Photoinduced electron and energy transfer processes in a bichromophoric pyrene-perylene bisimide system

The synthesis and photophysical properties of a system consisting of a bay-functionalized perylene bisimide, containing four appended pyrene and two coordinating pyridine units, and its reference system are described. A complete study of their photophysical properties was obtained using UV-vis absorption, steady state and time-resolved emission, and femtosecond transient absorption. Analysis of the data, obtained from time-resolved emission and femtosecond transient absorption spectroscopy, showed the presence of both photoinduced electron and energy transfer processes. A high yield (>90%) and fast photoinduced energy transfer (ken ≈ 6.2 × 109 s-1) is followed by efficient electron transfer (70%, ket ≈ 6.6 × 109 s-1) from the pyrene units to the perylene bisimide moiety. The energy donor-acceptor distance, R = 8.6 Å, is calculated from the experimental energy transfer rate using Förster theory. Temperature-dependent time-resolved emission spectroscopy showed an increase of the acceptor emission lifetime with decreasing temperature. It also indicates the presence of different conformations because two different electron transfer barriers (0.08 and 0.42 eV) were found. These barrier values were corroborated by a theoretical analysis of the energetics of the process using Marcus theory, indicating average donor-acceptor distances of 4.5 Å (room temperature) to 11 Å (at low T).