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RESEARCH PRODUCT

Ultrafast Twisting of the Indoline Donor Unit Utilized in Solar Cell Dyes: Experimental and Theoretical Studies

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Previous time-resolved measurements on D149, the most-studied dye of the indoline family, had shown a fast time-component of 20–40 ps that had tentatively been attributed to structural relaxation. Using femtosecond transient absorption, we have investigated the isolated indoline donor unit (i.e., without acceptor group) and found an ultrafast decay characterized by two lifetimes of 3.5 and 23 ps. Density functional theory calculations show π-bonding and π*-antibonding character of the central ethylene group for the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), respectively. The LUMO is localized on the flexible vinyl-diphenyl region of the donor unit and a twisting process is assumed to occur as a deactivation process for the excited molecule. This is confirmed by multireference second-order perturbation theory (CASSCF/CASPT2) calculations of the lowest-lying excited state, in which it is shown that torsion of the ethylene bond to 96° and pyramidalization to ca. 100° lead to a...