## What is/are Ultrafast Singlet?

Ultrafast Singlet - By using femtosecond transient absorption spectroscopy, two dynamic processes are observed: an ultrafast singlet → triplet intersystem crossing (<0.^{[1]}Ultrafast singlet fission has the potential to facilitate highly efficient photovoltaics through the multiplication of excitons in organic molecular architectures.

^{[2]}Large couplings from charge-transfer (CT) state mediation were invoked to explain the ultrafast singlet fission rate observed in crystalline polyacene, but its experimental confirmation is still lacking.

^{[3]}Here, combining excited-state time-domain Raman spectroscopy and tree-tensor network state simulations, we construct the full 108-atom molecular movie of ultrafast singlet fission in a pentacene dimer, explicitly treating 252 vibrational modes on 5 electronic states.

^{[4]}

## ultrafast singlet fission

Ultrafast singlet fission has the potential to facilitate highly efficient photovoltaics through the multiplication of excitons in organic molecular architectures.^{[1]}Large couplings from charge-transfer (CT) state mediation were invoked to explain the ultrafast singlet fission rate observed in crystalline polyacene, but its experimental confirmation is still lacking.

^{[2]}Here, combining excited-state time-domain Raman spectroscopy and tree-tensor network state simulations, we construct the full 108-atom molecular movie of ultrafast singlet fission in a pentacene dimer, explicitly treating 252 vibrational modes on 5 electronic states.

^{[3]}