Intense Charge Transfer Band Research Articles

Structures and manifestation of ortho-, meta-, and para-NH2-substitution in the optical spectra of europium and terbium aminobenzoates

Four pairs of isostructural europium and terbium ortho-, meta, and para-aminobenzoates ([Ln(2-ABenz)3(H2O)], [Ln(3-ABenz)3(H2O)3], [Ln(3-ABenz)3(H2O)3]·3H2O, [Ln(4-ABenz)3(H2O)], (Ln = Eu, Tb; ABenz – aminobenzoate anion)) were investigated using X-ray diffraction and optical spectroscopy (luminescence and luminescence excitation spectra, as well as vibrational IR and Raman spectra). The crystal structures of [Tb(2-ABenz)3(H2O)], [Eu(3-ABenz)3(H2O)3], [Eu(3-ABenz)3(H2O)3]·3H2O, and [Eu(4-ABenz)3(H2O)] were determined by single crystal X-ray analysis. The [Eu(3-ABenz)3(H2O)3] structure constitutes a new type of lanthanide m-aminobenzoate in the R3 space group. The Ln3+ coordination polyhedron formed by three bidentate chelate carboxylic groups and three terminal water molecules in both structures of m-aminobenzoate can be described as a distorted three-capped trigonal prism. The influence of the incorporation of three solvate outer-sphere H2O molecules in the crystal lattice of m-aminobenzoate on the Eu3+ luminescence center was analyzed. Restructuring of the LnO9 coordination polyhedron results in a decrease in the distortions of the crystal field that appears as a loss of extra splitting of the Ln3+ electronic levels in [Ln(3-ABenz)3(H2O)3]·3H2O. The effect of the electron-donating NH2 group located in different positions on the benzene ring on process of the excitation energy transfer to Ln3+ ion is examined. The hypsochromic shift of an intense interligand charge transfer (ILCT) band in the Tb3+ excitation spectra of the sequence of o-, m-, and p-NH2-substituted compounds is observed. This shift is the consequence of the different electron density distribution depending on the NH2 position in the ligands. The predominant role of low-energy ligand–metal charge transfer (LMCT) states in the quenching of the luminescence of the europium o- and p-aminobenzoates in contrast with the europium m-aminobenzoates is discussed. Compounds including a supplementary Cl substituent on the benzene ring [Ln(2-A-5-Cl-Benz)3(H2O)n] (Ln = Eu, Tb) and [Eu(4-A-2-Cl-Benz)3(H2O)n] are also under consideration.

Ferrocenyl substituted triphenylamine based donor–acceptor molecular systems; synthesis, photophysical, electrochemical, and theoretical studies

A series of push–pull molecular systems 5–14 were designed, and synthesized by the Sonogashira cross-coupling, Knoevenagel condensation, and cycloaddition reactions. The donor ‘ferrocenyl substituted triphenylamine’ was kept constant, whereas the acceptors were varied (malononitrile, indanone, and tetralone). The electronic absorption spectra of the ferrocenyl substituted triphenylamine based donor–acceptor (D–A) compounds 5–9 exhibit intense intramolecular charge transfer (ICT) band in the visible region (415–502 nm). The incorporation of the tetracyanoethylene (TCNE) group in compounds 5–9 results in new D–A system 10–14, which exhibits the ICT band in the region 630–700 nm. The electrochemical studies suggest considerable donor–acceptor interaction. The computational studies reveal strong D–A interaction, and show good agreement with the experimental results.

Effect of the Zeolitic Matrix on the Reduction Process of Cu<sup>2+</sup> Cations in Clinoptilolite, Mordenite and Erionite

Three different zeolites (erionite, mordenite and natural clinoptilolite) were used to study influence of zeolite topology on the state of copper during ion-exchange and following reduction in hydrogen flow. This comparative study clearly demonstrates the influence of used zeolite matrices on the process of implantation of copper nanospecies. Starting from the ion-exchange, the alterations in the state of Cu2+ ions start to be evident due to variations of the intensity of charge transfer band. Copper ions start to reduce at specific temperatures depending on the type of zeolite matrix. Copper plasma resonance band change its shape and position for different zeolites. In the case of Cu-CLI samples this band change both the shape and position for different temperature of reduction. These observations permit to suggest that the mechanism of copper ion reduction and agglomeration to form copper nanoparticles noticeably depend on the type of zeolite matrix. This mechanism is more complex for the Cu CLI than for the Cu-MOR and Cu-ERI systems. Copper nanoparticles formed at low temperatures in the case of Cu-CLI samples undergo changes while temperature of reduction grow.

