First-principles determinations and investigations of the electronic absorption and third-order polarizability spectra of electron donor-acceptor chromophores tetraalkylammonium halide/carbon tetrabromide

Calculations on donor-acceptor molecular pairs of tetraalkylammonium halide/carbon tetrabromide complexes are provided to investigate structure/property-related linear and nonlinear optical properties by using the time-dependent density functional theory technique coupled with the sum-over-states method. The calculated energies of the first allowed electronic transition decrease, and the nonresonant third-order polarizabilities at the THG, EFISHG, and DFWM optical processes increase progressively from [DBU-H+Br-.CBr(4) to [NPr(4)Br.CBr(4)] to [NMe(4)Br.CBr(4)]. The obtained electronic absorption spectra show a progressive red shift with increasing donor strength from Cl to I for [NR(4)h.CBr(4)] (h = Cl, Br, and I). The charge transfers from the halogen donor to the carbon tetrabromide acceptor make significant contributions to the electronic absorption spectra in the low-energy zone and the third-order polarizabilities in the nonresonant frequency region. The counterion indirectly affects the electronic absorption and third-order polarizability spectra through the interactions between the donor and acceptor.     

Conformationally tuned large two-photon absorption cross sections in simple molecular chromophores

We investigate here the relationship between molecular architecture and two-photon absorption (TPA) processes in a class of alkyl-substituted 4-quinopyran chromophores. We find that TPA cross sections diverge as the one-photon gap energy nears one-half of the two-photon gap. The molecular strategy proposed here to tune these two-excitation gaps for maximizing TPA cross sections is to twist the molecule about the bond connecting the chromophore donor and acceptor phenylene fragments. Extremely large TPA cross sections, determined by the absorption bandwidth, can then be realized (imaginary part of the third-order polarizability approximately 2.6 x 10(5) x 10(-36) esu) for fundamental photon energies near 1.0 eV, when the torsional angle approaches 104 degrees. The required torsional angle is achieved by introduction of sterically encumbered 2,2',2' ',2' " tertiary alkyl substituents.     

Side-chain engineering of high-efficiency conjugated polymer photovoltaic materials

In recent years,conjugated polymers have attracted great attention in the application as photovoltaic donor materials in polymer solar cells(PSCs).Broad absorption,lower-energy bandgap,higher hole mobility,relatively lower HOMO energy levels,and higher solubility are important for the conjugated polymer donor materials to achieve high photovoltaic performance.Side-chain engineering plays a very important role in optimizing the physicochemical properties of the conjugated polymers.In this article,we review recent progress on the side-chain engineering of conjugated polymer donor materials,including the optimization of flexible side-chains for balancing solubility and intermolecular packing(aggregation),electron-withdrawing substituents for lowering HOMO energy levels,and two-dimension(2D)-conjugated polymers with conjugated side-chains for broadening absorption and enhancing hole mobility.After the molecular structural optimization by side-chain engineering,the2D-conjugated polymers based on benzodithiophene units demonstrated the best photovoltaic performance,with powerconversion efficiency higher than 9%.     

Folding of alternating dialkylsilylene-spaced donor-acceptor copolymers: the oligomer approach

A series of oligmers with donor-acceptor pairs separated by diisopropylsilylene (iPr(2)Si) spacers, composed of monomer 4b, dimer 5, trimer 6, and tetramer 7, were synthesized to scrutinize the folding behavior. Monomer 4a with a dimethylsilylene (Me(2)Si) spacer was also prepared for comparison. The 4-aminostyrene moiety was used as the donor and the stilbene moiety as the acceptor. Both steady-state and time-resolved fluorescence spectroscopic measurement were made. Regardless of the substituents on the silicon atom, the emission spectra of 4a and 4b exhibit both local excited (LE) emission of the acceptor chromophore and emission from the charge-separated state (CT emission), which are similar to that of the corresponding Me(2)Si-spaced copolymer 2a with the same donor and acceptor chromophores, but different from that of the copolymer with the iPr(2)Si spacer 2b. Dimer 5 behaves like 4 and 2a. As the chain length of the oligomers increases, the emission properties of the higher homologues become prone to that of 2b. Thus, tetramer 7 exhibits emission from the charge-transfer complex, which is essentially same as that of 2b. Moreover, charge-transfer absorptions emerge in 6 and 7. These results suggest that the folding nature of oligomers approaches that of the corresponding polymer, as the degree of oligomerization increases, and the electronic interactions between adjacent donor-acceptor pairs are controlled by the steric effect of the substituents on the silicon atoms and concomitant amplification of the stabilizing energy by extending the distance of the folding structure.     

