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Pyrazinoindole-Based Lewis-Acid/Base Assembly: Intriguing Intramolecular Charge-Transfer Switching through the Dual-Sensing of Fluoride and Acid Yonghyeon Baek, Youngjae Kwon, Chanyoung Maeng, Ji Hye Lee, Hyonseok Hwang, Kang Mun Lee, and Phil Ho Lee J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b02942 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Organic Chemistry

Pyrazinoindole-Based Lewis-Acid/Base Assembly: Intriguing Intramolecular ChargeTransfer Switching through the Dual-Sensing of Fluoride and Acid

Yonghyeon Baek,a,b,† Youngjae Kwon,a,† Chanyoung Maeng,a,b Ji Hye Lee,a Hyonseok Hwang,a Kang Mun Lee,a,* and Phil Ho Leea,b,* a

Department of Chemistry and Institute for Molecular Science and Fusion Technology, Kangwon National University, Chuncheon 24341, Kangwon, Republic of Korea b

National Creative Research Initiative Center for Catalytic Organic Reactions,

Kangwon National University, Chuncheon 24341, Kangwon, Republic of Korea †

These authors contributed equally to this work.

E-mail: [email protected], [email protected]

Table of Contents

ABSTRACT: Pyrazinoindole-based Lewis-acid/base assemblies are prepared through the use of regioselective formal [3 + 3] cycloaddition reactions and their intriguing photophysical properties are described. The assemblies exhibit strong emissions in THF solution, which are attributed to ACS Paragon Plus Environment

The Journal of Organic Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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through-space intramolecular charge-transfer (ICT) transitions between the branched Lewisacid/base moieties. Furthermore, these show ratiometrically color-change responses in PL titration experiments, which give rise to new colors through turn-on emissions ascribable to ICT transitions that alternate between the pyrazinoindole units and each triarylboryl or amino moiety, a consequence of the binding of the fluoride or acid. Pieces of filter paper covered by these assemblies demonstrated exhibited blue-shifted color changes when immersed in aqueous acidic solutions, suggesting that these are promising candidate indicators that detect acid through emissive color. Computational data for these assemblies and their corresponding adducts verify the existence of ICT transitions that alternate through fluoride or acid binding.

INTRODUCTION The development of various methodologies for the synthesis of N-heterocyclic compounds has led to considerable research into efficient functional materials for prominent optoelectronic applications.1 In particular, carbazole-based compounds have been widely used in organic lightemitting diodes (OLEDs) and photovoltaic cells due to their excellent thermal stabilities that originate from their structural rigidities and intriguing electronic properties that are based on the electron-rich nature of their carbazole units.2 These carbazole derivatives were further improved through systematic combinations with a variety of functional groups, thereby broadening their applicabilities to optoelectronics.3 Nevertheless, the discovery and development of novel electronic materials based on other N-heterocyclic compounds that exhibit more-efficient photophysical properties are required because the performance of carbazole-based materials has become saturated. In a continuous effort to explore novel classes of N-heterocyclic compounds with intriguing photophysical properties,4 we recently reported a novel regioselective method for the synthesis of pyrazinoindoles, which are similar to their electronically abundant carbazole congeners.5 In addition, indoloquinoxalines bearing pyrazinoindolyl moieties were recently reported to be effective host and fluorescent materials.6 Stimulated by these results, we envisaged that pyrazinoindole derivatives

