Characterization by time-resolved UV/Vis and infrared absorption spectroscopy of an intramolecular charge-transfer state in an organic electron-donor-bridge-acceptor system

A long-lived intramolecular charge-separated state in an electron-donor-acceptor molecule is characterized by time-resolved visible and infrared absorption spectroscopy. Bands that can be assigned to the negatively charged acceptor chromophore can be clearly observed in the time-resolved IR spectrum. In contrast, the electronic absorption spectrum of the radical anion lacks characteristic absorption bands. The IR spectra can be interpreted on the basis of harmonic vibrational frequencies and intensities calculated using B3LYP ab initio methods for model systems of the separate electron donor and acceptor units. This result indicates that the charge-separated state, in spite of the small distance between electron donor and acceptor, can be represented well as an intramolecular radical ion pair.