Effects of Solvent Dielectric on Thermally Activated Delayed Fluorescence: A Predictive Computational Polarization Consistent Approach

We study computationally thermally activated delayed fluorescence (TADF) in donor–acceptor compounds. The relevant electronic excited states that are strongly affected by the dielectric environment are treated by a polarization consistent framework. The high fidelity potential energy surfaces are used following a quantum-mechanical Fermi’s golden rule (FGR) picture to calculate rates of intersystem crossing (ISC) and reverse intersystem crossing (RISC). To demonstrate the potency of the approach, we consider isomers of benzonitrile functionalized tert-butyl-substituted dimethylacridine (DMAC-BN), which were recently found to perform well as TADF emitters. The calculated excited state energies that appear to reproduce well measured spectral trends with respect to the dielectric constant are used to parametrize ISC/RISC FGR rates. The calculated rates reproduce well measured rates, whereas semiclassical based rates are grossly underestimated. In particular, we find in agreement with the recent experimental study [Phys. Rev. Appl.2019, 12, 044021] that the ortho and meta isomers are significantly more effective as TADF emitters. The computational framework provides valuable insight at the molecular level into RISC rates and therefore can contribute to the design of materials of increased TADF efficiency

Contact Geometry Symmetry Dependence of Field Effect Gating in Single-Molecule Transistors

The geometric aspects for the functionality of a molecule-based field effect transistor (FET) are analyzed. A computational study is performed on molecular models involving a well-defined conjugation plane coupled to gold-based electrodes through thiol bonding. Transport gating of the FET is shown to depend on a symmetry-breaking effect induced by the gating field. This effect is also related to the orientation of the field relative to the gold−thiol bonds, the molecular conjugation plane, and the overall symmetry of the device. First, it is found that the presence of a center of inversion in the bulk-coupled molecular system results in the cancellation of the transisting response. Second, a mirror plane of the molecule−bulk system, which includes the transport vector, will cancel the gating response to fields oriented perpendicular to that mirror plane. The symmetry properties are determined for the bulk contacted molecular junction

Conductance of a Cobalt(II) Terpyridine Complex Based Molecular Transistor: A Computational Analysis^†

A recent experiment, in which a molecular transistor based on the coordination chemistry of cobalt(II) and organic self-assembled monolayers is formed by means of self-aligned lithography, is analyzed with a computational approach. The calculations reveal that a complex involving two cobalt(II) ions bridged by acetate ions can effectively span the nanogap. This bridged complex is shown to be both more flexible and more conductive than the alternative structure involving a single cobalt(II) ion. The single cobalt(II) ion complex is the more stable structure in a nonconfined environment (i.e., in solution) but is found to be less effective at connecting the leads of the fabricated gap and is less likely to result in a conductive device

Conductance of a Cobalt(II) Terpyridine Complex Based Molecular Transistor: A Computational Analysis^†

A recent experiment, in which a molecular transistor based on the coordination chemistry of cobalt(II) and organic self-assembled monolayers is formed by means of self-aligned lithography, is analyzed with a computational approach. The calculations reveal that a complex involving two cobalt(II) ions bridged by acetate ions can effectively span the nanogap. This bridged complex is shown to be both more flexible and more conductive than the alternative structure involving a single cobalt(II) ion. The single cobalt(II) ion complex is the more stable structure in a nonconfined environment (i.e., in solution) but is found to be less effective at connecting the leads of the fabricated gap and is less likely to result in a conductive device

Antioxidative Triplet Excitation Energy Transfer in Bacterial Reaction Center Using a Screened Range Separated Hybrid Functional

