3 A NTHRACENE D ERIVATIVES W ITHOUT S PACERS
3.4 Synthesis of new Phosphoryl Anthracenes and Fluorescence Characterizations Fluorescence Characterizations
3.4.1 Bis(dialkylamino)phosphanylanthracenes
In addition to the 9-bis(dialkylamino)phosphoryl-10-methylanthracenes presented in 3.3, related compounds and precursor molecules were synthesized which will be described in the following.
9-Bis(dimethylamino)ohosphanylanthracene (29) and its oxidation products 30-32 were prepared by lithiation of 9-bromoanthracene in diethyl ether at –15°C. After subsequent reaction with dimethylaminochlorophosphane, the volatile solvent was evaporated, the crude product was re-dissolved in DCM and the precipitated lithium chloride was removed by filtration. By evaporation of the solvent, HAnP(NMe2)2 (29) was obtained as a dark red amorphous solid (Scheme 3-9).
Scheme 3-10: Synthesis of 29-32.
Recrystallization of HAnP(NMe2)2 (29) from toluene at –30°C yielded crystals which were suitable for X-ray diffraction experiments. The highly pure crystals of 29 obtained by recrystallization were used for oxidation reactions. The first oxidation of 29 was conducted using H2O2 in a solvent mixture of MeOH/DCM 1:1 at –15°C. HAnPO(NMe2)2 (30) was obtained by removal of the volatile solvent and recrystallization from MeOH/DCM. Recrystallization also provided crystals of sufficient quality for diffraction experiments. The oxidations using elemental sulfur and selenium were both carried out in toluene at 110°C. After 6 h the solutions were filtrated and concentrated to approximately 50% of the initial volume. Crystallization from toluene gave HAnPS(NMe2)2 (31) and HAnPSe(NMe2)2 (32) as yellow crystalline solids.
The crystal structures of 29-32 are depicted in Figure 3-52, crystallographic information on 29-32 is compiled in Table 3-16.
Figure 3-52: Crystal structures of 29-32.
Table 3-16: Space groups and selected bond lengths [Å] and angles [°] of 29-32.
29 30 31 32
Space gr. P21/n Space gr. Pna21 Space gr. P21/n Space gr. P21/c P1–C9 1.8614(14) P1–C9 1.8468(14) P1–C9 1.8363(12) P1–C9 1.8295(13)
P1–O1 1.4805(11) P1–S1 1.9587(5) P1–Se1 2.1056(4) P1–N1 1.6866(13) P1–N1 1.6607(12) P1–N1 1.6572(11) P1–N1 1.6651(12)
Folding 12.2 Folding 3.6 Folding 8.9 Folding 12.3
Twist 9.1 Twist 4.2 Twist 4.0 Twist 8.5
Despite the close structural relation among 29-32, none of the compounds are isostructural and they even crystallize in different space groups with exception of 29 and 31. The P–C and P–N distances are all in the expected range and differ only marginally among 29-32. Notably, the un-oxidized HAnP(NMe2)2 (29) has the longest P–C and P–N bonds of the four compounds. As expected, the angles around the phosphorous atom indicate that the lone pair in 29 requires the most space in comparison with the chalcogens in 30-32. In 29, all angles of the phosphorus bound substituents to one another are smaller than the ideal tetrahedral angle. The sulfur- and selenium-oxidized 31 and 32 also show pronounced deviation from the tetrahedral angle, while the angles surrounding the phosphorus atom in HAnPO(NMe2)2 (30) are
close to the ideal 109.45°. The deformations of the anthracene moieties also differ significantly among the four compounds. Because the deformation is closely linked to the steric strain applied by the phosphanyl substituent, folding and twist deformations can be explained by the orientation of this substituent. HAnPO(NMe2)2 (30) exhibits the clearly weakest deformation of the anthracene moiety, in terms of folding and of twist deformation. It also shows the smallest torsion angle of the chalcogen-phosphorous bond to the anthracene plane.
Because the P–O bond is located almost in the anthracene plane, the dimethylamino substituents are located on opposite sides of this plane, which leads to a fairly even distribution of steric strain.
This explains the weak distortion of the anthracene moiety. All other compounds show distinctly stronger deformation and all feature a nearly orthogonal torsion
In contrast to the compounds presented in 3.3, which all carry a methyl group in 10-position of the anthracene moiety, the majority of 29-32 do not adapt the typical
“head-to-tail” arrangement with parallel oriented anthracene planes. Instead, an
“edge-to-face” or “herringbone” like arrangement is found in the packing plots of 29, 30, and 32. This can be attributed to the absence of a methyl group in 10-position, which generally prevents an “edge-to-face” arrangement.
Figure 3-53: superposition of 29-32, transparency decreases from 29 to 32.
