Charge Transfer-Induced State Filling in CdSe Quantum Dot–Alizarin Complexes

Ultrafast transient absorption spectroscopy is applied to study the photoinduced processes of inorganic–organic CdSe quantum dot–alizarin hybrid complexes. The formation of a pronounced transient bleaching of the quantum dot excitonic transitions after selective photoexcitation of the surface-bound alizarin indicates an electron transfer from the alizarin excited state to the quantum dot 1S­(e) state. An electron transfer time of 19 ps is determined, which is independent of the alizarin concentration. A derivative-like spectral signature immediately after photoexcitation of the alizarin is explained by an excitation-induced level shifting of the QD electronic transitions. Our study demonstrates that the bleaching of the quantum dot excitonic transitions can be used to evaluate the charge transfer dynamics in the investigated hybrid complexes

Implementation and Evaluation of Data Analysis Strategies for Time-Resolved Optical Spectroscopy

Time-resolved optical spectroscopy plays a key role in illuminating the mechanisms of many fundamental processes in physics, chemistry, and biology. However, to extract the essential information from the highly complex time-resolved data, advanced data analysis techniques are required. Here we present the implementation strategies and the evaluation of the familiar global lifetime and target analysis as well as the not so widely adopted lifetime distribution analysis (LDA). Furthermore, we demonstrate the implementation of analysis strategies dealing with a number of artifacts inherently present in data from ultrafast optical experiments. The focus of the work is placed on LDA as it allows invaluable exploration depth of the kinetic information contained in the experimental data. We establish a clear regularization procedure for the use of LDA in ultrafast optical spectroscopy and evaluate the performance of a number of factors that play a role in the reliable reconstruction of lifetime distributions. Our results show that the optimal regularization factor can be determined well with the L-curve and the generalized cross-validation techniques. Moreover, the performance evaluations indicate that the most efficient regularization norm is the identity matrix. The analytical procedures described in this work can be readily implemented and used for the analysis of any time-resolved data

Ultrafast Dynamics of Photoisomerization and Subsequent Unfolding of an Oligoazobenzene Foldamer

Investigating and deciphering the dynamics of photoswitchable foldamers provides a detailed understanding of their photoinduced conformational transitions, resembling similar processes in photoresponsive biomacromolecules. We studied the ultrafast dynamics of the photoisomerization of azobenzene moieties embedded in a foldamer backbone and the resulting conformational helix–coil transition by time-resolved femtosecond/picosecond pump–probe spectroscopy in the visible and infrared region. During <i>E</i> → <i>Z</i> photoisomerization of the azobenzenes, the complexity of the photoinduced conformational transition of the pentameric foldamer <b>10</b>_<b>5</b> is reflected in distinct spectral characteristics and a 2-fold slower decay of the excited-state absorption bands compared to the monomer <b>M</b> (τ_4,foldamer = 20 ps, τ_4,monomer = 9 ps). Time-resolved IR experiments reveal the vibrational features of the monomer and the foldamer after photoexcitation, with an additional time constant for the foldamer (τ = 150 ps), indicating the initial steps of unfolding of the helical conformation, which are supported by density functional theory calculations. Our results record the overall sequence of photoinduced structural changes in the foldamer, starting from the initial ultrafast isomerization of the azobenzene unit(s) and ending with the complete unfolding on a later time scale. From our experiments, we could gain insight into the coupling of primary photoisomerization events (“cause”) and secondary unfolding processes (“effect”) in these oligoazobenzene foldamers

Real Time Observation of Ultrafast Peptide Conformational Dynamics: Molecular Dynamics Simulation vs Infrared Experiment

Employing nonequilibrium molecular dynamics (MD) simulations and transient infrared (IR) spectroscopy, a joint theoretical/experimental study on a water-soluble photoswitchable octapeptide designed by Renner et al. [Biopolymers 2002, 63, 382] is presented. The simulations predict the cooling of the hot photoproducts on a time scale of 7 ps and complex conformational rearrangements ranging from a few picoseconds to several nanoseconds. The experiments yield a dominant fast relaxation time of 5 ps, which is identified as the cooling time of the peptide in water and also accounts for initial conformational changes of the system. Moreover, a weaker component of 300 ps is found, which reflects the overall conformational relaxation of the system. The virtues and the limitations of the joint MD/IR approach to describe biomolecular conformational rearrangements are discussed

