Lowest Unoccupied Molecular Orbital Levels Research Articles

Thiophene- and Carbazole-Substituted N-Methyl-Fulleropyrrolidine Acceptors in PffBT4T-2OD Based Solar Cells.

The impact of fullerene side chain functionalization with thiophene and carbazole groups on the device properties of bulk-heterojunction polymer:fullerene solar cells is discussed through a systematic investigation of material blends consisting of the conjugated polymer poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2-octyldodecyl)-2,2′;5′,2″;5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T-2OD) as donor and C60 or C70 fulleropyrrolidines as acceptors. The photovoltaic performance clearly depended on the molecular structure of the fulleropyrrolidine substituents although no direct correlation with the surface morphology of the photoactive layer, as determined by atomic force microscopy, could be established. Although some fulleropyrrolidines possess favorable lowest unoccupied molecular orbital levels, when compared to the standard PC71BM, they originated OPV cells with inferior efficiencies than PC71BM-based reference cells. Fulleropyrrolidines based on C60 produced, in general, better devices than those based on C70, and we attribute this observation to the detrimental effect of the structural and energetic disorder that is present in the regioisomer mixtures of C70-based fullerenes, but absent in the C60-based fullerenes. These results provide new additional knowledge on the effect of the fullerene functionalization on the efficiency of organic solar cells.

Spectroscopic and Theoretical Investigation of Color Tuning in Deep-Red Luminescent Iridium(III) Complexes.

A series of heteroleptic, neutral iridium(III) complexes of the form [Ir(L)2(N^O)] (where L = cyclometalated 2,3-disubstituted quinoxaline and N^O = ancillary picolinate or pyrazinoate) are described in terms of their synthesis and spectroscopic properties, with supporting computational analyses providing additional insight into the electronic properties. The 10 [Ir(L)2(N^O)] complexes were characterized using a range of analytical techniques (including 1H, 13C, and 19F NMR and IR spectroscopies and mass spectrometry). One of the examples was structurally characterized using X-ray diffraction. The redox properties were determined using cyclic voltammetry, and the electronic properties were investigated using UV–vis, time-resolved luminescence, and transient absorption spectroscopies. The complexes are phosphorescent in the red region of the visible spectrum (λem = 633–680 nm), with lifetimes typically of hundreds of nanoseconds and quantum yields ca. 5% in aerated chloroform. A combination of spectroscopic and computational analyses suggests that the long-wavelength absorption and emission properties of these complexes are strongly characterized by a combination of spin-forbidden metal-to-ligand charge-transfer and quinoxaline-centered transitions. The emission wavelength in these complexes can thus be controlled in two ways: first, substitution of the cyclometalating quinoxaline ligand can perturb both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital levels (LUMO, Cl atoms on the ligand induce the largest bathochromic shift), and second, the choice of the ancillary ligand can influence the HOMO energy (pyrazinoate stabilizes the HOMO, inducing hypsochromic shifts).

Fluorinated Thiophene-Phenylene Co-Oligomers for Optoelectronic Devices.

Organic optoelectronics requires materials combining bright luminescence and efficient ambipolar charge transport. Thiophene-phenylene co-oligomers (TPCOs) are promising highly emissive materials with decent charge-carrier mobility; however, they typically show poor electron injection in devices, which is usually assigned to high energies of their lowest unoccupied molecular orbitals (LUMOs). A widely used approach to lower the frontier orbitals energy levels of a conjugated molecule is its fluorination. In this study, we synthesized three new fluorinated derivatives of one of the most popular TPCOs, 2,2'-(1,4-phenylene)bis[5-phenylthiophene] (PTPTP) and studied them by cyclic voltammetry, absorption, photoluminescence, and Raman spectroscopies. The obtained data reveal a positive effect of fluorination on the optoelectronic properties of PTPTP: LUMO levels are finely tuned, and photoluminescence quantum yield and absorbance are increased. We then grew crystals from fluorinated PTPTPs, resolved their structures, and showed that fluorination dramatically affects the packing motif and facilitates π-stacking. Finally, we fabricated thin-film organic field-effect transistors (OFETs) and demonstrated a strong impact of fluorination on charge injection/transport for both types of charge carriers, namely, electrons and holes. Specifically, balanced ambipolar charge transport and electroluminescence were observed only in the OFET active channel based on the partially fluorinated PTPTP. The obtained results can be extended to other families of conjugated oligomers and highlight the efficiency of fluorination for rational design of organic semiconductors for optoelectronic devices.

