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Mar 22, 2018 - Sudhakar Pagidi, Neena K. Kalluvettukuzhy, and Pakkirisamy Thilagar*. Department of Inorganic and Physical Chemistry, Indian Institute ...
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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Tunable Self-Assembly and Aggregation-Induced Emission Characteristics of Triarylboron-Decorated Naphthalimides Sudhakar Pagidi, Neena K. Kalluvettukuzhy, and Pakkirisamy Thilagar* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: Understanding the structure−property and structure− morphology correlations of organic luminescent materials is essential in order to employ such materials for desired applications. In this regard, we prepared a library of novel triarylboron-tethered N-boryl-1,8naphthalimides (TAB-NPIs) 1−6 with variations in their steric and electronic factors. Compounds 1−6 are weakly luminescent in molecularly dispersed state; however, they are strongly luminescent in the aggregated state (THF−H2O mixture). The presence of sterically hindered triarylboryl units in 1−6 endowed these molecules with aggregation-induced emission (AIE) characteristics by preventing cofacial arrangements of NPI moieties, as shown by the supramolecular interactions in the crystal lattice. These structural features preclude excimer formation which can be detrimental to the radiative process in these molecules. Promisingly, these compounds undergo self-assembly and form interesting morphologies such as sheets (1), fibers (2), tubes (3), vesicles (4), flowers (5), and porous films (6). These types of morphologies are rarely observed in purely organic materials; evidently, the morphology of these compounds strongly depends on their molecular conformations and steric and electronic factors.



INTRODUCTION

displays, molecular switches, forensic applications, and biological estimations.5−10 Of various organic molecular materials, 1,8-naphthalimides (NPIs) have received considerable attention in recent times owing to their potential applications in molecular sensors,11 optoelectronic materials,12 DNA intercalators,13 anticancer agents,13,14 and live cell imaging.15 Despite their great advantages, the solid-state luminescence quantum efficiencies of NPI derivatives are very low due to aggregation-caused quenching (ACQ), caused by intermolecular face-to-face π−π interactions.16 Research groups have studied the AIE characteristics of NPIs by incorporating AIE-active pendants, in which the optical properties are governed by the AIE pendants.17 Very recently we investigated the aggregation-induced emission enhancement (AIEE) characteristics of various NPI derivatives in the absence of AIE pendants. These studies revealed that a delicate balance between molecular flexibility and intermolecular interactions is the right recipe for achieving AIEE from NPIs.18 Tuning the optical properties of fully carbon based conjugated systems by integrating boron into them is an attractive stratagem to develop materials with advanced properties and applications such as nonlinear optics,19 lightemitting devices,20 sensors,21 bioimaging,22 and external stimuli-responsive materials.23,26 Recently, we became inter-

Well-defined self-assemblies of organic luminescent materials have received special attention because of their admirable properties and efficient performances, which depend not only on their supramolecular organizations but also macroscopic dimensions.1−3 However, the self-assemblies are formed by various noncovalent interactions that facilitate the consumption of part of the excitation energy through nonradiative decay channels, which ultimately results in the quenching of the luminescence.4 Determination of the cumulative effect of such noncovalent interactions on controlling and tuning the morphology and luminescence of organic self-assembled structures remains elusive. In general, conventional planar πconjugated organic molecules exhibit interesting morphologies but suffer from luminescence quenching in the solid/condensed state due to strong π−π interactions, which restricts their applications in modern technologies.4 One important approach to overcome this disastrous problem is by designing molecular systems with a propeller geometry. Such molecular symmetries can effectively suppress the undesirable molecular motions/ intermolecular interactions and invoke strong luminescence in the solid/condensed state.5 This concept is popularly known as aggregation-induced emission (AIE).5a,b Numerous research groups have adopted this concept and developed molecules which are luminescent in the condensed/solid state and expanded the scope of purely organic luminescent molecules in high-tech applications such as light-emitting diodes, lasers, © XXXX American Chemical Society

Received: March 22, 2018

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DOI: 10.1021/acs.organomet.8b00166 Organometallics XXXX, XXX, XXX−XXX

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significantly affected by the nature of the spacer between TAB and NPI units. The molecular structures of 1 and 4−6 were confirmed by single-crystal X-ray diffraction studies. Optical Properties. Dilute (10 μM) THF solutions of 1−6 displayed absorption in the region 280−360 nm (Figure 1, left).

ested in developing boron-based multiple emissive molecular systems.24 During the course of our study we have established that tweaking the molecular conformation by judiciously perturbing the electronic structure of luminophores has a significant effect on their optical properties.25,26 Having studied optical and structural features of TAB19−26 and NPI12−18 individually, we set out to investigate the optical and morphological characteristics of TAB-NPI conjugates. We envisioned that, by carefully balancing the intermolecular interactions via controlling the steric and electronic factors between intrinsically propeller TAB and planar NPI moieties, one can develop luminescent materials with tunable morphologies. Accordingly, a series of TAB-NPIs 1−6 was designed and synthesized. Our anticipation was realized, as these compounds showed interesting AIE attributes and luminescent selfassemblies with a range of morphologies such as sheets, tubes, fibers, and vesicles. These intriguing results are reported in this article.

