J. Phys. Chem. B 2008, 112, 16367–16381
16367
Investigation of Macrocyclic Polymers as Artificial Light Harvesters: Subpicosecond Energy Transfer in Poly(9,9-dimethyl-2-vinylfluorene) Jerainne M. Johnson,† Rong Chen,‡ Xiyi Chen,§ Amy C. Moskun,| Xi Zhang,⊥ Thieo E. Hogen-Esch,# and Stephen E. Bradforth*,× Department of Chemistry and Loker Hydrocarbon Institute, UniVersity of Southern California, UniVersity Park, Los Angeles, California 90089-0482 ReceiVed: July 4, 2008; ReVised Manuscript ReceiVed: October 5, 2008
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. Steadystate 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 Fo¨rster 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. 1. Introduction In recent years there has been considerable interest in conjugated polymer systems for their applications in electroluminescent devices (e.g., organic light-emitting diodes (OLED), optical displays, and solar sensitizers).1-4 The contributions of their semiconducting and optical properties along with ease of synthesis make them very attractive systems for these applications. In particular, polyfluorene has been the focus of interest, but the lack of control of chain ends has proved to be a problem in their properties and applications. Nonconjugated repeating fluorene systems such as poly(9,9-dimethyl-2-vinylfluorene), PDMVF,5 and polybenzofulvenes6 are also of interest as they have distinct photophysical characteristics,5,6 and they too can act as charge conductors.7-10 In addition, nonconjugated PDMVF and similar poly-vinyl aromatic polymers can be synthe* To whom correspondence should be addressed. † Current address: National Institute of Standards and Technology, Gaithersburg, MD 20899. ‡ Current address: Department of Chemistry, The State University of New York at Stony Brook, Stony Brook, NY 11794. § Current address: University of Connecticut Health Center, Farmington, CT 06030. | Current address: Department of Environmental Science, Whittier College, Whittier, CA 90608. ⊥ Current address: Michigan Molecular Institute, 1910 Saint Andrews Rd, Midland, MI 48640. # Loker Hydrocarbon Institute and Department of Chemistry. × Department of Chemistry.
sized as macrocycles which leads to deployment of chromophores in a similar architecture to natural photosynthetic assemblies. It is thus of interest to characterize and understand their excitation hopping dynamics and see how well they mimic the biological design in their energy transport. In particular, X-ray crystallographic studies have revealed the structural details of the light harvesting complexes (LH1 and LH2) of purple photosynthetic bacteria.11-13In LH2, bacteriochlorophyll molecules (BChl) are bound to the proteins and deployed in a symmetrical ring structure with 18 BChl units arranged in a pinwheel-type fashion.11 An additional nine BChls units are bound in a symmetrical arrangement lying flat in a second plane. Nature has optimized light harvesting in these photosynthetic organisms using chromophores that are principally arranged by noncovalent interactions. The electronic couplings between the BChl chromophores in LH2 have recently been calculated.14,15 Using the collective electronic oscillator approach, Tretiak et al. give the magnitude of the electronic coupling strengths of B850-B850, B800-B850, and B800-B800 as ∼400, ∼40, and ∼25 cm-1, respectively.15 The strength of these interactions implies rapid energy transfer between neighboring BChl molecules. Fluorescence depolarization, transient absorption, annihilation, and photon echo studies reveal energy hopping between B850 molecules, on the order of 100-250 fs16-19 at room temperature. Hopping rates of 0.5-1 ps have also been reported for B800-B800 and B800-B850 molecules.20-24
10.1021/jp806250k CCC: $40.75 2008 American Chemical Society Published on Web 11/24/2008
16368 J. Phys. Chem. B, Vol. 112, No. 51, 2008
Johnson et al.
Figure 1. (a) PM3 optimized structure of a PDMVF macrocycle with 12 chromophore units showing the covalent structure from the top (right panel) and the arrangement of the transition dipoles (shown as thick sticks) with respect to a plane. The transition dipoles are approximately coplanar. (b) The structure of the chromophore unit, DMF. The direction of the S1 f S0 transition dipole moment is along the long axis of the molecule is shown.