ACS Paragon Plus Environment

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would exhibit interesting photophysical characteristics. In order to understand the intrinsic photophysical nature of pyrazinoindolyl derivatives and to investigate their potential as attractive molecular scaffolds for sensory materials,7 we considered preparing systems based on pyrazinoindoles difunctionalized with both Lewis-acidic and basic moieties, which is an electronic environment that can promptly be altered by binding to anions or cations (Scheme 1). First, difunctionalized pyrazinoindoles (PI1 and PI2) were regioselectively synthesized through the formal [3 + 3] cycloadditions of a diazoindolinimine with 2H-azirines. In addition, bearing in mind that the luminescence responses of pyrazinoindolyl derivatives to alternating intramolecular electronic environments is worth analyzing for their potential applications as sensory materials, we investigated the fluoride- and acid-binding properties of PI1 and PI2 by titration experiments. Synthesis and characterization details, including molecular structures in the solid state and electronic-transition changes mediated by ion sensing, are provided and augmented by computational study. Scheme 1 Ion binding parhways of pyrazinoindole-based Lewis acid base assemblies. H+

N N

N

H+ N

N

F-

N N

N

F-

N ICT

R2

R1 Protonated

R2

R1

R2

R1

Fluorinated

R1 = B(Mes)2, R2 = NHEt2 [PI1 H] R1 = NHEt2, R2 = B(Mes)2 [PI2 H]

R1 = BF(Mes)2, R2 = NEt2 [PI1 F] R1 = NEt2, R2 = BF(Mes)2 [PI2 F]

R1 = B(Mes)2, R2 = NEt2, PI1 R1 = NEt2, R2 = B(Mes)2, PI2

Color-changed & turn-on emission by ion sensing

Fully reversible process

Controlled ICT

Regioselective synthesis

Acid indicator

RESULTS AND DISCUSSION Synthesis and characterization Pyrazinoindoles PI1 and PI2 functionalized with dimesitylboryl groups capable of binding to ACS Paragon Plus Environment


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anions, as well as diethylamino groups capable of binding to cations, were regioselectively prepared (Scheme 2). First, pyrazinoindoles 3a and 3b, which are oppositely substituted with bromo and iodo groups, were synthesized by regioselective formal [3 + 3] cycloaddition reactions in order to selectively introduce the coupling positions. Halides 3 were then selectively aminated at their iodides using the Buchwald-Hartwig coupling reaction to give 4a and 4b, after which dimesitylboryl groups were introduced at the remaining bromine positions with n-BuLi and Mes2BF, to provide the novel Lewis-acid/base-substituted pyrazinoindoles PI1 and PI2. Scheme 2 Regioselective syntheses of pyrazinoindole based assemblies, PI1 and PI2

NTs

1a

regioselective [3 + 3] cycloaddition

N

N2 N Me

Me N

cat. Rh

+

N

X

3a, X = I, Y = Br, 70% 3b, X = Br, Y = I, 67%

Y X

2a, X = I, Y = Br 2b, X = Br, Y = I

Y

Buchwald-Hartwig

cat. Pd Et2NH

Me N

Me N N

N

- N2, - TsH

1) n-BuLi

N

N

N

2) Mes2BF

X

X

Y

PI1, X = NEt2, Y = BMes2, 54% PI2, X = BMes2, Y = NEt2, 47%

Y

4a, X = NEt2, Y = Br, 60% 4b, X = Br, Y = NEt2, 50%

The solid-state structures of PI1 and PI2 were confirmed by single-crystal X-ray diffraction (Fig. 1, Tables S1 and S2 in the Supporting Information). The boron atom in each assemblies was observed to adopt a perfectly trigonal planargeometry, as evidenced by the sum of the three C–B–C angles (Σ (C–C–C) = 360°, PI1: 120.5°, 116.2° and 123.4°, and PI2: 119.3°, 122.7° and 118.0°). The nitrogen atom of each diethylamino group exhibited a distorted trigonal pyramidal arrangement. Importantly, X-ray analyses revealed that the phenyl ring bridging the dimesitylboryl and diethylamino groups in each structure is distorted with respect to the pyrazinoindole plane, with dihedral angles of 40.0° (borane-bridging phenyl in PI1), 38.8° (amine-bridging phenyl for PI1), 38.3° (borane-bridging in PI2), and 46.9° (amine-bridging phenyl in PI2). Although the features of the solid state structure of each assembly suggest that electron delocalization between the bridging


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phenyl group and the pyrazinoindole moiety is unfavorable, both moieties are electronically connected through the free rotation of the bridging phenyl group in solution, which is clearly supported by intramolecular charge-transfer (ICT) transition bands between the pyrazinoindole units and triarylboryl or anilino moieties (see photophysical properties, below).

