New light-harvesting roles of hot and forbidden carotenoid states in artificial photosynthetic constructs

We present results from fluorescence excitation and transient absorption spectroscopy on a series of artificial light harvesting dyads made up of a zinc phthalocyanine (Pc) covalently linked through a phenylamine to carotenoids (Car) with 8, 9, 10 or 11 conjugated double bonds, referred to as dyads 8-11. In dyad-10 and dyad-11, the energy transfer efficiency from the carotenoid S ₂ state to Pc was shown to depend on the amount of excess vibrational energy in the carotenoid S ₂ state. The carotenoid S ₂ state lifetimes in dyad-9 and dyad-10 were several times shorter than those of model carotenoids (car-n) in solution, indicating that energy transfer to Pc occurs from the S ₂ state. The S ₂ lifetimes lengthen with excess vibrational energy, and this correlates with a higher efficiency of energy transfer. We hypothesize that the higher energy transfer efficiency on excess vibrational excitation results either from a decreased internal conversion rate to lower-lying optically forbidden states, or from an enhanced coupling between vibrationally hot S ₂ states with Pc. Ultrafast transient absorption studies revealed S* state features with unprecedented characteristics: In dyad-9, the S* state had a lifetime of only 100 fs and was observed to operate as the major mediator of energy transfer between Car and Pc. In contrast, the contribution to the energy transfer process by the optically forbidden S ₁ state was negligible (5%). In dyad-10, neither S* nor S ₁ appear to play a role in the energy transfer process to Pc, and all Car to Pc energy transfer proceeds through S ₂. Many of the observed phenomena may be a consequence of the unusually strong electronic coupling between Car and Pc observed in the past on these particular systems.