Real Time Evolution and Quantification of the PF β-phase

To further elucidate the mechanism for the formation of the β-phase in the hybrid matrix, the emission and excitation spectra and the pictures were collected for DU-PF-0.01 along the course of the gelation process (Fig. 3.12). The PF chains dissolved in the sol mixture (t = 0) adopt the α-phase conformation. Upon addition of the gelation regents (EtOH, HCl and H2O), the

condensation of the silica network during the sol-gel reaction induces the formation of the β-phase. This is confirmed by an increase in the intensity of the characteristic peak of the β-phase centred at 435 nm in the excitation spectrum, and by the red-shift and the growth of a new peak centred at 480 nm in the emission spectrum. Both peaks increase in intensity with time, reaching their maximum as the condensation reaction reaches completion. The evolution of the β-phase over time is consistent with the transition from a fluid to a more confined environment for PFO-OH as the sol−gel transition proceeds, which promotes π−π stacking of the conjugated backbone. Similar behaviour has been observed in the literature, where the gelation process of PFO in a methylcyclohexane (MCH) solution (concentration of 1.0 wt. %) was induced by aging the sample at 20 °C.45 Since MCH is a poor solvent for PFO, the chains of the CP already start to interact after 10 min of aging.

Figure 3.12. (a) Emission ( ex = 360 nm) and (b) excitation ( em = 500 nm) spectra and (c) pictures of DU-

PF-0.01 recorded as a function of time after initiation of the sol-gel process (t = 0 → 143 h).

The interactions between PFO macromolecules increase in intensity and number with time, to the point at an aging time of 2160 min, the original PFO/MCH solution had turned into a highly viscous gel.

In an attempt to quantify the contribution of the β-phase to the optical properties of each PF-grafted ureasil hybrid, Gaussian multi-peak fitting was performed on the excitation spectra. The final excitation spectrum of each PF-functionalised ureasil hybrid arises from three contributions: (i) PFO-OH adopting the β-phase, (ii) PFO-OH adopting the α-phase, and (iii) either the DU-PF- 0 or the TU-PF-0 matrix. To take into account all of these contributions, Gaussian fits were first performed on the excitation spectrum of PFO-OH in solution (Fig. 3.13a) and on the blank ureasil (Fig. 3.13b). The obtained fits results were then used to provide initial values for the peak centres to fit the DU-PF-x and TU-PF-x excitation spectra. The fits for all samples can be found in the Appendix (Fig. A3.5).

Figure 3.13. Area-normalised excitation spectrum ( em = 480 nm) and corresponding Gaussian fits of the

spectral components associated with the vibronic modes of PFO-OH, where νn refers to the vibrational

mode, n, and the blue and purple components of the ureasil of (a) PFO-OH (10-6 mol dm-3, THF), (b) DU-

PF-0 and (c) DU-PF-0.01. The shaded area corresponds to the pure β phase component. (d) % β-phase contribution (open symbols) and photoluminescence quantum yields (solid symbols) as a function of the PFO-OH wt% for the DU-PF-x and TU-PF-x series. The solid and dashed lines serve only to guide the eye.

The excitation spectrum of PFO-OH in the isolated α-phase conformation (Fig. 3.13a) can be resolved into four components: the 0-0 electronic transition (~3.1 eV) and other three peaks (~3.3, ~3.4 and ~3.8 eV) which can be ascribed to the higher order vibronic contributions.46 Analysis of the excitation spectrum of DU-PF-0 (Fig. 3.13b) confirms that its emission arises from the combination of the purple component (~3.5 eV) and the blue component (~3.2 eV) as discussed previously.42

The components for the PF-grafted hybrids (Fig. 3.13c and A3.4) were assigned as follows: (i) the β-phase peak (~2.9 eV), (ii) the α-phase 0−0 electronic transition at ∼3.1 eV; (iii) and (iv) are overlapping contributions from the ν1 vibronic transition of the α-phase (∼3.3 eV) and

the ureasil emission (∼2.9−3.4 eV); and (v) higher order vibronic bands (∼4.2 eV) associated with the PF. We note that, while for pure PF-based systems the content of the β-phase is generally

assessed by subtraction of the absorption spectrum of the mixed-phase solution/thin film from the spectrum of the same sample adopting a pure α-phase, the strong overlap between the components of the ureasil and those of the PF prevent us for isolating each contribution in the excitation spectrum of the PF-grafted hybrids in this case. However, as previously shown by Knaapila et al.,47 Gaussian fits enable us to isolate the peak corresponding to the β-phase and quantify its relative contribution to the global excitation spectrum. Similar results were obtained for samples in the entire DU-PF-x and TU-PF-x series (Fig. A3.4) and the percentage of β-phase in each sample is shown in Fig. 3.13d.

For both sets of samples, the amount of β-phase initially increases with the concentration (1.9% to 5.1% and 2.5% to 7.5% for TU-PF-x and DU-PF-x, respectively). As the concentration of the CP is further increased, the percentage of β-phase moderatly decreases to 4.1% and 4.6% for DU-PF-0.1 and TU-PF-0.1. Fig. 3.13d also presents the photoluminescence quantum yields (ΦPL)

for the samples as a function of concentration. The ΦPL of ureasil is 1.5 ± 0.2% and 3.0 ± 0.1% for

DU-PF-0 and TU-PF-0 respectively, in good agreement with the literature,37, 48 while for PFO-OH in THF, the ΦPL is 85.3% (± 2.2%), comparable to the values reported for PFO in a dilute solution

of a good solvent.49 As mentioned above, the general decrease in intensity of the 0-0 emission peak indicates the presence of self-absorption. The values of ΦPL were therefore corrected from this

effect using a procedure previously described in literature (see details in Chapter 2).50 For both di- ureasil and tri-ureasil hybrids, the incorporation of the CP results in a drastic increase in the ΦPL at

any of the investigated concentrations, ranging from 56.8 to 66.4% for DU-PF-x and from 80.2 to 82.6% for TU-PF-x respectively. These values indicate that the PF chains are homogeneously distributed in the ureasil matrices, which inhibits the undesired aggregation phenomena which often leads to a generalised quenching of the emission for CPs incorporated in a host material in the solid state.51 The lowest value of ΦPL was measured for DU-PF-0.05 (56.8 ± 1.2%), to which

corresponds the highest percentage of β-phase (7.5%). It has previously been shown for PF films that a high β-phase contribution can lower the ΦPL due to the formation of fluorenone chemical