Ab Initio Modeling of Mixed-Dimensional Heterostructures: A Path Forward

Understanding the electronic structure of mixed-dimensional heterostructures is essential for maximizing their application potential. However, accurately modeling such interfaces is challenging due to the complex interplay between the subsystems. We employ a computational framework integrating first-principles methods, including GW, density functional theory (DFT), and the polarizable continuum model, to elucidate the electronic structure of mixed-dimensional heterojunctions formed by free-base phthalocyanines and monolayer molybdenum disulfide. We assess the impact of dielectric screening across various scenarios, from isolated molecules to organic films on a substrate-supported monolayer. Our findings show that while polarization effects cause significant renormalization of molecular energy levels, band energies and alignments in the most relevant setup can be accurately predicted through DFT simulations of the individual subsystems. Additionally, we analyze orbital hybridization, revealing potential pathways for interfacial charge transfer. This study offers new insights into hybrid inorganic/organic interfaces and provides a practical computational protocol suitable for scaled-up studies.

Protonated Organic Semiconductors: Origin of Water-Induced Charge-Trap Generation

Despite dramatic improvements in the electronic characteristics of organic semiconductors, the low operational stability of organic field-effect transistors (OFETs) hinders their direct use in practical applications. Although the literature contains numerous reports on the effects of water on the operational stability of OFETs, the underlying mechanisms of trap generation induced by water remain unclear. Here, a protonation-induced trap generation of organic semiconductors is proposed as a possible origin of the operational instability in organic field-effect transistors. Spectroscopic and electronic investigation techniques combined with simulations reveal that the direct protonation of organic semiconductors by water during operation may be responsible for the trap generation induced by bias stress; this phenomenon is independent of the trap generation at an insulator surface. In addition, the same feature occurred in small-bandgap polymers with fused thiophene rings irrespective of their crystalline ordering, implying the generality of protonation induced trap generation in various polymer semiconductors with a small bandgap. The finding of the trap-generation process provides new perspectives for achieving greater operational stability of organic field-effect transistors.

Role of Dielectric Screening in Calculating Excited States of Solvated Azobenzene: A Benchmark Study Comparing Quantum Embedding and Polarizable Continuum Model for Representing the Solvent

The low energy excited states of the conformational isomers of solvated azobenzene are calculated with several DFT methods accounting for the solute-solvent interaction implicitly with the polarizable continuum model or explicitly with subsystem DFT. For the latter, embedding potentials are calculated for 21 sampled snapshots of the solvent molecules. First, we find that accounting for the solvent implicitly or explicitly has little effect on the predicted cis-trans S₁ excitation energy gap. Second, we find that azobenzene's S₁ cis and trans energies are accurate as long as a screened range-separated hybrid exchange-correlation functional is employed. Finally, we also tested a simplified workflow whereby a single, averaged, embedding potential is used. Unfortunately, we find larger deviations against the experiment for the simplified workflow. This highlights a basic flaw in the approach, where the time scale of solvent averaging is much longer than that of the solute's electronic polarization.

Binding Energy of Triplet Excitons in Nonfullerene Acceptors: The Effects of Fluorination and Chlorination

One strategy to improve the photovoltaic properties of nonfullerene acceptors (NFAs), employed in state-of-art organic solar cells, is the rational fluorination or chlorination of these molecules. Although this modification improves important acceptor properties, little is known about the effects on the triplet states. Here, we combine the polarizable continuum model with an optimally tuned range-separated hybrid functional to investigate this issue. We find that fluorination or chlorination of NFAs decreases the degree of the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) overlap along these molecules. Consequently, the energy gap between T₁ and S₁ states, ΔE_ST = E_S1 - E_T1, also decreases. This effect reduces the binding energy of triplet excitons, which favors their dissociation into free charges. Furthermore, the reduction of ΔE_ST can contribute to mitigating the losses produced by the nonradiative deactivation of the T₁ excitons. Interestingly, although Cl has a lower electronegativity than F, chlorination is more effective to reduce ΔE_ST. Since the chlorination of NFAs is easier than fluorination, Cl substitution can be a useful approach to enhance solar energy harvesting using triplet excitons.

