Solid State Structures

The crystal structures of ClAnBIPC2 (59) and BrAnBMes2 (60) were also determined.

Despite considerable efforts, no crystals of ClAnBCat (58) of sufficient quality could be obtained. ClAnBIPC2 (59) crystallizes in the orthorhombic space group P212121, and the asymmetric unit contains one molecule of 59. All B–C-bonds are nearly identical at 1.58 Å, which is a common bond distance between carbon an boron.

Figure 3-100: Solid state structure of ClAnBIPC2 (59), hydrogen atoms are omitted for clarity.

The angles around the boron atom almost perfectly match the expected 120° angle.

Surprisingly, none of the six methyl groups show rotational disorder. Despite the bulky substituents at the boron atom, the anthracene moiety is weakly distorted, showing folding and twist angles of only 2.6° and 1.5°. This can be attributed to the nearly symmetrical alignment of the substituents on opposite sides of the anthracene plane (Figure 3-100, left). The torsion angle of the B1–C15-bond to the anthracene plane is nearly right-angled at 86.0°. The boron atom in 10-position and the chlorine atom in 9-position are only minimally displaced from the C9^…C10 vector.

The steric demand of the boron bound substituents is so large that it prevents the molecules from coming close to one another in the packing motif of 59. Even the typical “head-to-tail” orientation found for most anthracene derivatives is inhibited.

Thus, there is no π-π-overlap, because the fluorophores are so far apart. The only interaction found is an aromatic C–H^…π bond from hydrogen atom in 3-position to a peripheral ring of the adjacent anthracene moiety. It measures 2.742 Å while enclosing an angle of 67.2° with the ring plane (Figure 3-100, right) which can be considered comparatively strong due to the fairly short distance and the steep angle.

Table 3-36: Selected bond lengths [Å] and angles [°] of 59.

B1–C10 1.582 (4) B1–C15 1.580 (4) C10–B1–C15 120.0 (3) C10–B1–C25 119.7 (3) C4a–C10–B1–C15 86.0(3)

Folding 6.0

Twist 1.5

Figure 3-101: Left: nearly orthogonal orientation of the boron bound substituents to the anthracene plane in 59 (hydrogen atoms are omitted for clarity); right: C–H^…π interaction between two anthracene

moieties in 59 (boron bound substituents and hydrogen atoms in 5-8-position are omitted for clarity).

BrAnBMes2 (60) also crystallizes in the orthorhombic space group P212121. In contrast to 59, here the asymmetric unit contains two independent molecules which differ in their geometries. They will be referred to as molecule 1 and molecule 2. While the bond distances and angles surrounding the boron atom in molecule 1 and 2 are very similar and in the expected range, the deformation of the fluorophore is clearly stronger in molecule 1. This is reflected by the folding angle of 10.1°, which is nearly twice as large as in molecule 2 (5.4°). The measured twist angles of the anthracene moiety are nearly identical for both molecules (4.6° / 4.8°). Furthermore the boron atom is located notably outside the C9^…C10 axis in molecule 1, assuming a position almost 0.3 Å above the anthracene plane. In contrast to ClAnBCy2 (59), the torsion angles of the boron bound substituents to the anthracene plane are far from orthogonal in both molecule 1 and 2. They measure 47.9° and 126.4°, respectively. This leads to unbalanced steric strain applied by the mesityl groups, which again leads to distinctly stronger deformation of the fluorophore than observed for 59.

Unlike 59, the packing motif of BrAnBMes2 (60) produces a multitude of intermolecular interactions. Though the steric demand of the mesityl substituents also prevents a “head-to-tail” orientation of the molecules and therewith hinders π-π-overlap, there are several C–H^…π interactions present in the solid state structure of 60.

The mesityl substituents offer numerous aromatic and aliphatic hydrogen atoms for interactions of this kind. A total of three C–H^…π interactions originating from aromatic anthracene protons are found (Table 3-39). The clearly weakest is an offset face-to-face interaction (Figure 3-102, left), which has nearly parallel orientation and has a distance of 3.26 Å, which makes it one of the shortest face-to-face interactions found throughout this entire thesis. Two more sp² type C–H^…π bonds are found between the hydrogen atoms in 1,2-position of molecule 2 to an adjacent π-system. (Figure 3-102, center) These can be considered fairly strong due to distances/angles of 2.812 Å/56.9°

and 2.920 Å/52.5°, respectively.

