The formed Z-stilbene@PCN was suspended in cyclohexane and irradiated with
light of a wavelength of 400–500 nm for several hours. Remarkably, this resulted in
a nearly 100% Z-to-E photoconversion without the formation of any photo-induced
by-products (e.g., dihydrophenanthrene or photooxidized products). Selective isomerization of Z- to E-stilbene only occurred in the presence of the PCN. In absence
of the network or the single network components (ligand or ZnI 2 ), no photoconversion was found. Furthermore, after removal of the stilbene@PCN crystals,
the isomerization stopped and by adding pyrene as a blocking agent to the reaction
mixture, a slower photo-conversion was observed. The E/Z ratio increases faster
within the crystals than in the supernatant. Therefore, the authors concluded that Zto-E photoisomerization exclusively occurs within the pores of the PCN. Additionally, the E-stilbene within the pores exchanges with the unreacted Z-stilbene in the
supernatant solution.
Conclusively, these results demonstrate the successful photo-conversion inside
the pores of a PCN with a selective formation of E-stilbene without any photoinduced by-products and, thus, demonstrate the first photoswitching of a photochromic dye inside a porous coordination network/MOF.
2.2 Azobenzene and Its Derivatives in MOFs
2.2.1 Bulk Material
In contrast to stilbenes, several studies have been published on azobenzene and
derivatives incorporated in MOFs, both in bulk materials and in thin films. In the
following, the results will be presented in order of their publication date. It should be
emphasized that the first work by Kitagawa and co-workers was published
ca. 2 years after the pioneering work of Fujita and co-workers on stilbene@PCN
(Sect. 2.1.1).
In 2012, Kitagawa and co-workers expanded the field of switch@MOF systems
to azobenzene (AZB) as photoactive part [126]. Via a gas-phase loading process, EAZB was incorporated into the flexible porous coordination polymer (PCP MOF)
[(Zn 2 (terephthalate) 2 (triethylenediamine))] n (also referred to as DMOF-1) [127]. By
elemental and thermogravimetric analyses, the composition of AZB@DMOF-1 (the
authors denoted this hybrid material as 1 ⊃ AB) was determined. Additionally,
XRPD measurements on the composite material showed a change of the host
structure originating from the successful inclusion of the photochromic dye. The
XRPD patterns of pure DMOF-1 and AZB-loaded DMOF-1 are depicted in Fig. 4a,
b, respectively.
In comparison to benzene@DMOF-1, a similar diffraction pattern of
AZB@DMOF-1 is observed, where the connectivity of the overall framework is
preserved, but a rhombic net is formed [127]. The authors concluded that the change
from a tetragonal to an orthorhombic crystal system accompanied by a shrinkage of
the cell volume was a result of the AZB embedment.
Photoactive Molecules within MOFs
117
light of a wavelength of 400–500 nm for several hours. Remarkably, this resulted in
a nearly 100% Z-to-E photoconversion without the formation of any photo-induced
by-products (e.g., dihydrophenanthrene or photooxidized products). Selective isomerization of Z- to E-stilbene only occurred in the presence of the PCN. In absence
of the network or the single network components (ligand or ZnI 2 ), no photoconversion was found. Furthermore, after removal of the stilbene@PCN crystals,
the isomerization stopped and by adding pyrene as a blocking agent to the reaction
mixture, a slower photo-conversion was observed. The E/Z ratio increases faster
within the crystals than in the supernatant. Therefore, the authors concluded that Zto-E photoisomerization exclusively occurs within the pores of the PCN. Additionally, the E-stilbene within the pores exchanges with the unreacted Z-stilbene in the
supernatant solution.
Conclusively, these results demonstrate the successful photo-conversion inside
the pores of a PCN with a selective formation of E-stilbene without any photoinduced by-products and, thus, demonstrate the first photoswitching of a photochromic dye inside a porous coordination network/MOF.
2.2 Azobenzene and Its Derivatives in MOFs
2.2.1 Bulk Material
In contrast to stilbenes, several studies have been published on azobenzene and
derivatives incorporated in MOFs, both in bulk materials and in thin films. In the
following, the results will be presented in order of their publication date. It should be
emphasized that the first work by Kitagawa and co-workers was published
ca. 2 years after the pioneering work of Fujita and co-workers on stilbene@PCN
(Sect. 2.1.1).
In 2012, Kitagawa and co-workers expanded the field of switch@MOF systems
to azobenzene (AZB) as photoactive part [126]. Via a gas-phase loading process, EAZB was incorporated into the flexible porous coordination polymer (PCP MOF)
[(Zn 2 (terephthalate) 2 (triethylenediamine))] n (also referred to as DMOF-1) [127]. By
elemental and thermogravimetric analyses, the composition of AZB@DMOF-1 (the
authors denoted this hybrid material as 1 ⊃ AB) was determined. Additionally,
XRPD measurements on the composite material showed a change of the host
structure originating from the successful inclusion of the photochromic dye. The
XRPD patterns of pure DMOF-1 and AZB-loaded DMOF-1 are depicted in Fig. 4a,
b, respectively.
In comparison to benzene@DMOF-1, a similar diffraction pattern of
AZB@DMOF-1 is observed, where the connectivity of the overall framework is
preserved, but a rhombic net is formed [127]. The authors concluded that the change
from a tetragonal to an orthorhombic crystal system accompanied by a shrinkage of
the cell volume was a result of the AZB embedment.
Photoactive Molecules within MOFs
117