Fig. 1 X-ray crystal structures of PI1 (left) and PI2 (right) (40% thermal ellipsoids). H atoms are omitted for clarity.

Photophysical properties To investigate the photophysical properties of the pyrazinoindole-based ion acceptor assemblies (PI1 and PI2), UV−vis absorption and photoluminescence (PL) spectroscopy in THF were performed at room temperature (green solid lines in Figs. 2 and 3 for PI1 and PI2, respectively, and Table 1). Both PI1 and PI2 exhibited dominant low-energy absorption bands at 405 nm and 416 nm, respectively, which are associated with ICT transitions between their pyrazinoindolyl units and anilino moieties (diethylaminophenyl, see computational results below).The high-energy absorptions in the regions centered at 320 nm in the spectra of PI1 and PI2 are assignable to typical dominant π(phenylene) → pπ(B) ICT transitions in the boryl moieties, as are usually observed for other triarylboranes. The PL spectra of the two assemblies in THF exhibit strong broad and structureless emission bands at λem = 528 nm (PI1) and 584 nm (PI2) (green solid lines in Figs. 2b and 3b). In particular, the PL spectra apparently display solvent-dependent emission bands; these bands are dramatically red-shifted from the blue (462 nm for PI1 in cyclohexane, Figs. 2b and 3b,



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Table 1) or green (514 nm for PI2) emissive regions, to the yellow (567 nm for PI1 in DMSO) or orange (618 nm for PI2) regions with increasing solvent polarity, indicative of perfect positive solvatochromism. The observed emissive behavior suggests that the excited states of these pyrazinoindoles have significant polar character, as evidenced by the observed ICT transitions. Similarly, the solvatochromic-emission studies reveal straight lines of positive slope (red lines in Figs. 2c and 3c) when the Stokes shifts (from 3720 cm−1 in cyclohexane to 6790 cm−1 in DMSO for PI1, and from 4700 cm−1 in cyclohexane to 7540 cm−1 in DMSO for PI2) are plotted against the empirical solvent polarity parameter [ET(30)].8 These very large stokes-shift, of over 100 nm, further verify and support the assignment of ICT characteristics9 to these Lewis-acid/baseassembled systems. Consequently, these emissive bands are associated with through-space ICT transitions10 between the triarylboryl and anilino moieties (see computational studies, below). The decay lifetimes (τ) of the emissions from the assembies in THF were determined to be 2.33 ns for PI1, and 9.21 ns for PI2, which are assignable to fluorescence (Table 1, and Figs. S1a and S2a in the Supporting Information).

Fig. 2 (a) UV-vis absorption and (b) PL spectra of PI1 (3.00 × 10−5 M) in various organic solvents. (c) Stokes shift as a function of the empirical solvent-polarity parameter [ET(30)].


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Fig. 3 (a) UV-vis absorption and (b) PL spectra of PI2 (3.00 × 10−5 M) in various organic solvents. (c) Stokes shift as a function of the empirical solvent-polarity parameter [ET(30)]. Changes in optical properties upon titration The ability of fluoride ions to bind to the boron center in each assemblies was investigated by UV-vis absorption and PL titration experiments (Fig. 4). The addition of increasing amounts of fluoride (tetrabutylammonium fluoride, TBAF) to THF solutions of PI1 and PI2 resulted in gradually quenched absorption bands at around 320 nm, which is attributed to reductions in the intensities of the triarylboryl-centered ICT transitions. Interestingly, these PL titration experiments revealed tremendously enhanced emissions at 497 nm for PI1, and 510 nm for PI2, leading to blueshifted colors. In particular, following fluoride titration, 3.7-fold and 1.3-fold enhancements in absolute quantum yield (ФPL) were determined as 0.78 for PI1