Achieving Adjustable Multifunction Based on Host-Guest Interaction-Manipulated Reversible Molecular Conformational Switching

Small molecules that are capable of toggling between multiple and definite conformational states under external stimuli have great potential for use in molecular switches or sensors. However, currently developed regulation approaches for these switchable molecules mostly involve covalent bond-breaking/reforming processes, thereby inevitably producing byproducts or causing fatigue accumulation. Herein, we report a simple but successful model whose molecular conformation can be precisely manipulated between stretched and folded forms by employing host-guest interactions with rigid macrocycles, thus avoiding possible side reactions and fatigue accumulation and possessing excellent reversibility. Moreover, the conformation states of this molecule can be visualized and identified by luminous readout, endowing it with real-time self-reporting features. Furthermore, this controllable and reversible conformational conversion is accompanied by various valuable functions, including controllable multicolor emission; ratiometric fluorescent thermosensing with high temperature resolution, excellent reversibility, lock/unlock switching, and especially linear detection range tunability; and in addition real-time intracellular temperature sensing and imaging, disclosing the intriguing microscopic "conformation-function" relationship based on a single molecule.

Electronic Spectra of C60 Films Using Screened Range Separated Hybrid Functionals

We study computationally the electronic spectra of C₆₀ thin films using the recently developed density functional theory (DFT) framework combining a screened range separated hybrid (SRSH) functional with a polarizable continuum model (PCM). The SRSH-PCM approach achieves excellent correspondence between the frontier orbital's energy levels and the ionization potential and electron affinity of the molecular system at the condensed phase and consequently leads to high quality electronic excitation energies when used in time-dependent DFT calculations. Our calculated excited states reproduce the experimentally main reported spectral peaks at the 3.6-4.6 eV energy range and when addressing excitonic effects also reproduce the red-shifted spectral feature. Notably, we analyze the low-lying peak at 2.7 eV and associate it to an excitonic state.

Tunable lifetimes and efficiencies of room temperature phosphorescent liquids by modulating the length and number of alkyl chains

Organic room temperature phosphorescence (RTP) liquid composites exhibit the potential to make innovative changes in large area flexible lighting applications, and it is extremely challenging to achieve high-efficiency RTP in pure organic solvent-free liquid systems. The excited state properties and inner lighting mechanisms of these composites are unclear; therefore, a theoretical perspective to design high efficiency RTP liquids with tunable lifetime is highly desired. Herein, we systematically investigate the photophysical properties of a series of long swallow-tailed bromonaphthalimide (BT unit) molecules by the newly proposed optimally tuned range-separated (RS) functional method, and a state-of-the-art RTP molecule with an absolute quantum yield (ΦRTP) of 57.1% and a lifetime (τ) of 160 ms in solvent-free liquid is obtained. Moreover, theoretical results show that the energy gap between the lowest singlet excited state (S1) and triplet excited state (T1) can be reduced and the non-radiative energy consumption process can be restricted by modulating the length and number of alkyl chains in organic RTP molecules. Thus, a wise molecular design strategy is proposed and five additional efficient RTP molecules with tunable lifetimes (43, 19, 136, 0.11 and 0.005 ms) and efficiencies (11.3%, 6.8%, 5.9%, 0.2% and 0.05%) are theoretically proposed. This study sheds light on the relationship among molecular structure, lifetime and efficiency, and can provide an important prototype to explore high-efficiency RTP by pure organic solvent-free liquid systems.

