The excited-state dynamics of the radical anion of perylene (Pe) generated upon bimolecular photoinduced electron transfer (PET) with a donor was investigated using broadband pump–pump–probe spectroscopy. It was found to depend on the age of the anion, that is, on the time interval between the first pump pulse that triggers PET and the second one that excites the ensuing Pe anion (Pe^{•–}). These differences, observed in acetonitrile but not in tetrahydrofuran, report on the evolution of the PET product from an ion pair to free ions. Two photoinduced charge recombination pathways of the ion pair to the neutral Pe*(S₁) + donor state were identified: one occurring in a few picoseconds from Pe^{•–}*(D₁) and one taking place within 100–200 fs from Pe^{•–}*(D_n>1). Both processes are sensitive to the interionic distance over different length scales and thus serve as molecular rulers.

The excited-state dynamics of the push–pull azobenzene Methyl Orange (MO) were investigated in several solvents and water/glycerol mixtures using a combination of ultrafast time-resolved fluorescence and transient absorption in both the UV-visible and the IR regions, as well as quantum chemical calculations. Optical excitation of MO in its trans form results in the population of the S₂ ππ* state and is followed by internal conversion to the S₁ nÏ€* state in ∼50 fs. The population of this state decays on the sub-picosecond timescale by both internal conversion to the trans ground state and isomerisation to the cis ground state. Finally, the cis form converts thermally to the trans form on a timescale ranging from less than 50 ms to several minutes. Significant differences depending on the hydrogen-bond donor strength of the solvents, quantified by the Kamlet Taft parameter α, were observed: compared to the other solvents, in highly protic solvents (α > 1), (i) the viscosity dependence of the S₁ state lifetime is less pronounced, (ii) the S₁ state lifetime is shorter by a factor of ≈1.5 for the same viscosity, (iii) the trans-to-cis photoisomerisation efficiency is smaller, and (iv) the thermal cis-to-trans isomerisation is faster by a factor of ≥10³. These differences are explained in terms of hydrogen-bond interactions between the solvent and the azo nitrogen atoms of MO, which not only change the nature of the S₁ state but also have an impact on the shape of ground- and excited-state potentials, and, thus, affect the deactivation pathways from the excited state.

Ultrafast photochemical reactions in liquids occur on similar or shorter time scales compared to the equilibration of the optically populated excited state. This equilibration involves the relaxation of intramolecular and/or solvent modes. As a consequence, the reaction dynamics are no longer exponential, cannot be quantified by rate constants, and may depend on the excitation wavelength contrary to slower photochemical processes occurring from equilibrated excited states. Such ultrafast photoinduced reactions do no longer obey the Kasha–Vavilov rule. Nonequilibrium effects are also observed in diffusion-controlled intermolecular processes directly after photoexcitation, and their proper description gives access to the intrinsic reaction dynamics that are normally hidden by diffusion. Here we discuss these topics in relation to ultrafast organic photochemical reactions in homogeneous liquids. Discussed reactions include intra- and intermolecular electron- and proton-transfer processes, as well as photochromic reactions occurring with and without bond breaking or bond formation, namely ring-opening reactions and cis–trans isomerizations, respectively.

Using photoinduced bimolecular electron transfer reactions as example we demonstrate how diffusion controlled bimolecular chemical reactions can be studied in a model-free manner by quantitatively combining different ultrafast spectroscopical tools.

Acylgermanes have been shown to act as efficient photoinitiators. In this investigation we show how dibenzoyldiethylgermane 1 reacts upon photoexcitation. Our real-time investigation utilizes femto- and nanosecond transient absorption, time-resolved EPR (50 ns), photo-chemically induced dynamic nuclear polarization, DFT calculations, and GC-MS analysis. The benzoyldiethylgermyl radical G• is formed via the triplet state of parent 1. On the nanosecond time scale this radical can recombine or undergo hydrogen-transfer reactions. Radical G• reacts with butyl acrylate at a rate of 1.2 ± 0.1 × 10⁸ and 3.2 ± 0.2 × 10⁸ M^–1 s^–1, in toluene and acetonitrile, respectively. This is _Ëœ1 order of magnitude faster than related phosphorus-based radicals. The initial germyl and benzoyl radicals undergo follow-up reactions leading to oligomers comprising Ge–O bonds. LC-NMR analysis of photocured mixtures containing 1 and the sterically hindered acrylate 3,3-dimethyl-2-methylenebutanoate reveals that the products formed in the course of a polymerization are consistent with the intermediates established at short time scales.

