Boranes with Ultra-High Stokes Shift Fluorescence - ACS Publications

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Boranes with Ultra-High Stokes Shift Fluorescence S. Joel Cassidy,† Ian Brettell-Adams,† Louis E. McNamara,‡ Mallory F. Smith,† Michael Bautista,† Hongda Cao,† Monica Vasiliu,† Deidra L. Gerlach,† Fengrui Qu,† Nathan I. Hammer,*,‡ David A. Dixon,*,† and Paul A. Rupar*,† †

Department of Chemistry and Biochemistry, University of Alabama, Tuscaloosa, Alabama 35487, United States Department of Chemistry and Biochemistry, University of Mississippi, Oxford, Mississippi 38677, United States

‡

Organometallics Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/10/18. For personal use only.

S Supporting Information *

ABSTRACT: The synthesis and optical characterization of 9-(2,6bis(methoxymethyl)phenyl) borafluorene (BMMP-BF) are reported. NMR spectroscopic data and single-crystal X-ray diffraction data of BMMP-BF show intramolecular chelation by the 2,6-bis(methoxymethyl)phenyl moiety via a boron−oxygen dative bond. The optical spectra of BMMP-BF are unusual in that absorption is entirely in the UV region (λmax = 284 nm), yet fluorescence occurs at 536 nm. This equates to a Stokes shift of 2.05 eV (16 500 cm−1) and is among the highest Stokes shifts ever reported for a small molecule. Density functional theory (DFT) calculations show that the boron−oxygen dative bond in BMMPBF is ruptured in the excited state and that emission occurs from a trigonal planar boron geometry. This bond cleavage and the concurrent planarization of the boron center are responsible for the high Stokes shift. Two borafluorenes related to BMMP-BF were also examined: 9-(2,6-bis((methylthio)methyl)phenyl) borafluorene (BMTMP-BF) and 9-(2,6-bis(tert-butoxymethyl)phenyl) borafluorene (BtBuMP-BF). Both BMTMP-BF and BtBuMP-BF have optical properties similar to those of BMMP-BF with remarkably large Stokes shifts. Finally, BMMP-BF-(2T)2, which possesses bithiophene moieties on the 2 and 7 positions of a BMMP-BF core, was also synthesized and studied. The absorption spectrum of BMMP-BF-(2T)2 is red-shifted compared to BMMP-BF. BMMP-BF-(2T)2 was found to exhibit dual emissions rather than the single, high Stokes shift emission of BMMPBF. DFT calculations suggest that the dual emissions of BMMP-BF-(2T)2 arise due to radiative relaxation from two different structures in the excited state.

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Chart 1. Structures of Representative Borafluorenes21,24

INTRODUCTION Conjugated systems containing boron atoms are widely studied for their interesting and unusual optical properties. For instance, boron-dipyrromethene (BODIPY) and related molecules are commonly used as highly emissive and stable fluorescent dyes.1−3 Taking advantage of the empty p-orbital on three-coordinate boron, boron-conjugated molecules have also been widely developed as Lewis acid-based sensors in the detection of anions and neutral Lewis bases.4−6 In addition, the incorporation of boron into conjugated polymers has been reported,7−9 with recent examples being utilized in the creation of high efficiency organic solar cells.10 Borafluorenes are the boron analogues of fluorene and are an important class of boron-containing conjugated systems (Chart 1). Borafluorenes are more Lewis acidic and have higher electron affinities compared to structurally similar, nonannulated boranes and have been studied under a number of different contexts, including for use as Lewis acid catalysts,11,12 fluoride sensors,13 as emissive species,14 and as precursors to more complex boron-containing molecules.15−20 Although many borafluorenes with three-coordinate boron centers are air-sensitive, seminal work by Yamaguchi demonstrated that © XXXX American Chemical Society

steric protection around the boron center can render borafluorenes air-stable.13,21 Our own interests in the area have involved studying structure−property relationships in borafluorene small molecules and the synthesis of borafluorene containing conjugated polymers.22,23 In this contribution, we report on a series of novel borafluorenes in which the boron is four-coordinate due to chelation by pincerlike ligands. This chelation leads to unusual dynamics upon photoexcitation, resulting in extremely large fluorescence Stokes shifts and, in one case, dual fluorescence. Received: July 4, 2018

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DOI: 10.1021/acs.organomet.8b00460 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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RESULTS AND DISCUSSION

Table 1. Selected Bond Lengths and Angles for BMMP-BF, BtBuMP-BF, BMTMP-BF, and BMMP-BF-Br2 from singlecrystal X-ray Diffraction Experimentsa

We set out to synthesize a borafluorene with the OCO chelating 2,6-bis(methoxymethyl)phenyl (BMMP) ligand to enhance the stability of these usually reactive molecules. Examples of BMMP complexes with lithium,25 magnesium,26 aluminum,27 and tin28 have been previously reported, and all show the ether oxygen atoms coordinating with the metal center. We assumed that this would also be the case for boron and reasoned that chelation to the boron center should improve the stability of the borafluorene, as has been seen with other four-coordinate borafluorenes.29,30 BMMP-BF was synthesized by reacting BMMP-Li25 with Cl-BF31 (Scheme 1). BMMP-BF was isolated as a colorless powder that is airstable in the solid state and in solution over the course of several months and can be purified by column chromatography.

compound BMMP-BF

BMTMP-BF

BtBuMP-BF

BMMP-BF-Br2

B−X (Å)

∑C−B−C (deg)

X=O 1.637(4) 1.717 X=S 2.0291(13) 2.116 X=O 1.6823(18) 1.775 X=O 1.615(5)

339.3 345.5 345.7 346.7 334.6 340.6 340.4

a

Numbers in italics were computed at the B3LYP/DZVP2 level of theory.

