Absorption and Isomerization of Azobenzene Guest Molecules in Polymeric Nanoporous Crystalline Phases

: PPO co-crystalline (CC) ﬁlms including azobenzene guest molecules have been prepared and characterized by WAXD, FTIR and UV-Visible measurements. Isomerization reactions of azobenzene (photo-induced trans to cis and spontaneous cis to trans) included in α and β nanoporous-crystalline (NC) phases leading to CC phases, or simply absorbed in amorphous phase have been studied on thick and thin ﬁlms. Spectroscopic analysis shows that photo-isomerization of azobenzene occurs without expulsion of azobenzene guest molecules from crystalline phases. Sorption studies of α and β NC ﬁlms immersed into photo-isomerized azobenzene solution reveal a higher selectivity of the β NC phase toward cis azobenzene isomer than the α NC phase, inducing us to propose the β NC phase as particularly suitable for absorbing spherically bulky guest molecules.

In particular, starting from s-PS or PPO CC phases, by using simple extraction procedures, NC phases presenting crystalline density lower than those of corresponding amorphous phases (named δ and ε for s-PS [13,14] or α and β for PPO [15,16]) can be obtained. As for PPO, the two NC phases (α and β), showing crystalline density of 0.93 g/cm 3 and 0.95 g/cm 3 lower than that of the corresponding amorphous phase (1.04 g/cm 3 ) [17], are particularly interesting since they present a higher sorption and diffusivity of most suitable guest molecules with respect to δ and ε NC phases of s-PS. This behavior can be assigned mainly to the contribution of NC phases but also to the high free volume of the PPO amorphous phase [12].
It is well to highlight that polymeric nanoporous crystalline (NC) phases present many advantages with respect to nanoporous amorphous polymers as well as with respect to inorganic and organometallic nanoporous materials. Particularly relevant are simple processing conditions (low pressures and temperatures) and processing techniques (both from melt and solutions) [18], leading to a large variety of morphologies like fibers, sheets, films, membranes, foams, gels and aerogels [19][20][21][22][23]. Moreover, NC polymers present the further advantage of showing identical cavities that allow molecular selectivity [24][25][26][27][28][29][30] comparable to those of NC inorganic and organometallic materials. An additional interesting, rather unique characteristic of NC polymers is their capability to control the orientation of their guest molecules at macroscopic level. In particular, by using different crystallization procedures, the orientation of both host polymeric co-crystalline phases and guest molecules can be easily realized [31].
It is known that inclusion of chromophores as well as of photoreactive guest molecules in confined lattices is increasingly taken into account for developing advanced materials with new electronic and optical properties [45][46][47][48]. As for s-PS, intramolecular solid-state reactions [49] have been performed involving guest molecules absorbed in NC phases (leading to CC phases). In particular, photo-isomerization of norbornadiene to quadricyclane occurred into the δ co-crystalline phase (CC) [49] as well as the photo-isomerization of azobenzene guest molecules included inside the δ and ε CC phases occurred without expulsion of guest molecules from s-PS co-crystalline lattices [50]. In the case of irradiationinduced polymerization of benzyl methacrylate in the s-PS co-crystalline phase [51], instead, it was evidenced that polymerization products were expelled from the crystalline host lattice leading to a rearrangement of the crystalline phase that became a mesomorphic phase.
As is well known, photochromic molecular switches such as azobenzene [52], spiropyran [53], or arylazopyrazole [54] are responsive molecules that isomerize upon irradiation with light. Achieving fine control of these compounds' isomerization is important since they can be crucial components for the development of functional photoresponsive materials [55,56], light-powered molecular machines [57][58][59], and in photopharmacology [60], where spatiotemporal control of molecular transitions/reactions is needed. In particular, azobenzene, represents a very interesting example of molecular switch, its reversible trans→cis isomerization has been largely studied in solution [61], in the gas phase [62][63][64], as well as in amorphous and crystalline phases [65][66][67] and it is still actually investigated for many applications, from the development of active materials to the design of stimuli-responsive molecular systems and light-powered molecular machines [68][69][70].
To fully understand its potential, it becomes important to control the mechanism and kinetics of photo-induced isomerization; one possible strategy involves the confinement of photo-responsive switches such as azobenzene molecules within confined molecular environments, which may allow control over their light induced conversion.
In this paper the formation of co-crystalline phases of azobenzene with PPO have been studied.
Isomerization reactions (both photo-induced trans→cis and spontaneous cis→trans) of azobenzene as a guest of PPO co-crystalline phases or simply absorbed in the amorphous phase have been analyzed by FTIR and UV-Visible measurements on thick and thin films.
Sorption studies on photo-isomerized azobenzene solution have been also performed in order to evaluate a possible selectivity of α or β NC phases toward cis or trans azobenzene isomers.
Carbon tetrachloride, benzene, chloroform, mesitylene, acetonitrile, methyl-ethyl ketone, methanol and azobenzene were supplied by Aldrich and used without any purification.
Semicrystalline PPO films with a thickness of nearly 50 µm, presenting α and β cocrystalline (CC) phases both with c// orientation were obtained by casting procedure at room temperature from 1.5 wt% carbon tetrachloride and benzene solution, respectively.
By immersion of CC PPO films in pure acetonitrile for 1 h, followed by acetonitrile desorption, carbon tetrachloride and benzene guest molecules were removed leading to PPO films showing α and β nanoporous crystalline (NC) phases.
Amorphous PPO films were achieved from 1.5 wt% chloroform solution at 60 • C, followed by solvent removal.
All cast films, semicrystalline and amorphous, have been immersed in 2% azobenzene/methanol solution for 30 min.
Axially amorphous oriented films were obtained by stretching PPO amorphous films in the temperature range 220-230 • C up to draw ratio 4-5. Films exhibiting axial orientation of α NC phase were obtained by axial stretching of PPO/mesitylene CC films at 170-175 • C up to draw ratio 4 and by subsequent mesitylene removing through sorption/desorption procedures of acetonitrile at room temperature for 2 h [71]. PPO/mesitylene CC films, with thickness of nearly 150 µm, were obtained by casting from 3.5 wt% mesitylene solution at room temperature. Film stretching experiments were conducted by a dynamometer INSTRON 4301, with controllable speed and temperature, up to draw ratio λ = l/l 0 (where l is the final length of the film) of 4-5 and by using elongation rate in the range 3-10 mm/min.
Amorphous and NC axially stretched films were immersed in a 0.2 wt% azobenzene/methanol solution for 48 h leading to amorphous and CC films with 1.5 wt% and 6 wt% of azobenzene content, respectively. NC axially stretched films were also immersed in a 2 wt% azobenzene/methanol solution for 12 h at room temperature leading to CC film with 12 wt% of azobenzene.
Amorphous PPO films with thickness of nearly 0.1 µm were obtained by spin-coating of PPO chloroform solution (1.5 wt%) onto quartz surfaces at a spin-rate of 1500 rpm. PPO α and β NC films were obtained by exposure of amorphous PPO films to carbon tetrachloride and methyl-ethyl ketone vapors at 60 • C for 30 min, respectively, then all films have been immersed in 0.2 wt% azobenzene/methanol solution for 30 min.
The content of azobenzene molecules was evaluated by thermogravimetric measurements (TGA), with a TA instrument TGA Q500 with heating rate of 10 • C/min, in the temperature range of 20-600 • C, under nitrogen gas flow.

