The good overlap of the absorption and emission spectra of the BASF dyes shown in Figure 9.5 forecasts efficient energy transfer in a mixture of these dyes. This will lead to an increase of the absorption efficiency in the dye layer and, at the same time, the emission wavelength can be shifted towards the red end of the spectrum, helping improve the operation of fluorescent collectors and down-shifting.
Examples of combined absorption and fluorescence spectra for different BASF dye doped PMMA mixtures fabricated by spin coating on glass substrates are shown in Figure 9.8. These mixtures of dyes can be used in fluorescent or down-shifting collectors. The normalised absorption and
Figure 9.6 Semi-log plots of the dependence of fluorescence efficiency on the concentration of the dye in the fluorescent layer for: (a) Rhodamine 101; and for (b) Violet-V570 and Yellow-Y083.
emission spectra show efficient energy transfer between two and three dye systems, and the ability to shift to the red end of the spectrum the absorbed energy. Nearly all emission in these dye mixtures occurs from the acceptor dye in the layer with estimated energy transfer efficiency in the order of 60-80%. In collectors fabricated by spin coating, the dye concentrations are kept high in order to achieve high absorbance. Thus, the fluorescence efficiency is reduced somewhat due to quenching and possible aggregate formation. In thicker PMMA cast collectors the concentrations can be kept
Figure 9.7 Fluorescence spectra for Yellow-Y083 dye in the fluorescent layer at different dye concentrations (ppm). The monomer fluorescence spectrum is reduced with increasing concentration (arrow pointing down). The onset of the excimer emission is observed for dye layer concentration above 800 ppm and the fluorescence spectrum is shifted to the red and the fluorescence increases (arrow pointing up).
low, less than 1000 ppm, for mixture of dyes without sacrifice in fluorescence efficiency.41 Keeping the concentration low to avoid fluorescence quenching but at the same time increasing the proximity of the molecules together to achieve efficient energy transfer is a challenge in standard solution method approaches and different methods are required.
In Figure 9.9 a different system is shown consisting of two LB dye monolayers which consist of donor and acceptor molecules. The energy transfer efficiency of the dye monolayer, measured by time-resolved fluorescence, gives an efficiency of 80%.
Similar systems have been developed during the past years using LB films45 and dye-loaded zeolites10 and are finding their way in applications to collectors. Figure 9.10(a) shows a cylindrical nanochannel structure in zeolites which can accommodate individual dyes and create an artificial photonic system with efficient energy transfer between the dyes. Calzaferri7’10 has pioneered the research field in artificial light harvesting by developing hierarchically organised structures based on one-dimensional channel materials such as zeolites and mesoporous silicas. His research has produced artificially photonic antenna systems which can be used as building blocks for solar energy conversion devices such as fluorescent collectors.46
Figure 9.10(b) illustrates a schematic based on the arrangement of dye molecules (D-donor) in J-aggregates packed in a brickstone work arrangement using LB films. The optical properties of J-aggregates are dramatically
Figure 9.8 (a) Normalised absorption and emission spectra of different mixture of
dyes with efficient energy transfer at different donor : acceptor ratios deposited as a thin polymer film via spin coating: (a) V570 and Y083; (b) Y083 and O240; (c) V570, Y083 and O240; and (d) Y083, O240 and R305. The excitation wavelength was 370 nm in (a), (b) and (c) and 440 nm in (d).
different from those of their individual molecules. Because the molecules in the J-aggregate are tightly packed, their oscillator strengths are strongly coupled and as a result coherent excitons are created within the monolayer. An acceptor dye (A-acceptor) can be incorporated in the aggregate monolayer which can act as an energy acceptor. J-aggregates can mimic light harvesting arrays45 and appear to manifest efficient quenching in acceptor : donor mixing ratios as high as (1 : 10,000).47