Scheme 1. Synthesis of BMMP-BF

The 11B NMR spectrum of BMMP-BF consists of a single resonance at 16 ppm, which is indicative of coordination of a BMMP oxygen atom to the boron center (Supporting Information, Figure S3). For comparison, aryl-borafluorenes with three-coordinate boron atoms have 11B NMR resonances between 50 and 70 ppm.23 However, the 1H NMR spectrum of BMMP-BF contained only a single set of resonances for the methylmethoxy groups, which is inconsistent with a static fourcoordinate boron center (Supporting Information, Figure S1). We suspect that the boron is four-coordinate in solution, but there is rapid intramolecular ligand exchange between the two ether groups on the BMMP ligand relative to the 1H NMR spectroscopic time scale. Aluminum complexes with the BMMP ligand also show identical intramolecular exchanges in solution.27 Low-temperature (−40 °C, limited by solubility) 1 H NMR experiments of BMMP-BF were not successful in reversing the coalescence of signals attributed to the methylmethoxy moieties in different environments. Single-crystal X-ray diffraction (XRD) images of BMMP-BF showed that, in the solid state, one of the oxygen atoms approaches the boron at a distance of 1.637(4) Å (Table 1, Figure 1). This boron−oxygen single bond is longer than a typical B−O covalent bond (ca. 1.3−1.5 Å)32 but still alters the environment around the boron atom away from planarity, with the boron atom adopting a distorted tetrahedral geometry. The absorption spectra of 9-aryl-borafluorenes with threecoordinate boron centers typically consist of an intense, short wavelength absorption due to the biphenyl π to π* transition. This is followed by a much weaker long-wavelength absorption due to the π-to-vacant B p-orbital transition, and, as a consequence, 9-aryl-borafluorenes tend to be pale yellow-green in color.13,21,23 In contrast, the absorption spectrum of BMMP-BF consists only of a short wavelength absorption, with a maximum at 284 nm, that extends to 325 nm (Figure 2A and Table 2). The difference in absorption spectra can be explained by the fact that one of the oxygen atoms of the

Figure 1. Single-crystal X-ray structures of (A) BMMP-BF, (B) BtBuMP-BF, (C) BMTMP-BF, and (D) BMMP-BF-Br2 (50% ellipsoids). Hydrogen atoms were omitted for clarity.

BMMP ligand is coordinated to the boron center in the ground state, thereby serving as a Lewis base and donating electron density into the otherwise vacant p-orbital on boron. Other four-coordinate boron-containing borafluorenes show similar absorption spectra.14 Although the absorption spectrum of BMMP-BF is like that of known four-coordinate borafluorenes, its emission spectrum more closely resembles that of three-coordinate borafluorenes (Figure 2A). Specifically, the maximum fluorescence intensity occurs at 536 nm. On the basis of this and the absorption maximum at 284 nm, the fluorescence of BMMP-BF has a remarkably large Stokes shift of 16 500 cm−1 (Table 2), which is among the largest Stokes shifts ever reported for a small molecule.33 We hypothesized that the large Stokes shift of BMMP-BF arises due to photodissociation of the B−O Lewis acid−base dative bond in the excited state, with BMMP-BF emitting from a geometry in which the boron is three-coordinate. Furthermore, we propose that this bond dissociation is accompanied by charge transfer from the biphenyl π system to the p-orbital on boron, in a process termed bond-cleavageB


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Organometallics

The similarity in fluorescent lifetimes between BMMP-BF and Mes*BF suggests that the long lifetime of BMMP-BF is not due to the structural rearrangement in the excited state but is intrinsic to three-coordinate aryl borafluorenes. Furthermore, we hypothesize that the steric bulk of the aryl groups, which are rotated orthogonal to the plane of the borafluorene moiety, prevent quenching of the long-lived excited state. The lower quantum yield of BMMP-BF (Φ = 0.03 in dichloromethane (DCM)) compared to Mes*BF (Φ = 0.35 in tetrahydrofuran (THF))21 could be a result from inefficient B−O bond breaking in the excited state and more efficient nonradiative relaxation enabled by the less bulky substituents of BMMP compared to Mes*. Density functional theory (DFT) and time-dependent (TD) DFT35,36 calculations using the hybrid B3LYP37−39 exchangecorrelation functional with the DFT-optimized DZVP2 basis sets40 were performed to gather further support for our hypothesis that the emission from BMMP-BF occurs from a BICT transition, which produces an excited-state structure in which the boron is three-coordinate.41 The computed groundstate structure of BMMP-BF closely matches the XRD determined four-coordinate boron structure (Table 1, Figures 3 and 4). In the ground state, a conformer with a three-

Figure 2. Normalized absorption (solid lines) and normalized emission (dashed lines) spectra of BMMP-BF, BMTMP-BF, BtBuMP-BF, and BMMP-BF-(2T)2. Spectra were acquired in DCM at 25 °C.

