1997
J . Phys. Chem. 1986, 90, 1997-1998
Nonphotochemical Hole Burning of Self-Aggregated Dimers of Chlorophyll a in Polystyrene T. P. Carter and G. J. Small* Ames Laboratory-VSDOE and Department of Chemistry, Iowa State University, Ames, Iowa 5001 1 (Received: March 3, 1986)
Nonphotochemical hole burning is reported for several burn frequencies throughout the inhomogeneousprofiles of chlorophyll bands in polystyrene films. The holes burned at T = 1.8 K are stable and show no spontaneous hole filling at short times (-1 h). The hole width associated with the lowest dimer component has a value of 0.16 f 0.03 cm-' at 1.8 K. a Q,(M) dimer
Application of a variety of spectroscopic techniques to isolated reaction centers (RC), intact photosynthetic organisms, and artificial chlorophyll (Chl) and bacteriochlorophyll (Bchl) aggregates has produced important insights.'V2 Notable is the postulation of the chlorophyll special pair (Chl,,, Bchl,,) as the primary electron donor in RC.334 It was, in part, a comparison of lowresolution optical absorption spectra of artificial dimers of chlorophyll with those of the R C which led to this postulation. Recently, a crystal structure determination of the R C isolated from Rhodopseudomonas uiridis has identified a substructure which appears to be the B ~ h l , , . ~ High-resolution optical spectra for artificial aggregates would be valuable for the interpretation of spectra from in vivo systems or isolated RC. We report here the first observation of nonphotochemical hole burning (NPHB)6 for a self-aggregated dimer of Chl a, Chl a? in polystyrene. Holes burnt into the lowest energy Q, component (DJof the dimer absorption are narrow in contrast with the transient holes observed for the lowest energy absorption component of the isolated R C of Rps. s p h a e r o i d e ~ . We ~ ~ ~also observe NPHB in the region corresponding to the absorption of the upper dimer component, D,. The N P H B apparatus has a burn and reading resolution of 0.001 and 0.08 cm-I, respectively.I0 Details of the Chl a2/ polystyrene film preparation are given elsewhere." The absorption spectrum is shown in Figure 1. The D,and D, components lie a t 14 655 and 14 990 cm-I. The Chl a monomer transition lies a t 15 040 cm-I in polystyrene.I2 Hole burning was performed at the burn frequencies (wB)indicated by arrows. The inset shows a hole (-5% OD change) burned with wB = 14515 cm-'. For (1) Katz, J. J.; Shipman, L. L.; Cotton, T. M.; Janson, T. R. In The Porphyrins, Vol. V, Part C, Dolphin, D., Ed.; Academic Press: New York,
1978.
(2) Katz, J. J.; Hindman, J. C. In Biological Events Probed by Ultrafast Laser Spectroscopy, Alfano, R. R., Ed.; Academic Press: New York, 1982. (3) McElroy, J. D.; Mauzerall, D. D.; Feher, G . Biochim. Biophys. Acta 1972. ~,267. ~. , 363. (4) Norris, J. R.; Uphaus, R. A.; Crespi, H. Acad. Sci. U.S.A. 1971, 68, 625. ~
L.; Katz, J. J. Proc. Natl.
( 5 ) Deisenhofer, J.; Epp, 0.;Miki, K.; Huber, R.; Michel, H. J. Mol. Biol.
1984. 180. 385.
( 6 ) Small, G. J. In Modern Problems in Solid State Physics. Molecular Spectroscopy, Agranovich, V. M., Hochstrasser, R.M., Ed.; North-Holland: Amsterdam, 1983. (7) By self-aggregated we mean Chl a molecules interacting via the ring V keto-Mg interaction which had been shown to preferentially produce dimers in dry nonpolar solvents at room temperature and at concentrations up to M. See ref 1. (8) Meech, S. R.;Hoff, A. J.; Wiersma, D. A. Chem. Phys. Lett. 1985, 121, 287. (9) Boxer, S. G.; Lockhart, D. J.; Middendorf, T . R. Chem. Phys. Lett., in press. (IO) Fearey, B. L.; Carter, T. P.; Small, G . J. J . Phys. Chem. 1983, 87, 3590. ._. -.
(1 1) Carter, T. P. Ph.D. Dissertation, Iowa State University, 1986. Cotton, T. M.; Loach, P. A.; Katz, J. J.; Ballschmiter, K. Photochem. Photobiol. 1978, 27,
735.
(12) Carter, T. P.;Small, G . J . Chem. Phys. Lett. 1985, 120,
178.
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Figure 1. Absorption spectrum of Chl u2 in a polystyrene film at 1.8 K. The arrows mark the energies at which holes were burned. The inset shows a typical hole at the lowest burn frequency used; the hole was burned for 30 s with 1.5 mW/cm2 and corresponds to a 5% change in optical density. Dry film used, 100 fim thickness.
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the same burn intensity and burn time the hole depth decreases slowly with increasing wB,reaching a value of -2% at W B = 15 129 cm-'. These results, together with the fact that the contribution of the D, absorption tail at uB = 14 515 cm-' is
