Intense and Stable Near-Infrared Emission from Light-Emitting Electrochemical Cells Comprising a Metal-Free Indacenodithieno[3,2-b]thiophene-Based Copolymer as the Single Emitter

Abstract

We report on the synthesis, characterization, and application of a series of metal-free near-infrared (NIR) emitting alternating donor/acceptor copolymers based on indacenodithieno[3,2-b]thiophene (IDTT) as the donor unit. A light-emitting electrochemical cell (LEC), comprising a blend of the copolymer poly[indacenodithieno[3,2-b]thiophene-2,8-diyl-alt-2,3-diphenyl-5,8-di(thiophen-2-yl)quinoxaline-5,5′-diyl] and an ionic liquid as the single-layer active material sandwiched between two air-stable electrodes, delivered NIR emission (λ peak = 705 nm) with a high radiance of 129 μW/cm 2 when driven by a low voltage of 3.4 V. The NIR-LEC also featured good stress stability, as manifested in that the peak NIR output from a nonencapsulated device after 24 h of continuous operation only had dropped by 3% under N 2 atmosphere and by 27% under ambient air. This work accordingly introduces IDTT-based donor/acceptor copolymers as functional metal-free electroluminescent materials in NIRemitting devices and also provides guidelines for how future NIR emitters should be designed for further improved performance.

On the Design of Host–Guest Light‐Emitting Electrochemical Cells: Should the Guest be Physically Blended or Chemically Incorporated into the Host for Efficient Emission?

“…Figure a presents the chemical structure of the host and guest compounds. The poly(indacenodithieno[3,2‐ b ]thiophene) (PIDTT) homopolymer was selected for the host because its planar backbone facilitates efficient charge transport while the hexylphenyl side groups ensure good solubility for low‐cost solution processing . 4,7‐Bis(4,4‐bis(2‐ethylhexyl)‐4 H ‐silolo[3,2‐ b :4,5‐ b ′]dithiophen‐2‐yl)benzo[ c ][1,2,5]‐thiadiazole (SBS) was chosen for the guest emitter, since it features a donor–acceptor–donor architecture with a narrow energy gap that is compatible with the PIDTT host in that it facilitates efficient host‐to‐guest energy transfer.…”

“…Materials and Synthesis : The dibromo‐substituted SBS and SB R S monomers and the PIDTT host were synthesized according to the procedure described in a recent publication 13b. The bis(trimethylstannyl)‐substituted indacenodithieno[3,2‐ b ]thiophene monomer (IDTT‐Sn), the dibromo‐substituted indacenodithieno[3,2‐b]thiophene monomer (IDTT‐Br), and 2‐octylthiophene were purchased from Solarmer Energy, Inc., while the other reagents and solvents were obtained from Sigma‐Aldrich.…”

“…SBS* :13b,18 In a 10 mL two‐neck flask, dibromo‐substituted SBS (0.200 g, 0.18 mmol), Pd 2 (dba) 3 (0.0032 g, 0.004 mmol), and tri( o ‐tolyl)phosphine (0.0043 g, 0.014 mmol) were subjected to four vacuum/N 2 backfill cycles and dissolved in dry THF (5 mL). 2‐Octylthiophene (0.191 g, 0.53 mmol) was added and the mixture was stirred at 80 °C for 24 h. The reaction mixture was poured over water and extracted with DEE (3 × 100 mL) and washed with water (3 × 100 mL).…”

“…PIDTT‐SBS_C0.5, PIDTT‐SBS_C2.5, and PIDTT‐SBS_C5.0 :13b,20 The amount of IDTT‐Sn was constant 0.5 equiv. (0.1345 g, 0.100 mmol), while the amounts of IDTT‐Br and dibromo‐substituted SBS were varied from 0.495 equiv.…”

Combining Benzotriazole and Benzodithiophene Host Units in Host–Guest Polymers for Efficient and Stable Near‐Infrared Emission from Light‐Emitting Electrochemical Cells

“…Albeit promising, these studies suffer from that a high‐frequency operation depends on a converter circuit in mobile applications, that Ru is a rare and expensive compound, and that the measured lifetime of the Ru‐based NIR‐LEC was very limited at a mere 20 min. Our group recently reported on NIR‐LECs based on an emissive host–guest copolymer with indacenodithieno[3,2‐ b ]thiophene (IDTT) as the single host unit, which delivered NIR emission centered at 725 nm with a high radiance of 263 µW cm −2 when driven by a constant current …”