Generating laser-pulse enantiomers.

We present an optical setup capable of mirroring an arbitrary, potentially time-varying, polarization state of an ultrashort laser pulse. The incident beam is split up in two and the polarization of one beam is mirrored by reflection off a mirror in normal incidence. Afterwards, both beams are recombined in time and space such that two collinear ultrashort laser pulses with mutually mirrored polarization, i.e., laser-pulse enantiomers, leave the setup. We employ the Jones formalism to describe the function of the setup and analyze the influence of alignment errors before describing the experimental implementation and alignment protocol. Since no wave plates are utilized, broadband pulses in a large wavelength range can be processed. In particular, we show that the setup outperforms broadband achromatic wave plates. Furthermore, since the two beams travel separately through the optical system they can be blocked independently. This opens the possibility for circular dichroism, ellipsometry, and anisotropy spectroscopy with shot-to-shot chopping and detection schemes as well as chiral coherent control applications.

Tracing the steps of photoinduced chemical reactions in organic molecules by coherent two-dimensional electronic spectroscopy using triggered exchange.

We establish coherent triggered-exchange two-dimensional (TE2D) electronic spectroscopy as an expansion of pump-repump-probe transient absorption spectroscopy and uniquely elucidate the role of higher-lying electronic states in ultrafast photochemistry. As an example, this is demonstrated for a molecular switch present in two ring-open conformations. The formation of a new species-the radical cation-is observed and its precursor state is identified via TE2D.

Ultrafast charge-transfer dynamics of donor-substituted truxenones.

The photophysics of two donor-substituted truxenone derivatives has been studied by femtosecond time-resolved transient absorption spectroscopy. The systems consist of a central truxenone acceptor with three triarylamine (TARA) branches which act as electron donors. Upon excitation in the visible regime an electron is transferred from the donor to the acceptor, generating a charge-separated state. This state can be probed via the characteristic absorption of the TARA radical cation around 700 nm. A second absorption band around 420 nm exhibits the same kinetics and is assigned to an absorption of the radical anion of the truxenone moiety. The back electron transfer and the recovery of the ground state can be interpreted within the framework of Marcus theory. To study the dependence of the back electron transfer on the electronic coupling, the distance between the donor and the acceptor was adjusted. Two solvents were employed, dimethylsulfoxide and dichloroethane. A biexponential decay of the bands assigned to the charge-separated state was observed, with time constants in the picosecond range. Surprisingly, the rates for electron back transfer do not follow the simple picture of the donor-acceptor distance being the determining factor. The observations are explained within a model that additionally takes steric interactions between the donor and the acceptor into account.

Precise and rapid detection of optical activity for accumulative femtosecond spectroscopy.

We present polarimetry, i.e. the detection of optical rotation of light polarization, in a configuration suitable for femtosecond spectroscopy. The polarimeter is based on common-path optical heterodyne interferometry and provides fast and highly sensitive detection of rotatory power. Femtosecond pump and polarimeter probe beams are integrated into a recently developed accumulative technique that further enhances sensitivity with respect to single-pulse methods. The high speed of the polarimeter affords optical rotation detection during the pump-pulse illumination period of a few seconds. We illustrate the concept on the photodissociation of the enantiomers of methyl p-tolyl sulfoxide. The sensitivity of rotatory detection, i.e. the minimum rotation angle that can be measured, is determined experimentally including all noise sources to be 0.10 milli-degrees for a measurement time of only one second and an interaction length of 250 µm. The suitability of the presented setup for femtosecond studies is demonstrated in a non-resonant two-photon photodissociation experiment.

Substituted Tris(2-pyridylmethyl)amine Ligands for Highly Active ATRP Catalysts.

The synthesis and application of a very active catalyst for copper-catalyzed atom transfer radical polymerizations (ATRP) with tris([(4-methoxy-2,5-dimethyl)-2-pyridyl] methyl)amine (TPMA*) ligand is reported. Catalysts with TPMA* ligands are approximately 3 orders of magnitude more active than those with tris(2-pyridylmethyl)amine (TPMA). Catalyst activity was evaluated by cyclic voltammetry, stopped-flow, and ATRP kinetics. Catalysts with TPMA* ligands perform better than those with TPMA ligands, especially at low catalyst concentrations.

Determination of ATRP Equilibrium Constants under Polymerization Conditions.

Atom transfer radical polymerization (ATRP) equilibrium constants (KATRP) were measured during polymerization of methyl acrylate (MA) with CuIBr/CuIIBr2 in either dimethyl sulfoxide (DMSO) or acetonitrile (MeCN) in the presence of either tris(2-pyridylmethyl)amine (TPMA) or tris[2-(dimethylamino)ethyl]amine (Me6TREN) as the ligand and with ethyl 2-bromopropionate as the initiator. The ln(KATRP) values changed linearly with the volume fraction of solvents in the reaction medium, allowing extrapolation of the values for KATRP to bulk conditions, which were 2 × 10-9 and 3 × 10-8 for TPMA and Me6TREN ligands at 25 °C, respectively. The temperature effect on KATRP values was studied in MA/MeCN = 1/1 (v/v) with TPMA as the ligand in the temperature range from 0 to 60 °C. The KATRP values increased with temperature providing ΔH = 36 kJ mol-1 in MeCN.

