In-solution Fluorescence

Even if only exposed to daylight, solutions of BrAnBMes2 (60) reveal a green-blue fluorescent glow, which underlines the conspicuous fluorescent behavior of this compound class (Figure 3-95).

Figure 3-96: Left side: DCM solution of BrAnBMes₂ (60), under exposure to daylight (far left), fluorescent glow highlighted with an arrow; under irradiation with UV light, λ = 366 nm (mid. Left). Right side: DCM

solution of BrAnBCat (58); under exposure to daylight (mid. right), under irradiation with UV light, λ = 366 nm (far right).

Figure 3-97: Left: normalized excitation (red) and emission (green) spectra of ClAnBCat (58); right:

emission spectra of ClAnBCat (58) at 274 nm (cyan), 359 nm (blue), 379 nm (green), and 396 nm (red).

This first impression is confirmed by fluorescence experiments which certify the strong fluorescence of ClAnBCat (58). Though the emission spectrum shows a small shoulder at 425 nm, a defined band structure is not visible, very much like observed for the majority of alkyl- and arylphosphane substituted compounds inspected before.

The excitation spectrum exhibits a single broad band, and the excitation and emission maxima are well separated by ~50 nm. The Emission maximum is also red-shifted by nearly 50 nm compared to unsubstituted anthracene (Figure 3-96).

Figure 3-98: left: normalized excitation (red) and emission (green) spectra of ClAnBCy2 (59); right:

normalized excitation (red) and emission (green) spectra of BrAnBMes2 (60).

When changing from strongly electron withdrawing oxygen substituents at the boron atom as in ClAnBCat (58) to alkyl substituents in ClAnBIPC2 (59), these drastic differences become apparent in the fluorescence spectra (Figure 3-97, left).

Accordingly, excitation and emission spectra of ClAnBIPC2 (59) differ significantly from

those of 58. The excitation spectrum now shows two maxima, which are separated by over 100 nm.

Also the emission spectrum features a typical anthracene band structure and the emission

Moving on to BrAnBMes2 (60), the boron bound substituents are now aromatic, and hence of clearly more electron withdrawing character than the aliphatic substituents in (59). This alteration is again visible in the fluorescence properties of 60. Both the excitation and emission spectrum bear resemblance to those of ClAnBCat (58), showing a single broad excitation band and a broad emission band lacking vibrational structures. The emission maximum at 455 nm is red-shifted even farther than the emission maximum of 58. The only serious difference lies in the distance between excitation and emission maximum, which adds up to only 36 nm for 60.

Derivable tendencies are that boron bound +I-alkyl substituents lead to emission properties similar to un-substituted anthracene and to a small gap between excitation and emission maxima. This gap increases the stronger the electron withdrawing character of the substituents becomes (59 < 60 < 58). In comparison to a corresponding phosphanyl anthracene which also carries a halogen substituent in 9-position, the fluorescence phenomena of ClAnBCat (58) and BrAnBMes2 (60) can be regarded as similar in respect to the resulting emission wavelengths (Figure 3-98). In terms of emission intensity, the boranyl anthracenes produce largely stronger fluorescence than their phosphane substituted counterparts. This finding can explained by the fact that there is a clearly smaller probability of electron transfer from the electron deficient boranyl substituent to the excited fluorophore than from the electron rich phosphanyl substituent. Hence, the observed quenching in solution is stronger for compounds carrying the latter substituent.

Figure 3-99: Normalized emission spectra of ClAnBCat (58, cyan), ClAnBIPC2 (59, blue), BrAnBMes2 (60, green), and BrAnPPh2 (9, red).

In order to function as a potential sensor, the observed fluorescence phenomena of 58-60 must be altered to a detectable degree by formation of a Lewis acid/base adduct, resulting in a concrete “yes/no” signal. By estimating the effect of the used substituents, 58-60 can be ordered by Lewis acidity. The aliphatic, aromatic and hetero atomic substituents lead to an order of ClAnBIPC2 (59) < BrAnMes2 (60) < ClAnBCat (58) of acidity. The steric demand of the substituents also differs significantly, from the very bulky isopinocampheyl substituent, to the fairly bulky mesityl substituent to the planar and undemanding catachole substituent, which leads to a reverse order of the compounds 58-60 in terms of steric shielding of the boron atom: ClAnBIPC2 (59) >

BrAnMes2 (60) > ClAnBCat (58). These two factors combined lead to a very weak expected tendency of adduct formation for ClAnBIPC2 (59), a slightly stronger tendency for BrAnMes2 (60) and the clearly strongest for ClAnBCat (58).

To monitor the adduct formation in situ via fluorescence spectroscopy, solutions of 58-60 in non-Lewis basic solvents (hexane, heptane, toluene) were prepared. The analyte solutions of Lewis bases were prepared in corresponding solvents. The following Bases were used: MeCN, NEt3, TMEDA, PPh3, THF. These donors were chosen due to their variety in steric demand (from linear MeCN to bulky PPh3) and due to the different heteroatoms included. Because amines are in general potential quenchers of anthracene fluorescence, the applied concentrations were kept low. All experiments were carried out under inert gas atmosphere to avoid decomposition of 58-60 by oxidation or hydrolysis.

Unfortunately, no noteworthy alterations of the emission properties upon addition of Lewis bases could be found for any of the compounds. A possible explanation is that the acid/base interaction is too weak in solution. This may be assigned to too weak Lewis acidity of 58-60. Even the supposed strongest Lewis acid ClAnBCat (58) does not offer optimum electron pare acceptor properties, as the boron bound oxygen atoms bear lone pairs which can interact with the empty p-orbital of the boron atom, reducing its electron deficiency (Scheme 3-38).

Scheme 3-39: Interaction of an oxygen lone pair with the empty p-orbital of the boron atom.

Additionally, the steric demand of the boron bound substituents in 59 and 60 is quite large which may also hinder the adduct formation. Moreover, the low concentrations of the sample and analyte solutions might decelerate the adduct formation to a degree that in situ fluorescence measurements are not suitable for detecting this process. Finally, the interaction between Lewis acid and base may generally be too weak to alter the electronic properties of the fluorophore in solution.

Despite these unsatisfying results, the general concept presented above is not disproved. Synthetic alterations of the boranyl anthracenes could solve the problems stated, or at least verify the crucial factors inhibiting the function of such a detecting device. Possible solutions will be addressed in 3.6.5.