Figure 1. Absorption (left) and luminescence (right) spectra of TABNPIs 1−6 (concentration 10 μM, λex 330 nm) in THF solution (λem ∼394, ∼379, ∼400, ∼425, ∼390, and ∼400 nm, respectively, for 1−6).



RESULTS AND DISCUSSION Synthesis and Characterization. The general synthesis procedure used for the preparation of compounds 1−6 is illustrated in Scheme 1. Borylaniline precursors (R-NH2; 1a−

These absorption peaks can be attributed to the combined transitions of π(aryl) → pπ(B) and π → π* of both TAB and NPI moieties. The absorption spectral features of all the compounds appear to be similar, indicating that the effect of subtle changes in torsion angle between NPI and TAB is negligible. However, there is an increase in extinction coefficient observed for 6 due to the presence of two naphthalimide units. To understand the electronic structures of 1−6, DFT (density functional theory) calculations were performed using the B3LYP hybrid functional and 6-31G(d) basis set for all the atoms as incorporated in Gaussian 09 software.30 Optimization of the molecular structures with consequent frequency tests provided structures which closely resembled those obtained by single-crystal X-ray diffraction studies. The optimized geometries showed high dihedral angles between NPI and the spacer aryl moiety as follows: 1 (68°), 2 (86°), 3 (78°), 4 (77°), 5 (77°), and 6 (66°). These values clearly indicate that the TAB and NPI units in this molecules adopt a twisted conformation along the molecular long axis (Figure S26). Further, ground-state geometries were optimized by including THF and water solvents using the polarizable continuum model (PCM), and it was found that the dihedral angles are increased slightly in comparison to those under vacuum (Table S1). The highest occupied molecular orbital (HOMO) in all compounds, except for 3, was delocalized on the aryl moieties (mesityl in case of 1, 2, 5, and 6 and duryl in the case of 4) attached to boron; in the case of 3, the HOMO resides largely on the duryl spacer. However, the lowest unoccupied molecular orbital (LUMO) is delocalized on the naphthalimide unit in all of the cases (Figure S27). In order to rationalize the experimentally observed UV−vis absorption bands for 1−6, time-dependent density functional theory (TDDFT) calculations were performed. Due to the orthogonal conformation of the NPI and TAB units, the overlap between the frontier molecular orbitals is negligible; as a result the corresponding transitions are very weak. The molecular orbitals contributing to the major absorptions with significant oscillator strength are shown in Figure 2 (Tables S2−S4). The molecular orbital coefficient distribution involved in transitions clearly indicates that the absorption bands in compounds 1−6 arise from the combination of the π(aryl)−pπ(B) transition of the boryl unit and π−π* transitions of the naphthalimide unit.

Scheme 1. General Synthesis Procedure (Top) and the Chemical Structures (Bottom) of TAB-NPI Conjugates 1− 6a

a

Reaction yields are given in parentheses.

6a) were synthesized by following our previous methodologies.26 TAB-NPIs 1−6 were synthesized in moderate yields by heating a mixture of 1,8-naphthalic anhydride, the respective borylaniline, and imidazole in the presence of a catalytic amount of zinc acetate for ∼6 h at around 120 °C. Compounds 1−6 are stable under ambient conditions. All of the compounds were soluble in common organic solvents such as dichloromethane, ethyl acetate, chloroform, and tetrahydrofuran (THF) but are insoluble in water. Compounds 1−6 were characterized by NMR spectroscopy (1H, 13C, and 11B) and high-resolution mass spectrometry (HRMS). Compounds 1−6 showed broad 11 B resonances in the region 73−77 ppm and were not B

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Figure 2. Molecular orbitals of 1−6 involved in electronic transitions (DFT and TD-DFT calculations performed under vacuum).

