Singlet exciton fission (SEF) is a spin-allowed conversion of a photogenerated singlet exciton into two triplet excitons through a multiexcitonic triplet pair <sup>1</sup>(TT) intermediate, thereby creating two excitons per photon. Acene dimers have been studied extensively over the years to explore the mechanism of intramolecular SEF. In our previous study, we have elucidated the role of spin density delocalization for a phenyl diketopyrrolopyrrole (DPP) bridged pentacene dimer. In the present study we investigate the difference in efficiency, mechanistic pathway and charge transfer mediation by exciting the molecule at different wavelengths. The DPP-bridge itself being a chromophore modifies the absorption range of the pentacene dimer explicitly to incorporate a DPP centric transition in addition to the red shifted pentacene absorption. The transient absorption spectroscopic measurements reveals the triplet absorption feature evolving as well as decaying at slower rate compared to direct pentacene-centric excitation. Global analysis reveals the presence of an additional state which converts to pentacene singlet like absorption, implying possible energy transfer process occurring prior to SEF when excited with higher energy photons (~400-500 nm). Our contribution not only elucidates the possibility of improving the color tunability for singlet fission dimers beyond their normal spectral range but also illustrates the multiple mechanistic pathways leading to SEF within the same molecular system.

Excitation energy dependence of singlet fission in diketopyrrolopyrrole bridged pentacene dimer

ACS Spring Meeting 2021

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