and 0.34 for PI2 respectively,

compared to the ФPL values (0.21 for PI1 and 0.27 for PI2, Table 1) determined prior to the addition of fluoride. These dramatic changes and turn-on emission patterns unambiguously indicate that fluoride binding to the boron center interrupts the through-space ICT transition between the triarylboryl and anilino moieties, and triggers a new ICT transition from the pyrazinoindolyl to the anilino unit. Furthermore, the radiative decay constants (kr = ФPL/τ) for PI1 and PI2 were calculated to be 9.0 × 107 and 2.9 × 107 s−1 respectively, while those following fluoride titration were determined to be 8.2 × 108 and 1.8 × 108 s−1, respectively (τ values for PI1 and PI2 following fluoride titration were calculated to be 0.95 and 1.84 ns respectively, Figs. S1b and S2b). These results strongly indicate that the radiative-decay mechanism involving through-space ICT transitions is more efficient than that between the pyrazinoindolyl and anilino units; furthermore, fluoride binding efficiently facilitates the emissive mechanism in each system. Table 1 Photophysical Data for PI1 and PI2 λabsa/nm

λex

(ε × 10−3 M−1 cm−1)

/nm

λem /nm Cyclo-

AceTolb

THFb

Hexb PI1

319 (29.3), 405 (27.2)

336

462

KF

KH

DMSOb

filmc

(× 104)

(× 102)

567

491

4.9

8.3

Φema,d

τa/ns

0.21

2.33

toneb 493

528

544



PI2 ac

320 (22.4), 416 (5.9)

349

514

545

584

602

618

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541

5.0

8.4

0.27

9.21

= 3.0 × 10−5 M in THF. bc = 3.0 × 10−5 M. cMeasured in the film state (5 wt% doped on PMMA).

dAbsolute

PL quantum yield.

In addition, the linear decrease in the intensities of the absorption band at around 320 nm for each compound, and the Job's plot (Fig. S3) based on the PL-titration data clearly indicate 1:1 binding between the boron center and the fluoride ion in each system. The binding constants (KF) for the complexes were determined to be 4.9 × 104 M−1 (PI1) and 5.0 × 104 M−1 (PI2) (insets in Fig. 4, Table 1), which is one order of magnitude lower than that for trimesitylborane [3.3(±0.5) × 105 M−1].11 The low K values for the complexes in this study are believed to originate from the abundant electron densities of the pyrazinoindole moieties. The dimesitylboryl moiety is well-known to be selective for cyanide anions as well as fluoride ions over other anions.12 Titration experiments involving PI1 or PI2, and tetrabutylammonium cyanide (TBACN) in THF clearly exhibited the same turn-on behavior and blue-shifted spectral changes (PI1: λem = 528 nm to 496 nm, PI2: λem = 584 nm to 508 nm, Fig. S4 for PI1 and S5 for PI2) as those observed with fluoride, with similar binding constants (KCN = 4.7 × 104 and 4.8 × 104 M−1 for PI1 and PI2, respectively) to those determined for fluoride.

Fig. 4 Changes in the UV-vis absorption (left) and PL spectra (right) of (a) PI1 and (b) PI2 solutions (3.00 × 10−5 M) in THF upon the addition of TBAF (0 to 3.54 × 10−5 M). The left insets show absorbances at 319 nm (PI1) and 320 nm (PI2) as functions of [F−]. The lines correspond to binding isotherms calculated with KF = 4.9 × 104 M−1 (PI1) and 5.0 × 104 M−1 (PI2). The right insets show changes in the emission colors of PI1 and PI2 under a hand-held UV lamp (λex = 365 nm) following titration. ACS Paragon Plus Environment

