Theoretical Study on Optoelectronic Properties of 1,4-Dithiazole-5,10-dihydrophenazine and Its B ← N-Fused Derivatives.

Recently, Liu et al. reported 1,4-dithiazole-5,10-dihydrophenazine (DTDHP) and its B ← N-fused derivative (DTHDHP-BF2), which were expected to show excellent optoelectronic properties (Angew. Chem. Int. Ed. 2022, 61, e202205893). However, their charge-transport performance and luminescence emission mechanisms have not been revealed. In this work, we used density functional theory (DFT) calculations to investigate the optoelectronic properties of DTDHP and DTHDHP-BF2 and analyzed the influence of the introduction of -BF2 on the basic parameters governing charge transport and injection in detail. Our calculation results showed that adding -BF2 could stabilize the frontier molecular orbitals and decrease the reorganization energies associated with electron transport due to the formation of B ← N bonds, and the intermolecular electronic couplings are greatly enhanced owing to the strong intermolecular F···H interactions. Based on the master equation coupled with the Marcus-Hush electron transfer theory, we theoretically predicted the charge transport properties of DTDHP and DTHDHP-BF2. The optimum hole mobility (3.87 cm2 V-1 S-1) and electron mobility (1.52 cm2 V-1 S-1) of DTHDHP-BF2 are, respectively, 3 and 9 times as high as the corresponding optimum values of compound DTDHP. Moreover, the assignments of multiple fluorescence bands in the experiment were confirmed by time-dependent density functional theory (TDDFT) calculations. The simulated emission spectra indicate that the experimental fluorescence maxima at 687 nm originates from the S1 → S0 transition of the double proton transfer phototautomer (T2H) of DTDHP, and the shoulder peak at ∼660 nm may be related to the excited-state single-proton transfer phototautomer (T1H); for DTHDHP-BF2, the experimental fluorescence maxima at 687 nm should be attributed to normal Stokes shifted emission, and the shifted fluorescence with a peak at 751 nm originates from the emission of the photodissociation product of DTHDHP-BF2.

Theoretical Insights into the Luminescence and Sensing Mechanisms of N,N'-Bis(salicylidene)-[2-(3',4'-diaminophenyl)benzthiazole] for Copper(II).

The intramolecular proton transfer (IPT) reaction potential energy surfaces (PESs) of N,N'-bis(salicylidene)-[2-(3',4'-diaminophenyl)benzthiazole] (BTS) in the S0 state and S1 state are constructed. It is found that the IPT reactions in the ground state hardly take place due to the high reaction energy barrier for single-proton (6.3 kcal/mol) and double-proton transfer (14.1 kcal/mol) reactions and low backward reaction energy barriers for single-proton (1.9 kcal/mol) and double-proton transfer (1.2 kcal/mol) reactions. In comparison, an excited-state intramolecular single-proton transfer reaction is a barrierless and exothermic process, and thus, single-proton transfer tautomer T1H contributes most to the fluorescence emission. Based on the analysis of PESs, the experimental absorption and emission spectra are reproduced well by the calculated vertical excitation energies of BTS and its photoisomerization products, and the triple fluorescence emission profile in the experiment is reassigned unequivocally. Furthermore, thermodynamic analysis of the BTS-Cu(II) complex shows that the dinuclear complex (C1) with Cu(II) coordinating with O and N atoms of the hydrogen bonds is the most thermodynamically stable structure, and the intramolecular hydrogen bonding structure in BTS is destroyed due to the chelation of Cu(II) and BTS; as a result, the IPT reaction of C1 in S0 and S1 states is significantly inhibited. The inhibitor of Cu(II) in the IPT reaction plays a major role in fluorescence quenching.

Quantitative Prediction of Charge Mobilities and Theoretical Insight into the Regulation of Site-Specific Trifluoromethylethynyl Substitution to Electronic and Charge Transport Properties of 9,10-Anthraquinone.

