Long-lived triplet state charge separation in novel piperidine-bridged donor-acceptor systems

Two bichromophoric systems are presented that contain an N-alkylnaphthalimide electron acceptor and a 4-methoxyaniline (3a) or an aniline (3b) electron donor, respectively. Upon photoexcitation of 3a in cyclohexane electron transfer occurs in the singlet manifold to afford the short-lived (tau(f) = 0.75 ns) (1)(D+-A(-)) state in ca. 70% yield. An important decay pathway of this D+-A(-) state consists of intersystem crossing (ISC) to yield a triplet state localized on the naphthalimide moiety (D-(3)A). In a slightly more polar solvent like di-n-butyl ether, an equilibrium between D-(3)A and (3)(D+-A(-)) is observed by means of transient absorption spectroscopy. Both species decay with an overall decay time of ca. 1 mu s. Thus, upon changing the spin multiplicity of the D+-A(-) state from singlet to triplet, an increase of its lifetime by three orders of magnitude is observed. In more polar solvents like dioxane, THF, and acetonitrile the (3)(D+-A(-)) State is the only species observed in the transient absorption spectrum, with decay times of ca. 1, 0.5, and 0.1 mu s, respectively. The D-(3)A state is the precursor state for the (3)(D+-A(-)) state in these solvents. It is proposed that, upon increasing solvent polarity, the singlet charge-separation process is retarded as a result of the large driving force (-Delta G(s) degrees > 1 eV), which allows the triplet pathway (D-(1)A --> D-(3)A --> (3)(D+-A(-))) to compete effectively. Compound 3b possesses a somewhat weaker donor chromophore than 3a resulting in a smaller driving force. The decay of the locally excited singlet state of 3b occurs mainly via charge separation in the singlet manifold (D-(1)A --> (1)(D+-A(-))). Only in the very polar solvent acetonitrile does the triplet pathway become competitive, and evidence is found for the formation of (3)(D+-A(-)).