title: Photoinduced intramolecular electron transfer and triplet energy transfer in a steroid-linked norbornadiene-carbazole dyad.
abstract: Bichromophoric compound 3 beta-((2-(methoxycarbonyl)bicyclo[2.2.1]hepta-2,5-diene-3-yl)carboxy)androst-5-en-17 beta-yl-[2-(N-carbazolyl)acetate] (NBD-S-CZ) was synthesized and its photochemistry was examined by fluorescence quenching, flash photolysis, and chemically induced dynamic nuclear polarization (CIDNP) methods. Fluorescence quenching measurements show that intramolecular electron transfer from the singlet excited state of the carbazole to the norbornadiene group in NBD-S-CZ occurs with an efficiency (Phi SET) of about 14 % and rate constant (kSET) of about 1.6 x 10(7) s-1. Phosphorescence and flash photolysis studies reveal that intramolecular triplet energy transfer and electron transfer from the triplet carbazole to the norbornadiene group proceed with an efficiency (TET + TT) of about 52 % and rate constant (kTET + kTT) of about 3.3 x 10(5) s-1. Upon selective excitation of the carbazole chromophore, nuclear polarization is detected for protons of the norbornadiene group (emission) and its quadricyclane isomer (enhanced absorption); this suggests that the isomerization of the norbornadiene group to the quadricyclane proceeds by a radical-ion pair recombination mechanism in addition to intramolecular triplet sensitization. The long-distance intramolecular triplet energy transfer and electron transfers starting both from the singlet and triplet excited states are proposed to proceed by a through-bond mechanism.
authors.affiliation: Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences, Beijing, P R China.
authors.forename: Li-Ping
authors.initials: LP
authors.lastname: Zhang
authors.forename: Bin
authors.initials: B
authors.lastname: Chen
authors.forename: Li-Zhu
authors.initials: LZ
authors.lastname: Wu
authors.forename: Chen-Ho
authors.initials: CH
authors.lastname: Tung
authors.forename: Hong
authors.initials: H
authors.lastname: Cao
authors.forename: Yoshifumi
authors.initials: Y
authors.lastname: Tanimoto
mesh_terms: D000736:Androstenes / Q000138:chemical synthesis / Q000737:chemistry; D002227:Carbazoles / Q000138:chemical synthesis / Q000737:chemistry*; D004735:Energy Transfer; D008163:Luminescent Measurements; D009682:Magnetic Resonance Spectroscopy; D009636:Norbornanes / Q000138:chemical synthesis / Q000737:chemistry*; D010777:Photochemistry; D010782:Photolysis; D013050:Spectrometry, Fluorescence
chemical_list: D000736:Androstenes; D002227:Carbazoles; D009636:Norbornanes