Preparation of Cu–mordenite by ionic exchange reaction under milling: A favorable route to form the mono-(μ-oxo) dicopper active species

Cu-exchanged mordenite samples were prepared using both solid state and aqueous solution ionic reactions. The obtained materials were then characterised from UV–vis, XPS, and TPR data in order to obtain information on the state of copper in the exchanged samples. Three types of copper species were identified, surface clusters of Cu2O, hydrated Cu(II) ions with two slightly different coordination environments, and mono (μ-oxo) dicopper core, [Cu2O]2+, located in β type channels for samples activated on heating in the presence of oxygen. The formation of this last species, active for the methane conversion into methanol, was found to be particularly favored for the solid state ionic exchange reaction, and it is detected as an intense charge transfer band at 400nm in the recorded UV–vis spectra and a well-defined peak at 936.34eV of binding energy in the XPS spectra. The stabilisation of this dicopper core is the main distinctive difference between the exchange reactions in the solid state and in solution. The appearance of this bridged binuclear Cu2+ species was also detected in the recorded TPR profiles. According to the recorded CO2 adsorption isotherms, the ionic exchange process modifies the accessible pore volume but the material preserves its porous features. The ability of that core for the low temperature methane conversion into methanol was confirmed recording chromatographic profile at different temperatures.

Structural Investigation and Photoluminescent Properties of Gadolinium(III), Europium(III) and Terbium(III) 3-Mercaptopropionate Complexes

This work reports on the synthesis, crystallographic determination and spectroscopic characterization of gadolinium(III), terbium(III) and europium(III) 3-mercaptopropionate complexes, aqua-tris(3-mercaptopropionate)lanthanide(III)--[Ln(mpa)3(H2O)]. The Judd-Ofelt intensity parameters were experimentally determined from emission spectrum of the [Eu(mpa)3(H2O)]complex and they were also calculated from crystallographic data. The complexes are coordination polymers, where the units of each complex are linked together by carboxylate groups leading to an unidimensional and parallel chains that by chemical interactions form a tridimensional framework. The emission spectrum profile of the [Eu(mpa)3(H2O)] complex is discussed based on point symmetry of the europium(III) ion, that explains the bands splitting observed in its emission spectrum. Photoluminescent analysis of the [Gd(mpa)3(H2O)] complex show no efficient ligand excitation but an intense charge transfer band. The excitation spectra of the [Eu(mpa)3(H2O)] and [Tb(mpa)3(H2O)] complexes do not show evidence of energy transfer from the ligand to the excited levels of these trivalent ions. Therefore the emission bands are originated only by direct f-f intraconfigurational excitation of the lantanide(III) ions.

Synthesis, Characterization, and Electron-Transfer Processes in Indium Ferrocenyl-Containing Porphyrins and Their Fullerene Adducts