Degradation of bromobenzene via external electric field

Bromobenzene is one of the organic pollutants that damage the natural environment and poses a serious threat to human health. Therefore, it is meaningful to study its degradation characteristics under the electric field. In this paper, density functional theory (DFT) at BPV86/6-311G (d, p) level are employed for the study of C–Br bond distance, total energy, charge distribution, dipole moment, lowest unoccupied molecular orbital (LUMO) level, highest occupied molecular orbital (HOMO) level, energy gap and potential energy surface (PES) of bromobenzene in external electric field ([Formula: see text]15.43[Formula: see text]V[Formula: see text][Formula: see text][Formula: see text]nm[Formula: see text]–15.43[Formula: see text]V[Formula: see text][Formula: see text][Formula: see text]nm[Formula: see text]). It shows that as the electric field increases, the C–Br bond tends to break. The changes in the HOMO level and the LUMO level result in a rapid drop in the energy gap. In addition, the dissociation barrier gradually decreases. When the applied electric field reaches 15.43[Formula: see text]V[Formula: see text][Formula: see text]nm[Formula: see text], the dissociation barrier disappears completely, which means that the C–Br bond is broken and bromobenzene is degraded.

Giant Conductance Enhancement of Intramolecular Circuits through Interchannel Gating

Summary For neutral intramolecular circuits with two constitutionally identical branches, a maximum 4-fold increase in total conductance can be obtained according to constructive quantum interference (CQI). For charged intramolecular circuits, however, the strong electrostatic interactions entangle the quantum states of these two parallel pathways, thus introducing complicated transport behavior that warrants experimental investigation of the intramolecular circuit rules. Here, we report that a tetracationic cyclophane with parallel channels exhibits a 50-fold conductance enhancement compared with that of a single-channel control, an observation that supplements intramolecular circuit law in systems with strong Coulombic interactions. Flicker noise measurements and theoretical calculations show that strong electrostatic interactions between charged parallel channels—serving as the chemical gate to promote the effective conductance of each channel—and CQI boosts the total conductance of the two-channel circuit. The molecular design presented herein constitutes a proof-of-principle approach to charged intramolecular circuits that are desirable for quantum circuits and devices.

Donor-acceptor random regioregular π-conjugated copolymers based on poly(3-hexylthiophene) with unsymmetrical monothienoisoindigo units.

Donor–acceptor π-conjugated random copolymers based on regioregular poly(3-hexylthiophene), rr-P3HT, with unsymmetrical monothienoisoindigo moieties were obtained by direct arylation polycondensation of 2-bromo-3-hexylthiophene with unsymmetrical monothienoisoindigo motifs under the optimized conditions [palladium-immobilized on thiol-modified silica gel with chloride counter anions, PITS-Cl (2.5 mol%), PivOH (1.0 equiv.), K2CO3 (3.0 equiv.), DMAc, 100 °C, 24 h]. Incorporation of unsymmetrical monothienoisoindigo electron-acceptor units into the polymers tuned their highest occupied and lowest unoccupied molecular orbital levels, which were close to those of the hole transport material (PEDOT) and electron transport material (PCBM), respectively, in thin-film organic solar cells. Alkyl chains of the unsymmetrical monothienoisoindigo units in the polymers tuned their macrostructural order, resulting in the observation of crystalline patterns and specific absorption peaks in thin films. An organic solar cell containing the most crystalline random copolymer showed an efficiency of 1.91%.

High efficiency green perovskite light-emitting diodes based on exciton blocking layer

In recent years, metal halide perovskite materials, owing to their excellent photoelectric properties including high photoluminescence quantum yield, high color purity, tunable band gap, etc., have been regarded as new-generation lighting sources and are widely used to fabricate perovskite light-emitting diodes (PeLEDs). Though great progresses have been made in recent years, neither the efficiency nor stability has not yet reached the requirements of commercialization. Thus, further improvement is needed. In this work, a small organic molecule material, namely 4,4'-cyclohexylidenebis[N,N-bis(p-tolyl)aniline] (TAPC) with a wide bandgap and a good hole transport ability, is used as an exciton blocking layer by utilizing the spin-coating method to improve the stability and efficiency of PeLEDs. Highly efficient and stable CsPbBr<sub>3</sub> PeLEDs are finally realized. The physical mechanism related to the improved electroluminescence performance is investigated thoroughly. Firstly, the stepped energy level alignment is formed, since the highest occupied molecular orbital energy level (HOMO) of TAPC is located between the HOMO of (3,4-ethylenedioxythiophene):poly(p-styrene sulfonate) (PEDOT: PSS) and the valence band of CsPbBr<sub>3</sub>, which is beneficial to hole injection and transport. Meanwhile, the lowest unoccupied molecular orbital level of TAPC is high enough to prevent electrons from leaking into the anode effectively and confine electrons and excitons well in the emitting layer. Secondly, the introduction of the TAPC layer can avoid the direct contact between the perovskite light emitting layer and the strong acidic layer of PEDOT:PSS, thereby eliminating the related excitons quenching, which can further increase the radiative recombination.

