series of donor–acceptor type poly(arylene ethynylene)s (PAEs) have been synthesized through Sonogashira polycondensation. The polymers consist of an electron donating 9,9-bis(2-ethylhexyl)-9H-fluorene, triphenylamine, 1,4-dialkoxybenzene or 9-(2-ethylhexyl)-9H-carbazole unit and an electron accepting 2,5-bis(2-ethylhexyl)-3,6-diphenylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (EH-DPP) unit, alternately connected through ethynyl bridge. The polymers exhibit weight-average molecular weights ( M w) up to 68, 500 and are soluble in chlorobenzene, dichlorobenzene, THF, chloroform and toluene. A brilliant red solution with absorption maxima between 491 and 500 nm was observed for all the polymers. An intense red fluorescent with photoemission maxima between 551 and 571 nm was observed from polymer solutions. The polymers showed good thermal stability with decomposition temperature more than 260 °C at 5% weight loss. Onset oxidation potentials of the polymers were observed between 1.30 and 1.58 V with HOMO energy levels in the range of −6.10 to −6.38 eV. The OLED devices were fabricated with configuration of ITO/PEDOT:PSS/polymer/LiF/Al for all the polymers and EL maxima between 666 and 684 nm were observed.

Molecular designing of novel ternary copolymers of donor–acceptor polymers using genetic algorithm

An efficient designing route to novel ternary copolymers consisting of polypyrrole (PPy), polythiophene (PTh) and polyfuran (PFu) is developed with the help of genetic algorithm. Using the band structure results obtained from ab initio crystal orbital (CO) calculations, the electronic structures and conduction properties of real ternary copolymers based on donor acceptor type polymers are investigated. The electron rich heterocyclic rings in the backbone chain of the copolymer are joined together by electron withdrawing groups Y, carbonyl group (>C O) and difluoromethylene group (>C CF 2) in an attempt to design the conducting polymer with lowest band gap. A comparative study of various electronic properties is presented. The effects of substitution on the behaviour and properties of the copolymers as well as on the density of states (DOS) are discussed. Band gap decreases as a result of substitution on the polymer backbone chain due to decrease in ionization potential and increase in electron affinity values. This is expected to enhance the intrinsic conductivity of the resulting copolymer. Use of alternate donor acceptor moieties within the repeat units should maximize the extended π conjugation.

Alternative Donor−Acceptor Stacks from Crown Ethers and Naphthalene Diimide Derivatives: Rapid, Selective Formation from Solution and Solid State Grinding

The atypical 1:2 complexation between an electron-rich crown ether host and electron-deficient naphthalene diimide-based guests led to the formation of alternative donor-acceptor (ADA) stacks. The ADA stacks can be expediently obtained in high yield as polycrystalline aggregates from solution. More remarkably, the high degree of organization has also been realized in a simple solid-to-solid mechanical grinding process. The solid-state structures have been verified by solid-state NMR spectroscopy, single crystal, and powder X-ray diffraction analysis. The current findings not only provide convenient ways of obtaining novel donor-acceptor stacks involving a macrocyclic host but also represent an important step in transferring electroactive host-guest systems from solution to the solid state.

Indolo[3,2-b]carbazole-based alternating donor–acceptor copolymers: synthesis, properties and photovoltaic application

Two new donor–acceptor (D–A) type copolymers, poly{5,11-di(1-decylundecyl)indolo[3,2-b]carbazole-3,9-diyl-alt-4,7-dithien-2-yl-2,1,3-benzothiadiazole-5′,5″-diyl} (PIC-DTBT) and poly{5,11-di(1-decylundecyl)indolo[3,2-b]carbazole-3,9-diyl-alt-4,7-di(2,2′-bithien-5-yl)-2,1,3-benzothiadiazole-5′,5″-diyl} (PIC-DT2BT) were synthesized. The polymers have an alternating structure consisting of a large-size fused aromatic ring, indolo[3,2-b]carbazole (IC), as the donor segment and 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT) or 4,7-di(2,2′-bithien-5-yl)-2,1,3-benzothiadiazole (DT2BT) as the acceptor segment. The absorption spectra and the electrochemical properties of PIC-DTBT and PIC-DT2BT indicate that these copolymers have suitable energy levels for photovoltaic application. Polymer photovoltaic devices based on blends of the copolymers and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) have a high open-circuit voltage (>0.9 V) under the illumination of AM 1.5 (100 mW cm−2). The higher power conversion efficiency (PCE) of PIC-DT2BT (2.07%) compared to that of PIC-DTBT (1.47%) suggests that DT2BT could be a promising building block for the efficient D–A type copolymers if the low solubility is overcome by the molecular design of the donor segment.