Three new indium(III) tetra- and penta(ferrocenyl)-substituted porphyrins of the general formula XInTFcP [X = Cl(-), OH(-), or Fc(-); TFcP = 5,10,15,20-tetraferrocenylporphyrin(2-); Fc = ferrocene] have been prepared and characterized by UV-vis, magnetic circular dichroism (MCD), (1)H, (13)C, 2D, and variable-temperature NMR spectroscopy, as well as elemental analysis. Molecular structures of the ClInTFcP, FcInTFcP, and FcInTFcP@4C60 complexes were determined by X-ray crystallography with the last compound being not only the first example of a C60 adduct to the organometallic porphyrins but also the first structure in which organometallic porphyrin antennas intercalated into four electron-transfer channels. The electronic structures and relative energies of individual atropisomers, as well as prospective electron-transfer properties of fullerene adducts of XInTFcP complexes, were investigated by the Density Functional Theory (DFT) approach. Redox properties of XInTFcP complexes were investigated using electrochemical (CV and DPV), spectroelectrochemical, and chemical oxidation approaches. Electrochemical experiments conducted in low-polarity solvent using noncoordinating electrolyte were crucial for the sequential oxidation of ferrocene substituents in XInTFcP compounds. In agreement with DFT calculations, the axial ferrocene ligand in FcInTFcP, with direct In-C σ-bond has a 240 mV lower oxidation potential compared to the first oxidation potential for equatorial ferrocene substituents connected to the porphyrin core. The first equatorial ferrocene oxidation process in all XInTFcP complexes is separated by at least 150 mV from the next three ferrocene based oxidations. The second, third, and fourth redox processes in the ferrocene region are more closely spaced. The addition of the bulky axial ferrocene ligand results in significantly larger rotational barriers for equatorial ferrocene substituents in FcInTFcP compared to the other complexes and leads to better defined redox waves in cyclic voltammetry (CV) and differential pulse voltammetry (DPV) experiments. Mixed-valence compounds of the general formula [XInTFcP](n+) (n = 1, 2) were observed and characterized by spectroelectrochemical and chemical oxidation approaches. In all cases, the presence of the intense intervalence charge transfer (IVCT) bands associated with the oxidation of a single equatorial ferrocene substituent were detected in the NIR region confirming the presence of the iron-based mixed-valence species and suggesting long-range metal-metal coupling in the target systems. The resulting data from the mixed-valence [XInTFcP](n+) (n = 1, 2) complexes matched very closely to the previously reported MTFcP and metal-free poly(ferrocenyl)porphyrins and were assigned as Robin and Day Class II mixed-valence compounds.

Class III Delocalization and Exciton Coupling in a Bimetallic Bis‐ligand Radical Complex

The geometric and electronic structure of an oxidized bimetallic Ni complex incorporating two redox-active Schiff-base ligands connected via a 1,2-phenylene linker has been investigated and compared to a monomeric analogue. Information from UV/Vis/NIR spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, and density functional theory (DFT) calculations provides important information on the locus of oxidation for the bimetallic complex. The neutral bimetallic complex is conformationally dynamic at room temperature, which complicates characterization of the oxidized forms. Comparison to an oxidized monomer analogue 1 provides critical insight into the electronic structure of the oxidized bimetallic complex 2. Oxidation of 1 provides [1˙](+), which is characterized as a fully delocalized ligand radical complex; the spectroscopic signature of this derivative includes an intense NIR band at 4500 cm(-1). Oxidation of 2 to the bis-oxidized form affords a bis-ligand radical species [2˙˙](2+). Variable temperature EPR spectroscopy of [2˙˙](2+) shows no evidence of coupling, and the triplet and broken symmetry solutions afforded by theoretical calculations are essentially isoenergetic. [2˙˙](2+) is thus best described as incorporating two non-interacting ligand radicals. Interestingly, the intense NIR intervalence charge transfer band observed for the delocalized ligand-radical [1˙](+) exhibits exciton splitting in [2˙˙](2+), due to coupling of the monomer transition dipoles in the enforced oblique dimer geometry. Evaluating the splitting of the intense intervalence charge transfer band can thus provide significant geometric and electronic information in less rigid bis-ligand radical systems. Addition of excess pyridine to [2˙˙](2+) results in a shift in the oxidation locus from a bis-ligand radical species to the Ni(III) /Ni(III) derivative [2(py)4](2+), demonstrating that the ligand system can incorporate significant bulk in the axial positions.

X-shaped organic dyes with a quinoxaline bridge for use in dye-sensitized solar cells

Two novel X-shaped organic sensitizers (FNE48 and FNE49) with a quinoxaline bridge have been designed and synthesized. The structural difference between the two sensitizers is the number of anchoring groups. The relationship between the chemical structures and their optoelectronic properties has been systematically investigated. It is found that a broader and more intense charge transfer band is observed for sensitizer FNE48, with a single anchoring group, in comparison to sensitizer FNE49, which contains twin anchoring groups. Consequently, a high power conversion efficiency (η) of 6.2% is achieved for a dye-sensitized solar cell (DSSC) based on sensitizer FNE48 using a liquid electrolyte. Moreover, a η of 5.2% is obtained for a quasi-solid-state DSSC based on sensitizer FNE48 and this remains at 100% of the initial value after continuous light irradiation for 1500 h.

