Intramolecular charge transfer in elongated donor-acceptor conjugated polyenes

Intramolecular charge transfer (ICT) has been investigated by electro-optical absorption measurements for two series of donor-acceptor polyenes of increasing length. Each molecule undergoes an increase in dipole moment upon photoexcitation. The experimental results indicate that lengthening the polyenic chain linking the donor and acceptor end groups results in a bathochromic shift of the ICT absorption band and induces marked increases of both excited state and transition dipoles, whereas the ground-state dipole is relatively unaffected. As a result, marked photoinduced changes in the dipole moment (up to 33 D) were obtained with the longest molecules (up to 27 Å) leading to highly polar excited states. Elongated donor—acceptor polyenes are therefore particularly interesting compounds for long-distance and low-energy intramolecular charge transfer.