Photoinduced Electron and Energy Transfer in Π-Conjugated Systems

Doctoral thesis, 2015

Photosynthesis is a fascinating process that provides food and oxygen. In this thesis, focus is on understanding and controlling the early processes of photosynthesis, namely photoinduced energy and electron transfer. Indeed, mimicking these processes in synthetic systems could open to a wide range of applications going from renewable energy production to molecular devices for information exchange. In this work, covalently linked π‒conjugated systems provide a means to get a better understanding of the mechanisms and factors that govern transfer of charge and energy in molecular systems. Photoinduced electron transfer (ET) is investigated in series of donor‒acceptor dyads Pn‒C60 and triads Fc‒Pn‒C60 employing butadiyne‒linked zinc porphyrin oligomers denoted Pn (n = 1‒8) as photoactive electron donor and π‒conjugated bridging structures (wires). In the triads, temperature dependence study of the recombination of the long‒range charge separated state Fc•+‒Pn‒C60•- provides new insights into the mechanistic nature of the charge transport linked to its wire‒like behavior. At high temperatures (> 280 K), the weakly distance‒dependent charge transport arises from coherent electron tunneling for the entire series. At low temperatures, crossover to incoherent hole hopping accompanied by a stronger distance dependence of the charge transport is observed in long Fc‒Pn‒C60 (n = 2‒4). Being able to tune the ET rate in donor‒acceptor systems is another property which is highly desirable for applications of molecules in actual devices. Here, in the long Pn‒C60 (n = 4, 6) dyads, we demonstrate the possibility of varying the rate of electron transfer between the photoexcited porphyrin oligomer (1Pn*) and the fullerene (C60) by either optical or chemical control of the conformation of the porphyrin chain. Additionally, we show that temperature could potentially be used to control the conformation of the porphyrin oligomer. Below 170 K, all studied systems Pn, Pn‒C60 and Fc‒Pn‒C60 spontaneously form highly‒ordered planar aggregates. The thesis also discusses photoinduced excitation energy transfer (EET) in the Pn systems and two anthracene dendrimers. In the latter, despite weak through‒bond electronic coupling, signs of an ultrafast EET between the anthracene dendrons are observed by time‒resolved fluorescence anisotropy. For the Pn systems, pump intensity‒dependent transient absorption measurements reveal at early times (< 30 ps) singlet‒singlet annihilation that is the characteristic of partially coupled systems.