Herein, we systematically studied the electronic and conducting properties of 9,10-anthraquinone (AQ) and its derivatives and discussed the substitute-site effects on their organic field-effect transistor (OFET) properties in detail. Our calculation results show the influence of different substitute sites on the ionization potential (IP), electronic affinity (EA), reorganization energy (λ), electronic couplings (V), and anisotropic mobility (µ) of semiconducting materials, which mainly originates from the variations of the frontier molecular orbital charge distributions, the steric hindrance, and the conjugate degree. Combining quantum-chemical calculations with charge transfer theory, we simulated the intermolecular hopping rate in the organic crystals of AQ derivatives and predicted the fluctuation range of three-dimensional (3D) anisotropic charge carrier mobility for the first time. Our calculation results well reproduced the experimental observations and provided evidence for the determination of the optimal OFET conduction plane and channel direction relative to the crystal axis.

Theoretical study of charge-transport and optical properties of organic crystals: 4,5,9,10-pyrenedi-imides.

This work presents a systematic study of the conducting and optical properties of a family of aromatic di-imides reported recently and discusses the influences of side-chain substitution on the reorganization energies, crystal packing, electronic couplings and charge injection barrier of 4,5,9,10-pyrenedi-imide (PyDI). Quantum-chemical calculations combined with the Marcus-Hush electron transfer theory revealed that the introduction of a side chain into 4,5,9,10-pyrenedi-imide increases intermolecular steric interactions and hinders close intermolecular π-π stacking, which results in weak electronic couplings and finally causes lower intrinsic hole and electron mobility in t-C5-PyDI (µh = 0.004 cm2 V-1 s-1 and µe = 0.00003 cm2 V-1 s-1) than in the C5-PyDI crystal (µh = 0.16 cm2 V-1 s-1 and µe = 0.08 cm2 V-1 s-1). Furthermore, electronic spectra of C5-PyDI were simulated and time-dependent density functional theory calculation results showed that the predicted fluorescence maximum of t-C5-PyDI, corresponding to an S 1→S 0 transition process, is located at 485 nm, which is close to the experimental value (480 nm).

Charge-transport properties of 4-(1,2,2-tri-phenyl-vinyl)-aniline salicylaldehyde hydrazone: tight-packing induced molecular 'hardening'.

Based on first-principles calculations, the relationship between molecular packing and charge-transport parameters has been investigated and analysed in detail. It is found that the crystal packing forces in the flexible organic molecule 4-(1,2,2-triphenylvinyl)-aniline salicylaldehyde hydrazone (A) can apparently overcome the dynamic intramolecular rotations and the intramolecular steric repulsion, effectively enhancing the molecular rigidity and decreasing the internal reorganization energy. The conducting properties of A have also been simulated within the framework of hopping models, and the calculation results show that the intrinsic electron mobility in A is much higher than the corresponding intrinsic hole mobility. These theoretical investigations provide guidance for the efficient and targeted control of the molecular packing and charge-transport properties of organic small-molecule semiconductors and conjugated polymeric materials.

A DFT Study on the Electronic Structures and Conducting Properties of Rubrene and its Derivatives in Organic Field-Effect Transistors.

We systematically studied the electronic structures and conducting properties of rubrene and its derivatives reported recently, and disscussed the influences of electron-withdrawing groups and chemical oxidation on the reorganization energies, crystal packing, electronic couplings, and charge injection barrier of rubrene. Hirshfeld surface analysis and quantum-chemical calculations revealed that the introduction of CF3 groups into rubrene decreases the H···H repulsive interaction and increases intermolecular F···H/H···F attractive interactions, which resulted in the tight packing arrangement and the increase of the electronic couplings, and finally cause the higer intrinsic hole-mobility in bis(trifluoromethyl)-dimethyl-rubrene crystal (µh = 19.2 cm2 V-1 s-1) than in rubrene crystal (µh = 15.8 cm2 V-1 s-1). In comparison, chemical oxidation reduces charge-carrier mobility of rubrene crystal by 2~4 orders of magnitude and increased the hole and electron injection barrier, which partly explains the rubrene-based field-effect transistor performance degrades upon exposure to air. Furthermore, we also discussed the influence of structural parameters of carbon nanotube (CNT) electrode on charge injection process, which suggests that the regulation of CNT diameters and increasing in thickness is an effective strategy to optimize CNT work functions and improve n-type OFET performances based on these organic materials.

