32 research outputs found
Threshold-like Complexation of Conjugated Polymers with Small Molecule Acceptors in Solution within the Neighbor-Effect Model
In some donor-acceptor blends based on conjugated polymers, a pronounced charge-transfer complex (CTC) forms in the electronic ground state. In contrast to small-molecule donor-acceptor blends, the CTC concentration in polymer: acceptor solution can increase with the acceptor content in a threshold-like way. This threshold-like behavior was earlier attributed to the neighbor effect (NE) in the polymer complexation, i.e., next CTCs are preferentially formed near the existing ones; however, the NE origin is unknown. To address the factors affecting the NE, we record the optical absorption data for blends of the most studied conjugated polymers, poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and poly(3-hexylthiophene) (P3HT), with electron acceptors of fluorene series, 1,8-dinitro-9,10-antraquinone (DNAQ), and 7,7,8,8-tetracyanoquinodimethane (TCNQ) in different solvents, and then analyze the data within the NE model. We have found that the NE depends on the polymer and acceptor molecular skeletons and solvent, while it does not depend on the acceptor electron affinity and polymer concentration. We conclude that the NE operates within a single macromolecule and stems from planarization of the polymer chain involved in the CTC with an acceptor molecule; as a result, the probability of further complexation with the next acceptor molecules at the adjacent repeat units increases. The steric and electronic microscopic mechanisms of NE are discussed
Spectroscopic assessment of charge mobility in organic semiconductors
Rapid progress in organic electronics demands new highly efficient organic
semiconducting materials. Nevertheless, only few materials have been created so
far that show reliable band-like transport with high charge mobilities, which
reflects the two main obstacles in the field: the poor understanding of charge
transport in organic semiconductors (OSs) and the difficulty of its
quantification in devices. Here, we present a spectroscopic method for
assessment of the charge transport in organic semiconductors. We show that the
intensities of the low-frequency Raman spectrum allow calculation of the
dynamic disorder that limits the charge carrier mobility. The spectroscopically
evaluated mobility clearly correlates with the device charge mobility reported
for various OSs. The proposed spectroscopic method can serve as a powerful tool
for a focused search of new materials and highlights the disorder bottleneck in
the intrinsic charge transport in high-mobility organic semiconductors
Walking around Ribosomal Small Subunit: A Possible âTourist Mapâ for Electron Holes
Despite several decades of research, the physics underlying translationâprotein synthesis at the ribosomeâremains poorly studied. For instance, the mechanism coordinating various events occurring in distant parts of the ribosome is unknown. Very recently, we suggested that this allosteric mechanism could be based on the transport of electric charges (electron holes) along RNA molecules and localization of these charges in the functionally important areas; this assumption was justified using tRNA as an example. In this study, we turn to the ribosome and show computationally that holes can also efficiently migrate within the whole ribosomal small subunit (SSU). The potential sites of charge localization in SSU are revealed, and it is shown that most of them are located in the functionally important areas of the ribosomeâintersubunit bridges, Fe4S4 cluster, and the pivot linking the SSU head to its body. As a result, we suppose that hole localization within the SSU can affect intersubunit rotation (ratcheting) and SSU head swiveling, in agreement with the scenario of electronic coordination of ribosome operation. We anticipate that our findings will improve the understanding of the translation process and advance molecular biology and medicine
Tuning of Molecular Electrostatic Potential Enables Efficient Charge Transport in Crystalline Azaacenes: A Computational Study
Neighbor Effect in Complexation of a Conjugated Polymer
Charge-transfer
complex (CTC) formation between a conjugated polymer and low-molecular-weight
organic acceptor is proposed to be driven by the neighbor effect.
