Energy Transfer in the Azobenzene-Naphthalene Light Harvesting System

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 I and 4 exhibit virtually quantitative ET efficiency. (5) Quenching data of I (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.