Polarized PL of PFO-Al 2 O 3 composite film

Figure 4.3.Top-surface SEM image of 15%-Al-Ti-700^oC.

oriented vertically, but a few PFO chains are lying across over the several nanopillars (as indicated by blue arrows).

The used mesoporous alumina film with vertical mesoporosity here was prepared according to Chapter 3.¹⁵ The obtained mesoporous alumina film possessed vertical mesospace throughout the film thickness (Figure 4.5). N2 adsorption-desorption isotherm proved the formation of the mesopores with uniform size of 10 nm (Figure 4.6), which were created by enclosing 4-fold- or 6-fold-arranged pillared J-alumina nanowires.¹⁵

This size was enough for PFOs to easily penetrate into the mesopores by capillary force, because the mesopore size was much larger than the width of 9,9-di-n-octylfluorene unit (ca. 1 nm). By casting PFO onto the mesoporous substrates, the PFO was adsorbed on and moved into the mesopores. The PFO penetrated inside the mesopores still sustained even after the washing process probably due to cooperative noncovalent interactions among PFOs and alumina framework. The “PFO/Al2O3-vertical” composite film was further studied as follows.

Figure 4.5. Tilted cross-sectional SEM image of mesoporous alumina film with vertical mesoporosity.

Figure 4.6. N2 adsorption-desorption isotherm for mesoporous alumina films with cage-type mesopores (Red) and vertical mesoporosity (Blue), respectively. The pore size distribution curves calculated by a Barrett-Joyner-Halenda (BJH) method are also shown as an inset image. The adsorbed volume is normalized by the film volume.

Polarized PL measurement of the film was carried out with a 325-nm laser. The PL spectra can provide important information the orientation of conjugated polymer PFO in the film. As previously shown in Figure 4.4, the PFO-containing films were excited from an in-plane direction and the emission spectra were detected by CCD camera. The emission spectra for PFO/Al2O3-vertical in Figure 4.7a can be mainly deconvoluted into three Gaussian peaks, locating at 417 nm, 438 nm, and 466 nm (For details, see Figure 4.8). These peaks can be assigned to be 0-0 band for amorphous phase emission, and 0-1 and 0-2 bands for E-phase emission, respectively. In both P-and S-polarized excitation, the relative ratios of each peak are almost constant, but the intensity of the P-polarized spectra was much higher than that of the S-polarized spectra, indicating that the PFO main chains were oriented normal to the substrate.

The polarization ratio (P) can be calculated by the following equation (P = IP/IS,

where IP and IS indicate PL intensity at 430 nm excited by P- and S-polarized light, respectively.). The value was calculated to be 3.23, showing high alignment degree of conjugated polymer chains. In the previous study, horizontally aligned conjugated polymer films have been prepared by several techniques.^2,3,4,5,16 In those cases, the polarization ratios (IS/IP) were used for understanding the alignment degrees of the polymer chains. The values were largely varied, depending on the applied techniques.

When Langmuir-Blodgett, stretching, or direct rubbing technique is applied, the polarization ratios in the polymer films ranged from 1.0 to 4.0.17,18,19,20,21 Therefore, the polarization ratio value in this study (3.23) was relatively higher value than those reported previously. In common, “anisotropy” has been used to evaluate polarized luminescence of anisotropic films. The “polarization ratio” used in this study also can be used for anisotropic fluorescence measurements, especially, for aligned several fluorescent polymers on substrates including polyfluorene, polypheylenevinylene, polythiophene, and polysilane. In the present work, in order to compare the orientation degree of the polyfluorene in the mesopores with other polymer systems reported previously, I used the “polarization ratio” rather than “anisotropy”.

As comparison, flat substrate (without mesopores) was used (marked as

“PFO/Al2O3-flat”). The polarization ratio (P) was measured to be around 1.00. As seen in PFO/Al2O3-flat spectra (Figure 4.7c), the P-polarized spectra were consistent with the S-polarized spectra, indicating no PL anisotropy. It is understood that the 1D polymer chains are twisted and randomly oriented on the flat substrate. In contrast, when cage-type mesoporous alumina film prepared with the same F127 surfactant (marked as “PFO/Al2O3-cage”) was used as non-vertically mesopored substrate, luminescence anisotropy was confirmed in which ISwas higher than IP(Figure 4.7b).

