Light-Harvesting Phthalocyanine–Diketopyrrolopyrrole Derivatives: Synthesis, Spectroscopic, Electrochemical, and Photochemical Studies

A new family of light-harvesting zinc phthalocyanine (ZnPc)–diketopyrrolopyrrole (DPP) hybrids have been synthesized and characterized. The absorption spectral measurements showed that the major absorptions of DPP (450–600 nm) are complementary to those of zinc phthalocyanine (300–400 and 600–700 nm). Therefore, the designed hybrids absorb over a broad range in the visible region. The geometric and electronic structures of the dyads were probed by initio B3LYP/6-311G methods. The majority of the HOMOs were found to be located on the ZnPc, while the majority of the LUMOs were on the DPP units. The DPP units serve as the antenna, which upon excitation undergo efficient singlet–singlet energy transfer to the attached ZnPc units. The formed singlet ZnPc, in turn, donates its electron to the electron-deficient DPP forming the low-lying radical ion pairs ZnPc^.+–DPP^.−(energy=1.44–1.56 eV as calculated from the electrochemical measurements). The excited-state events were confirmed by using a transient absorption technique in the picosecond–microsecond time range, as well as a time-resolved emission technique. The rates of energy transfer from the singlet DPP to ZnPc were found to be extremely fast >10¹⁰s⁻¹, while the rates of electron transfer from the singlet excited state of ZnPc to DPP were found to be 3.7–6.6×10⁹s⁻¹.