The fluorescence spectra and intensities of fluorescent samples are dependent on the optical density of the samples and the geometry of fluorescence detection.34 Of particular importance is fluorescence quenching at high dye concentrations, and the fluorescence emitted by the collectors provides a good vehicle how this quenching can be quantified. A simple method is to observe the fluorescence intensities at the red end of the spectrum where we can ensure that the re-absorption is negligible.42 At that wavelength range the fluorescence intensity is proportional to concentration except for any excitation energy loss due to quenching. Thus, a plot of the fluorescence intensity per absorption efficiency as a function of concentration should be constant, with any deviation observed due to quenching effects. Examples of these plots are shown in Figure 9.6.
Figure 9.6 shows fluorescence quenching plots for Violet (V570), Orange (O240) and Rhodamine 101 dyes. In the case of Rhodamine 101 there is a rapid drop in the fluorescence efficiency for concentrations higher than 7 x 10~4 M. For the BASF dyes V570 and O240, the plots show a decrease in fluorescence efficiency when the concentration reaches around 1000 parts per million (ppm). The decrease in fluorescence efficiency occurs because, as the dye molecules come closer together with increased concentration, they
Figure 9.4 Normalised absorption and emission spectra of selected BASF Lumogen organic dyes embedded in PMMA.
can transfer their excitation energy to each other which greatly increase nonradiative relaxation losses. In a recent study the Lumogen F dyes have shown to have very high fluorescent yields (near 100%) in plastic plate luminescent collectors and with no significant fluorescence concentration quenching, even at concentrations as high as 1000 ppm.41 In some instances, however, thin film collectors (fabricated, for example, by spin coating) require high dye doping in order to achieve high absorbance values which inevitably results in fluorescence quenching.
Increasing further the concentration of the dye in the polymer film can result in the presence of aggregates. The fluorescence spectra of the yellow dye (Y083) at high concentration show a significant change on their spectral shape which is indicative of the formation of aggregates (Figure 9.7). This is typical behaviour of excimer (excited state dimer) formation observed in perylene dyes.43 The emission spectrum of the Y083 dye changes drastically above 800 ppm signifying the onset of aggregate formation in the layer. Excimer emission is red-shifted with respect to the absorption spectrum of
Figure 9.5 Normalised absorption and emission spectra of BASF dyes (donor : acceptor) spin coated PMMA films: (a) V570 and Y083; (b) Y083 and O240; (c) O240 and R305; and (d) Y083 (excimer emission) and R305.
the dye leading to a large Stokes shift and hence a decrease in re-absorption. Excimers can have high fluorescence quantum yields, which makes them potentially a useful candidate for application in fluorescent collectors.44