Polymeric ion sensors with multiple detection modes achieved by a new type of click chemistry reaction

The rapid growth of the click chemistry concept enables the production of a wide variety of functional polymers. Among the new generation of click chemistry reactions, the highly efficient addition reactions between electron-rich alkynes and cyano-based acceptors, referred to as alkyne-acceptor click chemistry, have found promising application possibilities as polymeric chemosensors. The donor-acceptor chromophores, formed by this click chemistry reaction, feature intense charge-transfer (CT) bands in the visible region, but they are hardly fluorescent. Importantly, the chromophores possess two different nitrogen atoms, namely the aniline nitrogen and cyano nitrogen. The recognition of some specific metal cations by different nitrogen atoms in the polymers led to different modes of changes in the absorption spectra. For example, the hard acid of Fe(3+) ion was recognized by the aniline nitrogen, resulting in a decrease in the CT bands. On the other hand, the soft acid of the Ag(+) ion was captured by the cyano nitrogen, leading to a bathochromic shift in the CT bands. Some specific anions, such as CN(-), F(-), and I(-) ions, were also recognized by a chemodocimetric detection mode, discoloring the original solutions. When the CT bands decreased upon the addition of analytes, the polymers were found to serve as turn-on fluorescent sensors. In this perspective, the detailed detection modes of the new polymeric chemosensors are fully described.

イオン認識能を有する機能性高分子のクリック合成

The rapid growth of the click chemistry concept produced a wide variety of functional polymers. In this research field, polymeric ion sensors, which can recognize specific metal cations or anions, have mainly been synthesized by the Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC). However, due to several disadvantages suggested for the CuAAC, the development of new types of click chemistry reactions has recently started. Among them, it was found that the high-yielding addition reactions between electron-rich alkynes and cyano-based acceptors offer promising application possibilities as polymeric chemosensors. The donor-acceptor type products feature intense charge-transfer bands in the visible region. The recognition of some specific metal cations or anions resulted in well-defined visible colorimetric changes in the optical spectra.

Synthesis and characterization of redox-active charge-transfer complexes with 2,3,5,6-tetracyanopyridine (TCNPy) for the photogeneration of pyridinium radicals.

The heteroaromatic polynitrile compound tetracyanopyridine (TCNPy) is introduced as a new electron acceptor for the formation of deeply colored charge-transfer complexes. In MeCN, TCNPy is characterized by a quasireversible one-electron-reduction process at −0.51 V (versus SCE). The tetracyanopyridine radical anion undergoes a secondary chemical reaction, which is assigned to a protonation step. TCNPy has been demonstrated to generate 1:1 complexes with various electron donors, including tetrathiafulvalene (TTF) and dihydroxybenzene derivatives, such as p-hydroquinone and catechol. Visible- or NIR-light-induced excitation of the intense charge-transfer bands of these compounds leads to a direct optical electron-transfer process for the formation of the corresponding radical-ion pairs. The presence of available electron donors that contain protic groups in close proximity to the TCNPy acceptor site opens up a new strategy for the photocontrolled generation of pyridinium radicals in a stepwise proton-coupled electron-transfer (PCET) sequence.

Synthesis and photoluminescence characteristics of Dy3+ doped NaY(WO4)2 phosphors

The novel phosphor powders of NaY(WO4)2 doped with Dy3+ were synthesized by solid state reaction. X-ray diffraction analysis showed that the phosphors sintered at 900°C for 6h were a pure NaY(WO4)2 phase for all the Dy3+ doping concentrations. The room temperature excitation spectra of the phosphors vary with the Dy3+ concentration and consist of an intense charge transfer band of WO42− groups and weak f–f transition absorption peaks of Dy3+. The photoluminescence spectra, excited at the peak wavelength of charge transfer band, exhibit three bands centered at 488, 575 and 662nm, which originate from the transitions of 4F9/2→6H15/2 (blue), 4F9/2→6H13/2 (yellow) and 4F9/2→6H11/2 (red) of Dy3+, respectively. The effects of Dy3+ concentration on luminescence intensity and the yellow-to-blue intensity ratio of the phosphors were investigated. The different behaviors for the doping concentration dependence of luminescence intensity resulted from the different kinds of excitation was discussed.

