Effects of replacing carbamate with alkyl side chains on the properties and temperature sensing performance of hemi-isoindigo-based polymers

Previously, we developed several carbamate side chain-substituted hemi-isoindigo (HID)-based π-conjugated polymers, which demonstrated excellent sensitivity and stability as the sensing layers in chemiresistive temperature sensors. This work investigated the effects of the side chains on the HID units by changing the carbamate to alkyl side chains. Specifically, a series of 2-ethylhexyl-substituted HID polymers, poly(3-((3'',4'-bis(dodecyloxy)-[2,2':5',2''-terthiophen]-5-yl) methylene)-1-(2-ethylhexyl)indolin-2-one-6,5”-diyl) (PTAB), poly(3-((3'',4'-bis(dodecyloxy)-3,4-dimethoxy-[2,2':5',2''-terthiophen]-5-yl) methylene)-1-(2-ethylhexyl)indolin-2-one-6,5”-diyl) (PMAB), and poly(3-((7-(3,3'-bis(dodecyloxy)-[2,2'-bithiophen]-5-yl)-2,3-dihydrothieno[3,4-b] [1,4]dioxin-5-yl)methylene)-1-(2-ethylhexyl)indolin-2-one-6,5”-diyl) (PEAB) were synthesized, and their properties and temperature sensing performance were compared with their counterpart carbamate-substituted HID polymers, poly(2-ethylhexyl-3-((3'',4'-bis(dodecyloxy)-[2,2':5',2''-terthiophen]-5-yl)methylene)-2-oxoindoline-1-carboxylate-6,5”-diyl) (PTEB), poly(2-ethylhexyl-3-((3'',4'-bis(dodecyloxy)-3,4-dimethoxy-[2,2':5',2''-terthiophen]-5-yl)methylene)-2-oxoindoline-1-carboxylate-6,5”-diyl) (PMEB), and poly(2-ethylhexyl-3-((7-(3,3'-bis(dodecyloxy)-[2,2'-bithiophen]-5-yl)-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)methylene)-2-oxoindoline-1-carboxylate-6,5”-diyl) (PEEB), and their thermally annealed products, poly(3-((3'',4'-bis(dodecyloxy)-[2,2':5',2''-terthiophen]-5-yl)methylene)indolin-2-one-6,5”-diyl) (PTNB), poly(3-((3'',4'-bis(dodecyloxy)-3,4-dimethoxy-[2,2':5',2''-terthiophen]-5-yl)methylene)indolin-2-one-6,5”-diyl) (PMNB), and poly(3-((7-(3,3'-bis(dodecyloxy)-[2,2'-bithiophen]-5-yl)-2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)methylene)indolin-2-one-6,5”-diyl) (PENB). The highest occupied molecular orbital energy (E_HOMO) level and crystallinity of PEAB are very similar compared to PEEB. Chemiresistor devices with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) (PEAB:F4TCNQ) fabricated on flexible plastic substrates exhibited a high temperature coefficient of resistance (TCR) of −1.09% °C⁻¹, although the value is lower than that (−1.92% °C⁻¹) of the device based on PENB:F4TCNQ. The device based on PEAB:F4TCNQ also showed excellent stability with no performance degradation over 1 month, which is similar to the device based on PENB:F4TCNQ. On the other hand, PTAB and PMAB showed significantly higher E_HOMO levels and crystallinity compared to their counterpart polymers. Sensors based on PTAB:F4TCNQ and PMAB:F4TCNQ showed TCR values of −1.02% °C⁻¹ and −1.15% °C⁻¹, respectively, which are lower than their corresponding annealed carbamate-substituted HID polymers. PTAB has a much lower E_HOMO level (−4.95 eV) than that of PTNB (−4.69 eV) and is more crystalline than the latter, which should lead to poorer stability of the doped complex PTAB:F4TCNQ. Surprisingly, PTAB:F4TCNQ showed much better long-term stability than PTNB:F4TCNQ. It was considered that the hydrophobic alkyl side chains in PTAB can help prevent the interaction of water in the air with the PTAB:F4TCNQ complex, thereby stabilizing the complex. This study provided new insights into the design principles of conjugated polymers for printed and flexible temperature sensors.