Article pubs.acs.org/IC
Blue Fluorescence from BF2 Complexes of N,O-Benzamide Ligands: Synthesis, Structure, and Photophysical Properties Minoru Yamaji,*,† Shin-ichiro Kato,†,⊥ Kazuhiro Tomonari,‡ Michitaka Mamiya,‡ Kenta Goto,§ Hideki Okamoto,∥ Yosuke Nakamura,† and Fumito Tani§ †
Division of Molecular Science, Graduate School of Science and Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan Education Program of Materials and Bioscience, Graduate School of Science and Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan § Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 819-0395, Japan ∥ Division of Earth, Life, and Molecular Sciences, Graduate School of Natural Sciences and Technology, Okayama University, Okayama 700-8530, Japan
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‡
S Supporting Information *
ABSTRACT: Small molecules having intense luminescence properties are required to promote biological and organic material applications. We prepared five types of benzamides having pyridine, pyridazine, pyrazine, and pyrimidine rings and successfully converted them into three types of the difluoroboronated complexes, Py@BAs, as novel blue fluorophores. Py@BA having a pyridine moiety (2-Py@BA) showed no fluorescence in solution, whereas Py@BAs of pyridazine and pyrazine moieties (2,3-Py@BA and 2,5-Py@BA, respectively) emitted blue fluorescence with quantum yields of ca. 0.1. Transient absorption measurements using laser flash photolysis of the Py@BAs revealed the triplet formation of 2,3- and 2,5-Py@BAs, while little transient signal was observed for 2-Py@BA. Therefore, the deactivation processes from the lowest excited singlet state of fluorescent 2,3- and 2,5-Py@BAs consist of fluorescence and intersystem crossing to the triplet state while that of the nonfluorescent Py@BA is governed almost entirely by internal conversion to the ground state. Conversely, in the solid state, 2-Py@BA emitted intense fluorescence with a fluorescence quantum yield as high as 0.66, whereas 2,3- and 2,5-Py@BAs showed fluorescence with quantum yields of ca. 0.2. The crystal structure of 2-Py@BA took a herringbone packing motif, whereas those for 2,3- and 2,5-Py@BAs were twodimensional sheetlike. On the basis of the difference in crystal structures, the emission mechanism in the solid state was discussed.
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with fluorescence quantum yields of ca. 0.8 depending on the substituted positions.11 We also determined fluorescence quantum yields in the solid state of these BF2DKs ranging from 0.05 to 0.25, which were smaller than those in solution. Ikeda and co-workers reported unique “excited multimer” formation of BF2 complexes of dibenzoylmethane derivatives.12 Their fluorescence quantum yields varied from ca. 0.2 to 0.7 depending on the molecular aggregation state in the single crystals. Research and development for highly efficient fluorophores are required for their application to organic solid-state electronic devices, such as organic light-emitting devices (OLEDs). However, there are very few applications of BF2DK molecules to OLEDs,13,14 although tremendous efforts have been devoted to investigating BF2DKs in the solid state, such as single crystals and polymer dispersion films. To design new BF2 complexes with efficient emission properties in the solid state, especially blue emission, we need to create boron
INTRODUCTION Fluorescent probes are widely used in the field of biological and organic material science.1 Much attention has been paid to difluoroboronated complexes ligated with dipyrromethane and dibenzoylmethane skeletons as small and intensely fluorescent systems. One of the former is known as a derivative of 4,4difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY), having high fluorescence quantum yields, high extinction coefficients, sharp absorption and fluorescence emission spectra, and high light and chemical stability.2−4 Similarly, aromatic difluoroboronated β-diketones (BF2DKs) emit intense fluorescence not only in solution but also in the solid state.5−7 It is reported that varying the arene systems using biphenyl, naphthyl, and anthryl moieties as the chromophores in BF2DKs is effective for modulating the photophysical properties.8−10 Fluorescence quantum yields of these BF2DKs in solution vary from 1 × 10−3 to unity depending on the chromophores and the substituents.10 In our previous research, we prepared BF2DKs having phenacene moieties such as phenanthrene and chrysene and showed that they emitted intense fluorescence in solution © 2017 American Chemical Society
Received: August 5, 2017 Published: September 26, 2017 12514
DOI: 10.1021/acs.inorgchem.7b02013 Inorg. Chem. 2017, 56, 12514−12519
Article
Inorganic Chemistry
