Singlet Fission from Upper Excited States of Bodipy Crystalline Film and Single Crystal

The well-known boron-dipyrromethene (bodipy) dyes have excellent photophysical and photochemical properties, which are significant for practical use in many fields. meso-Phenyl-substituted bodipy (PhBDP) as one of the most classical bodipy derivatives was investigated in the forms of amorphous film, crystalline film, and single crystal by using time-resolved fluorescence and femtosecond transient absorption techniques. The fluorescence of both PhBDP films is strongly quenched because of J-aggregation, which is randomly distributed in the amorphous film but well arranged in the crystalline film. In the crystalline film, the triplet states were populated from the upper excited singlet states (3.54 eV) of the nanoaggregates via singlet fission (SF) within 30 fs. Moreover, SF was also detected after excitation of the lower singlet states (2.58 eV) which are below the SF threshold (3.28 eV). This phenomenon was explained in terms of two-quantum absorption. However, in the amorphous film, the SF process was not observed, indicating the crucial impact of the nanoaggregate morphology on the formation of SF. In the single crystal, SF was also generated owing to the molecular slip-stacked packing which is optimal for the delocalization of excitation. It facilitates exciton fission and energy transfer between the neighboring PhBDP dimer moieties as well. Our findings of PhBDP solids provide a significant theoretical and experimental basis for exploring new SF systems. The prominent quenching, efficient SF, and well-characterized energy transfer of PhBDP solid expands the applications of PhBDP-based organic materials in optoelectronic devices.