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Meanwhile, the addition of excess strong acid (tetrafluoroboronic acid, HBF4) to THF solutions of PI1 and PI2 resulted in the protonation of the anilino moieties and decreases in the intensities of the absorption bands involved in the ICT transitions from the anilino to the pyrazinoindolyl moieties, while also dramatically increasing the intensities of the high-energy emissions centered at 443 nm for PI1 and 451 nm for PI2 (Fig. 5, Table 1). These features induce switching in the emission colors to deep blue and decreases in ФPL (0.08 for PI1 and 0.13 for PI2 following acid titration) when compared to those of the neutral molecules prior to acid titration. Although the kr value for PI1 following acid titration was reduced to 3.3 × 107 s−1, that for PI2 was determined to be 6.6 × 107 s−1, which is a 2.3-fold enhancement over that of neutral PI2 (τ values for PI1 and PI2 following acid titration were calculated to be 0.95 and 1.84 ns respectively, Figs. S1c and S1c). Consequently, protonation of the anilino moiety of PI2 facilitates an efficient emission-decay mechanism.

Fig. 5 Changes in the UV-vis absorption (left) and PL spectra (right) of (a) PI1 and (b) PI2 solutions (3.00 × 10−5 M) in THF upon the addition of HBF4 (0 to 1.92 × 10−3 M). The left insets show absorbances at 405 nm (PI1) and 416 nm (PI2) as functions of [H+]. The lines correspond to the binding isotherms calculated with KH = 8.3 × 102 M−1 (PI1) and 8.4 × 102 M−1 (PI2). The right insets show changes in the emission colors of PI1 and PI2 under a hand-held UV lamp (λex = 365 nm) following titration. Interestingly, the fluorescent responses to both acid and fluoride in these assemblies are fully reversible by applying the opposite trigger ion (Figs. S6–S9). The fluoride adduct of each assembly formed by the addition of a slight excess of TBAF is simply transformed into the neutral compound



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by the addition of acid and finally, the acid adduct is formed by gradually increasing concentration of added acid. These result in a dramatic change in the emission color, from sky blue (fluoride adduct of PI1) or green (fluoride adduct of PI2) to deep blue (each acid adduct). Similarly, the emission band can be restored by the addition of fluoride after treatment of these assemblies with excess acid, resulting in red color shifts, from blue to sky blue (fluoride adduct of PI1) or green (fluoride adduct of PI2). These reversible processes, which involve fluoride and acid adducts, occur consecutively without the degradation of the assemblies, as clearly evidenced through changes in specific peaks in the 1H NMR spectra (Fig. 6). First, PI1 was dissolved in THF-d8 and its 1H NMR spectrum (A) was acquired; this spectrum was used as the control. Spectrum B was obtained following addition of 1.5 equivalents of tetrabutylammonium fluoride (TBAF) to A; this resulted in a shift of the peak corresponding the aryl protons of the mesitylene moiety, from 6.81 to 6.43 ppm, indicating that fluoride was bound to the dimesitylboryl unit. To confirm reversibility, excess HBF4 was added dropwise to B, which resulted in a shift of the aryl peak of the mesitylene moiety, from 6.43 to 6.81 ppm. These results show that the fluoride in the Mes2B–F complex is effectively quenched, and that the present system is reversible. The peak corresponding to the methylene protons in the Et2N moiety was also observed to shift from 3.40 to 3.43 ppm when excess HBF4 was added, suggesting that the Et2N group had been protonated. To provide an accurate comparison, the 1H NMR spectrum (D) obtained after addition of 2.0 equivalents of HBF4 to PI1 was used. The observation that the 1H NMR spectra of C and D were identical confirmed that the acid had protonated the diethylamino group. NMR studies were then conducted on PI2, which gave similar results to those observed for PI1. Specifically, PI2 was dissolved in THF-d8 to furnish its 1H NMR spectrum (A’), which was used as the control. The spectrum (B’) was acquired after adding 1.5 equivalents of TBAF to A’, which revealed that the mesitylene-aryl peak had shifted from 6.81 to 6.43 ppm, suggesting that fluoride was bound to the mesitylboryl moiety. To confirm reversibility, excess HBF4 was added dropwise to B’, which resulted in a shift of the mesitylene-aryl peak from 6.43 to


