Molecular Donor–Bridge–Acceptor Strategies for High-Capacitance Organic Dielectric Materials

Closed-loop transfer enables artificial intelligence to yield chemical knowledge

Artificial intelligence-guided closed-loop experimentation has emerged as a promising method for optimization of objective functions1,2, but the substantial potential of this traditionally black-box approach to uncovering new chemical knowledge has remained largely untapped. Here we report the integration of closed-loop experiments with physics-based feature selection and supervised learning, denoted as closed-loop transfer (CLT), to yield chemical insights in parallel with optimization of objective functions. CLT was used to examine the factors dictating the photostability in solution of light-harvesting donor-acceptor molecules used in a variety of organic electronics applications, and showed fundamental insights including the importance of high-energy regions of the triplet state manifold. This was possible following automated modular synthesis and experimental characterization of only around 1.5% of the theoretical chemical space. This physics-informed model for photostability was strengthened using multiple experimental test sets and validated by tuning the triplet excited-state energy of the solvent to break out of the observed plateau in the closed-loop photostability optimization process. Further applications of CLT to additional materials systems support the generalizability of this strategy for augmenting closed-loop strategies. Broadly, these findings show that combining interpretable supervised learning models and physics-based features with closed-loop discovery processes can rapidly provide fundamental chemical insights.

Recent Structural Engineering of Polymer Semiconductors Incorporating Hydrogen Bonds

Highly planar, extended π-electron organic conjugated polymers have been increasingly attractive for achieving high-mobility organic semiconductors. In addition to the conventional strategy to construct rigid backbone by covalent bonds, hydrogen bond has been employed extensively to increase the planarity and rigidity of polymer via intramolecular noncovalent interactions. This review provides a general summary of high-mobility semiconducting polymers incorporating hydrogen bonds in field-effect transistors over recent years. The structural engineering of the hydrogen bond-containing building blocks and the discussion of theoretical simulation, microstructural characterization, and device performance are covered. Additionally, the effects of the introduction of hydrogen bond on self-healing, stretchability, chemical sensitivity, and mechanical properties are also discussed. The review aims to help and inspire design of new high-mobility conjugated polymers with superiority of mechanical flexibility by incorporation of hydrogen bond for the application in flexible electronics.

Symmetry controlled photo-selection and charge separation in butadiyne-bridged donor–bridge–acceptor compounds

Electron transfer (ET) in donor–bridge–acceptor (DBA) compounds featuring alkyne bridges depends strongly on the torsion angle between the donor and acceptor.

Quantum Interference and Substantial Property Tuning in Conjugated Z-ortho-Regio-Resistive Organic (ZORRO) Junctions

Coherence is a significant factor in nanoscale electronic insulator technology and necessitates an understanding of the structure-property relationship between constructive and destructive quantum interference. This is particularly important in organic dielectric circuitry, which is the subject of this work. It is known that molecular wires composed of (i) meta-substituted phenylene rings, (ii) cross-conjugated double bonds (orthogonal to the molecular long axis), and (iii) single bonds can dramatically reduce electrical transmission. Here we add to these tools the use of an unexplored molecular shape to create strong and counterintuitive interference: a fully conjugated molecular wire with a structure that is forced back on itself in a Z shape, thereby exhibiting remarkably low conductance (G = 0.43 × 10^-9 S) even though the phenylene arrangements are ortho- rather than meta-disposed. We call these Z-shaped molecules having ultralow conduction Z-ortho-regio-resistive organics (ZORROs). Here we analyze a series of ZORRO molecules and find them to have significant insulating properties in the coherent electron-transport regime due to interfering transmission pathways in the phenylene rings. Importantly, we find that both electron-withdrawing (fluorine) and electron-donating (methoxy) substituents enhance the transmission, which is not desirable. The former is due to the suppression of the destructive quantum interference at the F site, thereby enhancing the overall transmission, much like a Büttiker probe. The latter is due to a methoxy unit resonance additive effect, akin to oxygen doping, and positively contributes to the transmission. We then examine the effects of replacing the phenylene rings with 4,5- and 3,4-disubstituted thiophenes and how this ZORRO modification further reduces the transmission. An ultralow conductance of 0.13 × 10^-9 S and a relatively high dielectric constant (ε_r) of ∼5 are predicted for the 3,4-thiophene ZORRO derivative, which closely resembles two cross-conjugated units, making it an intriguing candidate for a gate dielectric material.

