State Charge Transfer Complex Research Articles

Critical dopant concentrations govern integer and fractional charge-transfer phases in doped P3HT

The conjugated polymer poly(3-hexylthiophene) (P3HT) p-doped with the strong acceptor tetrafluorotetracyanoquinodimethane (F4TCNQ) is known to undergo ion-pair (IPA) formation, i.e. integer-charge transfer, and, as only recently reported, can form ground state charge-transfer complexes (CPXs) as a competing process, yielding fractional charge transfer. As these fundamental charge-transfer phenomena differently affect doping efficiency and, thus, organic-semiconductor device performance, possible factors governing their occurrence have been under investigation ever since. Here, we focus on the role of a critical dopant concentration deciding over IPA- or CPX-dominated regimes. Employing a broad, multi-technique approach, we compare the doping of P3HT by F4TCNQ and its weaker derivatives F2TCNQ, FTCNQ, and TCNQ, combining experiments with semi-classical modeling. IPA, CPX, and neutral-dopant ratios (estimated from vibrational absorption spectroscopy) together with electron affinity and ionization energy values (deduced from cyclic voltammetry) allow calculating the width of a Gaussian density of states (DOS) relating to the highest occupied molecular orbital in P3HT. While a broader DOS indicates energetic disorder, we use grazing-incidence x-ray diffraction to assess spatial order. Our findings consider the proposal of nucleation driving IPA formation and we hypothesize a certain host-dopant stoichiometry to be key for the formation of a crystalline CPX phase.

Single‐Solution Doping Enabling Dominant Integer Charge Transfer for Synergistically Improved Carrier Concentration and Mobility in Donor–Acceptor Polymers

AbstractChemical doping of donor–acceptor (D–A) polymers is essential for their usage in highly efficient optoelectronic devices. The crucial challenge remains to synergistically improve the carrier concentration and mobility of these polymers via a single solution doping method. Here, a D–A polymer, Pg32T‐OTz is designed and synthesized, containing a weak‐acceptor–strong‐donor backbone with nonpolar (alkoxy) and polar (ethylene glycol) side chains. Pure integer charge transfer (ICT) is shown to occur in 2,3,5,6‐tetrafluoro‐tetracyanoquinodimethane (F4TCNQ)‐doped D–A polymer at a low doping level. It is shown that the ordered edge‐on orientations of ICT structures overcome the Coulomb interactions and endow a long‐range hole delocalization, while few charge transfer complex states form in amorphous regions and bridge the crystalline ICT structures at high doping level, creating a network that sustains efficient charge transport. As a result, simultaneously high carrier concentration and high mobility are realized for F4TCNQ‐doped Pg32T‐OTz and thus a high electrical conductivity up to 550 S cm−1 is approached, which is the highest value among any doped polymers via a single‐solution doping process. This work demonstrates that the modulation of the acceptor strength combined with side‐chain engineering is an effective molecular design strategy to promote both the doping efficiency and carrier transport property of D–A polymers.

W196 and the β-Hairpin Motif Modulate the Redox Switch of Conformation and the Biomolecular Interaction Network of the Apoptosis-Inducing Factor.

The human apoptosis-inducing factor (hAIF) is a moonlight flavoprotein involved in mitochondrial respiratory complex assembly and caspase-independent programmed cell death. These functions might be modulated by its redox-linked structural transition that enables hAIF to act as a NAD(H/+) redox sensor. Upon reduction with NADH, hAIF undergoes a conformational reorganization in two specific insertions—the flexible regulatory C-loop and the 190-202 β-harpin—promoting protein dimerization and the stabilization of a long-life charge transfer complex (CTC) that modulates its monomer-dimer equilibrium and its protein interaction network in healthy mitochondria. In this regard, here, we investigated the precise function of the β-hairpin in the AIF conformation landscape related to its redox mechanism, by analyzing the role played by W196, a key residue in the interaction of this motif with the regulatory C-loop. Mutations at W196 decrease the compactness and stability of the oxidized hAIF, indicating that the β-hairpin and C-loop coupling contribute to protein stability. Kinetic studies complemented with computational simulations reveal that W196 and the β-hairpin conformation modulate the low efficiency of hAIF as NADH oxidoreductase, contributing to configure its active site in a noncompetent geometry for hydride transfer and to stabilize the CTC state by enhancing the affinity for NAD+. Finally, the β-hairpin motif contributes to define the conformation of AIF's interaction surfaces with its physiological partners. These findings improve our understanding on the molecular basis of hAIF's cellular activities, a crucial aspect for clarifying its associated pathological mechanisms and developing new molecular therapies.