A combination of sub-nanosecond photoexcitation and femtosecond supercontinuum probing is used to extend femtosecond transient absorption spectroscopy into the nanosecond to microsecond time domain. Employing a passively Q-switched frequency tripled Nd:YAG laser and determining the jitter of the time delay between excitation and probe pulses with a high resolution time delay counter on a single-shot basis leads to a time resolution of 350 ps in picosecond excitation mode. The time overlap of almost an order of magnitude between fs and sub-ns excitation mode permits to extend ultrafast transient absorption (TA) experiments seamlessly into time ranges traditionally covered by laser flash photolysis. The broadband detection scheme eases the identification of intermediate reaction products which may remain undetected in single-wavelength detection flash photolysis arrangements. Single-shot referencing of the supercontinuum probe with two identical spectrometer/CCD arrangements yields an excellent signal-to-noise ratio for the so far investigated chromophores in short to moderate accumulation times.

Unambiguous evidence for the formation of excited ions upon ultrafast bimolecular photoinduced charge separation is found using a combination of femtosecond time-resolved fluorescence up-conversion, infrared and visible transient absorption spectroscopy. The reaction pathways are tracked by monitoring the vibrational energy redistribution in the product after charge separation and subsequent charge recombination. For moderately exergonic reactions, both donor and acceptor are found to be vibrationally hot, pointing to an even redistribution of the energy dissipated upon charge separation and recombination in both reaction partners. For highly exergonic reactions, the donor is very hot, whereas the acceptor is mostly cold. The asymmetric energy redistribution is due to the formation of the donor cation in an electronic excited state upon charge separation, confirming one of the hypotheses for the absence of the Marcus inverted region in photoinduced bimolecular charge separation processes

Ultrafast photochemical processes can occur in parallel with the relaxation of the optically populated excited state toward equilibrium. The latter involves both intra- and intermolecular modes, namely vibrational and solvent coordinates, and takes place on timescales ranging from a few tens of femtoseconds to up to hundreds of picoseconds, depending on the system. As a consequence, the reaction dynamics can substantially differ from those usually measured with slower photoinduced processes occurring from equil-ibrated excited states. For example, the decay of the excited-state population may become strongly nonexponential and depend on the excitation wavelength, contrary to the Kasha and Vavilov rules. In this article, we first give a brief account of our current understanding of vibrational and solvent relaxation processes. We then present an overview of important classes of ultrafast photochemical reactions, namely electron and proton transfer as well as isomerization, and illustrate with several examples how nonequilibrium effects can affect their dynamics.

The time resolution of photon detection systems is important for a wide range of applications in physics and chemistry. It impacts the quality of time-resolved spectroscopy of ultrafast processes and has a direct influence on the best achievable time resolution of time-of-flight detectors in high-energy and medical physics. For the characterization of photon detectors, it is important to measure their exact timing properties in dependence of the photon flux and the operational parameters of the photodetector and its accompanying electronics. We report on the timing of silicon photomultipliers (SiPM) as a function of their bias voltage, electronics threshold settings and the number of impinging photons. We used ultrashort laser pulses at 400 nm wavelength with pulse duration below 200 fs. We focus our studies on different types of SiPMs (Hamamatsu MPPC S10931-025P, S10931-050P and S10931-100P) with different SPAD sizes (25μm, 50μm and 100μm) coupled to the ultrafast discriminator amplifier NINO. For the SiPMs, an optimum in the time resolution regarding bias and threshold settings can be reached. For the 50μm type, we achieve a single photon time resolution of 80 ps sigma, and for saturating photon fluxes better than 10 ps sigma.

The activities of our research group in the field of photoinduced electron transfer reactions are discussed and illustrated by several examples

Polarization-sensitive ultrafast infrared measurements on photoinduced electron transfer in donor-acceptor pairs in polar acetonitrile show distinct contributions from loose and tight ion pairs. Highly anisotropic signals from tight ion pairs reveal the importance of mutual orientation of the reactants (see picture) and thus the need to refine theoretical models based on spherical species that solely involve reaction distances.