Spectroscopy properties of Eu3+ doped Ca9R(VO4)7 (R=Bi, La, Gd and Y) phosphors by sol–gel method

An attempt has been made to synthesize the Eu3+-activated Ca9R(VO4)7 (R=Bi, La, Gd, and Y) red emitting phosphors by a modified sol–gel process. The phase formation processes of these samples have been characterized by thermogravimetric analysis (TGA) and x-ray diffraction (XRD). The optical excitation spectra of Ca9R(VO4)7 samples showed an intense charge transfer band (CTB) along with a group of typical sharp lines from Eu3+, and the emission spectra have been dominated by the electric dipole 5D0–7F2 transition peaked at 614nm. The parameters for the synthesis of these phosphors including their annealing temperature and concentration of Eu3+ ions have also been optimized. No concentration quenching is found in this class of samples, and the Ca9Eu(VO4)7 phosphor shows the strong red light. In addition, the thermal stability of phosphor Ca9Eu(VO4)7 was also investigated, and the results indicate that this class of phosphors can be served as a potential red emitting phosphor candidate for LEDs.

Synthesis and photoluminescence of Tb3+ Activated NaY(WO4)2 phosphors

The novel yellowish green phosphor powders of NaY(WO4)2 doped with Tb3+ were prepared by solid-state reaction. The powder samples were characterized by X-ray diffraction and photoluminescence. X-ray diffraction analysis showed that the phosphors sintered at 900°C for 6h were a pure NaY(WO4)2 phase for all the Tb3+ doping concentrations. The room temperature excitation spectra vary with the Tb3+ concentration and consist of an intense charge transfer band of WO42− group and weak intra-4f8 transition absorption peaks of Tb3+ ions. The photoluminescence spectra, excited at the peak wavelengths of charge transfer bands, consist of the characteristic Tb3+ emission transitions from 5D3 and 5D4 excited levels to 7FJ (J=3–6) levels. The dependence of luminescence intensity on the Tb3+ concentration in NaY(WO4)2:Tb phosphors was investigated.

Study of concentration dependence of luminescent properties for Eu3+ doped CaWO4 red phosphors

Eu3+ doped CaWO4 phosphors of different concentrations are prepared by a co-precipitation method. The X-ray diffraction patterns and field emission scanning electron microscopy images are investigated. The excitation spectra, emission spectra and the curves of fluorescence decays of the samples are measured. The Judd-Ofelt (J-O) parameters, quantum efficiencies of the 5D0 level of Eu3+ and color coordinates are calculated. The dependences of relative intensity of charge transfer band, J-O parameter and quantum efficiency on doping concentration are discussed. The photoluminescence properties of Eu3+ doped CaWO4 luminescent material are studied. The results indicate that the 616 nm red emission of Eu3+5D07F2 transition can be effectively excited by 394.5 nm and 465 nm with high excitation efficiency and the concentration quenching is high. So the CaWO4: Eu red phosphor may have a potential application for white light emitting diode.

A High Molar Extinction Coefficient Mono-Anthracenyl Bipyridyl Heteroleptic Ruthenium(II) Complex: Synthesis, Photophysical and Electrochemical Properties

In our quest to develop good materials as photosensitizers for photovoltaic dye-sensitized solar cells (DSSCs), cis-dithiocyanato-4-(2,3-dimethylacrylic acid)-2,2'-bipyridyl-4-(9-anthracenyl-(2,3-dimethylacrylic)-2,2'-bipyridyl ruthenium(II) complex, a high molar extinction coefficient charge transfer sensitizer, was designed, synthesized and characterized by spectroscopy and electrochemical techniques. Earlier studies on heteroleptic ruthenium(II) complex analogues containing functionalized oligo-anthracenyl phenanthroline ligands have been reported and documented. Based on a general linear correlation between increase in the length of π-conjugation bond and the molar extinction coefficients, herein, we report the photophysical and electrochemical properties of a Ru(II) bipyridyl complex analogue with a single functionalized anthracenyl unit. Interestingly, the complex shows better broad and intense metal-to ligand charge transfer (MLCT) band absorption with higher molar extinction coefficient (λmax = 518 nm, ε = 44900 M−1cm−1), and appreciable photoluminescence spanning the visible region than those containing higher anthracenyl units. It was shown that molar absorption coefficient of the complexes may not be solely depended on the extended p-conjugation but are reduced by molecular aggregation in the molecules.