Spectroscopy and quantum-chemical calculations of nitro-bis-bipyridyl complexes of ruthenium(II) with 4-substituted pyridine ligands

The luminescence, absorption, and luminescence excitation spectra of complexes cis-[Ru(bpy)2(L)(NO2)]+ [bpy = 2,2′-bipyridyl, L = pyridine, 4-aminopyridine, 4-dimethylaminopyridine, 4-picoline, isonicotinamide, or 4,4′-bipyridyl] in alcoholic (4 : 1 EtOH–MeOH) solutions are studied at 77 K. A linear correlation is established between the energy of the lowest electronically excited metal-toligand charge transfer state d π(Ru) → π*(bpy) of the complexes and the pKa parameter of the free 4-substituted pyridines used as ligands L. The B3LYP/[6-31G(d)+LanL2DZ(Ru)] hybrid density functional method is used to optimize the geometry of complexes and calculate their electronic structure and the charge distribution on the atoms of the nearest environment of ruthenium(II) ions. It is shown that there exists a mutually unambiguous correspondence between the charge on the nitrogen atom of ligands L coordinated in the complex and the pK a parameter of ligands. The calculated energies of the electronically excited metal-to-ligand charge transfer states of complexes linearly (correlation coefficient 0.99) depend on the charge on the nitrogen atom of ligands L, which completely agrees with the experimental data.

Photophysical Studies on Covalently-linked Naphthalene and TEMPO Free Radical Systems: Observation of a Charge Transfer State in the Ground State.

A series of molecules containing a naphthalene chromophore and a stable free radical 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) covalently linked by a spacer group of different lengths have been synthesized. In n-hexane solution, their photophysical behavior was studied and compared with a system of freely moving naphthalene and the free radical TEMPO. The linked molecules showed strong quenching of the singlet and triplet states of the naphthalene moiety, compared to when naphthalene and TEMPO were not linked. The quenching efficiency decreased with increasing the length of the spacer group. In addition, new electronic absorption and emission bands, along with the usual bands of the individual moieties, were also seen. These news bands have been attributed to the formation of electron donor-acceptor charge-transfer complexes in the ground state, arising from the interaction between the two moieties in close proximity. The photophysical dynamics of the linked molecules has been rationalized by assuming the existence of two types of population of the linked molecules: folded and extended. The ground state complex formation is proposed to occur only in the folded conformation of the linked molecules. To our knowledge, this is possibly the first example of a ground state charge-transfer complex formation involving a TEMPO free radical and naphthalene.

Thin Film Photovoltaics with Organic Metal Halide Peroviskites

Organic metal halide perovskites have now emerged as new and promising candidates for developing low cost thin film solar cells with efficiencies in the range of 20.1% certified efficiency. The recent advances of CH3NH3PbI3 based thin film solar cells have manly focused on the design architecture and improving the stability. Both TiO2 and Al2O3 mesoscopic architectures provide robust support to design high efficiency thin film solar cells. The excited state dynamics leading to the charge separation remains to be fully understood. Through comparative spectral analysis between the absorption features of charge transfer complexes in the solution phase and the final solid-state perovskite films, we are able to fully classify the absorption features in the excited state of CH3NH3PbI3 across the transient absorption spectrum recorded following laser pulse excitation. The excited state of CH3NH3PbI3 characterized through transient absorption spectroscopy show dual behavior, viz., charge transfer complex state and charge separated state.1,2 The interplay between these two different excited states leads to efficient charge separation. Insights into the charge recombination kinetics and band filling aspects of CH3NH3PbI3 provide new ways to tune the efficiency of perovskite solar cells. These spectroscopic studies provide further insight into the optoelectronic behavior of methyl ammonium lead iodide films. Excited state properties that elucidate the exceptional photovoltaic properties will be discussed. Acknowledgments The research described herein is supported by the Office of Basic Energy Sciences, U.S. Department of Energy. Selected Publications Christians, J. A.; Fung, R.; Kamat, P. V., An Inorganic Hole Conductor for Organo-Lead Halide Perovskite Solar Cells. Improved Hole Conductivity with Copper Iodide. J. Am. Chem. Soc. 2014, 136, 758–764.Manser, J. S.; Kamat, P. V., Band Filling with Charge Carriers in Organometal Halide Perovskites. Nature Photonics 2014, DOI: 10.1038/NPHOTON.2014.1171.

