Ultrafast Radiative Recombination of Singlet Excitons in Conjugated Polymer Aggregates Modulated by the Intrachain Dissociation–Interchain Delocalization Competition

We readily modulated relaxation pathways of singlet (S1) excitons in push–pull conjugated polymer (CP, D18) aggregates by controlling the spin-coating process. Transient absorption spectroscopy results reveal that intrachain (localized) S1 excitons are efficiently converted into emissive interchain-delocalized excitons in an H-type excitonic coupling-dominant film. Conversely, a notable stimulated emission band emerges and red-shifts toward the steady-state photoluminescence peak within 1 ps in a film with enhanced J-type excitonic coupling (intrachain ordering). This indicates that the weakened interchain S1 delocalization leads to ultrafast intrachain S1 radiative recombination during relaxation. The competitive interplay between this weakened interchain delocalization and the intrachain ordering-driven S1 dissociation limits efficient progression into any of the dominant exciton deactivation pathways at early times. Thus, a kinetic bottleneck is induced, consequently increasing the probability of subpicosecond S1 radiative recombination. Our findings provide a molecular design strategy to minimize exciton energy dissipation in CP aggregates.