Source: https://pubs.rsc.org/en/content/articlehtml/2019/qm/c8qm00578h
Timestamp: 2019-04-19 01:00:42+00:00

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We report herein that 4,5-bis(diarylamino)phthalimides exhibit efficient solid-state emission. The phthalimides were easily prepared from dimethyl 4,5-bis(diarylamino)phthalates or commercially available 4,5-dichlorophthalic acid via a few steps. The absorption spectra of the phthalimides in toluene showed strong bands at 337–374 nm and weak bands possessing two shoulders at 378–455 nm. Toluene solutions of the phthalimides fluoresced in the greenish blue to orange-yellow region with good-to-high quantum yields. In contrast, the phthalimides showed no emission in DMSO. Based on the observation, phthalimide was demonstrated to serve as a turn-on type fluorescent probe for hydrazine. The phthalimides dispersed in a thin film of poly(methyl methacrylate) and in powder form fluoresced in the blue-to-green and green-to-orange region, respectively, with high quantum yields. As the electron-donating ability of the diarylamino moieties increased, the emission spectra in solution and the solid states were red-shifted. The density functional theory calculations confirm that the photo-excitation involves an intramolecular charge transfer from diarylamino groups to imide-carbonyl moieties.
Because of the recent advances in organic light-emitting devices (OLEDs), much attention has been paid to the design and development of π-conjugated compounds that exhibit highly efficient fluorescence in the solid state.1 The advances in organic light-emitting field-effect transistors,2 semiconducting lasers,3 and solid-state fluorescence sensing4 also rely on the development of organic solid-state emitters. Hence, studies on solid-state luminescence are pursued to develop novel luminophores that emit visible light in the solid state with high efficiency.
Phthalimide frameworks are easy to synthesize, are thermally stable, and exhibit good electron-accepting ability. Accordingly, various kinds of small molecules and polymeric compounds with phthalimide moieties are developed for application in organic transistors5 and photovoltaic cells.6 Examples of phthalimide-based luminescent materials are also available, which include N-(aminoalkyl)phthalimides,7N-aryl-4-(dialkylamino)phthalimides,8N-cyclohexyl-3-hydroxy- and N-cyclohexyl-3,6-dihydroxyphthalimides,9 4-aryl-N-(4-trifluorophenyl)phthalimides,10 oxydiphthalimides,11 4,5-dicarbazolyl-N-cyclohexylphthalimides,12 4-halo-N-(4-trifluorophenyl)phthalimides,13 and phthalimide-based polyimides.14 However, fluorescent properties reported for the known luminescent phthalimides were mostly those in solution. The precedents of phthalimides that exhibit fluorescence in the solid state are very limited, and the reported fluorescence quantum yields in the solid state, such as crystal, powder, and neat film, were generally lower than 0.3,15 except for 2,2′-(1S,2S)-1,2-cyclohexanediylbis[5,6-di-9H-carbazol-9-yl-1H-isoindole-1,3(2H)-dione] (Φneat film = 0.41)12 and 5-(dimethylamino)-2-(4-methylphenyl)-1H-isoindole-1,3(2H)-dione (Φcrystal = 0.58).8c Thus, the development of phthalimides that efficiently fluoresce in the solid state with high quantum yields exceeding over 0.3 remains unexplored.
During our research on the development of diaminophenylene-cored luminogens that exhibit efficient solid-state fluorescence,16 we recently demonstrated that 4,5-bis(diarylamino)terephthalates emitted blue light with good quantum yields in powder form and in a poly(methyl methacrylate) (PMMA) film.17 The electronic structure of the diaminoterephthalates can be translated as 1,2-bis(acceptor)-4,5-bis(donor)benzene. Based on this analysis, we designed 4,5-bis(diarylamino)phthalimides 1 as novel fluorophores, in which an imide functionality is employed as the equivalent of two acceptors in the 1,2-bis(acceptor)-4,5-bis(donor)benzene framework (Fig. 1). We report herein the synthesis, structures, photophysical properties, and theoretical calculations of 1, showing that 1 can serve as highly efficient solid-state emitters in the blue-to-orange spectral region with quantum yields ranging from 0.30 to 0.57.
