Titration Experiments

To fathom whether the presence of metal cations affects the fluorescence properties of phosphanyl- and phosphorylanthracenes in solution, selected compounds of 1-14 were subjected to titration experiments in which the emission of the respective compound was monitored as a function of the applied cation concentration. To account for the low sample concentrations and the comparatively weak donor strength of the monodentate 1-14, a non-donating solvent was chosen to assure interaction between the investigated compounds and the metal ions. Donating solvents would most likely exclusively coordinate the cations due to the enormous excess of solvent molecules compared to dissolved phosphoryl anthracenes. Therefore again DCM was chosen as a solvent. The poor solubility of most metal salts was of course a limiting factor, which was overcome by the discovery that zinc bromide is well soluble in DCM. Hence, this solvent/cation combination was chosen for the following experiments.

10^-4M solutions of HAnPSⁱPr2 (2), HAnPPh2 (4), HAnPSPh2 (5), HAnPSePh2 (6), MeAnPiPr2 (11), and MeAnPPh2 (13) in DCM were prepared and each was titrated with 1 mM ZnBr2 solution until a threefold excess of ZnBr2 was reached.

Figure 3-15: Left: emission of 2 upon titration with ZnBr2 at λEm = 438 nm (blue) and λEm = 485 nm (red); right:

stacked emission spectra of 2 upon titration with ZnBr2.

For HAnPSⁱPr2 (2), which had shown a fairly narrow emission maximum ant short wavelength compared to other sulfur oxidized compounds, the addition of Zn²⁺ leads to a broadening of the emission maximum and a notable increase of the formerly weak

0,00E+00

emission (Figure 3-15). The increase of emission is strong up to and addition of ca. 1.5 equivalents of ZnBr2 and then gradually weakens. The red-shift of the emission maximum from 438 nm to 485 nm is also remarkable, which is illustrated by the run of the curves in Figure 3-15 (left) which show the change of emission intensity for two different emission wavelengths which correspond to the maximum emission wavelengths before and after the titration. Though overall still comparatively weakly fluorescent, the addition of Zn²⁺ appears to inhibit a fluorescence quenching mechanism and also appears to alter the electronic properties of the compound.

Figure 3-16: Titration of HAnPPh₂ (4) (10^-4M in DCM) with ZnBr₂, λ_Em = 456nm.

The titration of HAnPPh2 (4) with ZnBr2 triggers a similar increase of emission intensity as observed for 2 (Figure 3-16). This increase of emission is in accordance with the reports of Yip et al. who observed an increase of emission of their closely related 9,10-bis(diphenylphosphanyl)anthracene upon complexion of gold(I),^[65] which they assigned to an inhibition of electron transfer between the phosphane and the fluorophore by binding of gold(I). A minor shift of the emission maximum by 5 nm is also found. The slight decrease of emission intensity at the beginning of the titration can be explained by the formation of local concentration maxima around the metal ions which are present at a much lower concentration than the phosphorylanthracene molecules at the beginning of the experiment. The electrostatic attraction of cations and donors leads to accumulation of molecules around the ions. In these areas of

higher concentration the collision factor is therewith also higher, which probably leads to the observed quenching.

Figure 3-17: Titration of MeAnPⁱPr2 (11) with ZnBr2.

Surprisingly, MeAnPⁱPr2 (11) showed completely different behavior upon titration than the previously investigated compounds. The initially fairly strongly fluorescent compound exhibits a strong decrease of emission intensity up to an addition of ca. 0.5 equivalents of Znr2 (Figure 3-17). From that point onward the emission stays at a

The behavior of HAnPSPh2 upon titration with ZnBr2 is again very similar to that of HAnPSⁱPr2 (2), showing a strong emission increase (Figure 3-18). The observed red-shift of emission is on the other hand much weaker than for 2.

The observed phenomena differed strongly among one another, which makes the deduction of a clear tendency tedious. While both sulfur oxidized compounds showed an increase of emission intensity, the un-oxidized compounds exhibited contrary behavior. The increase of the emission of HAnPPh2 (4) confirms the reports of Yip et al., while the strong quenching of MeAnPⁱPr2 (11) does not appear conclusive.

Nevertheless these experiments have shown that phosphanyl- and phosphorylanthracenes show sensitivity towards the presence of metal ions which is a finding which will find further application in the course of this thesis.

3.2 9,10-Bis(diphenylthiophosphoryl)anthracene (SPAnPS)

One of only few known anthracene based materials which exhibit strong solid state fluorescent was reported by Fei et al. in 2003.[44b, 44c] Their 9,10-bis(diphenylthio-phosphoryl)anthracene (SPAnPS) was the first reported anthracene derivative which showed sensory properties in the solid state. By intercalation of toluene into the solid state structure in a host/guest complex, a nearly T-shaped exciplex between co-crystallized toluene molecules and the anthracene fluorophore is formed, which exhibits intense solid state fluorescence. When the lattice solvent is removed under heating and reduced pressure, the observed emission is virtually completely quenched.

This way, Fei et al. were able to show the importance of the lattice solvent as a crucial factor for the occurrence of solid state fluorescence.

The work on this compound and its intercalation structures was continued by Schwab,^[59b] who succeeded in crystallizing SPAnPS from various aromatic solvents. The resulting intercalation structures were also characterized structurally, but solid state fluorescence was only vaguely monitored by visual examination under exposure of the compounds to UV-light at a fixed wavelength of 366 nm. Accurate fluorescence experiments were not conducted. Also, neither Fei nor Schwab have quantified structural properties of their compounds such as deformation of the fluorophore or intermolecular interactions. Hence, the alignment of structural features and observed solid state fluorescence was for the most part not possible.

Though Fei and Schwab recognized the presence of the lattice solvent within the structure as a requirement for solid state fluorescence, they were not able to identify whether the direct interaction of solvent molecules with the fluorophore or the conformation of the SPAnPS molecule resulting from the intercalation of solvent were the vital factors for strong emission. Fei et al. came to the conclusion that the excited dimer which is formed by C–H^…π bonding between the α-hydrogen atom of the intercalated toluene molecule and the fluorophore is crucial (Figure 3-19)^{[44b, 44c]}, while Schwab concluded that the transoid orientation of the P=S-bonds in the SPAnPS molecule is the predominant factor.^[59b] Moreover, the method used by Fei et al. to prove the dependency of solid state fluorescence on presence of toluene lattice solvent is problematic. By evaporating the solvent from the crystals under reduced pressure, the entire crystalline arrangement and packing motif is destroyed.

Crystallization of the SPAnPS molecule without lattice solvent was not achieved, which would provide a valid comparison of an equally ordered crystalline arrangement.

The preparation of new intercalation structures of SPAnPS, determination of the resulting solid state structures, quantification of structural features and assignment of measured solid state emission are therefore important steps in understanding of solid state fluorescence of SPAnPS.