compounds and the analytical data are given in the Supporting Information. Absorption, Emission, and Transient Absorption Measurements. Absorption spectra were recorded on a JASCO V-550 spectrophotometer. Emission spectra in solution were recorded on a Hitachi F-4010 fluorescence spectrophotometer. The fluorescence quantum yields and fluorescence spectra in the solid state were measured with a Hamamatsu Photonics absolute PL quantum yield measurement system (C9920-02). Fluorescence lifetimes (τf) were determined with a Hamamatsu Photonics fluorimeter system (Quantaurus Tau) on the basis of the time-correlated single-photon counting method. Solution samples in a quartz cell with a 1 cm optical path length were prepared in the dark, and purged with Ar when necessary. The third harmonic (355 nm) from a Nd:YAG laser system (LT-2137 from Lotis-TII) was used for laser flash photolysis as the excitation source. The details of the detection system for the time profiles of the transient absorption have been reported elsewhere.16 Transient absorption spectra were obtained using a Unisoku USPT1000-MLT system, which provides a transient absorption spectrum with one laser pulse. The obtained transient spectral data were analyzed by using the least-squares best-fit method. Theoretical Calculations. The calculation was performed at the DFT level with the use of the Gaussian 09 software package.17 The fully optimized geometries of the Py@BAs were calculated by using the 6-31G(d) basis set with the B3LYP method under vacuum.18−22 Atom coordinates for the optimized geometries are deposited in the Supporting Information. Time-dependent DFT (TD-DFT) calculations23−25 were carried out at the TD B3LYP/6-31+G(d) level26,27 by using the optimized ground-state geometries.
complexes different from BODIPYs and BF2DKs by developing new ligands. Currently, an efficient and durable solid-state blue emitter is required. We have been motivated to create a blue emitter with boron complexes different from BODIPYs and BF2DKs. In this context, we started to design ligand systems chelating a boron atom with nitrogen and oxygen atoms as a hybrid feature between BODIPY and BF2DK which have ligation systems with two nitrogen atoms and two oxygen atoms, respectively. Hachiya et al. prepared difluoro(amidopyrazinato-O,N)boron derivatives by mimicking the molecular structures of cypridina oxyluciferin.15 In the present research, we were successful in preparing ligands of benzamides having pyridine, pyridazine, pyrazine, and pyrimidine rings and attempted to synthesize difluoroboron complexes with these ligands (see Chart 1). Chart 1. Molecular Structures and Abbreviations for the Ligands and Difluoroboronated Complexes (Py@BA) Used in This Study
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RESULTS AND DISCUSSION Absorption and Emission Measurements. Figure 1 shows absorption and emission spectra of the Py@BAs prepared in the present work.
Three of the five ligands were successfully converted to the corresponding BF2 complexes. The emission features of the prepared BF2 complexes (Py@BAs) in solution and the solid state were investigated on the basis of fluorescence and transient absorption measurements with the aid of theoretical computations. Crystallographic studies of Py@BAs were also performed to understand the emission mechanism in the microcrystalline state.
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EXPERIMENTAL SECTION
Analytical Instruments. NMR spectra were recorded on NMR System 400 and 600 MHz spectrometers. High-resolution fast atom bombardment mass spectra (HR-FAB-MS) were measured with 3nitrobenzyl alcohol as a matrix and recorded on a double-focusing magnetic sector mass spectrometer (JEOL JMS-700). Melting points were measured with a YANACO micro melting point apparatus. Materials. Chloroform (spectroscopic grade) from Wako and ethanol (EtOH, spectroscopic grade) from Kishida were used as solvents for the spectroscopic measurements without further purification. General Procedures for Preparing the Ligands and Py@BA. The ligands (benzamide) were prepared by a reaction between the appropriate amino-substituted pyridine, pyrimidine, or pyrazine and benzoyl chloride. The prepared ligands were refluxed in benzene for 1 h in the presence of boron trifluoride diethyl etherate. We obtained 2-, 2,3-, and 2,5-Py@BAs, whereas no difluoroboron complexes of 2,4-L and 2,6-L were formed. The precipitated product was collected and washed with benzene. Details of the synthetic procedures for all of the
Figure 1. Absorption (black) and fluorescence (blue) spectra in chloroform at 295 K and phosphorescence spectra (red) in ethanol at 77 K obtained for 2-Py@BA (a), 2,3-Py@BA (b), and 2,5-Py@BA (c). The emission spectra are not corrected.