Prediction of Excited-State Properties of Oligoacene Crystals Using Fragment-Based Quantum Mechanical Method

A fundamental understanding of the excited-state properties of molecular crystals is of central importance for their optoelectronics applications. In this study, we developed the electrostatically embedded generalized molecular fractionation (EE-GMF) method for the quantitative characterization of the excited-state properties of locally excited molecular clusters. The accuracy of the EE-GMF method is systematically assessed for oligoacene crystals. Our result demonstrates that the EE-GMF method is capable of providing the lowest vertical singlet (S₁) and triplet excitation energies (T₁), in excellent agreement with the full-system quantum mechanical calculations. Using this method, we also investigated the performance of different density functionals in predicting the excited-state properties of the oligoacene crystals. Among the 13 tested functionals, B3LYP and MN15 give the two lowest overall mean unsigned errors with reference to the experimental S₁ and T₁ excitation energies. The EE-GMF approach can be readily utilized for studying the excited-state properties of large-scale organic solids at diverse ab initio levels.

Dielectric Screening Meets Optimally Tuned Density Functionals

A short overview of recent attempts at merging two independently developed methods is presented. These are the optimal tuning of a range-separated hybrid (OT-RSH) functional, developed to provide an accurate first-principles description of the electronic structure and optical properties of gas-phase molecules, and the polarizable continuum model (PCM), developed to provide an approximate but computationally tractable description of a solvent in terms of an effective dielectric medium. After a brief overview of the OT-RSH approach, its combination with the PCM as a potentially accurate yet low-cost approach to the study of molecular assemblies and solids, particularly in the context of photocatalysis and photovoltaics, is discussed. First, solvated molecules are considered, with an emphasis on the challenge of balancing eigenvalue and total energy trends. Then, it is shown that the same merging of methods can also be used to study the electronic and optical properties of molecular solids, with a similar discussion of the pros and cons. Tuning of the effective scalar dielectric constant as one recent approach that mitigates some of the difficulties in merging the two approaches is considered.

Theoretical exploitation of acceptors based on benzobis(thiadiazole) and derivatives for organic NIR-II fluorophores

Small-molecule dyes with fluorescence emission in the second near-infrared (NIR-II) region (1000-1700 nm) have attracted considerable attention in the biomedical and bioimaging fields due to their greater imaging depths, better spatial resolution, and higher signal-to-background ratios. However, currently reported organic NIR-II fluorophores are still limited and there is great demand to develop other novel NIR-II fluorophores besides benzobisthiadiazole (BBT)-based fluorophores. More importantly, there is a lack of an appropriate level of theory capable of providing both efficient and accurate predictions of the electronic structures of organic NIR-II fluorophores. In this work, successful application of time-dependent density functional theory (TDDFT) using optimally-tuned range-separated functionals for calculations of both absorption and fluorescence spectral properties has been demonstrated, compared with the available experimental data. A series of thiadiazole-based acceptors (A) and derivatives based on the D-A-D skeleton are designed coupled with the triphenylamine donor (D). The structure-property relationships for these fluorophores are thus revealed by analyzing their ground (S0) and excited (S1) state geometries, frontier molecular orbitals (HOMO and LUMO), HOMO-LUMO energy gaps, oscillator strengths, hole-electron distributions and fluorescence wavelengths. It is suggested that the existence of a hypervalent structure leading to a much lower LUMO level and accompanying significant hole-electron separation plays a key role in the red-shift of fluorescence emission in the NIR-II region. In addition, the substitution of BBT oligomers and analogues as acceptor cores is an efficient way to achieve both red-shifted fluorescence wavelengths and enhanced oscillator strengths. The present work provides a reliable and efficient theoretical tool for predicting the related electronic and spectral properties of organic fluorophores and future screening out of potential candidates for excellent NIR-II molecular fluorophores.

