Stimuli-responsive π-conjugated materials present opportunities for chemical sensing, whereby through interaction with an analyte the π-conjugated system undergoes a change in molecular geometry and/or electronic structure which can be detected as a change in either the optical or electrical characteristics. Here, a naphthalene diimide donor–acceptor conjugated polymer, poly­(2,7-bis­(2-decyl­tetradecyl)-4-methyl-9-(5′-methyl-[2,2′-bithiazol]-5-yl)­benzo­[lmn]­[3,8]-phenanthroline-1,3,6,8­(2H,7H)-tetraone) (PNDI2Tz), is reported as an acid sensing material. Shifts in the UV–vis spectroscopic signature of PNDI2Tz in the presence of protic and Lewis acids were investigated. In addition, PNDI2Tz-based n-channel organic field-effect transistors (OFETs) were fabricated and shown to respond to the gas phase Lewis acid, boron trifluoride (BF3), whereby the transistors reproducibly turn off in the presence of 60 ppm BF3.