There have been several attempts to construct synthetic models for LH2 to reproduce its efficient light harvesting properties. Dendrimers are one such example of a polymer design.25-27 These systems, however, do not reproduce the chromophore topology of the bacterial light harvesting proteins since energy is funneled to/from the core rather than transferring excitation between equivalent chromophores geometrically deployed in a plane. Although systems have been developed with similar architectures,28 we are unaware that any have been demonstrated to imitate LH2 in these functional aspects. Our goal has been to mimic bacterial light harvesters in terms of architecture and functionality using two-dimensional macrocyclic polymers. These 2-D macrocyclic systems have both topological similarities to the design in LH2 and demonstrate similar energy transport. For the current work, we concentrate on the simplest system with only one spectroscopically active pendent chromophore, a poly-vinyl aromatic homopolymer, poly(9,9-dimethyl-2-vinylfluorene) (henceforth referred to as PDMVF) consisting of 9,9-dimethylfluorene (DMF) repeating chromophore units. Such poly-vinyl aromatic polymers can be synthesized with 10-150 chromophore groups (or larger).5,29 Figure 1 shows a molecular mechanics structure of a PDMVF macrocycle with 12 repeating chromophore units. Figure 1 also shows the structure of the individual pendent chromophore unit, DMF. As in LH2, an identical chromophore unit is repeated around a circle, and these chromophores are deployed in an approximate plane. Of course, the smaller macrocycles are the most likely systems to mimic these architectural features of the natural light harvesting systems. The choice of the repeat chromophore unit is motivated by the fact that DMF absorbs in the near-UV region just inside the solar spectrum (300 nm), making it potentially useful for
solar energy harvesting. It has a reasonably high extinction coefficient (10 000 cm-1 M-1) and a relatively long excitedstate lifetime (∼6 ns) that will allow for many energy hops. In addition, fluorene is a very well spectroscopically characterized chromophore, and DMF is expected to exhibit only slight variations. In addition, these polymers would potentially have very little negative impact on the environment since they are composed entirely of hydrocarbons, thus eliminating the environmental issue of heavy metal waste. The organization of this paper is as follows. First we characterize the steady-state spectroscopy, fluorescence lifetime, and reorientation dynamics of the free chromophores, fluorene, DMF, and 9,9-dimethyl-2-ethylfluorene (DMEF). The same techniques are then applied to matched linear and macrocyclic polymers containing the pendent chromophore DMF. The depolarization exhibited by the polymers is shown to be virtually independent of contributions from the reorientational dynamics of the free chromophore, and depolarization in the macrocycles is shown to depend weakly on ring size. The measurement helps to characterize the hopping rate for energy transport in the polymer and the approximate conformation of the macrocycle in solution. By comparison with energy transport simulations for model structures, it is confirmed that the macrocyclic polymers mimic LH2 both in the time scale of energy transport and in the geometric pathways for the excitation flowing through the macromolecule. 2. Experimental Section Steady State. Steady-state absorption measurements were made using a Cary 50 UV/vis spectrophotometer. Steady-state fluorescence measurements were made on a PTI Quanta Master
Macrocyclic Polymers as Artificial Light Harvesters model C-60SE spectrofluorometer (configured with 2 nm excitation and detection bandpass). The concentration of sample solutions for steady-state fluorescence measurements was ∼5 mg/L. This corresponds to an optical density of 0.2-0.3 in a 1 cm cell at the fluorescence excitation wavelength (300 nm). For absorption measurements, solution concentrations were 50-100 mg/L, and 1 mm cells were used. Time Resolved. Time-resolved fluorescence was recorded using a time-correlated single photon counting (TCSPC) apparatus (Becker and Hickl SPC 630) used in tandem with a 200 kHz Ti-sapphire regenerative amplifier laser system (Coherent RegA 9000).30 The 280 nm excitation pulse was generated by frequency doubling 560 nm output of a 400 nm pumped type I optical parametric amplifier (OPA). The fluorescence emission was detected by a micrcohannel plate photomultiplier tube (Hamamatsu R3809U-50) coupled with a 0.125 m double monochromator (CVI Laser Corp.) with a slit width of 0.6 mm (detected bandpass is 2 nm). The instrument response function (IRF) for these fluorescence experiments was 25 ps. The sample solutions were contained in a quartz cuvette cell with path length of 1 cm and continuously stirred. For TCSPC measurements, the solution concentration had an optical density of 0.1 at 280 nm (polymer concentration ∼ 1.7 mg/L). Excitation intensities for TCSPC were
