Broad-Band N∧N Pt(II) Bisacetylide Visible Light Harvesting Complex with Heteroleptic Bodipy Acetylide Ligands
Pt(II) dbbpy bisacetylide (dbbpy = 4,4′-di(tert-butyl)-2,2′-bipyridine) complex (Pt-1) with two different Bodipy ligands was prepared with the goal to attain broad-band visible light absorbing, efficient funneling of the photoexcitation energy (via resonance energy transfer, RET) to the energy acceptor and high triplet formation quantum yields. Construction of the above-mentioned molecular structural motif is challenging because two different arylacetylide ligands are incorporated in the complex; normally two homoleptic acetylide ligands were used for this kind of N∧N Pt(II) complexes. A reference complex with trans bis(tributylphosphine) Pt(II) bisacetylide protocol (Pt-4) was prepared for comparison of the photophysical properties. The two different Bodipy ligands in Pt-1 and Pt-4 constitute singlet/triplet energy donor/acceptor, as a result the harvested photoexcitation energy can be funneled to the triplet state confined on one of the two Bodipy ligands. The photophysical properties of the complexes were studied with steady state UV–vis absorption and luminescence spectroscopies, femto- and nanosecond transient absorption spectroscopies, cyclic voltammetry, as well as DFT/TDDFT calculations. Fluorescence/phosphorescence dual emission were observed for the complex. The ultrafast intramolecular singlet/triplet energy transfer in Pt-1 was confirmed by the transient absorption spectroscopy (kFRET = 2.6 × 1011 s–1, ΦFRET = 87.1%) followed by an intersystem crossing (kISC = 1.9 × 1010 s–1), and the triplet state lifetime (τT) is 54.1 μs. The reference complex Pt-4 shows drastically different kinetics with kFRET = 6.9 × 1010 s–1, ΦFRET = 81.0%, kISC = 5.83 × 109 s–1, and τT = 147.9 μs. Different singlet oxygen (1O2) quantum yields (ΦΔ = 75% and 70%) and triplet state quantum yields (ΦT = 91% and 69%, respectively) were observed for complexes Pt-1 and Pt-4.
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