In this way, the energy and electron transfer mechanisms can be assessed in terms of a number of discrete reaction intermediates. A comprehensive review of global and target analysis techniques has been published (Van Stokkum et al. 2004). In the next section, we illustrate a few examples of time-resolved experiments and data analysis. We will start with the description of elementary energy transfer processes in artificial systems followed by more complex examples in natural light-harvesting compounds. Example 1: the light-harvesting function of carotenoids Carotenoids play an important role in light-harvesting antennae, not only in photoprotection but also by harvesting blue and
green light and transferring the excited-state energy to nearby (B)Chls (Frank selleck et al. 1999; Polivka and Sundström 2004; Ritz et al. 2000). Carotenoids have a complicated SB203580 solubility dmso excited-state manifold: they have a SN-38 purchase strongly allowed transition from the ground state (which has Ag − symmetry in ideal polyenes) to a state with Bu + symmetry called S2. This transition is responsible for their strong absorption of blue-green light. Below the S2 state lies the optically forbidden S1 state that has Ag − symmetry, along with a number of additional optically
forbidden states, the physical nature of which remains unclear (Polivka and Sundström 2004). Ultrafast spectroscopy has proven to be a valuable tool to map out the energy transfer pathways from carotenoid to (B)Chl and understand these processes at the molecular level. In particular, simple artificial photosynthetic light-harvesting systems have given important insights into the physical mechanisms that underlie the various energy transfer and relaxation processes (Berera et al. 2007; Kodis et al. 2004; Marino-Ochoa et al.
2002). Figure 3a shows a minimal artificial light-harvesting mimic suitable for the study of the light-harvesting role of carotenoids. The model system, referred to as dyad 1, is made up of two moieties: a carotenoid with nine conjugated double bonds in its π-electron system and a phthalocyanine (Pc) molecule. The Pc molecule has a maximal absorption at 680 nm (called selleckchem the Q band), and it acts as a Chl a mimic. The carotenoid to Pc energy transfer efficiency is very high in this particular dyad, ~90% (Berera et al. 2007). Fig. 3 a Molecular structure of a carotenophthalocyanine light-harvesting dyad 1. b Evolution-associated difference spectra (EADS) that result from a global analysis on transient absorption experiments on dyad 1. The excitation wavelength was 475 nm. c Kinetic traces at 560 nm (upper panel) and 680 nm (lower panel). d Kinetic scheme that describes the excited-state processes in dyad 1 upon carotenoid excitation. Solid lines denote energy transfer, dotted denote internal conversion, dashed denotes intersystem crossing processes. Source: Berera et al.