A layer-by-layer deposition process has been carried out for two oppositely charged water-soluble perylene diimide dyes without the use of intervening polyelectrolyte layers. The strong pi-pi interactions between the perylene moieties help stabilize the layers and simultaneously diminish the fluorescence quantum yield of the array without strongly affecting the absorption or fluorescence spectra. There is an alternation of fluorescence intensity according to which perylene species is on the outer layer, which is interpreted as the effect of facile energy transfer between the perylenes.
Recently the synthesis of water-soluble and fluorescent perylene diimides has been reported (Müllen, K.; et al. Angew. Chem., Int. Ed. 2004, 43, 1528; Chem.-Eur. J. 2004, 10, 5297). We have characterized the photophysics of two of these compounds (anionic n-PDI, CAS Reg. No. 694438-88-5. and cationic p-PDI, CAS Reg. No. 817207-4-7) in pure water, dimethyl sulfoxide (DMSO), and aqueous NaCl. These studies, supported by molecular dynamics simulations, have led to the conclusion that these compounds form weakly interacting aggregated species in pure water. n-PDI and p-PDI have been incorporated in polyelectrolyte films of poly(styrene sulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDAC) following the layer-by-layer (LBL) methodology. The optical density and fluorescence intensity of the PDI-LBL films grew linearly with the number of layers, and the PDI was not extracted by subsequent polyelectrolyte deposition. The PDI fluorescence quantum yield was substantially diminished in these films, which we interpret as a self-quenching effect, enhanced by inter- and intralayer energy transfer. Energy-transfer studies to the incorporated cationic dye Brilliant Green (BG) has demonstrated that the BG resides in the same PSS-rich region as p-PDI and is largely excluded from the region that contains n-PDI (PDAC-rich).
Multilayer films of water-soluble anionic and cationic perylene diimide (PDI) moieties have been prepared using the molecular layer-by-layer method described in an earlier publication (Tang, T. J.; Qu, J. Q.; Müllen, K.; Webber, S. E. Langmuir 2006, 22, 26-28) and the fluorescence intensity compared with and without a base layer prepared using an anionic terrylene diimide dye (n-TDI), which serves as an energy-trapping layer for the PDI exciton. The fluorescence quenching data could be fit equally well to a modification of a model used by Kuhn to describe energy transfer from a J aggregate or a model developed by Kenkre and Wong to describe excitonic transfer. For both models, we obtain a characteristic energy-transfer distance on the order of 5.4 nm. Fluorescence quenching of the PDI via a single Förster energy-transfer step to the n-TDI layer is ruled out on the basis of the observed power-law dependence. We also consider a model in which the excitation is trapped at the outermost surface. This model provides a reasonable fit to the data only if the Kuhn relationship is used.
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