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 (τf ) 0.75 ns) 1(D+-A-) state in ca. 70% yield. An important decay pathway of this D+-Astate consists of intersystem crossing (ISC) to yield a triplet state localized on the naphthalimide moiety (D-3A). In a slightly more polar solvent like di-n-butyl ether, an equilibrium between D-3A 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+-Astate 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-3A 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 (-∆GS° > 1 eV), which allows the triplet pathway (D-1A f D-3A f 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-1A f 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-).