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6.81 ppm. These results indicate that the fluoride in the Mes2B–F complex is quenched and that the present system is reversible. In addition, the peak corresponding to the methylene protons of the Et2N group was observed to shift from 3.39 to 3.71 ppm by the addition of excess HBF4, consistent with protonation of the Et2N group. The 1H NMR spectrum (D’) after the addition of 2.0 equivalents of HBF4 to PI2 was acquired for comparison. The 1H NMR spectra of C’ and D’ were identical, which provided assurance that the acid had protonated the diethylamino group. In addition, the 19F NMR spectra of B and B’ exhibited sharp peaks at 174.4 ppm (B) and 172.3 ppm (B’) (Figs. S10‒S11), which are typical of fluoride-coordinated triarylboranes,13 which provides strong evidence for complete fluoride–borane binding. H+

Me N N

Me N

N

N

N

BMes2

H .

H+

Me N

N

FEt2N

Me N

H+

N

N

N

N

FEt2N

+

F .

[PI1 H]

BMes2

Me N N

N

FEt2N

BMes2

-

Mes2B

Mes2B

+

NEt2

F .

[PI2 H]

PI1

[PI1 F]

N

F-

NEt2 H .

Me N

H+

Mes2B

NEt2

-

PI2

[PI2 F] a'

a

H+

H+

c D

PI1

[PI1.H]+

D' c

a

H+

[PI1.F]- [PI1.H]+

C

b'

b d d

a'

d'

H+

c' b'

C'

a d

[PI2.F]-

[PI2.H]+

d'

a'

d

d

PI1

B'

[PI1.F]-

b' [PI2.F]-

PI2

a'

a

b' b

b A

d'

d'

F−

b B

d'

d'

d

b

F−

c'

[PI2.H]+

PI2

PI1

A'

PI2

Fig. 6 1H NMR spectra showing changes in the chemical shifts of protons in PI1 and PI2 upon the addition of TBAF and HBF4 in THF-d8. A) PI1 in THF-d8. B) PI1 with TBAF (1.5 equiv.) in THFd8. C) HBF4 (4.0 equiv.) was added to B. D) HBF4 (2.0 equiv.) was added to PI1 in THF-d8. (a: signal corresponding to the proton on the aryl group of the dimesitylboryl moiety. b: signal corresponding to the methylene protons of the diethylamino group. c: H2O. d: n-Bu). A’) PI2 in THF-d8. B’) PI2 with TBAF (1.5 equiv.) in THF-d8. C’) HBF4 (4.0 equiv.) was added to B’. D’) HBF4 (2.0 equiv.) was added to PI2 in THF-d8. (a’: signal corresponding to the proton on the aryl group of the dimesitylboryl moiety. b’: signal corresponding to the methylene protons of the



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diethylamino group. c’: H2O. d’: n-Bu).