Turn-on fluorescence probes based on pyranine/viologen charge-transfer complexes for the determination of nucleotides

The formation of ground state charge-transfer complexes between pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid) and viologen (paraquat) derivatives is utilized for the design of novel fluoroionophores for the determination of phosphate species, particularly of nucleotides. The strong quenching of the pyranine fluorescence by viologen-type charge transfer acceptors can be countermanded if these are functionalized with triethylammonium groups that serve as recognition elements for phosphate anions. We report on the fluorogenic responses of these water-soluble molecular probes in presence of different phosphates. Absorbance measurements give additional information on the charge transfer complex formation and the interaction with nucleotides. The experimental data show that these aggregates form attractive, simple and versatile fluorescence turn-on probes for nucleoside triphosphates. The reversibility of the fluorescence response is demonstrated by means of an enzymatic model assay using ATPase for the decomposition of adenosine triphosphate.

Orientation-Dependent Electronic Structures and Optical Properties of the P3HT:PCBM Interface: A First-Principles GW-BSE Study

In this work, the effect of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) orientation on the electronic and optical properties of the regioregular poly(3-hexylthiophene) (P3HT):PCBM blend interface was studied by means of first-principles density functional theory calculations with G0W0 approximation plus the Bethe–Salpeter equation (BSE). The band structures and photoresponse are shown to depend on the PCBM orientation. The origin of the two main optical absorption peaks is determined, and the effect of PCBM rotation on optical properties is revealed. The calculated lowest charge transfer complex state energy, exciton binding energy, and the absorption spectrum for the flat-lying model are in good agreement with the experimental values, which indicates the flat-lying structure is the predominant interface structure in the experiments. The lowest-energy configuration is also determined as the flat-lying orientation in our calculations. Our results further suggest that the dissociation of excitons and charge transfer at the interface is more efficient for the PCBM flat-lying orientation than that for the upright-standing one, which provides a possible explanation for the increased performance of the P3HT:PCBM devices after a thermal annealing treatment.

Photocurrent spectroscopic studies of diketopyrrolopyrrole-based statistical copolymers

Diketopyrrolopyrrole (DPP) containing copolymers have gained a lot of interest in organic optoelectronics with great potential in organic photovoltaics. In this work, DPP based statistical copolymers, with slightly different bandgap energies and a varying fraction of donor-acceptor ratio are investigated using monochromatic photocurrent spectroscopy and Fourier-transform photocurrent spectroscopy (FTPS). The statistical copolymer with a lower DPP fraction, when blended with a fullerene derivative, shows the signature of an inter charge transfer complex state in photocurrent spectroscopy. Furthermore, the absorption spectrum of the blended sample with a lower DPP fraction is seen to change as a function of an external bias, qualitatively similar to the quantum confined Stark effect, from where we estimate the exciton binding energy. The statistical copolymer with a higher DPP fraction shows no signal of the inter charge transfer states and yields a higher external quantum efficiency in a photovoltaic structure. In order to gain insight into the origin of the observed charge transfer transitions, we present theoretical studies using density-functional theory and time-dependent density-functional theory for the two pristine DPP based statistical monomers.

Identifying the magnetoconductance responses by the induced charge transfer complex states in pentacene-based diodes

We investigate the magnetoconductance (MC) responses in photocurrent, unipolar injection, and bipolar injection regimes in pentacene-based diodes. Both photocurrent and bipolar injection contributed MC responses show large difference in MC line shape, which are attributed to triplet-polaron interaction modulated by the magnetic field dependent singlet fission and the intersystem crossing of the polaron pair, respectively. By blending 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane into pentacene, all the MC responses are suppressed but the MC response at unipolar injection regime is enhanced, which is attributed to the induced charge transfer complex states (CT complex states). This work identify the MC responses between single carrier contributed MC and exciton related MC by the induced CT complex states.

Why are the Interaction Energies of Charge-Transfer Complexes Challenging for DFT?

The description of ground state charge-transfer complexes is highly challenging. Illustrative examples include large overestimations of charge-transfer by local and semilocal density functional approximations as well as inaccurate binding energies. It is demonstrated here that standard density functionals fail to accurately describe interaction energies of charge-transfer complexes not only because of the missing long-range exchange as generally assumed but also as a result of the neglect of weak interactions. Thus, accounting for the missing van der Waals interactions is of key importance. These assertions, based on the evaluation of the extent of stabilization due to dispersion using both DFT coupled with our recent density-dependent dispersion correction (dDsC) and high-level ab initio computations, reflect the imperfect error-cancellation between the overestimation of charge-transfer and the missing long-range interactions. An in-depth energy decomposition analysis of an illustrative series of four small ambidentate molecules (HCN, HNC, HF, and ClF) bound together with NF3 provides the main conclusions, which are validated on a prototypical organic charge-transfer complex (i.e., tetrathiafulvalene-tetracyanoquinodimethane, TTF-TCNQ). We establish that the interaction energies for charge-transfer complexes can only be properly described when using well-balanced functionals such as PBE0-dDsC, M06-2X, and LC-BOP-LRD.