OVERLAPPING ALTERNATIVE DONOR SPLICE SITES IN THE HUMAN GENOME

Over 50% of donor splice sites in the human genome have a potential alternative donor site at a distance of three to six nucleotides. Conservation of these potential sites is determined by the consensus requirements and by its exonic or intronic location. Several hundred pairs of overlapping sites are confirmed to be alternatively spliced as both sites in a pair are supported by a protein, by a full-length mRNA, or by expressed sequence tags (ESTs) from at least two independent clone libraries. Overlapping sites may clash with consensus requirements. Pairs with a site shift of four nucleotides are the most abundant, despite the frameshift in the protein-coding region that they introduce. The site usage in pairs is usually uneven, and the major site is more frequently conserved in other mammalian genomes. Overlapping alternative donor sites and acceptor sites may have different functional roles: alternative splicing of overlapping acceptor sites leads mainly to microvariations in protein sequences; whereas alternative donor sites often lead to frameshifts and thus either yield major differences in the protein sequence and structure, or generate nonsense-mediated decay-inducing mRNA isoforms likely involved in regulated unproductive splicing pathways.

Synthesis and electroluminescent properties of a phenothiazine‐based polymer for nondoped polymer light‐emitting diodes with a stable orange‐red emission

AbstractA novel phenothiazine‐based polymer was synthesized through the Heck reaction of 3,7‐divinyl‐N‐octyl‐phenothiazine with 4,7‐dibromo‐2‐octylbenzotriazole according to the alternating donor–acceptor strategy. The polymer was characterized with 1H NMR, infrared spectroscopy, gel permeation chromatography, cyclic voltammetry, ultraviolet–visible spectroscopy, and fluorescence spectroscopy. With the polymer used as an active layer, three nondoped polymer light‐emitting diodes (PLEDs) with a double‐layer configuration were fabricated by the spin‐coating approach with different thermal annealing processes. The emission maximum in electroluminescent spectra was stabilized at 616 nm. The maximum luminance reached 2432 cd/m2. The coordinate value of Commission International de l'Eclairage 1931 in the double‐layer PLEDs after the thermal treatment was nearly stabilized at (x, y) =(0.62, 0.38). Additionally, the luminous efficiency of device II reached a balanceable state with an increase in the current. Therefore, the polymer had an orange‐red emission with stable chromaticity coordinates under different driving voltages. Finally, a nondoped device with a stable luminous efficiency and chromaticity was obtained. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4867–4878, 2007

Theoretical Electronic Structure and Properties of Alternating Fluorene–Acceptor Conjugated Copolymers and Their Model Compounds

The theoretical geometries and electronic properties of fluorene (F) based alternating donor–acceptor conjugated copolymers and their model compounds were studied by the density function theory (DFT) at the B3LYP level with 6–31G or 6–31G** basis set. The acceptors investigated in this study include thiazole (TZ), thiadiazole (TD), thienopyrazine (TP), thienothiadiazole (TT), thiadiazolothienopyrazine (TTP), quinoxaline (Q), benzothiadiazole (BT), pyrazinoquinoxaline (PQ), thiadiazoloquinoxaline (TQ), and benzobisthiathiadiazole (BB). The torsional angle, bridge bond length, bond length alternation, and intramolecular charge transfer were simulated and correlated with the electronic properties, i.e., HOMO, LUMO level, and band gap. The geometries of fluorene-based donor–acceptor alternating copolymers and their model compounds are significantly affected by the structure of acceptors, particularly the ring size on the backbone. The electronic properties of the polymers and their model compounds are well correlated with the acceptor strength, coplanarity of the backbone, and intramolecular charge transfer. The theoretical study suggests that the electronic properties of alternating fluorene–acceptor conjugated copolymers could be tuned by the geometries or acceptor strength. Hence, these proposed copolymers could have potential applications as light-emitting diodes (LEDs), transparent conductor, or photovoltaic devices.

