# Investigations of the Energy Transfer
## 4 Discussions and Conclusions  
The current work reported the ultrafast kinetic components of the excitation energy transfer in the phycobilisome extracted from A. platensis, as well as in its structural components, phycocyanin and allophycocyanin, which was reported here for the first time. The aim was to coherently connect the time dynamics in these \"simpler\" structural components in order to understand the more complex behavior of the phycobilisome, in view of its further use as solar concentrator for a photo voltaic device. The time-resolved fluorescence investigations on phycobilproteins resolved the fluorescence lifetime of the emitting chromophores and resulted in values of 1.34 ns for phycocyanin and 1.8 ns for allophycocyanin. A component on the time scale of a few hundreds of ps was resolved additionally in each case; however, this was limited by the temporal resolution of our time-resolved fluorescence system. Transient absorption experiments were further employed to identify the ultrafast energy transfer time components. We summarized in Table 1 the decay times obtained for each phycobiliprotein and the phycobilisome. The energy transfer process within the phycobiliprotein was explained using three time components. The longest time component in each phycobiliprotein was assigned to the fluorescence lifetime of the final emitting chromophores that decay through radiative and non-radiative processes. Two other kinetic components were identified: a middle component lasting some tens of picoseconds and a short one in the sub-picosecond range. This ultrashort component in phycocyanin was assigned to the energy transfer between \(\alpha\)-84 and \(\beta\)-84 chromophores. In allophycocyanin, a faster time component of less than 400 fs was detected and attributed to the localization of the excitation energy on one of the two chromophores. The middle component obtained for phycocyanin was assigned to the equilibration energy among spectrally differing \(\beta\)-84 chromophores, depending on the actual chromophore-protein arrangement. In the case of allophycocyanin, the middle component was attributed to the energy transfer between pairs of low and high-energy phycocyanobilin chromophores,based on the two minima observed in the evolution-associated spectra, which corresponded to their absorption. The energy transfer process was further mapped within the phycobilisome composed of phycocyanin rods and an allophycocyanin core and was explained using four decay times. An ultrashort component of 850 fs was assigned to the excitation energy transfer between \(\alpha\)-84 and \(\beta\)-84 chromophores inside phycocyanin hexamers. The ultrashort decay time of 370 fs obtained for allophycocyanin did not find a correspondent in the case of phycobilisome, which was mainly due to the phycobiliproteins configuration, but also because the pump pulse should be mostly absorbed by the phycocyanin. Therefore, this kinetic component would be superimposed by other energy transfer processes. The second component detected in the phycobilisome (14 ps) was shorter than the middle ones observed in the phycobiliproteins. The intra-rod phycocyanin equilibration is expected to take place on a shorter time scale due to the phycobiliproteins configuration in the assembled phycobilisome. Additionally, the energy transfer between the phycocyanobilin chromophores within the hexamers of phycocyanin and, respectively, the trimers of allophycocyanin, should be overlapped by the dominant energy transfer between the two phycobilproteins. A third decay time of 122 ps was identified in the phycobilisome, which indicated the rod to core energy transfer to the final emitting phycocyanobilin chromophores in the allophycocyanin core, that play the role of the terminal emitter. From there, the system returned to the ground state through radiative and non-radiative decay in more than 1 ns.