FORMATION OF EXTENDED AND FOLDED CHARGE SEPARATED STATES OF DONOR-SPACER-ACCEPTOR MOLECULES WITH FLEXIBLE AND SEMIRIGID SIGMA-BOND SPACERS

Charge separation resulting from photoexcitation of donor-spacer-accep tor molecules, D(S)A, with an anilino (An) donor, a naphthalene (N) or cyanonaphthalene (NCN) acceptor, and either flexible (trimethylene) o r semirigid (piperidine) four sigma-bond spacers, has been investigate d by time resolved microwave conductivity and fluorescence techniques. For the An/N pair in cyclohexane, close approach of the donor and acc eptor moieties is necessary for charge separation. For the larger driv ing force An/NCN couple, charge separation can occur in the fully exte nded configuration. In benzene charge separation occurs in the extende d configuration for both D/A pairs. For the trimethylene spacer molecu les, contact exciplexes are rapidly formed either by prior folding or by harpooning. For the semirigid spacer, electrostatically-driven fold ing, involving inversion of the piperidine ring, can occur subsequent to long-distance charge separation on a time scale of several nanoseco nds.