X-ray Diffraction
Two-dimensional (2D) Wide Angle X-ray Diffraction (WAXD) patterns were performed by D8 QUEST Bruker diffractometer and the equatorial profiles were collected along the equatorial direction of the 2D patterns.

Fourier Transform Infrared Spectroscopy (FTIR)
Infrared spectra were conducted at 2.0 cm −1 resolution with a Vertex 70 Bruker spectrophotometer. A total of 32 scans were performed and averaged to reduce the noise level. Polarized infrared spectra were obtained by using a SPECAC 12,500 polarizer.
As described in detail in a previous report [36], satisfactory evaluation of the degree of crystallinity (X c ) for PPO samples, can be obtained by FTIR spectral subtraction procedures based on the use of the 475 cm −1 band, according to the following relationship: where l and l are the thickness of the sample and of an amorphous PPO film, respectively. The degree of orientation of the chain axes of the host crystalline helices with respect to the draw direction was evaluated by using the axial orientation factor as reported in [17]: where θ is the angle between the chain axis and the transition moment vector of the vibrational mode and S h is the order parameter of the host crystalline phase as calculated by the formula: where R = A // /A ⊥ is the dichroic ratio, and A // and A ⊥ are the measured absorbance for polarization plane parallel and perpendicular to the draw direction, respectively.

UV-Vis Spectroscopy
UV-Visible spectra were recorded on Varian Cary 50 spectrophotometer in the range 200-600 nm with a scan rate of 600 nm/min.

Trans → Cis Photo-Isomerization
PPO/azobenzene co-crystalline samples were irradiated by a xenon lamp (BLX equipment from BioLink) at 365 nm, with a power of 65 W and an energy of 99.66 mJ/cm 2 at room temperature.

Formation of PPO/Azobenzene Co-Crystalline Phases
PPO co-crystalline (CC) films including azobenzene guest molecules were prepared by dipping α or β nanoporous crystalline (NC) films (~50 µm thick) in azobenzene solutions, using methanol as carrier. In particular, FTIR spectra in the wavenumber range 1100-400 cm −1 of α and β NC as well as of amorphous films, before (gray lines) and after immersion in 2 wt% azobenzene/methanol solution for 30 min are reported in Figure 1 (red, blue and black lines, respectively).
brational mode and Sh is the order parameter of the host crystalline phase as calculated by the formula: where R = A///A⊥ is the dichroic ratio, and A// and A⊥ are the measured absorbance for polarization plane parallel and perpendicular to the draw direction, respectively.

UV-Vis Spectroscopy
UV-Visible spectra were recorded on Varian Cary 50 spectrophotometer in the range 200-600 nm with a scan rate of 600 nm/min.

