UV-Vis and PL Spectroscopy

The chemical structures of V-BPy and BPy-POSS are similar to butterflies. The flexible ether linkages between pyrene rings and POSS moiety result in two conformational isomers of enthalpy-favor close form and entropy-favor open form as shown in Figure 2-1. For UV-Vis and PL spectroscopic studies, the uniform and transparent thin films of Py-OH, V-BPy and BPy-POSS were fabricated through spin coating process.

Before investigating the role of the pyrene in the photoluminescent analyses, we need to analyze the effect of the discrepancy between chemical structures of Py-OH, V-BPy, and BPy-POSS have little effect upon the UV-absorbance of pyrene functional groups. As shown in Figure 4-6, the UV-Vis absorption spectra of the

V-BPy and BPy-POSS in dilute solution (dichloromethane, 10^-5 M) are very similar to that of Py-OH. These peaks at 313.4, 327.6, and 344.0 nm indicate that there is coupling of the vibronic features corresponding to the transitions from ν=0 to ν’= 0, 1, 2, where ν and ν’ are the quantum vibrational numbers of the ground and excited states, respectively.⁶⁷ The photoluminescent spectra of dichloromethane solutions (10^-5 M) of Py-OH, V-BPy, and BPy-POSS excited by light source with wavelength of 343 nm are shown in Figure 4-7. The spectra of the Py-OH monomers without excimer emission (470.7 nm) showing two strong sharp bands at 378.8 and 398.0 nm and two additional shoulder bands at 416.5 and 446.8 nm are almost the same as those of monofunctional pyrene because it is well-dispersed within solvent.^81,67,108 The spectrum of the bispyrenyl V-BPy solution shows the enthalpy-favor result with two relatively weak emission bands of pyrene monomers (open form) at 378.8 and 398.0 nm similar to Py-OH and exhibits a strong typical excimer emission (close form) at 470.7 nm.62,81,67,108-109 The emission intensity ratio of the excimer band to the monomer band (I470.7/I378.8) was calculated to be about 5.2. It is notable that the excimer emission of V-BPy close forms results from intramolecular pyrene-pyrene stacking (π-π interaction) through the thermal rotation of flexible ether linkages. In contrast, the spectrum of the BPy-POSS solution shows result of two relatively strong emission bands of pyrene monomers at 385.5 and 404.2 nm which are red-shifted about 6.5 nm from that of Py-OH or V-BPy because of the effect of POSS moiety, and additionally exhibits a medium typical excimer emission at 470.7 nm.62,81,67,108-109

The emission intensity ratio of the excimer band to the monomer band (I470.7/I385.5) was reduced to be about 0.63. Kim et al¹⁰⁸ indicated that as the size of the aggregate of pyrene-monofunctional compounds increased, the emission shifted toward the higher wavelengths in accordance with the previous experimental and theoretical works that face-to-face stacking interactions in aromatic π-systems resulted in an

increase in the intensity of the transitions and in turn led to a red shift of photoluminescence spectra.^110-111 Herein, colloidal POSS moieties tend to dynamic aggregation, inducing the change in the photoluminescence spectra that conformation isomers with excimer at 470.7 nm, monomer emission at 385.5 and 404.2 nm. The monomer emission of BPy-POSS came along with the intermolecular excimer due to the steric hindrance arising from the aggregation of bulk POSS moieties, which can be observed by DLS. One BPy-POSS possesses two pyrene rings; when one pyrene ring associates with a pyrene ring of other BPy-POSS, the other pyrene ring could be steric hindered then they can not have inter- and intra- associated with other pyrene rings, thus the monomer emission of PL spectra was observed at 385.5 and 404.2 nm.

Figure 4-8 illustrates PL spectra of thin films of V-BPy and BPy-POSS were annealed at 150 °C. In the spectrum of annealed V-BPy thin film, the monomer emission at 396.5, 418.9 and 444.7 nm appeared due to ether bond rotation; and there are still a strong excimer emissions at 473.7 nm and 500.3 nm. In the spectrum of annealed BPy-POSS thin film, the shape remains as the same as pristine thin film does, indicating that the POSS moiety hindered the rotation of ether bond and the packing of BPy-POSS is stable.

300 325 350 375 400 425 450 0.0

0.2 0.4 0.6 0.8 1.0

344.0 nm 327.6 nm

313.4 nm

(a) Py-OH (b) V-BPy (c) BPy-POSS

Absorbance (a.u.)

Wavelength(nm)

Figure 4-6. UV-Vis spectra of dichloromethane solutions (10^-5 M) of (a) Py-OH, (b) V-BPy, and (c) BPy-POSS.

350 400 450 500 550 600 650 0.0

0.2 0.4 0.6 0.8 1.0

1.2 385.5 nm 398.0 nm 378.8 nm

Monomer Emission of I and II

Excimer at 470.7 nm

Intensity (a.u.)

Wavelength(nm)

(a) Py-OH (b) V-BPy (c) BPy-POSS

Figure 4-7. Normalized emission and excitation spectra of dichloromethane solutions (10^-5 M) of (a) Py-OH, (b) V-BPy, and (c) BPy-POSS.

350 400 450 500 550 600 650 Figure 4-8. Normalized emission and excitation spectra of thin film samples of (a)

Py-OH, (b) V-BPy, (c) BPy-POSS, (d) thermal annealed V-BPy, and (e) thermal annealed BPy-POSS.

4.2.4 Dynamic Light Scattering (DLS)

Lu et al. used CS Chem 3D software (MM2 calculations) for molecular modeling to calculate the cubic size of POSS (ca. 0.53 nm); the same method was used to calculate the length of isobutyl groups (ca. 0.43 nm) (Figure 4-9). The data show that the diameter of i-Bu POSS without aggregation is about 1.4 nm. As shown in Figure 4-10, the average diameter of these aggregations is about 3.72 nm. This indicates that i-Bu POSS exists in a form of slight aggregation in the toluene solution. Comparing to i-Bu POSS, Chem 3D software calculates the diameter of bispyrenyl group about 1.213 nm, and the diameter of BPy-POSS is about 2.17 nm. The DLS data of BPy-POSS indicates that there’s a large aggregation in the toluene solution; the average diameter

of these large aggregation is about 22.4 nm (Figure 4-11). The reason of forming BPy-POSS aggregation is the aggregation of POSS colloid and the inter- and intra- π-π stacking of pyrene rings.

~1.213nm

~0.53nm ~0.43nm

Si O C (a) (b) (c)

Figure 4-9. Chem 3D MM2 calculations and 3D structures of (a) a POSS core, (b) the iso-butyl group of i-Bu POSS, (c) bispyrenyl group of BPy-POSS.

Figure 4-10. Particle distribution of i-Bu POSS by DLS.

Figure 4-11. Particle distribution of BPy-POSS by DLS.