Py@BAs showed large absorption bands with molar absorption coefficients (ε) in the magnitude of 104 dm3 mol−1 cm−1 (see Table 1), which is typical for π,π* transitions. 2,3-and 2,5-Py@BAs provided weak fluorescence at 295 K, whereas 2-Py@BA showed no fluorescence in solution. The maximum wavelengths (λflu) of fluorescence are given in Table 1. Phosphorescence from Py@BAs was observable in EtOH at 77 K. On the basis of the quantum yields (Φf) and lifetimes (τf) of fluorescence determined in CHCl3, the fluorescence rates (kf), quantum yields (Φnr), and rates (knr) for nonradiative processes were evaluated by eqs 1−3, respectively. 12515
DOI: 10.1021/acs.inorgchem.7b02013 Inorg. Chem. 2017, 56, 12514−12519
Article
Inorganic Chemistry Table 1. Photophysical Parameters of Py@BA Obtained in Chloroform compound
ε/dm3 mol−1 cm−1 (λabs/nm)
λflu/nm
Φf
τf/ns
kfa/108 s−1
Φnrb
knrc/109 s−1
ETd/kcal mol−1
λsolid flu /nm
Φsolid f
τT/μs
2-Py@BA 2,3-Py@BA 2,5-Py@BA
19500 (335) 15000 (328) 19800 (354)
N/A 392 403
0 0.05 0.13
N/A 0.62 0.82
N/A 0.81 1.6
1.0 0.95 0.87
N/A 1.5 1.1
64.3 63.6 61.5
415 397 426
0.66 0.18 0.16
2.1 5.0 12.0
Determined by the equation kf = Φfτf−1. bDetermined by the equation Φnr = 1 − Φf. cDetermined by the equation knr = Φnrτf−1. dDetermined from the origin of the phosphorescence spectrum obtained in ethanol at 77 K.
a
k f = Φf τf −1
(1)
Φnr = 1 − Φf
(2)
k nr = Φnr τf −1
(3)
The obtained photophysical parameters are given in Table 1. The kf value obtained for 2,5-Py@BA was twice as large as that for 2,3-Py@BA, whereas their ε values are close to each other. These results indicated that the Strickler−Berg relationship between ε and kf values is applicable to the present Py@ BA systems.28 The absence of fluorescence from 2-Py@BA may be because of the nonradiative rate, which would consist of internal conversion from the S1 to the ground state and intersystem crossing to the triplet (T1) state, substantially larger than that for the kf value (∼107 s−1). The relaxation processes of Py@BAs will be discussed later. Figure 2 shows fluorescence spectra in the crystalline solid of Py@BA. Figure 3. Transient absorption spectra taken at 100 ns upon 355 nm laser pulsing in the Ar-purged chloroform solution of 2-Py@BA (a), 2,3-Py@BA (b), and 2,5-Py@BA (c) at 295 K. Insets give time profiles of the transient signals.
Transient absorption spectra were measured in the 390−720 nm wavelength region, in Ar-purged chloroform. The intensities of the transient absorption signals decreased with decay lifetimes (τT) in the microsecond time region. The τT data are given in Table 1. In aerated chloroform, the decay was accelerated. This fact indicates that the transient signals are due to triplet−triplet (T-T) absorption. Observation of the triplet state in solution for 2,3- and 2,5-Py@BAs shows that ISC from the S1 to the T1 state competes with the fluorescence process. On the basis of the facts of no fluorescence and little triplet formation for 2-Py@BA, the S1 state deactivates mainly by internal conversion (IC) to the ground state. The IC process of aromatic molecules in excited states is generally induced by vibrational and rotational motions. The difference between 2and 2,3- or 2,5-Py@BAs is the number of nitrogen atom(s) in the heterocycle moiety. It is noteworthy that the difference in the number of the nitrogen atom(s) in the heterocycle moieties caused a large difference in their relaxation processes, from the S1 states although the precise mechanism has not been determined at present. DFT Calculations. To get an insight into the photophysical features of Py@BAs, DFT and TD-DFT calculations were performed at the B3LYP/6-31+G(d) level.26,27 The calculated results are summarized in Table 2. Calculated molecular orbitals, the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), for Py@BAs under vacuum are shown in Figure 4. In the optimized structures of Py@BAs, the HOMO and LUMO are located over the ligand moieties. The S0 → S1 transitions of Py@BAs contributed from the HOMO →
Figure 2. Fluorescence spectra of 2-Py@BA (a), 2,3-Py@BA (b), and 2,5-Py@BA (c) in the solid state at 295 K.
