Wavelength-dependent Intramolecular Singlet Fission Through An Excimer-like Intermediate

Singlet Fission (SF) into two triplets offers exciting avenues for high-efficiency photovoltaics and optically initializable qubits. While the chemical space of SF chromophores is ever-expanding, the underlying mechanistic details of electronic-nuclear motions accompanying SF are often glossed over. Rigid SF dimers with well-defined orientations are helpful to decipher such details. Here, using polarization-controlled white-light two-dimensional and pump-probe spectroscopies, we investigate a new class of contorted naphthalenediimide dimers, recently reported to have a favorable intramolecular SF (iSF) pathway. 2D cross-peaks directly identify the two Davydov components of the dimer. 2D maps reveal that excitation of either Davydov component leads to an intermediate state which is generated within our instrument response. This intermediate proceeds to form a relaxed TT1 state whose formation kinetics is strongly dependent on which Davydov component is excited. We also find that the intermediate formation and relaxation is vibronically coherent with enhanced quantum beats only in the TT1 photoproduct, suggesting that intermolecular twisting and ruffling coordinates are strongly displaced upon TT1 formation. Polarization anisotropy directly tracks electronic motion during these steps and curiously reveals minimal electronic reorientation during TT1 formation. A likely hypothesis for this observation is that significantly mixed singlet-triplet electronic character is maintained throughout the nuclear evolution away from the Franck-Condon geometry towards relaxed TT1 without any reduction in the singlet electronic character. Such a mixing can introduce triplet annihilation channels and can therefore prevent the formation of long-lived high-spin triplets. Synthetic design of iSF dimers should aim minimize this electronic mixing.