Charge-transfer excitons in anthracene crystals and their role in optical charge carrier generation

The energy of an electron-hole pair in anthracene is calculated for a complete range of separations and the resulting change-transfer exciton potentials are used to analyze optical charge carrier generation as a function of wavelength and temperature.     

Charge-transfer interactions in cyclophanes

Charge-transfer interactions in cyclophane systems are reviewed. The majority of the work covered involves intermolecular complexation, with both donor and acceptor moieties existing within the same molecule. Studies have also been performed on intermolecular complexes, mainly tetracyanoethylene:cyclophane complexes. Host-guest complexes involving charge-transfer are also discussed. Other areas covered include solvent effects, substituent effects, and theoretical calculations.     

Charge-transfer and spin dynamics in DNA hairpin conjugates with perylenediimide as a base-pair surrogate

A perylenediimide chromophore (P) was incorporated into DNA hairpins as a base-pair surrogate to prevent the self-aggregation of P that is typical when it is used as the hairpin linker. The photoinduced charge-transfer and spin dynamics of these hairpins were studied using femtosecond transient absorption spectroscopy and time-resolved EPR spectroscopy (TREPR). P is a photooxidant that is sufficiently powerful to quantitatively inject holes into adjacent adenine (A) and guanine (G) nucleobases. The charge-transfer dynamics observed following hole injection from P into the A-tract of the DNA hairpins is consistent with formation of a polaron involving an estimated 3-4 A bases. Trapping of the (A 3-4) (+*) polaron by a G base at the opposite end of the A-tract from P is competitive with charge recombination of the polaron and P (-*) only at short P-G distances. In a hairpin having 3 A-T base pairs between P and G ( 4G), the radical ion pair that results from trapping of the hole by G is spin-correlated and displays TREPR spectra at 295 and 85 K that are consistent with its formation from (1*)P by the radical-pair intersystem crossing mechanism. Charge recombination is spin-selective and produces (3*)P, which at 85 K exhibits a spin-polarized TREPR spectrum that is diagnostic of its origin from the spin-correlated radical ion pair. Interestingly, in a hairpin having no G bases ( 0G), TREPR spectra at 85 K revealed a spin-correlated radical pair with a dipolar interaction identical to that of 4G, implying that the A-base in the fourth A-T base pair away from the P chromophore serves as a hole trap. Our data suggest that hole injection and transport in these hairpins is completely dominated by polaron generation and movement to a trap site rather than by superexchange. On the other hand, the barrier for charge injection from G (+*) back onto the A-T base pairs is strongly activated, so charge recombination from G (or even A trap sites at 85 K) most likely proceeds by a superexchange mechanism.     

Charge-transfer complexes in homogeneous catalysis

It is shown that these complexes can be formed between the reactants in redox reactions. Oxidizing and reducing agents are classified on the basis of charge-transfer capacity, which is defined by the integral for the overlap between the filled orbital of the reductant and the free one of the oxidant. Unfilled diffuse d-orbitals most readily give such complexes. Homogeneous catalysis of a redox reaction involves charge transfer in either direction between the catalyst (which has partly filled d-orbitals) and one of the reagents. The transfer is very much dependent on the orientation, so activators play a large part.     

Charge-transfer derivatization in fast-atom-bombardment mass spectrometry

The formation of charge-transfer complexes as derivatization reactions for fast-atom- bombardment (f.a.b.) mass spectrometry has been investigated. The donor N,N,N′,N′- tetramethyl-1,4-phenylenediamine (TMPD) and the acceptor 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) were studied. The f.a.b, spectrum of this complex in glycerol first yielded the cation radical, TMPD⁺ at m/z 164. However, with time the dominant ion becomes (TMPD+H)⁺. Results suggest that although a strong charge-transfer complex is formed, protonation of the donor molecule occurs whenever possible. In dimethylsulfoxide, initial f.a.b, spectra contain (TMPD+H)⁺, but as solvent evaporates, this ion is supplanted by the cation radical. Increased abundances of the cation radicals for charge- transfer complexes are observed in the absence of solvent or with the use of aprotic solvents. Characteristic ultraviolet/visible spectra of charge-transfer complexes clarify the competing processes of electron and proton transfer, and their time dependence. Charge- transfer derivatization is used to increase signals for the donor cation radicals of anthracene/picric acid, pyrene/picric acid, and indole/trinitrofluorenone.     

