Singlet Fission within Diketopyrrolopyrrole Nanoparticles in Water

Nanoparticles (NPs) of the singlet fission chromophore 3,6-bis(5-phenylthiophen-2-yOpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (PhTDPP) having average hydrodynamic diameters of 63-193 nm were prepared by rapidly injecting variable concentrations of PhTDPP solutions in tetrahydrofuran into water. These PhTDPP NPs are stable over months in water and exhibit fluorescence quantum yields << 1%. Femtosecond transient absorption spectroscopy shows that singlet fission is more rapid in smaller NPs, likely reflecting their greater surface area-to-volume ratio and consequent exposure of more molecules to the high dielectric aqueous environment. These observations suggest that charge transfer states, whose energy is sensitive to the dielectric constant of the surrounding medium, serve as virtual intermediates in PhTDPP NP singlet fission. However, the lifetime of the triplet excitons produced by singlet fission is longest in the larger NPs having greater long-range order, which allows the triplet excitons to diffuse further from one another thus slowing triplet-triplet annihilation.