Anisotropic electron-transfer mobilities in diethynyl-indenofluorene-dione crystals as high-performance n-type organic semiconductor materials: remarkable enhancement by varying substituents.

In this study, the electron-transfer properties of alkynylated indenofluorene-diones with various substituents (SiMe3, SiPr3, and SiPh3) that function as n-type organic semiconductors were comparatively investigated at the first-principles DFT level based on the Marcus-Hush theory. The reorganization energies are calculated by the adiabatic potential-energy surface method, and the coupling terms are evaluated through a direct adiabatic model. The maximum value of the electron-transfer mobility of SiPr3 is 0.485 cm(2) V(-1) s(-1), which appears at the orientation angle of the conducting channel on the reference plane a-b near to 172°/352°. The predicted maximum electron mobility value of SiPr3 is nearly 26 times larger than that of SiPh3. This may be attributed to the largest number of intermolecular π-π interactions. In addition, the mobilities in all three crystals show remarkable anisotropic behavior. The calculated results indicate that SiPr3 could be an ideal candidate as a high-performance n-type organic semiconductor material. Our investigations not only give us an opportunity to completely understand the charge transport mechanisms, but also provide guidelines for designing materials for electronic applications.

Quantitative prediction of charge mobilities of π-stacked systems by first-principles simulation.

This protocol is intended to provide chemists and physicists with a tool for predicting the charge carrier mobilities of π-stacked systems such as organic semiconductors and the DNA double helix. An experimentally determined crystal structure is required as a starting point. The simulation involves the following operations: (i) searching the crystal structure; (ii) selecting molecular monomers and dimers from the crystal structure; (iii) using density function theory (DFT) calculations to determine electronic coupling for dimers; (iv) using DFT calculations to determine self-reorganization energy of monomers; and (v) using a numerical calculation to determine the charge carrier mobility. For a single crystal structure consisting of medium-sized molecules, this protocol can be completed in ∼4 h. We have selected two case studies (a rubrene crystal and a DNA segment) as examples of how this procedure can be used.

Electronic structure and microscopic charge-transport properties of a new-type diketopyrrolopyrrole-based material.

Recently, diketopyrrolopyrrole (DPP)-based materials have attracted much interest due to their promising performance as a subunit in organic field effect transistors. Using density functional theory and charge-transport models, we investigated the electronic structure and microscopic charge transport properties of the cyanated bithiophene-functionalized DPP molecule (compound 1). First, we analyzed in detail the partition of the total relaxation (polaron) energy into the contributions from each vibrational mode and the influence of bond-parameter variations on the local electron-vibration coupling of compound 1, which well explains the effects of different functional groups on internal reorganization energy (λ). Then, we investigated the structural and electronic properties of compound 1 in its isolated molecular state and in the solid state form, and further simulated the angular resolution anisotropic mobility for both electron- and hole-transport using two different simulation methods: (i) the mobility orientation function proposed in our previous studies (method 1); and (ii) the master equation approach (method 2). The calculated electron-transfer mobility (0.00003-0.784 cm(2) V(-1) s(-1) from method 1 and 0.02-2.26 cm(2) V(-1) s(-1) from method 2) matched reasonably with the experimentally reported value (0.07-0.55 cm(2) V(-1) s(-1) ). To the best of our knowledge, this is the first time that the transport parameters of compound 1 were calculated in the context of band model and hopping models, and both calculation results suggest that the intrinsic hole mobility is higher than the corresponding intrinsic electron mobility. Our calculation results here will be instructive to further explore the potential of other higher DPP-containing quinoidal small molecules.