Formation of a CTC on the polymer chain results in an increased probability
of new CTC formation near the existing one. We present an analytical
model for CTC distribution considering the neighbor effect, based
on the principles of statistical mechanics. This model explains the
experimentally observed threshold-like dependence of the CTC concentration
on the acceptor content in a polymer:acceptor blend. It also allows
us to evaluate binding energies of the complexes
Charge Transport in Organic Semiconducting Crystals Exhibiting TADF: Insight from Quantum Chemical Calculations
Luminophores featuring thermally activated delayed fluorescence (TADF) are the workhorses of the third- and fourth-generation OLEDs. While these compounds have usually been used as dopants embedded in the host, non-doped TADF OLEDs have recently shown significant progress as well and have attained performances comparable to those of the host-dopant systems. For efficient operation of non-doped OLEDs, the charge transport in neat films and single crystals of TADF luminophores is important; however, this issue was nearly unexplored theoretically. In the current study, we calculated the charge-carrier mobilities in four single crystals of TADF luminophores that have different molecular packing motifs. Specifically, in one of them both the donor and acceptor moieties form uniform Ï-stacks, while in the others the donors (acceptors) show alternating lateral shifts along the stacks; the difference in the molecular packing resulted in the difference in the transfer integrals between the molecules. The reorganization energies differed as well by up to four times for the studied crystals. As a result, the charge mobilities varied from 0.001 to ~0.3 cm2/(Vâs), with the largest being predicted for the crystal of the luminophore that consisted of a rigid donor and acceptor. We anticipate that the results obtained will be useful in the design of TADF luminophores for non-doped OLEDs, OLETs, and other organic light-emitting devices
High-Mobility Naphthalene Diimide Derivatives Revealed by Raman-Based In Silico Screening
Charge transport in crystalline organic semiconductors (OSCs) is considerably hindered by low-frequency vibrations introducing dynamic disorder in the charge transfer integrals. Recently, we have shown that the contributions of various vibrational modes to the dynamic disorder correlate with their Raman intensities and suggested a Raman-based approach for estimation of the dynamic disorder and search for potentially high-mobility OSCs. In the present paper, we showcase this approach by revealing the highest-mobility OSC(s) in two series of crystalline naphthalene diimide derivatives bearing alkyl or cycloalkyl substituents. In contrast to our previous studies, Raman spectra are not measured, but are instead calculated using periodic DFT. As a result, an OSC with a potentially high charge mobility is revealed in each of the two series, and further mobility calculations corroborate this choice. Namely, for the naphthalene diimide derivatives with butyl and cyclopentyl substituents, the estimated room-temperature isotropic electron mobilities are as high as 6 and 15 cm2 V–1 s–1, respectively, in the latter case even exceeding 20 cm2 V–1 s–1 in a two-dimensional plane. Thus, our results highlight the potential of using the calculated Raman spectra to search for high-mobility crystalline OSCs and reveal two promising OSCs, which were previously overlooked
Threshold-like complexation of conjugated polymers with small molecule acceptors in solution within the neighbor-effect model
Intrachain Aggregation of Charge-Transfer Complexes in Conjugated Polymer:Acceptor Blends from Photoluminescence Quenching
Recent
studies of conjugated polymer donorâacceptor blends
show that the donor and acceptor can form a weak charge-transfer complex
(CTC) in the electronic ground state, and these CTCs can significantly
change the photophysics in the blend. In this work, we study photoluminescence
quenching in model polymer acceptor blends of polyÂ(methoxy,5-(2âČ-ethyl-hexyloxy-1,4-phenylene-vinylene))
(MEH-PPV) with TNF (2,4,7-trinitrofluorenone) in solution. Our experimental
data show that the observed strong increase in the CTC concentration
with acceptor content results only in a moderate quenching enhancement.
We propose an extended SternâVolmer relation to model photoluminescence
quenching in conjugated polymers with statistically homogeneous distribution
of CTCs over polymer chains. We compare the experimental data with
the model and conclude that the CTCs are not randomly distributed
within a chain but form intrachain CTC clusters. These findings imply
that the CTCs can influence the morphology of donorâacceptor
blends, which is of paramount importance for the performance of organic
solar cells
Inhibiting Low-Frequency Vibrations Explains Exceptionally High Electron Mobility in 2,5-Difluoro-7,7,8,8-tetracyanoquinodimethane (F<sub>2</sub>âTCNQ) Single Crystals
Organic
electronics requires materials with high charge mobility.
Despite decades of intensive research, charge transport in high-mobility
organic semiconductors has not been well understood. In this Letter,
we address the physical mechanism underlying the exceptionally high
band-like electron mobility in F<sub>2</sub>-TCNQ (2,5-difluoro-7,7,8,8-tetracyanoquinodimethane)
single crystals among a crystal family of similar compounds F<sub><i>n</i></sub>-TCNQ (<i>n</i> = 0, 2, 4) using
a combined experimental and theoretical approach. While electron transfer
integrals and reorganization energies did not show outstanding features
for F<sub>2</sub>-TCNQ, Raman spectroscopy and solid-state DFT indicated
that the frequency of the lowest vibrational mode is nearly twice
higher in the F<sub>2</sub>-TCNQ crystal than in TCNQ and F<sub>4</sub>-TCNQ. This phenomenon is explained by the specific packing motif
of F<sub>2</sub>-TCNQ with only one molecule per primitive cell so
that electronâphonon interaction decreases and the electron
mobility increases. We anticipate that our findings will encourage
investigators for the search and design of organic semiconductors
with one molecule per primitive cell and/or the poor low-frequency
vibrational spectrum