Table 3-17: selected torsion angles [°] in 29-32.
29 LP–P1–C9–C9a* 74.65(11)
Figure 3-54: C-H…π stabilized “edge-to-face” arrangement in 29 (left), 30 (center) and 32 (right).
The absence of a methyl group in 10-position allows the hydrogen atoms in 4,5- and 10-position to interact with the aromatic π-systems of adjacent anthracene moieties, as depicted in Figure 3-54. The resulting C-H…π bonds stabilize this arrangement and appear to be energetically favoured compared to the optional π stacked form which would also be accessible. In all three compounds two C-H…π bonds per molecule are formed, which are listed in Table 3-18.
The fact that these are all sp2 C-H…π bonds - combined with the very short distances and fairly large angles to the π system - underlines the strength of these interactions. They are in terms of bond distance among the strongest interactions of this kind throughout this entire thesis.
The packing motif of HAnPS(NMe2)2 (31) on the other hand shows the typical “head-to-tail”
arrangement with phosphoryl substituents directed in opposite directions. This is quite surprising because 31 does not show any specific structural or geometrical features which would justify a completely different arrangement than its closely related derivatives. The π-π distance measures 3.47 Å, which is fairly short. The observed π-π overlap is ~65% which is also one of the largest overlaps of phosphoryl anthracenes encountered so far (Figure 3-55).
Table 3-18:C-H…π Interactions in 29, 30, and 32.
C-H…π Interactions [Å] / [°]
29 2.642 / 56.6 2.851 / 54.8 30 2.509 / 61.7 2.654 / 67.8 32 2.644 / 59.4 2.765 / 53.3
Figure 3-55: π-π interaction in the structure of HAnPS(NMe2)2 (31): side view (left), top view (right).
The solid state emission properties of HAnPS(NMe2)2 (31) were also investigated.
Both the broad excitation band and the single emission band which shows no vibrational structure (Figure 3-56, left) are very similar to the excitation and emission spectra of the 10-methyl substituted MeAnPS(NMe2)2 (21) and MeAnPS(NEt2)2 (23) (c.f.
2.2).
Figure 3-56: left: normalized solid state excitation (red) and emission (green) spectra of HAnPS(NMe2)2
(31); right: normalized solid state emission spectra of MeAnPS(NMe2)2 (21) (green) and HAnP(SNe2)2 (31) (red).
In comparison to its 10-methyl substituted counterpart, HAnPS(NMe2)2 (31) shows a nearly identical excitation spectrum, an identical maximum excitation wavelength of 449 nm, but a red-shifted emission maximum. (Figure 3-56, right) The emission maxima differ by 14 nm. Hence, excitation and emission maxima are farther separated for 31. While 21-28 were also structurally closely related, varying maximum emission
wavelengths could in no case be traced back to π-π interactions (c.f. 3.3) which have repeatedly been shown to induce red-shifts of emission.[42c, 50, 53-54] Although π-π overlap ranges from 0% to 35% amongst 21-28, absolutely no correlation between overlap and maximum emission wavelengths was found. For 31 and 21, however, there is a correlation.
Though the observed bathochromic shift between 21 and 31 could be thought to be caused by the absence of the methyl substituent in 10-position in the structure of 31, this can be regarded as unlikely because a second substituent in 10-position has been shown to rather promote bathochromic shifts of emission in 3.1. Thus, intermolecular interactions appear to be the most probable cause of the red-shift of emission of 31.
And indeed, 21 and 31 differ largely in terms of π-π overlap: 21 features an overlap of 35%, while 31 shows an overlap of 65%. Because the π-π distances of 3.51 and 3.49 Å are almost identical, the interaction found in the structure of 31 can be considered nearly twice as strong as the one in 21. These two compounds are the first in this thesis to confirm the consequences of π-π overlap described in literature.
Table 3-19: solid state fluorescence properties of 21 and 31.
λmax (Ex) [nm] λmax (Em) [nm] Irel.
21 449 481 1
31 449 495 0,36
Figure 3-57: maximum solid state emission spectra of HAnPS(NMe2)2 (31) (red) and MeAnPS(NMe2)2 (21) (green).
This suggests that this red-shifting effect is by no means negligible for phosphoryl anthracenes, but that it first becomes relevant and its consequences detectable from a certain degree of π-π overlap onward. For 21-28, which all exhibit an overlap of 35% or less, the effects of π-π interaction appear to be comparatively weak and are therefore outnumbered by other effects. In the case of HAnPS(NMe2)2 (31) they are strong enough to dominate this compound’s fluorescence properties.
This also becomes manifest in the measured emission intensity. Both MeAnPS(NMe2)2 (21) and HAnPS(NMe2)2 (31) feature similarly strong deformations of the fluorophore (Table 3-20).
