Most azobenzene-containing materials, such as most polymers or organic solvents, have strong absorption bands in the mid-IR range, especially in the so-called
fingerprint region between 600 cm
À1 and 1,500 cm
À1 and, thus, making them
virtually nontransparent for IR light. As a result, the AZB photoswitching has only
rarely been investigated by vibrational spectroscopy. Therefore, Ruschewitz and
co-workers presented one of these rare examples of precise AZB vibrational
data [129].
Generally, azobenzene is switched between its two isomers by irradiation with
UV light. However, the necessity of UV light that might interfere with the host
matrix is a severe drawback. Additionally, the absorption bands of E- and Zazobenzene overlap, which leads to photostationary states so that a complete transformation of one isomer to the other is not possible. By modifying azobenzene with
suitable substituents, separation of the absorption bands might be achieved [135],
e.g., introduction of fluorine atoms in the phenyl rings of AZB. For tF-AZB,
photostationary states with almost quantitative switching (91% Z and 86% E) were
reached, when dissolved in acetonitrile [136]. Remarkably, these isomerization
processes are triggered by light in the visible region. Hence, UV light is no longer
obligatory to induce the E-to-Z transformation and vice versa [136].
As a consequence, Ruschewitz and co-workers extended their work to the incorporation of fluorinated azobenzenes into MOFs [137]. Fifteen new photochromic
hybrid materials were synthesized via a gas phase loading process. As host materials,
the authors chose MOF-5 [55], MIL-68(In) [132], MIL-68(Ga) [132], MIL-53(Al)
[133], and MIL-53(Ga) [134], similar to their investigations on pristine azobenzene
[129]. As photochromic part, various fluorinated azobenzenes were embedded,
namely, ortho-tetrafluroroazobenzene (tF-AZB), 4H, 4H’-octafluoroazobenzene
(oF-AZB), and perfluoroazobenzene (pF-AZB), and their optical properties inside
the host scaffolds were studied. The authors confirmed the successful guest inclusion
by XRPD measurements and proved that pF-AZB 0.34 @MIL-53(Al) and pFAZB 3.57 @MOF-5 did not release any guest molecules at ambient conditions by
DSC/TG analyses. In a next step, the crystal structure of pF-AZB 0.34 @MIL-53
(Al) was solved in order to understand the underlying host-guest interactions and
the associated photochromic properties, as in contrast to AZB embedded in MIL-53
(Al), an E-to-Z isomerization of pF-AZB occurred within this MOF host. Solving the
crystal structure of such composite materials is not straightforward, since possible
disorder of the guest must be considered. Therefore, low temperature measurements
are to be preferred, and, additionally, the initial state of the dye molecule should be
close to 100% E or Z. Both requirements are met for the mentioned system leading to
the crystal structure shown in Fig. 9. pF-AZB perfectly fits into the pores of MIL-53
(Al), taking the respective van der Waals radii into account. However, in contrast to
AZB 0.5 @MIL-53(Al) the occupancy of the guest sites in the pores of the MOF is
significantly lower in pF-AZB 0.34 @MIL-53(Al), thus giving the guests in the latter
more space for the photoswitching process.
Additionally, the authors thoroughly studied the underlying host-guest interactions (Fig. 9, right). Firstly, O À HÁÁÁF interactions (shortest O À HÁÁÁF distance:
213 pm) seem to dominate, and only weak O À HÁÁÁπ interactions are observed
122
H. A. Schwartz and U. Ruschewitz
fingerprint region between 600 cm
À1 and 1,500 cm
À1 and, thus, making them
virtually nontransparent for IR light. As a result, the AZB photoswitching has only
rarely been investigated by vibrational spectroscopy. Therefore, Ruschewitz and
co-workers presented one of these rare examples of precise AZB vibrational
data [129].
Generally, azobenzene is switched between its two isomers by irradiation with
UV light. However, the necessity of UV light that might interfere with the host
matrix is a severe drawback. Additionally, the absorption bands of E- and Zazobenzene overlap, which leads to photostationary states so that a complete transformation of one isomer to the other is not possible. By modifying azobenzene with
suitable substituents, separation of the absorption bands might be achieved [135],
e.g., introduction of fluorine atoms in the phenyl rings of AZB. For tF-AZB,
photostationary states with almost quantitative switching (91% Z and 86% E) were
reached, when dissolved in acetonitrile [136]. Remarkably, these isomerization
processes are triggered by light in the visible region. Hence, UV light is no longer
obligatory to induce the E-to-Z transformation and vice versa [136].
As a consequence, Ruschewitz and co-workers extended their work to the incorporation of fluorinated azobenzenes into MOFs [137]. Fifteen new photochromic
hybrid materials were synthesized via a gas phase loading process. As host materials,
the authors chose MOF-5 [55], MIL-68(In) [132], MIL-68(Ga) [132], MIL-53(Al)
[133], and MIL-53(Ga) [134], similar to their investigations on pristine azobenzene
[129]. As photochromic part, various fluorinated azobenzenes were embedded,
namely, ortho-tetrafluroroazobenzene (tF-AZB), 4H, 4H’-octafluoroazobenzene
(oF-AZB), and perfluoroazobenzene (pF-AZB), and their optical properties inside
the host scaffolds were studied. The authors confirmed the successful guest inclusion
by XRPD measurements and proved that pF-AZB 0.34 @MIL-53(Al) and pFAZB 3.57 @MOF-5 did not release any guest molecules at ambient conditions by
DSC/TG analyses. In a next step, the crystal structure of pF-AZB 0.34 @MIL-53
(Al) was solved in order to understand the underlying host-guest interactions and
the associated photochromic properties, as in contrast to AZB embedded in MIL-53
(Al), an E-to-Z isomerization of pF-AZB occurred within this MOF host. Solving the
crystal structure of such composite materials is not straightforward, since possible
disorder of the guest must be considered. Therefore, low temperature measurements
are to be preferred, and, additionally, the initial state of the dye molecule should be
close to 100% E or Z. Both requirements are met for the mentioned system leading to
the crystal structure shown in Fig. 9. pF-AZB perfectly fits into the pores of MIL-53
(Al), taking the respective van der Waals radii into account. However, in contrast to
AZB 0.5 @MIL-53(Al) the occupancy of the guest sites in the pores of the MOF is
significantly lower in pF-AZB 0.34 @MIL-53(Al), thus giving the guests in the latter
more space for the photoswitching process.
Additionally, the authors thoroughly studied the underlying host-guest interactions (Fig. 9, right). Firstly, O À HÁÁÁF interactions (shortest O À HÁÁÁF distance:
213 pm) seem to dominate, and only weak O À HÁÁÁπ interactions are observed
122
H. A. Schwartz and U. Ruschewitz