Injecting electronic excitation energy into an artificial antenna system through an Ru2+ complex

The Ru2+ complex [Ru(bpy)2(bpy-ph4-Si(CH3)3)]2+ can be electrostatically bound to the negatively charged channel entrances of dye-loaded zeolite L crystals where it acts as a functional stopcock molecule. Impressive electronic triplet-singlet excitation energy transfer from the Ru2+ complex to the acceptor dye oxazine 1 (Ox1) located inside the channels can be observed when the donor molecule is selectively excited. Time-resolved luminescence experiments have been performed on the separate components and on the assembled donor-acceptor material. The luminescence lifetime of the Ru2+ complex attached to the zeolite is reduced by a factor of 30 when Ox1 acceptor molecules are present. The fluorescence decay of Ox1 incorporated in zeolite L is single exponential with a lifetime of 3 ns. The much longer lifetime in zeolite L than in solution is due to the fact, that the diethyl groups are sterically restricted when the dye is inside the host.     

Interfacing strong electron acceptors with single wall carbon nanotubes

The complementary use of steady-state and time-resolved spectroscopy in combination with electrochemistry and microscopy are indicative of mutual interactions between semiconducting SWNTs and a water-soluble strong electron acceptor, i.e., perylenediimide. Significant is the stability and the strong electronic coupling of the perylenediimide/SWNT electron donor-acceptor hybrids. Several spectroscopic and spectroelectrochemical techniques, i.e., Raman, absorption, and fluorescence, confirmed that distinct ground- and excited-state interactions occur and that kinetically and spectroscopically well characterized radical ion pair states form within a few picoseconds.     

Donor/Acceptor-Substituted Tetraethynylethenes: Systematic Assembly of Molecules for Use as Advanced Materials

A comprehensive series of tetraethynylethenes (= 3,4-diethynylhex-3-ene-1,5-diynes, TEEs) bearing electrondonating (p-methoxyphenyl or p-aminophenyl) and/or electron-accepting (p-nitrophenyl) groups was prepared via [Pd]-catalyzed Sonogashira cross-coupling reactions. The electronic and photonic properties of these molecules were investigated with a special emphasis on the effects caused by degree and pattern of donor/acceptor substitution around the central TEE core. This analysis showed that intramolecular donor-acceptor interactions, as evidenced by a long-wavelength charge-transfer band, are considerably more effective in TEEs 44 and 46 , with trans and cis, linearly-conjugated electronic pathways between donor and acceptor, than in 11 , with a geminal, cross-conjugated electronic pathway. UV/VIS Spectroscopy revealed a steady bathochromic shift of the longest-wavelength absorption band (λ_max) as the number of donor-acceptor conjugation paths increased upon changing from bis-arylated ( 11, 44 , and 46 ) to tetrakis-arylated ( 14, 31 , and 35 ) TEEs. The position of the longest-wavelength absorption was also found to be strongly dependent on the nature of the N-substituents in the R₂NC₆H₄-donor groups. Electronic emission spectroscopic investigations demonstrated a considerable solvent dependency of the fluorescence of donor-acceptor-substituted TEEs such as 11 or 44 , in agreement with the presence of highly polarized excited states in these molecules. Correspondingly, fluorescence spectra of TEEs bearing only donor or acceptor substituents showed little solvent dependency. The large majority of the donor/acceptor-substituted TEEs are thermally and environmentally stable molecules. They can be stored for months as solids in the air at room temperature, and many decompose only upon heating to temperatures above 200°. X-Ray analysis showed the conjugated C-atom scaffolds of 44, 46 , and 67 to be essentially planar, whereas the aryl substituents in 28 and 30 are rotated out of the plane of the TEE core by varying degrees.     

