INTRAMOLECULAR CHARGE-TRANSFER IN ELONGATED DONOR-ACCEPTOR CONJUGATEDPOLYENES

Intramolecular charge transfer (ICT) has been investigated by electro- optical absorption measurements for two series of donor-acceptor polye nes of increasing length. Each molecule undergoes an increase in dipol e moment upon photoexcitation. The experimental results indicate that lengthening the polyenic chain linking the donor and acceptor end grou ps results in a bathochromic shift of the ICT absorption band and indu ces marked increases of both excited state and transition dipoles, whe reas the ground-state dipole is relatively unaffected. As a result, ma rked photoinduced changes in the dipole moment (up to 33 D) were obtai ned with the longest molecules (up to 27 Angstrom) leading to highly p olar excited states. Elongated donor-acceptor polyenes are therefore p articularly interesting compounds for long-distance and low-energy int ramolecular charge transfer.