Trans → Cis Photo-Isomerization
PPO/azobenzene co-crystalline samples were irradiated by a xenon lamp (BLX equipment from BioLink) at 365 nm, with a power of 65 W and an energy of 99.66 mJ/cm 2 at room temperature.
It is apparent that the amount of azobenzene absorbed in α and β NC films was higher than that absorbed in amorphous sample and, by thermogravimetric measurements, it was evaluated close to ≈8.8 wt%, ≈9.6 wt% and ≈4 wt% for α, β and amorphous films, respectively.
The degree of crystallinity, evaluated by FTIR measurements, for α and β films was 42% and~46%, respectively. X-ray diffraction patterns of PPO CC films of Figure 1A,B, taken with a two-dimensional diffractometer, are shown in Figure 2A,B; for comparison, patterns of α and β nanoporous crystalline phases are also reported in Figure 2A',B' respectively. Equatorial EDGE profiles are shown in Figure 2a It is apparent that the amount of azobenzene absorbed in α and β NC films was higher than that absorbed in amorphous sample and, by thermogravimetric measurements, it was evaluated close to ≈8.8 wt%, ≈9.6 wt% and ≈4 wt% for α, β and amorphous films, respectively.
The degree of crystallinity, evaluated by FTIR measurements, for α and β films was ~42% and ~46%, respectively. X-ray diffraction patterns of PPO CC films of Figure 1A,B, taken with a two-dimensional diffractometer, are shown in Figure 2A,B; for comparison, patterns of α and β nanoporous crystalline phases are also reported in Figure 2A',B' respectively. Equatorial EDGE profiles are shown in Figure 2a It is apparent that diffraction profiles of PPO films with azobenzene (red and blue lines) exhibited 2θ values similar to NC films (black lines), however, variations in relative intensity of diffraction peaks, indicating the formation of PPO/azobenzene CC phases, were visible [72]. Moreover, patterns in Figure 2 presented diffraction arcs centered on the equator for 2θ < 16° and on the meridional for 2θ > 16°, indicating the occurrence of c// orientation for PPO films [72]. The degree of c// orientation, evaluated by azimuthal scan of the 001 reflection (χ (001)) of the EDGE pattern of Figure 2, was close to 0.6 for both CC and NC α form films and it was close to 0.5 for CC and NC β form films.
Additional information relative to PPO/azobenzene CC phases could be obtained from axially stretched films. In particular, two-dimensional WAXD patterns (A,B,C) and the corresponding equatorial EDGE profiles of axially stretched PPO films, exhibiting NC α phase before (A,a) and after sorption of different amount of azobenzene ~6 wt% (B,b) and 12 wt% (C,c) are shown in Figure 3.
Diffraction profiles of films with azobenzene (3b,3c) exhibited 2θ values similar to NC films (3a); however, a strong reduction of relative intensity for 100 peak as well as an intensity enhancing for 310 peak, by increasing the azobenzene content, confirmed the formation of the PPO/azobenzene CC phase with inclusion of azobenzene guest molecules into the nanoporous α crystalline phase. It is apparent that diffraction profiles of PPO films with azobenzene (red and blue lines) exhibited 2θ values similar to NC films (black lines), however, variations in relative intensity of diffraction peaks, indicating the formation of PPO/azobenzene CC phases, were visible [72].
Moreover, patterns in Figure 2 presented diffraction arcs centered on the equator for 2θ < 16 • and on the meridional for 2θ > 16 • , indicating the occurrence of c// orientation for PPO films [72]. The degree of c// orientation, evaluated by azimuthal scan of the 001 reflection (χ (001) ) of the EDGE pattern of Figure 2, was close to 0.6 for both CC and NC α form films and it was close to 0.5 for CC and NC β form films.
Additional information relative to PPO/azobenzene CC phases could be obtained from axially stretched films. In particular, two-dimensional WAXD patterns (A,B,C) and the corresponding equatorial EDGE profiles of axially stretched PPO films, exhibiting NC α phase before (A,a) and after sorption of different amount of azobenzene~6 wt% (B,b) and 12 wt% (C,c) are shown in Figure 3.
Diffraction profiles of films with azobenzene (3b,3c) exhibited 2θ values similar to NC films (3a); however, a strong reduction of relative intensity for 100 peak as well as an intensity enhancing for 310 peak, by increasing the azobenzene content, confirmed the formation of the PPO/azobenzene CC phase with inclusion of azobenzene guest molecules into the nanoporous α crystalline phase.
Axially stretched co-crystalline PPO films were also characterized by infrared linear dichroism measurements.
FTIR spectra in the wavenumber range 1100-400 cm −1 , taken with polarization plane parallel (thin blue line) and perpendicular (thick red line) to the draw direction of PPO films presenting an α NC phase (A), CC phase with~6 wt% of azobenzene (B) and amorphous phase with~1.5 wt% of azobenzene (C) are reported in Figure 4, respectively. Azobenzene peaks labelled with (*) are also shown. Chemistry 2021, 3, FOR PEER REVIEW 6 Axially stretched co-crystalline PPO films were also characterized by infrared linear dichroism measurements.
FTIR spectra in the wavenumber range 1100-400 cm −1 , taken with polarization plane parallel (thin blue line) and perpendicular (thick red line) to the draw direction of PPO films presenting an α NC phase (A), CC phase with ~6 wt% of azobenzene (B) and amorphous phase with ~1.5 wt% of azobenzene (C) are reported in Figure 4, respectively. Azobenzene peaks labelled with (*) are also shown.  [17,72]. The degree of orientation (fc,IR) of α stretched film, evaluated on the dichroism of the isolated peak at 495 cm −1 , was ~0.45.  Axially stretched co-crystalline PPO films were also characterized by infrared linear dichroism measurements.
FTIR spectra in the wavenumber range 1100-400 cm −1 , taken with polarization plane parallel (thin blue line) and perpendicular (thick red line) to the draw direction of PPO films presenting an α NC phase (A), CC phase with ~6 wt% of azobenzene (B) and amorphous phase with ~1.5 wt% of azobenzene (C) are reported in Figure 4, respectively. Azobenzene peaks labelled with (*) are also shown.  Vibrational peaks associated to trans azobenzene isomer absorbed in α film are also apparent in Figure 4B and were dichroic, (see for instance peaks at 1070 (⊥), 690(⊥), 545(⊥), 524 (//) cm −1 ), confirming the inclusion of trans azobenzene isomer as guest in the α phase, while they were merely detectable and not dichroic if absorbed in amorphous stretched films, (see peak at 690 in Figure 4C).
The order parameter S, evaluated on vibrational peaks at 690 and 524 cm −1 of trans azobenzene isomer absorbed in CC films, associated to torsion τ (C-C) and to out of plane γ (C-H) (at 690 cm −1 ) or to in plane bending δ (C-N), δ (C-C), and δ (N=N) (at 524 cm −1 ) [73], are S g,690 = −0.05 and S g,524 = +0.21, respectively. The obtained results also let us affirm that trans azobenzene guest molecules were included as guest in the α PPO CC phase.