Electronic structure and properties of alternating donor–acceptor conjugated copolymers: 3,4-Ethylenedioxythiophene (EDOT) copolymers and model compounds

The electronic structure and properties of 3,4-ethylenedioxythiophene (EDOT) based alternating donor–acceptor conjugated copolymers and their model compounds were studied by the density functional theory (DFT) at the B3LYP level with 6-31G or 6-31G** basis set. The acceptors investigated include thiazole (Z), thiadiazole (D), thienopyrazine (TP), thienothiadiazole (TD), thiadiazolothienopyrazine (TPD), quinoxaline (BP), benzothiadiazole (BD), pyrazinoquinoxaline (BPP), benzobisthiadiazole (BDD), and thiadiazoloquinoxaline (BDP). The torsional angle, intramolecular charge transfer, bridge bond length, and bond length alternation were analyzed and correlated with the electronic properties. It was found that the geometries of the donor–acceptor materials were significantly affected by the ring size and intramolecular charge transfer. The HOMO level, LUMO level, and band gap of the model compounds were well correlated with the acceptor strength. However, the electronic properties of the copolymers did not vary systematically with the acceptor strength due to the change in geometry from model compound to polymer. The aromatic geometry of EDOT–TP model compound is transformed to quinoid in the corresponding copolymer and results in a small band gap ( E g) of 0.97 eV. Large intramolecular charge transfer and the small bond length alternation in the EDOT–BDP copolymer resulted in an E g of 0.7 eV. Hence, these two polymers could have potential applications for transparent conductors or photovoltaic devices. The small effective masses and large HOMO and LUMO bandwidths of PEDOT–TP and PEDOT–BDP make them potential materials for ambipolar thin film transistors. The theoretical results suggest that both the acceptor strength and the stable geometry contribute significantly to the electronic properties of alternating donor–acceptor conjugated copolymers.

Donor–acceptor copolymers for red‐ and near‐infrared‐emitting polymer light‐emitting diodes

AbstractWe report a comparative study of two organic soluble, vinylene‐based, alternating donor–acceptor copolymers with 1,4‐(2,5‐dihexadecyloxyphenylene) as the donor; the acceptor is either a 2,5‐linked pyridine or a 5,8‐linked 2,3‐diphenylpyrido[3,4‐b]pyrazine. The polymers are synthesized via a Heck coupling methodology from a dihalo monomer and a divinyl monomer to yield number‐average molecular weights of 16,000 g/mol for the pyridine polymer (PPyrPV) and 6500 g/mol for the pyridopyrazine polymer (PPyrPyrPV), with high solubility in common chlorinated solvents and lower solubility in less polar solvents (e.g., tetrahydrofuran). Thin‐film measurements show band gaps of 2.2 and 1.8 eV for PPyrPV and PPyrPyrPV, respectively. Both polymers exhibit photoluminescence in solution and in the solid state and exhibit electroluminescence when incorporated into light‐emitting diodes. In this case, a broad red emission centered at 690 nm for PPyrPV and a near‐infrared emission centered at 800 nm for PPyrPyrPV have been observed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1417–1431, 2005

Developments in the chemistry and band gap engineering of donor–acceptor substituted conjugated polymers

Abstract This paper reviews the tools to manipulate and minimize the band gap of conjugated (co)polymers. The effects of minimization of the bond length alternation and of the incorporation of donor-π-acceptor units are discussed in particular. A systematic study of a series of alternating donor–acceptor oligomers has revealed the achievable limits and allowed for a prognosis of the performance of corresponding copolymers. The decisive influence of the LUMO eigenvectors of the acceptors on the band gap is highlighted. A new synthesis of benzo[1,2-c:4,5-c]bis[1,2,5]thiadiazole containing donor–acceptor monomers and their acid-catalyzed polymerization are described. Finally, band gap tuning by appropriate substitution, conformational adjustment and mesoscopic ordering is discussed.