Covalently Bonded Pillared Layered Bromoplumbate with High Thermal Stability: High Capacitance Gain after Photoinduced Electron Transfer

Improving the stability and photoelectric properties are current aims in the field of inorganic-organic hybrid lead halides. In this work, a new covalently bonded pillared layered bromoplumbate, [Pb₃Br₆(CV)]_n, was prepared using a photoactive zwitterion viologen, N,N'-4,4'-bipyridiniodipropionate (CV), as a ligand. It has a high thermal stability in air and shows a remarkable increase of capacitance after photoinduced electron transfer (PIET). The observed dielectric switch ratio of ∼689% at 3 kHz exceeded all reported values at room temperature for switchable dielectric materials. The capacitance gain was derived from the redistribution of electrons after PIET from the lead-halide layer to organic π-aggregates, so that the activation energy of hopping polarizability was significantly reduced.

Can the Dielectric Constant of Fullerene Derivatives Be Enhanced by Side-Chain Manipulation? A Predictive First-Principles Computational Study

The low efficiency of organic photovoltaic (OPV) devices has often been attributed to the strong Coulombic interactions between the electron and hole, impeding the charge separation process. Recently, it has been argued that by increasing the dielectric constant of materials used in OPVs, this strong interaction could be screened. In this work, we report the application of periodic density functional theory together with the coupled perturbed Kohn–Sham method to calculate the electronic contribution to the dielectric constant for fullerene C₆₀ derivatives, a ubiquitous class of molecules in the field of OPVs. The results show good agreement with experimental data when available and also reveal an important undesirable outcome when manipulating the side chain to maximize the static dielectric constant: in all cases, the electronic contribution to the dielectric constant decreases as the side chain increases in size. This information should encourage both theoreticians and experimentalists to further investigate the relevance of contributions to the dielectric constant from slower processes like vibrations and dipolar reorientations for facilitating the charge separation, because electronically, enlarging the side chain of conventional fullerene derivatives only lowers the dielectric constant, and consequently, their electronic dielectric constant is upper bound by the one of C₆₀.

Chain Length Dependence of the Dielectric Constant and Polarizability in Conjugated Organic Thin Films

Dielectric materials are ubiquitous in optics, electronics, and materials science. Recently, there have been new efforts to characterize the dielectric performance of thin films composed of molecular assemblies. In this context, we investigate here the relationship between the polarizability of the constituent molecules and the film dielectric constant, using periodic density functional theory (DFT) calculations, for polyyne and saturated alkane chains. In particular, we explore the implication of the superlinear chain length dependence of the polarizability, a specific feature of conjugated molecules. We show and explain from DFT calculations and a simple depolarization model that this superlinearity is attenuated by the collective polarization. However, it is not completely suppressed. This confers a very high sensitivity of the dielectric constant to the thin film thickness. This latter can increase by a factor of 3-4 at reasonable coverages, by extending the molecular length. This significantly limits the decline of the thin film capacitance with the film thickness. Therefore, the conventional fit of the capacitance versus thickness is not appropriate to determine the dielectric constant of the film. Finally, we show that the failures of semilocal approximations of the exchange-correlation functional lead to a very significant overestimation of this effect.

Tuning the ambipolar charge transport properties of tricyanovinyl-substituted carbazole-based materials

The ambipolar charge transport properties of a series of push–pull carbazole-based semiconductors are here evaluated. The ambipolar characteristics depend on the supramolecular organization. Experimental results were confirmed and justified through the X-ray analysis of single crystals and by theoretical calculations.

Energy Gap between the Poly-p-phenylene Bridge and Donor Groups Controls the Hole Delocalization in Donor-Bridge-Donor Wires

