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.

SFRP Synthesis of Acenaphthylene Oligomers and Block Copolymers. Potential Light Harvesting Structures

“…In our studies we found, in particular, 11 that the efficiency of energy transfer (ET) is controlled by the specific molecular architecture linking D and A units in the LH model system. This is illustrated in Scheme 1: Donor D groups absorb light energy which is then transferred from the excited D (D*) to the acceptors A; D groups in the case of polymers and particularly peripherally-substituted starburst or dendrimeric polymers, have been likened to "antennae."…”

“…Our strategy uses the approach of controlled polymerization via SFRP in conjunction with azo-unimer 1 8,11,13 containing an azo functionality (a suitable acceptor moiety for LH studies) and the TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl) moiety for reversibly capping the growing chain with acenaphthylene (AcNp) monomer. Chlorobenzene (PhCl) is added as a diluent to suppress self-polymerization of AcNp, as well as to control molecular weight (MW) and polydispersity (PD = M w /M n ), of the growing oligomer, 4 (eq.…”

“…13 We have previously demonstrated the importance of Donor-Acceptor geometries that can be modulated through control of oligomer/polymer molecular architecture. 11 Through SFRP in the current study we have thus prepared a series of oligomers/polymers of controlled molecular weights, and relatively low polydispersities. The present flexible approach allows us to incorporate various Donor-Acceptor moieties in order to explore the strong reducing potential of various Donor excited states.…”

“…[1][2][3][4][5][6][7] Part of our ongoing research in the area of materials science [8][9][10][11] involves investigation of the underlying processes of light harvesting, including absorption of light by suitable donor (D) moieties and efficient energy (and/or) electron transfer to acceptor (A) moieties in LH model molecules.…”

Synthesis and characterization of photo- and pH-responsive nanoparticles containing amino-substituted azobenzene

“…Mp: 126.7-128.0 C. 1 A solution of methacryloyl chloride (0.53 mL, 0.0055 mol) and dichloromethane (10 mL) was then added dropwise from a dropping funnel and triethylammonium salt in the form of smog was observed immediately.…”