Trans → Cis Photo-Isomerization of Azobenzene Molecules
Isomerization, photo-induced trans → cis or spontaneous cis → trans, of azobenzene being guest of PPO co-crystalline phases or simply absorbed in PPO amorphous phases were studied at room temperature by both FTIR and UV-Visible measurements.
FTIR measurements were performed on cast PPO films as well as on axially stretched films. FTIR spectra in the wavenumber range 800-500 cm −1 of azobenzene absorbed in CC as well as in amorphous films (~50 µm thick) before and after photo-isomerization are reported in Figure 5. apparent in Figure 4B and were dichroic, (see for instance peaks at 1070 (⊥), 690(⊥), 545(⊥), 524 (//) cm −1 ), confirming the inclusion of trans azobenzene isomer as guest in the α phase, while they were merely detectable and not dichroic if absorbed in amorphous stretched films, (see peak at 690 in Figure 4C).
The order parameter S, evaluated on vibrational peaks at 690 and 524 cm −1 of trans azobenzene isomer absorbed in CC films, associated to torsion τ (C-C) and to out of plane γ (C-H) (at 690 cm −1 ) or to in plane banding δ (C-N), δ (C-C), and δ (N=N) (at 524 cm −1 ) [73], are Sg,690 = −0.05 and Sg,524 = +0.21, respectively. The obtained results also let us affirm that trans azobenzene guest molecules were included as guest in the α PPO CC phase.

Trans → Cis Photo-Isomerization of Azobenzene Molecules
Isomerization, photo-induced trans → cis or spontaneous cis → trans, of azobenzene being guest of PPO co-crystalline phases or simply absorbed in PPO amorphous phases were studied at room temperature by both FTIR and UV-Visible measurements.
FTIR measurements were performed on cast PPO films as well as on axially stretched films.
FTIR spectra in the wavenumber range 800-500 cm −1 of azobenzene absorbed in CC as well as in amorphous films (~50 μm thick) before and after photo-isomerization are reported in Figure 5. . FTIR spectra in the wavenumber range 800-500 cm −1 of azobenzene absorbed in PPO films presenting α (A), β (B) and amorphous phases (C) before (red, blue and black lines) and after isomerization (green lines). FTIR spectra of starting PPO films presenting α NC phase, β NC phase as well as amorphous phase are also reported (gray lines).
Peaks associated to trans azobenzene isomer at 524, 545, 690, ~755 cm −1 and at ~775 cm −1 are shown. After photo-isomerization for 1 h, a decrease of trans isomer peaks as well as an increase of cis isomer peaks at ~756 cm −1 and at 701 cm −1 were clearly apparent indicating the occurrence of trans → cis isomerization on PPO thick films.
The degree of isomerization [75], evaluated on the reduction of intensity of trans isomer peak at 524 cm −1 associated to in plane bending δ (C-N), δ (C-C), and δ (N=N) [73] vibrational mode, was close to ~40% for α and β films and to ~50% for amorphous PPO films.
Additionally informative were polarized FTIR spectra of axially oriented films since, evaluating the linear dichroism of both trans and cis azobenzene isomers before and after Figure 5. FTIR spectra in the wavenumber range 800-500 cm −1 of azobenzene absorbed in PPO films presenting α (A), β (B) and amorphous phases (C) before (red, blue and black lines) and after isomerization (green lines). FTIR spectra of starting PPO films presenting α NC phase, β NC phase as well as amorphous phase are also reported (gray lines).
Peaks associated to trans azobenzene isomer at 524, 545, 690,~755 cm −1 and at 775 cm −1 are shown. After photo-isomerization for 1 h, a decrease of trans isomer peaks as well as an increase of cis isomer peaks at~756 cm −1 and at 701 cm −1 were clearly apparent indicating the occurrence of trans → cis isomerization on PPO thick films.
The degree of isomerization [75], evaluated on the reduction of intensity of trans isomer peak at 524 cm −1 associated to in plane bending δ (C-N), δ (C-C), and δ (N=N) [73] vibrational mode, was close to~40% for α and β films and to~50% for amorphous PPO films.
Additionally informative were polarized FTIR spectra of axially oriented films since, evaluating the linear dichroism of both trans and cis azobenzene isomers before and after isomerization, it was possible to establish if the isomerization, into a confined molecular environment, occurred with or without expelling the cis isomer from the crystalline lattice.
Polarized FTIR spectra of α PPO film (150 µm thick) exhibiting CC phase with trans azobenzene showing a degree of orientation f c,IR ≈ 0.45 (evaluated according to ref. [17]), as prepared, after trans → cis photo-isomerization and after spontaneous cis → trans isomerization are reported in Figure 6A-C respectively. to out of plane bending γ (C-H) vibrational mode of the cis isomer [73,74] was clearly apparent; the presence of dichroism (Sg,701 = +0.03) let us suppose the inclusion of cis isomer inside the α NC crystalline phase of PPO. Moreover, since after 30 days a complete spontaneous return from cis to trans isomer occurred ( Figure 6C) without changing the dichroism of peaks at 690 cm −1 and 524 cm −1 mainly due to the trans isomer [73,74], we can conclude that trans → cis → trans isomerization occurred without expulsion of azobenzene guest molecules from PPO co-crystalline lattice. Figure 6. Polarized FTIR spectra in the wavenumber range 800-500 cm −1 taken with polarization plane parallel (thin blue line) and perpendicular (thick red lines) to the draw direction for α PPO stretched films with ≈3 wt% of azobenzene before isomerization (A); after 1 h of isomerization (B); after 30 days from isomerization (C).

