Long-range electron and excitation energy transfer in donor–bridge–acceptor systems

Donor–bridge–acceptor (D-B-A) systems, either as supermolecules or on surfaces, have been extensively studied with respect to long-range electron (ET) and excitation energy (EET) transfer. In more recent years, the main research objective has been to develop knowledge on how to construct molecular-based devices, with predetermined electron transfer properties, intended for application in electronics and photovoltaics. At present, such construction is in general hampered for several reasons. Most importantly, the property of a D-B-A system is not a simple linear combination of properties of the individual components, but depends on the specific building blocks and how they are assembled. An important example is the ability of the bridge to support the intended transfer process. The mediation of the transfer is characterized by an attenuation factor, β, often viewed as a bridge specific constant but which also depends on the donor and the acceptor, i.e. the same bridge can either be poorly or strongly conducting depending on the donor and acceptor. This review gives an account of the experimental exploration of the attenuation factor β in a series of bis(porphyrin) systems covalently linked by bridges of the oligo(phenyleneethynylene) (OPE) type. Attenuation factors for ET as well as for both singlet and triplet EET are discussed. A report is also given on the dependence of the transfer efficiency on the energy-gap between the donor and bridge states relevant for the specific transfer process. The experimental variation of β with varying donor and acceptor components is shown for a range of conjugated bridges by representative examples from the literature. The theoretical rationalization for the observed variation is briefly discussed. Based on the Gamow tunneling model, the observed variations in β-values with varying donors and acceptors for the same bridges is simulated successfully simultaneously as the observed energy-gap dependence is modelled.     

Reorganization energy of supramolecular donor–acceptor dyad of octaethylporphyrin isomers and axial-coordinated acceptor: Experimental and computational study

The internal reorganization energy (λ_V) of photoinduced electron transfer (ET) in the supramolecular donor–acceptor dyads of 2,3,7,8,12,13,17,18-octaethylporphinatozinc(II) (ZnOEP) and electron acceptor ligands was compared with those of structural isomers, 2,3,7,8,11,12,17,18-octaethylhemiporphycenatozinc(II) (ZnHPc) and 2,3,6,7,12,13,16,17-octaethylporphycenatozinc(II) (ZnPcn). First, ET process of the supramolecular donor–acceptor dyads of ZnOEP was investigated by means of the transient absorption spectroscopy mainly. The formation of supramolecular dyads was confirmed by absorption spectral change, from which the association constant was estimated. The ET process was confirmed by the observation of radical cation of ZnOEP during the laser flash photolysis. The ET rates of these dyads are in the order of ZnOEP > ZnHPc > ZnPcn, when the driving forces for ET are similar to each other. From the free energy dependence of ET rates, the λ_V values of OEP and its isomers were estimated. The estimated λ_V value was in the order of ZnOEP < ZnHPc < ZnPcn. This tendency was reproduced by calculation at B3LYP/6-31G(d) and BHandHLYP/6-31G(d) levels. The origin of this tendency was discussed on the basis of the structural change during the ET process.     

Donor–acceptor perylenediimide–ferrocene conjugates: synthesis,photophysical, and electrochemical properties

Donor–acceptor, perylenediimide–ferrocene conjugates have been synthesized by Suzuki, and Sonogashira coupling reactions. The photophysical and electrochemical properties of these conjugates are discussed. It has been shown that fluorescence as well as the electron affinity of the perylenediimide can be tuned by attaching the appropriate ferrocenyl derivatives.     

Hydrogen splitting by pyramidalized 13–15 donor–acceptor cryptands: A computational study

A series of new donor–acceptor cryptands, where pyramidalized donor (azaadamantane) and acceptor (bora/ala/adamantane) molecules are spatially oriented toward each other and linked via aromatic spacer, are constructed and computationally studied at M06-2X and ωB97X-D levels of theory. Kinetic stability of the perfluorinated bora- and ala-adamantane with respect to F migration to group 13 element is demonstrated. The effectiveness of the constructed cryptands, featuring pyramidalized perfluorinated acceptor moieties, in the heterolytic splitting of molecular hydrogen is predicted. Hydrogen splitting is highly exothermic and exergonic and is accompanied by small activation barriers. The most promising candidates for the experimental studies are identified. © 2019 Wiley Periodicals, Inc.     

