Ultrafast photochemistry of photosynthetic reaction centre mimics

© 2013 Dr. Benjamin Edward RobothamIn this work the photoinduced electron transfer (ET) behaviour of several photosynthetic reaction centre (RC) mimics was investigated. Particular attention was given to the decay pathways following excitation of the porphyrin S2 state. The systems studied are linked either by covalent or supramolecular means or a combination of both. The redox-active chromophores include zinc(II) tetraphenylporphyrin (ZnTPP), tin(IV) tetraphenylporphyrin (SnTPP), buckminsterfullerene (C60 ), an amino-substituted naphthalene diimide (ANDI) and several N-substituted naphthalene diimides (NDIs). A boron dipyrromethene (BODIPY) is employed as an energy transfer (EnT) partner. The time scales of the investigated processes required the use of ultrafast transient absorption (TA) measurements. The design and construction of the first implementation of a multichannel broadband detection TA instrument using a high repetition rate (94 kHz) amplified laser source is described. A hybrid global target analysis method, which allows TA data to be fitted by both sequential and branched decay scheme models, is also presented. In non-covalently bound ZnTPP:C60 complexes ET originating directly from the porphyrin S2 state is observed. Rapid charge recombination partially reforms both the ZnTPP S1 state as well as the ground state species. In a covalently linked ZnTPP-ANDI dyad a very similar behaviour is observed, although in this case charge recombination returns the ZnTPP S1 state near quantitatively. In both cases a non-thermalised charge separated state can be inferred. If a further thermalised charge separated state is energetically accessible from the porphyrin S1 state, a second ET step to the same acceptor can occur. In supramolecular complexes of ZnTPP with either dipyridine naphthalene diimide (pyrNDI) or diisoquinoline naphthalene diimide (isoquinNDI), ET commencing from the former’s S1 state to the NDI acceptors is observed. Isoquinoline coordination to the porphyrin metal centre, as it occurs in the ZnTPP:isoquinNDI complex, has not been previously reported as an assembly method for RC mimics. In this system the possibility of a very similar higher excited state behaviour as in the ZnTPP-ANDI dyad, i.e. ET from the porphyrin S2 state followed by charge recombination to itsS1 state, is inferred. ET from the porphyrin S2 state does not require a covalent link to the employed acceptor, although a small chromophore separation and the proximity of the frontier orbitals thereby ensured are a necessity. Finally, the photophysics of an NDI2-SnTPP:(OPh-BODIPY)2 array and several related model systems was investigated. In the NDI2-SnTPP:(OH)2 triad, ET from SnTPP to an NDI moiety is exclusively observed for porphyrin S2-excitation. The product of subsequent charge recombination, either the porphyrin S1 state or the ground state species, is strongly dependent on the solvent polarity. In the arrays containing both SnTPP and BODIPY the former’s S2 state can undergo EnT to the latter’s S1 state. However, rapid ET from the interlocated phenolate linker moiety to SnTPP is the dominant quenching process of the S2 state in these arrays