title: Theoretical investigation of the excited states of coumarin dyes for dye-sensitized solar cells.
abstract: Using time-dependent density functional theory (TD-DFT), configuration interaction single (CIS) method, and approximate coupled cluster singles and doubles (CC2) method, we investigated the absorption spectra of coumarin derivative dyes (C343, NKX-2388, NKX-2311, NKX-2586, and NKX-2677), which have been synthesized for efficient dye-sensitized solar cells. The CC2 calculations are found in good agreement with the experimental results except for the smallest coumarin dye (C343). TD-DFT underestimates the vertical excitation energy of the larger coumarin dyes (NKX-2586 and -2677). Solvents (methanol) are found to induce a red shift of the vertical excitation energies, and their effects on the molecular geometry and the electronic structure are examined in detail. The deprotonated form of coumarin is also investigated, where a blue shift of the vertical excitation energies is observed.
authors.affiliation: Department of Applied Chemistry, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Kurashige
authors.forename: Takahito
authors.initials: T
authors.lastname: Nakajima
authors.forename: Saki
authors.initials: S
authors.lastname: Kurashige
authors.forename: Kimihiko
authors.initials: K
authors.lastname: Hirao
authors.forename: Yoshinori
authors.initials: Y
authors.lastname: Nishikitani