Ultrafast infrared transient absorption spectroscopy is used to study the photoinduced bimolecular electron transfer reaction between perylene in the first singlet excited state and 1,4-dicyanobenzene in acetonitrile and dichloromethane. Following vibrational marker modes on both donor and acceptor sides in real time provides direct insight into the structural dynamics during the reaction. A band narrowing on a time scale of a few tens of picoseconds observed on the antisymmetric CN stretching vibration of the dicyanobenzene radical anion indicates that a substantial part of the excess energy is channeled into vibrational modes of the product, despite the fact that the reaction is weakly exergonic. An additional narrowing of the same band on a time scale of several hundreds of picoseconds observed in acetonitrile only is interpreted as a signature of the dissociation of the geminate ion pairs into free ions.

The ultrafast ground state recovery (GSR) dynamics of the radical cation of perylene, Pe^•+, generated upon bimolecular photoinduced electron transfer in acetonitrile, has been investigated using pump−pump−probe spectroscopy. With 1,4-dicyanobenzene as electron acceptor, the free ion yield is substantial and the GSR dynamics of Pe^•+ was found to depend on the time delay between the first and second pump pulses, Δt₁₂, i.e., on the “ageâ€� of the ion. At short Δt₁₂, the GSR dynamics is biphasic, and at Δt₁₂ larger than about500 ps, it becomes exponential with a time constant around 3 ps. With trans-1,2-dicyanoethylene as acceptor, the free ion yield is essentially zero and the GSR dynamics of Pe^•+ remains biphasic independently of Δt₁₂. The change of dynamics observed with 1,4-dicyanobenzene is ascribed to the transition from paired to free solvated ion, because in the pair, the excited ion has an additional decay channel to the ground state, i.e., charge recombination followed by charge separation. The rate constants deduced from the analysis of these GSR dynamics are all fully consistent with this hypothesis.

The dynamic Stokes shift of coumarin 153 has been measured in two room-temperature ionic liquids, 1-(3-cyanopropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-propyl-3-methylimidazolium tetrafluoroborate, using the fluorescence up-conversion technique with a 230 fs instrumental response function. A component of about 10−15% of the total solvation shift is found to take place on an ultrafast time scale < 10 ps. The amplitude of this component is substantially less than assumed previously by other authors. The origin of the difference in findings could be partly due to chromophore-internal conformational changes on the ultrafast time scale, superimposed to solvation-relaxation, or due to conformational changes of the chromophore ground state in polar and apolar environments. First three-pulse photon-echo peak-shift experiments on indocyanine green in room-temperature ionic liquids and in ethanol indicate a difference in the inertial component of the early solvent relaxation of <100 fs.

The fluorescence dynamics of perylene in the presence of tetracyanoethylene in acetonitrile was studied experimentally and theoretically, taking into consideration that the quenching is carried out by remote electron transfer in the Marcus inverted region. The initial stage was understood as a convolution of the pumping pulse with the system response accounting for the fastest (kinetic) electron transfer accompanied by vibrational relaxation. The subsequent development of the process was analyzed with differential encounter theory using different models of transfer rates distinguished by their mean square values. The single channel transfer having a bell-shaped rate with a maximum shifted far from the contact produces the ground state ion pair. It was recognized as inappropriate for fitting the quenching kinetics at moderate and long times equally well. A good fit was reached when an additional near contact quenching is switched on, to account for the parallel electron transfer to the electronically excited state of the same pair. The concentration dependence of the fluorescence quantum yield is well fitted using the same rates of distant transfer as for quenching kinetics while the contact approximation applied to the same data was shown to be inadequate.

The charge recombination dynamics of the ion pairs formed upon electron-transfer quenching of perylene by tetracyanoethylene in acetonitrile has been investigated using ultrafast fluorescence upconversion, transient absorption, and transient grating techniques. For this donor/acceptor pair, charge separation is highly exergonic (ΔG_CS= −2.2 eV), but charge recombination is weakly exergonic (ΔG_CR = −0.6 eV). It was found that for more than 90% of the ion pair population, charge recombination is ultrafast and occurs in less than 10 ps. This decay component could not be observed in a previous investigation with a lower time resolution. The results indicate that the primary quenching product is a contact ion pair and not a solvent-separated ion pair as generally assumed for highly exergonic electron-transfer quenching processes. A possible explanation for this apparent divergence is that the contact ion pair is initially formed in an electronic excited state. Only a very minor fraction of the ion pair population undergoes the slow charge recombination predicted by Marcus theory for weakly exergonic charge-transfer processes (normal region).

Several aspects of ultrafast photochemistry in the condensed phase are discussed and illustrated by three examples from our laboratory.