Exciton Spectra and the Microscopic Structure of Self-Assembled Porphyrin Nanotubes

Abstract

Exciton spectra and the microscopic structure of self-assembled porphyrin nanotubes Vlaming, S. M.; Augulis, R.; Stuart, M. C. A.; Knoester, J.; van Loosdrecht, P. H. M. Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands ReceiVed: September 16, 2008; ReVised Manuscript ReceiVed: December 11, 2008 The optical properties of tubular aggregates formed by self-assembly of zwitterionic meso-tetra(4-sulfonatophenyl)porphyrin (TPPS 4 ) molecules are studied through a combination of experimental and theoretical techniques. The interest in these systems, with diameters of 18 nm and lengths extending up to micrometers, derives from their strong interaction with light and their similarity to natural light-harvesting systems for photosynthesis. The absorption and linear dichroism spectra are obtained in the spectral region from 300 to 750 nm, which includes the exciton bands deriving from the molecular B (Soret) as well as the Q transitions. We demonstrate that a Frenkel exciton model which takes into account the four dominant molecular excited states (B x , B y , Q x , and Q y ) provides a good global fit to the experimental spectra. From comparison between theory and experiment, we propose a detailed molecular structure within the nanotube.

Exciton Level Structure and Dynamics in Tubular Porphyrin Aggregates

“…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).…”

“…34 As in the case of the B-band, these two excitons have different polarizations: the transition dipole of the lowest-energy one is parallel, while the higher energy one is perpendicular to the cylinder axis. 31 The first vibronic replicas of both aforementioned excitonic states give rise to the third peak, at around 630 nm with no clear polarization. We performed temperature dependent absorption and photoluminescence measurements, the results of which are presented in Figure 2c.…”

“…9,[21][22][23][24] The structure of TPPS4 tubular aggregates has been characterized by small angle X-ray scattering, atomic force microscopy (AFM), and cryo-electron microscopy. [25][26][27][28][29][30][31][32][33] TPPS4 tubular aggregates are found to be cylindrical structures with a diameter of approximately 16-18 nm 31,33 and a length up to several microns. As shown in Figure 1, these tubular aggregates can be viewed as a rolled-up 2D…”

Disorder-induced exciton localization and violation of optical selection rules in supramolecular nanotubes

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14] Examples of molecules that in solution self-assemble into aggregates of a tubular shape are carbocyanine molecules with hydrophobic and hydrophilic side groups [2][3][4][5][6][7] and porphyrin derivatives. [8][9][10][11][12] Both these molecules yield aggregates with a diameter of the order of 10 nm and a length of up to microns, which explains why they are often referred to as molecular nanotubes. Their shape and size resemble natural light-harvesting systems in green sulphur bacteria, 1,[13][14][15][16][17][18] while these systems also have a comparable geometry to previously studied helical polymers.…”

“…33 Because of the large size of tubular aggregates, several recent papers have used the coherent potential approximation (CPA) to account for the effect of disorder on the spectra. 6,12 As the CPA does not allow for a proper description of localization, the breaking of the selection rules and the distribution of dipole orientations cannot be studied within this approach. Rather, investigating the nature of the localized exciton states requires numerical simulations, a technique applied in Refs.…”