Work Group Prof. Dr. F. Temps

Photochromic Molecular Switches

Photochromic molecular switches receive vast attention in chemistry, physics, nanotechnology and biology because of their enormous application potential as, e.g., tiny light-driven molecular manipulators, actuators and engines, as smallest optical memory and logical devices in data storage and information technology, or in superresolution imaging and nanoscopy beyond the optical diffraction limit. Key to the novel functions and applications is that photochromic molecules may be reversibly interconverted between two isomeric states with different chemical structures and consequentially different molecular, optical,electronical and/or magnetical properties by irradiation with light of two different colors.

Prerequisite for the design and application of improved photoswitches is a deeper understanding of the underlying ultrafast photo-induced molecular reaction dynamics determining the ensuing photochemistry. In the context of the Collaborative Research Centre "Function by Switching" (CRC 677) at CAU Kiel, we apply our entire toolbox of ultrafast femtosecond time-resolved spectroscopy methods to elucidate the dynamics of photo-induced switching processes in a variety of important classes of molecules, from azobenzenes and diazozines to fulgides and spiropyranes/spirooxazines, and in different environments, from "free" molecules in solution to switches in polymer colloids, amorphous thin films, on gold nanoparticles, or in monolayers on quartz surfaces.

  

Furylfulgides

Fulgides and fulgimides are highly useful molecular switches for applications in, e.g., data storage devices and, in particular, super-resolution microscopy, where they allow us to control the fluorescence state (on or off) of an attached fluorophore by Förster Resonance Energy Transfer (FRET). Their pronounced photochromism rests on the pericyclic ring opening and ring closure reactions between the central 1,3,5-hexatriene (HT) and 1,3-cyclohexadiene (CHD) moieties (see Scheme below). The almost planar closed (C) form shows absorption already in the VIS spectral range, whereas the open (E) isomer is non-planar and typically absorbs only in the near UV. Upon irrradiation at appropriate wavelengths, their mutual interconversion is fully photoreversible in both directions. Furthermore, the molecules stand out for their thermal irreversibility in combination with high photostability. Applications are, however, disfavored by the existence of a third form, the (Z) isomer, which is produced by photoisomerization around the C=C double bond in the HT moiety and by relatively low quantum yields for the desired E - C isomerization. 

C-E-Z Reaction Cycle of Fulgides
 
Photo-induced conversion reactions between the (C), (E), and (Z) isomers of methyl furylfulgide.

 

In an extensive research thrust in collaboration with the Mattay group at the University of Bielefeld, we have studied the complete C - E - Z photoswitching cycles of a series of chemically modified furylfulgides with vastly improved photoswitching dynamics. The E-to-Z reaction could be virtually stopped, while the E-to-C photoisomerization time could be accelerated to ~ 50 fs. The successful synthetic efforts in the Mattay group were rationally guided by the results of our femtosecond spectroscopy experiments. Several interesting applications are awaiting further studies.

Important Papers:

  • F. Renth, R. Siewertsen, F. Temps, "Enhanced Photoswitching and Ultrafast Dynamics in Structurally Modified Photochromic Furylfulgides",  Int. Rev. Phys. Chem. 32, 1 - 38 (2013). DOI: 10.1080/0144235X.2012.729331
  • R. Siewertsen, F. Strübe, J. Mattay, F. Renth, F. Temps, "Tuning of Switching Properties and Excited-State Dynamics of Fulgides by Structural Modifications", Phys. Chem. Chem. Phys. 13, 3800 - 3808 (2011). DOI: 10.1039/C0CP01890B 

  

Diazocines

Azobenzene (AB) and its derivatives are of great interest as molecular photoswitches because of the large-amplitude structural changes between their elongated (E) and more compact (Z) forms, the reversibility of their transformations, and their high photostabilities, which guarantee large numbers of switching cycles. The photochromic properties of AB are, however, far from ideal, because the S1(nπ*) electronic absorption bands of the E and Z isomers peak at practically the same wavelength (λ = 450 - 440 nm) and differ significantly only in their oscillator strengths. 

We (re)discovered a "bridged azobenzene", 5,6-dihydrodibenzo[c,g][1,2]diazocine, which had not been investigated in detail as a photoswitch. Static UV/VIS and femtosecond transient absorption studies showed that this diazocine exhibits far superior photoswitching properties than plain AB.

E and Z Diazozine 
 
The Z and E isomers of 5,6-dihydrodibenzo[c,g][1,2]diazocine.

 

Since its rediscovery by us, diazocine and diazocine derivatives have become favorite photoswitches in the CRC 677 "Function by Switching" at CAU Kiel. Current work in our group includes studies of diazocine derivatives, related bridged compounds predicted by theoretical work in the Hartke group and synthesized in the Herges group, and investigations of the photoswitching dynamics of diazocines embedded in PMMA polymers prepared in collaboration with the Staubitz group and the group of Walter Richtering and Felix Plamper at RWTH Aachen.

