Sensitizing the Sensitizer: The Synthesis and Photophysical Study of Bodipy−Pt(II)(diimine)(dithiolate) Conjugates

The dyads 3, 4, and 6, combining the Bodipy chromophore with a Pt(bpy)(bdt) (bpy = 2,2′-bipyridine, bdt = 1,2-benzenedithiolate, 3 and 6) or a Pt(bpy)(mnt) (mnt = maleonitriledithiolate, 4) moiety, have been synthesized and studied by UV−vis steady-state absorption, transient absorption, and emission spectroscopies and cyclic voltammetry. Comparison of the absorption spectra and cyclic voltammograms of dyads 3, 4, and 6 and those of their model compounds 1a, 2, 5, and 7 shows that the spectroscopic and electrochemical properties of the dyads are essentially the sum of their constituent chromophores, indicating negligible interaction of the constituent chromophores in the ground state. However, emission studies on 3 and 6 show a complete absence of both Bodipy-based fluorescence and the characteristic luminescence of the Pt(bpy)(bdt) unit. Dyad 4 shows a weak Pt(mnt)-based emission. Transient absorption studies show that excitation of the dyads into the Bodipy-based 1ππ* excited state is followed by singlet energy transfer (SEnT) to the Pt(dithiolate)-based 1MMLL′CT (mixed metal-ligand to ligand charge transfer) excited state (τSEnT3 = 0.6 ps, τSEnT4 = 0.5 ps, and τSEnT6 = 1.6 ps), which undergoes rapid intersystem crossing to the 3MMLL′CT state due to the heavy Pt(II) ion. The 3MMLL′CT state is then depopulated by triplet energy transfer (TEnT) to the low-lying Bodipy-based 3ππ* excited state (τTEnT3 = 8.2 ps, τTEnT4 = 5 ps, and τTEnT6 = 160 ps). The transition assignments are supported by TD-DFT calculations. Both energy-transfer processes are shown to proceed via a Dexter electron exchange mechanism. The much longer time constants for dyad 6 relative to 3 are attributed to the significantly poorer coupling and resonance of charge-separated species that are intermediates in the electron exchange process.