Figure 3-102: Left: solid state structure of BrAnBMes₂ (60), molecule 1 (hydrogen atoms are omitted for clarity); right: superposition of molecule 1 and molecule 2 (dashed), hydrogen atoms are omitted for clarity.

Table 3-37: Selected bond lengths [Å] and angles [°] of molecule 1

Table 3-38: Selected bond lengths [Å] and angles [°] of molecule 2

B1–C9 1.592 (5) B1–C9 1.580 (5)

B1C15 1.575 (5) B1C15 1.579 (5)

C9B1C15 116.9 (3) C9B1C15 118.4 (3)

C15B1C24 121.0 (3) C15B1C24 121.0 (3)

Folding 10.1 Folding 5.4

Twist 4.8 Twist 4.6

Figure 3-103: Aromatic C-H^…π interactions in the structure of BrAnBMes2 (60). Left: parallel C-H^…π interaction between molecule 1 and 2 (mesityl substituents are omitted for clarity); center: C-H^…π interactions of the hydrogen atoms in 2,3-position (mesityl substituents are omitted for clarity); right:

C-H^…π interaction of a mesityl substituent (residual mesityl groups are omitted for clarity).

Additionally, a medium strength sp³ C–H^…π interaction is found which measures 2.755 Å while enclosing a steep angle of 81.6° with the ring plane. The proposed strongest C–H^…π bond is found between an aromatic proton of a mesityl substituent and a peripheral ring of the anthracene moiety of molecule 1. Though it is not the shortest interaction at 2.889 Å, the angle of 82.7° is close to the optimum orthogonal orientation (Figure 3-102, right). The pronounced differences in geometry and intermolecular interactions should also influence the solid state fluorescence phenomena of 59 and 60, as will be shown in 3.6.4.

Furthermore, compounds 58-60 were also crystallized from solvent mixtures containing the donors used for the in-solution experiments in 3.6.2 (MeCN, NEt3, TMEDA, THF, PPh3). This way it was intended to monitor the adduct formation in solid

Table 3-39: C-H^…π interactions in the structure of 60.

state by co-crystallization of the respective donor and potentially alter the solid state fluorescence properties. In practice, it was only possible to isolate crystals of either the mere boranylanthracenes or of decomposition products (Figure 3-103).

Figure 3-104: Decomposition products of ClAnBIPC₂ (59) (left), and of ClAnBCat (58) (right, bottom).

This shows the sensitivity of ClAnBCat (58) and ClAnBIPC2 (59) towards hydrolysis. Even small amounts of moisture contained in the assumed anhydrous donors were sufficient to hydrolyze significant amounts of the boranyl anthracenes. Also traces of lithium chloride originating from the salt elimination during synthesis appear to promote decomposition of 58 and 59.

Though lithium chloride was removed by filtration, very small particles are capable of passing through the filter, which explains traces of lithium salt in the products. The only compound that stayed intact independent of the donor bases added was BrAnBMes2 (60). This underlines the stability of aromatic substituted anthryl boranes compared to aliphatic or heteroatomic substituted derivatives, which will be taken up in 3.6.5.

3.6.4 Solid State Fluorescence

The air sensitive solid state samples of 58-60 were prepared in an argon glove box and measured immediately after removal from the inert gas atmosphere to avoid oxidation or decomposition.

Figure 3-105: Left: normalized solid state excitation (red) and emission (green) spectra of ClAnBCat (58);

right: normalized solid state excitation (red) and emission (green) spectra of ClAnBIPC2 (59).

Compared to the in-solution fluorescence, both excitation and emission maxima of 58-60 are clearly red-shifted (Figure 3-104). All compounds show a broad excitation band of over 100 nm width, which has similarly been observed in the solid state fluorescence of phosphanyl and phosphoryl anthracenes (c.f. 3.2/3.3). The difference to in-solution measurements is particularly pronounced for ClAnBIPC2 (59) which exhibits two narrow and well separated excitation maxima in solution. Several other tendencies comply well with the findings of previous in-solution experiments.