Important Papers:

  • R. Siewertsen, J. B. Schönborn, B. Hartke, F. Renth, F. Temps, "Superior Z - E and E - Z Photoswitching Dynamics of Dihydrodibenzodiazocine, a Bridged Azobenzene, by S1(nπ*) Excitation at λ = 387 and 490 nm", Phys. Chem. Chem. Phys.13, 1054 - 1063 (2011). DOI: 10.1039/C0CP01148G
  • R. Siewertsen, H. Neumann, B. Buchheim-Stehn, R. Herges, C. Näther, F. Renth, F. Temps, "Highly Efficient Reversible Z - Photoisomerization of a Bridged Azobenzene with Visible Light through Resolved S1(nπ*) Absorption Bands", J. Am. Chem. Soc. 131, 15594 - 15595 (2009). DOI: 10.1021/ja906547d

   

Multi-azobenzene switches

In functional systems, photoswitches are typically embedded in complex environments, where the close proximity of the chromophores leads to cooperative phenomena, e.g. steric interactions, excitonic coupling, charge-transfer (CT) and direct electronic coupling in π-conjugated systems. All of these mechanisms may compete with the desired photoisomerization. To elucidate the ensuing influences, we investigate prototypical photoswitchable multi-azobenzene compounds, such as bis[4-(phenylazo)-phenyl]amine (BPAPA) and tris[4-(phenylazo)phenyl]amine (TPAPA), where the AB units are connected via an amino linker to enable electronic coupling between the chromophores, in comparison to the parent compound 4-aminoazobenzene (AAB) for reference.

BPAPA and TPAPA
 
Structures of bis[4-(phenylazo)-phenyl]amine (BPAPA) and tris[4-(phenylazo)phenyl]amine (TPAPA).

  

Important Papers:

  • T. G. Gopakamur, T. Davran-Candan, J. Bahrenburg, R. J. Maurer, F. Temps, K. Reuter, R. Berndt, "Broken Symmetry of an Adsorbed Molecular Switch Determined by Scanning Tunneling Spectroscopy", Angew. Chem. Int. Ed. 52, 11007 - 11070 (2013). DOI: 10.1002/anie.201305027
  • J. Bahrenburg, C. M. Sievers, J. B. Schönborn, B. Hartke, F. Renth F. Temps, C. Näther, F. D. Sönnichsen, "Photochemical Properties of Multi-Azobenzene Compounds", Photochem. Photobiol. Sci. 12, 511 - 518 (2013). DOI:  10.1039/C2PP25291K

  

Azobenzenes in polymer networks

The photoswitching dynamics of azobenzene (AB) and many azobenzene derivatives have been rather extensively studied in solution by ultrafast time-resolved methods for some time. For many applications, however, the ABs have to be embedded in complex environments such as liquid crystals, polymers, thin films or composite materials, where the photoswitching has not been investigated in much detail to date although the light-induced transformations may be strongly affected by the surrounding matrix. 

Azobenzenes in Polymers
 
Disperse Red 1 (DR1, left) attached to a side chain and 4,4'-bis(acetamido)-azobenzene (BAAB, right) in the main chain of PBMA.

 

In order to elucidate the important mechanisms under the much more complex conditions in polymer micronetworks than in solution, we are investigating the trans-to-cis photoisomerization dynamics of AB derivatives in cross-linked colloidal polymethylmethacrylate (PMMA) and polybutylmethacrylate (PBMA) micronetworks by femtosecond time-resolved fluorescence up-conversion spectroscopy. Our results show huge differences in the dynamics of azobenzenes linked to polymers compared to free azobenzenes in solution.

Important Papers:

  • J. Bahrenburg, F. Renth, F. Plamper, W. Richtering, F. Temps, "Femtosecond Spectroscopy Reveals Huge Differences in the Photoisomerization Dynamics of Azobenzenes Linked to Polymers and Azobenzenes in Solution," Phys. Chem. Chem. Phys.16, 11549 - 11554 (2014). DOI: 10.1039/C4CP01196A

  

Molecular switches in polymer films

In order to apply femtosecond spectroscopy to molecular switches in thin glasses and polymer films, we have developed a new scanning pump-probe setup which allows us to perform single-shot dynamics measurements. 

This new setup enables us to do very exciting new femtosecond experiments!

Important Papers:

  • K. Röttger, S. Wang, F. Renth, J. Bahrenburg, F. Temps, "A Femtosecond Pump-Probe Spectrometer for Dynamics in Transmissive Polymer Films", Appl. Phys. B 118, 185 - 193 (2015). DOI: 10.1007/s00340-014-5967-y