Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Stimuli-Responsive B/N Lewis Pairs Based on the Modulation of B−N Bond Strength Qinggao Hou,† Lijie Liu,† Soren K. Mellerup,‡ Nan Wang,† Tai Peng,† Pangkuan Chen,*,† and Suning Wang*,†,‡ †
Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology of China, Beijing 102488, China ‡ Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
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S Supporting Information *
ABSTRACT: A series of robust organoboranes with electronically tunable functionality of B/N Lewis pairs has been achieved. These compounds feature a B/N-containing core in which the interactions between the B and N atoms are modulated as a result of the structural flexibility of the nonconjugated backbone. Examination of the substituent effects of the Lewis base moiety reveals that bulky or aryl substituents favor the dynamic switching of the B−N bond in response to external stimuli, such as heat or mechanical pressure, leading to emission color modulation. This work provides a new, straightforward proof of concept toward new switchable materials design based on tunable electronic interactions.
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Scheme 1. Selected Examples of Responsive Systems Based on B ← X Bonds
ue to many intriguing examples of autonomic response to various external stimuli in biological processes,1,2 synthetic materials of dynamic chemical systems that mimic responsive behavior in nature have attracted much research attention. These materials can reversibly change their thermal, optical, or mechanical properties upon stimulus applied such as light, temperature, pH, and pressure.3−5 Recently, facile synthesis and property evaluation of responsive architectures have been sought toward various emerging applications.6−8 Switchable building blocks and dynamic functionality are the key to achieving optical-responsive materials. Several frequently investigated adaptive systems include azobenzenes, stilbenes, diarylethenes, and spiropyran derivatives. Their operations are based on reversible photoswitching of either conformational (Z/ E) isomers or open/closed ring structures concomitant with visual color change on exposure to light irradiation.9 Organoboranes, as typical luminescent chromophores, have captured broad interest in areas of light-emitting materials, sensors, and devices.10,11 It is well-established that the addition of nucleophiles to Lewis acidic luminescent boron systems leads to Lewis acid/base complexes with attenuated or quenched emission. Recently, main-group-based ambipolar systems are attracting much research interest because of their great potential as new responsive materials and for catalytic applications.12−14 Several classes of coordination-driven systems involving a B ← X bond (X = N, O, or P; see examples in Scheme 1) were disclosed recently, unveiling the intrinsic mechanism of switching photophysical properties in such systems.11c,13c,15 A common feature of these previously reported systems is that they rely on either a relatively weak donor or a constrained boron unit to achieve complete rupture/formation of a B ← X bond upon stimulation. In contrast, systems that involve partial rupture of a © XXXX American Chemical Society
B ← X bond in response to external stimuli remain rare, and experimental data illustrating the impact of the subtle B ← X bond strength change on photophysical properties of intramolecular Lewis pairs are still scarce. To address this, we designed a series of B/N Lewis pairs/ adducts (3a−3d, Scheme 1) confined by a flexible framework. The unique feature of this new system is that it allows fine-tuning of the B ← N bond strength by varying the substituent group (R) on the nitrogen atom and using external stimuli. We found that the sterics and electronics of the R group have a great influence on the B ← N bond strength and the dynamic and stimuli-responsive behavior of molecules in this system. The details are presented herein. The synthetic methodology for these compounds is simple and straightforward, as shown in Scheme 2. First, starting Received: August 29, 2018
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DOI: 10.1021/acs.orglett.8b02774 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 2. Ring-Closing Synthesis of B/N Lewis Pairs
materials 1a−1d were prepared by reaction of appropriate amines/anilines with commercially available 2-bromobenzyl bromide under basic conditions using modified procedures.16 Subsequently, TipB(OMe)2 (2) was used as the boron source and attached to scaffold of 1 via electrophilic substitution using n-BuLi at −30 °C, producing air-stable compounds 3a−3d as colorless solids in 41−59% isolated yields after standard workup and purification by column chromatography on silica gel (see Supporting Information (SI)). The new molecules were fully characterized by 1 H, 13 C, and 11 B NMR and HRMS spectroscopic analyses. Crystal structures of 3a−3d were determined by single-crystal X-ray diffraction analysis at 180 K (Figure 1). Bond angles about
Figure 2. Absorption (left) and emission (right) spectra in THF (c = 1.0 × 10−4 M, λex = 272 nm for 3a and 3b and 281 nm for 3c and 3d).
show major ππ* transition bands at 272 nm for 3a and 3b, and at 280 nm for 3c and 3d. Adducts 3c and 3d also show a weak absorption band at ∼380 nm. Compounds 3a−3d show bright fluorescence with contrasting quantum yields (ΦFL) of 3, 17, 99, and 69% in THF for 3a−3d, respectively (Table S7). Molecules that have a Ph (3c) or butyl-Ph (3d) bound to the nitrogen atom clearly have a brighter fluorescence. Furthermore, based on the DFT-calculated NBO values (Figure 1), ΦFL values appear to increase with a decreasing B−N bond strength. From hexane to DMF, compounds 3c and 3d (∼110 nm) display a bathochromic shift much greater than that of 3a and 3b (∼80 nm), indicative of a more pronounced CT character in 3c and 3d (Figure S15). TD-DFT calculations (B3LYP, 6-31g(d)) were carried out, and the results are summarized in Tables S9−S12. Vertical excitations to the first three excited states have similar energies and weak oscillator strengths for all four compounds involving similar orbital contributions. This is illustrated for 3c in Figure 3
Figure 1. Crystal structures of 3a−3d (green, B; blue, N) and a list showing the observed (180 K) and calculated B−N bond lengths (Å) (DFT-optimized structures), and the NBO Wiberg bond indices for the B−N bonds of optimized structures (DFT, B3LYP-6-31g(d)). Hydrogen atoms are omitted for clarity.
B and N in the crystal structures show a significant deviation from the typical value of 120° for ideally sp2-hybridized geometry. These observed deviations indicate the existence of a four-coordinated B center. At 180 K, the B−N bond lengths are comparable in 3a−3d but are greater than those of typical B−N bonds (
