Separation of Scattering and Absorption Contributions in UV/Visible Spectra of Resonant Systems Norberto Micali, †,‡ Francesco Mallamace, ‡,§ Mariangela Castriciano, ⊥ Andrea Romeo, ⊥ and Luigi Monsu ´ Scolaro* ,‡,⊥ Istituto di Tecniche Spettroscopiche, ITS-CNR, Messina, Italy, INFM, Unita ` di Messina, Messina, Italy, Dipartimento di Fisica, Universita ` di Messina, Messina, Italy, and Dipartimento di Chimica Inorganica, Chimica Analitica, e Chimica Fisica, Universita ` di Messina and Istituto di Chimica dei Prodotti Naturali (ICTPN-CNR), Sezione di Messina, Messina, Italy Resonance light scattering (RLS) is a phenomenon due to an enhancement of the scattered light in close proximity to an absorption band. The effect is easily detectable in the case of strongly absorbing chromophores, which are able to interact, thus leading to large aggregates (Paster- nack,R. F.; Collings, P. J. Science 1995, 269, 935). The measurement of absorption spectra from solutions con- taining such resonant systems can lead to misleading results. In this paper, a simple method is described to obtain absorption spectra of aggregated species with a fairly good correction of the scattering component. The RLS spectrum, obtained using a common spectrofluorim- eter, is correlated to the extinction spectrum of the same sample, allowing for an estimation of the scattering contribution to the total extinction spectrum. The method has been successfully applied both on real samples containing aggregated chromophores, such as porphyrins, chlorophyll a and gold colloids, and by simulating extinc- tion spectra. Aggregation of small molecules is an area of active research because of the many implications in different fields of chemistry, biology and physics. 1 A particular interest has been addressed to systems in which arrays of chromophores are structurally orga- nized. These supramolecular assemblies exhibit physicochemical properties, which are important from a fundamental point of view and are suitable for possible technological applications, for example, materials with enhanced nonlinear optical susceptibili- ties. 2 In biological systems, ordered assemblies of bacteriochloro- phylls are found in the light-harvesting complexes LH-I and LH- II of purple bacteria 3-6 and in the chlorosomes of green photosynthetic bacteria. 7 Several mimetic aggregates based on synthetic porphyrins have been tailored as light harvesting systems for artificial photosynthesis and as molecular devices. 8,9 Recently, nano- and micrometric porphyrin wheels, potentially mimicking these natural arrays, have been obtained by simple solvent evaporation. 10,11 A different problem is the formation of supramolecular assemblies of charged porphyrins on oppositely charged polymeric matrixes, for example DNA, 12-14 RNA, 15,16 and polypeptides. 17-21 In the absence of a templating support, the buildup of highly structured H- or J-aggregates, corresponding to the limiting cases of parallel units aligned face-to-face or edge- to-edge, respectively, has been reported. 