Computationally calculated absorption values are consistent with the experimentally observed values. Dilute solutions of 1−6 in THF are weakly emissive (Figure 1, right). The luminescence quantum yields calculated for 1−6

in THF are 1.2%, 0.07%, 0.09%, 0.04%, 0.3%, and 0.5%, respectively,29 and are consistent with the above observation. The structureless broad weak luminescence characteristics of compounds 1−6 can be ascribed to involvement of a polar CT C

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Organometallics state in the emission process. The excitation spectra of compounds 1−6 showed a red shift in comparison to the respective absorption spectra. These results clearly indicate that the excited states involved in both absorption and emission are different. Further, the maxima corresponding to λCT computed from TD-DFT calculations (Figure S25) matches well with observed excitation spectral maxima. These results directly corroborate the above inference of the involvement of polar CT emissive states in 1−6. To rationalize the root cause for the excited-state deactivation process, excited-state geometry optimization studies were carried out. The results obtained clearly indicated that 1−6 undergo enormous structural reorganization in the electronically excited state (Table S5). Thus, it can be argued that the geometrical reorganization in the excited state is the major deactivation pathway for the photoexcited luminophores and is responsible for the experimentally observed feeble emission characteristics of 1− 6. Therefore, it would be interesting to see the luminescence characteristics of these compounds in the aggregated/ condensed state, where the imposed physical constraints will restrict the geometrical reorganizations, which may invoke a radiative decay pathway to elicit the strong emission. Aggregation-Induced Emission (AIE) Characteristics. The AIE features of 1−6 were studied in a THF−water mixture, to control the solvent polarity and extent of solute aggregation. With increasing water fraction from 0% to 60% in THF solutions of 1−6, no significant changes in their luminescence intensity was observed. An instant enhancement in fluorescence was observed when the water fraction was increased to 70%. These results clearly indicate that the aggregation of 1−6 starts after the addition of 70% of water and is the cause for the enhancement in the emission characteristics. A successive increase in water fraction ( f w (v%) = 80 or 90) leads to a further increase in the luminescence (Figure 3 and Figure S28). The luminescence intensity of solutions of 2−4 with f w (v%) = 90 is significantly lower in comparison to that of the respective solutions with f w (v%) = 80, which could be due to the sudden precipitation of aggregates with higher water content. Compounds in a molecularly dispersed state exhibit weak luminescence due to the active rotations of the aryl rotors that consume the part of excited energy and promote the nonradiative deactivation; in addition, there could be an involvement of nonradiative electron transfer from the electron-rich mesityl/duryl unit to NPI. The luminescence efficiency of compounds gradually decreases with increasing number of electron-donating methyl substituents on the TAB unit. These results clearly indicate that the luminescence of these compounds depends not only on the steric crowding but also on electronic factors. In the aggregated state, intermolecular interactions not only hampered free rotation of these units but also altered the electron transfer process by modifying the molecular confirmations. This consequently restored the radiative decay process. In comparison to their respective dilute solutions, compounds 1, 2, 5, and 6 (∼64, ∼ 77, ∼ 63, and ∼59 nm, respectively) showed a bathochromic shift in the aggregated state, while a hypsochromic shift was observed for 3 (∼10 nm) and 4 (∼58 nm) (Table S6). The observed different Stokes shift for aggregates of the former (1, 2, 5, and 6) and latter (3 and 4) can be attributed to the difference in their cumulative intermolecular interactions. The subtle changes in the torsion angle among TAB-NPI in compounds 1 (phenyl spacer), 2 (xylenyl spacer), and 3 (duryl spacer) have a significant effect on the AIE features. For instance, compound

Figure 3. Luminescence spectra of 1 (top) and 5 (bottom) in THF with increasing water fraction ( f w (v%); concentration 100 μM, λex 330 nm). The inset shows the images of compounds under UV-light illumination (λex 365 nm).

1 showed 290-fold enhancement in emission intensity while 2 and 3 exhibited only 9- and 44-fold enhancement, respectively, in comparison to their respective molecularly dispersed solutions. Further, replacing the distant two mesityl groups in 3 with duryl groups in compound 4 led to a significant decrease in the luminescence enhancement from 44 to 27. These results further substantiate our claim that the luminescence of these compounds not only depends on the steric crowding but is also subject to electronic factors. There is a 200-fold difference in enhancement observed between 1 (p-phenylene bridge) and 5 (m-phenylene bridge). The reason could be well understood by analyzing the supramolecular interactions in the crystal state (vide infra). Compound 1 forms very short C−H···O bonds (2.422 Å) in comparison to 5 (2.712 Å), and the strength of these interactions stiffens the molecules in the aggregated state and boosts the emission. A further decrease in the enhancement was observed in compound 6 (40-fold) (Figure S22). Hence, it can be concluded that, in the condensed state, the imposed physical constraints and the intrinsic propeller geometry of the triarylborane unit avoid the close π···π stacking interactions of naphthalimides (NPI) and carbonyl (CO) of NPI to form strong intermolecular interactions with neighboring molecules; the synergetic effects of both in 1−6 endows these molecules with unique AIE characteristics. This may open up a new avenue for the developments of novel AIE luminophores. To further understand the size distribution of the aggregates, DLS (dynamic light scattering) measurements were carried out for both dilute solutions and aggregates. In the molecularly D

