Delocalization and Electron Transfer in Conjugated Porphyrin Oligomers
Event details
| Date | 09.10.2014 |
| Hour | 16:30 › 17:30 |
| Speaker |
Prof. Bo Albinsson Department of Chemistry and Biotechnology, Chalmers University of Technology, 412 96 Göteborg, Sweden |
| Location | |
| Category | Conferences - Seminars |
The photophysical properties of a series of butadiyne linked conjugated zinc porphyrin oligomers have been investigated, specifically on how they are influenced by conformations and topologies. Conformational dependence was studied extensively on the porphyrin dimer, P2. The butadiyne linker allows almost free rotation of the individual porphyrin moieties and it was shown that a broad distribution of different dihedral conformations was contributing to the observed spectral properties of P2. Further, it was shown that twisted and planar conformations of P2 could be selectively excited due to their well separated electronic transitions.1 Control of the conformational distribution could potentially be utilized in many interesting applications, and this was demonstrated by constructing a molecular based photonic memory in2 which a pyridine appended DTE photochrome was used to optically switch the planarization of P2 on and off. Conformation was also dramatically influenced by aggregation of two porphyrin oligomers (trimers and larger) upon slowly cooling to around 160 K.3
The topological aspects were studied by comparing the properties of a linear and a circular porphyrin hexamer. Experiments showed that the fluorescence quantum yield of the circular hexamer was 40 times smaller than the linear counterpart. This was explained by the symmetry change associated with bending the linear oligomer, which makes the S0→ S1 transition symmetry forbidden for circular topologies.4
The electron transfer reactions in the fullerene appended porphyrin oligomers, Pn-C60, were studied thoroughly. It was shown how excitation of different conformations greatly affects the rates and efficiency of the charge separation. Twisted conformers of P2-C60 showed a 4 times faster charge separation rate compared to planar conformers.5 For longer oligomers excitation of twisted conformers can have the opposite effect since it could localize the excitation energy too far away from the fullerene
electron acceptor for efficient charge separation.6
1. Winters, M. U.; Kärnbratt, J.; Eng, M.; Wilson, C. J.; Anderson, H. L.; Albinsson, B., Photophysics of a butadiyne- linked porphyrin dimer: Influence of conformational flexibility in the ground and first singlet excited state. J. Phys. Chem. C 2007, 111 (19), 7192-7199.
2. Kärnbratt, J.; Hammarson, M.; Li, S.; Anderson, Harry L.; Albinsson, B.; Andréasson, J., Photochromic Supramolecular Memory With Nondestructive Readout. Angew. Chem. -Int. Ed. 2010, 49 (10), 1854-1857.
3. Kärnbratt, J.; Gilbert, M.; Sprafke, J. K.; Anderson, H. L.; Albinsson, B., Self-Assembly of Linear Porphyrin Oligomers into Well-Defined Aggregates. The Journal of Physical Chemistry C 2012, 116 (37), 19630-19635.
4. (a) Hoffmann, M.; Karnbratt, J.; Chang, M. H.; Herz, L. M.; Albinsson, B.; Anderson, H. L., Enhanced pi conjugation around a porphyrin[6] nanoring. Angew. Chem.-Int. Ed. 2008, 47 (27), 4993-4996; (b) Sprafke, J. K.; Kondratuk, D. V.; Wykes, M.; Thompson, A. L.; Hoffmann, M.; Drevinskas, R.; Chen, W.-H.; Yong, C. K.; Kärnbratt, J.; Bullock, J. E.; Malfois, M.; Wasielewski, M. R.; Albinsson, B.; Herz, L. M.; Zigmantas, D.; Beljonne, D.; Anderson, H. L., Belt-Shaped π- Systems: Relating Geometry to Electronic Structure in a Six-Porphyrin Nanoring. Journal of the American Chemical Society 2011, 133 (43), 17262-17273.
5. Winters, M. U.; Kärnbratt, J.; Blades, H. E.; Wilson, C. J.; Frampton, M. J.; Anderson, H. L.; Albinsson, B., Control of electron transfer in a conjugated porphyrin dimer by selective excitation of planar and perpendicular conformers. Chem.- Eur. J. 2007, 13 (26), 7385-7394.
6. Kahnt, A.; Kärnbratt, J.; Esdaile, L. J.; Hutin, M.; Sawada, K.; Anderson, H. L.; Albinsson, B., Temperature Dependence of Charge Separation and Recombination in Porphyrin Oligomer–Fullerene Donor–Acceptor Systems. J. Am. Chem. Soc. 2011, 133 (25), 9863-9871.
Practical information
- General public
- Free
Organizer
- Dr Frank van Mourik