Applications as emissive acid indicators The PL spectra of PI1 and PI2 in their film states (5 wt% doped on PMMA) exhibited slightly blue-shifted emission bands compared to those in THF solution, which were in the green region (λem = 491 nm) for PI1 and the yellow (λem = 541 nm, Table 1 and Fig. S12) for PI2. The filmsample emissive colors are nearly identical to those of pieces filter paper covered with PI1 or PI2 (the making method are described in experimental) when held under a hand-held UV lamp (λex = 365 nm), as shown in Fig. 7. To further investigate the changes in fluorescence responses as functions of pH, the emission colors of filter-paper samples covered with PI1 or PI2 were examined after immersion of each into standard aqueous solution at pH 1.0, 2.0, 3.0, and 5.0 for 5 s (Fig. 7). The emissive colors exhibited by PI1 and PI2 were observed to gradually blue shift with decreasing pH, resulting in distinctive deep-blue emissions at pH 1.0. Indeed, the PL spectra of the samples following immersion into the acidic solutions exhibited distinctly enhanced emissions in the blue region (at about 438 nm for PI1 and 445 nm for PI2, Figs. 7a and 7b) that increased with increasing acidity and, at the same time, the intensities of the original PL emissions were observed to gradually decrease with increasing acidity. These experimental results and emissive features are strong evidence that the aniline moieties in PI1 and PI2 are protonated at the interfaces between the solid state and the aqueous acidic solution. Consequently, these materials are promising indicator candidates that detect acidity through blue-shifted changes in emissive color.


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Fig. 7 PL spectra (λex = 336 nm for PI1 and 349 nm for PI2) of filter paper covered with (a) PI1 (orange solid line) and (b) PI2 (orange solid line), and PL spectra acquired following immersion of each sample into standard aqueous pH solutions (pH = 1.0, 2.0, 3.0, and 5.0) and drying in air. The upper insets show the emission color changes of filter-paper pieces covered with PI1 and PI2 after immersion into standard solutions when viewed under a hand-held UV lamp (λex = 365 nm).

Theoretical caluculation To obtain better insight into the origins of the electronic transitions in these assemblies and their alternating behavior upon binding to fluoride or acid, the ground (S0) and first-excited (S1) states of PI1 and PI2, and their anionic and cationic adducts ([PI1·F]−, [PI1·H]+, [PI2·F]− and [PI2·H]+) were optimized using density functional theory at the B3LYP/6-31G(d) level of theory, after which electronic transitions were examined by time-dependent DFT (TD-DFT) calculations (Fig. 8, Table 2, Figs. S13–S18, and Tables S3–S14). The calculated electronic transitions involving the S0 and S1 states for these assemblies and their adducts are in good agreement with the experimentally observed absorption and emission bands. The calculated results for the optimized structures in the ground (S0) state reveal that the major contributions to the low-energy absorptions (below 400 nm) are mainly due to HOMO → LUMO+1 transitions (λabs = 420 nm for PI1 and 412 nm for PI2; Fig. 8a and Table 2). While the HOMOs of both PI1 and PI2 are predominantly localized on their diethylanilino (Et2NPh) moieties (78.2% for PI1 and 79.6% for PI2, Tables S4 and S10), the LUMO+1 is spatially dominant over each pyrazinoindolyl unit (83.6% for PI1 and 75.2% for PI2), indicating that the low-energy absorptions originate from intramolecular charge transfer (ICT) between the anilino and pyrazinoindolyl moieties. On the other hand, the major high-energy transitions for the absorption regions above 400 nm are mainly attributed to HOMO−1 (for PI1) and HOMO−2 (for PI2) → LUMO transitions. The orbital contributions of HOMO−1 in PI1 and HOMO−2 in PI2 are mainly localized on the pyrazinoindolyl moieties (> 70%, Tables S4 and S10), whereas the LUMOs are mainly localized on the triarylboryl units (dimesitylboryl, >62%), resulting