induced intramolecular charge transfer (BICT). The formation of a BICT state was recently invoked by Tanaka and Chujo to explain the origins of dual emission in related borafluorene systems.34 Initial experimental support for this hypothesis comes from the similarities of the absorption spectrum of BMMP-BF to other four-coordinate borafluorenes29 and the similarities of the emission spectrum of BMMP-BF to threecoordinate aryl-borafluorenes.13,21 In addition, the emission spectra of BMMP-BF exhibits bathochromic shifts in more polar solvents (Supporting Information, Figure S18), which is consistent with the proposed presence of a charge-transfer excited state. Fluorescence lifetime measurements provide further support to the hypothesis of emission of BMMP-BF from a threecoordinate state. We synthesized the previously reported Mes*BF (Mes* = 2,4,6-tri-tert-butylphenyl; Chart 1)21 and measured its fluorescence lifetime in nondeoxygenated solvent to be remarkably long at 150 ns (Supporting Information, Figure S19). The fluorescence lifetime of BMMP-BF was also found to be unusually long at 122 ns, suggesting a similarity in their emission processes (Supporting Information, Figure S20).

Figure 3. DFT-calculated excitation and emission for BMMP-BF and reorganization energies.

Table 2. Select Absorption and Emission Data of BMMP-BF, BMTMP-BF, BtBuMP-BF, and BMMP-BF-(2T)2 experimental absorptiona λmax, nm (log ε)

experimental emission,a nm (τ, ns) [Φf]

experimental Stokes shift,a eV [cm−1]

calculated absorptionb λmax, nm (error)c

BMMP-BF

284 (4.21)

536 (122) [0.026]

2.05 [16500]

BMTMP-BF

284 (4.04)

BtBuMP-BF

286 (4.01)

BMMP-BF-(2T)2

408 (4.33)

539 (132) [0.023] 530 (116) [0.033] 446 (0.50) 639 (4.38) [0.018]

2.06 [16600] 1.99 [16100] 0.45 [3600] 1.10 [8900]

311d 290 (0.09 eV) 321d 291 (0.10 eV) 313d 292 (0.09 eV) 454 (0.31 eV)

compound

calculated emission,b nm (error)c

calculated Stokes shift (eV)b

581 (0.17 eV)

2.13

581 (0.17 eV)

2.13

575 (0.12 eV)

2.09

550 (0.53 eV) 753 (0.29 eV)

0.48 1.08

a

Measured in DCM at room temperature. bCalculated at the B3LYP/DZVP2 level of theory. cThe error is the absolute difference between measured and calculated values. dCalculated to be very weak. C


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Organometallics

resemble the HOMO and LUMO of the three-coordinate conformer on the ground-state surface (Figure 4C,D) and are consistent with charge transfer to the boron p-orbital. The calculated Stokes shift of 2.13 eV is within 0.1 eV of the experimental shift of 2.05 eV. We ruled out other potential explanations for the large Stokes shift of BMMP-BF. The emission spectrum of BMMPBF did not change with concentration, suggesting that exciplex formation is not responsible for the apparent large Stokes shift. There is minimal change in emission upon excitation at different wavelengths, indicating that absorption and fluorescence are occurring from the same species and not an impurity. Finally, attempts at excitation using wavelengths greater than the absorption region of BMMP-BF (i.e., >330 nm) did not result in emission, which excludes the possibility of emission occurring from a longer-wavelength absorbing impurity or from a different structural conformation of BMMP-BF. We note that boron-containing compounds with large Stokes shifts have previously been reported. For example, Yamaguchi reported a series of compounds containing B−N bonds in which the large Stokes shifts arise due to twistinginduced charge transfer.43 The breaking of dative bonds involving boron have recently been observed in other boron-containing conjugated systems. For example, Yamaguchi and co-workers observed this in a pyridine adduct of a partially fused trinaphthylborane (Chart 2), which resulted in the presence of two different emissive

Figure 4. Visualization of the (A) HOMO and (B) LUMO of BMMP-BF in its lowest-energy conformation. Visualization of the (C) HOMO and (D) LUMO of BMMP-BF in a geometry in which the boron atom is three-coordinate.

coordinate boron center was also found as a minimum but is predicted to be ΔH(0 K) = 7.7 kcal/mol higher in energy, so the four-coordinate structure of BMMP-BF is clearly the dominant conformation present at room temperature. The energy difference between the four-coordinate and threecoordinate structures on the ground-state potential energy surface of ∼8 kcal/mol is consistent with the 1H NMR spectrum showing rapid equilibration between two fourcoordinate structures on the NMR time scale. The predominantly highest occupied molecular orbital (HOMO) → lowest unoccupied molecular orbital (LUMO) vertical excitation for BMMP-BF was calculated to be 311 nm, but it is predicted to be very weak (Table 2, Figure 3 and Supporting Information, Figure S27). The HOMO is delocalized over the C atoms of the borafluorene, and the LUMO is also delocalized over the borafluorene with contributions from the B and the O in the dative bond (Figure 4A,B). A much more intense transition is predicted at 290 nm, which is in excellent agreement within 0.1 eV of the observed absorption maximum of 284 nm (Table 2). The more intense transition has a number of different contributions, including HOMO → LUMO+1, HOMO → LUMO, and HOMO−3 → LUMO. The LUMO+1 is delocalized over the two six-member rings of the borafluorene, and HOMO−3 is the corresponding occupied orbital for the LUMO+1 (Supporting Information, Figure S24). Geometry optimization of the BMMP-BF S1 excited state at the TD-DFT (B3LYP) level produced a structure in which the B−O dative bond is dissociated and the boron has adopted a trigonal planar environment (Figure 3; use of the CAM-B3LYP42 functional led to the same result). Attempts to find a local minimum for the S1 excited state with a four-coordinate boron center were not successful, suggesting that, in the S1 state, the B−O dative bond is not stable. Vertical relaxation from the geometrically optimized S1 state, in which the boron is three-coordinate, corresponds to a photon emitted with a wavelength of 581 nm, which is within 0.2 eV of the observed fluorescence of BMMP-BF at 536 nm (Table 2 and Figure 3). The two orbitals making up the excited singlet state

