Intrachain triplet energy transfer in platinum-acetylide copolymers

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(-CC-Ar-CC-)], 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(-CC-T-CC-))x-(Pt(PBu3)2(-CC-P-CC-))(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 >> 10(8) 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.