Charge-transfer exciton transitions in anthracene-PMDA crystal

The reflection spectrum of the first singlet charge transfer transition of anthracene-PMDA (pyromellitic dianhydride) crystal has been obtained at temperatures close to 2 K. The spectrum is polarized along the stack axis and consists of a series of vibronic bands, approximately 500 cm^?1 wide, with impressed fine structure arising from intermolecular phonon modes. This fine structure is particularly prominent for the 00 band which starts with a zero-phonon line which is the most dominant feature of the entire spectrum. The zero-phonon line has a reflectivity of approximately 0.37 and a line width less than 5 cm^?1. This is the first time that an absolute reflection spectrum of a charge transfer transition with such vibronic details has been obtained. A preliminary discussion of the nature of the transition is presented.     

Charge-transfer excited state dynamics in DNA hairpins substituted with an ethylenylpyrenyl-dU electron source and halo-dU traps

In this work nine DNA hairpins (HPs) are studied at room temperature to observe their pyrene(*+)/dU(*-) CT excited-state dynamics following photoexcitation at 355 nm with a 25 ps laser pulse. The HPs are 18-24 bases long, have a central tetra-T loop, and have a single U(PE) (5-(2-pyren-1-yl-ethylenyl)-2'-deoxyuridine) substitution in the central region of their stems. Three of the HPs are also substituted with 5-XdU traps, where X = Br or F, to learn about the effects of these traps on CT excited-state lifetimes and emission quantum yields in U(PE) substituted HPs. The combination of lengthened average CT lifetime and enhanced CT emission quantum yield in HPs with excess electron traps compared to HPs lacking traps strongly suggests that excess electrons are injected into the DNA stem at pyrimidine sites external to U(PE) as well via charge separation within U(PE) itself. Furthermore, the increased CT emission quantum yield in HPs with traps compared to HPs without traps implies that externally injected electrons can migrate to uracil in U(PE) (i.e., Py(*+)dU) and thus indirectly form the emissive Py(*+)dU(*-) CT state of U(PE).     

Charge-transfer transitions in protein circular dichroism calculations

Charge-transfer transitions in proteins play a key role in many biophysical processes, from the behavior of redox proteins to photochemical reactions. We present ab initio calculations on a model dipeptide and more approximate calculations of the electronic excited states of proteins which, taken together, provide the most definitive assignment and characterization of charge-transfer transitions in proteins to date. We have calculated from first principles the electronic circular dichroism (CD) spectra of 31 proteins on the basis of their structures. Compared to previous studies, we achieve more accurate calculated CD spectra between 170 and 190 nm, owing mainly to the importance in alpha-helices of a charge-transfer transition from the lone pair on one peptide group to the pi* orbital on the next peptide group.     

Charge-transfer complexes of meso-substituted porphines

Charge-transfer (CT) complexes of 5,10,15,20-tetramethyl-21H,23H-porphine [H₂(tmp)] and 5,10,15,20-tetraphenyl-21H,23H-porphine [H₂(tpp)] have been prepared with TCNQ-type (TCNQ = 7,7,8,8-tetracyanoquinodimethane) acceptors. The complexes crystallize in a mixed-stacked structure. The electronic state of the complexes has been investigated by combining structural geometry information of the acceptors with vibrational spectroscopy data. The complexes were found to possess neutral ground states. The difference between the donor oxidation potential and the acceptor reduction potential (ΔE) also supports this designation of their electronic states. The CT absorption energy shows a linear correlation with ΔE, which is expected for CT complexes in their neutral ground states. The frontier orbitals of the porphyrin donor that participate in the CT interactions have been examined by calculating the overlap integral between the donor occupied molecular orbitals and acceptor LUMO in the complexes. In the H₂(tmp) and H₂(tpp) complexes, a_2u- and a_1u-type porphyrin HOMO and next-HOMO, respectively, are suggested to both be contributors to the establishment of π–π* CT interactions and formation of the complex.     

Charge-transfer interactions in tris-donor-tris-acceptor hexaarylbenzene redox chromophores

Symmetric- and asymmetric hexaarylbenzenes (HABs), each substituted with three electron-donor triarylamine redox centers and three electron-acceptor triarylborane redox centers, were synthesized by cobalt-catalyzed cyclotrimerization, thereby forming compounds with six- and four donor-acceptor interactions, respectively. The electrochemical- and photophysical properties of these systems were investigated by cyclovoltammetry (CV), as well as by absorption- and fluorescence spectroscopy, and compared to a HAB that only contained one neighboring donor-acceptor pair. CV measurements of the asymmetric HAB show three oxidation peaks and three reduction peaks, whose peak-separation is greatly influenced by the conducting salt, owing to ion-pairing and shielding effects. Consequently, the peak-separations cannot be interpreted in terms of the electronic couplings in the generated mixed-valence species. Transient-absorption spectra, fluorescence-solvatochromism, and absorption spectra show that charge-transfer states from the amine- to the boron centers are generated after optical excitation. The electronic donor-acceptor interactions are weak because the charge transfer has to occur predominantly through space. Moreover, the excitation energy of the localized excited charge-transfer states can be redistributed between the aryl substituents of these multidimensional chromophores within the fluorescence lifetime (about 60?ns). This result was confirmed by steady-state fluorescence-anisotropy measurements, which further indicated symmetry-breaking in the superficially symmetric HAB. Adding fluoride ions causes the boron centers to lose their accepting ability owing to complexation. Consequently, the charge-transfer character in the donor-acceptor chromophores vanishes, as observed in both the absorption- and fluorescence spectra. However, the ability of the boron center as a fluoride sensor is strongly influenced by the moisture content of the solvent, possibly owing to the formation of hydrogen-bonding interactions between water molecules and the fluoride anions.     

