Singlet Fission is an extensively studied spin-allowed mechanism that has been reported to reduce thermal loses factored in the calculation of Shockley-Queisser limit and can therefore enhance the theoretical efficiency limit of photovoltaic devices. However, the mechanisms under which this process happens, whether intermolecular or intramolecular, are not well understood. In this regard, we report a comparative study between rigid−bridged (planar) and flexible−bridged (twisted) perylene diimide (PDI) trimer systems to highlight the role of the π−bridge linker in activating intramolecular singlet exciton fission (iSEF). We use time-resolved spectroscopic measurements to show how a slight structural variation of the π−bridge of multichromophoric PDI systems strongly affects the triplet yield and triplet formation rate. Triplet formation proceeds via conventional intersystem crossing for the planar trimer as evidenced by its triplet yield of 16 % and triplet production in the nanosecond timescale. On the other hand, we find clear evidence of highly efficient (170 %) and fast (few hundred picoseconds) iSEF taking place in the twisted trimer. A fused ring connection induces strong coupling among the PDI units as in the planar system and this inhibits SEF completely due to a strong competition with other ultrafast processes, such as intramolecular charge transfer. Our results demonstrate that a rotatable π−bridge, capable of lowering the coupling between the chromophores, is necessary to activate iSEF in multichromophoric systems. The quantum chemical simulations prove that the entropic gain allowed by multiple possibilities of accommodating the correlated triplet pair on adjacent PDIs in the twisted trimer is a crucial determinant for multiexciton triplet generation. Successive multiexciton triplet separation occurs in the flexible−bridged PDI due to weak coupling among the units, and degenerate double triplet and quintet states. Our results represent a significant step forward in structure−property understanding and may direct the design of new efficient iSEF materials.

Activating intramolecular singlet exciton fission by altering π−bridge flexibility in perylene diimide trimers for organic solar cells

ACS Spring Meeting 2021

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