Selective Sorption of Azobenzene in PPO Films
In order to evaluate the capability of α and β NC phases of PPO to include cis azobenzene isomer inside their crystalline lattice, PPO films, presenting NC (α and β) or amorphous phases, were immersed 30 min in a photo-isomerized 2% azobenzene/methanol solution (irradiated by 365 nm light for 4 h) and analyzed by FTIR measurements.
FTIR spectra reported in Figure 7 indicated that both α and β NC films absorbed higher amounts of trans and cis azobenzene isomers (see peaks at 690 and 701 cm −1 respectively) with respect to amorphous film, as well as that the β NC phase absorbed higher amounts not only of the trans isomer but also of the cis isomer also with respect to α NC phase. Dichroic peaks associated to trans azobenzene isomer at 690, and 524 cm −1 with S g,690 = −0.05 and S g,524 = +0.24 respectively, shown in Figure 6A, confirming the inclusion of the trans isomer into the CC phase. After photo-isomerization a peak at 701 cm −1 associated to out of plane bending γ (C-H) vibrational mode of the cis isomer [73,74] was clearly apparent; the presence of dichroism (S g,701 = +0.03) let us suppose the inclusion of cis isomer inside the α NC crystalline phase of PPO. Moreover, since after 30 days a complete spontaneous return from cis to trans isomer occurred ( Figure 6C) without changing the dichroism of peaks at 690 cm −1 and 524 cm −1 mainly due to the trans isomer [73,74], we can conclude that trans → cis → trans isomerization occurred without expulsion of azobenzene guest molecules from PPO co-crystalline lattice.

Selective Sorption of Azobenzene in PPO Films
In order to evaluate the capability of α and β NC phases of PPO to include cis azobenzene isomer inside their crystalline lattice, PPO films, presenting NC (α and β) or amorphous phases, were immersed 30 min in a photo-isomerized 2% azobenzene/methanol solution (irradiated by 365 nm light for 4 h) and analyzed by FTIR measurements.
FTIR spectra reported in Figure 7 indicated that both α and β NC films absorbed higher amounts of trans and cis azobenzene isomers (see peaks at 690 and 701 cm −1 respectively) with respect to amorphous film, as well as that β NC phase absorbed higher amounts not only of the trans isomer but also of the cis isomer also with respect to α NC phase.
The amount of cis and of trans azobenzene isomers absorbed in α, β and in amorphous films, evaluated by thermogravimetric measurements, corresponded roughly to~1 wt%, 2.5 wt% and~0.5 wt% for the cis isomer and to~4 wt%,~5 wt% and~3 wt% for the trans isomer, respectively.
This result, indicating a higher selectivity of β NC phase with respect to α NC phase to absorb cis azobenzene isomer, never observed for other guest molecules so far, can be particularly relevant in order to propose the β NC phase as the PPO nanoporous crystalline phase particularly suitable for absorbing spherically bulky guest molecules. The amount of cis and of trans azobenzene isomers absorbed in α, β and in amorphous films, evaluated by thermogravimetric measurements, corresponded roughly to ~1 wt%, ~2.5 wt% and ~0.5 wt% for the cis isomer and to ~4 wt%, ~5 wt% and ~3 wt% for the trans isomer, respectively.
This result, indicating a higher selectivity of β NC phase with respect to the α NC phase to absorb cis azobenzene isomer, never observed for other guest molecules so far, can be particularly relevant in order to propose the β NC phase as the PPO nanoporous crystalline phase particularly suitable for absorbing spherically bulky guest molecules.

