DEPOLARIZED RESONANCE LIGHT-SCATTERING BY PORPHYRIN AND CHLOROPHYLL-AAGGREGATES

A quantum mechanical model is developed for the observed resonance enh ancement of light scattering by aggregates of electronically interacti ng chromophores. Aggregate size, monomer oscillator strength, extent o f electronic coupling, and aggregate geometry are all important determ inants of intensity in resonance light scattering (RLS) spectra. The t heory also predicts the value of the depolarization ratio (rho(V)(90)) of RLS for a given aggregate geometry. These results are used to inte rpret the RLS depolarization ratios of four aggregates: tetrakis(4-sul fonatophenyl)porphine aggregated at low pH (rho(V)(90) = 0.17 at 488 n m), ns-bis(N-methylpyridinium-4-yl)-diphenylporphinato copper(II) aggr egated in 0.2 M NaCl solution (rho(V)(90) = 0.13 at 450 nm) and on cal f thymus DNA(rho(V)(90) = 0.20 at 454 nm), and chlorophyll a aggregate s in formamide/water (rho(V)(90) = 0.23 and 0.32 at 469 and 699 nm, re spectively). The analysis is consistent with a J-aggregate geometry fo r all four systems. Furthermore, the specific values of rho(V)(90) all ow us to estimate the orientation of the monomer transition dipoles wi th respect to the long axis of the aggregate. We conclude that depolar ized resonance light scattering spectroscopy is a powerful probe of th e geometric and electronic structures of extended aggregates of strong chromophores.