Samples of 2-Py and 2,3-Py@BAs for emission measurement were recrystallized from a mixture of chloroform and n-hexane, whereas that of 2,5-Py@BA was recrystallized from toluene. The fluorescence maxima (λsolid flu ) in the solid state given in Table 1 are red-shifted in comparison with those in solution. It is a typical observation that the wavelength of an emission band in solution differs from that in the solid state.12,29 The fluorescence quantum yields (Φsolid ) in the solid state were f determined, and the data are also given in Table 1. The Φsolid f values of 2,3- and 2,5-Py@BAs are greater than those (Φf) in solution. In particular, 2-Py@BA showed a large value of Φsolid f although it displayed no fluorescence in solution. Transient Absorption Measurements. In order to investigate the deactivation processes from the S1 state, laser flash photolysis of Py@BAs was carried out in chloroform. Transient absorption spectra obtained upon 355 nm laser pulsing in chloroform solution of Py@BAs are displayed in Figure 3. 12516
DOI: 10.1021/acs.inorgchem.7b02013 Inorg. Chem. 2017, 56, 12514−12519
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Inorganic Chemistry Table 2. Calculated Photophysical Data of Py@BAs under Vacuuma compound
HOMO/eV
LUMO/eV
S0 → S1 transitionb
λtrc/nm
fd
coeffe
2-Py@BA 2,3-Py@BA 2,5-Py@BA
−6.83 (−6.84) −7.15 (−7.15) −7.10 (−7.04)
−2.53 (−2.53) −2.90 (−2.88) −3.04 (−3.00)
π,π* π,π* π,π*
321 (328) 328 (330) 339 (350)
0.5579 (0.7686) 0.2644 (0.5041) 0.4532 (0.6298)
0.69566 (0.70179) 0.69584 (0.70249) 0.69992 (0.70402)
a Data in parentheses are calculated results considering the dielectric constant of CHCl3. bCharacter of the transitions to the excited singlet states with the lowest excitation energies. cWavelength estimated from transition energy. dOscillator strength for the S0 → S1 transition. eFor a transition from HOMO to LUMO.
nitrogen-containing heteroaromatics and the benzene rings being 3.8, 7.1, and 5.9°, respectively. In the crystals of Py@BAs, two neighboring molecules form the π-stacked dimer structures in an anti, head-to-tail manner. The intermolecular distances between the center of the heteroaromatic moiety of the molecule and the mean plane of the π skeleton of the facing molecule in the dimers of 2-Py@BA, 2,3-Py@BA, and 2,5-Py@ BA are 3.40, 3.60, and 3.37 Å, respectively, indicating that the π−π stacking interactions are efficient. Figure 6 shows the packing diagrams of the crystal structures of 2-Py@BA, 2,3-Py@BA, and 2,5-Py@BA. Any π-stacked dimers of 2-Py@BA, 2,3-Py@BA, and 2,5Py@BA interact with other dimers through π−π stacking interactions to form one-dimensional (1D) column structures along the a, b, and a axes, respectively. While the 1D columns in the crystal of 2-Py@BA take a slanted, slipped-stacked form, those of 2,3-Py@BA and 2,5-Py@BA take a slightly zigzag form. Noticeably, 2-Py@BA, 2,3-Py@BA, and 2,5-Py@BA differ in their overall packing motifs despite the similarity of their πstacked dimers. In the case of the crystal of 2-Py@BA, the 1D columns are packed in a herringbone motif as a result of multipoint CH−O (2.71 Å) and F−π (3.20 and 3.22 Å) interactions. On the other hand, the columns of 2,3-Py@BA assemble together by CH−O (2.81 Å) and CH−N (2.43 Å) interactions to afford two-dimensional (2D) sheetlike structures. Similar to the crystal structure of 2,3-Py@BA, the columns of 2,5-Py@BA have short contacts with each other by CH−N (2.63 Å) and CH−F (2.58 Å) interactions to form 2D sheetlike structures; toluene molecules are sustained by CH−π interactions (2.81−3.05 Å). In the crystal structures of 2,3-Py@ BA and 2,5-Py@BA, no noticeable F−π interaction was observable, implying that the CH−N and/or CH−F interactions prevail over F−π interactions. Consequently, these findings show that the difference in the heteroaromatic moieties, namely, pyridine, pyridazine, and pyrazine rings in 2Py@BA, 2,3-Py@BA, and 2,5-Py@BA, respectively, is reflected by their supramolecular arrangement in the solid state. In the present work, we determined Φsolid (=0.66) in the f solid state for photoluminescence for 2-Py@BA, which was greater than those for 2,3- and 2,5-Py@BAs (0.16−0.18). By analogy to Φf, Φsolid is expressed as f
