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The entatic state denotes a distorted coordination geometry of a complex from its typical arrangement that generates an improvement to its function. The entatic-state principle has been observed to apply to copper electron-transfer proteins and it results in a lowering of the reorganization energy of the electron-transfer process. It is thus crucial for a multitude of biochemical processes, but its importance to photoactive complexes is unexplored. Here we study a copper complex—with a specifically designed constraining ligand geometry—that exhibits metal-to-ligand charge-transfer state lifetimes that are very short. The guanidine–quinoline ligand used here acts on the bis(chelated) copper(I) centre, allowing only small structural changes after photoexcitation that result in very fast structural dynamics. The data were collected using a multimethod approach that featured time-resolved ultraviolet–visible, infrared and X-ray absorption and optical emission spectroscopy. Through supporting density functional calculations, we deliver a detailed picture of the structural dynamics in the picosecond-to-nanosecond time range.

The prototype operation of an ultrafast write–readout–erase–readout cycle of an all‐optical system based on photochromic indolylfulgides is demonstrated. In the employed dye the molecular structure is switched using light of different wavelengths between two thermally stable states to allow binary encoding of information. Non‐destructive readout of the bit states using infrared light completes the scheme of an all‐optical memory. For ultrafast operation femtosecond light pulses are applied and it is demonstrated that two consecutive write/erase processes separated by less than 4 ps still allow the defined readout of the bit state. The short time between the write/erase and readout actions demonstrates that an all‐optical data storage system based on indolylfulgides may be operated at memory clock rates that exceed 250 GHz.

Photo-excited xanthone is known to undergo ultrafast intersystem crossing (ISC) in the 1 ps time domain. Correspondingly, its fluorescence quantum yield in most solvents is very small (∼10−4). Surprisingly, the quantum yield in water is 100 times larger, while ISC is still rapid (∼1 ps), as seen by ultrafast pump probe absorption spectroscopy. Temperature dependent steady state and time resolved fluorescence experiments point to a delayed fluorescence mechanism, where the triplet 3nπ* state primarily accessed by ISC is nearly isoenergetic with the photo-excited 1ππ* state. The delayed fluorescence of xanthone in water decays with a time constant of 700 ps, apparently by internal conversion between the 3nπ* state and the lowest lying triplet state 3ππ*.

Structural changes of peptides containing the azobenzene dye 4-aminomethyl-phenylazobenzoic acid (AMPB) are studied with ultrafast spectroscopy. AMPB peptides are a new class of molecules where the photoisomerizable dye azobenzene is linked to the peptide moiety via a flexible methylene spacer. The ultrafast reactions in the femtosecond to nanosecond time domain are investigated for the optical switch AMPB, a linear and cyclic octapeptide, and a bicyclic octapeptide containing an additional disulfide bridge. These molecules with increasing conformational constraints are studied for the cis to trans and the trans to cis photoreactions. For the cis to trans reaction the isomerization of the chromophore occurs fast in the 1-ps range, whereas it is slower (10-ps range) in the trans to cis reaction. In all peptides the structural changes of the chromophore lead to modifications in the peptide structure in the 10-ps–1-ns time range. The results indicate that the chromophore AMPB acts simultaneously as a fast molecular switch and as a sensor for initial conformational dynamics in the peptide. Experiments in the mid-infrared range where the structural changes of the peptide backbone are directly observed demonstrate that the essential part of the structural dynamics in the bicyclic AMPB peptide occurs faster than 10ns.

Femtosecond spectroscopy in combination with site-directed mutagenesis has been used to study the dynamics of primary electron transfer in native and 12 mutated reaction centers of Blastochloris (B) (formerly called Rhodopseudomonas) viridis. The decay times of the first excited state P* vary at room temperature between of 0.6 and 50ps, and at low temperatures between 0.25 and 90ps. These changes in time constants are discussed within the scope of nonadiabatic electron transfer theory using different models: 1) If the mutation is assumed to predominantly influence the energetics of the primary electron transfer intermediates, the analysis of the room temperature data for the first electron transfer step to the intermediate P+BA− yields a reorganization energy λ=600±200cm−1 and a free energy gap ΔG ranging from −600cm−1 to 800cm−1. However, this analysis fails to describe the temperature dependence of the reaction rates. 2) A more realistic description of the temperature dependence of the primary electron transfer requires different values for the energetics and specific variations of the electronic coupling upon mutation. Apparently the mutations also lead to pronounced changes in the electronic coupling, which may even dominate the change in the reaction rate. One main message of the paper is that a simple relationship between mutation and a change in one reaction parameter cannot be given and that at the very least the electronic coupling is changed upon mutation.

