A theoretical description of charge reorganization energies in molecular organic P-type semiconductors

Periodically Twisted Molecular Wires Based on a Fused Unit for Efficient Intramolecular Hopping Transport

Realizing efficient long-distance intramolecular charge transport based on a hopping mechanism is a key challenge in molecular electronics. In hopping transport, a smaller reorganization energy (λ) and energy difference between hopping sites (ΔEhs) should lead to a smaller activation energy and faster charge transfer. However, the development of π-extended molecules that meet these requirements is challenging. In this study, we successfully synthesized several nanometer-scale π-extended molecules composed of a fused π-conjugated unit as a hopping site for reducing λ. Conformational twists between fused units effectively localize π-conjugation in each unit, contributing to reducing ΔEhs. The expected electronic structures of the oligomers were confirmed using spectroscopic and electrochemical measurements. Single-molecule conductance measurements exhibited higher conductance and lower activation energy than those of nonfused oligothiophenes. First-principles calculations indicated that smaller λ and ΔEhs values explain the high conductance. These results highlight the efficiency of the proposed molecular design for effective intramolecular hopping transport.

Non-adiabatic molecular dynamics simulations provide new insights into the exciton transfer in the Fenna-Matthews-Olson complex

A trajectory surface hopping approach, which uses machine learning to speed up the most time-consuming steps, has been adopted to investigate the exciton transfer in light-harvesting systems. The present neural networks achieve high accuracy in predicting both Coulomb couplings and excitation energies. The latter are predicted taking into account the environment of the pigments. Direct simulation of exciton dynamics through light-harvesting complexes on significant time scales is usually challenging due to the coupled motion of nuclear and electronic degrees of freedom in these rather large systems containing several relatively large pigments. In the present approach, however, we are able to evaluate a statistically significant number of non-adiabatic molecular dynamics trajectories with respect to exciton delocalization and exciton paths. The formalism is applied to the Fenna-Matthews-Olson complex of green sulfur bacteria, which transfers energy from the light-harvesting chlorosome to the reaction center with astonishing efficiency. The system has been studied experimentally and theoretically for decades. In total, we were able to simulate non-adiabatically more than 30 ns, sampling also the relevant space of parameters within their uncertainty. Our simulations show that the driving force supplied by the energy landscape resulting from electrostatic tuning is sufficient to funnel the energy towards site 3, from where it can be transferred to the reaction center. However, the high efficiency of transfer within a picosecond timescale can be attributed to the rather unusual properties of the BChl a molecules, resulting in very low inner and outer-sphere reorganization energies, not matched by any other organic molecule, e.g., used in organic electronics. A comparison with electron and exciton transfer in organic materials is particularly illuminating, suggesting a mechanism to explain the comparably high transfer efficiency.

Theory for nonlinear conductivity switching in semiconducting organic ferroelectrics

In this work, the ferroelectric and semiconducting properties of the organic semiconducting ferroelectric benzotrithiophene tricarboxamide (BTTTA), and especially their nonlinear coupling, are theoretically investigated. BTTTA is an exponent of a small class of semiconducting organic ferroelectrics for which experiments have established a surprising polarization direction dependence of the bulk conductivity at finite fields. First, molecular dynamics (MD) simulations are used to investigate the occurrence and, under the influence of an external electric field, the inversion of the macroscopic electric dipole that forms along the axis of supramolecular columns of BTTTA. The MD results are consistent with the experimentally observed ferroelectric behavior of the material. Building on the MD results, a QM/MM scheme is used to investigate the charge carrier mobility in the quasi-1D BTTTA stacks in the linear and non-linear regimes. Indeed, at finite electric fields, a clear resistance switching effect was observed in the form of a hole mobility that is a factor ∼2 larger for antiparallel orientations of the polarization and field than for a parallel orientation. This phenomenon can be understood as a microscopic ratchet that is based on the non-equilibrium interaction between the (oriented) dipoles and the (direction of the) charge transport.

