Photoinduced Electron Transfer Reactions in a Porphyrin-Viologen Complex: Observation of S2 to S1 Relaxation and Electron Transfer from the S2 State

Andersson, M. 2000. Tuning Electron Transfer Reactions by Selective Excitation in Porphyrin Acceptor Assemblies. Acta Universitatis Upsaliensis. Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 578. 58 pp. Uppsala. ISBN 91-554-48437. This thesis concerns electron transfer reactions from different excited states in porphyrins, and the effect of changing the energy of the link connecting the donor and acceptor. Photoinduced electron transfer, and subsequent processes were studied using ultrashort laser pulses and nanosecond laser flash photolysis. Excitation of Zn(II)-porphyrins in the Soret band lead to population of the higher lying S2 state. The lifetime and transient absorption spectrum was measured for the S2 state. When an electron acceptor was attached to the Zn(II)-porphyrin, either as an ion pair, or covalently bound through an amide link, electron transfer was found to compete with S2 to S1 relaxation. In the ion pair, electron transfer was faster than 200 fs, with a lifetime of the charge separated state of 1.3 ps. Further, in the covalently linked dyad, the Zn(II) porphyrin triplet state was repopulated from a charge transfer state. In [2]-rotaxanes, the Zn(II) porphyrin donor (ZnP) and Au(III) porphyrin acceptor (AuP) are not connected by a direct covalent link. Selective excitation of either the ZnP or the AuP resulted in rapid electron transfer from the ZnP to the AuP. The bis-phenanthroline link connecting the different porphyrins was changed by coordination of Cu(I) or Ag(I). Electron transfer from the ZnP singlet was unaffected by coordination of either Ag(I) or Cu(I), while electron transfer to the AuP triplet was in the Ag(I) link found to occur by an enhanced superexchange, and by a sequential mechanism in the Cu(I) coordinated link. Mikael Andersson, Department of Physical Chemistry, Uppsala University, Box 532, SE-751 21 Uppsala, Sweden  Mikael Andersson 2000 ISSN 1104-232X ISBN 91-554-4843-7 Printed in Sweden by Akademitryck AB, Edsbruk, 2000