The as-prepared mesoporous alumina film prepared with F127 possesses Im-3m mesostructure whose <001>-direction is oriented perpendicular to substrate, according to our previous report. By calcination at 400°C to remove surfactants, 60%

shrinkage along caxis is occurred, forming distorted Im-3mmesostructure. Therefore, ellipsoidal cage-type mesopores are formed in the used Al2O3-cage film. The size of

cage-type mesopores was around 8 nm in diameter, as detected by N2

adsorption-desorption isotherm (Figure 4.6). The long axis of the ellipsoidal mesopores are oriented parallel to the substrate and their ellipsoidal mesopores are connected through the micropores each other (Figure 4.9). Appearance of a small hysteresis loop was caused by the presence of spherical mesopores connected with microporosity (Figure 4.6). Based on the above background, I suppose that the polymer chains tend to align horizontally to the substrate. This is the reason for the luminescence anisotropy observed in Figure 4.7b.

Figure 4.7. PL spectra for (a) PFO/Al2O3-vertical, (b) PFO/Al2O3-cage, (c) PFO/Al2O3-flat, and (d) PFO/Al2O3-hexagonal. The band at 417 nm can be assigned to be 0-0 band for amorphous phase emission. The bands at 438 nm, and 466 nm can be assigned to be 0-1 and 0-EDQGVIRUȕ-phase emission, respectively.

Figure 4.8. Relative peak areas of 0-1 band to 0- EDQG IRU ȕ-phase emission, calculated by peak deconvolution. The band at 417 nm can be assigned to be 0-0 band for amorphous phase emission. When the PFO amorphous phase is formed, the 0-0 band for amorphous phase emission is observable. This band has no correlation with other band peaks. On the other hand, the bands at 438 nm and 466 nm can be assigned to be 0-1 and 0-EDQGVIRUȕ-phase emission, respectively. The peak area for 0-1 band strongly correlates with that for 0-2 band. From calculation by waveform separations, the relative peak areas of 0-1 band to 0-2 band were almost 2 for all the films.

Figure 4.9.(Top) Illustration showing <100> view of distorted Im-3mmesostructure.

(Bottom) Illustration showing <110> view of distorted Im-3m mesostructure and actual cross-sectional TEM image.

Furthermore, 2D-hexagonally ordered mesoporous alumina film prepared by using Brij 58 (as marked as “PFO/Al2O3-hexagonal”) was used as another non-vertically mesopored substrate. Experimental procedure and detailed characterization data is shown in supporting information. As seen in top-surface SEM image, all the 1D-mesochannels were lying to the substrate surface (Figure 4.10a). The pore-to-pore distance was measured to be around 4 nm by powdery TEM image (Figure 4.10b). The polarization ratio was drastically decreased to be 0.23 (for PFO/Al2O3-hexagonal), meaning that the PFO chains were thought to be oriented parallel along the lying 1D-mesochannels. Also, the 0-0 band for amorphous phase emission, which was observed in other samples, entirely disappeared. All the peaks were originated from E-phase emission. It was revealed that the 1D straight mesochannels increased the packing degree of the PFO chains (i.e., the PFO chains were crystallized to some extent.). In contrast, PFO/Al2O3-vertical showed the emission band for amorphous phase at 420 nm (Figure 4.7a). As we described above,

the vertically oriented mesospace with 10 nm were created by enclosing the pillared nanopillars. Thus, this mesoporosity can be regarded as a sort of “inverse mesospace”

of a 2D-hexagonal mesostructure with “1D mesochannels”. Such “inverse mesospace” cannot provide isolated pore space. Therefore, several PFO chains were thought to lie across over the several nanopillars (as indicated by the blue arrows in Figure 4.4), thereby detecting the formation of the PFO amorphous phase.

Figure 4.10. (a) Top-surface SEM and (b) TEM images for 2D-hexagonally ordered mesoporous alumina film prepare with Brij 58.