Transient Magnetic Field Effect of Photoexcitations in Donor–Acceptor Organic Semiconductors

We report transient photoinduced absorption (t-PA) and magnetic field (B)-dependent t-PA (t-MPA(B)) in a pristine low band gap π-conjugated copolymer composed of donor and acceptor moieties, namely, the poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thi-eno[3,4-b]thiophenediyl]]) (or PTB7) used in photovoltaic applications. Unlike traditional π-conjugated polymers in which the primary photoexcitations are singlet excitons (SE), in pristine PTB7 we find at short times coexistence of two primary photoexcitation species, namely, SE and triplet–triplet (TT) pair. Both species are photogenerated directly from the ground state and are spin-correlated. Although the TT pair decomposes into two separate triplet excitons (TEs) in ∼100 ps, the separated TE spins are still entangled up to ∼6 μs. At longer times, the t-MPA(B) response of the surviving TEs shows transient narrowing effect, which is attributed to a distribution of the TE size.