Excess energy absorbed by photosystems (PSs) can result in photoinduced oxidative damage. Transfer of such energy within the core pigments of the reaction center in the form of triplet excitation is important in regulating and preserving the functionality of PSs. In the bacterial reaction center (BRC), the special pair (P) is understood to act as the electron donor in a photoinduced charge transfer process, triggering the charge separation process through the photoactive branch A pigments that experience a higher polarizing environment. At this work, triplet excitation energy transfer (TEET) in BRC is studied using a computational perspective to gain insights into the roles of the dielectric environment and interpigment orientations. We find in agreement with experimental observations that TEET proceeds through branch B. The TEET process toward branch B pigment is found to be significantly faster than the hypothetical process proceeding through branch A pigments with ps and ms time scales, respectively. Our calculations find that conformational differences play a major role in this branch asymmetry in TEET, where the dielectric environment asymmetry plays only a secondary role in directing the TEET to proceed through branch B. We also address TEET processes asserting the role of carotenoid as the final triplet energy acceptor and in a mutant form, where the branch pigments adjacent to P are replaced by bacteriopheophytins. The necessary electronic excitation energies and electronic state couplings are calculated by the recently developed polarization-consistent framework combining a screened range-separated hybrid functional and a polarizable continuum mode. The polarization-consistent potential energy surfaces are used to parametrize the quantum mechanical approach, implementing Fermi’s golden rule expression of the TEET rate calculations

Controlling the Emissive Activity in Heterocyclic Systems Bearing CP Bonds

The photophysical properties of a series of heteroatom substituted indoles are explored to identify chemical means to control their emissive activity. In particular, we consider impacts of changes in the conjugated backbone, where the CN bonds of benzoxazoles are replaced by CP bonds (benzoxaphospholes). The effects of extending the π-conjugation, incorporating various secondary heteroatoms (X–CP), and enforcing planar rigidity are also examined. Our computational analysis explains the higher fluorescence efficiency observed with extended π-conjugation and highlights the importance of maintaining molecular planarity at both ground- and emissive-state geometries

Calculating Off-Site Excitations in Symmetric Donor–Acceptor Systems via Time-Dependent Density Functional Theory with Range-Separated Density Functionals

Time-dependent density functional theory with range-separated hybrid functionals is used to calculate off-site excitations involving transitions between spatially separated orbitals in weakly coupled systems. Although such off-site excitations involve charge transfer, orbital degeneracy in symmetrical systems results in linear combinations of off-site excitations with equal weights and therefore zero net charge-transfer character. Like other types of off-site excitations, such “hidden” charge-transfer excitations are not accurately captured by conventional density functionals. We show that the recently introduced Baer–Neuhauser–Livshitz range-separated hybrid functional accurately characterizes such hidden off-site excitation energies via applications to the ethene dimer model system and to dye-functionalized silsesquioxanes

The Effect of Interfacial Geometry on Charge-Transfer States in the Phthalocyanine/Fullerene Organic Photovoltaic System

The dependence of charge-transfer states on interfacial geometry at the phthalocyanine/fullerene organic photovoltaic system is investigated. The effect of deviations from the equilibrium geometry of the donor–donor–acceptor trimer on the energies of and electronic coupling between different types of interfacial electronic excited states is calculated from first-principles. Deviations from the equilibrium geometry are found to destabilize the donor-to-donor charge transfer states and to weaken their coupling to the photoexcited donor-localized states, thereby reducing their ability to serve as charge traps. At the same time, we find that the energies of donor-to-acceptor charge transfer states and their coupling to the donor-localized photoexcited states are either less sensitive to the interfacial geometry or become more favorable due to modifications relative to the equilibrium geometry, thereby enhancing their ability to serve as gateway states for charge separation. Through these findings, we eludicate how interfacial geometry modifications can play a key role in achieving charge separation in this widely studied organic photovoltaic system

Solvated Charge Transfer States of Functionalized Anthracene and Tetracyanoethylene Dimers: A Computational Study Based on a Range Separated Hybrid Functional and Charge Constrained Self-Consistent Field with Switching Gaussian Polarized Continuum Models

We benchmark several protocols for evaluating the energies of excited charge transfer (CT) states of organic molecules dissolved in polar liquids. The protocols combine time-dependent density functional theory using range-separated hybrid functionals, constrained density functional theory, dispersion corrected functional, and a dielectric continuum model for representing the solvent. We compare the different protocols against well-established experimental measured charge transfer state energies in solvated dimers of functionalized anthracene and tetracyanoethylene. We find that using the range-separated hybrid functional for the charge-transfer state energies and the combination of constrained density functional theory with the recently improved switching Gaussian polarizable continuum model (PCM) provide good agreement with the experimental values of the solvated CT states. We also find that using dispersion corrected solvated geometries for the weakly coupled donor–acceptor dimers considered here leads to improved agreement with experimental measured values