4 Photoactivatable Compounds
4.2 Introduction to Photoswitches
The second class of photoactivatable molecules discussed in this thesis are photochromic compounds also known as photoswitches. The term photochromism, first introduced by Hirshberg, is distinguished by the reversible switching between at least two isomers.171 The switching from one to the other molecular entity can be induced by UV-, visible or IR-light, while the back reaction can be initiated either photochemically or thermally.
The most prominent molecules exhibiting photochromism are stilbenes, azobenzenes and spiropyranes.135In this work the photodynamics, in particular the ultrafast deactivation of the excited state, of the fluorophore (BODIPY) coupled to dithienylethene (DTE) and a water-soluble fulgimide were investigated.
4.2.1 Dithienylethene
Due to the excellent properties regarding photo- and thermal stability, strong resistance to fatigue and an extraordinary high quantum yield, the representative of the class of diarylethenes attracted considerable attention to the material sciences. These character-istics are exploited for example in optoelectronic devices such as storage systems, since they demand high thermal stability and an efficient write/delete procedure.45,172–175 The photochromism of DTE is defined by a ring-closing reaction upon UV-irradiation and a ring-opening reaction after photoexcitation with visible light.
conrotatory disrotatory
∆H hν
Figure 4.9: Ring-closing reaction of 1,6-dimethyl-1,3,5-hexatriene yield different isomers de-pendent on the excitation. A disrotatory movement of the orbitals can be achieved thermally while a photoinduced excitation would lead to a conrotatory ring-closure.
The substantial molecular motif for this cyclization is a 1,3,5-hexatriene which is formed to a 1,3-cyclohexadiene. The mechanism of this reaction on the example of 1,6-dimethyl-1,3,5-hexatriene, depicted in Figure 4.9, can be described with the lin-ear combination of atomic orbitals (LCAO) and the Woodward-Hoffmann-rules.176–178 The formation of the 1,3-cyclohexadiene derivative can be either initiated thermally or by light, enclosing a disrotatory or a conrotatory cyclization of the orbitals and yield different configurations, respectively.
4.2 Introduction to Photoswitches Another approach to exploit the efficient switching between the open and the closed form is the modulation of fluorescence, where the energy from a jointed photoexcited fluorophore is transferred on the closed DTE and thus the fluorescence is quenched. If DTE is present in its open form an energy transfer is not observed but the radiative relaxation of the fluorophore.179–181
UV vis S
N B N F F
S O
F F
F F FF
S
B N N
S
O
F F F
F
F F F F
Figure 4.10: Molecular structure of the BODIPY-DTE which are linked via a phenyl-ehtinyl-phenyl-subunit. The photochromic character is derived by the DTE-photoswitch with the cleavable bond in the closed ring depicted in red.
The investigation of the modulation of such a cooperative dyad system between a fluorophore and the DTE photoswitch was part of this work. Boron-dipyrromethene (BODIPY, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) served as fluorophore in the in-vestigated system. It is distinguished by very sharp absorption and emission bands exhibiting a high extinction coefficient as well as a high fluorescence quantum yield.
The BODIPY moiety is linked via a phenyl-ethinyl-phenyl-unit to the DTE photoswitch (Figure 4.10). The results concerning the molecular dyad can be found in Section 7.1.
4 Photoactivatable Compounds 4.2.2 Fulgides and Fulgimides
The other photochromic compound investigated in this work is based on the scaffold of a fulgide. Fulgides were first reported by Stobbe who found a change in color upon irradiation of the anhydrides of 1,3-butadiene-2,3-dicarboxylic acid. He introduced the name fulgide due to the intense color of the closed form (from latin fulgere = shine).
Based on the substitution of the methylene hydrogen atoms with methyl groups, fulgides become thermally irreversible.182 Furthermore, the photoswitch offers many sites for versatile modifications providing a tailored photoswitch for designated applications.
The photochemical reactions of fulgides encompass an intramolecular rearrangement of free electron pairs as depicted in Figure 4.11, as was first reported by Santiago and Becker.183
Figure 4.11: Photochromic reactions of fulgides. A ring-closing reaction to the C-isomer (right, brown) is only accessible through the E-isomer (middle, green) which is in a photochem-ical equilibrium with the non-cyclizable Z-isomer (left, blue).
The cyclization reaction yields the closed isomer (C-isomer) which is only accessible via the isomer where the two outer double bonds are parallel oriented to each other.
In this case, it is the E-isomer (middle, green) which is in equilibrium with the non-cyclizable Z-isomer (left, blue). In analogy to DTE, the photochemical ring-opening reaction as well as the 6π-electrocyclization follows a conrotatory movement of the orbitals in the context of the Woodward-Hoffmann rules.176–178
However, the presence of the non-cyclizable isomer and the chemical instability caused by the hydrolysis reaction of the succinic anhydride renders a less efficient switching and confines its applications to non-aqueous media. In order to prevent the formation of the non-cyclizable isomer, and therefore to pattern a more compatible switching reaction, indolylfulgides were introduced, exhibiting a relatively bulky substituent the formation of the non-cyclizable open form can be inhibited. Furthermore, the replacement of the succinic anhydride with a succinimid ring entails an improvement of water-solubility
4.2 Introduction to Photoswitches introducing the indolylfulgimide to a wide range of applications in aqueous environ-ments.184 The investigated water-soluble indolylfulgimide and its photochemical reac-tions are depicted in Figure 4.12.
N
Figure 4.12: Molecular structure of the water-soluble indolylfulgimide and its photochemical reactions including three isomers. The cyclized isomer (C-isomer, brown) is only accesible through the Z-isomer (blue), while the green E-isomer is non-cyclyzable.
Due to the solubility-mediating carboxylic acid group at the inner double bond, the priority of this substituent is now higher. Therefore, the isomer which undergoes the electrocyclization reaction bears now a Z-configuration, while the E-isomer is now the non-cyclizable entity. The detailed synthesis of the water-soluble indolylfulgimide is reported in the literature.185 The results regarding the photoswitch can be found in Section 7.2.
4 Photoactivatable Compounds