Investigation of Macrocyclic Polymers as Artificial Light Harvesters: Subpicosecond Energy Transfer in Poly(9,9-dimethyl-2-vinylfluorene)

Abstract

The spectroscopy and dynamics of a novel molecular architecture that mimics natural light harvesting have been characterized. The deployment of 9,9-dimethyl-2-fluorenyl (DMF) chromophores in atactic macrocyclic poly(9,9-dimethyl-2-vinylfluorene) is similar to that in the light harvesting antenna LH2 of the purple photosynthetic bacteria. A variety of spectroscopic probes are used to study the dynamics in these novel polymer systems. The number of chromophores is tuned from 12-142 identical chromophore units. Steady-state absorption and emission measurements, time-resolved fluorescence, and ultrafast transient absorption anisotropy techniques provide evidence for distinct differences in the photophysics of matching molecular weight linear and cyclic polymers and of the occurrence of energy transfer in these polymers. There is direct evidence of energy transfer in these macrocycles manifested in the depolarization decay components, which are characterized by two exponentials and are substantially faster than observed for reorientation of the free DMF chromophore. The time constants for the macrocycles are 700-900 fs and 7-8 ps and are size dependent; the biexponential decay arises from conformational and stereochemical disorder and can be well described by a master equation simulation assuming Förster incoherent hopping on model polymer structures. The results suggest energy hopping between adjacent chromophores on a 1 ps time scale. The pathway for energy migration is shown to be primarily between nearest neighbors along the cyclic backbone, but there is a considerable spread in the site-to-site hopping rates. Small cycles adopt a pseudoplanar ring type arrangement of the chromophore transition dipoles as observed in bacterial light harvesting antenna, and it is found that the linear polymers also show similar short-range planarity of transition dipoles. Overall, it is found that such small macrocyclic polymers possess excellent characteristics for light harvesting among identical chromophores and behave as a circular photonic wire.