Correlation effects in the optical spectra of porphyrin oligomer chains: Exciton confi nement and length dependence

Journal of Chemical Physics 138, 024312 (2013)

Correlation effects in the optical spectra of porphyrin oligomer chains: Exciton confi nement and length dependence

Conor Hogan, Maurizia Palummo, Johannes Gierschner,, Angel Rubio

Excited states of ethylene-linked free-base porphyrin oligomers and polymer are studied using many-body perturbation theory (MBPT) within the GW approximation and the Bethe-Salpeter equation. Trends in the electronic levels with oligomer length are analysed and the correct long- range behaviour in the band gap is obtained. High polarizabilities and strong red-shifts in the optical absorption peaks are predicted in agreement with observations on other strongly conju- gated oligoporphyrins. We explain these trends by means of spatial and spectral analyses of the exciton character. Although Wannier-Mott and charge-transfer excitons are identi ed in the optical spectra, the strongest polarizabilities are actually associated with small, tightly-bound excitons (Frenkel-like), in contrast with expectations. Furthermore, the common procedure of extrapolating polymer properties from oligomer calculations is examined from an MBPT perspective.

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http://dx.doi.org/10.1063/1.4773582

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