Fig. 1 Molecular structures of 4,5-bis(diarylamino)phthalimides 1.
Designed phthalimides 1 were easily prepared from dimethyl 4,5-bis(diarylamino)terephthalates (aryl: C6H5, 4-CF3C6H4)17 or commercially available 4,5-dichlorophthalic acid via two or three steps (Scheme 1). Hydrolysis of the dimethyl esters with lithium hydroxide followed by condensation with aniline derivatives gave 1a–1f in good-to-high yields. Phthalimides 1g–1i were synthesized through a three-step sequence involving condensation of 4,5-dichlorophthalic acid with amine, Pd-catalyzed amination with aniline derivative,18 and Cu-catalyzed arylation. Decomposition temperatures, at which 5% mass loss occurs, of 1 ranged from 247 °C to 336 °C, indicating that 1 are thermally stable.
The absorption spectra of 1 in toluene are shown in Fig. 2.19 Each spectrum shows a strong absorption band at 300–370 nm and a weaker and broad band at 360–500 nm. Based on the DFT calculations, the weaker bands at longer wavelengths are ascribed to the ICT from the two diarylamino moieties to the two imide-carbonyl groups (vide infra). The spectra of 4,5-bis[(4-CF3C6H4)2N] derivatives 1f–1h are significantly blue-shifted compared with those of bis[(C6H5)2N] derivatives 1a–1e, while bis[(4-tert-butylC6H4)2N] derivative 1i shows a red-shifted spectrum with respect to that of 1a. This dependence of the absorption spectra on the electron-donating ability of an Ar2N group is consistent with the ICT nature of the optical transition of 1. Compared with the absorption spectra of the corresponding phthalates, the spectra of 1 are bathochromically shifted, indicating that the HOMO–LUMO gaps of 1 are narrower than those of the corresponding phthalates. Considering that the electron-withdrawing effect of an aminocarbonyl group, i.e. an amide moiety, is weaker than that of an alkoxycarbonyl group, the ICT attribute in 1 is expected to be weaker than those in the corresponding phthalates, which should induce a blue shift in the absorption spectra of 1 as compared with those of the corresponding phthalates. However, the results are contradictory. These findings suggest that the co-planarity of the central benzene ring and the two acceptors (aminocarbonyl groups), which contribute to enhanced π-conjugation, is more important for the HOMO–LUMO gaps than the ICT character.
The fluorescence spectra of 1 in toluene are shown in Fig. 3, and the data are summarized in Table 1. In toluene, 1a–1e having (C6H5)2N groups as donors exhibited green fluorescence at 522–556 nm with good-to-high quantum yields (Φ = 0.33–0.68). Greenish blue emission was observed for 1f–1h having weaker electron-donating (4-CF3C6H4)2N groups compared with a (C6H5)2N group, and 1i with stronger electron-donating (4-tert-butylC6H4)2N groups showed orange-yellow emission. Moreover, when the solvent was changed from toluene to THF and chloroform, the spectra of 1a exhibited a red shift according to the solvent polarity (in THF: λem = 561 nm, Φ = 0.19; in chloroform: λem = 601 nm, Φ = 0.10), suggesting that the excited state had a charge-separated nature (Fig. S5, ESI†). This behavior of the emission colors/spectra, which depend on the electron-donating ability of amino groups and solvent polarity, is consistent with the ICT mechanism.
a λ em: wavelength of emission maximum; Φ: absolute fluorescence quantum yield determined with a calibrated integrating sphere system. b Measured at 10−5 M.