Effect of steric hindrance and number of substituents on the transfer and interface properties of Y-shaped hole-transporting materials for perovskite solar cells

Alkyl sulfoxide groups were introduced into the branch chain terminals of a hole-transporting material (HTM) Z34 with different numbers and positions to design four new Y-shaped HTMs: ZT1, ZT2, ZT3 and ZT4. The effects of steric hindrance and number of substituents on the transfer and interface properties of the Y-shaped HTMs were investigated theoretically. Calculations reveal that the introduction of alkyl sulfoxide increases the distribution of intramolecular holes and orbital overlap between the HOMOs of the dimers. The electronic coupling was greatly improved owing to the increased distribution of holes and orbital overlap. ZT1 shows small steric hindrance when one alkyl sulfoxide is introduced into the top branch chain, which leads to translation π-π stacking. ZT2 and ZT4 show slightly greater steric hindrance when two or four alkyl sulfoxide groups are introduced into the side branch chains, which leads to face-to-face stacking. While ZT3 shows large steric hindrance when three alkyl sulfoxide groups are introduced into the top and side branch chains, which causes head-to-head stacking. With the increase in number of alkyl sulfoxide groups, the steric hindrance of the molecule increases and the hole mobility decreases. ZT1 achieves the highest hole mobility (2.63 × 10-2 m2 V-1 s-1) that is two orders of magnitude higher than that of Z34 (1.36 × 10-4 m2 V-1 s-1) owing to the optimal balance between the number of alkyl sulfoxide groups and steric hindrance. The HTM/CH3NH3PbI3 adsorbed system was also simulated to characterize the interface properties. Enhanced interface interaction was achieved in the HTM/perovskite systems of ZT2 and ZT3. The orbital distribution of the HTM/perovskite cluster indicates that the new HTMs can promote hole migration and prevent internal electron-hole recombination. The present work not only evaluates the reliable relationship between the structure and properties of new HTMs, but also provides a valuable design strategy for efficient Y-shaped HTMs.

Computational Approaches for Organic Semiconductors: From Chemical and Physical Understanding to Predicting New Materials

While a complete understanding of organic semiconductor (OSC) design principles remains elusive, computational methods─ranging from techniques based in classical and quantum mechanics to more recent data-enabled models─can complement experimental observations and provide deep physicochemical insights into OSC structure-processing-property relationships, offering new capabilities for in silico OSC discovery and design. In this Review, we trace the evolution of these computational methods and their application to OSCs, beginning with early quantum-chemical methods to investigate resonance in benzene and building to recent machine-learning (ML) techniques and their application to ever more sophisticated OSC scientific and engineering challenges. Along the way, we highlight the limitations of the methods and how sophisticated physical and mathematical frameworks have been created to overcome those limitations. We illustrate applications of these methods to a range of specific challenges in OSCs derived from π-conjugated polymers and molecules, including predicting charge-carrier transport, modeling chain conformations and bulk morphology, estimating thermomechanical properties, and describing phonons and thermal transport, to name a few. Through these examples, we demonstrate how advances in computational methods accelerate the deployment of OSCsin wide-ranging technologies, such as organic photovoltaics (OPVs), organic light-emitting diodes (OLEDs), organic thermoelectrics, organic batteries, and organic (bio)sensors. We conclude by providing an outlook for the future development of computational techniques to discover and assess the properties of high-performing OSCs with greater accuracy.

Efficient Surface Hopping Approach for Modeling Charge Transport in Organic Semiconductors