Resonance hyper-Raman excitation profiles and two-photon states of a donor-acceptor substituted polyene

Resonance Raman and resonance hyper-Raman spectra and excitation profiles have been measured for a "push-pull" donor-acceptor substituted conjugated polyene bearing a julolidine donor group and a nitrophenyl acceptor group, in acetone at excitation wavelengths from 485 to 356 nm (two-photon wavelengths for the nonlinear spectra). These wavelengths span the strong visible to near-UV linear absorption spectrum, which appears to involve at least three different electronic transitions. The relative intensities of different vibrational bands vary considerably across the excitation spectrum, with the hyper-Raman spectra showing greater variation than the linear Raman. A previously derived theory of resonance hyper-Raman intensities is modified to include contributions from purely vibrational levels of the ground electronic state as intermediate states in the two-photon absorption process. These contributions are found to have only a slight effect on the hyper-Rayleigh intensities and profiles, but they significantly influence some of the hyper-Raman profiles. The absorption spectrum and the Raman, hyper-Rayleigh, and hyper-Raman excitation profiles are quantitatively simulated under the assumption that three excited electronic states contribute to the one- and two-photon absorption in this region. The transition centered near 400 nm is largely localized on the nitrophenyl group, while the transitions near 475 and 355 nm are more delocalized.     

Two heterocyclic merocyanine classes and their optical properties in relation to the donor-acceptor strength of end substituents

The relationship between the first electronic transition energy of heterocyclic merocyanines and donor-acceptor properties of their end substituents has been quailtatively treated and analytically substantiated in terms of the classical valence bond model and the biradicaloid theory. A simple graphical technique has been suggested to classify donor-acceptor-substituted compounds in relation to their electronic structure and to predict the effect of donor-acceptor strength on their optical behavior. In this context, merocyanines can be divided into two classes which differ in charges and electron populations of their donor and acceptor moieties and exhibit mirror-like responses to the variation of donor-acceptor strength. Two families of new heterocyclic merocyanines have been synthesized and their experimentally observed spectroscopic properties have been rationalized using the approach developed.     

Effects of heterocyclic ring and amino-ethyl-amino group on the electronic and photophysical properties of a triphenylamine-pyrimidine dye

Introduction of a heterocyclic ring and an amino-ethyl-amino group to donor-acceptor (D-A) type photosensitive dyes can modulate the lifetime of the charge separation generated in the D-A dyes as well as their electronic and UV-vis absorption properties. Here we perform density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations to study 11 derivatives of a triphenylamine-pyrimidine, namely MTPA-Pyc, in order to improve the performance of MTPA-Pyc as solar cell sensitizers. Five heterocyclic rings and an amino-ethyl-amino group were introduced on the styryl moiety of MTPA-Pyc. The results show that the introduction of heterocyclic rings generally causes an absorption red shift, but the absorption intensity reduces as a result of the increase in the dihedral angle between the donor and acceptor. Further, introduction of an amino-ethyl-amino group to these dyes with a heterocyclic ring modification disrupts the conjugation between the donor and acceptor, which does not benefit the absorption but may have the potential to increase the lifetime of charge separation of the dyes. We identify 2 out of 11 dyes that have the best potential for solar cell applications.     

Tetrathiafulvalene derivatives as NLO-phores: synthesis, electrochemistry, Raman spectroscopy, theoretical calculations, and NLO properties of novel TTF-derived donor-pi-acceptor dyads.

Novel pi-conjugated donor-acceptor chromophores, based on the strong electron-donating tetrathiafulvalene moiety and different electron-withdrawing acceptors, exhibit large second-order optical nonlinearities. The effect of increasing the length of the polyenic spacer and the influence of the nature of the acceptor moiety on the NLO properties have been studied by using the electric field-induced second-harmonic generation (EFISH) technique as well as by semiempirical and ab initio theoretical calculations. A charge-transfer band has been observed in the absorption spectra of these D-pi-A compounds that undergoes an hypsochromic shift when increasing the number of vinylenic spacer units connecting both donor and acceptor moieties. The degree of the intramolecular charge transfer from the donor to the acceptor has also been analyzed by means of Raman spectroscopy.     