Electronic, optical, and charge transfer properties of donor–bridge–acceptor hydrazone sensitizers

The ground state geometries have been computed by using density functional theory (DFT) at B3LYP/6-31G*, B3LYP/6-31G**, and PCM-B3LYP/6-31G* level of theories. The highest occupied molecular orbitals (HOMOs) are delocalized on whole of the molecule and the lowest unoccupied molecular orbitals (LUMOs) are localized on the tricarbonitrile. The lowest HOMO and LUMO energies have been observed for Dye1 while highest for Dye4. The LUMO energies of Dye1–Dye4 are above the conduction band of TiO₂ and HOMOs are below the redox couple. The absorption spectra have been computed in solvent (methanol) and without solvent by using time-dependant DFT at TD-B3LYP/6-31G*, TD-B3LYP/6-31G**, and PCM-TD-B3LYP/6-31G* level of theories. The calculated maximum absorption wavelengths of the spectra in methanol are in good agreement with experimental evidences. The maximum absorption wavelengths of new designed sensitizers are red shifted compared to parent molecule. The electronic coupling constant and electron injection have been computed by first principle investigations. The improved electronic coupling constant and electron injection revealed that new modeled systems would be efficient sensitizers.     

New donor–acceptor copolymers with ultra-narrow band gap for photovoltaic application

Two novel donor–acceptor (D–A) copolymers P1 and P2 with the thiazoloquinoxaline repeating acceptor moiety and different donor moieties of benzo[1,2-b:4,5-b']dithiophene and isomeric benzo[2,1-b:3,4-b']dithiophene have been prepared. The polymers show light absorption at 300–1200 nm and a band gap width of 0.98 and 1.14 eV, respectively. The energies of the HOMO (–5.42 and–5.29 eV) and LUMO (–3.90 and–3.83 eV) levels of polymers P1 and P2 have been determined. The absorption maximum for polymer P1 in the long-wavelength region is red-shifted by 161 nm, which is caused by stronger charge transfer in P1 as compared with P2. This fact indicates that the benzo[1,2-b:4,5-b']dithiophene structural moiety has a higher electron-donating ability than the benzo[2,1-b:3,4-b']dithiophene moiety. The red shift of the absorption spectrum of polymer P1 in comparison with that of P2 indicates that interchain π–π stacking interactions are more efficient in P1 than in P2.     

Temperature dependence of photoinduced electron transfer in hydrogen-bonded donor–acceptor systems

We have studied the temperature dependence of photoinduced electron transfer (PET) reactions in three hydrogen-bonded donor–acceptor systems in the range 220–298 K. For the hydrogen-bonded system in the normal region, the PET rate constant was found to increase with increase in temperature. For the two systems in the inverted region, the rate constants were nearly independent of temperature. We have analyzed the results using electron transfer theories.     

Heteroatom substitution-induced asymmetric A–D–A type non-fullerene acceptor for efficient organic solar cells

Research on asymmetric A–D–A structured non-fullerene acceptors has lagged far behind the development of symmetric counterpart. In this contribution, by simply replacing one sulfur atom in indacenodithiophene unit with a selenium atom, an asymmetric building block Se PT and a corresponding asymmetric non-fullerene acceptor Se PT-IN have been developed. Asymmetric Se PT-IN achieved a high efficiency of 10.20% in organic solar cells when blended with PBT1-C, much higher than that of symmetric TPT-IN counterpart(8.91%). Our results demonstrated an effective heteroatom substitution strategy to develop asymmetric A–D–A structured non-fullerene acceptors.     

Modelling the molecular weight distribution in terpolymerization systems with donor–acceptor complexes

In this work we present, for the first time, a mathematical development of the moments of the molecular weight distribution in terpolymerization systems where donor–acceptor complexes are formed and the propagating reactions are carried out according to the complex participation model. The resulting set of equations is applied to the system formed by vinyl chloride (VC), vinyl acetate (VAc) and maleic anhydride (MA), in order to show the use of the equations and the type of information that might be obtained from them.     

