Structure-Performance Correlations in Spray-Processable Green Dioxythiophene-Benzothiadiazole Donor-Acceptor Polymer Electrochromes

The continuing search for relevant structure-property relationships in the area of organic electronics is expected to impact both intrinsic material performance capability and the viability of their implementation in a broad range of device applications. Cathodically coloring pi-conjugated polymer electrochromes represent a class of materials potentially attractive for low-cost and nonemissive flexible display devices including e-paper. Nonetheless, both the synthetic access to a full range of visible colors and the ability to produce solution-processable systems that switch rapidly and durably from a colored neutral state to a highly transmissive doped state upon electrochemical oxidation require that material structure-property relationships be carefully examined. In this report, we correlate molecular structure effects, redox properties, and electrochromic performance for a series of rationally designed neutral-state green polymers composed of electron-rich 3,4-dioxythiophene (DOT) units and the electron-deficient core 2,1,3-benzothiadiazole (BTD). While homopolymers synthesized from 3,4-alkylenedioxy-bridged monomers including 3,4-ethylenedioxythiophene (EDOT) and 3,4-propylenedioxythiophenes (ProDOT) have shown particularly desirable redox-switching properties in the early years of electrochromic polymer development, their "unbridged" dialkoxythiophene counterparts (DalkOTs) have not raised the same initial interest. Herein, it is shown that low band gap systems relying on DalkOT units and electron-deficient BTD cores could represent viable alternatives to their ProDOT-based counterparts in electrochemical devices involving green-to-transmissive switching electrochromes. Interestingly, provided the set of materials examined in this study, the long-term switching stability of the ProDOT-co-BTD system remains superior to that of its polymeric analog relying on DalkOTs - exhibiting less than 15% loss of contrast over 20,000 switching cycles (atmospheric conditions). Long-term cycle life is further demonstrated in a window-type device integrating the ProDOT-co-BTD system. DFT calculations performed at the B3LYP/6-31G** level suggest subtle variations in the energy-band structure of the polymer repeat-units and predict the existence of the dual band of optical absorption exhibited by the low-band gap polymers.