Structural,magnetic, and optoelectronic properties of (diimine)(dithiolato)platinum(II) and -palladium(II) complexes and their charge-transfer adducts with nitrile acceptors

Two new diimine dithiolato complexes, (dbbpy)Pt(dmid), 1, and (dbbpy)Pd(dmid), 2, were prepared and characterized (dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine; dmid = 2-oxo-1,3-dithiole-4,5-dithiolate). Both complexes interact with the nitrile acceptor TCNQ, and 1 also interacts with TCNQF(4) and TCNE (TCNQ = 7,7,8,8-tetracyanoquinodimethane; TCNQF(4) = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane; TCNE = tetracyanoethylene) to form supramolecular 2:1 charge-transfer solids that stack in the manner -DDADDADDA- (D = electron donor; A = electron acceptor). All compounds have been fully characterized by magnetic, spectroscopic, electrochemical, and single-crystal X-ray crystallographic analyses. Magnetic susceptibility studies of the charge-transfer compounds revealed that the platinum-based complexes exhibit temperature-independent paramagnetism of approximately 10(-3) emu/mol. The donor complexes exhibit continuous absorption bands across the UV/visible and into the NIR region. Upon interaction with the nitrile acceptors, the extinction coefficients of the absorption bands increase and the energies of some d-d transitions in the NIR region change. The donor-acceptor compounds possess desired spectral features for solar cell dyes, but low conversion efficiencies resulted when a representative compound was tested in a TiO(2) solar cell. The results, however, serve to illustrate that the donor-acceptor interactions persist in solution and the adsorption of the dye molecules to the semiconductor surface occurs in the absence of typical anchoring groups. Evaluation of the spectral and electrochemical data for the title compounds and the results of the preliminary solar cell study serve as guides for future research in choosing promising candidates for efficient solar cell dyes.     

APPLICATIONS OF ELECTRONIC ENERGY TRANSFER IN SOLUTION*,†

Abstract— lntermolecular transfer of electronic energy is a powerful tool for obtaining information about excited molecules in solution. Proper application of excitation transfer requires giving careful attention to the spectroscopic properties of the donor and acceptor and accounting for all possible donor-acceptor interactions of consequence. Water presents no particular difficulties so that electronic energy transfer methods can be applied to the photochemistry of biologically important molecules in aqueous solvents.     

Achieving over 16% efficiency for single-junction organic solar cells

To achieve high photovoltaic performance of bulk hetero-junction organic solar cells(OSCs), a range of critical factors including absorption profiles, energy level alignment, charge carrier mobility and miscibility of donor and acceptor materials should be carefully considered. For electron-donating materials, the deep highest occupied molecular orbital(HOMO) energy level that is beneficial for high open-circuit voltage is much appreciated. However, a new issue in charge transfer emerges when matching such a donor with an acceptor that has a shallower HOMO energy level. More to this point, the chemical strategies used to enhance the absorption coefficient of acceptors may lead to increased molecular crystallinity, and thus result in less controllable phase-separation of photoactive layer. Therefore, to realize balanced photovoltaic parameters, the donor-acceptor combinations should simultaneously address the absorption spectra, energy levels, and film morphologies. Here, we selected two non-fullerene acceptors, namely BTPT-4F and BTPTT-4F, to match with a wide-bandgap polymer donor P2F-EHp consisting of an imidefunctionalized benzotriazole moiety, as these materials presented complementary absorption and well-matched energy levels. By delicately optimizing the blend film morphology, we demonstrated an unprecedented power conversion efficiency of over 16% for the device based on P2F-EHp:BTPTT-4F, suggesting the great promise of materials matching toward high-performance OSCs.     