The photodissociation of triiodide into diiodide and iodine in ethanol solution was studied by a novel femtosecond pump−probe technique. The dispersed emission of the triiodide sample was recorded as a function of the delay between a 407 nm pump pulse and a 815 nm probe pulse. The most prominent feature of the resulting time dependent spectra is a strong anti-Stokes (with respect to the probe wavelength) contribution. This contribution rises with a delay of ~500 fs and decays on the 2 ps time scale. The decay of the anti-Stokes spectrum is discussed in terms of the cooling dynamics of nascent diiodide for which the technique presented here gives a more direct access than transient absorption spectroscopy.

Anti-Stokes Raman scattering after resonant infrared excitation with femtosecond light pulses was used to monitor the vibrational excitation of the CH-stretching modes of liquid CH2CCl2 and CH3CCl3. Selective excitation and probing allowed determination of the various time-constants for the redistribution between the CH-stretching modes and their relaxation. For the symmetric CH-stretching mode of CH2CCl2 relaxation times of 3.5 and 9.5 ps are determined for the transfer to overtones and to the asymmetric CH-stretching mode, respectively. For CH3CCl3 these time constants are 5.0 and 4.7 ps.

Transient absorption experiments with probing in the blue part of the spectrum (440−500 nm) are performed for the electron-transfer system oxazine 1 in N,N-dimethylaniline. The experiments with high time resolution of better than 20 fs show pronounced modulations due to high-frequency wave packet motion (600 cm ^-1) in the excited electronic state of the oxazine 1 molecule before electron transfer. Additionally, modulations due to wave packet motion in the product state of oxazine 1 after complete charge separation are observed. The absorption transients related to the electron transfer from N,N-dimethylaniline to oxazine 1 are clearly nonmonoexponential with time constants of 30 and 80 fs. This behavior points to a strong adiabaticity of the electron-transfer reaction with a high relevance of nuclear motions.

Fluctuating pulse durations considerably influence the time resolution of probing experiments. The commonly used arithmetic averaging the experimental data changes the shape of the signal curves and gives longer decay times in the wings. Methods are presented to reduce the effect of fluctuating pulse durations.

Coherent Raman probe scattering experiments are performed to study dynamical processes of polyatomic liquids at 300 K. For single homogeneous transitions the dephasing timeT2 is readily obtained from time resolved investigations. Spectral studies show an interesting time dependent shift in scattered frequency. After the excitaiton the vibrating molecules are shown to relax freely with their resonance frequency. Multiple, equally spaced transitions exhibit a beating phenomenon which provides the dephasing time and the frequency interval between neighboring vibrational states. Inhomogeneously broadened systems do not allow a ready determination of the dephasing time by the present probing technique. Previous experiments on the subject have to be reconsidered.

The presented theoretical model for a mode-locked Nd-glass laser simultaneously takes into account dynamics of the mode-locking dye, amplification saturation and radiation background. A systematic variation of laser parameters gives insight into the pulse formation process and allows to improve the laser design. The calculations show that it should be possible to decrease considerably the duration of light pulses of a mode-locked Nd-glass laser. Using a new mode-locking dye with a switching time of τ=2.7×10−12s we obtained stable laser operation and a pulse duration of 1.7×10−12s.

