Article pubs.acs.org/Organometallics
Bistable Polyaromatic Aminoboranes: Bright Solid State Emission and Mechanochromism Neena K. Kalluvettukuzhy and Pakkirisamy Thilagar* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India S Supporting Information *
ABSTRACT: Reported herein are the synthesis, structure, and intriguing optical characteristics of four new polyaromatic aminoboranes (1−4) bearing dimesitylboron (Mes2B) as electron-accepting unit(s) and diphenylamine (Ph2N) as electron-donating unit(s). These compounds are strongly fluorescent in the solid state. Crystalline samples of 1 and triarylboranedecorated aminoboranes 3 and 4 were found to be blue emitters in the solid state. Compounds 1 and 2 showed aggregation-induced emission (AIE) and aggregation-induced emission color switching, respectively, while 3 and 4 exhibited aggregation-induced emission enhancement. Compounds 1 and 2 showed fascinating mechanofluorochromism upon grinding, and such fluorescence changes are due to a crystalline−amorphous phase transition, as confirmed by powder X-ray diffraction studies (PXRD). Interestingly, a ground sample of 2 was found to be stable and did not revert back upon removal of external stress even after the sample was kept over a long period of time under ambient conditions (more than 6 months). “IPC” was written on a substrate of 2, and the part that was touched showed fluorescence different from the rest of the substrate, which could be erased by heating, demonstrating its capability for rewritable data storage devices. The effect of steric and electronic factors on the optical properties of molecules was corroborated by DFT computational studies.
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INTRODUCTION Recently, a great deal of research effort has been devoted to modulate the optical properties of π-conjugated systems by substituting a CC fragment with an isoelectronic and isosteric BN unit.1 Intense research efforts on BN-based materials are largely driven by their potential applications in several fields ranging from materials science to biology.1−3 The first breakthrough in the field of BN/CC isosterism was led by Dewar and co-workers in the early 1950s and 1960s.4 Since then, several research groups have made seminal contributions to the field.1−21 To name a few, the groups of Ashe,6 Piers,7 Liu,8 Braunschweig,9 Müllen,10 Bettinger,11 and Nakamura and Hatakeyama12 have independently made significant contributions to BN heterocyclic chemistry. Very recently, numerous organic−inorganic hybrid polymers containing BN fragments have been elegantly developed by research groups of Jäkle,13 Pei,14 Perepichka,15 Yamamoto,16 and Helten.17 In spite of the enormous research progress in this field, the development of the chemistry of acyclic aminoboranes is not as significant as that of azaborine and BN-incorporated PAH systems.1−21 In the early 1970s Williams and co-workers studied the synthesis and preliminary optical properties of various acyclic aminoboranes.18 Later in 2007, Wang and co-workers exploited acyclic aminoboranes as anion sensor and electroluminescent material.19 In 2012, Yamguchi et al. observed the large Stokes shift from a series of acyclic aminoboranes with cyclic amine donors and explained that the large Stokes shift is due to the twisted intramolecular charge transfer (TICT) phenomena.20 © XXXX American Chemical Society
Very recently, Helten et al. demonstrated the synthesis and unique optical characteristics of oligo-/polyiminoboranes.17 Despite these developments, knowledge about the solid-state luminescent aminoboranes has been elusive.21 Recently we became interested in investigating the optical characteristics of luminescent compounds in both the solid state and solution by constraining the molecular conformations.21,22 Very recently, we demonstrated the solid-state emission, mechanoluminescence, and triboluminescence properties of a series of aminoboranes.21 In recent years, smart materials, which change their color upon external stimuli, have attracted considerable attention due to their potential applications in niche technology. Bright solid-state emission and good emission color contrast are desirable features of mechanochromic materials.23,24 Considering the market demand for external stimuli-responsive materials, we explored the potential of aminoboranes as a new class of external stimuli-responsive materials by modulating their optical characteristics by introducing electron-rich and electron-deficient substituents onto the B−N unit. For these studies we have synthesized the four new TAAB (tetraarylaminoboranes) 1−4 and studied their external stimuli-responsive optical characteristics. These compounds exhibit intriguing optical properties such as aggregaSpecial Issue: Tailoring the Optoelectronic Properties of Organometallic Compounds with Main Group Elements Received: May 2, 2017
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DOI: 10.1021/acs.organomet.7b00332 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics tion-induced emission (AIE), high-contrast mechanoluminescence, and bright solid-state emission. The unique mechanoluminescence characteristics of 1 and 2 have been successfully exploited for rewritable data storage. All of these intriguing results are reported in this paper.
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RESULTS AND DISCUSSION Synthesis and Characterization. Compounds 1−4 (Chart S1 in the Supporting Information) were synthesized by following the procedures reported very recently by Thilagar et al.21a (Scheme 1). Compounds 1, 3, and 4 were prepared by Scheme 1. Synthesis of Aminoboranes 1−4
Figure 1. Absorption (top) and emission spectra (bottom) of compounds 1−4 measured in THF solvent (concentration 10−5 M and λex 350 nm). The inset shows the emission spectra of compounds 1 and 2.
lithiation of the corresponding diarylamine (1a, 3a, and 4a) with 1 equiv of n-BuLi at −78 °C followed by quenching with bis(mesityl)fluoroborane. Compound 2 was prepared by a Buchwald−Hartwig cross-coupling reaction between 2a and diphenylamine. All compounds were characterized by 1H, 13C, and 11B NMR spectroscopy and high-resolution mass spectrometry (HRMS). Compounds 1−4 are soluble in common organic solvents such as toluene, dichloromethane, ethyl acetate, chloroform, and tetrahydrofuran and insoluble in water. All compounds were stable under ambient conditions. Thermogravimetric analysis revealed that 1−4 were stable up to ∼300, ∼370, ∼325, and ∼370 °C, respectively, without any noticeable weight loss. 11 B NMR spectra of 1 and 2 showed broad peaks around ∼50.3 and ∼48.2 ppm, respectively, which correspond to the B atom bonded to N. The presence of an additional triarylboron unit in 3 and 4 can be well established from their 11B NMR spectra. CDCl3 solutions of 3 and 4 showed two 11B resonances in the regions ∼48−50 and 69−71 ppm. The upfield 11B resonances (∼50.3 and ∼48.3 ppm) could be assigned to the B atom of B− N unit, and the peaks around ∼69−71 ppm were ascribed to the triarylboron unit, respectively. The 11B resonances corresponding to the B atom of the B−N unit in 1−4 are in line with those of the reported aminoboranes with considerable B−N π-bonding characteristics.21,25 The molecular structures of compounds 1−4 were confirmed by single-crystal X-ray diffraction studies. Absorption and Fluorescence Studies. Compounds 1− 4 showed two strong absorption bands lying in the region of 200−400 nm (Figure 1 (top) and Table S1 in the Supporting Information). The higher energy band (
