Intramolecular energy transfer in a synthetic dendron-based light harvesting system

• We synthesize a new light harvesting dendron suitable for single molecule studies.
• The two green-fluorescent donor dyes undergo efficient FRET to the red-fluorescent acceptor dye.
• A combined FRET/superexchange mechanism explains all energy transfer pathways between the visible light-absorbing dyes and the UV-absorbing backbone.
• The dendron can occupy several different conformations, which are simulated computationally and visualized on a gold surface by scanning tunneling microscopy.

Single-molecule experiments based on Förster resonant energy transfer (FRET) or on single molecule absorption spectroscopy (SMA) are now capable of studying energy funneling, exciton blockade, singlet fission, and a variety of other processes that involve multiple photoactive groups interacting on a single molecular backbone. Here, we present synthesis and optical characterization of a new dendron functionalized with two green donor dyes (Cy3) and one red acceptor dye (Cy5) through flexible linkers. We describe in detail the synthesis of the conjugated network and the flexible dye coupling. Characterization of the dendron and of control molecules with fewer donors or no acceptor by ensemble absorption and emission spectroscopy shows that the system is capable of light harvesting, producing an intramolecular FRET signal from the acceptor greater than expected from a single donor. We also investigate intramolecular energy transfer upon UV excitation of the conjugated backbone. The photophysical behavior of this light harvesting dendron can be rationalized by a simple Förster/superexchange model. Simulations and scanning tunneling microscopy of single dendron molecules show that the dyes can fold over onto the dendron, creating a heterogeneous distribution of conformations suitable for single molecule studies of light harvesting.

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