A beta-naphthaleneimide-modified terthiophene exhibiting charge transfer and polarization through the short molecular axis. Joint spectroscopic and theoretical study

We present a new terthiophene derivative substituted at the thienyl beta positions with a naphthalenediimide functionalization. The UV-vis absorption and emission spectroscopic properties as well as the electrochemical properties have been discussed to describe its electronic structure. The vibrational Raman data are used to inspect the molecular architecture. All of the experimental data are supported by quantum chemical calculations with different approaches. A close comparison with other terthiophenes available in the literature is conducted, always stressing the effect of the relative orientation of the donor and acceptor groups either with long-axis or short-axis polarizations. The electronic structure of the molecule has been understood in terms of the HOMO-LUMO absolute energy values. A considerable reduction of the band gap from the constituting units to the studied molecule is detected, although electronic interaction is not optimal for this configuration. Fluorescence quenching is interpreted by the possibility of intersystem crossing and internal conversion. Finally, Raman spectroscopy gives additional information of the distribution of conjugation along the molecular domain, of subtle conformational effects, and of the intermolecular interaction by means of temperature-dependent Raman data. This study provides guidelines for controlling the electronic structure and for exploring new strategies pursuing improved dyes.     

Controlling conformations in alternating dialkylsilylene-spaced donor-acceptor copolymers by a cooperative Thorpe-Ingold effect and polymer folding

A series of dialkylsilylene-spaced copolymers 6 and 7, which contain Me(2)Si and iPr(2)Si spacer groups, respectively, and have alternating donor and acceptor chromophores, have been designed and regioselectively synthesized by hydrosilylation. The ratio of the donor and acceptor chromophores for each repeat unit is 2:1, and the two donor chromophores are linked by a trimethylene bridge. A 4-aminostyrene moiety is used as the donor and a series of acceptor chromophores with different reduction potentials are employed. Both steady-state and kinetic measurements reveal that the photoinduced electron transfer (PET) in 6 obeyed the Marcus theory in which normal and inverted regions are observed. On the other hand, the iPr(2)Si-spaced copolymers 7 exhibit absorption and emission from the charge-transfer complexes exclusively due to ground-state interactions between the donor and acceptor chromophores. The discrepancy in photophysical behavior may have arisen from the difference in distance between the adjacent donor and acceptor chromophores. The bulkiness of the substituents on the silicon atom (i.e., Me versus iPr) may exert the Thorpe-Ingold effect on the local conformation around the silicon atom. The differences in the small energetic barriers for each of the conformational states may be amplified by extending the distance of the folding structure, which results in perturbing the conformation of the polymers. These results suggest that the electronic interactions between adjacent donor-acceptor pairs in these copolymers are controlled by the synchronization of the substitution effect and corresponding polymeric structures.     

Bifunctional charge transfer operated fluorescent probes with acceptor and donor receptors. 2. Bifunctional cation coordination behavior of biphenyl-type sensor molecules incorporating 2,2':6',2' '-terpyridine acceptors

Based on donor (D)-acceptor (A) biphenyl (b) type molecules, a family of fluorescent reporters with integrated acceptor receptors and noncoordinating and coordinating donor substituents of varying strength has been designed for ratiometric emission sensing and multimodal signaling of metal ions and protons. In part 2 of this series on such charge transfer (CT) operated mono- and bifunctional fluorescent devices, the cation coordination behavior of the sensor molecules bpb-R equipped with a proton- and cation-responsive 2,2':6',2' '-terpyridine (bp) acceptor and either amino-type donor receptors (R = DMA, A15C5 = monoaza-15-crown-5) or nonbinding substituents (R = CF(3), H, OMe) is investigated employing the representative metal ions Na(I), Ca(II), Zn(II), Hg(II), and Cu(II) and steady-state and time-resolved fluorometry. The bpb-R molecules, the spectroscopic behavior and protonation behavior of which have been detailed in part 1 of this series, present rare examples for CT-operated bifunctional fluorescent probes that can undergo consecutive and/or simultaneous analyte recognition. The analyte-mediated change of the probes' intramolecular CT processes yields complexation site- and analyte-specific outputs, i.e., absorption and fluorescence modulations in energy, intensity, and lifetime. As revealed by the photophysical studies of the cation complexes of these fluoroionophores and the comparison to other neutral and charged D-A biphenyls, the spectroscopic properties of the acceptor chelates of bpb-R and A- and D-coordinated bpb-R are governed by CT control of an excited-state barrier toward formation of a forbidden charge transfer state, by the switching between analytically favorable anti-energy and common energy gap law type behavior, and by the electronic nature of the ligated metal ion. This accounts for the astonishingly high fluorescence quantum yields of the acceptor chelates of bpb-R equipped with weak or medium-sized donors and the red emission of D- and A-coordinated bpb-R observed for nonquenching metal ions.     

