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.

Exciton Level Structure and Dynamics in Tubular Porphyrin Aggregates

“…Two of us have shown previously 34 , using a simplified model that disregarded the effects of the static disorder, that this approximation reproduces well the optically active one-exciton states of TPPS4 tubes.…”

“…We described the aggregate structure using a model geometry which was proposed in previous work 31 and was shown to reproduce the structure of the experimental absorption and linear dichroism spectra 34 . We refer the reader to the original papers for details, and only give a brief summary here.…”

“…The B band originates from the degenerate B x and B y transitions; the Q band consist of two peaks, which originate from Q x,y (0-1) and Q x,y (0-0) transitions, respectively. 31,34 Upon aggregation, the B band is split into a very narrow red-shifted peak at 490 nm (2.53 eV) and a wider peak at 423 nm (2.93 eV); these two peaks originate from different B-band excitons and, as typical for the cylindrical aggregates 51 , they have different polarizations: the low-energy component is polarized parallel to the cylinder axis, while the high-energy component is polarized perpendicular to the aggregate axis.…”

“…Figure 2a shows the absorption and fluorescence spectra of TPPS4 aggregates and monomers in solutions at room temperature, which are consistent with previous spectroscopic studies. 31,34 . The monomer has two prominent absorption bands: the B band at 436 nm (2.85 eV), and the Q band at 647 nm (1.92 eV).…”

“…31 The optical properties of these tubular aggregates have been investigated by both experimental and theoretical methods. 31,34 To realize the potential of tubular aggregates as biomimetic photosynthetic antennae for efficient energy transport, it is crucial to probe and understand the level structure and dynamics of the excitons in these systems. Questions have remained about the degree of exciton localization, energy relaxation pathways and timescales, energy transport mechanisms, and the occurrence of superradiant and dark states.…”