The dependence of photoinduced energy transfer on the orientation of the acceptor with respect to the π-plane of the donor in naphthalene-linked crown ether–Tb3+ complexes

Abstract

Abstract The photoinduced energy transfer (ET) from naphthalene (N) to Tb 3+ has been studied in the complexes of Tb 3+ ion with 2,3-naphtho-17-crown-5 ether(I), 2,3-naphtho-20-crown-6 ether(II), 1,8-naphtho-21-crown-6 ether(III) and 1,5-naphtho-22-crown-6 ether(IV), respectively, using nitrate (NO 3 − ) ion as the counter anion in EtOH glass at 77 K. The ligands are so designed that the Tb 3+ ion can be complexed with a predetermined orientation with respect to the naphthalene molecular plane. In systems I and II, the Tb 3+ ion is along the Z -axis; in system III, it is along the Y -axis and in IV, it is along the X -axis, where Z - and Y - are the molecular in-plane long and short axes of the naphthalene molecular plane respectively and X - is the out-of plane axis perpendicular to the naphthalene molecular plane. Present studies indicate that the efficiency of energy transfer (ET) and the quenching of naphthalene phosphorescence show a strong dependence on the orientation of the acceptor metal ion (Tb 3+ ) with respect to the π-plane of the donor naphthalene moiety. The ET studies suggest that an exchange mechanism involving the lowest (ππ * ) triplet state of N and the 5 D 4 state of Tb 3+ ion is predominantly operating. Our observation further indicates that for a given orientation in a complex the emission intensity of the various transitions ( 5 D 4 → 7 F J , J =2–6) for Tb 3+ , vis-a-vis ET efficiency varies considerably with Δ J values (=0, +1 and +2).