Light scattering experiments are usually performed at wavelengths away from absorption bands, but for species that aggregate, enhancements in light scattering of several orders of magnitude can be observed at wavelengths characteristic of these species. Resonance light scattering is shown to be a sensitive and selective method for studying electronically coupled chromophore arrays. The approach is illustrated with several examples drawn from porphyrin and chlorin chemistry. The physical principles underlying resonance light scattering are discussed, and the advantages and limitations of the technique are reviewed.

The interactions of tetrakis(4-N-methylpyridyl)-porphine (H2TMpyP-4) and its copper(II), nickel(II), zinc(II), cobalt(III), iron(III), and manganese(III) derivatives with several nucleic acids have been investigated. Spectrophotometric titrations of H2TMpyP-4 and Cu(II)TMpyP-4 with the synthetic polymer poly(dG-dC) could be analyzed by a nearest-neighbor exclusion model leading to n approximately equal to two base pairs and equilibrium constants of 7.7 X 10(5) M-1 and 8.0 X 10(5) M-1, respectively. The other metal derivatives [except for the nickel(II) porphyrin] do not provide sufficiently large color changes with poly(dG-dC) to allow analysis. In contrast, all of these porphyrins interact with poly(dA-dT) and DNA. For those porphyrins investigated, the binding profiles are not adequately fit by a nearest-neighbor exclusion model but have profiles suggesting that cooperativity effects are important. Spectral and circular dichroic experiments both suggest base specificity. With calf thymus DNA, the copper(II) and nickel(II) derivatives show prominent negative circular dichroism (CD) features and large red shifts and hypochromicity of the Soret absorption band characteristic of GC specificity, as demonstrated with the synthetic polymer. The other metal derivatives show prominent positive induced visible CD features with small red shifts and hypochromicity of the absorption bands in the Soret region characteristic of AT specificity. Only the metal-free derivative has a conservative CD spectrum suggesting distribution among GC and AT sites.

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The self-aggregation of cationic porphyrins in aqueous solution has been studied by means of absorption, 1 H NMR, and resonance light scattering spectroscopy. The aim of the present study is clarification of the factors which most influence porphyrin self-aggregation in water. Cationic 5,10,15,20-tetrasubstituted porphyrins [PorSub 4 : PC3Py (Sub ) -(CH 2 ) 3 -Py + Cl -), PC5Py (Sub ) -(CH 2 ) 5 -Py + Cl -), PC7Py (Sub ) -(CH 2 ) 7 -Py + Cl -), TPPOC2Py (Sub ) p-C 6 H 4 -O(CH 2 ) 2 -Py + Br -), and TPPOC3Py (Sub ) p-C 6 H 4 -O(CH 2 ) 3 -Py + Br -), where Py + ) N-alkylpyridinium] were used. PC3Py forms a dimer in aqueous solution with or without an added inorganic salt. In the presence of KNO 3 , dimer formation of PC3Py is dominated by a large and negative enthalpy change. The entropy change for aggregation increases with increasing concentrations of added KNO 3 , while enthalpy changes are almost constant. The thermodynamic parameters suggest that dimer formation of PC3Py is the result of a large enthalpic gain due to extended van der Waals interactions, in cooperation with enhanced hydrophobic interactions. Electrostatic repulsion suppresses further association. However, other cationic porphyrins studied show evidence of higher self-aggregate formation. X-ray crystallographic studies of PC5Cl (Sub ) -(CH 2 ) 5 -Cl), a precursor of PC5Py, show the formation of a slipped face-to-face dimer as the basic unit for forming a crystal lattice, while that of TPP (Sub ) -C 6 H 5 ) indicates the absence of face-to-face stacking interaction in the crystals. Judging from these results, it is assumed that the slipped face-to-face dimer is the unit of the higher self-aggregates of PC5Py or PC7Py in water with or without KNO 3 . Elongated alkyl chains of PC5Py and PC7Py may be responsible by enhancing hydrophobic interaction of the porphyrins studied. TPPOC2Py alone forms a J-aggregate, characterized by a sharp and red-shifted Soret band and a strongly enhanced resonance light scattering signal. Higher self-aggregates having face-to-face arrangement are formed with TPPOC3Py; these provide no enhanced RLS feature. The present study reveals the dependence of aggregate formation of water-soluble porphyrins on the peripheral mesosubstituents.