Photoinduced Reorientation and Polarized Fluorescence of a Photoalignable Liquid Crystalline Polymer

Featured Application

Birefringent film, photoalignment layer, polarized light-emitting film.

Abstract

Thermally stimulated photoinduced reorientation of liquid crystalline (LC) polymethacrylate composed of a phenyl benzoate mesogen connected with N-benzylideneaniline (NBA2) end moiety exhibits a significant molecular reorientation (D > 0.7) when the film is exposed to linearly polarized 313 nm light and subsequently annealed in the LC temperature range of the material. Hydrolysis of the NBA2 end moieties yields an oriented mesogen with phenylamine moieties without distorting the oriented structure. In situ condensation of 2-hydroxybenzaldehyde derivatives and phenylamine moieties yields oriented N-salicylideneaniline side groups. The resultant film displays a polarized fluorescence with a polarization ratio up to 3.4.

1. Introduction

Photoalignment using photoresponsive polymeric films is a dust-free process applicable to display and diffraction devices. Compared with the mechanical stretching and rubbing process, photoalignment easily fabricates a molecular orientation pattern. Many types of photoresponsive polymers are available for photoalignment [1,2,3,4,5]. Among them, axis-selective photoisomerization of azobenzene-containing polymers has been intensely studied as the photoalignment layer for low-molecular liquid crystals (LCs) and photoinduced molecular reorientation using a linearly polarized light [6,7,8,9,10,11]. Cinnamate, coumarin, and phenyl benzoate photosensitive groups in the polymeric films show axis-selective photo-crosslinking and photorearrangement, which are suitable for the photoalignment layer in LCs. However, these photoreacted films typically have a small optical anisotropy due to lack of the molecular reorientation [12,13,14,15,16,17,18]. In some cases, this small anisotropy is amplified at an elevated temperature due to thermally induced self-organization when the photosensitive film possesses LC characteristics [19,20]. This phenomenon is also observed in azobenzene-containing LC polymer (LCP) films [20,21,22].

N-benzylideneaniline (NBA) derivatives show trans-cis photoisomerization similar to azobenzene derivatives. Several polymers with NBA derivative side groups have been synthesized [23,24,25]. Similar to azobenzene-containing LCPs, LCPs with NBA side groups (NBA1 and NBA2 in P1 and P2 in Figure 1) display a significant thermally stimulated molecular reorientation using linearly polarized 313 and 365 nm light [26,27]. Copolymers with a methacrylate composed of benzoic acid side groups show thermally stimulated cooperative molecular reorientation [28,29]. The NBA side groups in these reoriented films can be hydrolyzed at elevated temperatures under humid conditions, yielding oriented films with free benzaldehyde (from NBA1) or phenylamine (from NBA2) side moieties [28,29]. In situ condensation with phenylamine or 2-hydroxybenzaldehyde derivatives reforms different types of NBA or N-salicylideneaniline derivatives, generating reoriented films with high functionalities, such as high birefringence, thermal stability, and polarized fluorescence [28,29]. However, the hydrolyzed-oriented films should exhibit the LC characteristics to maintain the oriented structure upon thermal hydrolysis. Therefore, the content of NBA in the copolymer is limited due to the non-LC characteristics after hydrolysis of NBA1 and 2 in the copolymer.

To improve the content of in situ functionalization of the photoaligned NBA-containing LCP films, we explored P3, which shows a sufficient photoinduced reorientation behavior [30]. Hydrolyzed P3 exhibits LC characteristics due to the presence of phenyl benzoate mesogenic moieties. In situ condensation of the hydrolyzed film with higher inherent birefringent phenylamine derivatives improves the birefringence of the oriented film up to 0.44. However, an NBA-containing LCP composed of the benzoate mesogenic side groups with the opposite C=N direction (P4) has yet to be synthesized. In situ condensation with 2-hydroxybenzaldehyde derivatives after hydrolysis of a reoriented P4 film should yield a higher composition of oriented N-salicylideneaniline derivatives.

