Letter pubs.acs.org/macroletters
Polyimides with Heavy Halogens Exhibiting Room-Temperature Phosphorescence with Very Large Stokes Shifts Kenta Kanosue and Shinji Ando* Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, Ookayama 2-12-1-E4-5, Meguro-ku, Tokyo 152-8552, Japan S Supporting Information *
ABSTRACT: Semiaromatic polyimides (PIs) and imide compounds containing heavy halogens (Br, I) in pyromellitic moieties were designed and synthesized to examine their photoluminescence properties. Solutions of imides and PI films exhibited reddish-color room-temperature phosphorescence (RTP) with very large Stokes shifts (ca. 10000 cm−1). In addition, the PI films showed small-Stokes-shifted fluorescence emissions at around 540 nm with absorption bands arising from aggregated PI chains at 400−500 nm. Enhanced phosphorescence observed for the PI films under vacuum indicates that the RTP lifetime is significantly influenced by the triplet−triplet energy transfer to atmospheric oxygen. These PIs with very-large-Stokes-shifted RTP are applicable as spectral conversion materials in displays, photovoltaic devices, and crop cultivators, as well as to oxygen/air sensors.
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plexes,12 and triplet states (phosphorescence).13 In our previous studies, we synthesized semiaromatic PIs containing hydroxyl (−OH) groups in the dianhydride moieties that undergo ESIPT.8,10 Because of significant structural transformation in the excited state, these PIs exhibit reddish fluorescence emission with very large Stokes shifts (ν > 8500 cm−1) under UV irradiation. However, these ESIPT PIs require multistep synthetic procedures to introduce −OH groups into the dianhydride moieties. In addition, there is a structural limitation; −OH groups must be introduced into the positions spatially close to the hydrogen acceptor groups, that is, carbonyl oxygen in the imide bonds. In order to circumvent the synthetic difficulties as well as enhance the Stokes shift, innovative design concepts are required for highly photoluminescent nextgeneration PIs. Most recently, room-temperature phosphorescence (RTP), which is emitted from organic compounds without heavy metals, has attracted increasing attention owing to its various functional applications, such as cellular imaging and temperature and oxygen sensors.14−17 In general, the lowest triplet excited state (T1) lies at a lower energy level than the lowest singlet excited state (S1), resulting in a larger Stokes shift of phosphorescence compared to that of fluorescence. RTP has been investigated mainly for heavy-metal complexes in which efficient spin−orbital couplings facilitate intersystem crossing (ISC).13 On the other hand, phosphorescence from purely
olyimides (PIs) are well-known as a class of superengineering plastics possessing high thermal, mechanical, and environmental stabilities originating from their rigid chemical structures and strong intermolecular interactions.1 They have been applied in a wide range of industries, including the automotive, microelectronics, photonics, and aerospace industries. Recently, the photoluminescence properties of solid PI films have been studied extensively owing to their possible applications as spectral converters in displays, photovoltaic devices, and crop cultivators.2−10 The photoluminescence of aromatic PI films has not received attention for a long time because of their very low fluorescence quantum yields (Φ). For example, the Φ value of a PI film prepared from pyromellitic dianhydride and bis(4-aminophenyl)ether (PMDA/ODA), which is commercialized as Kapton-H, has been reported to be 9.7 × 10−7.3 In 2009, we reported that a semiaromatic PI film derived from 1,4-bis(3,4-dicarboxyphenoxy)benzene dianhydride and 4,4′-diaminocyclohexylmethane (HQDEA/ DCHM) exhibited strong blue fluorescence emission with a Φ value of 0.11.5 This high Φ value was attributable not only to effective suppression of charge-transfer (CT) interactions between electron-donating diamine and electron-accepting dianhydride moieties, but also to enhancement of the locally excited π−π* transition and suppression of concentration quenching by a steric effect of the dianhydride structure. Although the Φ values of fluorescent PI films have been improved dramatically,5−7 enlarged Stokes shifts (ν) have been expected for spectral conversion applications. Several methods have been known to enhance the Stokes shifts of organic compounds and polymers, such as those utilizing excited-state intramolecular proton transfer (ESIPT),6,8−11 excited com© XXXX American Chemical Society
Received: August 20, 2016 Accepted: October 31, 2016
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DOI: 10.1021/acsmacrolett.6b00642 ACS Macro Lett. 2016, 5, 1301−1305
Letter