Enhancement the photovoltaic performance of conjugated polymer based on simple head-to-head alkylthio side chains engineered bithiophene

In this article, three novel and simple molecular structure with donor-acceptor (D-A) type copolymers via only head-to-head alkoxy (OR) and/or alkylthio (SR) side chains onto the bithiophene (BT) as donor units and fluorinated benzotriazole (FBTA) as acceptor unit, namely, PBTOR-FBTA, PBTOSR-FBTA and PBTSR-FBTA, were successfully designed and synthesized. The impacts of sulfur-oxygen (S⋯O) or sulfur-sulfur (S⋯S) noncovalent interactions on their physicochemical properties, molecular stacking, carrier mobility, morphologies of blend films, as well as their photovoltaic performance were deeply and systematically studied. The introduction of SR side-chains suddenly lowered the highest occupied molecular orbital (HOMO) energy levels, blue-shifted absorption, enhanced π-π stacking, as well as improved morphology of the photoactive layer blends in comparison with the reference polymer without SR side-chain. Polymer solar cells (PSCs) were fabricated to estimate their photovoltaic performance of the polymers. Under an optimized blend ratio of PBTSR-FBTA:PC₇₁BM (1:0.8, w/w), the PBTSR-FBTA-based device exhibits a higher power conversion efficiency (PCE) of 6.25%, which is about 3.34 and 1.87 folds than that of the PBTOR-FBTA and PBTOSR-FBTA-based devices, respectively. Our research results demonstrate that the modification of the simple and low-cost SR side chains is an effective strategy to improve the photovoltaic performance of the polymers.     

Phenothiazine-phenylquinoline donor-acceptor molecules: effects of structural isomerism on charge transfer photophysics and electroluminescence

Large differences in the intramolecular charge-transfer fluorescence quantum yields and electroluminescence efficiencies were observed among the isomeric donor-acceptor molecules 2-(4-phenyl-2-quinolyl)-10-methylphenothiazine (2PQMPT) and 3-(4-phenyl-2-quinolyl)-10-methylphenothiazine (3PQMPT). In solution, the 2PQMPT isomer had a larger positive solvatochromism and thus a greater degree of charge transfer, whereas 3PQMPT had a larger fluorescence quantum yield (71%) compared to 2PQMPT (46%). High brightness (23750 cd/m(2)) and high efficiency (8.18 cd/A, 4.45 lm/W, 2.42% external quantum efficiency at 1015 cd/m(2)) green electroluminescence was achieved from 3PQMPT diodes. In contrast, green light-emitting diodes with lower brightness (8900 cd/m(2)) and efficiencies (4.79 cd/A, 2.36 lm/W, 1.41% external quantum efficiency at 690 cd/m(2)) were obtained from 2PQMPT. The two isomeric donor-acceptor molecules had identical HOMO (5.1 eV) and LUMO (2.4 eV) energy levels derived from electrochemistry. Density functional theory (DFT) calculations provided insights into the molecular geometry, electronic structures, and properties of the donor-acceptor isomers. These results demonstrate the pronounced influence of the donor/acceptor connection on the charge-transfer emission efficiency of donor-acceptor molecules and the performance of solid-state light-emitting devices based on them.     

Photoinduced charge generation and recombination dynamics in model donor/acceptor pairs for organic solar cell applications: a full quantum-chemical treatment

This work focuses on two fundamental processes in organic solar cells-exciton dissociation and charge recombination-and describes how quantum-chemical calculations can be exploited to estimate the molecular parameters that determine the rates of these processes. The general concepts behind our approach are illustrated by considering a donor-acceptor complex made of a phthalocyanine (electron donor) molecule and a perylene (acceptor) molecule. The results highlight how the relative rates of the two processes depend on the dimensionality of the molecules, their relative positions, the symmetry of the relevant electronic levels, and the polarity of the medium. It is shown, for instance, that highly symmetric configurations of the complex can strongly limit charge recombination; this emphasizes the need for a fine control of the supramolecular organization at organic-organic interfaces in donor-acceptor blends.