title: Shape of the Q band in the absorption spectra of porphyrin nanotubes: Vibronic coupling or exciton effects?
abstract: Absorption and linear dichroism spectra of self-assembled tubular aggregates of TPPS(4) porphyrin are studied theoretically with special emphasis on the low energy part of the spectra (the Q band region) where the coupling with intramolecular vibrations is pronounced. The model Hamiltonian includes both the excitonic coupling between four molecular electronic excited states contributing to the porphyrin Q and B bands as well as the intermediate-strength linear exciton-phonon coupling to one effective high-frequency molecular vibrational mode. Good agreement between the calculated and experimental spectra is obtained. The results allow us to identify the nature of the peaks observed in the Q band region of the aggregate's absorption spectrum; we show that the two most prominent peaks within the Q band originate from two different excitonic subbands. It is shown that the coupling between the Q and B bands plays an important role and the vibronic coupling affects the details of the absorption lineshape.
authors.affiliation: Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, AG Groningen 9747, The Netherlands. a.u.stradomska@rug.nl
authors.forename: Anna
authors.initials: A
authors.lastname: Stradomska
authors.forename: Jasper
authors.initials: J
authors.lastname: Knoester