Photochromic materials - synthesis and photochemistry

Controlling the interfacial properties of organic materials is important for many technological applications. Self-assembled monolayers (SAMs) adsorbed on metal surfaces offer the possibility to construct inorganic-organic supramolecular architectures. The surface properties of organic films can be influenced by the packing and orientation of the terminal functional groups. Controlling the conformation of the adsorbates offers an additional way for manipulating the interfacial properties and / or reactivities of the SAMs. Therefore, photochromic systems based on organic materials which are covalently linked with normal alkanethiols are expected to form SAMs. Such systems might be switched by light and are of special interest to molecular electronics for instance as information recording device. We focus on aromatic azo-compounds which are known to show reversible trans-cis photoisomerization at the N=N-bond. The azo-compound is thiol-functionalized onto gold substrates and surface properties are characterized by AFM and STM. To understand both the properties of the excited electronic states and the collective switching behaviour of the excited azo-chromophor NEXAFS- and time-resolved laser-synchroton pump-probe experiments are performed.

Well-defined model interfaces of different structure and length of the n-alkanethiole chain functionalized with substituted azobenzenes that exhibit different transition dipole moments will be prepared.

Another very interesting photochromic system based on a tetraanthraporphyrazine that can insert and release molecular oxygen. Oxygen is covalently linked within the anthracene moiety as epidioxy bridge. Release of oxygen occurs when the compound is excited by consecutive two-photon absorption in the Q-band region. Up to four molecules of oxygen can be ejected. It is a challenge to link such a molecule with an appropriate spacer.

1.

2.

Fig. 1 Absorption spectra of porphyrazine 1, tetraepidioxyporphyrazine 2, and copper tetra-tert-butylphthalocyanine t₄PcCu. This reference phthalocyanine shows that the epidioxy bridges are attached to the middle part of the anthracene moiety in 2.

Such photochromic systems are studied by metallic tips within the project “ultrahigh resolution near-field microscopy” (colloboration with 3-02 UP-2 (M. Raschke)). Similar systems are studied too by time-resolved IR spectroscopy within the project “ultrashort time spectroscopy of biological systems” (collaboration with SfB 450 UP-B9 (K. Heyne, FU)).

Novel dyes for ophthalmic surgery - investigation of stained surfaces mimicing the internal limiting membrane (ILM)

At present, two dyes are commonly used to assist ophthalmic surgery. The removal of the internal limiting membrane (ILM) using these dyes has been shown to improve visual and anatomical success rates. The ILM layer is partially peeled after staining the ILM surface with these dyes. However, because of limitation of dyes a systematic evaluation of other dyes for intraocular use is urgently necessary.

• The dye should absorb in the visible region between 500-650 nm of the spectrum because of the high sensitivity of the human eye in this range
  
• The staining material must exhibit a large absorption coefficient. Large extinction coefficients allow for the injection a significantly lower amount of the dye. As a consequence the formation of aggregates is suppressed
  
• The dye should show a high solubility in water and balanced saline, but should not be concentration dependent. In most cases, this effect is accompanied with a dramatic shift of the absorption maximum and / or with the appearance of new absorption band
  
• The material should have a high photochemical stability
  
• The triplet quantum yield should be as low as possible, avoiding singlet oxygen formation that can lead to irreversible oxidation of the ILM or other tissue or to photodegradation of the dye itself. (The lifetime of singlet oxygen both in water and in the solid state is very short)
  
• The staining material should have minimal dark toxicity
  
• The dye must exhibit very good adsorption properties towards the target tissue

[1] Haritoglou, C., Yu, A., Freyer, W., Priglinger, G., Alge, C., Eibl, K., May, C.A., Welge-Luessen, U., Kampik A. Invest. Ophthalmol. Vis. Sci. 2005, 46, 3315-3322.
[2] Haritoglou, C., Freyer, W., Kampik, A. Graves Archive of Ophthalmology, submitted.
[3] Haritoglou, C., Freyer, W. Patent is applied.

In collaboration with Ch. Haritoglou and A. Kampik, Department of Ophthalmology LMU, Munich