Design,synthesis and charge transport properties of conjugated copolymers based on 2,6-azulene and bi-thieno[3,4-c]pyrrole-4,6-dione
Azulene,a nonalternant and nonbenzenoid hydrocarbon,has drawn increasing attention for constructing optoelectronic materials owing to its unique electronic structure and physicochemical properties.It is still a challenge to control the dipole orientation of azulene units in the backbone of 2,6-azulene-based conjugated polymers.Herein,three 2,6-azulene and bi-thieno[3,4-c]pyrrole-4,6-dione(BTPD)based conjugated copolymers P(AzBTPD-1),P(AzBTPD-2),and P(AzBTPD-3)with different dipole arrangements of azulene moieties were synthesized by direct arylation polymerization,where the rational design of the monomers allows for the achievement of the precisely controlled orientation of azulene units in the polymer main chain.The dipole arrangements of 2,6-azulene units were random for P(AzBTPD-1),head-to-head and tail-to-tail-arranged for P(AzBTPD-2)and head-to-tail-arranged for P(AzBTPD-3).High-temperature gel permeation chromatography of P(AzBTPD-1),P(AzBTPD-2),and P(AzBTPD-3)at 150℃ with 1,2,4-trichlorobenzene as the eluent gave average molecular weight values of 20.8,20.7,and 24.1 kDa,respectively,with the corresponding polydispersity index values of 2.22,2.48,and 2.17,respectively.All three polymers have similar molecular weights,thereby the influence of molecular weight can be ignored.UV-vis absorption spectra and cyclic voltammetry were performed to evaluate the optoelectronic properties of these three polymers.The maximum absorption wavelength of P(AzBTPD-1),P(AzBTPD-2),and P(AzBTPD-3)in thin film showed red shifts(8,11 and 17 nm)relative to those in chloroform solution.The largest red shift of 17 nm of P(AzBTPD-3)indicated its strong intermolecular interactions in solid state.The energy levels of highest occupied molecular orbital(HOMO)and lowest unoccupied molecular orbital(LUMO)of P(AzBTPD-1),P(AzBTPD-2),and P(AzBTPD-3)were-5.27/-3.56 eV,-5.27/-3.58 eV and-5.27/-3.59 eV,respectively,which were acquired by cyclic voltammetry measurements.Due to the electron-withdrawing property of BTPD,these three polymers did not show obvious proton responsiveness,and their UV-vis absorption spectra show no obvious change upon protonation.To investigate the effect of azulene dipole arrangements in the polymeric backbone on the charge transport properties of these polymers,bottom-gate and top-contact organic field-effect transistor(OFET)devices based on these three polymers were fabricated.Under nitrogen atmosphere,all three polymers showed unique n-channel charge transport behaviors.The thermal annealed OFETs based on P(AzBTPD-1),P(AzBTPD-2),and P(AzBTPD-3)showed electron mobilities of 0.011,0.019 and 0.027 cm2 V-1 s-1,respectively.The higher electron-transport ability of P(AzBTPD-3)is consistent with the better degree of polymer order and the lower LUMO energy level obtained from cyclic voltammetry measurement.Examination of thin films via atomic force microscopy(AFM)provides the evidence of the best morphology of P(AzBTPD-3)among three polymers,which is in accordance with its best device performance.The root mean square roughness(RMS)values of thin films of these three polymers decreased after thermal annealing treatment,indicating the improved thin film morphology of these three polymers,which is consistent with the gradually enhanced performance of these polymers'OFET devices.Therefore,regulation the dipole arrangements of 2,6-azulene units in the polymeric backbone is an effective strategy for obtaining high-performance organic optoelectronic materials.Our work not only presented an efficient strategy to achieve the precisely controlled structural regularity of 2,6-azulene-based conjugated polymers,but also gave new insights for the synthetic chemistry of polymers with low-symmetrical conjugated monomers and the study of their structure-property relationships.
万方数据
维普
