Investigations on the ultrafast electron injection mechanism from the dye alizarin to wide band gap semiconductor colloids in aqueous medium are presented, combined with detailed studies on population, depopulation, and relaxation phenomena in trap states and their influence on the injection process. Because of the very strong electronic coupling between dye and semiconductor in an alizarin/TiO 2 system, a very fast electron injection from the excited dye to the conduction band of TiO 2 is expected. Our measurements show an injection time τ inj < 100 fs, suggesting that the electron transfer follows an adiabatic mechanism. Furthermore, we present experiments over a wide spectral range on the recombination reaction of the electron in the conduction band of the semiconductor colloid and the dye cation to the ground state. We find highly multiphasic recombination dynamics with time constants from 400 fs to the nanosecond time scale. The nonexponential character of the recombination reaction is attributed to fast relaxation processes. The crucial contribution of surface trap states and their influence on the observed dynamics was investigated with alizarin adsorbed on the insulating substrate ZrO 2 . Since the conduction band edge lies far above (≈1 eV) the S 1 state of alizarin, the electron injection into this band is completely suppressed. Despite this fact our spectroscopic investigations show that on ultrafast time scales the formation of an alizarin cation occurs. This observation, is explained by fast electron injection into surface trap states near the docking site on the colloid. For the alizarin/ZrO 2 system the time scale for the injection into these traps is determined to be faster than 100 fs. The relaxation processes in the traps and the repopulation of the S 1 state occur within 450 fs, the subsequent ground-state relaxation takes 160 ps. The ultrafast injection dynamics into the traps, recorded for alizarin/ ZrO 2 , underlines the importance of surface states for the initial charge separation also for systems with a lower band edge such as TiO 2 . We show that in the dye/ZrO 2 system the process of electron injection is not suppressed but "stopped" after the ultrafast transition into trap states. It is therefore a valuable system for probing the electron dynamics in surface states.

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Electron transfer from organic dye molecules to semiconductor-colloidal systems is among the fastest reported charge-separation reactions. We present investigations on alizarin complexing the surface of TiO 2 semiconductor colloids in solution. Because of the very strong electronic coupling between the sensitizer and the semiconductor in the alizarin/TiO 2 system, very fast electron injection from the photoexcited dye to the conduction band of TiO 2 occurs. The real-time observation of the injection process is achieved by transient absorption spectroscopy using a 19-fs excitation pulse provided by a pump pulse from a noncollinear optical parametric amplifier and a probe pulse from a quasi-chirp-free supercontinuum. An injection time τ inj of 6 fs can be unambiguously derived in three different ways from the experimental data: (i) analysis of individual transients at spectral positions without contributions from subsequent reactions (relaxation, recombination); (ii) global fitting procedure for 31 wavelengths over a wide spectral range; and (iii) calculation of the S* state and comparison to the "nonreactive" system alizarin/ZrO 2 . The spectral signature of the 6-fs kinetic component can be assigned to electron transfer from the excited dye molecule to the TiO 2 colloid. Even for this strongly coupled system, we propose a localized excitation with a subsequent adiabatic electron transfer reaction that is, to our knowledge, the fastest electron-transfer reaction that has been directly measured by transient spectroscopy.

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In this Minireview, we describe the function of the bacterial reaction centre (RC) as the central photosynthetic energy-conversion unit by ultrafast spectroscopy combined with structural analysis, site-directed mutagenesis, pigment exchange and theoretical modelling. We show that primary energy conversion is a stepwise process in which an electron is transferred via neighbouring chromophores of the RC. A well-defined chromophore arrangement in a rigid protein matrix, combined with optimised energetics of the different electron carriers, allows a highly efficient charge-separation process. The individual molecular reactions at room temperature are well described by conventional electron-transfer theory.