A time-resolved coherent anti-Stokes Raman technique is demonstrated that yields a spectral resolution beyond the linewidth obtained in spontaneous Raman spectroscopy. Two picosecond dye lasers, independently tunable with low timing jitter, are used. The coherent material excitation is generated by long pump pulses and monitored by short delayed probing pulses. Transiently narrowed Raman spectra are recorded by scanning the lasers over the resonances of interest and simultaneously monitoring the coherent scattering signal as a function of the excitation frequency. The technique is applied to molecular vibrations in liquids. The differences between the technique presented and standard coherent anti-Stokes Raman spectroscopy are demonstrated.

The modification of reaction centers from Rhodobacter sphaeroidesby the introduction of pheophytins instead of bacteriopheophytins leads to interesting changes in the primary photosynthetic reaction: long-living populations of the excited electronic state of the special pair P* and the bacteriochlorophyll anion B−Ashow up. The data allow the determination of the energetics in the reaction center. The free energy of the first intermediate P+B−A, where the electron has reached the accessory bacteriochlorophyll BAlies ≈ 450 cm−1below the initially excited special pair P*.

The spectral properties of the excited electronic state of the reaction centers of Rhodopseudomonas(Rps.) viridisare studied by dichroic transient absorption spectroscopy with sub-picosecond time resolution. The theoretical analysis of the experimental results allows the assignment of the transient absorption from two dimer bands of the special pair and show its excitonic coupling to other pigments.

Femtosecond spectroscopy shows that primary electron transfer in photosynthetic reaction centres is an ultrafast stepwise reaction where the electron is transferred via a chain of pigments. In a first reaction step, the electron is transferred from the special pair to the accessory bacteriochlorophyll with a time constant of ca. 3 ps. A second, subpicosecond reaction carries the electron to a bacteriopheophytin before it reaches the quinone QAwithin 200 ps. Experiments on native and mutated reaction centres yield information on energetics, reorganization energies and electronic coupling of the reaction centre. A consistent theoretical treatment of the data shows that standard non–adiabatic theory describes well the primary electron transfer process at room temperature. However, for a special fast reacting mutant at low temperatures, the limits of non–adiabatic theory may be reached.

The initial electron transfer steps in the photosynthetic reaction center of the purple bacterium Rhodobacter sphaeroides have been investigated by femtosecond time-resolved spectroscopy. The experimental data taken at various wavelengths demonstrate the existence of at least four intermediate states within the first nanosecond. The difference spectra of the intermediates and transient photodichroism data are fully consistent with a sequential four-step model of the primary electron transfer: Light absorption by the special pair P leads to the state P*. From the excited primary donor P*, the electron is transferred within 3.5 +/- 0.4 ps to the accessory bacteriochlorophyll B. State P+B- decays with a time constant of 0.9 +/- 0.3 ps passing the electron to the bacteriopheophytin H. Finally, the electron is transferred from H- to the quinone QA within 220 +/- 40 ps.

The influence of phase-modulation on femtosecond time-resolved coherent Raman scattering is investigated theoretically and experimentally. The coherent Raman signal taken as a function of the spectral position shows unexpected temporal oscillations close to time zero. A theoretical analysis of the coherent Raman scattering process indicates that the femtosecond light pulses are amplitude and phase modulated. The pulses are asymmetric in time with more slowly decaying trailing wings. The phase of the pulse amplitude contains quadratic and higher-order contributions.

The primary electron transfer has been investigated by femtosecond time-resolved absorption spectroscopy in two chemically modified reaction centers (RC) of Rhodobacter sphaeroides, in which the monomeric bacteriochlorophylls BA and BB have both been exchanged by 13(2)-hydroxybacteriochlorophyll a or [3-vinyl]-13(2)-hydroxybacteriochlorophyll a. The kinetics of the primary electron transfer are not influenced by the 13(2)-hydroxy modification. In RCs containing [3-vinyl]-13(2)-hydroxybacteriochlorophyll a the primary rate is reduced by a factor of 10.

