Charge separation and singlet fission in covalently linked diketopyrrolopyrrole derivatives and triphenylamine triad in solution

Abstract

A covalently linked push-pull type triad containing two unsymmetrical electron donors, triphenylamine (TPA) and benzothiophene (BT), and diketopyrrolopyrrole (DPP), an electron acceptor, through thiophene spacer has been synthesized (BT-DPP-TPA(7)) and ultrafast relaxation dynamics of the triad has been explored in solution phase by various spectroscopic methods. Steady-state and time-resolved emission studies show the efficient fluorescence quenching of the DPP entity of as prepared BT-DPP-TPA(7) triad. The negative free energy values comprising the redox potentials and singlet state energy of BT-DPP-TPA(7) revealed the probability of electron transfer from the singlet ground state of TPA to the excited singlet state of DPP. Femtosecond transient absorption (fsTA) spectroscopic studies confirmed the formation of charge separation state by detecting triphenylamine radical cation as electron-transfer transients. The rate of charge separation, kCS, is (109−108 s−1) observed to be increasing from nonpolar to polar solvents and the rate of charge recombination, kCR, was found to be slower (μs time scale) in polar solvents like DMF and chloroform, than that in hexane, non-polar solvent (ns time scale). This novel aspect could be due to asymmetrically designed push-pull type triad, a feature that was not evident in push-pull triad constructed using symmetric TPA as electron donors. Furthermore, fsTA studies also demonstrate, for the first time, that the DPP derivatives, TDPP(4) and BT-DPP-TPA(7), both undergo singlet fission (SF) event (S→TT) in 100−200 ps time scale in solution phase for the solution of concentration above ∼100 μM. These results may pave the new avenue for device design comprising DPP derivatives.