Effect of Weak Coupling on the Absorption Spectrum of an Aggregate of Chromophores

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Abstract
A theory is developed for the absorption spectrum of chromophores in a molecular aggregate with weak coupling in which they interact through their vibronic transition moments while vibrating independently. The change upon aggregation of the oscillator strength of a vibronic line, due to interaction with other vibronic lines of the same electronic band or other bands, is treated by first‐order perturbation theory. Equations are derived which relate the aggregate extinction coefficient of a band, as a function of wave‐number, to the monomer chromophore spectra and certain factors for the interaction of electronic transition moments in the aggregate. The interaction of degenerate electronic transition moments can be evaluated from the experimental monomer and aggregate spectra, and used in cases of simple aggregate geometry to determine whether weak coupling applies to the aggregate. The effects of weak coupling upon the shapes and oscillator strengths of bands are illustrated by an example which shows that weak coupling of two overlapping bands in the spectrum of a planar dye molecule may explain the hypsochromic shift of the wave‐number of maximum absorption observed upon aggregation of the dye.