Photoinduced charge separation and charge recombination in terthiophene-acetylene-fullerene linked dyads

Abstract

Charge separation (CS) and recombination (CR) processes in terthiophene-fullerene linked dyads with bridges (3T-br n-C 60-R, bridge-1 (br1) = C C , and bridge-2 (br2) = (CH 2) 5 C C ; R = CN or Me) were investigated by fluorescence up-conversion method and transient absorption measurement in benzonitrile (PhCN) and toluene. With photoexcitation of the 3T moiety in 3T-br n-C 60-R, the CS process takes place fast in the region of (0.14–3.5) × 10 12 s −1 via 13T*-br n-C 60-R. In the case of 3T-br1-C 60-R, the CS process takes place fast via one step before the vibrational relaxation of the 13T* moiety, whereas in the case of 3T-br2-C 60-R, the CS process occurs two steps competing with the vibrational relaxation and after the relaxation. Such difference can be interpreted by the rigidity of the bridges. The 3T +-br n-C 60 −-R states were confirmed by the transient absorption spectra in the (1.0–3.6) × 10 3 ps region. The lifetimes of the radical ion-pair ( τ RIP) were evaluated to be 10–25 ps for 3T +-br1-C 60 −-R in PhCN. In toluene, the τ RIP values became longer ((1.2–4.7) × 10 2 ps) than those in PhCN. In both solvents, the τ RIP values are longer for R = Me than that for R = CN. In the case of 3T-br2-C 60-R in which the methylene chain inserted between 3T and acetylene bridge, the τ RIP values are longer than the corresponding values for of 3T +-br1-C 60 −-R, i.e., (0.91–1.2) × 10 3 ps in PhCN and (1.5–3.6) × 10 3 ps in toluene for R = CN and Me, respectively. It is revealed that the radical ion pairs last for longer in toluene than those in PhCN, suggesting that the CR process in the inverted region of the Marcus parabola, which was also supported by the substituent effect, i.e., unstable 3T +-br n-C 60 −-Me prolongs longer than stable 3T +-br n-C 60 −-CN. Drastic prolongation of τ RIP for 3T +-br2-C 60 −-R indicates that the electronic coupling for CR through the normal methylene group in 3T-br2-C 60-R is smaller than the π-conjugated system in 3T-br1-C 60-R.