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phenyl spacer between the TAB and NPI units). Further, this could be the reason for the greatly diminished AIE effect of the compounds containing more sterically hindered spacer groups (2−4). In the solid state, intermolecular C−H- - -π (C−H and πcloud of naphthalimide units) and C−H- - -O (C−H of the naphthalimide or C−H of the methyl group of the boryl moiety and the oxygen of the imide unit) interactions between adjacent molecules generate a 2D sheetlike structure, and propagation of these interactions between the adjacent sheets generates a 3D supramolecular network (Figure 5). The metric parameters

dispersed state, no detectable particles were observed. However, in the condensed state, aggregates with an average hydrodynamic radii of ∼151, ∼153, ∼114, ∼155, ∼185, and ∼162 nm were observed for 1−6, respectively (Figure S30 and Table S7). Thus, the enhanced emission in the aggregated state in comparison to dilute solutions can be attributed to the formation of luminescent nanoaggregates. Further, TRF (timeresolved fluorescence) measurements were carried out to understand the luminescence lifetime characteristics. The longer average lifetimes of 1−6 in the aggregated state (1/9 THF/H2O) in comparison to the respective molecularly dispersed state further support the involvement of a greater number of molecules in the formation of aggregates, which are responsible for the luminescence enhancement18 (Figure S31 and Table S8). Single-Crystal X-ray Analysis. To further understand the solid-state structures and their supramolecular interactions, crystallizations of 1−6 were carried out in different organic solvents. Single crystals of 1 and 4−6 suitable for single-crystal X-ray diffraction studies were obtained by slow evaporation of the respective dichloromethane solutions under ambient conditions. The molecular structures of compounds 1 and 4− 6 are shown in Figure 4, and the crystallographic refinement

Figure 5. Intermolecular interaction diagram of compounds 1 (top), 4 (middle), and 5 (bottom).

observed for these intermolecular interactions (in the range 2.60−3.00 Å) are consistent with the values reported for such interactions elsewhere.27,28 In the crystal structures of 1 and 4− 6, no intermolecular face-to-face π- - -π interactions between NPI units were observed. It can be speculated that the C−H- - π and C−H- - -O intermolecular interactions assist the formation of nanoaggregates as well as restrict the intramolecular rotations/vibrations in the condensed state, thereby conferring these molecules with intriguing AIEE signatures. The absence of face-to-face intermolecular π- - -π interactions in these molecules precludes the excimer formation which can be detrimental to the radiative process in this molecules.4 Scanning Electron Microscopy and Solid-State Emission Studies. Irrespective of the difference in their molecular structure composition (steric and electronic factors), similar intermolecular interactions (C−H- - -π and C−H- - -O) were observed in the crystal structures of 1 and 4−6. This intrigued us to probe the effect of molecular conformations on the morphologies of 1−6 in the solid state. Scanning electron microscopy (SEM) studies were carried out to understand the morphology of these materials. Compounds 1−6 (10 mg) were dispersed in spectroscopic grade MeOH (5 mL) by sonication

Figure 4. Molecular structures of 1 (top right), 4 (top left), 5 (bottom left), and 6 (bottom right). Hydrogen atoms are omitted for clarity.

data are summarized in Table S9. Compounds 1, 5, and 6 crystallized in a monoclinic crystal lattice with space group P21/ c, whereas 4 crystallized in an orthorhombic crystal lattice with space group P212121. The smallest dihedral angle was noted between TAB and NPI moieties in 1 (64.25°) in comparison to 4 (78.13°), 5 (78.19°), and 6 (74.62°). The metric parameters are in close resemblance with their respective optimized structures. It is obvious that, in the crystal environment, various intermolecular interactions can change the geometry of the molecules quite substantially, and hence slight differences in torsion angle of the compounds in the crystal and optimized geometry were observed. The duryl spacer in 4 is highly puckered, possibly due to the steric congestion imposed by the presence of four additional methyl groups. This could be the possible reason for the distinct optical features observed for 3 and 4 (with duryl spacer between TAB and NPI units) in comparison to other compounds 1, 5, and 6 (with a simple E

