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The electronic properties and second-order nonlinear optical (NLO) responses of B/N-doped zigzag graphene nanoribbon (ZGNR) have been investigated using quantum chemistry methods. The electron-deficient B atoms prefer to form π-conjugation with the C atoms nearby along the B-doped zigzag edge. On the other hand, the electron-rich N atoms with radical characteristics weaken the conjugated bonding effects in the N-doped ZGNR. The NLO response of the ZGNR is enhanced by doping only one zigzag edge with B or N atoms. The conjugated B-doped zigzag edge takes the role of electron donor, while the N-doped zigzag edge serves as electron acceptor, giving rise to the discordant impact on the second-order NLO response of the BN-doped ZGNR.

The photophysical properties of organoboron quinolate derivatives can be modified readily by manipulating the coordination environment around the central boron atom. This class of compounds applied in organic light-emitting diodes (OLEDs) materials has been studied by quantum chemistry. To reveal the relationship between the structures and properties of these electroluminescent materials, the ground- and excited-state geometries were optimized at the B3LYP/6-31G(d) and CIS/6-31G(d) levels, respectively. The ionization potentials and electron affinities were computed. The mobilities of hole and electron in these compounds were studied computationally based on the Marcus electron transfer theory. The maximum absorption and emission wavelengths of these compounds were calculated using the time-dependent density functional theory method. The solvent effect on the absorption and emission wavelengths of these compounds was also considered by a polarizable continuum model. These results show that boron compounds which containing both the hydroxyquinoline/hydroxybenzoquinoline as ligand and O/S in position X follow the rule, that is, the emission shifts to longer wavelength as covalent nature of the boron–ligand bonding is increased. Meanwhile, the negative HOMO and IPs decrease but the negative LUMO and EAs increase by substitution of O with S in position X. It was deduced that both the hole- and electron-injection abilities are improved by substituting S in place of O in position X. After chemical modification in position R 2 with electron-donating properties of NH2 or 1,4-diethynyl-2,5-dihexyloxybenzene, introduced 1,4-diethynyl-2,5-dihexyloxybenzene improves both the hole- and electron-transfer rate, which leads to better equilibrium property. It can be concluded that the better equilibrium property depends on the conjugated length of side chain in position R 2. Moreover, exchanging the substituents R 1 and R 2 in BNO1a and BNO1’a can slightly change the hole-transfer rate by 0.04 eV. According to these calculations, series BNO and BNS can be applied as electron transport and hole transport materials at the same time. Specially, series BNO2 and BNS have better performance than Mes2B[p-4,4’-biphenyl-NPh(1-naphthyl)] (BNPB) in both the hole- and electron-injection ability.

The photophysical properties of a series of multifunctional compounds applied in organic light-emitting diode (OLED) materials have been studied by quantum chemistry. These compounds have been integrated by an electron and hole transporting component as well as an emitting component into the donor–π–acceptor (D–π–A) structures. To reveal the relationship between the structures and properties of these multifunctional electroluminescent materials, the ground- and excited-state geometries were optimized at the B3LYP/6-31G(d), HF/6-31G(d), and CIS/6-31G(d) levels, respectively. The ionization potentials and electron affinities were computed. The mobilities of hole and electron in these compounds were studied computationally based on the Marcus electron transfer theory. The maximum absorption and emission wavelengths of compounds 1–4 were calculated by time-dependent density functional theory method. As a result of these calculations, it was concluded that the electron injections of compounds 2–6 are much easier than Mes2B[p-4,4′-biphenyl-NPh(1-naphthyl)] (BNPB) due to the introduction of the thiophene group, anthracene group, and N=N as a part of the π-conjugated bridge, compounds 5 and 6 can act as electron transport and hole transport materials, respectively. Compounds 1 and 2 have higher electron mobility and light-emitting efficiency as compared to compounds 3 and 4. Compounds 3 and 4 have quite longer fluorescence lifetimes than compounds 1 and 2 due to the larger Stoke’s shifts.

