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 (τ 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 μs. 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 μs, 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 (−Δ G S ° > 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 - ).