Figure 3-106: Solid sample of BrAnBMes2 (60) in daylight (left) and under exposure to UV light (λ = 366 nm) (right).

Table 3-40: Maximum excitation an emission wavelengths of 58, 59, and 60.

λmax (Ex) [nm] λmax (Em) [nm]

ClAnBCat (58) 525

ClAnBIPC2 (59) 413 463

BrAnBMes2 (60) 467 509

Figure 3-107: normalized solid state excitation (red) and emission (green) spectra of BrAnBMes₂ (60).

E.g. ClAnBIPC2 (59) solely shows a band structure in its emission spectrum, as it did in solution. Furthermore ClAnBCat (58) exhibits the largest gap between excitation and emission maximum – in this case of nearly 100 nm – as observed in solution. In terms of emission, ClAnBIPC2 (59) shows the maximum of shortest wavelength. This also confirms the results acquired in solution. In contrast, ClAnBCat (58) features a distinctly farther red-shifted emission maximum than BrAnBMes2 (60), which was reversed for in-solution experiments. The difference in maximum emission wavelengths between ClAnBIPC2 (59) and BrAnBMes2 (60) of 45 nm cannot be explained by differences in π-π overlap, since neither of the two compounds feature such interactions in their packing motif. Hence, other factors must account for this phenomenon.

Irel.

ClAnBCat (58) 0,37 ClAnBIPC2 (59) 0,18 BrAnBMes2 (60) 1,0

Figure 3-108: Solid state maximum emission spectra of ClAnBCat (58) (blue), ClAnBIPC₂ (59) (green), and BrAnBMes2 (60) (red).

The differences in emission intensity are just as striking. BrAnBMes2 (60) shows the clearly strongest emission intensity, followed by 58 and 59 (Figure 3-107). Taking into account the structural properties illustrated in 6.3.4, the observed intensities can be evaluated. Due to the lack of structural information on ClAnCat (58), this compound will be excluded from argumentation. The structural information is summarized in Table 3-41. Because the structure of BrAnBMes2 (60) contains two independent molecules the specified folding and twist angles are averaged values of both molecules.

Table 3-41: Summarized structural information on ClAnBCy2 (59) and BrAnBMes2 (60).

Folding [°] Twist [°] C–H^…π interactions [Å]/[°] Irel

ClAnBIPC2 2.6 1.5 1 x sp²: 2.742/67.2 0.18

BrAnBMes2 7.5 4.7 3 x sp²: 2.812/56.9; 2.920/52.5;

2.889/82.7 1 x sp³: 2.755/81.6 1.0 The weak emission intensity of ClAnBIPC2 (59) compared to BrAnBMes2 (60) intuitionally suggests a strong deformation of the anthracene moiety in 59. In fact, the opposite is the case. The weakly fluorescent 59 shows a by factor three smaller deformation of the fluorophore than the strongly fluorescent ClAnBMes2 (60). As stated earlier, the often cited effect of π-π overlap induced quenching is irrelevant here because neither of the packing plots exhibit such interactions. Again C–H^…π interactions are the only possibility of explaining this apparently contradictory result.

While ClAnBIPC2 (59) only forms a single C–H^…π bond, there are four such interactions present in the structure of BrAnBMes2 (60). Especially the aromatic interactions which are oriented almost orthogonal to the π system can be considered particularly strong and are similar to those found in SPAnPS@tol (15), which have been shown to generate a strongly fluorescent material by formation of a T-shaped exciplex. Thus, number and strength of the C–H^…π interactions on hand outnumber the effect of fluorophore deformation.

Figure 3-109: Solid state emission spectra of SPAnPS@tol (15, red), BrAnBMes2 (60, green), and ClAnBCat (58, blue).

The strong fluorescence of these compounds is further confirmed by comparison to SPAnPS@tol (15), which is one of the compounds with the strongest observed solid state fluorescence (Figure 3-108). Especially BrAnBMes2 (60) comes very close to 15 in terms of emission intensity proving the emission enhancing character of the C–H^…π bonded arrangement described above.