Intrachain Triplet Energy Transfer in Platinum−Acetylide Copolymers

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Abstract
A series of platinum−acetylide homo- and copolymers was prepared and characterized by using photophysical methods. The polymers feature repeat units of the type [trans-Pt(PBu3)2(C⋮CArC⋮C)], where Ar = 1,4-phenylene (P) or 2,5-thienylene (T). The properties of homopolymers that contain only the 1,4-phenylene or 2,5-thienylene repeat units were compared with those of random copolymers having the structure −[−(Pt(PBu3)2(C⋮CTC⋮C))x−(Pt(PBu3)2(C⋮CPC⋮C))(1-x)-] where x = 0.05, 0.15, and 0.25. Absorption and photoluminescence spectroscopy demonstrates that the singlet and triplet excitations localized on 1,4-phenylene units are higher in energy relative to those localized on the 2,5-thienylene units. The mechanism and dynamics of intrachain triplet energy transfer from 1,4-phenylene to the 2,5-thienylene repeats were explored in the copolymers. Photoluminescence and nanosecond transient absorption spectroscopy indicate that at room temperature P → T energy transfer is efficient and rapid (k ≫ 108 s-1), even in the copolymer that contains only 5% 2,5-thienylene repeat units. At 77 K, steady-state and time-resolved photoluminescence spectroscopy reveals that triplet energy transfer is much less efficient and a fraction of the triplet excitations is “trapped” on the high-energy 1,4-phenylene units. Intrachain energy transfer is believed to occur by two mechanisms, one involving P → T singlet energy transfer followed by intersystem crossing, whereas the other involves intersystem crossing prior to P → T triplet energy transfer. The relationship between the observed energy transfer efficiencies and mechanisms in the copolymers is discussed.