Benchmark study of ionization potentials and electron affinities of armchair single-walled carbon nanotubes using density functional theory

The intrinsic parameters of carbon nanotubes (CNTs) such as ionization potential (IP) and electron affinity (EA) are closely related to their unique properties and associated applications. In this work, we demonstrated the success of optimal tuning method based on range-separated (RS) density functionals for both accurate and efficient prediction of vertical IPs and electron affinities (EAs) of a series of armchair single-walled carbon nanotubes C_20n H₂₀ (n  =  2-6) compared to the high-level IP/EA equation-of-motion coupled-cluster method with single and double substitutions (IP/EA-EOM-CCSD). Notably, the resulting frontier orbital energies (-ε _HOMO and -ε _LUMO) from the tuning method exhibit an excellent approximation to the corresponding IPs and EAs, that significantly outperform other conventional density functionals. In addition, it is suggested that the RS density functionals that possess both a fixed amount of exact exchange in the short-range and a correct long-range asymptotic behavior are suitable for calculating electronic structures of finite-sized CNTs. Next the performance of density functionals for description of various molecular properties such as chemical potential, hardness and electrophilicity are assessed as a function of tube length. Thanks to the efficiency and accuracy of this tuning method, the related behaviors of much longer armchair single-walled CNTs until C₂₀₀H₂₀ were studied. Lastly, the present work is proved to provide an efficient theoretical tool for future materials design and reliable characterization of other interesting properties of CNT-based systems.

Assessment of Ab Initio and Density Functional Theory Methods for the Excitations of Donor-Acceptor Complexes: The Case of the Benzene-Tetracyanoethylene Model

The understanding of the excited-state properties of electron donors, acceptors and their interfaces in organic optoelectronic devices is a fundamental issue for their performance optimization. In order to obtain a balanced description of the different excitation types for electron-donor-acceptor systems, including the singlet charge transfer (CT), local excitations, and triplet excited states, several ab initio and density functional theory (DFT) methods for excited-state calculations were evaluated based upon the selected model system of benzene-tetracyanoethylene (B-TCNE) complexes. On the basis of benchmark calculations of the equation-of-motion coupled-cluster with single and double excitations method, the arithmetic mean of the absolute errors and standard errors of the electronic excitation energies for the different computational methods suggest that the M11 functional in DFT is superior to the other tested DFT functionals, and time-dependent DFT (TDDFT) with the Tamm–Dancoff approximation improves the accuracy of the calculated excitation energies relative to that of the full TDDFT. The performance of the M11 functional underlines the importance of kinetic energy density, spin-density gradient, and range separation in the development of novel DFT functionals. According to the TDDFT results, the performances of the different TDDFT methods on the CT properties of the B-TCNE complexes were also analyzed.

Thermally activated delayed fluorescence processes for Cu(i) complexes in solid-state: a computational study using quantitative prediction

Calculated fluorescence (k_F), phosphorescence (k_P), and ISC rate constants (kvib.ISC/RISC) with the vibronic spin–orbit coupling at 300 K for Cu(dppb)(pz₂Bph₂).

Basic photophysical analysis of a thermally activated delayed fluorescence copper(i) complex in the solid state: theoretical estimations from a polarizable continuum model (PCM)-tuned range-separated density functional approach

The calculated conversion and decay rates of the S₁and T₁states at 300 K.

Impact of Dielectric Constant on the Singlet-Triplet Gap in Thermally Activated Delayed Fluorescence Materials

Thermally activated delayed fluorescence (TADF) relies on the presence of a very small energy gap, ΔE_ST, between the lowest singlet and triplet excited states. ΔE_ST is thus a key factor in the molecular design of more efficient materials. However, its accurate theoretical estimation remains challenging, especially in the solid state due to the influence of polarization effects. We have quantitatively studied ΔE_ST as a function of dielectric constant, ε, for four representative organic molecules using the methodology we recently proposed at the Tamm-Dancoff approximation ωB97X level of theory, where the range-separation parameter ω is optimized with the polarizable continuum model. The results are found to be in very good agreement with experimental data. Importantly, the polarization effects can lead to a marked reduction in the ΔE_ST value, which is favorable for TADF applications. This ΔE_ST decrease in the solid state is related to the hybrid characters of the lowest singlet and triplet excited states, whose dominant contribution switches to charge-transfer-like with increasing ε. The present work provides a theoretical understanding on the influence of polarization effect on the singlet-triplet gap and confirms our methodology to be a reliable tool for the prediction and development of novel TADF materials.