One of the drawbacks of the electroluminescence (EL) polymers is that they are usually much better at accepting and transporting holes than electrons due to their inherent richness of π-electrons. One approach improving electron injection and transport in conjugated polymers is to incorporate moieties with high electron affinities. In this theoretical work, to gain an insight into the chemical structure-property relationships was aimed by controllable modification of the main chain structures. Two cyanovinylene derivatives with 2,7-fluorenylene and p-phenylene moieties in the main chains, namely, poly{(2,5-dimethoxy-p-phenylene-1,4-ylene)-alt-[1,2-bis(p-phenylene)-1- cyanovinylene]} (PPhCN) and poly{[9,9-dimethyl-2,7-fluorenylene]-alt-[1,2-bis(p-phenylene)-1-cyanovinylene]} (PFCN), were studied employing density functional theory (DFT) and time dependent density functional theory (TD-DFT) with B3LYP functional. The electronic properties of the neutral molecules, extrapolated ionization potentials (IP) and electron affinities (EA), and energy gaps were investigated in comparison with pristine poly(2,7- fluorenylene). From comparison with poly(2,7-fluorenylene) (PF), the 1,2-bis(p-phenylene)-1-cyanovi- nylene unit was found to be a good electron-withdrawing moiety for electronic materials and the incorporation of 1,2-bis(p- phenylene)-1-cyanovinylene resulted in a narrow band gap and a red shift of both the absorption and photoluminescence emission peaks. Most importantly, the LUMO energies of PFCN are around 1 eV lower than those of PF, which results in the decrement of EA about 0.9 eV, indicating that the 1,2-bis(p-phenylene)-1-cyanovinylene unit has significantly improved the electron-accepting properties of the copolymer PFCN. Substitution of 2,5-dimeth- oxy-p-phenylene for 9,9-dimethyl-2,7-fluorenylene induced larger band gaps and thus a blue-shift in absorption and emission peaks, which can be attributed to the better conjugated backbone in PFCN.

Poly(fluorene)-type materials are widely used in polymer-based emitting devices. One of the drawbacks of light-emitting diodes based on polyfluorene derivatives is the injection of holes from the anode due to the high ionization potential (IP) of most derivatives. Substitution by electron-donating alkoxy substituents or by adding charge carriers on the conjugated polymer's backbone produces a remarkable influence on its electrical and optical properties. In this contribution, we apply quantum-chemical techniques to investigate a family of pi-conjugated polymers with substituted dimethoxy groups at the 3,6 positions of the fluorene ring, namely, poly(2,7-(3,6-dimethoxy-fluorene)(PDMOF), poly(2,7-(3,6-dimethoxy-fluorene)-co-alt-fluorene (PDMOFF), and poly(2,7-(3,6-dimeth-oxy-fluorene)-co-alt-2,5-thiophene (PDMOFT). The electronic properties of the neutral molecules, HOMO-LUMO gaps (Delta(H)(-)(L)), in addition to the positive and negative ions, are studied using the B3LYP functional. The lowest excitation energies (E(g)) and the maximal absorption wavelength lambda(abs) of PDMOF, PDMOFF, and PDMOFT are studied by employing time-dependent density functional theory (TD-DFT) and the ZINDO semiempirical method. The IP, EA, and E(g) values of each polymer were obtained by extrapolating those of the oligomers to the inverse chain length equal to zero ((1)/(n)() = 0). The influence of the presence of methoxy groups on the fluorene moiety on the ionization potential is especially emphasized. The outcomes show that the HOMO energies of these systems under study increase by about 0.4 eV and the IP values decrease by about 0.3 eV compared to those of the corresponding polyfluorene. Both effects result in a reduction of the energy barrier for the injection of holes in related polymeric light-emitting devices and should contribute to the enhancement of their performances. Because of the cooperation with thiophene in PDMOFT, which results in a good planar conformation, both the hole-creating and electron-accepting abilities are improved.

The equilibrium geometries, electronic structures, one- and two-photon absorption (OPA and TPA) properties of macrocyclic thiophene derivatives [C[6T_A_E]n–Me (n = 1–3)] with different ring sizes have been investigated by quantum chemistry calculation. The results indicate that as the unit number increases, the π-conjugated effect enhances, OPA maximum wavelength (λ(1)max) shows a significant red-shift with increasing oscillator strength (ƒ0k), and the TPA cross-section value (δmax) also increases, the δmax value of C[6T_A_E]3–Me is twentyfold larger than that of the repeat unit C[6T_A_E]–Me. We have analyzed the internal factors influencing δmax such as transition dipole moments (M0k and Mkn), the product of oscillator strengths (ƒ0k × ƒkn), and the number of π electrons (Ne) in detail. In addition, it is demonstrated that intramolecular charge transfer (ICT) is one of the intrinsic factors, but not a dominant one for the increase in TPA cross-section with regard to this system.

Doubly ortho-linked cis-4,4′-bis(diarylamino)stilbene/fluorene hybrids have potential application in organic light emitting diodes. These organic molecules can function as efficient hole transport materials, with sky-blue emission and high efficiency and stability. To reveal the relationship between the properties and structures of these functional materials, we apply quantum-chemical techniques to investigate their optical properties and electronic structures. It is found that 8- and 8′-substituents give an antibonding contribution to the highest occupied molecular orbitals (HOMO), thus increasing the HOMO energy, implying an enhancement of hole-creating ability. Changes in ionization potentials confirm this presumption. The absorption and emission spectra exhibit red shifts to some extent with the various 8- and 8′-substituents, which are in good agreement with the experimental values. The Stoke shifts range moderately from 40 to 86 nm, due to the rigid spiral structure that hinders geometrical relaxation.