Channelrhodopsin-2 from Chlamydomonas reinhardtii is a lightgated ion channel. Over recent years, this ion channel has attracted considerable interest because of its unparalleled role in optogenetic applications. However, despite considerable efforts, an understanding of how molecular events during the photocycle, including the retinal trans-cis isomerization and the deprotonation/reprotonation of the Schiff base, are coupled to the channel-opening mechanism remains elusive. To elucidate this question, changes of conformation and configuration of several photocycle and conducting/nonconducting states need to be determined at atomic resolution. Here, we show that such data can be obtained by solid-state NMR enhanced by dynamic nuclear polarization applied to 15 N-labeled channelrhodopsin-2 carrying 14,15-13 C 2 retinal reconstituted into lipid bilayers. In its dark state, a pure all-trans retinal conformation with a stretched C14-C15 bond and a significant out-of-plane twist of the H-C14-C15-H dihedral angle could be observed. Using a combination of illumination, freezing, and thermal relaxation procedures, a number of intermediate states was generated and analyzed by DNP-enhanced solid-state NMR. Three distinct intermediates could be analyzed with high structural resolution: the early P 500 1 K-like state, the slowly decaying late intermediate P 480 4 , and a third intermediate populated only under continuous illumination conditions. Our data provide novel insight into the photoactive site of channelrhodopsin-2 during the photocycle. They further show that DNP-enhanced solid-state NMR fills the gap for challenging membrane proteins between functional studies and X-ray-based structure analysis, which is required for resolving molecular mechanisms.ince their discovery (1), channelrhodopsins (ChRs) have generated enormous interest because of the rapid development of their applications in optogenetics (2-7). Commonly, ChR2 from Chlamydomonas reinhardtii (8) and its variants are used thanks to their favorable expression levels. They are the only proteins known today functioning as light-gated ion channels (Fig. 1A). Like other microbial retinal proteins, they undergo a periodic photocycle. In ChRs, this photocycle is coupled to channel opening and closing as revealed in electrophysiological recordings (8). A chimera of ChR1 and ChR2 has been crystallized to yield a structure at 2.3-Å resolution (9). However, little is known on how this coupling functions on a molecular level, and a large number of studies based on visible (10-13), IR (11,[14][15][16][17][18][19], resonance Raman spectroscopy (20, 21), and EPR spectroscopy (22, 23) has been performed to address this question.The photocycles of microbial rhodopsins are usually compared with bacteriorhodopsin, the first discovered and most studied lightdriven proton pump (24). Without any illumination, microbial retinal proteins thermally equilibrate into a dark state (25). In the case of bacteriorhodopsin, for example, this state contains a mixture of two species terme...

Femtosecond time-resolved spectroscopy on model peptides with built-in light switches combined with computer simulation of light-triggered motions offers an attractive integrated approach toward the understanding of peptide conformational dynamics. It was applied to monitor the light-induced relaxation dynamics occurring on subnanosecond time scales in a peptide that was backbone-cyclized with an azobenzene derivative as optical switch and spectroscopic probe. The femtosecond spectra permit the clear distinguishing and characterization of the subpicosecond photoisomerization of the chromophore, the subsequent dissipation of vibrational energy, and the subnanosecond conformational relaxation of the peptide. The photochemical cis͞trans-isomerization of the chromophore and the resulting peptide relaxations have been simulated with molecular dynamics calculations. The calculated reaction kinetics, as monitored by the energy content of the peptide, were found to match the spectroscopic data. Thus we verify that all-atom molecular dynamics simulations can quantitatively describe the subnanosecond conformational dynamics of peptides, strengthening confidence in corresponding predictions for longer time scales.

Ultrafast IR spectroscopy is used to monitor the nonequilibrium backbone dynamics of a cyclic peptide in the amide I vibrational range with picosecond time resolution. A conformational change is induced by means of a photoswitch integrated into the peptide backbone. Although the main conformational change of the backbone is completed after only 20 ps, the subsequent equilibration in the new region of conformational space continues for times >16 ns. Relaxation and equilibration processes of the peptide backbone occur on a discrete hierarchy of time scales. Albeit possessing only a few conformational degrees of freedom compared with a protein, the peptide behaves highly nontrivially and provides insights into the complexity of fast protein folding.