Adenine–Au and adenine–uracil–Au. Non-conventional hydrogen bonds of the anions and donator–acceptor properties of the neutrals

This is a study of adenine–Au and adenine–uracil–Au (neutral, anionic and cationic), applying the B3LYP density-functional approach. In these systems, the interaction is directly related to the charge; so that as the metal atomic charge increases, the bond strength also increases. Neutral molecules are weakly bonded, the interaction in the case of cations is mainly electrostatic and in the case of the anions, the extra electron is localized on the metal atom and consequently, non-conventional hydrogen bonds are formed. In the case of adenine–Au (anion), the H dissociation energy is similar to the electron dissociation energy, and therefore both reactions may be possible. Moreover, the Au anionic atom modifies the hydrogen bonds of the uracil–adenine base pair. This may be significant in the study of point mutations that may occur in the Watson–Crick dimmer of nucleic basis. The electron-donator properties of these compounds are analyzed with the aid of the donator–acceptor map (DAM), previously described. Adenine–Au, uracil–Au and adenine–uracil–Au are more effective electron donors, but poorer electron acceptors than adenine, uracil and adenine–uracil. If the electron acceptor properties of carotenoids such as β-carotene and astaxanthin are compared, there are indications that astaxanthin may act as an oxidant instead of an antioxidant with the uracil–adenine base pair. The oxidation of nucleic acid bases by carotenoids may have important consequences, as oxidative damage of DNA and RNA appears to be linked to cancer. This is something that demands further studies and for this reason, work concerning the reactivity of carotenoids with DNA-nitrogen bases is in progress.     

Donor–acceptor interactions between resonance-excited silver nanoparticles and halide ions in water solutions

Donor–acceptor interactions between silver nanoparticles (NPs), resonance-excited by optical quanta of light, and halide ions are studied in aqueous solutions. It is shown that deactivation of the plasmon excitation of Ag NP proceeds according to the exchange mechanism of electron transfer. Plasmon excitation quenching constants are determined and a correlation between quenching and the donor properties of halide ions is found. The efficiency of electrostatic interaction between resonantly-excited Ag NPs and halide ions is studied, and their dipole moment is determined.

     

Stereospecific [2 + 2]-cross-photocycloaddition in a supramolecular donor–acceptor complex

A bis(propylammonium) derivative of (E)-4-(4-mercaptostyryl)pyridine, which was synthesized for the first time, forms a highly stable bimolecular complex with a bis(18-crown-6 ether) derivative of (E)-stilbene in solution owing to ditopic coordination via hydrogen bonds. The complex formation results in much faster deactivation of the excited states of both compounds, which is explained by photoinduced electron transfer from the stilbene derivative to the styrylpyridinium dye. Despite this, the complexed olefins undergo [2?+?2]-cross-photocycloaddition upon selective excitation of the dye to afford solely the syn-cycloadduct. The retro-photocycloaddition occurs readily upon UV irradiation of the cycloadduct and leads to the initial bimolecular complex.     

Pleated polymeric foldamers driven by donor–acceptor interaction and conjugated radical cation dimerization

Two naphthalene(NP) and bipyridinium(BIPY~(2+)) alternately incorporated polymers P1 and P2 have been prepared through the formation of dynamic hydrazone bonds. The polymers formed NP–BIPY~(2+)donor–acceptor interaction-induced pleated secondary structure. Upon reducing the BIPY~(2+)units to radical cation BIPY+, intramolecular dimerization of the BIPY+units induced the backbones to afford another pleated secondary structure. Adding electron-rich macrocyclic polyether bis-1,5-dinaphtho[38]crown-10 or electron-deficient cyclobis(paraquat-p-phenylene) cyclophane did not break the first foldamer by complexing the BIPY2+or NP units of the polymers, whereas the di(radical cationic)ring of the second cyclophane could break the second foldamer by forming threading complexes with the BIPY+units of the polymers.     

New opportunities in multiplexed optical bioanalyses using quantum dots and donor–acceptor interactions

In this study, we investigated the feasibility of using a novel volatile organic compound (VOC)-based metabolic profiling approach with a newly devised chemometrics methodology which combined rapid multivariate analysis on total ion currents with in-depth peak deconvolution on selected regions to characterise the spoilage progress of pork. We also tested if such approach possessed enough discriminatory information to differentiate natural spoiled pork from pork contaminated with Salmonella typhimurium, a food poisoning pathogen commonly recovered from pork products. Spoilage was monitored in this study over a 72-h period at 0-, 24-, 48- and 72-h time points after the artificial contamination with the salmonellae. At each time point, the VOCs from six individual pork chops were collected for spoiled vs. contaminated meat. Analysis of the VOCs was performed by gas chromatography/mass spectrometry (GC/MS). The data generated by GC/MS analysis were initially subjected to multivariate analysis using principal component analysis (PCA) and multi-block PCA. The loading plots were then used to identify regions in the chromatograms which appeared important to the separation shown in the PCA/multi-block PCA scores plot. Peak deconvolution was then performed only on those regions using a modified hierarchical multivariate curve resolution procedure for curve resolution to generate a concentration profiles matrix C and the corresponding pure spectra matrix S. Following this, the pure mass spectra (S) of the peaks in those region were exported to NIST 02 mass library for chemical identification. A clear separation between the two types of samples was observed from the PCA models, and after deconvolution and univariate analysis using N-way ANOVA, a total of 16 significant metabolites were identified which showed difference between natural spoiled pork and those contaminated with S. typhimurium.     