Time-resolved pump-and-probe experiments of reaction centers of the purple bacterium Rhodobacter sphaeroides (R26) in the mid-IR region between 1000 and 1800 cm-1 are recorded with a time resolution of 300-400 fs. The difference spectra of the states P*, P+HA-, and P+QA- with respect to the ground state P predominantly reflect changes of the special pair. They show positive and negative bands due to changes of distinct vibrational modes superimposed on a broad background of enhanced absorption. A number of certain bands can be assigned to the special pair P, to the bacteriopheophytin HA, and to the quinone QA. The temporal evolution of the IR absorbance changes is well described by the time constants known from femtosecond spectroscopy of the electronic states. Differences occur only at very early times, which are indicative of fast vibrational relaxation with a time constant of a few hundred femtoseconds.

Absorption spectra and light-induced absorbance changes of crystals from Rhodopseudomonas viridisreaction centers are recorded. A theoretical analysis of the absorption and circular dichroism spectra is presented, yielding a consistent picture of spectroscopic and structural information.

The primary electron transfer in reaction centers of Rhodobacter sphaeroides is studied by subpicosecond absorption spectroscopy with polarized light in the spectral range of 920-1040 nm. Here the bacteriochlorophyll anion radical has an absorption band while the other pigments of the reaction center have vanishing ground-state absorption. The transient absorption data exhibit a pronounced 0.9-ps kinetic component which shows a strong dichroism. Evaluation of the data yields an angle between the transition moments of the special pair and the species related with the 0.9-ps kinetic component of 26 +/- 8 degrees. This angle compares favorably with the value of 29 degrees expected for the reduced accessory bacteriochlorophyll. Extensive transient absorbance data are fully consistent with a stepwise electron transfer via the accessory bacteriochlorophyll.

Oscillatory components of the absorption changes in reaction centers from Rhodobacter sphaeroides are observed by femtosecond spectroscopy in the spectral region between 920 and 1100 nm covering the ranges of excited-state absorption and stimulated emission of the photoexcited primary donor and the long wavelength absorption band of the bacteriochlorophyll anion. These oscillations with a dominant frequency of 135 cm^-1 are damped on the 0.4−0.8 ps time scale. The observations fit well into a model with vibrational wave packet motion on the excited-state potential surface modulating stimulated emission. Amplitudes and phases of the oscillation do not show any peculiarity within the absorption band of the bacteriochlorophyll anion, indicating that the interaction of this wave packet with electron transfer to the accessory bacteriochlorophyll is not required to explain the experimental data.

A spectrometer system is presented for time-resolved probing in the midinfrared between 5 and 11 μm with a temporal resolution of better than 400 fs. Multichannel detection with HgCdTe detector arrays consisting of ten elements in combination with a high repetition rate permits one to record weak absorbance changes with an accuracy of 0.1 mOD.

Stable subpicosecond infrared pulses in the spectral region of 4.5–11.5 μm are generated by difference-frequency mixing in AgGaS_2. The system uses femtosecond pulses from a Ti:sapphire regenerative amplifier and from a tunable traveling-wave dye laser. The infrared pulses have a duration of 400 fs, an energy of more than 10 nJ, and a repetition rate of 1 kHz.

Pulses of 100-fsec duration are obtained by synchronous pumping of a colliding-pulse ring dye laser with a mode-locked Ar^+-ion laser. Stable operation of the synchronously pumped colliding-pulse mode-locked laser over hours was obtained by a suitable choice of the distance between the gain and the absorber in combination with an appropriate pump-pulse sequence.

The cyclobutane pyrimidine dimer (CPD) formed between two adjacent thymine bases is the most abundant DNA photolesion induced by UV radiation. The quantum yield of this reaction is on the order of ~1% in DNA. This small quantum yield hampers the study of damage formation in naturally occurring DNA. Investigations with increased accuracy become possible for a locked nucleotide model compound TLpTL which exhibits a quantum yield of about 10% for CPD formation. Time resolved IR spectroscopy on TLpTL and two other DNA model compounds (TpT and (dT)18) reveals that: (i) The absorption changes after ~1 ps are due to CPD photodamage. (ii) The quantum efficiency of CPD formation on the few picosecond time scale equals the quantum efficiency reported in stationary experiments. CPD photodamage formation in the investigated DNA constructs is thus predominantly formed from the primarily photoexcited singlet pp* state, whereas the triplet channel does not play an essential role.