Energy transfer and conformational dynamics in Zn–porphyrin dendrimers
Introduction
The study of dendrimers has attracted much attention in recent years due the wide range of possible applications in e.g. guest–host chemistry [1], optical data storage [2], [3], catalytic chemistry [4], environmental chemistry [5], [6], and biology [7], [8], [9], [10], [11], [12]. Porphyrin appended dendrimers have also been suggested as potential mimics of the natural photosynthetic light-harvesting (LH) antenna systems due to their structural consistency, and ability to transfer absorbed energy between subunits within the molecule. The dendrimers investigated in this Letter are designed for LH.
The processes occurring in the LH antennas of photosynthetic systems can be illustrated by those in the photosynthetic unit of purple bacteria, which consists of two ring-shaped LH pigment protein complexes – a peripheral LH2 antenna and a core LH1 antenna surrounding the reaction center [13]. Solar energy absorbed by the bacteriochlorophyll (BChl) molecules in the LH2 antenna migrates by sub-picosecond excitation energy transfer (ET) steps within the antenna complex before it is transferred to the LH1 ring (∼5 ps) and finally to the reaction center (∼35 ps) [14]. The efficient ET in the LH2 antenna system and the large absorption cross-section of the BChls are characteristics also desirable for an artificial photosynthetic system. Previous findings have illustrated efficient ET within peryleneimide dendrimers [15], [16], [17], [18], [19], [20], transition metal complexes [14], [21], [22], [23], [24], a novel bichromophoric system [2], Zn–porphyrin centered dendrimers [25], [26], [27], and free-base-porphyrin appended dendrimers [28].
The Zn–porphyrin dendrimers studied in this work are chosen as a mimic of the LH2 antenna system in purple bacteria due to their chemical stability and large absorption cross-sections. From studies of the free-base analog to G3P16 at 77 K, Yeow et al. [28] concluded that the ET is restricted to a dendron consisting of four porphyrin units (see Fig. 1). In this Letter we study the ET process using fluorescence anisotropy measurements at both room temperature and at 200 K, and compare the ET efficiency of the Zn–porphyrin appended dendrimers to that of their free-base analogs.
Section snippets
Experimental
The synthesis and purification of the Zn–porphyrin dendrimers is described elsewhere [29]. Solutions in THF were prepared with concentration such that the optical density at 430 nm was 0.1 mm−1 in the steady-state absorption measurements, less than 0.01 mm−1 in the steady state fluorescence measurements, and 0.1 mm−1 in the time-resolved fluorescence measurements (the specific concentrations are listed in Table 2). A 1-mm glass cuvette was used for time-resolved measurements. Fresh samples were
Structural and spectral characteristics of the Zn–porphyrin dendrimers
The dendrimers are characterized by a single-bonded nitrogen/carbon skeleton with the monomer G0P1 (0th generation, 1 porphyrin) attached at the ends. In this study the monomer, 1st, 3rd and 5th generation dendrimer with 1, 4, 16, and 64 porphyrin units, respectively, were used (see Fig. 1). The single-bonded skeleton is flexible, resulting in approximately spherically shaped dendrimers [28]. Gas-phase molecular-dynamics simulations show that the size of the porphyrin units prevents
Fluorescence anisotropy at room temperature
The fluorescence anisotropy is defined aswhere I∥ and I⊥ are the intensities of the fluorescence light polarized parallel and perpendicular to the excitation light, respectively. Directly after excitation, the anisotropy will be 0.4 if the transition dipole moments of the excited and the fluorescing states have the same orientation. The value of the anisotropy decreases if the porphyrin units change orientation or if ET between them takes place. The latter process
Acknowledgements
Financial support from the Swedish Research Council, the Swedish Energy Agency, the Knut and Alice Wallenberg Foundation, and the Magnus Bergwall Foundation is gratefully acknowledged. We further thank the Australian Research Council for a Discovery Research Grant (DP0208776) to M.J.C. Proofreading of the manuscript by Dr. Han-Kwang Nienhyus is highly appreciated.
References (34)
- et al.
J. Photochem. Photobiol. A
(2001) - et al.
Chem. Phys. Lett.
(1999) - et al.
Chem. Phys. Lett.
(1999) - et al.
Chem. Phys. Lett.
(2004) - et al.
Synth. Mater.
(2001) - et al.
Science
(1994) - et al.
J. Am. Chem. Soc.
(2003) - et al.
J. Am. Chem. Soc.
(2002) - et al.
Chem. Mater.
(1998) - et al.
J. Phys. Chem. B
(1998)
J. Am. Chem. Soc.
Langmuir
Nucleic Acids Res.
Macromolecules
J. Biomed. Mater. Res.
Bioconjugate Chem.
Sci. Am.
Cited by (34)
Novel PAMAM dendrimers with porphyrin core as potential photosensitizers for PDT applications
2018, Journal of Photochemistry and Photobiology A: ChemistryCitation Excerpt :Porphyrin-dendrimers (Pf-Ds) are interesting adduct compounds, because they merge the recognized optical properties of porphyrins with the versatility of the dendritic architecture in a unique class of macromolecules. Generally, two sub-classes of Pf-Ds can be distinguished, namely: 1) Pf-Ds with porphyrins covalently attached in the periphery, and 2) Pf-Ds from a porphyrin core [19–22]. A lot of reports deal with the synthesis methodologies and potential applications of these sensitizers [22–24].
Schiff base bridged biporphyrin: Synthesis, characterization and spectral properties
2014, Inorganic Chemistry CommunicationsFluorescence Lifetime Spectroscopy and Imaging of Visible Fluorescent Proteins
2009, Advances in Biomedical EngineeringFluorescence Lifetime Spectroscopy and Imaging of Visible Fluorescent Proteins
2008, Advances in Biomedical EngineeringSolvent induced control of energy transfer within Zn(II)-porphyrin dendrimers
2006, Chemical Physics LettersCitation Excerpt :The absorption spectra show no variation depending on the dendrimer size and only an insignificant solvent dependence. This indicates that there are no strong interactions between the Zn(II)-porphyrins within the same dendrimer in either of the solvents used [22,23]. Upon excitation, two fluorescence bands with a maximum at 600 nm (S1[0] → S0[0]) and 655 nm (S1[0] → S0[1]) are observed in both solvents.