Chart 2. Structuresa of a Pyridine-Coordinated, Partially Fused Trinaphthylborane, N−B Ladder Boranes, and Borafluorenes with Intramolecular Amine Coordination29,34,39,44,45

a

All of these compounds undergo dative bond dissociation upon photoexcitation.

species in solution.39,44 Pammer reported on the photodissociation of B−N intramolecular dative bonds ladder boranes.45 Tanaka and Chujo observed dual emissions from borafluorenes with intramolecular amine coordination (Chart 2).34,46 They attributed the two emissions as arising from two different structures in the excited state. The short-wavelength emission results from a structure in which the boron−nitrogen dative bond is intact and the long wavelength emission from a structure in which bond dissociation has occurred.29,34 Comparing Tanaka and Chujo’s borafluorene to BMMP-BF, the fact that BMMP-BF produces only one single, longwavelength emission is likely due to the reduced strength of the oxygen−boron dative bond compared to the nitrogen− boron dative bond in Chujo’s borafluorenes. The calculated dative bond energies at 298 K at the G3(MP2) level for BH3-R for R = NMe3, SMe2, and OMe2 are 38.0, 25.3, and 19.8 kcal/ mol, respectively, supporting this hypothesis.47 We also note that the emission quantum yields of Tanaka and Chujo’s borafluorenes are higher than that of BMMP-BF; we hypothesize that this is due to the presence of the two tertD


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Organometallics

state. DFT calculations predicted that the orbitals of BtBuMPBF are very similar to those of BMMP-BF, consistent with the spectral similarities (Supporting Information, Figure S25). We were initially surprised by the similarity of the optical spectra of BMTMP-BF to those of BMMP-BF; however, the calculated optical properties of BMTMP-BF are in excellent agreement with the experimental values. DFT calculations show that, in the ground state, the four-coordinate structure of BMTMP-BF is 5.1 kcal/mol more stable than the threecoordinate structure. Interestingly, the HOMO for BMTMPBF is localized on the sulfur atom that is not coordinated to boron (Figure 5a). The LUMO for BMTMP-BF is very similar

butyl groups on the aryl moiety that limit nonradiative relaxation in the borafluorenes reported by Tanaka and Chujo. Additional molecules related to BMMP-BF were synthesized to investigate how changes in the coordination environment around boron and changes in the conjugation of the molecule would impact optical properties (Scheme 2). Two borafluorScheme 2. Synthesis of BMTMP-BF, BtBuMP-BF, and BMMP-BF-(2T)2

enes with new chelating ligands were examined: BMTMP-BF, which has coordinating sulfur atoms in place of oxygens atoms, and BtBuMP-BF, which has tert-butoxy groups in place of the methoxy group in BMMP. In addition, a BMMP-BF derivative with bithiophene substituents on the 2 and 7 positions (BMMP-BF-(2T)2) was also investigated. BMTMP-BF and BtBuMP-BF were both synthesized from Cl-BF in a similar fashion to that of BMMP-BF (Scheme 2). To access BMMPBF-(2T)2, BMMP-BF was first reacted with N-bromosuccinimide (NBS) to form BMMP-BF-Br2, the structure of which was confirmed by NMR spectroscopy and single-crystal XRD (Table 1, Figure 1). A subsequent Stille coupling with BMMPBF-Br2 produced BMMP-BF-(2T)2 in modest yield (Scheme 2). The NMR spectroscopic properties of BMTMP-BF, BtBuMP-BF, and BMMP-BF-(2T)2 are very similar to those of BMMP-BF (see Supporting Information), which suggests that they all have four-coordinate boron atoms, with rapid intramolecular exchange of coordinating donor atoms. The single-crystal XRD structures of BMTMP-BF and BtBuMP-BF also show similar connectivity to that of BMMP-BF (Table 1, Figure 1). The B−O dative bond in BtBuMP-BF is 1.6823(18) Å, which is slightly longer than that observed in BMMP-BF, possibly due to the increased sterics of the tert-butyl group in BtBuMP-BF. In the case of BMTMP-BF, the B−S dative bond is notably longer at 2.0291(13) Å, a consequence of the larger size of sulfur compared to oxygen. There is a slight upfield shift in the 11B NMR spectrum of BMTMP-BF (5.3 ppm) compared BMMP-BF (16.1 ppm), suggesting a stronger interaction.48 Attempts at growing crystals of BMMP-BF(2T)2 suitable for single-crystal XRD were not successful. The absorption and fluorescence spectra of both BMTMPBF and BtBuMP-BF are nearly identical to those of BMMPBF (Figure 2b,c) and exhibit similarly long fluorescence lifetimes of 132 and 116 ns, respectively. For BtBuMP-BF, these observations suggest that the increased sterics of the tertbutyl group have negligible impact on the strength of the B−O dative bond or the dynamics of BtBuMP-BF in the excited