Nonhalogenated-Solvent-Processed Efficient Polymer Solar Cells Enabled by Medium-Band-Gap A-π-D-π-A Small-Molecule Acceptors Based on a 6,12-Dihydro-diindolo[1,2-b:10,20-e]pyrazine Unit.

In this contribution, a series of A-π-D-π-A small molecules (SMs), IPY-T-IC, IPY-T-ICCl, and IPY-T-ICF, containing the central donor unit (D) of 6,12-dihydro-diindolo[1,2-b:10,20-e]pyrazine (IPY), the π-conjugated bridge of thiophene, and the end-accepting group (A) of 3-(dic yanomethylidene)indol-1-one, 5,6-dichloro-3-(dicyanomethylidene)indol-1-one, or 5,6-difluoro-3-(dicyanomethylene)indol-1-one, were developed, characterized, and employed as the acceptor materials for polymer solar cells (PSCs). Influences of the different end-accepting groups on thermal properties, spectral absorption, energy levels, photovoltaic performance, and film morphology of these small-molecule acceptors (SMAs) were investigated in detail. These SMAs exhibit an excellent thermal stability and strong crystallization. The absorption spectra of these SMs mainly locate the wavelength between 400 and 700 nm, associated with the optical band gaps in the range of 1.75-1.90 eV. Compared with nonhalogenated IPY-T-IC, the halogenated SMAs IPY-T-ICCl and IPY-T-ICF present better absorption abilities, wider absorption region, and downshifted highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels. With regard to the complementary spectral absorption and matched HOMO/LUMO levels, PTB7-Th as a low-band gap polymer was chosen to be an electron donor to pair with these SMAs for fabricating bulk-heterojuntion PSCs. Under optimized conditions, among these SMAs, the PTB7-Th:IPY-T-IC-based PSC processed from a halogenated solvent system (chlorobenzene + 1-chloronaphthalene) delivers the best power conversion efficiency (PCE) of 7.32%, mainly because of more complementary spectral absorption, upper-lying LUMO level, higher and balanced carrier mobility, more efficiently suppressed trap-assisted recombination, better charge collection property, and blend morphology. Encouragingly, an improved PCE of up to 7.68% is achieved when the IPY-T-IC-based solar cell was processed from a nonhalogenated solvent system (o-xylene + 2-methylnaphthalene). In view of the large band gap of these IPY-based SMAs, the PCE of over 7.5% is notable and attractive for the related community. Our study argues that the IPY moiety is a potential electron-donating building moiety to develop medium-band-gap high-performance A-π-D-π-A SMAs for nonhalogenated-solvent-processed photovoltaic devices.

PffBT4T-2OD Based Solar Cells with Aryl-Substituted N-Methyl-Fulleropyrrolidine Acceptors.

Novel C60 and C70 N-methyl-fulleropyrrolidine derivatives, containing both electron withdrawing and electron donating substituent groups, were synthesized by the well-known Prato reaction. The corresponding highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels were determined by cyclic voltammetry, from the onset oxidation and reduction potentials, respectively. Some of the novel fullerenes have higher LUMO levels than the standards PC61BM and PC71BM. When tested in PffBT4T-2OD based polymer solar cells, with the standard architecture ITO/PEDOT:PSS/Active-Layer/Ca/Al, these fullerenes do not bring about any efficiency improvements compared to the standard PC71BM system, however they show how the electronic nature of the different substituents strongly affects the efficiency of the corresponding organic photovoltaic (OPV) devices. The functionalization of C70 yields a mixture of regioisomers and density functional theory (DFT) calculations show that these have systematically different electronic properties. This electronic inhomogeneity is likely responsible for the lower performance observed in devices containing C70 derivatives. These results help to understand how new fullerene acceptors can affect the performance of OPV devices.

