PLQY Self-Absorbance Correction

where y0 is the off-set from the baseline y, w is the width of the peak, xc is the centre of the peak

and A is the area of the peak. Using the area of each peak provided by the fitting, the degree of condensation of the siliceous network, C, was calculated from5:

⁄ ( ) (2.4) where %Tn is the percentage contribution of the area of Tn peak to the total area.

2.3.2.2 Fitting of FTIR Spectra

To examine the contributions from hydrogen bonding interactions associated with C=O stretching, the amide I bands (1610-1779 cm-1) of the FTIR spectra were deconvoluted. This was carried out by fitting multiple Gaussian bands to the specific region using the data analysis program OriginPro 8.5. The form of the peak is shown in Eqn. 2.3. The quality of all Gaussian fits were assessed qualitatively by comparing the cumulative fit peak (produced by summing the intensity of each of the fit peaks at a given x value) to the raw data. The quality of the fit was ensured quantitatively by confirming that the coefficient of determination, the R2 value, for each of the fits was above 0.999.

2.3.3

PLQY Self-Absorbance Correction

The measurement of PLQY of solid-state or thin film samples is more challenging than the corresponding solution measurements due to the fact that in the solid-state the PL is emitted anisotropically.6-8 This arises due to the distribution of the chromophores and corresponding emission dipole moments.7 The use of an integrating accessory has been popularised in an attempt to overcome this challenge.7,8 A schematic of the integrating sphere experimental set-up compared to the conventional front-face set-up used for solid-state luminescence is shown in Fig. 2.1.

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Figure 2.1. Schematic representation of the experimental set-up for (a) conventional front-face emission measurements of solid-state samples, and (b) the empty integrating sphere and (c) the integrating sphere containing a sample to be measured.

Illumination with monochromatic light at a selected wavelength is directed into the integrating sphere through a small entrance hole. The incident light strikes either the sample or the highly reflective Spectralon coating of the sphere, at which point it is emitted/reflected back into the sphere. A coated baffle prevents light from the excitation source being reflected directly to the detector. The geometry and reflective coating of the sphere ensures that emission in any direction from the sample is reflected around the sphere and eventually to the detector port.

For films with high refractive index and optically thick samples such as the di-ureasils measured in this thesis, waveguiding and reabsorption of the emitted photons may lead to further errors in the measured PLQY. Thus, PLQY measurements were performed using an integrating sphere, following the procedure reported by de Mello et al.7 and following this the measurements were corrected for self-absorption following the method put forward by Ahn et al.6 This approach takes into account the initial anisotropy of the emission and also any absorption and re-emission processes that occur to establish the true PLQY. Reabsorption and distortion of the measured spectrum can only occur in a region in which there is overlap between the absorption and emission spectra. Therefore, it is assumed that at the red-edge of the emission spectrum no spectral reshaping occurs. Thus, the shape in this region of spectrum measured in the integrating sphere matches the shape of the red edge of the true spectrum, F(λ) (obtained by measuring the photoluminescence for a sample whose spectrum is not distorted by self–absorption). Hence, the long wavelength emission is described by:

( ) _{( )} ( ) ( ) (2.5)

where F(λ) is the photoluminescence spectrum in photons per wavelength, normalised to

∫ ( ) and a is the probability of self-absorption of an emitted photon. The factor α represents an empirical factor that scales the true spectrum, F(λ), to an enhanced spectrum Fʹ(λ) whose red-edge is matched to that of the observed photoluminescence spectrum Fobs(λ).

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∫ ( )

∫ ( )

(2.6)

which provides the expression for the true photoluminescence quantum yield (PLQY):

( )

(2.7)

In an ideal situation, the true photoluminescence spectrum, F(λ), should be obtained by measuring a dilute solution of the lumophore ensuring that self-absorption does not occur. However, in CP doped di-ureasil systems, both the CP and the di-ureasil are photoluminescent and thus, no solution state equivalent exists. To overcome this, the true spectrum for each series is taken to be that of the lowest wt% sample measured in the front-face configuration (not in the integrating sphere) as in theory this sample should present minimal reabsorption.

This assumption was tested by comparison of the solution phase spectrum of the polymers with the lowest wt% sample measured in the front-face configuration; see Fig. 2.2 for DI-PBS-2.0 as an illustrative example.

Figure 2.2. Area-normalised solution phase emission spectrum for PBS-PFP in 25:75% v/v 1,4- dioxane/water (green dash line), the area normalised integrating sphere (IS) photoluminescence spectrum (Fobs(λ)) (black line), the area-normalised front-face (FF) photoluminescence spectrum (F(λ)) (red line) and

the scaled unattenuated photoluminescence spectrum (Fʹ(λ)) for DI-PBS-2.0. The scale factor, α, is adjusted so that Fʹ(λ) matches Fobs(λ) at λem = 500 nm and longer.

400 450 500 550 600 650 0.000 0.004 0.008 0.012 0.016

Int

en

sity

(a.u.)

Wavelength (nm)

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