Preparation of Thermally and Photochemically Immobilized N-type Conjugated Polymer Films via Quantitative Backbone Editing

Abstract

We report a series of n-type conjugated polymers based on PNDI-TfBTT and PNDIV-TfBTT backbones constructed from electron-deficient naphthalene diimide (NDI) and fluorinated benzothiadiazole (fBT) units, with PNDIV-TfBTT incorporating a vinylene spacer. Quantitative postpolymerization modification (PPM) via nucleophilic substitution replaced the fBT fluorine with thioether side chains, optionally containing azide groups. Thioether substitution improved solubility, while subtly changing the ordering of polymer films. Azide incorporation enabled both thermal and photochemical crosslinking, yielding insoluble and immobile films that retained good electron transport; although UV crosslinking initially reduced mobility, subsequent thermal annealing largely restored crystallinity and performance. This work underscores the utility of precise backbone editing to fine-tune the electronic and morphological properties of n-type polymers, offering new avenues for the fabrication of stable, patterned active layers in advanced organic electronic devices.

Keywords: Backbone editing; Conjugated polymer; Cross‐linking; N‐type material; Postpolymerization functionalization.

Scheme 1

a) Synthesis of n‐type polymers PNDI‐TfBTT and PNDIV‐TfBTT backbone polymers, and b) the synthesis of PNDI‐T(SR)BTT and PNDIV‐T(SR)BTT by PPM of the n‐type parent polymers using 1‐octanethiol.

Figure 1

Quantitative reaction of 1‐octanethiol with the PNDI‐TfBTT backbone. a) Structure of partially substituted PNDI‐T(SR)BTT. b) UV–vis absorption spectra in chlorobenzene (normalized to the peak at 350–370 nm) for the unsubstituted PNDI‐TfBTT (0 mol%), partially substituted (20, 40, 60, and 80 mol%) and fully substituted (100 mol%) PNDI‐T(SR)BTT polymers. c) ¹H NMR spectra of PNDI‐TfBTT as a function of increasing 1‐octanethiol inclusion along the backbone where the aromatic peaks for H_a were set to an integral of 2H and directly compared with XH_k. d) Measured (from ¹H NMR) XH_k relative intensities as a function of thiol concentration used in the reaction. Error bars indicate the peak integration signal divided by the signal‐to‐noise ratio.

Figure 2

a) UV–vis absorption spectra for PNDI‐TfBTT, PNDI‐T(SR)BTT, PNDIV‐TfBTT, and PNDIV‐T(SR)BTT in chlorobenzene, and b) as a spun‐cast thin film onto glass. c)–f) 2D GIWAXS diffraction images for c) PNDI‐TfBTT, d) PNDIV‐TfBTT, e) PNDI‐T(SR)BTT, and f) PNDIV‐T(SR)BTT thin films annealed at 200 °C. g) GIWAXS diffraction patterns (azimuthal integrations) for PNDI‐TfBTT, PNDIV‐TfBTT, PNDI‐T(SR)BTT, and PNDIV‐T(SR)BTT films annealed at 200 °C.

Figure 3

a), b) Transfer characteristics of a) PNDI‐TfBTT and b) PNDIV‐TfBTT OTFT devices. c) Electron mobility of PNDI‐TfBTT and PNDIV‐TBTT OTFT devices as a function of annealing temperature of the polymer thin film. d), e) Transfer characteristics of d) PNDI‐T(SR)BTT and e) PNDIV‐T(SR)BTT OTFT devices. f) Electron mobility of PNDI‐T(SR)BTT and PNDIV‐T(SR)BTT OTFT devices as a function of annealing temperature of the polymer thin film. Transistor channel length 30 µm and channel width 1000 µm.

Scheme 2

Postpolymerization modification of PNDI‐TfBTT with 3‐azidopropane‐1‐thiol in 10% and 100% relative molar equivalents.

Figure 4

Thermal crosslink studies of PNDI‐TfBTT, PNDI‐T(SAz)BTT – 10%, and PNDI‐T(SAz)BTT – 100%. a) Drop‐cast films were annealed at 150, 200, 250, and 300 °C for 30 mins under N₂ before immersing in chlorobenzene where the resulting solution was analyzed by UV–vis. b)–d) UV–vis absorption spectra for solutions from b) of PNDI‐TfBTT, c) PNDI‐T(SAz)BTT – 10%, and d) PNDI‐T(SAz)BTT – 100%. e) Schematic of the photocrosslinking studies where spin coated films were exposed to UV light (254 nm) for 60 mins under N₂ before immersion in chlorobenzene for 24 h. Photographs of the resulting uncoated glass (with most of the polymer having dissolved) and photopatterned film (with only the nonUV‐cured polymer having dissolved) for PNDI‐T(SAz)BTT – 10%, and PNDI‐T(SAz)BTT – 100%, respectively, are shown to the right of the schematic.

Figure 5

a), b) GIWAXS images a) and GIWAXS diffraction patterns (out‐of‐plane) b) of PNDI‐T(SAz)BTT – 10% films as‐cast (no thermal anneal), and after annealing at 200 and 300 °C, with and without pre‐exposure to UV light (254 nm for 60 min). c), d) GIWAXS images c) and GIWAXS diffraction patterns (out‐of‐plane) d) of PNDI‐T(SAz)BTT – 100% films as‐cast (no thermal anneal), and after annealing at 200 and 300 °C, with and without pre‐exposure to UV light (254 nm for 60 min).