Ternary organic solar cells with NC70BA as a third component material exhibit high open-circuit voltage and small energy losses

The ternary organic solar cells (OSCs) were fabricated with PBDB-T as donor and a blend of IEICO-4F and NC70BA as the acceptor and the power conversion efficiency (PCE) reaches 10.92%. Due to good compatibility and cascade LUMO level among PBDB-T, a large amount of IEICO-4F and a small amount of NC70BA (the ratio of IEICO-4F:NC70BA is 85:15), which beneficial for adjusting lowest unoccupied molecular orbital (LUMO) levels of blend acceptor and wide energy offset between the donor and acceptor materials, leading to the enhancement of the open-circuit voltage (VOC). The optimized ternary PBDB-T:IEICO-4F:NC70BA films is more efficient exciton dissociation and suppress charge carrier recombination than that of binary PBDB-T:IEICO-4F and PBDB-T:NC70BA films, leading to small energy losses. In addition, this approach maintains PCE without sacrificing short-circuit current density (JSC) and fill factor (FF), even if the weak long and near-infrared wavelength photon harvesting. A more than 10% PCE improvement is achieve by employing a ternary strategy in comparison to PBDB-T:IEICO-4F-based binary OSCs with a PCE of 9.87%. Simultaneously, the optimized ternary PBDB-T:IEICO-4F:NC70BA OSCs exhibit the excellent thermal stability and 78.8% initial PCE under thermal annealing treatment at 80 °C for 20 h.

Synthesis and characterization of topology-varied organic electroluminescent blue-light emitters based on carbazole and triphenylphosphine oxide derivatives

Two blue light emitters M-2EtCz and P-2EtCz based on carbazole and triphenylphosphine oxide were synthesized and characterized. The 5% weight loss temperatures of emitters M-2EtCz and P-2EtCz solid powder were 437 and 495 °C, and their photoluminescent quantum yield in solution were 53.93 and 75.78%, which were both significantly improved to those of parent EtCz-Ph. The introduction of triphenylphosphine oxide into EtCz-Ph can lower the lowest unoccupied molecular orbital levels of emitters M-2EtCz and P-2EtCz, while keeping their highest occupied molecular orbital levels unchanged. All the emitters exhibited blue light emission with aggregation-induced enhanced emission characteristic. The non-doped deep blue light-emitting diodes with maximum external quantum efficiencies of 2.68% were obtained for M-2EtCz, and the electroluminescent color purity of P-2EtCz was better than that of M-2EtCz. The doped device hosted by P-2EtCz achieved a maximum brightness of 10,891 cd/m2, external quantum efficiency of 2.04% and current efficiency of 5.65 cd/A.

A new medium-bandgap fused-[1]benzothieno[3,2-b][1]benzo-thiophene (BTBT) nonfullerene acceptor for organic solar cells with high open-circuit voltage

A new nonfullerene small molecule acceptor, namely DBTIC, based on an octocyclic thieno[3,2-b]thienodi(indenothiophene) unit using [1]benzothieno[3,2-b][1]-benzothiophene as the core unit, was developed. Despite the medium-bandgap of DBTIC (1.71 eV), a power conversion efficiency of 8.64% can be delivered by the solar cells combining DBTIC and a wide-bandgap polymer donor J52. The high open-circuit voltage (Voc) of 0.94 V is also rare for J52 based devices owing to the high-lying lowest unoccupied molecular orbital level of DBTIC. Moreover, using J71 with lower highest occupied molecular orbital level as polymer donor, a higher Voc up to 1.05 V can be achieved.

CdSe/ZnS Quantum-Dot Light-Emitting Diodes With Spiro-OMeTAD as Buffer Layer

We report a high-efficient green CdSe/ZnS quantum-dot light-emitting diode (QLED) using 2,2’,7,7’-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9’-Spirobifluorene (Spiro-OMeTAD) as an electron buffer layer. The high lowest unoccupied molecular orbital level of Spiro-OMeTAD suppresses the electron transfer and promotes the charge balance. The external quantum efficiency (EQE) of the green QLED device is increased by 1.93 times to 14.65%, and the brightness is increased by 1.37 times to 55 760 cd/m2. The results show that the Spiro-OMeTAD layer can effectively prevent excessive electron injection into QDs, and balance the combination of electron and hole carriers.

