π‑Electronic Co-crystal Microcavities with Selective Vibronic-Mode Light Amplification: Toward Förster Resonance Energy Transfer Lasing

π-conjugated
organic microcrystals often act as optical resonators
in which the generated photons in the crystal are confined by the
reflection at the crystalline facets and interfere to gain lasing
action. Here, we fabricate microcrystals from a mixture of carbon-bridged
oligo-para-phenylenevinylenes (COPVs) with energy-donor
(D) and energy-acceptor (A) characters. Upon weak excitation of the
single D–A co-crystal, Förster resonance energy transfer
(FRET) takes place, exhibiting spontaneous emission from A. In contrast,
upon strong pumping, stimulated emission occurs before FRET, generating
lasing action from D. Lasing occurs with single- and dual-vibronic
levels, and the lasing wavelength can be modulated by the doping amount
of A. Time-resolved spectroscopic studies reveal that the rate constant
of lasing is more than 20 times greater than that of FRET. Furthermore,
microcrystals, vertically grown on a Ag-coated substrate, reduce the
lasing threshold by one-fourth. This study proposes possible directions
toward organic solid FRET lasers with microcrystalline resonators.