Ladder-type indenefluorenylene-based conjugated polymers with RGB emission as efficient gain media for organic lasers: Design, synthesis, and optical gain properties

A set of novel ladder-type conjugated copolymers (2LF-An, 2LF-BT and 2LF-TBT) were designed, synthesized, and investigated as efficient gain media for organic lasers. The polymers were synthesized via Suzuki polymerization of indenefluorene units with varying blue, green, and red emitting units to afford RGB conjugated polymers with the emission wavelength ranging from 448 nm to 632 nm, covering the full visible spectrum. The thermal, morphological, photophysical, electrochemical, electroluminescent, and optical gain properties of the resulting conjugated polymers were systematically investigated and compared with those of the poly(9,9dioctylfluorene-co-benzothiadiazole) (F8BT) counterpart. With the ladder-type backbone structures incorporated, the resulting indenofluorene-based conjugated polymers exhibited enhanced optical and electrical properties with low ASE thresholds and high net gain coefficients. Organic light-emitting diodes (OLEDs) based on 2LF-An, 2LF-BT and 2LF-TBT were fabricated. The current efficiency of the 2LF-An, 2LF-BT and 2LF-TBT devices was 1.10, 3.11 and 0.50 cd/A, respectively, while the maximum brightness was 2772, 8582, 1682 cd/m², respectively. Low amplified spontaneous emission (ASE) thresholds were achieved for 2LF-An (20.90 μJ/cm²) and 2LF-BT (65.84 μJ/cm²) with high gain coefficients of 62.40 cm⁻¹ for 2LF-An and 66.07 cm⁻¹ for 2LF-BT, respectively. By blending 1% 2LF-TBT into 2LF-BT, high optical gain coefficients of g = 68 cm⁻¹ were recorded for 2LF-TBT with the ASE peak at 600 nm and the ASE threshold of 68 cm⁻¹. For comparison, the optical gain coefficient of F8BT was recorded at around 26.88 cm⁻¹ under identical test conditions. The results suggested that the incorporation of ladder-type indenofluorene chain structures played a positive role on improving the optical gain properties. More impressively, the low ASE thresholds were almost unchanged for the samples even upon annealing up to 200 ºC for the polymers, indicating excellent optical stability. The excellent optical stability, low ASE thresholds, and high net gain coefficients of the resulting polymers have promised their great potential to be used as efficient gain media for organic lasing.