Veil of the Charge Transfer State in Bay-Annulated Indigo-Based Donor-Acceptor Systems: Charge Separation versus Singlet Fission

Bay-annulatedindigo (BAI) is a new potential SF-active buildingblock, which has aroused great interest in the design of highly stablesinglet fission materials. However, singlet fission of unfunctionalizedBAI is inactive due to the inappropriate energy levels. Herein, weseek to develop a new design strategy by introducing the charge transferinteraction to tune the exciton dynamics of BAI derivatives. A newdonor-acceptor molecule (TPA-2BAI) and two control molecules(TPA-BAI and 2TPA-BAI) were designed and synthesized to unravel theveil of CT states in tuning the excited-state dynamics of BAI derivatives.Transient absorption spectroscopy studies show that CT states aregenerated immediately following the excitation. However, the low-lyingCT states induced by strong donor-acceptor interactions resultin them acting as trap states and inhibiting the SF process. Theseresults show that the low-lying CT state is detrimental to SF andprovide insight into the design of CT-mediated BAI-based SF materials.

Automated summary

Bay-annulated indigo (BAI) is a new potential building block for highly stable singlet fission materials. A new design strategy using charge transfer interaction was developed to tune the exciton dynamics of BAI derivatives. Experimental results show that strong donor-acceptor interactions lead to low-lying CT states that act as trap states and inhibit the singlet fission process.