Poly-p-phenylene wires are critically important as charge-transfer materials in photovoltaics. A comparative analysis of a series of poly-p-phenylene (^RPP_n) wires, capped with isoalkyl (^iAPP_n), alkoxy (^ROPP_n), and dialkylamino (^R2NPP_n) groups, shows unexpected evolution of oxidation potentials, i.e., decrease (-260 mV) for ^iAPP_n, while increase for ^ROPP_n (+100 mV) and ^R2NPP_n (+350 mV) with increasing number of p-phenylenes. Moreover, redox/optical properties and DFT calculations of ^R2NPP_n/^R2NPP_n^+• further show that the symmetric bell-shaped hole distribution distorts and shifts toward one end of the molecule with only 4 p-phenylenes in ^R2NPP_n^+•, while shifting of the hole occurs with 6 and 8 p-phenylenes in ^ROPP_n^+• and ^iAPP_n^+•, respectively. Availability of accurate experimental data on highly electron-rich dialkylamino-capped ^R2NPP_n together with ^ROPP_n and ^iAPP_n allowed us to demonstrate, using our recently developed Marcus-based multistate model (MSM), that an increase of oxidation potentials in ^R2NPP_n arises due to an interplay between the electronic coupling (H_ab) and energy difference between the end-capped groups and bridging phenylenes (Δε). A comparison of the three series of ^RPP_n with varied Δε further demonstrates that decrease/increase/no change in oxidation energies of ^RPP_n can be predicted based on the energy gap Δε and coupling H_ab, i.e., decrease if Δε < H_ab (i.e., ^iAPP_n), increase if Δε > H_ab (i.e., ^R2NPP_n), and minimal change if Δε ≈ H_ab (i.e., ^ROPP_n). MSM also reproduces the switching of the nature of electronic transition in higher homologues of ^R2NPP_n^+• (n ≥ 4). These findings will aid in the development of improved models for charge-transfer dynamics in donor-bridge-acceptor systems.

Computation of Dielectric Response in Molecular Solids for High Capacitance Organic Dielectrics

The dielectric response of a material is central to numerous processes spanning the fields of chemistry, materials science, biology, and physics. Despite this broad importance across these disciplines, describing the dielectric environment of a molecular system at the level of first-principles theory and computation remains a great challenge and is of importance to understand the behavior of existing systems as well as to guide the design and synthetic realization of new ones. Furthermore, with recent advances in molecular electronics, nanotechnology, and molecular biology, it has become necessary to predict the dielectric properties of molecular systems that are often difficult or impossible to measure experimentally. In these scenarios, it is would be highly desirable to be able to determine dielectric response through efficient, accurate, and chemically informative calculations. A good example of where theoretical modeling of dielectric response would be valuable is in the development of high-capacitance organic gate dielectrics for unconventional electronics such as those that could be fabricated by high-throughput printing techniques. Gate dielectrics are fundamental components of all transistor-based logic circuitry, and the combination high dielectric constant and nanoscopic thickness (i.e., high capacitance) is essential to achieving high switching speeds and low power consumption. Molecule-based dielectrics offer the promise of cheap, flexible, and mass producible electronics when used in conjunction with unconventional organic or inorganic semiconducting materials to fabricate organic field effect transistors (OFETs). The molecular dielectrics developed to date typically have limited dielectric response, which results in low capacitances, translating into poor performance of the resulting OFETs. Furthermore, the development of better performing dielectric materials has been hindered by the current highly empirical and labor-intensive pace of synthetic progress. An accurate and efficient theoretical computational approach could drastically decrease this time by screening potential dielectric materials and providing reliable design rules for future molecular dielectrics. Until recently, accurate calculation of dielectric responses in molecular materials was difficult and highly approximate. Most previous modeling efforts relied on classical formalisms to relate molecular polarizability to macroscopic dielectric properties. These efforts often vastly overestimated polarizability in the subject materials and ignored crucial material properties that can affect dielectric response. Recent advances in first-principles calculations via density functional theory (DFT) with periodic boundary conditions have allowed accurate computation of dielectric properties in molecular materials. In this Account, we outline the methodology used to calculate dielectric properties of molecular materials. We demonstrate the validity of this approach on model systems, capturing the frequency dependence of the dielectric response and achieving quantitative accuracy compared with experiment. This method is then used as a guide to new high-capacitance molecular dielectrics by determining what materials and chemical properties are important in maximizing dielectric response in self-assembled monolayers (SAMs). It will be seen that this technique is a powerful tool for understanding and designing new molecular dielectric systems, the properties of which are fundamental to many scientific areas.

Four-coordinate nickel(ii) and copper(ii) complex based ONO tridentate Schiff base ligands: synthesis, molecular structure, electrochemical, linear and nonlinear properties, and computational study

The properties of NLO active ternary Ni(ii)- and Cu(ii)-unsymmetrical Schiff base complexes were explored in concert with DFT calculations.