Triplet sensitization enables bidirectional isomerization of diazocine with 130 nm redshift in excitation wavelengths.

Diazocines are bridged azobenzenes with phenyl rings connected by a CH2-CH2 group. Despite this rather small structural difference, diazocine exhibits improved properties over azobenzene as a photoswitch and most importantly, its Z configuration is more stable than the E isomer. Herein, we reveal yet another unique feature of this emerging class of photoswitches. In striking contrast to azobenzenes and other photochromes, diazocine can be selectively switched in E → Z direction and most intriguingly from its thermodynamically stable Z to metastable E isomer upon successive excitation of two different triplet sensitizers present in solution at the same time. This approach leads to extraordinary large redshift of excitation wavelengths to perform isomerization i.e. from 400 nm blue to 530 nm green light (Z → E) and from 530 nm green to 740 nm far-red one (E → Z), which falls in the near-infrared window in biological tissue. Therefore, this work opens up of potential avenues for utilizing diazocines for example in photopharmacology, smart materials, light energy harvesting/storage devices, and out-of-equilibrium systems.

Studies of the First Electronically Excited State of 3-Fluoropyridine and Its Ionic Structure by Means of REMPI, Two-Photon MATI, One-Photon VUV-MATI Spectroscopy and Franck-Condon Analysis.

3-Fluoropyridine (3-FP) has been investigated by means of two-photon resonance-enhanced multi photon ionization (REMPI), mass-analyzed threshold ionization (MATI) and one-photon vacuum-ultraviolet (VUV) MATI spectroscopy. The aim was the determination of the effect of m-fluorine substitution on the vibronic structure of the first electronically excited and ionic ground state. The S1 excitation energy has been determined to be 35 064 ± 2 cm-1 (4.3474 ± 0.0002 eV). Strong evidence of a distinct vibronic coupling via ν16b and ν[Wag.out.,16a] to one or both of the lowest 1ππ* states has been found, which results in a warped S1 minimum structure with C1 symmetry. The adiabatic ionization energy of the ionic ground state (14a', nN-LP orbital) has been determined to be 76 579 ± 6 cm-1 (9.4946 ± 0.0007 eV), which is the first value reported for this state. The origin of the D1 state (4a'', π-orbital) is located close by at 77 129 cm-1 (9.5628 eV). As a result of the D0-D1 vicinity, the ionic ground state is coupled to the D1 state via ν[Wag.out.,16a] and ν10a, which induces a twisted D0 geometry with C1 symmetry. Furthermore, for the first time two-photon and one-photon MATI spectra are presented together, which yield a much better understanding of the ionic vibronic structure in comparison to either of these experiments alone.

Investigation of the complex vibronic structure in the first excited and ionic ground states of 3-chloropyridine by means of REMPI and MATI spectroscopy and Franck-Condon analysis.

3-Chloropyridine (3-CP) has been investigated by means of resonance-enhanced multi-photon ionization (REMPI) and mass-analyzed threshold ionization (MATI) spectroscopy to elucidate the effect of m-chlorine substitution on the vibronic structure of the first electronically excited and ionic ground states. The S1 excitation energy has been determined to be 34 840 ± 2 cm-1 (4.3196 ± 0.0002 eV) with a difference of less than 0.2 cm-1 between both isotopomers, which is the first reported value for this transition in the gas phase so far. The S1 state has been assigned to the 1π* ← n transition. It is subject to strong vibronic coupling via ν16b to one or both of the lowest 1ππ* states. In addition, strong coupling via at least one more non-totally symmetric vibration is very likely to exist but the vibration could not be identified yet. Overall, the coupling results in a minimum S1 structure with C1 symmetry. The adiabatic ionization energy of the nN-LP orbital (14a') has been determined to be 75 879 ± 6 cm-1 (9.4078 ± 0.0007 eV) with a difference of less than 2 cm-1 between the two isotopomers, which is the first value reported for this state so far. The ionic ground state exhibits a distinct vibronic coupling via ν16a and ν10a to either the D1 state (4a'') and/or D2 state (3a''), which results in a twisted D0 geometry with C1 symmetry. As a consequence of the warped geometry in both S1 and D0 states, very complicated MATI spectra were obtained when exciting S1 states at higher wavenumbers.

Axitinib: A Photoswitchable Approved Tyrosine Kinase Inhibitor.