Charge-transfer polymerization of butyl methacrylate

Butyl methacrylate (BuMA) can be polymerized by charge-transfer complexes formed by the interaction of ethanolamine (EA), BuMA, and carbon tetrachloride (CCl₄) in a non-aqueous solvent, such as N,N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO). The rate of polymerizationR _p is found to be linear with [BuMA] and proportional to both [CCl₄]^0.5 and [EA]^0.5 when [CCl₄]/[EA]≤1.R _p becomes independent of [CCl₄] when [CCl₄]/[EA]>1.R _p is proportional to [EA]^0.56 and to [BuMA]^1.30 when [CCl₄]>[EA]. The average rate constant at 30°C for the polymerization of BuMA in terms of monomer was 3.32×10⁻⁶ s⁻¹ when [CCl₄]/[EA]≤1, and 5.47×10⁻⁶ L/(mol s) when [CCl₄]/[EA]>1.     

Charge-transfer intensity in complexes with symmetrically equivalent acceptors

Mulliken's model for the intensity of charge-transfer absorption bands in donor/acceptor complexes is extended to transition-metal compounds. Attention is focused on the analysis of systems containing a central metal ion and symmetrically equivalent π-acceptor ligands. The theory is illustrated for a one-donor/two-acceptor complex in D_2d point symmetry. Results are stated for symmetries of commonly occurring transition-metal compounds that display well-documented metal-to-ligand charge-transfer transitions.     

Charge-transfer complexes of barbiturates and phenytoin

Simple and sensitive spectrophotometric methods are described, for the first time, for the determination of sodium salts of phenobarbital (1), thiopental (2), methohexital (3) and phenytoin (4). The methods are based on the reaction of these drugs as n-electron donors with the sigma-acceptor iodine and various pi-acceptors: 7,7,8,8-tetracyanoquinodimethane; 2,3-dichloro-5,6-dicyano-1,4-benzoquinone; 2,3,5,6-tetrachloro-1,4-benzoquinone; 2,3,5,6-tetrafluoro-1,4-benzoquinone; 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone; tetracyanoethylene and 2,4,7-trinitro-9-fluorenon. Depending on the solvent polarity, different coloured charge-transfer complexes and radicals were developed. Different variables and parameters affecting the reactions were studied and optimized. The formed complexes were examined by UV/VIS, infrared and (1)H-NMR. Due to the rapid development of colours at ambient temperature, the obtained results were used on thin layer chromatograms for the detection of the investigated compounds. Beer's plots were obeyed in a general concentration range of 1-400 mug ml(-1) for the investigated compounds with different acceptors. Interference from some co-formulated drugs was also studied. No interference was observed due to additives commonly present in the pharmaceutical preparations. The proposed methods could be applied successfully to the determination of the investigated compounds in pure and pharmaceutical dosage forms with good accuracy and precision, the recoveries ranged from 98.7+/-0.5 to 101.1+/-0.5%. The results were compared favourably with the official methods.     

Charge-transfer states in strongly coupled phthalocyanine fullerene ensembles

Fluorescence and transient absorption measurements show that in strongly coupled ZnPc-C60 and H2Pc-C60 dyads charge-separated states are formed; large -delta GCR degree and small lambda assist in stabilising ZnPc(.+)-C60.-/H2Pc(.+)-C60.-.     

Charge-transfer properties of lateral triphenylamine-dithienophosphole diads

Installation of an exocyclic triphenylamine group at the phosphorus center provides access to dithienophosphole materials with lateral charge-transfer (CT) ability. The degree of CT can be significantly manipulated not only via oxidation of the P-center but also surprisingly by alkylation of the 2,6-position of the scaffold.     

Charge-transfer complexes with trivalent phosphorus compounds

Charge-transfer complexes between triphenylphosphine and triethylphosphite as donors, and maleic, itaconic and citraconic anhydrides as acceptors are studied.