Crystal structural properties of a pinwheel compound: [3 6](1,2,3,4,5,6)cyclophane

In the crystal structures of [3 6](1,2,3,4,5,6)cyclophane 1 and the 1:TCNQ (1:1) complex, the cyclophane moiety is observed as the D 6 h structure because of the disorder of the molecules with C 6 h symmetry. In contrast, the C 6 h structure is observed in the crystals of the 1:TCNQ-F 4 (1:1) complex, and the complex shows an alternating donor–acceptor stacking with partial donor–acceptor overlap.

S · · · S Contact-assembled tetrathiafulvalene derivatives of copper(I) and silver(I) co-ordination polymers and organic radical cation salt

The reactions of 2-[4,5-bis(methylsulfanyl)-1,3-dithiol-2-ylidene]-4,5-bis(2-cyanoethylsulfanyl)-1,3-dithiole (CM-TTF) with copper(II) and silver(I) salts resulted in the isolation and structural characterization of S· · ·S contact assembled co-ordination polymers, [Cu(CM-TTF)][ClO4]·thf 1 and [Ag(CM-TTF)(CF3SO3)] 2, and radical cation salt, [CM-TTF][CuCl2] 3. Both 1 and 2 are 1∶1 metal–ligand complexes in which the metal ions and CM-TTF molecules are alternately linked forming two-dimensional networks and infinite linear chain structures, respectively, which are further reinforced by close intermolecular S· · ·S interactions. The structure of 3 consists of segregated stacks of CM-TTF˙+ donors and CuCl2– acceptors associated via short S· · ·Cl contacts leading to an alternate donor–acceptor–donor columnar arrangement. The complexes were additionally characterized by IR, UV/VIS and ESR spectra as well as electrical resistivity measurements. All the complexes and their corresponding air-oxidized species are poor electrical conductors, but they show typical charge-transfer ESR signals and electronic absorptions. Complex 3 was found to exhibit transition from antiferro- to ferro-magnetic coupling upon air oxidation.

Metal-organic ion pairs with dithiooxalate d 8 metal donors and viologens. Spectroscopic, structural and electronic characterization

Ion-pair charge-transfer complexes of the type {A 2+[M(dto) 2] 2−}, where A 2+ represents bipyridinium and phenanthrolinium compounds of different redox potential, M=Ni, Pd, Pt, and dto=1,2-dithiooxalate, were synthesized and characterized with respect to their spectroscopic, structural and electronic properties. A weak absorption band at the red-tail of the low-energy two-band system of the complex donor component can be attributed to the ion-pair charge-transfer (IPCT) transition type. As expected from electrochemical data, the IPCT band of these systems is markedly hypsochromically shifted compared with ion pairs containing dithiolene donors. The different nature of the highest occupied MO may offer an explanation for the distinct electro- and photochemical properties. Applying the Hush model reorganization energies are estimated in the range 107–125 kJ mol −1 for the thermal electron transfer from [Ni(dto) 2] 2− to the acceptor site. The amount of charge that is transferred from the donor to the acceptor ( α 2) is calculated as 10 −5–10 −6, lowered by 2–3 orders of magnitude compared with the metal dithiolenes. As shown by X-ray analysis of HEM[Ni(dto) 2] (HEM= N-2-hydroxyethyl- N′-methyl-4,4′-bipyridinium cation), the molecular structure consists of alternating donor–acceptor stacks with a typical plane-to-plane distance of 345 pm. In addition to supramolecular interactions both components are bonded by a hydrogen bridge between the OH group of the cation and the electron-rich oxygen of the dto ligand.