ACS Macro Letters organic molecules has been observed only under cryogenic conditions or in rigid matrices without local molecular motions.18−22 However, Hirata et al.14 recently realized efficient, long-lifetime, and various colored RTP from purely organic materials using host−guest systems. They utilized rigid steroidal host matrix to minimize the nonradiative decay pathways and the oxygen quenching, which results in highly RTP properties. In addition, it was recently reported that organic carbonyl compounds substituted with heavy halogens (Br, I) enhance the possibilities of ISC, which results in largeStokes-shifted RTP emission.15,23−25 Owing to the many advantages of purely organic RTP compounds, such as facile synthetic procedures, nontoxicity, and low environmental load in addition to large Stokes shifts, relevant studies are currently increasing. Takeuchi et al.23 reported that a fluorene derivative, 7-bromo-9,9-didodecylfluorene-2-carbaldehyde, exhibited RTP emission at 500 nm in argon-purged CHCl3. Kim et al.24 dispersed an RTP compound (Br6A) in an isotactic poly(methyl methacrylate) (iPMMA) guest matrix and prepared a microfluidic device, where the temperature gradient around the flowing channel can be monitored by phosphorescence intensities. Ventura et al.25 reported that 1,8-naphthalimide derivatives bearing a Br atom exhibited RTP emission at around 600 nm in air-free toluene. Chen et al.26 synthesized a polyacrylamide substituted by 4-bromonaphthalimide as a pendant group and found that the polymer powder showed a large-Stokes-shifted RTP emission with a high phosphorescence Φ value of 0.074. They attributed this high Φ value to the immobilization of phosphors and the shielding from oxygen owing to the interchain hydrogen bondings. Since PIs can be recognized as carbonyl compounds, introducing heavy halogens bearing effective spin−orbital coupling could make them candidates for novel organic RTP polymers. In addition, the rigid structure and strong intermolecular interaction of PIs are expected to be favorable to obtain efficient RTP emissions. In this letter, we report the synthesis and photoluminescence properties of novel RTP-emitting semiaromatic PIs containing Br or I in the dianhydride moiety (3Br-PI and 3I-PI; Chart 1c,e,
Figure 1. UV−vis absorption and photoluminescence emission spectra of PM-MC, 3Br-MC, and 3I-MC in CHCl3 (1.0 × 10−5 M). Excitation wavelengths (λex) were 316, 369, and 368 nm, respectively. Photo images were taken under white light (left) and UV (λ = 365 nm) irradiation (right). Note that the photoluminescence of PM-MC was too weak to be observed by naked eyes.
The synthetic and experimental details are described in the Supporting Information. Figure 1 shows the ultraviolet−visible (UV−vis) absorption and emission spectra of the model compounds dissolved in CHCl3 (1.0 × 10−5 M) at room temperature. Table 1 summarizes the peak wavelengths in the absorption (λabs) and emission (λem) spectra, Stokes shifts (ν), and photoluminescence quantum yields (Φ) observed in the solutions. PM-MC (without heavy halogens) showed an absorption band at 320 nm and small-Stokes-shifted fluorescence emission at 436 nm. We previously reported that the S0 → S1 transition of PM-MC can be attributed to the n−π* transition with a low oscillator strength, resulting in a very low Φ below 0.001 (Table 1).8 On the other hand, 3Br-MC and 3I-MC containing heavy halogens showed red-shifted absorption bands at 342 and 357 nm, respectively. These bathochromic shifts can be caused by the expansion of π-conjugation in the dianhydride moieties induced by electron-rich heavy halogens. In the emission spectra of 3Br-MC and 3I-MC, intense emission bands were clearly observed at around 590 nm with very large Stokes shifts over 10000 cm−1. Since the solution concentrations were low enough (1.0 × 10−5 M) to avoid the formation of ground- or excited-state complexes, these emissions must have originated from isolated molecules. In addition, these compounds are unlikely to undergo substantial conformational changes or
Chart 1. Chemical Structures of (a) PM-PI, (b) PM-MC, (c) 3Br-PI, (d) 3Br-MC, (e) 3I-PI, and (f) 3I-MC
Table 1. Photoluminescence Properties of PM-MC, 3Br-MC, and 3I-MC in CHCl3 (1.0 × 10−5 M)
respectively). Their properties were examined by comparing them with a PI without heavy halogens (PM-PI; Chart 1a). In addition, the corresponding imide compounds (PM-MC, 3BrMC, and 3I-MC; Chart 1b, d, and f, respectively) were synthesized as model compounds of the PIs to understand their photophysical mechanisms. Furthermore, to clarify the effects of atmospheric oxygen, photoluminescence measurements were conducted under vacuum conditions.
a
1302
imide compound
λabs (nm)
λem (nm)
ν (cm−1)
Φ
PM-MC 3Br-MC 3I-MC
320 342 357
436 591 593
8710a 10180 10310