Spontaneous Cis → Trans Isomerization of Azobenzene Molecules
Spontaneous cis→trans isomerization at room temperature of azobenzene molecules, being a guest of co-crystalline phases or dissolved in the amorphous phase have been studied, by FTIR and UV measurements, on PPO thick (~50 μm) and thin (~100 nm) films.
FTIR spectra, in the wavenumber range 720-670 cm −1 , of PPO/azobenzene thick films (~50 μm) presenting α, β CC as well as amorphous phases, after sorption of trans isomer (black lines), immediately after 1 h of irradiation at 365 nm (green lines) as well as during the time, leading to a spontaneous cis → trans isomerization, are reported in Figure 8A-C, respectively. The occurrence of trans → cis isomerization was clearly shown by the reduction of the intensity of peak at 691 cm −1 associated to the appearing of a peak at 701 cm −1 ; the spontaneous disappearance of it, during the time, clearly indicated the spontaneous return of cis azobenzene isomers to trans.
The half-time of cis → trans spontaneous isomerization at room temperature (τ½) was evaluated and it was ~48 h for azobenzene molecules included in PPO CC phases while it was five time faster (~10 h) for azobenzene molecules simply absorbed in PPO amorphous film. It is worth noting that the half-life time of cis → trans spontaneous isomerization of azobenzene dissolved in benzene is ~100 h [61], while in our experiment τ½ of cis→trans isomerization in methanol [76] it was ~240 h (see Figure S1 in supplementary materials).
The obtained τ½ values on PPO thick films were comparable to those obtained for azobenzene molecules in molecular structure types AlPO4-5 [65] and ZSM-5 [66] used as host materials. In particular, the half-life time τ½ of cis-azobenzene in AlPO4-5 composites was found to be ~100 h, whereas azobenzene in ZSM-5 showed a considerably shorter cisstate lifetime of ~6 h [66]. For host systems, like AlPO4-5 and ZSM-5, difference in channel size, geometry, polarity, and type of acidic sites influenced the relaxation of the cis state.