First-principles investigation of anisotropic electron and hole mobility in heterocyclic oligomer crystals.

Based on quantum chemistry calculations combined with the Marcus-Hush electron transfer theory, we investigated the charge-transport properties of oligothiophenes (nTs) and oligopyrroles (nPs) (n=6, 7, 8) as potential p- or n-type organic semiconductor materials. The results of our calculations indicate that 1) the nPs show intrinsic hole mobilities as high as or even higher than those of nTs, and 2) the vertical ionization potentials (VIPs) of the nPs are about 0.6-0.7 eV smaller than the corresponding VIPs of the nTs. Based on their charge-transport ability and hole-injection efficiency, the nPs have potential as p-type organic semiconducting materials. Furthermore, it was also found that the maximum values of the electron-transfer mobility for the nTs are larger by one-to-two orders of magnitude than the corresponding maximum values of hole-transfer mobility, which suggests that the nTs have the potential to be developed as promising n-type organic semiconducting materials owing to their electron mobility.

First-principles investigation of the electronic and conducting properties of oligothienoacenes and their derivatives.

Herein, we calculated reorganization energies, vertical ionization energies, electron affinities, and HOMO-LUMO gaps of fused thiophenes and their derivatives, and analyzed the influence of different substituents on their electronic properties. Furthermore, we simulated the angular resolution anisotropic mobility for both electron- and hole-transport, based on quantum-chemical calculations combined with the Marcus-Hush electron-transfer theory. We showed that: 1) styrene-group substitution can effectively elevate the HOMO energy level and lower the LUMO energy level, and therefore lower both the hole- and electron-injection barriers; and 2) chemical oxidation of the thiophene ring can significantly improve the semiconductor properties of the fused oligothiophenes through a decrease of the injection barrier and an increase in the charge-transfer mobility for electrons but without lowering their hole-transfer mobilities, which suggests that it may be a promising way to convert p-type semiconductors into ambipolar or n-type semiconductor materials.

Density functional theory study on electron and hole transport properties of organic pentacene derivatives with electron-withdrawing substituent.

Attaching electron-withdrawing substituent to organic conjugated molecules is considered as an effective method to produce n-type and ambipolar transport materials. In this work, we use density functional theory calculations to investigate the electron and hole transport properties of pentacene (PENT) derivatives after substituent and simulate the angular resolution anisotropic mobility for both electron and hole transport. Our results show that adding electron-withdrawing substituents can lower the energy level of lowest unoccupied molecular orbital (LUMO) and increase electron affinity, which are beneficial to the electron injection and ambient stability of the material. Also the LUMO electronic couplings for electron transport in these pentacene derivatives can achieve up to a hundred meV which promises good electron transport mobility, although adding electron-withdrawing groups will introduce the increase of electron transfer reorganization energy. The final results of our angular resolution anisotropic mobility simulations show that the electron mobility of these pentacene derivatives can get to several cm(2) V(-1) s(-1), but it is important to control the orientation of the organic material relative to the device channel to obtain the highest electron mobility. Our investigation provide detailed information to assist in the design of n-type and ambipolar organic electronic materials with high mobility performance.

Simulation of hole mobility in α-oligofuran crystals.

We investigated oligofuran (nF) (n=3, 4, 6) heterocyclic oligomers as p-type organic semiconductor materials, based on quantum chemistry calculations combined with the Marcus-Hush electron transfer theory. It was found that 6F single crystal, with a structure similar to that of 6T, possesses high hole-transfer mobility, which is nearly 17 times larger than that of 6T single crystal. In addtion, the ionization potential (IP) value of 6F is about 5.60 eV, that is, slightly smaller than the IP value of 6T (5.74 eV). The relatively small IP values ensure effective hole injection from the source electrode. Considering that 6T and functional oligothiophenes are active p-type semiconducting materials widely used in organic electronic devices, nFs and nF-based molecules have the potential to be developed as potential high efficiency p-type organic semiconducting materials.