22-30 J-aggregates are also common for a series of aromatic compounds, that is, cyanine * To whom correspondence should be addressed. Fax: +39-090-393756. E.mail: Monsu@ chem.unime.it. † Istituto di Tecniche Spettroscopiche. ‡ Unita ` di Messina. § Dipartimento di Fisica, Universita ` di Messina. ⊥ Dipartimento di Chimica Inorganica, Chimica Analitica, e Chimica Fisica, Universita ` di Messina and Istituto di Chimica dei Prodotti Naturali. (1) Lehn, J. M. In Supramolecular Chemistry; VCH: Weinheim, 1995. (2) Spano, F. C.; Mukamel, S. Phys. Rev. A 1989 , 40, 5783. (3) McDermott, G.; Prince, S. M.; Freer, A. A.; HawthornthwaiteLawless, A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs, N. W. Nature 1995 , 374, 517. (4) Karrash, S.; Bullough, P. A.; Ghosh, R. EMBO J. 1995 , 14, 631. (5) Cogdell, R. J.; Fyfe, P. K.; Barrett, S. J.; Prince, S. M.; Freer, A. A.; Isaacs, N. W.; McGlynn, P.; Hunter, C. N. Photosynth. Res. 1996 , 48, 55. (6) Pullerits, T.; Sundstrom, V. Acc. Chem. Res. 1996 , 29, 381. (7) Blankenship, R. E.; Olson, E. J. M.; Miller, M. In Anoxygenic Photosynthetic Bacteria; Blankenship, R. E., Madigan, M. T., Bauer, C. E., Eds.; Kluwer Academic Publishers: Dordrecht, 1995, 399. (8) Ward, M. D. Chem. Soc. Rev. 1997 , 26, 365. (9) Hayashi, T.; Ogoshi, H. Chem. Soc. Rev. 1997 , 26, 355. (10) Schenning, A. P. H. J.; Benneker, F. B. G.; Geurts, H. P. M.; Liu, X. Y.; Nolte, R. J. M. J. Am. Chem. Soc. 1996 , 118, 8549. (11) Hofkens, J.; Latterini, L.; Vanoppen, P.; Faes, H.; Jeuris, K.; Defeyter, S.; Kerimo, J.; Barbara, P. F.; Deschryver, F. C.; Rowan, A. E.; Nolte, R. J. M. J. Phys. Chem. B 1997 , 101, 10588. (12) Gibbs, E. J.; Tinoco, I., Jr.; Maestre, M. F.; Ellinas, P. A.; Pasternack, R. F. Biochem. Biophys. Res. Commun. 1988 , 157, 350. (13) Pasternack, R. F.; Bustamante, C.; Collings, P. J.; Giannetto, A.; Gibbs, E. J. J. Am. Chem. Soc. 1993 , 115, 5393. (14) Mukundan, N. E.; Petho, G.; Dixon, D. W.; Kim, M. S.; Marzilli, L. G. Inorg. Chem. 1994 , 33, 4676. (15) Pasternack, R. F.; Brigandi, R. A.; Abrams, M. J.; Williams, A. P.; Gibbs, E. J. Inorg. Chem. 1990 , 29, 4483. (16) Pasternack, R. F.; Gurrieri, S.; Lauceri, R.; Purrello, R. Inorg. Chim. Acta 1996 , 246, 7. (17) Pancoska, P.; Urbanova, M.; Bednarova, L.; Vacek, K.; Panschenko, V. Z.; Vasiliev, S.; Malon, P.; Kral, M. Chem. Phys. 1990 , 147, 401. (18) Nezu, T.; Ebert, G. Biopolymers 1991 , 31, 1257. (19) Gibbs, E. J.; Pasternack, R. F. J. Inorg. Organomet Polym. 1993 , 3, 77. (20) Purrello, R.; Monsu ` Scolaro, L.; Bellacchio, E.; Gurrieri, S.; Romeo, A. Inorg. Chem. 1998 , 37, 3647. (21) Purrello, R.; Monsu ` Scolaro, L.; Lauceri, R.; Gurrieri, S.; Romeo, A. J. Am. Chem. Soc. 1998 , 120, 12353. (22) Ohno, O.; Kaizu, Y.; Kobayashi, H. J. Chem. Phys. 1993 , 99, 4128. (23) Ribo `, J. M.; Crusats, J.; Farrera, J. A.; Valero M. L. J. Chem. Soc., Chem. Commun. 1994 , 681. (24) Pasternack, R. F.; Schaefer, K. F.; Hambright, P. Inorg. Chem. 1994 , 33, 2062. (25) Akins, D. L.; Zhu, H.-R.; Guo, C. J. Phys. Chem. 1994 , 98, 3612. Anal. Chem. 2001, 73, 4958-4963 4958 Analytical Chemistry, Vol. 73, No. 20, October 15, 2001 10.1021/ac010379n CCC: $20.00 © 2001 American Chemical Society Published on Web 09/12/2001