Fig. 4 (a) Optical images of amine-dissolved DMSO solution (10−4 M) and (b) fluorescence images of 1i-added DMSO solution: (1) none (control), (2) hydrazine monohydrate, (3) L-histidine methyl ester dihydrochloride, (4) D-glucosamine hydrochloride, (5) L-cysteine, (6) DL-homocysteine, (7) glutathione, (8) n-octylamine, (9) aniline, and (10) dibenzylamine.
The fluorescent behavior of 1 dispersed in the PMMA film (1 wt%) was investigated.23 The emission maxima (λem) of the 1-doped films were almost similar to those in toluene, except for 1e and 1i, whose emission maximum was blue-shifted by 18 and 24 nm, respectively (Table 1 and Fig. 5). The similarity of the luminescence spectra to those in toluene suggests that there are no electronic intermolecular interactions between 1 dispersed in the PMMA film and that the molecular conformations of 1 in the polymer matrix resemble those in solution. The blue-shifted spectra of 1e and 1i in PMMA film may imply that the molecular conformations are more twisted in polymer film than in solution. The quantum yields of 1-doped PMMA films were good to high (Φ = 0.39–0.66), indicating that 1 is a good candidate for dopants in light-emitting diodes and luminescent sensing films.
Fig. 7 (a) Fluorescence images of 1a, 1c, 1e, and 1g dispersed in the PMMA film; (b) fluorescence images of 1a, 1c, 1e, and 1g in powder, recorded under irradiation using a UV lamp (λex = 365 nm).
Single crystals suitable for X-ray diffraction analysis were obtained from the recrystallization of 1d from CH2Cl2/EtOH solution.24 The molecular and crystal structures are shown in Fig. 8. Compound 1d crystallizes in the monoclinic space group P21/c. As shown in Fig. 8a, each phenyl group of Ph2N moieties is directed upside down with respect to the central benzene ring. As the bond angles of carbon–nitrogen–carbon skeletons in Ph2N moieties are 117.88°, 118.90°, and 121.53°, and 117.33°, 117.44°, and 121.72°, respectively, the two nitrogen atoms are found to be sp2 hybridized; hence, the lone pair of electrons is not fully conjugated with the π-orbitals of the central benzene ring. The 2,4,6-Me3C6H2 group is oriented perfectly perpendicular (90.6°) to the benzene plane. Presumably due to the twisted molecular conformation, there is no π–π stacking in the crystal lattice (Fig. 8b), which aids in reducing the excited state energy loss through the Dexter mechanism. Instead, hydrogen bonds between the carbonyl groups and the benzene hydrogen of the central benzene ring are observed, which partially restrict the molecular motion leading to the loss of the excited state energy (Fig. 8c). Hence, one of the possible reasons for the efficient emission of powder 1d is such a packing motif.
Fig. 8 (a) Molecular and (b and c) crystal structures of 1d.
To gain insight into the electronic structure of 1, we carried out DFT calculations on 1a, 1f, 1f′ (Ar = 4-CF3C6H4, R = C6H5), and 1i at the B3LYP/cc-pVDZ level of theory25 using the Gaussian 09 package (revision D.01).26 The orbital drawings of the HOMOs and LUMOs are shown in Fig. 9, and the energies are listed in Table 2. In each 1, the HOMOs are primarily developed over the Ph2N moieties, the central benzene ring, and an imide nitrogen. In the case of 1f, the benzene ring of the imide moiety, i.e. 4-tert-butylphenyl group, also participates in the HOMO. As the HOMO of 1f′ is also extended over the phenyl group of an imide moiety, the spread of the HOMOs on aryl groups in the imide moiety of 1f and 1f′ is ascribed to the presence of the CF3 groups, but not to the tert-butyl group. Meanwhile, the LUMOs are localized on the central benzene and two carbonyl groups, and no lobes are spread over the benzene rings of the imide moiety in all cases. Time-dependent DFT (TD-DFT) calculations confirmed that the optical transition of 1 was the HOMO to LUMO transition. Thus, the photo-excitation of 1 involved ICT from two Ar2N groups to two carbonyl groups of the imide moiety. As shown in Fig. 2, the absorption spectra exhibited a red shift in the order of 1f, 1a, and 1i. The calculated energy gaps of the HOMOs and LUMOs (ΔEHOMO–LUMO) become smaller in the order of 1f, 1a, and 1i. The consistency between the trend of HOMO–LUMO energy gaps and the bathochromic shift of the absorption spectra demonstrates the validity of the DFT calculations as a method for analyzing the electronic structures of 1.