Spontaneous Cis → Trans Isomerization of Azobenzene Molecules
Spontaneous cis→trans isomerization at room temperature of azobenzene molecules, being a guest of co-crystalline phases or dissolved in the amorphous phase have been studied, by FTIR and UV measurements, on PPO thick (~50 µm) and thin (~100 nm) films.
FTIR spectra, in the wavenumber range 720-670 cm −1 , of PPO/azobenzene thick films (~50 µm) presenting α, β CC as well as amorphous phases, after sorption of trans isomer (black lines), immediately after 1 h of irradiation at 365 nm (green lines) as well as during the time, leading to a spontaneous cis → trans isomerization, are reported in Figure 8A-C, respectively. The occurrence of trans → cis isomerization was clearly shown by the reduction of the intensity of peak at 691 cm −1 associated to the appearing of a peak at 701 cm −1 ; the spontaneous disappearance of it, during the time, clearly indicated the spontaneous return of cis azobenzene isomers to trans.
The half-time of cis → trans spontaneous isomerization at room temperature (τ1 /2 ) was evaluated and it was~48 h for azobenzene molecules included in PPO CC phases while it was five time faster (~10 h) for azobenzene molecules simply absorbed in PPO amorphous film. It is worth noting that the half-life time of cis → trans spontaneous isomerization of azobenzene dissolved in benzene is~100 h [61], while in our experiment τ1 /2 of cis→trans isomerization in methanol [76] it was~240 h (see Figure S1 in supplementary materials).
The obtained τ1 /2 values on PPO thick films were comparable to those obtained for azobenzene molecules in molecular structure types AlPO 4 -5 [65] and ZSM-5 [66] used as host materials. In particular, the half-life time τ1 /2 of cis-azobenzene in AlPO 4 -5 composites was found to be~100 h, whereas azobenzene in ZSM-5 showed a considerably shorter cis-state lifetime of~6 h [66]. For host systems, like AlPO 4 -5 and ZSM-5, difference in channel size, geometry, polarity, and type of acidic sites influenced the relaxation of the cis state. Narrower channels enhanced the guest host interaction lowering the energy of the transition states involved in the cis-trans relaxation and hence having a stabilizing influence on the cis state as occurred for the AlPO 4 -5 and PPO CC phases. A higher concentration of acidic sites in the host system, as occurred for ZSM-5, may be responsible for the faster cis-relaxation (τ1 /2~6 h) [66]. For amorphous PPO films the absence of constrained channels or cavities could cause faster cis relaxation leading to a faster half-life time of τ1 /2~1 0 h with respect to half-life time occurring if azobenzene molecules were included in CC PPO films (τ1 /2~4 8 h). tion states involved in the cis-trans relaxation and hence having a stabilizing influence on the cis state as occurred for the AlPO4-5 and PPO CC phases. A higher concentration of acidic sites in the host system, as occurred for ZSM-5, may be responsible for the faster cis-relaxation (τ½ ~6 h) [66]. For amorphous PPO films the absence of constrained channels or cavities could cause faster cis relaxation leading to a faster half-life time of τ½ ~10 h with respect to half-life time occurring if azobenzene molecules were included in CC PPO films (τ½ ~48 h). Spontaneous cis → trans isomerization of azobenzene absorbed in CC and amorphous PPO thick films was also studied by UV measurements and reported in Figure 8A'-C' respectively.
It is well known that trans azobenzene absorption spectrum consists of two well separated bands in the UV visible region. The strong UV band at λ ~320 nm, comes from the allowed π → π* transition whereas the much weaker band in the visible region at λ ~450 nm corresponds to the forbidden n→π* transition [52,77]. As for the cis isomer, π → π* transitions at λ ~250 nm are weaker, but n → π* transition at λ ~450 nm absorbs more strongly than trans azobenzene. Spontaneous cis → trans isomerization of azobenzene absorbed in CC and amorphous PPO thick films was also studied by UV measurements and reported in Figure 8A'-C' respectively.
It is well known that trans azobenzene absorption spectrum consists of two well separated bands in the UV visible region. The strong UV band at λ~320 nm, comes from the allowed π → π* transition whereas the much weaker band in the visible region at λ 450 nm corresponds to the forbidden n→π* transition [52,77]. As for the cis isomer, π → π* transitions at λ~250 nm are weaker, but n → π* transition at λ~450 nm absorbs more strongly than trans azobenzene.
UV-visible spectra in the range 350-600 nm of PPO/azobenzene films (~50 µm) presenting CC and amorphous phases, after sorption of trans isomer (black lines), immediately after 1 h of irradiation at 365 nm (green lines) as well as during the time, leading to a spontaneous cis → trans isomerization, are reported in Figure 8A'-C', respectively. After 1 h of ultraviolet light exposure, a decrease in intensity of the 323 nm (out of scale) as well as an increase of the~446 nm band (green line) was apparent indicating that the photo-induced trans → cis isomerization occurred. Moreover, by collecting spectra at different time (colored lines) a decrease of the~446 nm band was shown indicating the spontaneous return of the cis azobenzene isomer to trans in all PPO films.
The half-times of cis-trans azobenzene spontaneous isomerization, evaluated on n → π* transition band at λ~446, were similar to those obtained by FTIR measurements.
Trans → cis photo-isomerization and spontaneous cis → trans isomerization of azobenzene molecules, being guest of PPO co-crystalline phases or dissolved in PPO amorphous phases, were studied at room temperature by UV-Visible measurements also on thin films (0.1-0.2 µm).
UV-Visible spectra in the spectral range between 275 and 500 nm of PPO films having a thickness of nearly 0.1 µm, presenting α and β NC as well as amorphous phases, after sorption from a 0.2 wt% azobenzene/methanol solution for 30 min (leading to CC phases) and 4 days of desorption at room temperature, are shown in Figure 9A-C (black lines).
senting CC and amorphous phases, after sorption of trans isomer (black lines), immediately after 1 h of irradiation at 365 nm (green lines) as well as during the time, leading to a spontaneous cis → trans isomerization, are reported in Figure 8A'-C', respectively. After 1 h of ultraviolet light exposure, a decrease in intensity of the 323 nm (out of scale) as well as an increase of the ~446 nm band (green line) was apparent indicating that the photoinduced trans → cis isomerization occurred. Moreover, by collecting spectra at different time (colored lines) a decrease of the ~446 nm band was shown indicating the spontaneous return of the cis azobenzene isomer to trans in all PPO films.
The half-times of cis-trans azobenzene spontaneous isomerization, evaluated on n → π* transition band at λ ~446, were similar to those obtained by FTIR measurements.
Trans → cis photo-isomerization and spontaneous cis → trans isomerization of azobenzene molecules, being guest of PPO co-crystalline phases or dissolved in PPO amorphous phases, were studied at room temperature by UV-Visible measurements also on thin films (0.1-0.2 μm).
UV-Visible spectra in the spectral range between 275 and 500 nm of PPO films having a thickness of nearly 0.1 μm, presenting α and β NC as well as amorphous phases, after sorption from a 0.2 wt% azobenzene/methanol solution for 30 min (leading to CC phases) and 4 days of desorption at room temperature, are shown in Figure 9A-C (black lines). UV-Visible spectra of Figure 9 display the high-intensity π → π* band in the UV region (at 318 nm) that clearly indicated the presence of the trans azobenzene isomer [78,79] in all PPO samples, whose amount was higher for the α and β CC phases, which were able to host in the crystalline lattice azobenzene molecules ( Figure 9A,B), and lower for the amorphous samples ( Figure 9C). A weaker band in the visible region at λ ~446 nm corresponding to the n → π* transition was also apparent.
After only 1 min of UV irradiation a strong reduction of the trans band at 318 nm as well as an increase of the weaker band associated at n → π* (green lines) were apparent, indicating the occurring trans → cis isomerization [78,80]. The degree of photo-isomerization [76], as evaluated on the basis of the absorbance at 318 nm [81] after 1 min of UV irradiation, was approximately ~55% for α and β CC phases and ~70% for azobenzene in amorphous phases.
Half-life time of cis → trans spontaneous isomerization as evaluated on the basis of the high-intensity π → π* band at 318 nm [69,81] was approximately ≈1 h for azobenzene enclosed in CC phases while it was faster (≈20 min) for azobenzene absorbed in amorphous films. Similar results have been obtained in s-PS azobenzene thin films showing τ½ close to 3 h for azobenzene included in CC phases and τ½ ≈1 h for azobenzene absorbed in amorphous s-PS films [50].
Moreover, it is worth noting that after 1 month of desorption at room temperature (see light blue curves in UV-vis spectra of Figure 9) a desorption of ~30% of azobenzene molecules being guest of PPO CC phases occurred whereas a much higher desorption of Figure 9. UV-Visible spectra in the range 275-500 nm of spin-coated PPO films exhibiting α CC (A) β CC (B) and amorphous (C) phases before (black lines), immediately after photo-isomerization with 365 nm light for 1 min (green lines) and during times (colored lines). For comparison, the spectra of PPO empty films (gray lines) are also reported.
UV-Visible spectra of Figure 9 display the high-intensity π → π* band in the UV region (at 318 nm) that clearly indicated the presence of the trans azobenzene isomer [78,79] in all PPO samples, whose amount was higher for the α and β CC phases, which were able to host in the crystalline lattice azobenzene molecules ( Figure 9A,B), and lower for the amorphous samples ( Figure 9C). A weaker band in the visible region at λ~446 nm corresponding to the n → π* transition was also apparent.
After only 1 min of UV irradiation a strong reduction of the trans band at 318 nm as well as an increase of the weaker band associated at n → π* (green lines) were apparent, indicating the occurring trans → cis isomerization [78,80]. The degree of photoisomerization [76], as evaluated on the basis of the absorbance at 318 nm [81] after 1 min of UV irradiation, was approximately~55% for α and β CC phases and~70% for azobenzene in amorphous phases.
Half-life time of cis → trans spontaneous isomerization as evaluated on the basis of the high-intensity π → π* band at 318 nm [69,81] was approximately ≈1 h for azobenzene enclosed in CC phases while it was faster (≈20 min) for azobenzene absorbed in amorphous films. Similar results have been obtained in s-PS azobenzene thin films showing τ1 /2 close to 3 h for azobenzene included in CC phases and τ1 /2 ≈1 h for azobenzene absorbed in amorphous s-PS films [50].
Moreover, it is worth noting that after 1 month of desorption at room temperature (see light blue curves in UV-vis spectra of Figure 9) a desorption of~30% of azobenzene molecules being guest of PPO CC phases occurred whereas a much higher desorption of nearly 85% was apparent if azobenzene molecules were simply absorbed in amorphous PPO films.
Finally, making a comparison with syndiotactic polystyrene (s-PS) the other commercial polymer capable of forming CC phases including azobenzene guest molecules, we can affirm that the main relevant result was the capability of NC PPO phases to absorb higher amounts of azobenzene guest molecules (also for few minutes of exposure time) with respect to NC s-PS phases. For instance, PPO and s-PS films,~50 µm thick, immersed for 30 min into a 2 wt% azobenzene/methanol solution, showed an azobenzene content close to~7.3 wt%,~9 wt% and~2.8 wt% for PPO films presenting NC α and β and amorphous phases, respectively; while only an azobenzene content of~1 wt% for s-PS films presenting δ NC phase (see supplementary materials).
The obtained results, i.e., the higher amount of azobenzene molecules stably included into CC PPO phases during the time, associated with the availability of PPO films to get different uniplanar orientation of the co-crystalline phases, with the polymer chain prevailing parallel or perpendicular [72] to the film plane, make particularly interesting CC PPO films that could be in principle suitable to develop active materials for the design of stimuli-responsive molecular systems and light-powered molecular machines.

