1994). In order to address this issue, ultrafast transient absorption spectroscopy was applied on the same artificial light-harvesting dyad as discussed previously, but with extended conjugated π-electron system of the carotenoid moiety with 10 or 11 C=C double bonds, implying lower excited-state energies (Fig. 4a). Strikingly, the Pc lifetime MM-102 in vitro is reduced from its natural lifetime of 3 ns
to 15–300 ps, depending on the length of the carotenoid’s conjugated π-electron system (Fig. 4b) and the solvent polarity. Furthermore, Berera et al. (2006) have demonstrated that the carotenoid S1 excited state acts as the acceptor of excited-state energy from the covalently linked Pc, as schematically shown in Fig. 4c, thereby providing an efficient channel for energy dissipation. Fig. 4 a Molecular structure of a carotenophthalocyanine
light-harvesting dyad 1, 2, and 3. The carotenoids of dyad 1, 2 and 3 contain 9, 10 and 11 conjugated C=C double bonds, respectively. b Upper panel: kinetic traces at 680 nm of dyad 1, 2, and 3 and a model Pc in ARS-1620 nmr tetrahydrofuran (THF). Lower panel: kinetic traces of dyad 3 dissolved in acetone detected at 480 nm (solid line) and 576 nm (dashed line). Excitation wavelength for b and d was 680 nm. c Kinetic scheme that describes the excited-state EX 527 nmr decay processes In dyad 2 and 3 upon Pc excitation. Solid line denotes energy transfer, dotted line denotes internal conversion process. d Evolution-associated difference spectra (EADS) that result from a global analysis on transient absorption experiments on dyad 3 dissolved in acetone. Source: Berera et al. (2006) A crucial aspect of Pc and Chl excited-state quenching by the carotenoid S1 state is the notion that such processes occur through a so-called inverted kinetic scheme, i.e., the quenching state S1 is slowly populated by rate constant kslow (in 15–300 ps)
and quickly depopulated Non-specific serine/threonine protein kinase with rate constant kfast (in ~5 ps). The latter time constant is inherent to the photophysics of the carotenoid S1 state, i.e., internal conversion to the ground state occurs on this timescale through efficient vibronic coupling between the ground and S1 states (Chynwat and Frank 1995). In such an inverted kinetic scheme, the donor (Pc) decays with a single rate constant kslow. The acceptor (carotenoid S1) will rise with rate constant kfast and decay in parallel with the donor with rate constant kslow, and reach a maximum transient concentration that remains low, and with sufficiently separated rate constants, it is approximately equal to kslow/kfast. Thus, in the specific case of the artificial light-harvesting dyads, the carotenoid S1 signal is expected to rise with a rate constant that corresponds to the internal conversion rate of S1 to the ground state and to have a low amplitude throughout the Pc excited-state lifetime.