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 (≈105 pump pulses), demonstrating the potential of COPV2-3-2 as a high optical gain material for advanced photonic applications.