Visible Light and Sunlight Photoinduced ATRP with ppm of Cu Catalyst.

Photochemically induced ATRP was performed with visible light and sunlight in the presence of parts per million (ppm) copper catalysts. Illumination of the reaction mixture yielded polymerization in case of 392 and 450 nm light but not for 631 nm light. Sunlight was also a viable source for the photoinduced ATRP. Control experiments suggest photoreduction of the CuII complex (ligand to metal charge transfer in the excited state), yielding a CuI complex, and a bromine radical that can initiate polymerization. No photoactivation of CuI complex was detected. This implies that the mechanism of ATRP in the presence of light is a hybrid of ICAR and ARGET ATRP. The method was also used to synthesize block copolymers and polymerizations in water.

Reaction dynamics of a molecular switch unveiled by coherent two-dimensional electronic spectroscopy.

Coherent two-dimensional electronic spectroscopy is usually employed on molecular species with fixed geometric configuration. Here we present two-dimensional Fourier-transform electronic spectra of dissolved 6,8-dinitro-1',3',3'-trimethylspiro[2H-1-benzopyran-2,2'-indoline] (6,8-dinitro-BIPS), a photochromic system present in two ring-open forms differing in the cis/trans configuration of a double bond, which both undergo a photoinduced ring closure. The two-dimensional spectra, recorded with 20 fs pump pulses centered at 605 nm and a supercontinuum probe covering the complete visible spectral range, allow for a detailed analysis of the photophysics and photochemistry of the two isomers and directly reveal that cis/trans isomerization among them does not play a major role. This experiment demonstrates the potential of two-dimensional electronic spectroscopy for reactive processes.

Ring-closure and isomerization capabilities of spiropyran-derived merocyanine isomers.

We report the photochemistry of two ring-open isomers, namely TTC and TTT, of a bidirectional photoswitchable spiropyran, 6,8-dinitro-1',3',3'-trimethylspiro[2H-1-benzopyran-2,2'-indoline] (6,8-dinitro BIPS). Both isomers are capable of ring closure after excitation with visible fs laser pulses, as disclosed by pump-wavelength-dependent transient absorption experiments in the visible spectral range. The main isomer TTC has its maximum absorption at 560 nm, whereas the minor isomer TTT is red-shifted (600 nm). The excited-state lifetimes differ strongly (τ ≈ 900 ps for TTT and τ ≈ 95 ps for TTC), nevertheless the quantum efficiencies for ring closure (40% for TTC and 35% for TTT) and isomerization (1-2% for TTC and 1-2% for TTT) are comparable. With regard to the bidirectional photoswitching capabilities, 6,8-dinitro BIPS is the first molecular switch based on a 6π-electrocyclic reaction where both ring-open isomers are capable of ring closure.

Ultrafast bidirectional photoswitching of a spiropyran.

We report on bidirectional photochemical switching of 6,8-dinitro-1',3',3'-trimethylspiro[2H-1-benzopyran-2,2'-indoline] (6,8-dinitro-BIPS) between the ring-closed spiropyran and the ring-open merocyanine form. This is studied by femtosecond three-color pump-repump-probe experiments. Both ring opening and ring closure are photoinduced. Completion of an entire cycle, consisting of opening and subsequent closure, can be achieved within 40 ps. A much shorter time (<6 ps) is needed for the converse cycle, consisting of initial ring closure and subsequent ring opening. Furthermore, we perform pump-probe experiments with ultraviolet/visible pump and visible/mid-infrared probe pulses for an unambiguous spectroscopic identification of the open and closed molecular forms. Following visible excitation of the ring-open molecules, ultrafast ring closure is observed directly in the mid-infrared. The quantum efficiencies for ring opening and ring closure starting from the respective equilibirum states are determined to be approximately 9% and 40%. These results show that 6,8-dinitro-BIPS is an ultrafast bidirectional molecular switch exhibiting a high quantum efficiency.

Ultrafast multisequential photochemistry of 5-diazo Meldrum's acid.

We disclose the light-induced dynamics and ultrafast formation of several photoproducts from the manifold of reaction pathways in the photochemistry of 5-diazo Meldrum's acid (DMA), a photoactive compound used in lithography, by femtosecond mid-infrared transient absorption spectroscopy covering several nanoseconds. After excitation of DMA dissolved in methanol to the second excited state S(2), 70% of excited molecules relax back to the S(0) ground state. In competing processes, they can undergo an intramolecular Wolff rearrangement to form ketene, which reacts with a solvent molecule to an enol intermediate and further to carboxylate ester, or they first relax to the DMA S(1) state, from where they can isomerize to a diazirine and via an intersystem crossing to a triplet carbene. For a reliable identification of the involved compounds, density functional theory calculations on the normal modes and Fourier transform infrared spectroscopy of the reactant and the photoproducts in the chemical equilibrium accompany the analysis of the transient spectra. Additional experiments in ethanol and 2-propanol lead to slight spectral shifts as well as elongated time constants due to steric hindrance in transient spectra connected with the ester formation channel, further substantiating the assignment of the occurring reaction pathways and photoproducts.