Efficient Intersystem Crossing in Heavy-Atom-Free Perylenebisimide Derivatives

Efficient intersystem crossing (ISC) in heavy-atom-free organic chromophores remains rare because of the lack of strong spin-orbit coupling effects in such compounds. Finding organic chromophores with ISC ability is important for applications in several areas, e.g., photocatalysis and photodynamic therapy. Herein, we report new perylenebisimide (PBI) chromophores with tetraphenylethynyl substituents at the 2,5,8,11-positions of the PBI core (ortho-positions, not the usually reported bay-positions of PBI), which show efficient ISC without the presence of any heavy atoms. Steady-state and picosecond-nanosecond transient absorption spectroscopies as well as time-dependent density functional theory computations were used to reveal the photophysical properties. For one of the PBI derivatives, excitation wavelength-dependent ISC was observed. The efficient ISC was attributed to the S-1/S-2 --> T-n (n > 1) processes. Photochemical reduction of the PBI derivatives in the presence of a sacrificial electron donor (triethanolamine) produced a stable PBI radical anion.