Design a thieno[3,2-b]thiophene bridged nonfullerene acceptor to increase open-circuit voltage, short-circuit current-density and fill factor via the ternary strategy

In this study, we report a new small molecule acceptor (named TT-4F) which uses 3,6-dimethoxylthieno[3,2-b]thiophene (TT) as the π-bridge. Addition of 0.05 weight ratio amount of TT-4F into the host binary blend of PTB7-Th:IEICO-4F, resulting in a ternary blend in a weight ratio of 1:1:0.05, enables increased open-circuit voltage (Voc), short-circuit current-density (Jsc), and fill-factor (FF) at the same time. Finally, 12.1% efficiency is obtained. Compared to the 3-(2-ethylhexyloxylthiophene) bridge on IEICO-4F, the additional methoxyl group on the TT-6 position is involved in the lowest unoccupied molecular orbital (LUMO) and the larger π-system on TT increases the electron-donating nature, both of which help to raise the LUMO level, one reason of the increased Voc. Upon addition of 0.05 TT-4F, the hole mobility is increased, the monomolecular recombination is reduced, and the charge dissociation and collection is enhanced. All of these contribute to the increased Jsc and FF.

Benzodipyrrole-2,6-dione-3,7-diylidenedimalononitrile Derivatives for Air-Stable n-Type Organic Field-Effect Transistors: Critical Role of N-Alkyl Substituent on Device Performance.

Benzodipyrrole-2,6-dione-3,7-diylidenedimalononitriles (BDPMs) were synthesized as active materials for the use in air-stable n-type organic field-effect transistors (OFETs), whose optical and electrochemical properties were examined. BDPM-based small molecules exhibit deep lowest unoccupied molecular orbital levels, which are required in air-stable n-type OFETs. An OFET device that was based on BDPM-But and fabricated by vapor deposition provided a maximum electron mobility of 0.131 cm2 V-1 s-1 under ambient conditions.

Robust random forest based non-fullerene organic solar cells efficiency prediction

Abstract Non-fullerene materials have attracted attention as high-performance molecular acceptors in organic solar cells (OSCs). A proper understanding of the energy level alignment between donors and non-fullerene acceptors is crucial for photoactive materials selection in designing high-performance non-fullerene OSCs. However, the quantitative assessment for the proper selection of donors and non-fullerene acceptors is still rarely studied, which is seen as time-consuming and complicated tasks. In this study, the optimized Random Forest model based on the electronic descriptors (e.g., highest occupied molecular orbitals levels, lowest unoccupied molecular orbitals levels, and band gap) provides the high predictive power, reaching the coefficient of determination (R2) of 0.85 and 0.80 for the training set and testing set, respectively. The use of machine learning approach benefits the development of non-fullerene OSCs in two ways: (1) it helps to extract complex correlation between various descriptors and device performance, and (2) it indicates that the band gap of acceptors is the more critical feature for improving the efficiency of non-fullerene OSCs. The machine-learning model for predicting the efficiency of non-fullerene OSCs (macroscopic performance) from frontier molecular orbital energy levels of the organic materials (microscopic properties) is developed, as an important guide to design the heterojunction blends and accelerate the research for non-fullerene OSCs.

Phthalonitrile-based bipolar host for efficient green to red phosphorescent and TADF OLEDs

A bipolar host material named 4-(9H-carbazol-9-yl)phthalonitrile (CzPN) was developed by combining carbazole and phthalonitrile as donor and acceptor units, respectively. The introduction of the phthalonitrile unit realized a relatively small energy gap of 3.00 eV as well as a deep lowest unoccupied molecular orbital level of −2.72 eV. The characterization of carrier-only devices demonstrated the bipolar charge transporting property of CzPN. With a triplet energy of 2.70 eV, CzPN was investigated as host in green, yellow, and red phosphorescent organic light-emitting diodes (PhOLEDs) and green thermally activated delayed fluorescence (TADF) devices. A low turn-on voltage of 2.8 V was achieved by all the four diodes. Both PhOLEDs and TADF devices exhibited high external quantum efficiency values over 20% with relatively small efficiency roll-offs. These findings demonstrate that CzPN is an excellent host material for both phosphorescent and TADF emitters, and phthalonitrile is a promising unit to develop universal host materials.

Impact of Incorporating Nitrogen Atoms in Naphthalenediimide-Based Polymer Acceptors on the Charge Generation, Device Performance, and Stability of All-Polymer Solar Cells.