Conclusions
Co-crystalline polymeric films of a commercial polymer, as PPO, with azobenzene, i.e., one of the most popular molecular switches, have been prepared and structurally characterized by WAXD, FTIR and UV-Vis measurements.
A comparison with amorphous films revealed a higher solubility as well as a higher stability of azobenzene molecules included as guest in PPO CC phases with respect to azobenzene molecules simply dissolved in amorphous phase.
FTIR linear dichroism measurements on axially oriented PPO films allowed us to establish the inclusion of both trans and cis isomer into α CC phase, as well as the occurrence of trans → cis photo-isomerization and spontaneous cis → trans isomerization without expulsion of azobenzene guest molecules from host co-crystalline lattice.
Sorption studies conducted on photo-isomerized azobenzene methanol solution have shown a higher solubility and selectivity of β NC phase toward cis azobenzene isomer with respect to amorphous and to α NC phase, making the β NC phase particularly promising for absorbing spherically bulky guest molecules and attempting confined photoreactions.
The whole set of data suggests that these films can be suitable for developing active materials for the design of stimuli-responsive molecular systems and light-powered molecular machines.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/chemistry3030078/s1, Figure S1: UV-Visible spectra in the range 350-550 nm of 0.2 wt% azobenzene/methanol solution before (black line), after photoisomerization (green line) and as collected at different times during spontaneous cis-trans isomerization (colored curves). Figure S2: UV-Visible spectra in the range 350-600 nm of α CC PPO (red line), β CC PPO (blue line), amorphous PPO (black line) and δ CC s-PS (green line) films after immersion for 30 min in 2 wt% azobenzene/methanol solution. PPO film spectra (gray line) is also reported for comparison.