The trajectory surface hopping (TSH) method is nowadays widely applied to study the charge/exciton transport process in organic semiconductors (OSCs). In the present study, we systematically examine the performance of two approximations in the fewest switched surface hopping (FSSH) simulations for charge transport (CT) in several representative OSCs. These approximations include (i) the substitution of the nuclear velocity scaling along the nonadiabatic coupling vector (NCV) by rescaling the hopping probability with the Boltzmann factor (Boltzmann correction (BC)) and (ii) a phenomenological approach to treat the quantum feedback from the electronic system to the nuclear system (implicit charge relaxation (IR)) in the OSCs. We find that charge mobilities computed by FSSH-BC-IR are in very good agreement with the mobilities obtained by standard FSSH simulations with explicit charge relaxation (FSSH-ER), however, at reduced computational cost. A key parameter determining the charge carrier mobility is the reorganization energy, which is sensitively dependent on DFT functionals applied. By employing the IR approximation, the FSSH method allows systematic investigation of the effect of the reorganization energies obtained by different DFT functionals like B3LYP or ωB97XD on CT in OSCs. In comparison to the experiments, FSSH-BC-IR using ωB97XD reorganization energy underestimates mobilities in the low-coupling regime, which may indicate the lack of nuclear quantum effects (e.g., zero point energy (ZPE)) in the simulations. The mobilities obtained by FSSH-BC-IR using the B3LYP reorganization energy agree well with experimental values in 3 orders of magnitude. The accidental agreement may be the consequence of the underestimation of the reorganization energy by the B3LYP functional, which compensates for the neglect of nuclear ZPE in the simulations.

Synergistic effects of side-functionalization and aza-substitution on the charge transport and optical properties of perylene-based organic materials: a DFT study

The physicochemical properties of organic materials are subject to the chemical structure of the molecular unit and the arrangement of molecules in a crystal.

Is a Single Molecule Sufficient to Determine the Internal Charge Trapping Energy in Crystalline Organic Semiconductors?

In silico diagnoses of charge-transfer efficiencies in organic semiconductors require the accurate computations of transport parameters. We show here that ignoring the molecular packing effects when computing the internal charge trapping energy may cause a severe deviation to the result deduced from the periodic crystal structure. This deviation can reach up to 100 meV in common organic materials. According to the semiclassical Marcus theory, this energy difference can lead to orders of magnitude change in the charge transfer rate. Studying from a total of 45 organic crystals, we find that single-molecule approximation though is adequate for rigid planar molecules yet it fails to describe the trapping energy for molecules that have inter-ring single bond(s) or are subjected to planarity changes during the transition from the isolated state to the embedded state. These results and conclusions may shed light on the removal of theory-experiment discrepancies on charge mobilities and lay the basis for the future fast and precise screening of high-performance organic semiconductors.

Active discovery of organic semiconductors

The versatility of organic molecules generates a rich design space for organic semiconductors (OSCs) considered for electronics applications. Offering unparalleled promise for materials discovery, the vastness of this design space also dictates efficient search strategies. Here, we present an active machine learning (AML) approach that explores an unlimited search space through consecutive application of molecular morphing operations. Evaluating the suitability of OSC candidates on the basis of charge injection and mobility descriptors, the approach successively queries predictive-quality first-principles calculations to build a refining surrogate model. The AML approach is optimized in a truncated test space, providing deep methodological insight by visualizing it as a chemical space network. Significantly outperforming a conventional computational funnel, the optimized AML approach rapidly identifies well-known and hitherto unknown molecular OSC candidates with superior charge conduction properties. Most importantly, it constantly finds further candidates with highest efficiency while continuing its exploration of the endless design space.

A quantitative structure-property study of reorganization energy for known p-type organic semiconductors

Intramolecular reorganization energy (RE), which quantifies the electron-phonon coupling strength, is an important charge transport parameter for the theoretical characterization of molecular organic semiconductors (OSCs). On a small scale, the accurate calculation of the RE is trivial; however, for large-scale screening, faster approaches are desirable. We investigate the structure-property relations and present a quantitative structure-property relationship study to facilitate the computation of RE from molecular structure. To this end, we generated a compound set of 171, which was derived from known p-type OSCs built from moieties such as acenes, thiophenes, and pentalenes. We show that simple structural descriptors such as the number of atoms, rings or rotatable bonds only weakly correlate with the RE. On the other hand, we show that regression models based on a more comprehensive representation of the molecules such as SMILES-based molecular signatures and geometry-based molecular transforms can predict the RE with a coefficient of determination of 0.7 and a mean absolute error of 40 meV in the library, in which the RE ranges from 76 to 480 meV. Our analysis indicates that a more extensive compound set for training is necessary for more predictive models.