Fig. 9 HOMO and LUMO drawings of 1a, 1f, 1f′, and 1i.
a Calculated at the B3LYP/cc-pVDZ level using the Gaussian 09 (revision D01). b ΔEexp: energy gap between HOMO and LUMO, estimated from the wavelength of the absorption edge.
We have developed 4,5-bis(diarylamino)phthalimides as novel luminophores that emit fluorescence with high efficiency in the solid state such as in powder form and in doped PMMA films. The solid-state emission color can be altered from blue to orange by changing the electron-donating properties of diarylamino moieties and a substituent on an imide nitrogen. The molecular design of phthalimides 1 is based on the concept that the construction of the 1,2-bis(acceptor)-4,5-bis(donor)benzene structure is effective for inducing twisted molecular conformation and ICT transition resulting in a large Stokes shift, both of which are essential for achieving highly efficient solid-state luminescence. The luminescent phthalimides are readily prepared and their molecular modification is easy, which are attractive features for future applications in solid-state light-emitting devices and chemical/biological sensing.
The sample (1 mg) was placed in a glass tube and dissolved in a toluene solution (1 mL) of PMMA (99 mg). The resulting solution was dropped onto a quartz plate (10 mm × 10 mm) and spin-coated at 300 rpm for 40 s; then the spinning speed was increased to 1000 rpm over a period of 60 s. The deposited film was dried for 12 h in air and under reduced pressure (6.7 × 10−2 Pa) for 4 h.
UV-visible absorption spectra were measured with a Shimadzu UV-2550 spectrometer. Fluorescence spectra and absolute quantum yields were recorded using a calibrated integrating sphere with a Hamamatsu Photonics C9920-02 Absolute PL Quantum Yield Measurement System.
Both density functional theory (DFT) and time-dependent DFT calculations were carried out at the B3LYP/cc-pVDZ level using the Gaussian 09 package (revision D.01). The initial geometries for structural optimizations were based on the molecular structure of 1d determined by X-ray diffraction analysis of a single crystal.
This work was supported by Grants-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan (KAKENHI: 15H03795, 15H00740, 15H00996, and 15K13671). We would like to thank Prof. Takashi Yumura (Kyoto Institute of Technology) for valuable discussion on theoretical results.
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Fluorescence quantum yields of the known phthalimides in the solid state are summarized in the ESI†.
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Absorption spectra of 1a in THF, chloroform, and DMSO, which are almost similar to those in toluene, are shown in Fig. S3 (ESI†).
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A plausible mechanism for hydrazine sensing is shown in the ESI†.
When, for example, the content of 1i in the PMMA film decreased from 1 wt% to 0.1 wt%, or increased to 5 and 10 wt%, the fluorescence quantum yields decreased from 0.46 to 0.40, 0.34, and 0.32.
Data were measured on a Bruker SMART APEX diffractometer (MoKα radiation, λ = 0.71073 Å) at 300 K. The structures were solved by direct methods using SHELXTL program and refined with full-matrix least-squares on F2. CCDC 1553093 (1d)†.
DFT calculations at the cam-B3LYP/cc-pVDZ level were also carried out and the results are summarized in Table S1 (ESI†). The HOMO–LUMO energy gaps determined by the calculations at the B3LYP/cc-pVDZ level were closer to the experimentally determined gaps than those at the cam-B3LYP/cc-pVDZ level.
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