Donor–acceptor nonradiative energy transfer mediated by surface plasmons on ultrathin metallic films

Selected properties of donor–acceptor energy transfer in the presence of surface plasmon coupled emission (SPCE) on metallic nanofilms are demonstrated. These properties of surface plasmon mediated energy transfer (SPMET) are for the first time compared to those of traditional energy transfer (ET) based on the same donor–acceptor system. The presence of plasmons significantly accelerates energy transfer as revealed by the results of fluorescence intensity decay. In particular, the rise time of acceptor fluorescence intensity upon donor excitation is 10 times shorter in the presence of SPCE. It is also observed that contrary to ET the sensibilized acceptor emission in SPMET is totally linearly polarized.     

Synthesis and photophysical properties of donor–acceptor system based bipyridylporphyrins for dye-sensitized solar cells

Bipyridylporphyrin derivatives possessing a porphyrin moiety as the electron donor and bipyridyl moiety as the electron–acceptor were designed and synthesized for dye-sensitized solar cells(DSSCs). The photophysical and electrochemical properties were investigated by absorption spectrometry and cyclic voltammetry. Density functional theory(DFT) was employed to study electron distribution. From the photovoltaic performance measurements, a maximum conversion efficiency(η) of 0.38% was achieved based on the bipyridylporphyrin ruthenium dye A7(J_(SC)= 1.33 mA/cm~2, V_(OC)= 0.45 V, FF = 0.64) under 1.5 irradiation(100 mW/cm~2).     

Gallium trichloride-mediated reactions of ‘double’ donor–acceptor cyclopropanes with alkenes and dienes

The reactions of ‘double’ donor–acceptor cyclopropanes containing a p- or m-phenylene moiety with alkenes or dienes in the presence of GaCl₃ comprise formation of gallium 1,2-zwitterionic intermediates, the structure of final products being substrate dependent. In contrast to the para-or meta-isomers, reaction of 2,2'-(1,2-phenylene)bis(cyclopropane-1,1-dicarboxylate) does not involve alkene and affords isomeric tricyclo[6.2.2.0^2,7]dodeca-2,4,6-triene-9,9,11,11-tetra-carboxylate, a product of intramolecular rearrangement.     

The gas–liquid tunable self-assembly properties of rod–coil diblock copolymer: donor–acceptor alternating structure served as rod segment

The rod–coil diblock copolymers in which the donor–acceptor alternating structures served as the rod segment were synthesized. The supramolecular self-assembly property of the copolymers was investigated in the methanol atmosphere. By changing the assembly condition, well-defined vesicles and porous films were produced, respectively. Pores with different size dispersions were obtained by tuning the methanol atmosphere. Moreover, porous films were also decorated on diverse substrates with nonplanar structures. The investigation on self-assembly properties of this rod–coil copolymer is the complementarity to the self-assembly of rod–coil copolymers. This is a very useful self-assembly method that can be used to prepare the self-assembly nanostructures with donor–acceptor alternating copolymers.     

Synthesis and properties of π-conjugated donor–acceptor macrocycles derived from phenanthrylene building blocks

Phenanthrylene-ethynylidene macrocycles combining electron donor and electron acceptor subunits in their shape-persistent fully conjugated core were synthesized. The donor subunits consisted of two 9,10-dialkoxyphenanthrenes linked either with 1,2-ethynylidene or 2,5-thienylene bridge. The acceptors were 9,10-phenanthroquinone and dibenzoquinoxaline and dibenzophenazines derived from it. Solvatochromic photoluminescence from intramolecular-charge-transfer (ICT) excited state was observed mainly in non-polar solvents. In more polar solvents, the excited states favor non-radiative relaxation. DFT calculated HOMO/LUMO energies of the macrocycles correlate well with spectroscopic and electrochemical data. In the series of substituted dibenzophenazine acceptors a good correlation with Hammett substituent constants σ_p^– was found.