Excited-state charge-transfer complexes (exciplexes) are actively exploited in organic optoelectronics to understand their nature in the solid state and improve device performance. However, the diffusion of exciplexes is not actively studied, despite its ever-growing importance in such devices due to the lack of apparent charge-transfer (CT) absorption. Here, we report, based on time-resolved photoluminescence spectroscopy, that energy transfer (ET) takes place directly from exciplexes to exciplexes; this is called exciplex-exciplex ET. Recent reports and our own observation of the subband-gap CT absorption in exciplex-forming systems support the direct exciplex-exciplex ET. The ET takes place dominantly via the Dexter-type exchange mechanism, as revealed by the exponential decrease of the ET rate constant with separation between the energy-donating exciplex and the energy-accepting exciplex.
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