Figure 4. HOMO and LUMO orbitals of Py@BAs in vacuum.
LUMO configurations can be assigned as a π,π* transition. The wavelengths (λtr) estimated from the calculated transition energies are in the 320−350 nm wavelength region and are similar to the experimental λabs values of the lowest energy absorption bands in CHCl3. The magnitude of the calculated oscillator strengths (f) for the S0 → S1 transitions is responsible for the allowed π,π* transition. It is known that the Strickler− Berg equation relates the fluorescence rate kf to the oscillator strength f.28 In the present study, we definitely observed no fluorescence from 2-Py@BA in solution, although its calculated f value was in the same order of magnitude as those of 2,3- and 2,5-Py@BAs. This consideration supports the proposed mechanism that the absence of fluorescence from 2-Py@BA in solution may derive from IC from the S1 to the ground state. X-ray Crystallography. We investigated the molecular structures by X-ray diffraction analyses of single crystals of 2Py@BA, 2,3-Py@BA, and 2,5-Py@BA for understanding the emission mechanisms in the microcrystalline state. Crystalline samples for 2-Py@BA and 2,3-Py@BA were prepared from a mixture of chloroform and n-hexane, whereas that for 2,5-Py@ BA was from toluene. While the crystals of 2-Py@BA and 2,3Py@BA contain no solvent molecule, the crystal of 2,5-Py@BA contains toluene, which was used in the recrystallization, in a 2:1 stoichiometry. Figure 5 shows the arrangement of neighboring molecules in the crystals of 2-Py@BA, 2,3-Py@ BA, and 2,5-Py@BA. Molecules of 2-Py@BA, 2,3-Py@BA, and 2,5-Py@BA take a highly planar form with the dihedral angles between the
Φsolid = k fsolid(k fsolid + k nrsolid)−1 f
(4)
where ksolid and ksolid f nr are respectively rates for fluorescence and nonradiative processes in the solid state. As the emission spectra in the solid state and the molecular geometries of the dimer of Py@BAs are similar to each other, ksolid in the solid f state may be in the same order of magnitude. The difference in the Φsolid values may thus be due to that in ksolid f nr , which closely correlates to molecular structures in the crystal in terms of energy dispersion through spin−lattice interaction. The dimers of 2-Py@BA in the crystal were packed in a herringbone pattern, whereas those of 2,3- and 2,5-Py@BAs were in one-
Figure 5. Arrangement of neighboring molecules in the crystal packing for 2-Py@BA (a), 2,3-Py@BA (b), and 2,5-Py@BA (c). The perspective drawings of 2-Py@BA, 2,3-Py@BA and 2,5-Py@BA are shown in the Supporting Information. 12517
DOI: 10.1021/acs.inorgchem.7b02013 Inorg. Chem. 2017, 56, 12514−12519
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Inorganic Chemistry
state, while that of 2-Py@BA is governed almost by IC to the ground state. The difference in the deactivation features between 2- and 2,3- or 2,5-Py@BAs originated from the heterocycle moieties of Py@BAs. In the solid state, these three Py@BAs emitted blue fluorescence. 