Photoexcitation dynamics in polythiophene/fullerene blends for photovoltaic applications

We used the transient and steady state photomodulation spectroscopies for studying the photoexcitations dynamics in blends of regio-regular poly(3-hexylthiophene) (RR-P3HT) and fullerene in a broad spectral range from 0.13 to 2.25eV. We found that both localized polarons and singlet excitons are instantaneously photogenerated in the blends. However the photogeneration process of delocalized polarons which contribute to the photocurrent proceeds in two steps: first, within a couple of ps the excitons generated in the polymer domains populate the charge transfer complex states at the RR-P3HT/fullerene interfaces; this is followed by the charge transfer ionization into delocalized charge polarons in the polymer and fullerene constituents within ∼20ps. In contrast, the localized polaron dynamics are unrelated with the excitons and delocalized polarons dynamics. We also report on the occurrence of ultrafast quantum interference anti-resonances between photoinduced infrared-active vibrations and the delocalized polaron band in the blends, which shows the delocalization character of the photogenerated charges that contribute to the photocurrent.

Charge transfer complex states in diketopyrrolopyrrole polymers and fullerene blends: Implications for organic solar cell efficiency

The spectral photocurrent characteristics of two donor-acceptor diketopyrrolopyrrole (DPP)-based copolymers (PDPP-BBT and TDPP-BBT) blended with a fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester (PCBM) were studied using Fourier-transform photocurrent spectroscopy (FTPS) and monochromatic photocurrent (PC) method. PDPP-BBT:PCBM shows the onset of the lowest charge transfer complex (CTC) state at 1.42 eV, whereas TDPP-BBT:PCBM shows no evidence of the formation of a midgap CTC state. The FTPS and PC spectra of P3HT:PCBM are also compared. The larger singlet state energy difference of TDPP-BBT and PCBM compared to PDPP-BBT/P3HT and PCBM obliterates the formation of a midgap CTC state resulting in an enhanced photovoltaic efficiency over PDPP-BBT:PCBM.

Temporal and thermal properties of optically induced instabilities in P3HT field-effect transistors

In this contribution we report on temporal and thermal properties of an optically induced instability in poly(3-hexylthiophene) (P3HT) based organic field-effect transistors (OFET) fabricated on a flexible polyethylene terephthalate (PET) substrate. By illuminating depleted p-type top-gate P3HT field-effect transistors with visible light a substantial shift of the threshold-voltage of up to +20V and an increase in the off-current by three orders of magnitude has been observed. Both phenomena, the threshold-voltage shift and the increase of the off-current, require the presence of oxygen and are persistent for days at room temperature. The effect is explained by the formation of a charge-transfer-complex (CTC) of P3HT and oxygen known from literature to act as an electron trap. Here, we discuss the temporal and thermal stability of the detrapping of photo-generated charge carriers from such traps and the loss of trapping sites by diffusion of oxygen out of the P3HT layer. It is demonstrated that the thermally activated detrapping of electrons from the oxygen induced CTC states is much faster than the removal of the oxygen induced traps.

Optical studies of the charge transfer complex in polythiophene/fullerene blends for organic photovoltaic applications

We studied the photophysics of regioregular polythiophene/${\text{C}}_{61}$ (RR-P3HT/PCBM) blend films utilized for organic photovoltaic applications using the femtosecond transient and steady-state photomodulation techniques with above-gap and below-gap pump excitations and electroabsorption spectroscopy. We provide strong evidence for the existence of charge transfer complex (CTC) state in the blend that is formed deep inside the optical gap of the polymer and fullerene constituents, which is clearly revealed in the electroabsorption spectrum with an onset at 1.2 eV. We identify this ``midgap'' band as the lowest lying CTC state formed at the interfaces separating the polymer and fullerene phases. With above-gap pump excitation the primary photoexcitations in the blend are excitons and polarons in the polymer domains that are generated within the experimental time resolution (150 fs), having distinguishable photoinduced absorption (PA) bands in the mid-IR. The photogenerated excitons subsequently decay within $\ensuremath{\sim}10\text{ }\text{ps}$, consistent with the polymer weak photoluminescence in the blend. In contrast, with below-gap pump excitation, a new PA band in the mid-IR is generated within our time resolution, which is associated with photogenerated species that decay into polarons at much later times; also no PA of excitons is observed. We interpret the photoexcitations as CT excitons, which with below-gap pump excitation are resonantly generated on the CTC states at the interfaces, as the first step for polaron generation, without involving intrachain excitons in the polymer phase. We found that the polarons generated with below-gap pump excitation are trapped at the interfaces with relatively long lifetime, and thus may generate polarons on the polymer chains and fullerene molecules with a different mechanism than with above-gap excitation. In any case the interfacial polarons generated with below-gap excitation do not substantially contribute to the photocurrent density in photovoltaic applications because of the relatively thin active layer using the traditional bulk heterojunctions design. This is shown by comparing the charge photogeneration efficiency spectrum measured on a fabricated photovoltaic solar cell with the polaron photogeneration action spectrum measured on a thick blend film.