Figure 5. Visualization of the (A) HOMO and (B) LUMO of BMTMP-BF in its lowest-energy conformation. Visualization of the (C) HOMO and (D) LUMO of BMTMP-BF in the ground-state geometry in which the boron atom is three-coordinate.

to that of BMMP-BF. The more intense transition in BMTMP-BF is composed of three close transitions involving the HOMO → LUMO+1, HOMO → LUMO+2, and HOMO−1 → LUMO+1. The LUMO+1 is localized on the external C atoms of the six-membered rings of the borafluorene and looks the same as that of BMMP-BF (Supporting Information, Figure S26). The LUMO+2 is located on the substituted phenyl ring in both, so there is some component of charge transfer occurring (Supporting Information, Figure S26). For BMTMP-BF, the HOMO−1 still has a contribution from the S but also has a significant contribution from the C atoms on the borafluorene that resemble the HOMO of BMMP-BF (Supporting Information, Figure S26). Despite these differences in the frontier molecular orbitals, the behavior of BMTMP-BF upon photoexcitation appears identical to that of BMMP-BF, as the only stable structure of BMTMP-BF in the S1 state has undergone BICT, where the boron atom is three-coordinate. This similarity partly results from the fact that the borafluorene contribution so important in BMMP-BF is also present in BMTMP-BF in terms of the occupied orbitals, and the unoccupied orbitals are quite similar for both. The two sets of spectra are energetically so similar because the different contributions from the orbital excitations and the dative bond energies in the reorganization process are effectively canceling each other out (Supporting Information, Figure S31). E


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Organometallics The absorption and emission spectra of BMMP-BF-(2T)2 are significantly different from those of BMMP-BF. The presence of the bithiophene moieties produces a red-shifted absorption compared to BMMP-BF, with an λmax of 408 nm (Figure 2D, Table 2) Interestingly, the fluorescence spectrum of BMMP-BF-(2T)2 features two distinct emissions centered at 446 and 639 nm, respectively (Figure 2D, Table 2). The short wavelength emission of BMMP-BF-(2T)2 has a much shorter lifetime (0.50 ns), whereas the longer wavelength emission has a lifetime of approximately a magnitude larger (4.38 ns; Supporting Information, Figure S23), although it is much shorter than the fluorescence lifetime for the emission of BMMP-BF. DFT calculations were performed on BMMP-BF-(2T)2 to determine the origins of the dual emissions. Ground-state geometry optimization of BMMP-BF-(2T)2 results in a structure like that of BMMP-BF, with an intact B−O dative bond. The ground-state three-coordinate boron conformer is 7.7 kcal/mol higher in energy compared to the four-coordinate geometry. The similarity between the energy differences for the four-coordinate and three-coordinate structures for BMMP-BF and BMMP-BF-(2T)2 shows that the extended conjugation of BMMP-BF-(2T)2 has negligible impact on the Lewis acidity of the boron atom. The HOMO for the four-coordinate BMMPBF-(2T)2 is a π-orbital that extends across the thiophenes and the borafluorene carbon atoms (Figure 6A) with the LUMO corresponding to an antibonding version of the HOMO (Figure 6B). The calculated very intense HOMO → LUMO vertical excitation is predicted to be at 454 nm, which is 0.31 eV longer than the experimental λmax value (Table 2). Interestingly, the potential energy surface of the BMMP-BF(2T)2 S1 state had two distinct stable structures. The first structure, which is lower in energy, consists of a trigonal boron center, with the B−O bond no longer intact due to the formation of the BICT state, as found for the other borafluorenes. The second, higher-energy (by ΔEele = 8.6 kcal/mol) structure, is more similar to the ground-state structure of BMMP-BF-(2T)2, as the boron atom is still four-coordinate due to the B−O dative bond. Thus, the stability of the two excited-state structures is reversed from that in the ground state by a value comparable to the energy difference in the ground state. The presence of two stable excited-state structures is consistent with the dual emission of BMMP-BF-(2T)2, with the long-wavelength emission arising from the B−O bond broken S1 structure and the shortwavelength emission arising from the B−O bond intact S1 structure. The DFT calculations predict that emitted photons from these two states would have wavelengths of 753 and 550 nm, respectively, which are both longer than the experimental (Table 2). Thus, the shorter-wavelength emission is likely due to the four-coordinate excited state with the shorter lifetime, whereas the longer wavelength with the longer lifetime is the resulting emission from the three-coordinate excited state. Apparently, the intense absorption for BMMP-BF involves more orbital mixing, and this leads to a longer lifetime than if the initial excitation is the direct HOMO → LUMO transition with a large intensity, as is found for BMMP-BF-(2T)2. The presence of a stable four-coordinate excited state could be due to the fact that this is the only compound where the excitation is completely HOMO → LUMO and localized on the carbon atoms of the borafluorene and thiophene substituents. The extended conjugation of BMMP-BF-(2T)2 may stabilize the π system in the excited state leading to a stable four-coordinate

Figure 6. Visualization of the (A) HOMO and (B) LUMO of BMMP-BF-(2T)2 in its lowest-energy conformation. Visualization of the (C) HOMO and (D) LUMO of BMMP-BF-(2T)2 in the groundstate geometry in which the boron atom is three-coordinate.