Still the observed emission intensity of 21 is by nearly factor three higher than of 31 (Figure 3-57). Again this can be ascribed to the stronger π-π overlap of 31. This has also been repeatedly reported in literature and was also introduction of the phosphane substituent. This was achieved by selective mono-lithiation of 9,10-dibromoanthracene at –15°C and subsequent reaction with methyl iodide.
Scheme 3-11: Synthesis of MeAnP(NMe2)2 (33) and MeAnP(NEt2)2 (34).
After aqueous work-up, the obtained 9-bromo-10-methylanthracene was lithiated a second time and then reacted with the respective chlorophosphane. After removal of precipitated lithium chloride and evaportation of the solvent, MeAnP(NMe2)2 (33) and MeAnP(NEt2)2 (34) were obtained as dark red highly viscous oils (Scheme 3-10). Due to their oily texture, it was not possible to crystallize 33 and 34 even at low temperatures.
The solid state structures could therefore not be acquired.
Table 3-20: Fluorophore deformations of 21 and 31.
Folding [°] Twist [°]
21 9.9 4.0
31 8.9 2.0
While the oxidations of 33 and 34 with elemental sulfur and selenium were successful (as described in 3.3), the oxidations using peroxide were problematic. The reaction conditions used were basically identical to those of previous oxidations with hydrogen peroxide: at –15°C in a solvent mixture of MeOH and DCM. If the peroxide solution is too concentrated or added to the un-oxidized phosphane too fast, decomposition of the compound is observed.
As depicted in Figure 3-58, the phosphorus bound amino groups were cleaved from the compound and have formed ammonium ions as counter ions to the hypophosphite anion. This underlines the reduced stability of P–N bonds compared to P–C bonds. Although in this case the instability of the P–N bonds has led to decomposition of the compound, this phenomenon can also by synthetically exploited, which will be shown in 3.4.4.
Finally, also symmetrical bis(dialkylamino)phosphanylanthracene derivatives were synthesized. Related symmetrical compounds have previously been utilized for the preparation of cyclic metal complexes, which also makes this compound class chemically relevant.[71]
Scheme 3-12: Synthesis of 35, 36, and 37.
Figure 3-58: Crystal structure of decomposition product 33a.
Di-lithiation of 9,10-dibromoanthracene and reaction with two equivalents of chlorophosphane yielded the symmetrical (Et2N)2PAnP(NEt2)2 (35), which can again be converted to its oxidation products by reaction with elemental sulfur or selenium in toluene at 110°C (Scheme 3-11). (Et2N)2SPAnPS(NEt2)2 (36) and (Et2N)2SePAnPSe(NEt2)2
(37) were obtained by crystallization from toluene. Though 35-37 have been previously synthesized and 36 and 37 have also been subjected to X-ray diffraction experiments for structure determination, these compounds were re-synthesized and listed here because they are important precursor molecules of metal complexes described in 3.5.
Moreover the crystallization of 35 was accomplished for the first time. Generally un-oxidized phosphanyl anthracenes have proven to be far more challenging to crystallize than their oxidized analogues. Especially the presence of numerous aliphatic substituents further hinders the crystallization progress, because many flexible substituents in a single molecule rarely simultaneously assume fixed ordered positions, which is a precondition for obtaining crystalline materials. Hence, most un-oxidized bis(diethylamino)phosphanylanthracenes are highly viscous oils. After a considerably long crystallization time of nearly three years, crystals of 35 which were suitable for diffraction experiments were obtained. It is the first un-oxidized bis(diethylamino)phosphanylanthracene that has been crystallized so far (Figure 3-59).
Table 3-21: selected bond lengths [Å]
Figure 3-59: Crystal structure of 35, only one of two independent molecules is depicted, hydrogen atoms are omitted.
As predicted, the numerous very flexible ethyl groups all assume different orientations in the crystal structure, which induces the low symmetry of the structure.
35 crystallizes in the triclinic space group P and two independent molecules of 35 are present in the asymmetric unit. The two molecules are similar regarding their conformations and deformations of the anthracene moieties, but not identical.
Molecule 1 exhibits a folding angle of 20.4° and a twist angle of 5.9°, while molecule 2 is folded by 21.9° and twisted by 3.4°. The overall deformation is nevertheless alike and in both cases fairly strong, which is not surprising in view of the steric strain supplied by two bulky substituents. The P–C and P–N distances also only deviate minimally between molecule 1 and 2 and are in the expected range. The steric demand of the substituents bring about such large intermolecular distances in the packing motif of 35, that virtually no noteworthy π-π or C-H…π interactions are found which is quite rare.