Positional Effects from σ-Bonded Platinum(II) on Intersystem Crossing Rates in Perylenediimide Complexes: Synthesis, Structures, and Photophysical Properties

In this investigation, the synthesis and photophysical properties of a series of new chromophores featuring Pt(II) σ-bonded to perylenediimide (PDI) cores are reported. A Pt(PPh₃)₂X (X = Cl, Br) moiety was attached to PDI in either the ortho or the bay position (2- or 1-positions respectively) or a Pt(PPh₃)₂ subunit was used to bridge two bay positions (1- and 12-positions) forming a Pt(II) cyclometalate. Through a combination of steady-state and transient absorption and photoluminescence spectroscopy, the excited-state dynamics of these molecules were revealed, indicating that the Pt atom location on the PDI has a substantial impact on observed intersystem crossing (ISC) rates. The ISC time constants for the bay-substituted and cyclometalated PDIs are between 2.67 and 1.29 ns, respectively, determined by the singlet fluorescence decays from the initially populated singlet excited states. In the case of the ortho-substituted PDI, ISC to the triplet state occurs on the ultrafast time scale with a time constant of 345 fs, determined through ultrafast transient absorption spectroscopy. In all instances, the measured PDI-based fluorescence quantum yields quantitatively correlate with the measured ISC rates leaving little doubt that variation in the Pt(II) substitution position(s) markedly influence the resultant photophysics. Electronic structure calculations suggest differing amounts of metal-based contributions in the lowest excited states as a function of substitution position, supporting the trend of the observed ISC rates.