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Organometallics (5 min), and the supernatant solution was drop-casted onto the silicon wafer. The above samples were allowed to evaporate slowly under ambient conditions and were annealed at 100 °C for 1 h prior to SEM imaging. Highly appealing sheets, fibers, tubes, and vesicular morphologies were observed for 1−4, respectively. A flowerlike surface morphology was observed for 5, while 6 exhibited a continuous porous thin film (Figure 6). The dimensions of the

Figure 7. Luminescence spectra of TAB-NPIs 1−6 in the solid state (λex 350 nm).

in both forms; however, in the case of 2−4, the values are different possibly due to the presence of different kinds of intermolecular contacts, arrangement of molecules in the aggregate, and size of aggregates. A study of the growth mechanism of these morphologies is in progress in our laboratory.



SUMMARY The design, facile synthesis, and AIEE properties of the new series of triarylboron-tethered N-aryl-1,8-naphthalimides (TABNPIs) 1−6 are reported. Compounds 1−6 displayed weak emission in solution; however, they exhibit strong luminescence in the aggregated state. We reasoned that the intrinsic propeller geometry of the boryl unit in 1−6 confers to these molecules unique AIE characteristics by preventing cofacial arrangements of NPI units. In the solid state, compounds 1 and 4−6 generate interesting supramolecular structures via intermolecular C− H- - -π and C−H- - -O interactions. The newly synthesized TAB-NPIs exhibited interesting self-assemblies, and the morphologies of these compounds were dictated by their molecular conformations. The dimensions and morphologyadjustable microstructures are highly desirable in practical applications.



Figure 6. Scanning electron microscope (SEM) images of the TABNPIs 1−6 (a)−(f), respectively).

EXPERIMENTAL SECTION

Materials and Methods. Chemicals were purchased from Aldrich (USA) and SDFCL (India) and were used as such unless otherwise specified. All reactions were carried out under an atmosphere of purified nitrogen using standard Schlenk techniques. The 1H and 13C NMR spectra were recorded at 25 °C on a Bruker Avance 400 MHz NMR spectrometer operating at a frequency of 400 MHz for 1H and 100 MHz for 13C. 1H NMR spectra were referenced to TMS (0.00 ppm) as an internal standard. Chemical shift multiplicities are reported as singlet (s), doublet (d), triplet (t), quartet (q), and multiplet (m). 13 C resonances were referenced to the CDCl3 signal at ∼77.6 ppm. High-resolution mass spectra (HRMS) were recorded on a Micromass Q-TOF high-resolution mass spectrometer by the electrospray ionization (ESI) method. Electronic absorption and emission spectra were recorded on a PerkinElmer LAMBDA 750 UV/visible spectrophotometer and Horiba JOBIN YVON Fluoromax-4 spectrometer, respectively. Solutions of all the compounds for spectral measurements were prepared using spectrophotometric grade solvents, a microbalance (±0.1 mg precision), and standard volumetric glassware. Quartz cuvettes with sealing screw caps were used for the solution-state spectral measurements. Solution-state quantum yields were calculated with reference to anthracence (ϕ = 0.27 in EtOH at 27 °C) as standard.29 The time-resolved luminescence (TRF) decay measurements were carried out at a magic angle using a nanosecond diode laser based time correlated single photon counting (TCSPC)

structures of 1−6 are in the micrometer range; which clearly shows that the microstructures are formed by self-assembly of several hundreds of molecules. These results clearly indicate that the morphologies of 1−6 are strongly dictated by the molecular conformations and steric and electronic features. Fluorescence microscopic images of self-assemblies of 1, 2, and 5 showed a blue color luminescence, while 3, 4, and 6 were weakly luminescent (Figure S32). To get further insight into this observation, the solid-state fluorescncce of compounds 1− 6 was studied. All of the compounds were dispersed in MeOH and drop-casted onto quartz plates and annealed (100 °C) prior to luminescnece measurements. Compounds 1−6 displayed emission at ∼443, ∼421, ∼442, ∼ 423, ∼440, and ∼460 nm, respectively, and the emissions were believed to be arise from their respective self-assemblies (Figure 7). In comparison to the solution state, the solid-state emission of all compounds except 4 is red-shifted because intermolecular interactions can stabilize them, leading to a reduction in band gap. The emission in the aggregated state (90/10 THF/water mixture) and in self-assemblies are virtually same in the case of 1, 5, and 6, possibly due to similar kind of interactions existing F