Photoinduced electron transfer between hen egg white lysozyme and anticancer drug menadione

The protein, hen egg white lysozyme, on photoexcitation undergoes electron transfer with menadione (2-methyl-1,4-naphthoquinone), a widely known anticancer drug. With the addition of menadione the fluorescence of lysozyme is quenched with the simultaneous formation of an excited state charge-transfer complex in the longer wavelength and a ground state complex. The former is further evident from laser flash photolysis studies, which indicate a tryptophan to menadione electron transfer. From fluorescence quenching studies the binding constant is found to be ∼1.7×10 4 M −1 with the corresponding changes in enthalpy (Δ H°) and entropy (Δ S°) as 12.24 kJ mol −1 and 124.12 J mol −1 K −1, respectively, indicative of an entropy-driven process. The circular dichroism studies also show some structural changes with increase in α-helical content in the protein on interaction with menadione. Finally, docking studies give some insight into the role of Trp 108 of lysozyme in the interaction.

Revisiting aromatic thiols effects on radical photopolymerization

Influence of thiols on kinetics of acrylate radical photopolymerization in the presence and absence of photoinitiators was studied. The kinetics was monitored by infrared absorption and photo-differential scanning calorimetry. It was found that aromatic thiols at concentrations, <0.5% (∼0.05 M), can accelerate radical photopolymerization. Initiation of radical photopolymerization by some aromatic thiols in the absence of conventional photoinitiators was observed. As well as expected inhibition of photopolymerization at higher concentration of aromatic thiols due to chain transfer was detected. A ground state charge-transfer complex formation between thiols and benzoin based photoinitiators was detected. Mechanism of thiols participation in photopolymerization involving reduction of oxygen inhibition and dissociation of charge-transfer complexes is discussed.

C60 fullerene and its molecular complexes under axial and shear deformation

We have studied the pristine C60 and its molecular complexes with the organicdonors bis(ethylenedithio) tetrathiafulvalene (BEDT-TTF or ET) andtetramethyltetraselenafulvalene (TMTSF) by means of ESR and Ramanspectroscopy at high pressure. The important changes in the ESR signal of C60were observed under axial pressure combined with shear deformation. It is shownthat the treatment at a anisotropic pressure of 4 GPa results in a reduction in thesymmetry of the C60 molecule and the formation of radicals. Treatment of themolecular complex of (ET)2·C60 at a pressure of ∼4.5 GPa and a temperature of150°C leads to the formation of C60 dimers. The Raman spectra of themolecular complex C60·TMTSF·2(CS2) were measured in situ at ambienttemperature and pressures up to 9.5 GPa. The pressure behaviour of the Ramanpeaks reveals singularity at 5.0 ± 0.5 GPa related to the softening andsplitting of some of the phonon modes. The residual softening of the Ag (2)mode is the same as in the case of KC60, which may be an indication that thetransition has a charge-transfer character, resulting in the formation of theC60−1anionic state charge-transfer complex.

Charge-transfer reactions of C 60 in surfactant-based complex fluid media

Surfactant-based complex fluid media enables the investigation of charge-transfer reactions between a hydrophobic C 60 molecule and hydrophilic solutes. The formation of a ground state charge-transfer complex between C 60 and ethylamine was demonstrated in an ionic surfactant/water/oil microemulsion, the value of the complexation of K = 1.1 M −1 was determined. The one-electron reduction of fullerene occurs in reaction with the excess electrons stored on nanometer-sized metal(Ag) or quantized semiconductor (TiO 2) particles in microemulsions.

Observation of a donor exciton band in silicon

Far-infrared transmission measurements on P-doped Si at donor concentrations below the metal-insulator transition show absorption bands from donor pairs. We identify the lowest energy band as charge transfer excitations from a donor to its neigbhor, forming excitons (D +D -) in the Mott-Hubbard gap. The results suggest that charge transfer states in complexes dominate the low-energy optical excitations as the insulator approaches the Mott density.