Acidochromic Behaviors of Indacenodithiophene-Based Conjugated Polymers Containing Azo, Imine, and Vinyl Bonds

Acidochromic materials possess significant potential for the development of molecular switches, acid sensors, smart displays, and erasable/reprintable media. The semiconductive nature of conjugated polymers exhibiting such a behavior makes them ideal for use in electronic devices. In this study, we present a comparative investigation of three indacenodithiophene-based conductive polymers, containing azo, imine, and vinyl bonds (namely, PIDT-BAB, PIDT-BIB, and PIDT-BVB, respectively). We examined the alterations in the spectral properties of these polymers upon exposure to trifluoroacetic acid (TFA). The acidochromic response of PIDT-BAB and PIDT-BIB is indicated by DFT calculations to occur via protonation at the nitrogen atom. PIDT-BIB demonstrated heightened sensitivity to TFA. Conversely, PIDT-BVB did not display acidochromic properties in the film but was responsive to TFA in solution through acid doping. Repeated exposure of polymer films was used to examine the robustness of the polymers over 50 cycles. DFT calculations showed an increase in the planarity of PIDT-BAB and PIDT-BIB backbones as a result of protonation. This effect was particularly strong in PIDT-BAB, resulting in an unusually large bathochromic shift of 510 nm. The corresponding pink-to-transparent transition is particularly interesting for applications in sensors. Our findings provide valuable guidelines for the design of conjugated polymers tailored for acidochromic devices.