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Organometallics fluorescence spectrometer from IBH (U.K.). Scanning electron micrographs (SEMs) were collected on a field emission scanning electron microscope (FESEM, FEI Inspect F50), and an energy dispersive X-ray spectroscopy apparatus (EDS, Oxford instrument) was attached to the FESEM. Optical microscopy images of compounds were captured on an Olympus 1X71 inverted fluorescence microscope. The samples were prepared by dispersing compounds in MeOH and then drop-casting on a glass plate. Single-crystal X-ray diffraction studies were carried out with a Bruker SMART APEX diffractometer equipped with a three-axis goniometer. The data were integrated using SAINT, and an empirical absorption correction was applied with SADABS. The structures were solved by direct methods and refined by full-matrix least squares on F2 using SHELXTL software. All nonhydrogen atoms were refined with anisotropic displacement parameters, while the hydrogen atoms were refined isotropically on the positions calculated using a riding model.30 Theoretical calculations were performed using the Gaussian 09 suite of quantum mechanical calculations. The hybrid Becke 3-Lee−Yang−Parr (B3LYP) exchange correlation functional was employed to predict the minimum energy molecular geometries of the compounds. Geometries were fully optimized in the gas phase at the B3LYP level of theory by using the 6-31G(d) basis set for all of the atoms. Frequency calculations were performed on each optimized structure using the same basis set to ensure that it was a minimum on the potential energy surface.31 Synthesis of Compounds 1−6. The borylanilines 1a−6a used for coupling with 1,8-naphthalic anhydride for the synthesis of TABNPIs 1−6 were prepared by following the reported procedures.26 2-(4-(Dimesitylboranyl)phenyl)-1H-benzo[de]isoquinoline1,3(2H)-dione (1). A mixture of 1,8-naphthalic anhydride (1.00 g, 5.00 mmol), 4-bis(mesityl)borylaniline (1.75 g, 5.14 mmol), zinc acetate (0.05 mmol), and imidazole (1.00 g, 15.00 mmol) were heated under reflux at ∼120 °C. After the complete consumption of reactants (∼6 h, as confirmed by TLC), the reaction mixture was cooled to room temperature and treated with 2 N aqueous HCl (50 mL). The resultant residue was extracted with dichloromethane (3 × 100 mL); the combined organic phases were washed with water (3 × 100 mL) and dried over anhydrous Na2SO4. The organic volatiles were removed under reduced pressure. The crude product was purified by column chromatography over silica gel, using hexane/EtOAc (70/30) as eluent, to obtain the analytically pure compound as a colorless solid: yield 75%. 1H NMR (400 MHz, CDCl3, 25 °C): δ (ppm) 8.65 (d, J = 8 Hz, 2H), 8.28 (d, J = 8.4 Hz, 2H), 7.82 (m, 2H), 7.76 (m, 2H), 7.32 (d, J = 8.4 Hz, 2H), 6.84 (s, 4H), 2.31 (s, 6H), 2.07 (s, 12H). 13C NMR (100 MHz, CDCl3, 25 °C): δ (ppm) 164.3, 146.1, 141.7, 141.0, 138.9, 138.7, 137.2, 134.4, 131.9, 131.7, 128.7, 128.4, 128.3, 127.2, 122.9, 23.6, 21.3. 11B NMR (376.5 MHz, CDCl3, 25 °C): δ (ppm) 75.7. ESI-MS (positive-ion mode): calcd for C36H32BNO2, 521.2526 Da; found, 544.2426 Da [M + Na]+. Anal. Calcd for C36H32BNO2: C, 82.92; H, 6.19; N, 2.69. Found C, 82.89; H, 6.23; N, 2.72. 2-(4-(Dimesitylboranyl)-3,5-dimethylphenyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (2). Compound 2 was synthesized by following a procedure similar to that for 1. Quantities involved in the preparation of 2 are as follows: 1,8-naphthalic anhydride (1.00 g, 5.00 mmol), 4-(dimesitylboryl)-3,5-dimethylaniline (1.80 g, 5.10 mmol), zinc acetate (0.05 mmol), imidazole (1.00 g, 15.00 mmol). The crude product was purified by column chromatography over silica gel, using hexane/EtOAc (70/30) as eluent, to obtain the title compound as a colorless solid: yield 68%. 1H NMR (400 MHz, CDCl3, 25 °C): δ (ppm) 8.64 (t, J = 7.6 Hz, 2H), 8.28 (m, 2H), 7.81 (m, 2H), 6.88 (s, 4H), 6.77 (d, J = 7.2 Hz, 2H), 2.28 (s, 6H), 2.09 (s, 6H), 2.07 (s, 6H), 2.02 (s, 6H). 13C NMR (100 MHz, CDCl3, 25 °C): δ (ppm) 164.7, 148.2, 144.1, 142.0, 141.3, 141.1, 139.9, 136.3, 134.6, 131.9, 129.2, 129.0, 127.9, 127.5, 123.4, 23.5, 23.4, 23.3, 21.7. 11B NMR (376.5 MHz, CDCl3, 25 °C): δ (ppm) 77.2. ESI-MS (positive-ion mode): calcd for C38H36BNO2, 549.2839 Da; found, 572.2735 Da [M + Na]+. Anal. Calcd for C38H36BNO2: C, 83.06; H, 6.60; N, 2.55. Found: C, 83.10; H, 6.53; N, 2.60. 2-(4-(Dimesitylboranyl)-2,3,5,6-tetramethylphenyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (3). Compound 3 was synthesized by

