Bodipy as probes for coherent electronic energy transfer investigations

Electronic energy transfer (EET) is a ubiquitous process that plays a crucial role in photosynthetic light harvesting systems (LHs) and holds important applications in artificial systems. Emerging experimental breakthroughs have showed that the dynamics of light harvesting are not fully described by a classical random- walk picture but quantum coherent transfer takes also place. 2D photon echo spectroscopy has revealed the occurrence of coherent EET processes in a protein from marine algae,1 moreover, such interesting studies are not restricted to biological systems, but are reported on synthetic systems.2,3 Nowadays, the mechanisms proposed to clarify the survival of coherence in photosynthetic systems are still under debate, therefore a better comprehension on the role and mechanism of long- lived quantum coherence phenomenon represents an intriguing challenge. In order to provide artificial spectroscopic probes for investigating this process, our attention is focused on the synthesis of boron- dypirrins (BODIPY) based structures which have attracted interest in many research fields due to a plethora of interesting and advantageous properties, i.e. intense absorption profile, the excellent thermal and photochemical stability, high fluorescence quantum yield, negligible triplet-state formation, good solubility, chemical versatility and fine- tuned spectroscopic properties by chemical modification.4 The investigated BODIPYs bear benzylidene functionalities at the alpha pyrrole positions providing a more extended p-electrons conjugation of the backbone which guarantees red-shifted absorption fallen within the bandwidth of the laser instrument. Our experimental evidences show that the increased complexity of the molecular structure of the BODIPY, bearing two benzylidene functionalities (Figure) could lead to different possible configurations for the molecule causing the opening of new non-radiative relaxation channels. Moreover, the 2D photon echo spectrum shows a complex pattern proving complex dynamics between electronic and vibrational degrees of freedom. It is likely that the presence of coherent vibronic effect could be promoted by interaction between the two aryl moieties. A detailed spectroscopic investigation has been carried out using a combination of UV/Vis absorption, static and time- resolved fluorescence and 2D photon echo spectroscopy. Synthetic procedures, NMR and mass data of BODIPY derivatives will be described on details.