Oligophenylene Tuning from Monomer to Trimer Enables Ultrafast Excited-State Dynamics and Ultrahigh Emission Cross...

Chromophores with short photoluminescence lifetimes are highly desirable for minimizing exciton-exciton annihilation, suppressing triplet accumulation, and enabling ultrafast modulation in applications such as time-resolved imaging, high-speed data transmission, and light fidelity (Li-Fi). However, achieving rapid excited-state dynamics without compromising emission efficiency remains a significant challenge. Here, we report three solution-processable oligophenylene-based chromophores, Monomer, Dimer, and Trimer, functionalized with phenylcarbazole end caps. Through systematic structure-property modulation, these chromophores exhibit remarkably short photoluminescence lifetimes (down to 0.43 ns), high molar absorption coefficients (up to 236,000 M^-1 cm^-1 at 399 nm), and large radiative decay rates (up to 2.08 × 10⁹ s^-1). In the context of organic lasing, they also show large emission cross sections (up to 1.01 × 10^-15 cm²), minimal spectral overlap between excited-state absorption and amplified spontaneous emission (ASE), and very low solid-state ASE thresholds (0.67-0.80 μJ cm^-2) in the deep blue region. Computational studies attribute these properties to extended π-conjugation and high HOMO-LUMO overlap along the predominantly planar oligophenylene backbone, which enhances π-delocalization and oscillator strengths. Electroluminescent studies through fabrication of organic light-emitting diodes gave external quantum efficiencies up to 3.2%. These findings highlight the effectiveness of molecular engineering in accelerating excited-state decay while maintaining efficient light emission, establishing a promising platform for next-generation lasers, optical communications, ultrafast displays, and broadband photonic technologies.