In this paper, a new photoalignable LCP (P4) is synthesized, its photoinduced reorientation behavior is investigated, and the polarized fluorescence of the photoaligned film by in situ exchange of NBA moieties to oriented N-salicylideneaniline derivatives is explored. A P4 film shows significant thermally stimulated molecular reorientation (D > 0.7) when using linearly polarized 313 nm light. Subsequent hydrolysis achieves an oriented film with phenylamine end groups. Coating 2-hydroxybenzaldehyde derivatives on the hydrolyzed film induces in situ condensation. This process yields oriented N-salicylideneaniline derivative side groups, which exhibit polarized fluorescence.

2. Materials and Methods

2.1. Materials

All starting materials were used as received from TCI. The supporting information (SI) includes the detailed synthesis of P4. The number (weight) average molecular weight of P4 is 41,000 (143,500). P4 reveals LC characteristics between 55 and 287 °C. These characteristics were confirmed by differential scanning calorimetry (DSC) and polarized optical microscopy (Figure S1). 2-Hydroxy-4-methoxybenzaldehyde (HBA), 4-formyl-3-hydroxybenzoic acid (FHA) and 2-hydroxy-4-cyanobenzaldehyde (CNA) for in situ condensation were used as received.

2.2. Photoreaction

Thin P4 films (190 nm thick) were prepared by spin-coating onto quartz or CaF₂ substrates from a THF solution. The films were exposed to a linearly polarized 313 nm light (13 mW/cm², Asahi Spectra Co., Japan, REX-250 with a glass plate placed at Brewster’s angle and a 313 nm bandpass filter) and subsequently annealed at elevated temperatures under a dry N₂ atmosphere to generate thermal amplification of the molecular reorientation.

2.3. Hydrolysis of Oriented Films and SA Formation

In situ hydrolysis of the oriented film was achieved by immersing the oriented film in an acetic acid/water solution at 50 °C. The degree of hydrolysis (DH) was estimated by the change in the absorption band of NBA at 325 nm. Then HBA (FHA, CNA) in an ethylene glycol solution (1.0 wt/vol%) was coated on the hydrolyzed film. The film was subsequently annealed at 100 °C for 10 min to form N-salicylideneaniline moieties and immersed in diethyl ether to remove excess HBA (FHA, CNA) molecules.

2.4. Characterization

¹H-NMR spectra using a Bruker, Japan, DRX-500 FT-NMR and FTIR spectra (Jasco FTIR-6600, Japan) confirmed the polymer. The molecular weight was measured by GPC (Jasco PU-2080). We used a Shodex column using THF as an eluent calibrated using polystyrene standards. Both a polarized optical microscope (Olympus BX51, Japan) equipped with a Linkam TH600PM heating stage and a DSC (Hitachi High Technologies, DSC-7200, Japan) were used to evaluate the thermal property. As a measure of the optical anisotropy of the film, a polarization absorption spectrometer (Hitachi U-3010 spectrometer, Japan, equipped with Glan–Taylor polarization prisms) and a polarization FTIR spectroscope equipped with a wire-grid polarizer were used. In-plane dichroism (D) is estimated as

D = (A_⊥ − A_||)/(A_⊥ + A_||)

(1)

where A_⊥ and A_|| are the absorbances perpendicular and parallel to the polarization (E) of linearly polarized light, respectively.

The polarized fluorescence of the film was evaluated using a fluorescence spectrophotometer (Hitachi F-7000, Japan) with a polarizer film in front of the detector. The excitation wavelength was 365 nm. The polarization ratio (P) of the fluorescence is estimated as

P = FL_⊥/FL_||

(2)

where FL_⊥ and FL_|| are the emission intensity parallel and perpendicular to the orientation direction, respectively.

3. Results and Discussion

3.1. Thermal and Spectroscopic Properties of P4

The C=N direction of P4 is opposite to that of P3. The chemical structure of P4 contains a mesogen with a phenylamine end group after the hydrolysis of NBA2 [30]. P4 exhibits LC characteristics similar to those of P3 [30]. P4 shows a high LC temperature range due to the long mesogenic side moieties compared with those of P1 and P2 [26]. The UV–VIS absorption property of P4 differs from that of P3 due to the opposite direction of the C=N bond connected to the phenyl benzoate moiety, even though those of P1 and P2 are similar to each other (Figure S2a–d).