boron structure. This is also consistent with the short fluorescence lifetime of this excited state, which has a similar geometry to the ground-state minimum, so that little reorganization of the molecule is needed to emit. Our proposed mechanism for dual emission is essentially identical to the mechanism that Tanaka and Chujo proposed for dual emission in their borafluorene systems.34

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CONCLUSIONS In summary, we have reported the synthesis of borafluorene BMMP-BF, 2,6-bis(methoxymethyl)phenyl (BMMP). BMMP-BF absorbs in the UV region (λmax = 284 nm) but emits at 536 nm, which is among the largest Stokes shifts ever reported (2.05 eV, 16 500 cm−1). The origin of the large Stokes shift is due to bond-cleavage-induced charge transfer (BICT) transition, where the boron−oxygen dative bond ruptures upon photoexcitation to produce an S1 structure in F


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Organometallics

nitrogen stream using an Oxford N-Helix cryostat (Oxford Cryosystems). The crystal was irradiated with graphite monochromated Mo Kα radiation, and a hemisphere of diffraction data was measured using a strategy of φ and ω scans with 0.5° frame widths. Data collection, unit cell determination, data reduction, and integration, absorption correction, and scaling were performed using the Apex 2 software suite from Bruker.52 The crystal structure was solved by direct methods and refined by full-matrix least-squares refinement against F2. Non-hydrogen atoms were located from the difference map. Hydrogen atoms were placed in calculated positions and allowed to ride on the carrier atom. Hydrogen atoms on methyl groups were refined using a riding rotating model. Space group determination, structure solution, and refinement were performed using the Bruker SHELXTL software package.53 For BMMP-BF, BMTMP-BF, and BtBuMP-BF, fluorescence lifetime was measured on a Fluorolog-3 spectrofluorometer (Horiba JobinYvon) with a DeltaDiode (280 nm, D-280, Horiba Scientific) as the excitation source and a picosecond photon detection module (PPD-850, Horiba Scientific) as the detector. For BMMP-BF-(2T)2 and Mes*BF, fluorescence lifetime curves were obtained using the 405 nm line of an LDH series 405B pulsed diode laser (pulse width ≈ 100 ps) as the excitation source, and emission was detected using a PicoQuant PDM series single-photon avalanche diode (time resolution ≈ 50 ps) and TimeHarp 260 time-correlated single-photon counter (25 ps resolution). The geometries were optimized at the DFT54 level with the hybrid B3LYP37,38 exchange-correlation functional first with the DFToptimized DZVP2 basis sets.40 Vibrational frequencies were calculated to show that the structures were minima. TD-DFT calculations35,36 were performed to analyze the UV−vis spectra at the same computational level in the gas phase. All calculations were done with Gaussian 09.41 Synthesis of BMMP-BF. 2-Bromo-1,3-bis(methoxymethyl)benzene (1.29 g, 5.26 mmol) was dissolved in THF (15 mL). The solution was cooled to −78 °C. Butyl lithium (1.6 M in hexane, 3.3 mL, 5.3 mmol) was added dropwise and allowed to react at −78 °C for 50 min. After this time, while maintaining the reaction temperature at −78 °C, a toluene (8 mL) solution of 9-chloroborafluorene (1.05 g, 5.29 mmol) was added dropwise. The solution was then warmed to room temperature and left to stir for 16 h. The solvent was then removed, and the crude product was purified by silica gel chromatography (gradient from 100% hexane to 100% DCM). BMMP-BF was isolated as a crystalline white solid (834 mg, 48.5%). Crystals suitable for single-crystal XRD were grown by dissolving BMMP-BF in a minimal amount of boiling hexanes before being placed in a −20 °C freezer for 12 h. 1H NMR (500 MHz, CDCl3) δ 7.65 (dt, J = 7.6, 0.9 Hz, 2H), 7.31−7.26 (m, 3H), 7.23 (d, J = 7.4 Hz, 2H), 7.19 (d, J = 6.8 Hz, 2H), 7.08 (td, J = 7.1, 1.0 Hz, 2H), 4.58 (s, 4H), 3.09 (s, 6H); 13C NMR (126 MHz, CDCl3) δ 149.95, 138.73, 131.42, 128.62, 126.85, 126.81, 122.61, 119.43, 58.54; 11B NMR (160 MHz, C6D6) δ = 16.06; high-resolution mass spectrometry (HRMS) (electron impact (EI)/ sector) m/z [M]+ Calcd for C22H21BO2 328.1635; Found 328.1639. Elemental analysis, C (Theory = 80.51, Found 80.10), H (Theory = 6.45, Found 6.96). The purity of the compound was established by 1H NMR spectroscopy (Figure S1). Synthesis of BtBuMP-BF. To a toluene solution (20 mL) of 2bromo-1,3-bis(tert-butoxymethyl)benzene (307 mg, 0.932 mmol), cooled to −78 °C, was added butyl lithium (1.6 M in hexane, 0.57 mL, 0.91 mmol) dropwise. The solution reacted at −78 °C for 1.5 h before warming to room temperature and reacting a further 1.5 h. The solution was cooled again to −78 °C. A toluene solution (10 mL) of 9-chloroborafluorene (189 mg, 0.952 mmol) was added dropwise, and the reaction mixture was stirred at −78 °C for 1 h. The solution was then warmed to room temperature and allowed to stir for 16 h. The solvent was then removed, and the crude product was purified by silica gel chromatography (eluting from 100% hexane to 100% DCM.) Recrystallized in boiling hexane produced BtBuMP-BF as a crystalline white solid (178 mg, 46.2%). The purified product was dissolved in DCM/hexane (1:4), and the solvent was allowed to slowly evaporate at room temperature to roughly half the original volume over a 16 h

