title: Energy transfer in the azobenzene-naphthalene light harvesting system.
abstract: We have investigated the model light harvesting systems (LHSs) A and B typifying energy transfer (ET) between a naphthalene, Np (donor, D), and an azobenzene, Az (acceptor, A), shown schematically in Scheme 2 . These models were actualized as the naphthyl azo molecules 1 and 4 containing a methylene tether (Scheme 1). The methoxy azo molecules 2 and 5, respectively, served as benchmarks for the assessment of ET. Photophysical data, including initial rate constants for photoisomerization (trans to cis, t-1 --> c-1, and cis to trans, c-1 --> t-1), the relevant c-1 --> t-1 quantum yields, and fluorescence quenching with free naphthalene, 3, as D were measured. Therefore, (1) irradiation of 3 at (270 nm) to give 3* generates fluorescence at 340 nm that is 65% quenched by the trans isomer of 2 (t-2) and 15% quenched by c-2. Comparable naphthalenic fluorescence of c-1 (LH model A) is quenched beyond detectability. (2) Rates of photoisomerization were determined spectrophotometrically for c-1 --> t-1 starting from the c-1 photostationary state as compared with the c-2 --> t-2 benchmark. (3) Progressing toward more complex LH systems, the initial rate constants, k(i), for c-4 --> t-4 (LH model B), were measured as compared with the c-5 --> t-5 benchmark. (4) A new criterion for ET (D --> A) efficiency emerges that combines k(i) (c --> t) ratios and light absorption on irradiation (at 270 nm) ratios. On the basis of this new criterion, both 1 and 4 exhibit virtually quantitative ET efficiency. (5) Quenching data of 1 (almost complete) and 4 (95%) and ET are discussed by comparison with the relevant model azoarenes, 2 and 5, respectively, and in terms of geometrical considerations. Implications for the extension of the results, notably the new criterion for ET efficiency, in these LH models A and B to the polymer and block copolymer D-(CRR')(n)-A and D-(CRR')(n)-A-(CR''R''')(m)-D targets are considered.
authors.affiliation: Department of Chemistry, Queen's University, Kingston, ON, K7L 3N6 Canada.
authors.forename: Dalia
authors.initials: D
authors.lastname: Abdallah
authors.forename: Jamie
authors.initials: J
authors.lastname: Whelan
authors.forename: Julian M
authors.initials: JM
authors.lastname: Dust
authors.forename: Shmaryahu
authors.initials: S
authors.lastname: Hoz
authors.forename: Erwin
authors.initials: E
authors.lastname: Buncel
mesh_terms: D001391:Azo Compounds / Q000737:chemistry*; D004735:Energy Transfer / Q000528:radiation effects; D005453:Fluorescence; D007700:Kinetics; D008027:Light; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D009281:Naphthalenes / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
chemical_list: D001391:Azo Compounds; D009281:Naphthalenes; C031721:naphthalene; C009850:azobenzene