3.2. Axis-Selective Photoreaction and Thermal Stimulation Using Linearly Polarized 313 nm Light

The photoinduced molecular reorientations in P1–P3 films using linearly polarized 365 or 313 nm light show significant thermally stimulated molecular reorientation (D > 0.7) [24,30]. Similarly, the P4 film displays a photoinduced molecular reorientation when using linearly polarized 313 nm light (Figure 2a,b). A_⊥ (A_||) initially increases (decreases), but the absorbances of both directions decrease when the exposure energy exceeds 90 J/cm². This is due to the out-of-plane motion and side photoreaction of the mesogenic side groups. The observed photoinduced optical anisotropy is much smaller than that for a P3 film [30]. Additionally, an increase at 380–420 nm is detected with a small anisotropy when the exposure energy exceeds 20 J/cm². The polarized FTIR spectra show decreases in the absorbances at 1725, 1600, 1250, and 1162 cm^–1 (vibration bands of Ph-COO-Ph) (Figure 2c). Changes in the UV–VIS and FTIR spectra are due to the axis-selective photo-Fries rearrangement of the phenyl benzoate moieties. The photoinduced in-plane reorientation initially occurs, but the simultaneous photo-Fries rearrangement of the phenyl benzoate moieties at a high exposure dose restricts the in-plane motion of the mesogenic side groups (Scheme 1).

Annealing the exposed P4 films generates a significant in-plane thermal stimulation of the orientation similar to the cases of P1–P3 films [26,27,28,29,30]. Figure 3a shows the change in the polarized absorption spectra of a P4 film after exposure to linearly polarized 313 nm light for 5 J/cm² and subsequent annealing at 120 °C for 10 min. The slight optical anisotropy (D₂₇₆ = 0.05, D₃₂₅ = 0.05) after exposure is amplified to D₂₇₆ = 0.72 and D₃₂₅ = 0.76. However, the thermal amplification is not effective when the exposure energy is 30 J/cm² (Figure 3b). A significant thermally stimulated molecular reorientation is attained when the exposure energy is 2–10 J/cm² (Figure 3c and Figure S3a,b), as well as subsequent annealing of the exposed films in the LC temperature range of P4 (Figure 3d). Random motion above the LC temperature of the film vanishes the photoinduced dichroism. This behavior is similar to the thermal stimulation of an exposed P3 film using linearly polarized 365 nm light. By contrast, the D value decreases when the exposure energy exceeds 30 J/cm², regardless of the annealing temperature (Figure 3c and Figure S3c,d). As the photoreaction of the P4 film with 313 nm light proceeds, it includes an axis-selective photo-Fries rearrangement of the mesogenic side groups, which may prevent thermal stimulation of the in-plane molecular reorientation.

3.3. In Situ Hydrolysis of the Oriented Films

Hydrolysis of the NBA moieties occurs under acidic conditions [31,32,33]. We previously reported that photoalignment of copolymethacrylate with NBA2 and benzoic acid side groups and subsequent annealing under humid air generates an oriented film with hydrolyzed phenylamine moieties [28]. The benzoic acid side groups act as an acid catalyst. Immersing the oriented P4 film in an acetic acid solution generates hydrolysis because P4 does not include an acidic moiety. However, hydrolysis does not occur when using only water (Figure S4a).