Transient absorption spectroscopy of a heteroaromatic donor–acceptor-π-conjugated 2,2′-bipyridine dye

We report on a study of the excited state dynamics of a heteroaromatic 4,4′-π-conjugated 2,2′-bipyridine dye, bearing conjugated π-excessive and π-deficient heteroaromatic rings as donor and acceptor substituents, by means of steady state and transient absorption spectroscopy in EtOH solutions and a room temperature. The ground state absorption shows an intense intramolecular charge transfer band in the visible region with a maximum at 515 nm. Excited state dynamics is measured by pumping at 480 nm at shorter wavelengths from the absorption maximum and probing at specific wavelengths corresponding to bleaching, stimulated emission and at the maximum of excited state absorption. The spectral evolution is also evaluated in the frequency domain as a function of the delay between pump and probe pulses. By means of SVD analysis two components with time constants around 2 ps and 150 ps are identified. In addition a relevant dynamical Stokes shift of the stimulated emission occurs in the first 60 ps which is attributed to solvent dynamics. Ab initio calculations allows to assign the ground spectral features and the excited state conformation. The geometry of the ground and of the first excited state are optimized both in the cisoid and in the transoid forms. The latter results more stable of about 0.4 eV.

The electronic spectra and structure of palladium(II) molecular complexes and clusters

The electronic absorption spectra of palladium(II) diacetate (PDA) complexes with phosphines and sulfides (D) with the composition Pd(OAc)2 · 2D (1: 2) contain an intense charge transfer band at λmax ∼ 300 nm (ɛ ∼ 15 000) and do not absorb in the region of 400 nm. Polynuclear compounds such as PDA trimer [Pd(OAc)2]3, trimer complexes with D, and four- and six-membered palladium metallocyclic compounds formed in the interaction of PDA with mercaptans absorb at longer wavelengths. The electronic absorption spectra of all the palladium polynuclear compounds (clusters) contain bands at λmax ∼ 400 nm (ɛ ∼ 1000). The appearance of these bands in the spectra of palladium clusters is evidence of the formation of chemical bonds between neighboring Pd atoms, although Pd…Pd distances substantially exceed the sum of the covalent radii of palladium atoms.

Synthesis and characterization of rhenium(I) complexes with the polypyridinic quinone functionalized electron acceptor ligand [3,2-a:2′,3′-c]-benzo[3,4]-phenazine-11,16-quinone, Nqphen

In this work, the synthesis and characterization of fac-[Re(CO)3(Nqphen)(L)]PF6 complexes is reported. Nqphen is the quinone substituted acceptor ligand [3,2-a:2′,3′-c]-benzo[3,4]-phenazine-11,16-quinone, and L represents the donor monodentate pyridine substituted ligands 4-tert-butylpyridine (t-Bupy), 4-methoxypyridine (MeO-py) or 10-(4-picolyl)phenothiazine (py-PTZ). The complexes were synthesized by refluxing in methanol the metal precursor fac-Re(CO)3(Nqphen)TfO (TfO=trifluoromethanesulphonate anion) with the corresponding L ligand. The UV–Vis spectra of the complexes are dominated by intense intraligand (IL) bands, and less intense metal ligand charge transfer (MLCT) bands with maxima in the 380–400nm region. The IR shows the typical pattern for tricarbonyl Re complexes with facial (fac) geometry. An additional v(CO) stretching band, attributed to the quinone fragment of Nqphen, is observed.Electrochemical data indicate that the acceptor capacity of Nqphen is increased in the complexes with regard to the free ligand. This effect is sensitive to the nature of the L ligand, following the order: MeO-py<t-Bupy<py-PTZ, indicating therefore that the donor capacity of L affects the rest of the molecule. The results obtained for the fac-[Re (CO)3(Nqphen)(pyPTZ)]PF6 complex here reported were compared with those observed for the homologous complex fac-[Re(CO)3(Aqphen)(L)]0/+, with Aqphen=12,17-dihydronaphtho[2,3-h]dipyrido[3,2-a:2′,3′-c]-phenazine-12,17-dione, and L=Cl−, TfO−, py-PTZ.

Photoinduced Electron Transfer in a Prototypical Mulliken Donor−Acceptor Complex: C2H4···Br2

We report a new dual nozzle late mixing scheme for the trapping and interrogation of prereactive donor−acceptor complexes that is used initially to examine photoinduced electron transfer in the prototypical Mulliken donor−acceptor (halogen-bonded) π-complex, C2H4···Br2. Excitation into the intense charge-transfer band of the complex leads exclusively to the anti conformer of the single reaction product, 1,2-dibromoethane, in agreement with the Mulliken theory of electron transfer.