The goal of photopharmacology is to develop photoswitchable enzyme modulators as tunable (pro-)drugs that can be spatially and temporally controlled by light. In this context, the tyrosine kinase inhibitor axitinib, which contains a photosensitive stilbene-like moiety that allows for E/Z isomerization, is of interest. Axitinib is an approved drug that targets the vascular endothelial growth factor receptor 2 (VEGFR2) and is licensed for second-line therapy of renal cell carcinoma. The photoinduced E/Z isomerization of axitinib has been investigated to explore if its inhibitory effect can be turned "on" and "off", as triggered by light. Under controlled light conditions, (Z)-axitinib is 43 times less active than that of the E isomer in an VEGFR2 assay. Furthermore, it was proven that kinase activity in human umbilical vein cells (HUVECs) was decreased by (E)-axitinib, but only weakly affected by (Z)-axitinib. By irradiating (Z)-axitinib in vitro with UV light (λ=385 nm), it is possible to switch it almost quantitatively into the E isomer and to completely restore the biological activity of (E)-axitinib. However, switching the biological activity off from (E)- to (Z)-axitinib was not possible in aqueous solution due to a competing irreversible [2+2]-photocycloaddition, which yielded a biologically inactive axitinib dimer.

Simulations of optically switchable molecular machines for particle transport.

A promising application for design and deployment of molecular machines is nanoscale transport, driven by artificial cilia. In this contribution, we present several further steps toward this goal, beyond our first-generation artificial cilium (Raeker et al., J. Phys. Chem. A 2012, 116, 11241). Promising new azobenzene-derivatives were tested for use as cilium motors. Using a QM/MM partitioning in on-the-fly photodynamics, excited-state surface-hopping trajectories were calculated for each isomerization direction and each motor version. The methods used were reparametrized semiempirical quantum chemistry together with floating-occupation configuration interaction as the QM part and the OPLSAA-L forcefield as MM part. In addition, we simulated actual particle transport by a single cilium attached to a model surface, with varying attachment strengths and modes, and with transport targets ranging from single atoms to multi-molecule arrangements. Our results provide valuable design guidelines for cilia-driven nanoscale transport and emphasize the need to carefully select the whole setup (not just the cilium itself, but also its surface attachment and the dynamic cilium-target interaction) to achieve true transport. © 2018 Wiley Periodicals, Inc.

Ultrafast dynamics of UV-excited trans- and cis-ferulic acid in aqueous solutions.

The ultrafast UV-induced processes of the neutral, anionic and dianionic forms of trans- and cis-ferulic acid (FA) in aqueous solution were studied by static and femtosecond time-resolved emission and absorption spectroscopy combined with quantum chemical calculations. In all cases, initial excitation populates the first 1ππ* state. For the dianionic cis-isomer cFA2-, electronic deactivation takes place with a time constant of only 1.4 ps, whereas in all other cases, excited-state deactivation happens more than ten times slower, on a time scale of ≈20 ps. The data suggest sequential de-excitation pathways, where initial sub-picosecond solvent rearrangement and structural changes are followed by internal conversion to an intermediate excited electronic state from which deactivation to the ground state proceeds. Considering the time scales, barrierless excited-state pathways are suggested only in the case of cFA2-, where the observed formation of the isomerisation photoproduct tFA2- provides clear evidence for a cis ⇄ trans isomerisation coordinate. In the other cases, pathways with an excited-state energy barrier, presumably along the same coordinate, are likely, given the longer excited-state lifetimes.

Full-Dimensional Excited-State Intramolecular Proton Transfer Dynamics of Salicylic Acid.

Salicylic acid (SAc) and its excited-state intramolecular proton transfer (ESIPT) capabilities have been studied both experimentally and theoretically by static calculations. However, to our knowledge, no radiationless pathway has been proposed so far. Instead, excited-state deactivation was only investigated via fluorescence. Therefore, we will present full-dimensional photodynamics of SAc using the floating-occupation configuration-interaction (FOCI) treatment with single and paired double excitations based on the semiempirical RM1 Hamiltonian. To further clarify mechanistic details, the potential energy surface (PES) is scanned along the proton transfer coordinates in one and two dimensions. The time-evolution of relevant degrees of freedom (DOF), quantum yields and isomer populations are evaluated from 200 surface-hopping trajectories. It was found that a deactivation pathway from the excited state to the ground state is indeed accessible through a conical intersection, via rotation of the carboxyl group. Together with the ESIPT process, this rotation can also interchange the protons of the two (formal) OH groups, which makes the overall dynamics still more complex. Our full-dimensional photodynamics study provides a comprehensive overview of all these entangled steps.

Simulating a molecular machine in action.

Using QM/MM methods, we have simulated the action of a simple molecular machine, a cilium. It consists of a platform for surface mounting, a photochemical motor unit, and a tail-like effector that amplifies the small-scale conformational change of the motor unit into a larger-scale beating motion usable for molecular transport. In this proof-of-principle application, we show that the techniques used here make it possible to perform such simulations within reasonable real time, if the device action is sufficiently fast. Additionally, we show that this molecular device actually works as intended for one isomerization direction. For the other direction, results are inconclusive, possibly because the total propagation times we can afford are too short to capture the complete event.