Design And Synthesis Of N-type Fused-Ring Molecules Based On B←N And Imide Units

Organic semiconductors are the material basis of organic electronics and can be divided into p-type semiconductors for hole transport and n-type semiconductors for electron transport.n-Type organic semiconductor material is an indispensable part of logic complementary circuit,and its development is relatively backward.n-Type organic semiconductors require low energy levels,which are usually achieved by introducing electron-deficient groups into theπ-conjugate skeleton.At present,imide is the most widely studied electron-deficient group,and the LUMO(Lowest Unoccupied Molecular Orbital)energy level of molecules can reach-4.0 e V.In order to reduce the electron injection barrier in contact with the metal electrode,the energy level of n-type organic semiconductor needs to be further reduced.We have carried out innovative research on the design and synthesis of low-level n-type organic small molecule semiconductors.We have designed and synthesized a series of n-type fused ring molecules containing two electron-deficient groups,namely boron nitrogen coordination bond(B←N)and imide.Through the synergistic action of boron nitrogen coordination bond and imide,the energy level of n-type organic small molecule can be reduced more effectively.It provides a new design idea and a new material system for the development of N-type organic small molecule materials.The main research contents and results are as follows:In the first part,a molecular design strategy of introducing two boron and nitrogen coordination bonds and two imides into pyrene-fused azahexacene is proposed.Through the isomerization of two B←N units,n-type fused ring molecules with center symmetry(c-BNI)and axisymmetric symmetry(a-BNI)are designed and synthesized.Both the two molecules have planar configuration,which is beneficial for closeπ-stacking in solid state and thus for charge transport.a-BNI has net dipole moment of zero,while c-BNI has a net dipole moment of 3.20 Debye with intramolecular charge transfer character.The maximum absorption peaks of c-BNI and a-BNI in toluene solution are 691 nm and 701 nm,respectively,and the absorption spectra of them in thin films are relatively wide,with two absorption peaks occurring at 716 nm and 660 nm.c-BNI and a-BNI exhibit deep-lying LUMO and HOMO(Highest Occupied Molecular Orbital)energy levels of-4.04 e V/-5.78 e V and-4.12e V/-5.95 e V,respectively,which are much lower than those of n-type molecules containing only two B←N units or two imide units reported in literatures.In the second part,on the basis of the work in the first part,we further reduce the energy level of molecules by extending theπ-conjugated length.we synthesis a large n-type all-fused-ring molecule,the two B←N units are incorporated to the core and the two imide units are incorporated to the termini of a dibenzo-azaacene skeleton consisting of 14 fused rings(nap-BNI).nap-BNI has planar configuration and delocalized LUMO and HOMO of the molecule.The increase of molecular conjugation length causes the maximum absorption wavelength of molecules in toluene solution to be redshifted by 30 nm(721 nm)and exhibits near-infrared fluorescence(maximum emission wavelength of 746 nm and fluorescence quantum efficiency of 12%).Compared with the solution state,the absorption spectrum of nap-BNI films is widened and blue-shifted(maximum absorption peak is 659 nm),and there is no fluorescence emission property,indicating that H aggregation is formed in nap-BNI films.The LUMO and HOMO levels of nap-BNI are as low as-4.16 e V and-5.91 e V,respectively.In the third part,based on the work of the first part,a design strategy is proposed to further reduce the molecular energy level by increasing the number of B←N units,and N-type dense ring molecules(4-BNI)with four B←N units and two imide groups are synthesized.4-BNI presents a curved configuration,and the intramolecular azabenzobenzene skeleton presents a dihedral angle of 23.6~o.4-BNI exhibited obvious near-infrared absorption in toluene solution,with a maximum absorption wavelength of 770 nm.Because of its curved surface configuration,the aggregation is weak in thin film,and the absorption spectrum is similar to the solution state.4-BNI shows low LUMO and HOMO levels of-4.77 e V and-6.39 e V,respectively.The 4-BNI doped P3HT films as p-dopant increased the electrical conductivity of P3HT films by 3 orders of magnitude,and the doping behavior was confirmed by UV-visible absorption spectra and electron paramagnetic resonance spectroscopy.In general,we proposed a molecular design idea to reduce the energy level of n-type organic small molecules through the synergistic action of boron and nitrogen coordination bonds and imide.A series of n-type fused ring molecules containing both boron and nitrogen coordination bonds and imide were synthesized,which reduced the LUMO level of molecules to-4.77 e V,close to the energy level limit of organic semiconductors.These low-level n-type fused ring molecules have a broad application prospect in field effect transistors,solar cells and other optoelectronic devices.