An important goal for solar energy conversion is efficient light harvesting. Synthetic light harvesters are still far from reaching unit quantum efficiency, in contrast to natural photosynthetic systems. Such a high quantum efficiency is the result of ultrafast (&lt;ps) coherent inter-pigment energy transfer. For such quantum energy transport, it is critical to find the right balance between (a) electronic coupling, resulting in coherences, (b) vibronic resonances and (c) coupling to the environment which causes decoherence. In this work, we investigated the excited state energy transfer in a series of cofacial porphyrin arrays. Porphyrins are fundamental building blocks of different natural light-harvesting complexes, which display an impressive variety of electronic and optical properties. With different substituents on each monomer of porphyrin dimers and different connectivity to the linker, we can fine tune the interaction between the cofacially stacked π-electronic systems and hence obtain synthetic control over the electronic energy gaps as well as the coupling strength. We used two-dimensional electronic spectroscopy to study the excited state dynamics of a series of porphyrin dimers with varying degrees of electronic coupling strengths and compared them to their constituent monomeric units. After Q-band excitation (S0→S1) the porphyrin monomers undergo intersystem crossing to form a long-lived triplet state. In dimers, an additional ultrafast relaxation channel has been found to be activated due to inter-excitonic relaxation (&lt;2 ps). A clear rise of the lower excitonic band is observed for all dimers. A change in connectivity of the monomeric units to the linker from β-meso to meso-meso has been found to enhance the electronic coupling and accelerate the inter-excitonic relaxation rate by three-fold. This is supported by the time-dependent density functional theory (TDDFT) calculations, which show the electron density in the excitons to be more delocalized over two monomeric units in the meso-meso linked dimer compared to the beta-meso linked dimers. In addition, a number of coherences beat frequencies (120, 255, 340, 400, 445, 850, 1145 cm-1) are found in all samples, which were analyzed and characterized with the help of TDDFT calculations.

Synthetic control of excited state energy transfer in cofacial porphyrin dimers

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.