Ultrafast Energy Transfer Induced Lasing From a Coplanar Donor‐Acceptor‐Donor Molecule in a Microspherical Cavity

Abstract Intramolecular excitation energy transfer (IET) dynamics and molecular rigidity of a donor‐acceptor system control the lasing efficiency of organic gain media. Herein, a novel molecule COPV2‐3‐2 , featuring carbon‐bridged oligo( para ‐phenylenevinylenes) (COPV) is reported. In this structure, COPV2 and COPV3 serve as photo‐absorptive donor and emissive acceptor monomer units, respectively, and two identical energy donors are connected to a coplanarly arranged acceptor through rigid orthogonal bridges. The rational design of the donor‐bridge‐acceptor‐bridge‐donor (D‐B‐A‐B‐D) molecule ensures precise control over IET and enables a four‐level energy system for efficient lasing at the acceptor emission band. Time‐resolved studies using femtosecond‐transient absorption spectroscopy unveil that COPV2‐3‐2 exhibits near unity intramolecular energy transfer with an ultrafast rate constant of 2.27 ps −1 . Additionally, singlet‐singlet annihilation (SSA) is also observed over a pump fluence of 115 µJ cm −2 , which can hinder population inversion. To suppress the lasing threshold below that for SSA, COPV2‐3‐2 is doped into a polystyrene matrix to fabricate a high Q ‐factor microspherical cavity. This microcavity exhibits whispering gallery mode (WGM) lasing with an averaged low lasing threshold (≈104 µJ cm −2 ), high Q ‐factors (≈1180), and excellent photostability half‐life (≈10 5 pump pulses), demonstrating the potential of COPV2‐3‐2 as a high optical gain material for advanced photonic applications.