Covalent organic frameworks (COFs) are a new emerging class of two-dimensional functional materials, and their unique electronic character plays a key role in practical applications. Here, to provide a useful strategy to tune the electronic character of COFs, doping effects with organic molecular acceptors, TCNQ, F4TCNQ, and TCNE, on the frontier orbital energy levels of sp2c-COF and COF366 mono- and bilayers are explored. Due to the charge transfer from COFs to dopants, the Fermi level is shifted to a lower energy and drops into the top valence band of sp2c-COF and COF366, and the VBM/CBM energy level of COFs is also decreased. Moreover, the increased electron density shifts the LUMO energy level of dopants to a higher energy. Then, two electronic states, the top valence band of COFs and the LUMO of the p-type dopants, are pinned around the Fermi level. It means that the organic molecular acceptor serves as an effective p-type dopant for COFs. In addition, it is confirmed that the stronger the electron-accepting ability of p-type dopants or the higher the surface density of the dopants, the larger the variation of the frontier energy levels of COF monolayers will be. Thereby, an overall linear correlation between the electronic property variations of COFs and the charge transfer amount from COFs to p-type dopants is observed. Our results proved that surface doping with organic molecular acceptors is a reliable approach to modulate the frontier energy level of COFs, which provides an effective strategy to optimize the performance of COF-based devices.

Diarylethene (DAE) is one of the most widely used functional unit for electrochromic or photochromic materials. To better understand the molecular modification effects on the electrochromic and photochromic properties of DAE, two modification strategies, substitution with functional groups or heteroatom, was investigated theoretically by density functional theory (DFT) calculations. It is found that red-shifted absorption spectra caused by decreased HOMO-LUMO energy gap and S0→S1 transition energy during the ring-closing reaction, will become more significantly by adding different functional substituents. In addition, for two isomers, the energy gap and S0→S1 transition energy decreased by heteroatom substitution of S atoms with O or NH, while are increased by replacing of two S atoms with CH2. For the intramolecular isomerization, one-electron excitation is the most effective way to trigger the O→C reaction, while the C→O reaction occurs most readily in the presence of one-electron reduction. Moreover, it is confirmed that substitution with strong electron donating groups (-OCH3/-NH2) or with one O/two CH2 heteroatoms will lead to a more favorable O→C reaction. Functionalized with strong electron-withdrawing groups (-NO2 and -COOH) or one/two NH heteroatom substitution will make the C→O reaction easier. Our results confirmed that the photochromic and electrochromic properties of DAE can be tuned effectively by molecular modifications, which provides theoretical guidance for the design of new DAE-based photochromic/electrochromic materials.

No abstract

Sulfone-based molecules are widely used as molecular building blocks for host materials in the emissive layers of organic light-emitting diodes (OLEDs). In this work, the electronic properties of dibenzo[b,d]thiophene 5,5-dioxide (DBTO) and its derivatives were investigated by quantum-chemical techniques to get more detailed information about the carbazole and biphenylamino functionalization effects on the charge injection property and triplet transition energy of such SO 2 -based host materials. Calculated results demonstrated that the charge injection property of DBTO can be tuned by different functionalization strategies. Vertical ionization potential (VIP) and vertical electron affinity (VEA) values are reduced by functionalization, especially in the case of biphenylamino substitution. The S 0 ! S 1 and S 0 ! T 1 * transitions, which are mainly dominated by the HOMO and LUMO energy levels, can also be tuned by functionalization. For the S 0 ! S 1 transition, the effect of carbazole substitution is larger than that of biphenylamino substitution, while the effect of carbazole substitution on the S 0 ! T 1 * transition energies of DBTO is smaller than that of biphenylamino substitution. This effect will be more obvious if the number of introduced functional groups increases. In addition, it was observed that the substitution effect at the meta-position of DBTO is much larger than that at the para-position.

No abstract