@Article{PhysChemChemPhys_16_5188, author = {D. Villamaina and M. Kelson and S. Bhosale and E. Vauthey}, title = {{Excitation wavelength dependence of the charge separation pathways in tetraphorphyrin-naphthalene diimide pentads}}, journal= {Phys. Chem. Chem. Phys.}, ISSN = {1463-9076}, volume= {16}, number= {11}, pages = {5188-5200}, url = {http://pubs.rsc.org/en/Content/ArticleLanding/2014/CP/c3cp54871f}, eprint= {http://www.unige.ch/sciences/chifi/publis/refs_pdf/ref01393.pdf}, doi= {10.1039/c3cp54871f}, abstract = {{The excited-state dynamics of two multichromophoric arrays composed of a naphthalene diimide centre and four zinc or free-base porphyrins substituted on the naphthalene core via aniline bridges has been investigated using a combination of stationary and ultrafast spectroscopies. These pentads act as efficient antennae as they absorb over the whole visible region, with a band around 700 nm, associated with a transition to the S$_1$ state delocalised over the whole arrays, and bands at higher energy due to transitions centred on the porphyrins. In non-polar solvents, population of these porphyrin states is followed by sub-picosecond internal conversion to the S$_1$ state. The existence of a charge-separated state located above the S$_1$ state could enhance this process. The decay of the S$_1$ state is dominated by non-radiative deactivation on the 100 ps timescale, most probably favoured by the small S$_1$-S$_0$ energy gap and the very high density of vibrational states of these very large chromophores. In polar solvents, the charge-separated state lies just below the S$_1$ state. It can be populated within a few picoseconds by a thermally-activated hole transfer from the S$_1$ state as well as via sub-picosecond non-equilibrium electron transfer from vibrationally hot porphyrin excited states. Because of the small energy gap between the charge-separated state and the ground state, charge recombination is almost barrierless and occurs within a few picoseconds. Despite their very different driving forces, charge separation and recombination occur on similar timescales. This is explained by the electronic coupling that differs considerably for both processes.}}, year = {2014} }