Tuning interfacial charge separation by molecular twist: a new insight into coumarin-sensitized TiO₂ films

Abstract

The 7-aminocoumarin class of dyes exhibits significant molecular twisting in their electronic excited state, which has important consequences on the reaction mechanism involving the intramolecular charge-transfer (ICT) state. In this work, the relationship between the molecular structure of coumarin dye and charge separation on TiO₂ surface has been explored using 7-(dialkylamino)coumarin-3-carboxylic acid dyes C343 and D1421. The alkylamino group in the form of tethered constraint geometry of the julolidyl ring of coumarin C343 and rotation free geometry of 7-N,N′-diethylamino moiety of coumarin D1421 compel planar and twisted ICT (TICT) excited states in polar solvents, respectively. The femtosecond transient absorption studies show that both the charge-transfer states participate in directional electron injection from coumarin dye into the conduction band of the TiO2 semiconductor substrate. The TICT state formation in the excited state minimizes the donor and acceptor orbital overlap and causes slow back electron-transfer reaction. Current–voltage and incident photon-to-current efficiency measurements verify the beneficial electronic decoupling of the TICT state producing superior photovoltaic response of dye-sensitized solar cells based on coumarin D1421. This work presents tuning of interfacial charge separation by molecular twist at the electron-donating site in D-π-A dye/TiO₂ systems.