following a procedure similar to that for 1. Quantities involved in the preparation of 3 are as follows: 1,8-naphthalic anhydride (1.00 g, 5.00 mmol), 4-(dimesitylboryl)-2,3,5,6-tetramethylaniline (2.00 g, 5.14 mmol), zinc acetate (0.05 mmol), imidazole (1.00 g, 15.00 mmol). The crude product was purified by column chromatography over silica gel, using hexane/EtOAc (75/25) as eluent, to obtain the title compound as a colorless solid: yield 72%.1H NMR (400 MHz, CDCl3, 25 °C): δ (ppm) 8.66 (d, J = 7.2 Hz, 2H), 8.30 (d, J = 8.84 Hz, 2H), 7.83 (t, J = 15.2 Hz, 2H), 6.77 (d, J = 6 Hz, 4H), 2.27 (s, 6H), 2.08 (s, 6H), 2.06 (s, 6H), 2.00 (s, 6H), 1.95 (s, 6H). 13C NMR (100 MHz, CDCl3, 25 °C): δ (ppm) 164.1, 149.4, 144.8, 141.5, 141.3, 139.8, 136.7, 134.8, 134.6, 132.3, 131.9, 131.1, 129.4, 129.3, 129.1, 127.5, 123.3, 23.6, 23.5, 21.7, 20.6, 15.0. 11B NMR (376.5 MHz, CDCl3, 25 °C): δ (ppm) 73.6. ESI-MS (positive-ion mode): calcd for C40H40BNO2, 577.3152 Da; found, 600.3117 Da [M + Na]+. Anal. Calcd for C40H40BNO2: C, 83.18; H, 6.98; N, 2.43. Found: C, 83.23; H, 6.91; N, 2.50. 2-(4-(Bis(2,3,5,6-tetramethylphenyl)boranyl)-2,3,5,6-tetramethylphenyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (4). Compound 4 was synthesized by following a procedure similar to that for 1. Quantities involved in the preparation of 4 are as follows: 1,8naphthalic anhydride (1.00 g, 5.00 mmol), 4-(bis(2,3,5,6tetramethylphenyl)boryl)-2,3,5,6-tetramethylaniline (2.20 g, 5.14 mmol), zinc acetate (0.05 mmol), imidazole (1.00 g, 15.00 mmol). The crude product was purified by column chromatography over silica gel, using hexane/EtOAc (80/20) as eluent, to obtain the analytically pure compound as an off-white solid: yield 60%. 1H NMR (400 MHz, CDCl3, 25 °C): δ (ppm) 8.67 (d, J = 7.6 Hz, 2H), 8.30 (d, J = 8.4 Hz, 2H), 7.83 (t, J = 16.4 Hz, 2H), 6.94 (s, 2H), 2.18 (s, 6H), 2.15 (s, 6H), 2.03 (s, 12H), 1.96 (s, 12H). 13C NMR (100 MHz, CDCl3, 25 °C): δ (ppm) 163.8, 150.1, 149.3, 136.8, 136.4, 136.2, 134.5, 134.3, 133.4, 133.2, 132.9, 131.9, 131.6, 130.8, 128.9, 127.2, 123.0, 20.4, 20.3, 19.4, 19.2, 14.8. 11B NMR (376.5 MHz, CDCl3, 25 °C): δ (ppm) 76.2. ESIMS (positive-ion mode): calcd for C42H44BNO2, 605.3465 Da; found, 628.3192 Da [M + Na]. Anal. Calcd for C42H44BNO2: C, 83.30; H, 7.32; N, 2.31. Found: C, 83.27; H, 7.36; N, 2.27. 2-(3-(Dimesitylboranyl)phenyl)-1H-benzo[de]isoquinoline1,3(2H)-dione (5). Compound 5 was synthesized by following a procedure similar to that for 1. Quantities involved in the preparation of 5 are as follows: 1,8-naphthalic anhydride (1.00 g, 15.00 mmol), zinc acetate (0.05 mmol), 3-(dimesitylboryl)aniline (1.75 g, 5.14 mmol), imidazole (1.00 g, 15.00 mmol). The crude product was purified by column chromatography over silica gel, using hexane/ EtOAc (80/20) as eluent, to obtain the title compound as an off-white solid: yield 70%. 