Carbazole-σ-sulfobenzimide derivative exhibiting mechanochromic thermally activated delayed fluorescence as emitter for flexible OLEDs: Theoretical and experimental insights

For the first time exploiting sulfobenzimide moiety as an acceptor unit, the new type of donor-σ-acceptor emitter exhibiting thermally activated delayed fluorescence (TADF) is demonstrated. In different solutions, the synthesized compound emits light resulting from either locally excited carbazole moiety or trough-space charge transfer (exciplex-like) between carbazole and sulfobenzimide units. In the solid state, this emitter demonstrates aggregation-induced emission enhancement and different emission colours due to its different conformations. The mechanoluminescence of the donor-σ-acceptor compound was observed and studied in detail by experimental and theoretical approaches including single-crystal and powder X-ray analyses. Electroluminescence of the different colours was observed when the compound was utilized as non-doped TADF emitter in rigid and flexible organic light-emitting diodes fabricated on glass or poly(ethylene terephthalate) substrates. The device fabricated on the rigid substrate exhibited the best performance with maximum current efficiency, power efficiency, and external quantum efficiency of 11.0 cd A−1, 3.0 lm W−1, and 4.3%, respectively. [Display omitted] The highlights of this work are as follows.•Design and synthesis of new type of donor-σ-acceptor emitter with the combination of mechanoluminescence, aggregation-induced emission enhancement and thermally activated delayed fluorescence.•Mechanochromic luminescence predicted by the theoretical analysis of conformers and supported by the results of powder X-ray and single crystal X-ray analyses.•Strongly media dependent blue TADF•Ionization potential of 6.08 eV and electron affinity of 2.65 eV favourable for hole and electron injection.•Bipolar charge-transport with balanced hole and electron mobilities of 3.2 × 10−6 cm2/V × s and 4.4 × 10−6 cm2/V × s respectively at the electric field of 4.43 × 105 V/cm.•Rigid and flexible OLEDs with external quantum efficiency reaching 4.3%.