2-Py@BA showed a Φsolid f value of 0.66, while for 2,3- and 2,5-Py@BAs, the Φsolid values f were ca. 0.17. Crystallographic studies of Py@BAs showed similarities and differences in crystal packing among Py@BAs. π-stacked dimer structures in an anti, head-to-tail manner were seen for these three Py@BAs. From the viewpoint of crystal structures, a herringbone motif was seen for 2-Py@BA, whereas a 1D column structure consisting of the dimer of the molecule was found for 2,3- and 2,5-Py@BAs. In the present work, we demonstrated that the lower number of nitrogens in the heterocyclic moiety for 2-Py@BA in comparison to that for 2,3and 2,5-Py@BAs substantially affects the photophysical properties in solution and the solid state and the crystal structures. The present results provide a strategy to produce a new blue emitter by means of N,O-ligation with boron atoms and display remarkable enhancement of fluorescence efficiency in the solid state: namely, for 2-Py@BA. Additionally, it is expected that one can modify the luminescence properties in solution and the solid state by replacing the phenyl ring in 2-Py@BA with various aromatic chromophores. Further studies are in progress.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02013. Precise synthesis procedures and analytical data for the compounds used in this work, decay profiles of fluorescence, results of DFT calculations including tables of atom coordinates for the optimized geometries, and 1 H and 13C NMR spectra of prepared compounds (PDF) Accession Codes
CCDC 1539204−1539205 and 1555564 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.
Figure 6. Molecular packing in the crystals for 2-Py@BA (a), 2,3-Py@ BA (b), and 2,5-Py@BA (c). The perspective drawings of 2-Py@BA, 2,3-Py@BA, and 2,5-Py@BA are shown in the Supporting Information. Toluene molecules included in the crystal of 2,5-Py@ BA are disordered.
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solid dimensional column structures. The knr value for a herringbone structure is smaller than that for two-dimensional sheetlike structures. It is noteworthy that replacement of nitrogen atoms in heterocycles of 2,3- and 2,5-Py@BAs with a C−H moiety affects the fluorescence character of 2-Py@BA in solution as well as in the solid state, where the large difference in the crystal structures emerges.
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.Y.:
[email protected]. ORCID
Minoru Yamaji: 0000-0001-9963-2136 Yosuke Nakamura: 0000-0002-6047-1336
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Present Address
CONCLUSION Five types of benzamides having pyridine, pyridazine, pyrazine, and pyrimidine rings were prepared, and three types of the difluoroboronated complexes, 2-, 2,3-, and 2,5-Py@s, were successfully produced. 2-Py@BA, which has a pyridine moiety, showed no fluorescence in solution, whereas 2,3- and 2,5-Py@ BAs emitted blue fluorescence with Φf = ca. 0.1. Transient absorption measurement using laser flash photolysis of the Py@ BAs revealed the triplet formation of 2,3- and 2,5-Py@BAs, while little transient signal was obtained for 2-Py@BA. Therefore, the deactivation processes from the S1 state of 2,3and 2,5- Py@BAs consist of fluorescence and ISC to the triplet
⊥
Department of Materials Science, School of Engineering, The University of Shiga Prefecture, 2500 Hassaka-cho, Hikone, Shiga 522-8533, Japan.
Author Contributions
All authors contributed equally. Funding
This work was supported by Grants-in-Aid for Scientific Research (JP26288032 and JP17K05976) from the Japan Society for the Promotion of Science (JSPS) and was partially supported by the association for the advancement of science and technology, Gunma University. 12518
DOI: 10.1021/acs.inorgchem.7b02013 Inorg. Chem. 2017, 56, 12514−12519
Article
Inorganic Chemistry Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Cooperative Research Program of the Network Joint Research Center for Materials and Devices to M.Y. and H.O., respectively. M.Y. acknowledges the technical staff at Kyushu University for performing the HRMS spectrometry of the new compounds under the Cooperative Research Program of the Network Joint Research Center for Materials and Devices.
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DOI: 10.1021/acs.inorgchem.7b02013 Inorg. Chem. 2017, 56, 12514−12519