Substitution of C atoms in a polymer backbone by N atoms allows for the facile tuning of the energy levels as well as the backbone conformation and packing structures of conjugated polymers. Herein, we report a series of three polymer acceptors (PAs) with N atoms introduced at different positions of the backbone and investigate how these N atoms affect the device performances of all-polymer solar cells (all-PSCs). The three PAs, namely, P(NDI2DT-BTT), P(NDI2DT-PTT), and P(NDI2DT-BTTz), are composed of naphthalenediimide (NDI)-based and benzothiadiazole (BT)-based derivatives (dithiophene-BT (BTT), dithiophene-thiadiazolepyridine (PTT), and dithiazole-BT (BTTz)). The PTT and BTTz units are synthesized by replacing the C atoms in BT and thiophene, respectively, with N atoms, which effectively tune the optical, electrochemical, and charge-transporting properties of the corresponding PAs. The all-PSCs using poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl))benzo[1,2-b:4,5-b']dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c']dithiophene-4,8-dione)] (PBDB-T) as a polymer donor and P(NDI2DT-PTT) as PA exhibit a significantly enhanced power conversion efficiency (PCE) of 6.95%, whereas the all-PSCs based on the other PAs show relatively lower PCEs (6.02% for PBDB-T:P(NDI2DT-BTT) and 1.43% for PBDB-T:P(NDI2DT-BTTz)). The high PCE of the PBDB-T:P(NDI2DT-PTT) device is due to the superior charge transfer and charge dissociation, resulting from the closely matched energy levels between PBDB-T and P(NDI2DT-PTT), as well as a more favorable bulk heterojunction morphology with improved miscibility. Importantly, the P(NDI2DT-PTT)-based all-PSC device shows improved air stability compared to the P(NDI2DT-BTT)-based device, which is most likely due to a decreased lowest unoccupied molecular orbital level of the PA. Our findings suggest that the incorporation of N atoms into the PAs is an effective strategy for improving the efficiency and stability of all-PSCs.

High‐Output Triboelectric Nanogenerator Based on Dual Inductive and Resonance Effects‐Controlled Highly Transparent Polyimide for Self‐Powered Sensor Network Systems

AbstractHigh‐output triboelectric nanogenerators (TENGs) are demonstrated based on polyimide (PI)‐based polymers by introducing functionalities (e.g., electron‐withdrawing and electron‐donating groups) into the backbone. The TENG based on 6FDA‐APS PI, possessing the most negative electrostatic potential and the low‐lying lowest unoccupied molecular orbital level, produces the highest effective charge density of about 860 µC m−2 in practical working conditions with the ion injection process. This may be ascribed to the excellent charge‐retention characteristics as well as the enhanced charge transfer capability, which increases the output power by 7 times compared to the commercially available Kapton film‐based TENG. Finally, a 6FDA‐APS‐driven sensor network system is demonstrated, providing the identity of three gases (H2, CO, and NO2) by illuminating the light‐emitting diodes within several seconds.

Linear Correlations between Adsorption Energies and HOMO Levels for the Adsorption of Small Molecules on TiO2 Surfaces

Adsorption is a fundamental step in catalysis on a solid surface, and therefore, its understanding is important for explaining its behavior. This work investigated the adsorption of various small molecules, including H2, N2, CO, CO2, CH4, NH3, H2O, H2S, dimethyl sulfoxide, alkanes, alkenes, alkynes, aromatic compounds, alcohols, aldehydes, ketones, nitriles, carboxylic acids, amides, and amines, on the anatase (101) and rutile (110) surfaces of TiO2, using periodic density functional theory calculations and statistical methods. Adsorption energies were computed at the same level of theory to obtain a clean and consistent data set. A linear relationship was observed between the adsorption energies of these molecules and their highest occupied molecular orbital (HOMO) levels, whereas no obvious correlation was evident for the lowest unoccupied molecular orbital levels. Improved correlations between the adsorption energies and HOMO levels were generated by dividing these molecules into two subgroups: hydrocarbons and heteroatom-containing compounds. Interactions between frontier molecular orbitals and the surfaces were considered, to gain a better understanding of the significant correlations that were identified. The data show that these relationships can be primarily ascribed to the interactions between the HOMO of the small molecule and conduction state of the TiO2 surface. The statistical analysis using machine learning demonstrated that the HOMO and dipole moment are the first and second most important properties, respectively, in terms of rationalizing and predicting the adsorption energies.