| The current development of n-type organic semiconductors is far lag behind that of p-type organic semiconductors,for n-type and p-type semiconductors are both components of logic circuits,the development of high-performance n-type semiconductors is an urgent problem to be solved.Bithiophene imide（BTI）hold advantages of containing electron-absorbing imide groups,good planarity and small steric hindrance,but the molecule contains two electron-rich thiophene rings,which reduces the electron affinity of the molecule,and the LUMO energy level of the molecule is not low enough,making the BTI unit likely to show p-type performance.In this paper,the main research work focuses on the BTI unit,introduces the active functional group aldehyde group on the BTI unit,and using the reaction of the aldehyde group to modify the BTI unit,increase the electron affinity of the molecule,and develop a series of new n-type organic semiconductors.The main contents of this paper are as follows:We report here a class of small molecular OTMs based on terminal cyanation of a bithiophene imide-based ladder-type heteroarene BTI2.It was found that the cyanation could effectively lower the lowest unoccupied molecular orbital（LUMO）level from-2.90 e V（BTI2）to-4.14 e V（BTI2-4CN）and thus lead to significantly improved n-doping efficiency.Additionally,the terminal cyano-functionalization can maintain the close packing and efficient intermolecular charge transfer between these cyanated molecules,thus yielding high electron mobilities of up to 0.40 cm2 V-1 s-1.Benefited from its low LUMO-enabled efficient n-doping and high electron mobility,an encouraging n-type electrical conductivity of 0.43 S cm-1 and power factor（PF）of6.34μW m-1 K-2 were achieved for tetracyanated BTI2-4CN,significantly outperforming those of its noncynated BTI2(<10-7 S cm-1,PF undetectable).These results suggest the great potential of the terminal cyanation strategy of ladder-type heteroarenes for developing high-performance small molecular OTMs.While the development of high-performing n-type polymer semiconductors remains a challenge,the involving of costly transition-metal catalysts and even toxic reagents during the preparation of these materials might also cause serious environmental issues.Herein,by devising two new aldehyde-functionalized bithiophene imide（BTI）-type building blocks,we design and synthesized two acceptor-acceptor type n-type polymers,BTI-BD and BTI2-BD,on the basis of an environmentally benign,transition metal-free and thus cost-effective aldol condensation.The resulting polymers exhibited planar and rigid backbone conformations and low LUMO energy levels close to-4.0 e V,thus leading to unipolar n-type tranport character and a highest electron mobility of 1.35×10-3 cm2 V-1 s-1 in organic field-effect transistors for the BTI2-BD polymer which features a higher molecular ordering in the solid states.Our results demonstrate that the aldol polycondensation strategy could serve as a cheaper and more ecofriendly alternative to the convential transition metal-catalyzed cross-coupling reactions for constructing n-type acceptor-acceptor polymers with unipolar n-type charge transport characteristics.Compared with A-A type n-type polymer semiconductors,D-A type polymer semiconductors are more widely studied.The push-pull effect between the donor and acceptor units in the D-A polymer can effectively promote the transfer of charge,while the donor unit in the D-A polymer has high reactivity and is easier to synthesis.Based on the aldehyde-based BTI unit,we constructed a strong acceptor unit with indole derivatives by dehydration condensation reaction,and then copolymerized with different donor units to obtain three D-A polymer semiconductors P-BTIID-T,P-BTIID-FT,and P-BTIID-Se.Since BTI and indole derivatives are electron-deficient units,the LUMO energy level of the three polymers is effectively reduced,and the field-effect transistors based on the three polymers are all electron transport,with electron mobility of 5.79×10-4 cm2 V-1 s-1. |
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