Solubility Stokes shift

HPTS is a highly water-soluble, pH-sensitive dye with a pK of 7.5 in aqueous solution [8], When in alkaline medium, pH > 7.5, acid-base equilibrium is totally displaced toward the anion form (3sPyO ) of the dye. The electronic character of 3sPyO remains unchanged after photo-excitation, and corresponds to a singlet-excited state [9], Fluorescence from this state undergoes a fast 0.4 ps Stokes shift and has a maximum at 515 nm and a lifetime of 5.3 0.1 ns [10],... 

Like many other pigment classes, DPP pigments fluoresce in solution. The Stokes shifts are in the range 10-15 nm and the fluorescence quantum yields ca. 60 In the solid state, some DPPs show a minor fluorescence. Through N substitution, both the solubility and the Stokes shift can be increased. 

As a direct consequence of N-alkylation excluding the possibility of intermolec-ular hydrogen bond formation in the solid state, the solubilities of DPPs were drastically raised upon N-substitution (e.g., solubility in DMF at 25 °C 100 mg/1 for 2, whereas 3300 mg/1 for 27). Most of the N-substituted DPP derivatives fluoresced strongly, and with appropriately substituted derivatives Stokes shifts of up to 70 nm at fluorescence quantum yields of up to 95% have been reported (see Section 11.3.2). 

The microscopic polarity in the interior of micellar aggregates is of intrinsic interest in surfactant research. A well-known approach to this information is the use of solvatochro-mic dye probes. For these chromophores, the wavelengths of absorption and emission (and hence the Stokes shift) depend on the environment in which they are dissolved. A large variety of dye molecules exhibiting this property has been tested with surfactant systems. However, inspection reveals that most of these dyes contain ionic groups and, therefore, are either water-soluble or tend to bind to the hydrophilic exterior of micellar aggregates. 

Shishino, Y., Yonezawa, T., Kawai, K Nishihara, H. (in press). Molten matrix sputtering synthesis of water-soluble luminescent Au nanopartides with a large Stokes shift, Chem. Commun. 

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