Read between the Molecules: Computational Insights into Organic Semiconductors

The performance and key electronic properties of molecular organic semiconductors are dictated by the interplay between the chemistry of the molecular core and the intermolecular factors of which manipulation has inspired both experimentalists and theorists. This Perspective presents major computational challenges and modern methodological strategies to advance the field. The discussion ranges from insights and design principles at the quantum chemical level, in-depth atomistic modeling based on multiscale protocols, morphological prediction and characterization as well as energy-property maps involving data-driven analysis. A personal overview of the past achievements and future direction is also provided.

Mechanistical Insights into the Bioconjugation Reaction of Triazolinediones with Tyrosine

The bioconjugation at tyrosine residues using cyclic diazodicarboxamides, especially 4-substituted 3 H-1,2,4-triazole-3,5(4 H)-dione (PTAD), is a highly enabling synthetic reaction because it can be employed for orthogonal and site-selective (multi)functionalizations of native peptides and proteins. Despite its importance, the underlying mechanisms have not been thoroughly investigated. The reaction can proceed along four distinctive pathways: (i) the S_EAr path, (ii) along a pericyclic group transfer pathway (a classical ene reaction), (iii) along a stepwise reaction path, or (iv) along an unusual higher order concerted pericyclic mechanism. The product mixtures obtained from reactions of PTAD with 2,4-unsubstituted phenolate support the S_EAr mechanism, but it remains unclear if other mechanisms also take place. In the present work, the various mechanisms are compared using high-level quantum chemistry approaches for the model reaction of 4 H,3 H-1,2,4-triazole-3,5(4 H)-dione (HTAD) with p-cresol and p-cresolate. In a protic solvent (water), the barriers of the S_EAr mechanism and the ene reaction are similar but still too high to explain the available experimental observations. This is only possible if the S_EAr reaction of cresolate with HTAD is taken into account for which nearly vanishing barriers are computed. This satisfactorily explains measured conversion rates in buffered aqueous solutions and the strong activation effects observed upon addition of bases.

Density Functional Theory Investigations of D-A-D' Structural Molecules as Donor Materials in Organic Solar Cell

Squaraine core based small molecules in bulk heterojunction organic solar cells have received extensive attentions due to their distinguished photochemical properties in far red and infrared domain. In this paper, combining theoretical simulations and experimental syntheses and characterizations, three major factors (fill factor, short circuit and open-cirvuit voltage) have been carried out together to achieve improvement of power conversion efficiencies of solar cells. As model material systems with D-A-D' framework, two asymmetric squaraines (CNSQ and CCSQ-Tol) as donor materials in bulk heterojunction organic solar cell were synthesized and characterized. Intensive density functional theory computations were applied to identify some direct connections between three factors and corresponding molecular structural properties. It then helps us to predict one new molecule of CCSQ'-Ox that matches all the requirements to improve the power conversion efficiency.

Enhancing charge mobilities in organic semiconductors by selective fluorination: a design approach based on a quantum mechanical perspective

Selective fluorination of organic semiconducting molecules is proposed as a means to achieving enhanced hole mobility. Naphthalene is examined here as a root molecular system with fluorination performed at various sites. Our quantum chemical calculations show that selective fluorination can enhance attractive intermolecular interactions while reducing charge trapping. Those observations suggest a design principle whereby fluorination is utilized for achieving high charge mobilities in the crystalline form. The utility of this design principle is demonstrated through an application to perylene, which is an important building block of organic semiconducting materials. We also show that a quantum mechanical perspective of nuclear degrees of freedom is crucial for a reliable description of charge transport.

The dimer-approach to characterize opto-electronic properties of and exciton trapping and diffusion in organic semiconductor aggregates and crystals

We present the recently developed dimer approach which seems to include all main effects determining the photo-physics of organic semiconductor aggregates.