Singlet fission in covalent dimers of methylene-locked 1,3-diphenyl-isobenzofuran: semiclassical simulations of nonadiabatic dynamics

We present surface hopping simulations of the nonadiabatic dynamics in three covalently bound dimers of the methylene-locked 1,3-diphenyl-isobenzofuran (ML-DPBF) chromophore, a modification of a molecule previously studied experimentally and computationally by Michl and coworkers (J. C. Johnson and J. Michl, Top. Curr. Chem. (Z), 2017, 375, 80). Our aim is to test the suitability for singlet fission of such dimers, as well as to study how the singlet fission dynamics is affected by the mutual arrangement of chromophores in the covalent assembly. Two of our investigated covalent dimers, namely D1 and D2, are newly designed dimers in which the chromophores are linked by two bridges so as to attain suitable stacking arrangements, while the third one (D0) is a linear dimer in which one -CH2- bridge connects the two chromophore units. From our simulations it turned out that D1 and D2 undergo singlet fission in a sub-picosecond time scale. The final populations of the double triplet state (TT) are large (0.74 and 0.84, respectively), but do not reach the limiting value of 1 because of a persistent exchange with the lowest dark excitonic state. On the other hand, our simulations for D0 indicate that this dimer does not undergo singlet fission in the 4 ps following the photoexcitation, mainly because of the weak interaction between chromophores. Our study, besides indicating the potential suitability for singlet fission of dimers D1 and D2, highlights the impact of the mutual arrangement of chromophores in determining the singlet fission efficiency. In fact, we show the importance of fine tuning not only the effective couplings that permit to populate the TT state, but also the exciton splitting of the lowest singlet states.