Tuning Stimulated Emission Properties of Oligofluorene-Based Gain Media via Non-conjugation Strategy

Abstract To uncover spiro-substitution effect in gain media, four types of spiro-terfluorene derivatives, in which two individual conjugation chains with different conjugation length were connected via spirofluorene-9,9'-xanthene (SFX) and spirobifluorene (SBF) units, have been designed and synthesized. All spiro-terfluorene derivatives display high photoluminescent quantum yield (PLQY) and radiative transition rate (Kr) in solution. The PLQY and Kr gradually increase in the spin-coating films of 2SFX-SBF, 3SFX-SBF and 4SFX-SBF, clearly indicating that the lateral spiro-substitution can effectively suppress aggregation-induced exciton quenching. As a result, 4SFX-SBF with two identical conjugation chains possesses the best amplified spontaneous emission (ASE) performance with a low threshold of 0.28 μJ/cm2 and high gain coefficient of 159 cm-1. In addition, the spiro-terfluorenes display outstanding thermal stability, and the ASE threshold only doubles after 150 oC annealing. Hence, the lateral spiro-substituted strategy would be an intriguing strategy to design high-gain and low-threshold media for organic lasers.