which the boron atom is three-coordinate. Two related molecules, namely, BtBuMP-BF (BtBuMP = 2,6-bis(tertbutoxymethyl) phenyl) and BMTMP-BF (BMTMP = 2,6bis((methylthio)methyl)phenyl), were synthesized and found to have structural and optical properties nearly identical to that of BMMP-BF, including having remarkably large Stokes shifts. The presence of tert-butyl groups appears to have little impact on the excited-state behavior of BtBuMP-BF. Despite having frontier molecular orbitals distinct from BMMP-BF, the optical properties of BMTMP-BF are nearly identical to those of BMMP-BF, due to a complex interplay of orbital and electronic interactions, as well as differences in the B−O and B−S dative bond dissociation energies. BMMP-BF, BMTMP-BF, and BtBuMP-BF all possess remarkably long fluorescence lifetimes in excess of 100 ns. The long lifetimes appear to be intrinsic to aryl borafluorenes, as the previously known Mes*BF (Mes* = 2,4,6-tri-tertbutylphenyl)21 was also found to have a fluorescence lifetime of 150 ns. Finally, BMMP-BF-(2T)2, which has bithiophene moieties on the 2 and 7 positions of BMMP-BF, was synthesized and characterized. Because of the additional conjugation caused by the presence of the bithiophene moieties, BMMP-BF-(2T)2 absorbs in the visible region (λmax = 408 nm). Unlike the other borafluorenes in this report, BMMP-BF-(2T)2 fluoresces at two distinct wavelengths. On the basis of DFT calculations, the short wavelength emission arises from an S1 structure in which the boron−oxygen dative bond remains intact and the longwavelength emission occurs due to a BICT transition caused by boron−oxygen dative bond cleavage. The presence of extended conjugation in BMMP-BF-(2T)2 appears to be responsible for stabilizing the π* orbital and thus for stabilizing the structure in which the boron−oxygen bond remains intact. The results of this study add to the growing evidence that, in the excited state, conjugated molecules with boron−donor dative bonds can undergo BICT transitions. Furthermore, results from this contribution and from Tanaka and Chujo’s recent work34 show that the occurrence of BICT transitions appear to be susceptible to factors including small changes in the electronic properties of a molecule and small changes in solvent environment. We believe that BICT in boroncontaining conjugated materials represents a new paradigm for the design of photoactive boron-containing conjugated materials that could be harnessed for sensor applications (as recently demonstrated by Chujo in viscosity sensing34), the development of novel photoacids, and the creation of high Stokes shift materials. BICT transitions may also contribute to the properties of conjugated polymers incorporating boranes with intramolecular coordination.49,50

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EXPERIMENTAL SECTION

General Experimental. All manipulations were performed under an anhydrous N2 atmosphere using standard Schlenk line and glovebox techniques unless stated otherwise. Solvents were dried by passing them through an alumina column and then stored over 4 Å molecular sieves. The following compounds were synthesized according to literature procedures: 2-bromo-1,3-bis(methoxymethyl)benzene,26 2-bromo-1,3-bis(tert-butoxymethyl)benzene,25 2-bromo1,3-bis((methylthio)methyl)benzene,51 Mes*BF,21 and 9-chloroborafluorene.31. Single-crystal XRD data were collected using a Bruker diffractometer equipped with a three-circle PLATFORM goniometer and an APEX II CCD camera (Bruker-AXS). The crystal was mounted on a glass fiber using silicone grease and cooled to 100 K under a cold G