Polarized absorption spectrum changes of oriented P4 films before and after hydrolysis are shown in Figure 4a,b. Hydrolysis was carried out by immersing in an acetic acid/water solution [(a) 1/5 (pH = 1.5) or (b) 1/3 (pH = 1.3) (vol/vol)] at 50 °C for 10 min, respectively. After hydrolysis, the absorption band at 325 nm decreases, and the absorption maxima shift to a shorter wavelength due to the formation of 4-aminophenyl benzoate mesogenic side groups. The FTIR spectrum decreases at 1621 and 1590 cm^–1 (N=C vibration), and new absorptions appear at 3385 and 3361 cm^–1 (Ph-NH₂ vibration) (Figure 4c). In these cases, the DH (D₂₆₀) values are 60% (0.64) and 95% (0.49) after hydrolysis when using 1/5 and 1/3 solution, respectively. The higher DH and lower D₂₆₀ values using 1/3 solution indicate that the hydrolysis is fast but the orientation structure gradually collapses when using a higher acidity solution. Additionally, the DH (D₂₆₀) value increases (decreases) as the acetic acid concentration or immersion time for the hydrolysis increases (Figure S4b,c). Nevertheless, the film maintains its orientation structure, even though most of the mesogenic groups are hydrolyzed (Figure S4c). This is because the hydrolyzed P4 film displays LC characteristics upon hydrolysis at 50 °C. This characteristic is confirmed by the change in the transmittance of the hydrolyzed film between crossed polarizers upon heating, exhibiting thermal stability up to 110 °C (Figure 5).

3.4. In Situ Formation of Oriented N-Salicylideneaniline Derivatives

The formation of oriented N-salicylideneaniline derivatives is carried out by coating HBA, FHA, or CNA in an ethylene glycol solution (1.0 wt/vol%) onto a hydrolyzed P4 film and subsequently annealing at 100 °C for 10 min. Figure 6 shows the changes in the polarized absorption spectrum of reoriented-hydrolyzed P4 films [DH = (a) 60% and (b) 94%] before and after the formation of the N-salicylideneaniline derivatives using HBA and their polarized fluorescence spectra. For both films, a new absorption band at 340 nm is observed, but its intensity differs. The DH = 94% film displays a larger absorbance at 340 nm due to the higher amount of N-salicylideneaniline derivative formation. It should be mentioned that both films show a similar D at 340 nm (D₃₄₀ = 0.72–0.76), but the initial D₂₆₀ value of the hydrolyzed film is lower for the higher DH film. This indicates that the thermal condensation process simultaneously induces thermally stimulated molecular reorientation of the formed N-salicylideneaniline moieties. Additionally, the use of FHA and CNA for the condensation shows similar results, where the D₃₄₀ values after condensation are 0.64 (FHA) and 0.65 (CNA) when using DH = 90% films (Figure 6c,d).

Polarized fluorescence is useful for polarized light-emitting or polarization-sensing devices [34,35]. Several studies have studied the polarized fluorescence of LC materials based on the photoalignment technique [34,35,36,37]. The N-salicylideneaniline derivative-formed films exhibit fluorescent emission with a large Stokes shift due to excited-state intermolecular proton transfer (ESIPT) (Figure 6a–d and insets) [38,39]. For HBA-introduced films, λ_max of the fluorescence is 530 nm, and a higher emission is observed when using a higher DH film (P = 2.3–2.4) because more oriented mesogens with N-salicylideneaniline moieties are formed. Similar polarized fluorescent behaviors are observed for the FHA- and CNA-introduced films (λ_max = 547 nm, P = 2.8 for FHA, λ_max = 540 nm, P = 3.4 for CNA). These fluorescent behaviors of oriented N-salicylideneaniline derivatives are similar to those obtained by in situ condensation using oriented copolymer films of NBA2 and benzoic acid side groups [28]. However, the fluorescent intensity is larger due to the higher content of N-salicylideneaniline derivative-containing mesogenic side groups.

4. Conclusions

Here, the synthesis and thermally stimulated molecular reorientation of a new photoalignable LCP composed of a phenyl benzoate mesogenic group connected with an NBA2 end moiety are investigated. Exposing to linearly polarized 313 nm light and subsequent annealing yields a film with a significant photoinduced molecular orientation. However, a photo-Fries rearrangement of the phenyl benzoate groups at a high exposure dose reduces the thermal stimulation performance of the molecular reorientation. Immersing the reoriented film in an acetic acid/water solution hydrolyzes the NBA2 moieties without affecting the oriented structure to form phenylamine end groups. In situ condensation of the phenylamine groups in the hydrolyzed film and 2-hydroxybenzaldehyde derivatives introduces oriented N-salicylideneaniline derivative side groups, which display polarized fluorescence at 530–547 nm with a polarization ratio up to 3.4.