Ultrafast Photoinduced Deactivation Dynamics of Proteorhodopsin

We report femtosecond time-resolved absorption change measurements of the photoinduced deactivation dynamics of a microbial rhodopsin in the ultraviolet–visible and mid-infrared range. The blue light quenching process is recorded in green proteorhodopsin’s (GPR) primary proton donor mutant E108Q from the deprotonated 13-<i>cis</i> photointermediate. The return of GPR to the dark state occurs in two steps, starting with the photoinduced 13-<i>cis</i> to all-<i>trans</i> reisomerization of the retinal. The subsequent Schiff base reprotonation via the primary proton acceptor (D97) occurs on a nanosecond time scale. This step is two orders of magnitude faster than that in bacteriorhodopsin, potentially because of the very high p<i>K</i>_A of the GPR primary proton acceptor

Three-State Fluorescence of a 2‑Functionalized Pyrene-Based RNA Label

The pyrene-based RNA-fluorescence label 2-(2-pyrenyl­ethynyl) adenosine (2PyA) shows triexponential fluorescence, which depends strongly on the excitation wavelength. Most strikingly, a structured, long-lived fluorescence is observed in solution at room temperature after excitation into the S2 state, which is shifted hypsochromically by 30 nm compared to excitation into the S1 state. This very unusual behavior is investigated in detail with steady-state and time-resolved emission spectroscopy, ultrafast transient absorption spectroscopy, and quantum chemical calculations with both wave functions (CC2-level) and density-functional theory (DFT). 2PyA is found to emit simultaneously from two different intramolecular charge transfer states (mesomeric and twisted, MICT and TICT) which are populated most efficiently via the S1 state and a pyrene-like locally excited (LE) state. Rotational momentum derived from excess excitation energy is required to populate twisted LE configurations. Therefore, the LE state is most efficiently accessible via excitation to the S2. The stabilization of the different substates is related to two distinct reaction coordinates: the adenine–pyrene distance and the adenine–pyrene tilt angle, respectively

Carotenoid Radical Cations as a Probe for the Molecular Mechanism of Nonphotochemical Quenching in Oxygenic Photosynthesis

Nonphotochemical quenching (NPQ) is a fundamental mechanism in photosynthesis which protects plants against excess excitation energy and is of crucial importance for their survival and fitness. Recently, carotenoid radical cation (Car•+) formation has been discovered to be a key step for the feedback deexcitation quenching mechanism (qE), a component of NPQ, of which the molecular mechanism and location is still unknown. We have generated and characterized carotenoid radical cations by means of resonant two color, two photon ionization (R2C2PI) spectroscopy. The Car•+ bands have maxima located at 830 nm (violaxanthin), 880 nm (lutein), 900 nm (zeaxanthin), and 920 nm (β-carotene). The positions of these maxima depend strongly on solution conditions, the number of conjugated CC bonds, and molecular structure. Furthermore, R2C2PI measurements on the light-harvesting complex of photosystem II (LHC II) samples with or without zeaxanthin (Zea) reveal the violaxanthin (Vio) radical cation (Vio•+) band at 909 nm and the Zea•+ band at 983 nm. The replacement of Vio by Zea in the light-harvesting complex II (LHC II) has no influence on the Chl excitation lifetime, and by exciting the Chls lowest excited state, no additional rise and decay corresponding to the Car•+ signal observed previously during qE was detected in the spectral range investigated (800−1050 nm). On the basis of our findings, the mechanism of qE involving the simple replacement of Vio with Zea in LHC II needs to be reconsidered

Ultrafast Charge Separation in Multiexcited CdSe Quantum Dots Mediated by Adsorbed Electron Acceptors

Ultrafast Charge Separation in Multiexcited CdSe Quantum Dots Mediated by Adsorbed Electron Acceptor