Organometallics

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period, yielding crystals suitable for XRD analysis. 1H NMR (500 MHz, CDCl3) δ = 7.62 (d, J = 6.9 Hz, 2 H), 7.14−7.24 (m, 7 H), 7.03 (td, J = 0.9, 7.2 Hz, 2 H), 4.43 (s, 4 H), 1.02 (s, 18 H); 13C NMR (125 MHz, CDCl3) δ = 148.64, 138.98, 131.65, 127.93, 126.82, 126.47, 121.29, 119.47, 81.45, 67.21, 27.54; 11B NMR(160 MHz, CDCl3) δ = 17.14; HRMS (EI/sector) m/z [M]+ calcd for C28H33BO2 412.2610; Found 412.2574; the purity of the compound was established by 1H NMR spectroscopy (Figure S4). Synthesis of BMTMP-BF. To a toluene solution (20 mL) of 2bromo-1,3-bis((methylthio)methyl)benzene (201 mg, 0.725 mmol) at −78 °C was added butyl lithium (1.6 M in hexane, 0.45 mL, 0.72 mmol) dropwise. The reaction was stirred at −78 °C for 1 h before warming to room temperature and stirred an additional 4 h. The solution was cooled again to −78 °C. A toluene solution (10 mL) of 9-chloroborafluorene (154 mg, 0.776 mmol) was added dropwise, and the reaction mixture was stirred at −78 °C for 1 h. The solution was then warmed to room temperature and allowed to stir for 16 h. The crude solution was filtered over neutral alumina before the solvent was removed, and the remaining residue was recrystallized in boiling hexane to produce BMTMP-BF as a white powder (92.6 mg, 35.4%). The purified product was dissolved in Et2O/hexane (1:4), and the solvent was allowed to slowly evaporate at room temperature to roughly half the original volume over a 16 h period, yielding crystals suitable for XRD analysis. 1H NMR (500 MHz, CDCl3) δ = 7.72 (d, J = 7.3 Hz, 2H), 7.21−7.31 (m, 7 H), 7.08 (td, J = 0.9, 7.2 Hz, 2 H), 3.67 (s, 4 H), 1.84 (s, 6 H); 13C NMR (125 MHz, CDCl3) δ = 149.15, 141.75, 131.37, 128.22, 127.45, 126.51, 126.17, 120.08, 41.42, 17.84; 11B NMR (160 MHz, CDCl3) δ = 5.31; electrospray ionization (ESI) MS m/z [M + H]+ Calcd for C22H21BS2+H+ 361.13; Found 361.2; elemental analysis C (Theory = 73.33, Found 73.15), H (Theory = 5.87, Found 6.00). The purity of the compound was established by elemental analysis. Synthesis of BMMP-BF-(Br)2. In the absence of light, BMMP-BF (164 mg, 0.5 mmol) and NBS (178 mg, 1.00 mmol) were dissolved in dichloromethane (50 mL). Silica (10 g) was added and formed a suspension upon stirring. The reaction was stirred in open air for 16 h in the dark. The crude material was filtered to collect the filtrate and dried. The product was then extracted using diethyl ether/water. The final product was recrystallized from ethyl acetate and isolated as white crystals (124 mg, 50%). 1H NMR (500 MHz, CD2Cl2) δ = 7.49 (d, J = 8.2 Hz, 2 H), 7.41 (dd, J = 1.9, 8.2 Hz, 2 H), 7.32 (dd, J = 6.9, 8.2 Hz, 1 H), 7.29 (d, J = 1.9 Hz, 2 H), 7.24 (d, J = 7.9 Hz, 2 H), 4.60 (s, 4 H), 3.12 (s, 6 H); 13C NMR (126 MHz, CD2Cl2) δ = 150.1, 147.3, 138.5, 134.1, 131.4, 127.2, 122.8, 121.7, 121.0, 77.7, 58.6 11B NMR (160 MHz, CDCl3) δ = 14.40; HRMS (EI/sector) m/z [M]+ Calcd for C22H19BBr2O2 483.9844; Found 483.9842. The purity of the compound was determined by 1H NMR spectroscopy (Figure S10). Synthesis of BMMP-BF-(2T)2. BMMP-(Br)2 (48.6 mg, 0.100 mmol), [2,2′-bithiophen]-5-yltrimethylstannane (98.7 mg, 0.300 mmol), and Pd(PPh3)4 (120 mg, 0.010 mmol) were added to toluene (10 mL). The mixture was heated to 110 °C for 16 h. After that time the reaction was cooled to room temperature. The product was purified via flash chromatography; dichloromethane/hexane = 2:1. BMMP-BF-(2T)2 was isolated as a yellow solid (32.0 mg, 49%). 1 H NMR (500 MHz, CDCl3) δ = 7.68 (d, J = 7.9 Hz, 2 H), 7.58 (dd, J = 1.9, 7.9 Hz, 2 H), 7.45 (d, J = 1.6 Hz, 2 H), 7.37 (dd, J = 6.6, 8.2 Hz, 1 H), 7.30 (d, J = 7.6 Hz, 2 H), 7.23−7.14 (m, 6 H), 7.12 (d, J = 3.8 Hz, 2 H), 7.02 (dd, J = 3.8, 5.0 Hz, 2 H), 4.67 (s, 4 H), 3.16 (s, 6 H); 13C NMR (126 MHz, CDCl3) δ = 149.5, 149.0, 144.4, 138.7, 137.7, 135.7, 132.4, 128.3, 127.8, 127.0, 126.2, 124.5, 124.1, 123.3, 122.9, 122.7, 119.9, 77.5, 58.6;); 11B NMR (160 MHz, CDCl3) δ = 15.53; HRMS (EI/sector) m/z [M]+ Calcd for C38H29BO2S4 656.1143; Found 656.1171. The purity of the compound was determined by 1H NMR spectroscopy (Figure S13).

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00460. Experimental details, NMR spectra, additional optical spectra, additional visualization of molecular orbitals, calculated geometry coordinates (PDF) Illustrated molecular structure (XYZ) Accession Codes

CCDC 1853562−1853565 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (N.I.H.) *E-mail: [email protected]. (D.A.D.) *E-mail: [email protected]. (P.A.R.) ORCID

Louis E. McNamara: 0000-0002-7706-6441 Hongda Cao: 0000-0002-0313-7956 Monica Vasiliu: 0000-0001-7573-4787 Fengrui Qu: 0000-0002-9975-2573 Nathan I. Hammer: 0000-0002-6221-2709 David A. Dixon: 0000-0002-9492-0056 Paul A. Rupar: 0000-0002-9532-116X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (Grant No. CHE1507566 (P.A.R.) and OIA-1539035 (N.I.H.)) and The Univ. of Alabama for financial support (P.A.R. and D.A.D.). The computational work (D.A.D. and M.V.) was supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, U.S. Department of Energy (DOE) under the DOE BES Catalysis Center Program by a subcontract from Pacific Northwest National Laboratory (KC0301050-47319). D.A.D. also thanks the Robert Ramsay Chair Fund of The Univ. of Alabama for support. P.A.R. thanks Prof. M. Bonizzoni for helpful discussion.

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REFERENCES

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