1H NMR (400 MHz, CDCl3, 25 °C): δ (ppm) 8.62 (m, 2H), 8.25 (m, 2H), 7.77 (t, J = 7.6 Hz, 2H), 7.63 (m, 4H), 6.82 (s, 4H), 2.28 (s, 6H), 2.07 (s, 12H). 13C NMR (100 MHz, CDCl3, 25 °C): δ (ppm) 164.7, 147.1, 141.7, 141.5, 139.2, 137.0, 136.8, 135.9, 134.6, 132.6, 132.1, 131.9, 129.5, 128.9, 128.6, 127.4, 123.2, 24.0, 21.6. 11 B NMR (376.5 MHz, CDCl3, 25 °C): δ (ppm) 73.5. ESI-MS (positive-ion mode): calcd for C36H32BNO2, 521.2526 Da; found, 544.2864 Da [M + Na]+. Anal. Calcd for C36H32BNO2: C, 82.92; H, 6.19; N, 2.69. Found: C, 82.87; H, 6.26; N, 2.76. 2,2′-(5-(Dimesitylboranyl)-1,3-phenylene)bis(1H-benzo[de]isoquinoline-1,3(2H)-dione) (6). Compound 6 was synthesized by following a a procedure similar to that for 1. Quantities involved for preparation of 6 are as follows: 1,8-naphthalic anhydride (0.25 g, 1.20 mmol), 5-(dimesitylboryl)benzene-1,3-diamine (1.10 g, 3.00 mmol), zinc acetate (0.05 mmol), imidazole (0.40 g, 6.25 mmol). The crude product was purified by column chromatography over silica gel, using hexane/EtOAc (60/40) as eluent, to obtain the title compound as a colorless solid: yield 60%. 1H NMR (400 MHz, CDCl3, 25 °C): δ (ppm) 8.62 (d, J = 7.02 Hz, 4H), 8.23 (d, J = 8.3 Hz, 4H), 7.75 (t, J = 7.7 Hz, 4H), 7.59 (s, J = 1.9 Hz, 2H), 7.45 (t, J = 2 Hz, 1H), 6.81 (br s, 4H), 2.24 (s, 6H), 2.14 (s, 12H). 13C NMR (100 MHz, CDCl3, 25 °C): δ (ppm) 164.3, 147.5, 141.8, 141.5, 139.4, 137.2, 136.3, 134.5, 133.2, 132.1, 132.0, 128.9, 128.7, 127.4, 123.3. 11B NMR (376.5 MHz, CDCl3, 25 °C): δ (ppm) 73.4. ESI-MS (positive-ion mode): calcd for C48H37BN2O4, 716.2846 Da; found, 739.2643 Da [M + Na]+. Anal. G

DOI: 10.1021/acs.organomet.8b00166 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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Calcd for C48H37BN2O4: C, 80.45; H, 5.20; N, 3.91. Found: C, 80.35; H, 5.27; N, 3.88.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00166. Characterization data, details of calculations, crystallographic data, and figures and tables as described in the text (PDF) Accession Codes

CCDC 1415602−1415605 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.



AUTHOR INFORMATION

Corresponding Author

*E-mail for P.T.: [email protected]. ORCID

Pakkirisamy Thilagar: 0000-0001-9569-7733 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS Financial support was provided by the Science and Engineering Research Board (SERB) of India. P.T and P.S thank the Science and Engineering Research Board (SERB) of India, and N.K.K. thanks the Indian Institute of Science (IISc) for a research fellowship.



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DOI: 10.1021/acs.organomet.8b00166 Organometallics XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.organomet.8b00166 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.8b00166 Organometallics XXXX, XXX, XXX−XXX