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in absorption processes that correspond to ICT transition from the pyrazinoindolyl to the triarylboryl moieties. Clearly, the calculated transitions indicate that the main absorption processes in PI1 and PI2 occur independently of the ICT transitions between the diethylanilino or triaryboryl and the pyrazinoindole moieties, respectively. Interestingly, the calculations for the anionic or cationic adducts in their ground (S0) states exhibit distinctive ion binding that extinguishes the corresponding ICT transition in each system. The major low-energy absorptions for the fluoride adducts, [PI1·F]− and [PI2·F]−, are assigned to HOMO → LUMO transitions (Table 2), indicating that ICT transitions occur between the anilino moieties (>60% for HOMO, Table S6 and S12) and the pyrazinoindolyl units (>90% for HOMO). On the other hand, the main absorptions for the protonated adducts, [PI1·H]+ and [PI2·H]+, involve HOMO → LUMO+1 (for [PI1·H]+, Table 2) or LUMO (for [PI2·H]+) ICT transitions between the pyrazinoindolyl units (>78% for HOMO, Table S8 and S14) and the triarylboryl moieties (>55% for LUMOs). The calculated results for the ground (S0) states of these adducts indicate that ion binding interrupts the associated ICT transition in each case, which results in the other ICT transition dominating. The DFT calculations on S1-optimized structures of these assemblies exhibit similar features (Fig. 8b and Table 2) to the intrinsic characteristics of their emission bands. The major contributions to the emissions below 450 nm in PI1 and PI2 are HOMO → LUMO transitions, which are assigned to through-space ICT-based emissions between the anilino and triarylboryl moieties because the HOMOs are localized on the anilino units (74% for PI1 and 75% for PI2, Table S4 and S10), whereas the LUMOs are positioned on the triaryboryl units (78% for PI1 and 70% for PI2). The DFT calculations on S1-optimized structures of these assemblies exhibit similar features (Fig. 8b and Table 2) to the intrinsic characteristics of their emission bands. The major contributions to the emissions below 450 nm in PI1 and PI2 are HOMO → LUMO transitions, which are assigned to through-space ICT-based emissions between the anilino and triarylboryl moieties because the HOMOs are localized on the anilino units (74% for PI1 and 75% for PI2, Table S4 and S10), whereas the LUMOs are positioned on the triaryboryl units (78% for PI1 and 70%


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for PI2). The calculated results for the ionic adducts in their S1 states reveal that their original emissive transitions are dramatically altered upon ion binding. While the emissions for the fluoride adducts, [PI1·F]− and [PI2·F]−, are closely related to HOMO → LUMO transitions (Fig. 8b and Table 2) that are attributable to ICT between their pyrazinoindolyl (>90% of LUMO, Table S6 and S12) and triarylboryl moieties (>63% of HOMO), those for the acid adducts, [PI1·H]+ and [PI2·H]+, mainly involve HOMO → LUMO+1 (for [PI1·H]+) or LUMO (for [PI2·H]+) transitions that are attributable to ICT between the triarylboryl (>65% of LUMO, Table S8 and S14) and pyrazinoindolyl moieties (>65% of HOMO). The insight obtained from the computational results involving the S1-optimized structures of these assemblies and their adducts strongly suggest that ion binding to each functionalized center in these assemblies accelerates the interruption of the associated ICT transition while reinforcing the ICT transition involving the other functionality, resulting in emissive-color changes. Furthermore, fluoride binding to the boron centers in PI1 and PI2 evoke a significantly more elevated LUMO level compared to the increase observed for the HOMO level (ΔE between the LUMOs of PI1 and [PI1·F]− or PI2 and [PI2·F]− = 0.59 eV or 0.76 eV; ΔE between the analogous HOMOs = 0.43 eV or 0.28 eV respectively, Fig. 8b), while acid binding to the amino center evokes a more lowered HOMO level compared to that observed for the LUMO level (ΔE between the HOMO PI1 and [PI1·H]+ or PI2 and [PI2·H]+ = 0.86 eV or 1.07 eV; ΔE between the analogous LUMOs = 0.16 eV or 0.53 eV, respectively). Consequently, the changes in the frontier-orbital levels upon ion binding widen the bandgap leading to blue-shifted emissive patterns for these assemblies.



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Fig. 8 Frontier molecular orbitals of PI1 and PI2, and their anionic and cationic adducts ([PI1·F]−, [PI1·H]+, [PI2·F]−, and [PI2·H]+) in (a) their ground states (S0) and (b) their first-excited singlet states (S1), with their DFT-calculated relative energies (isovalue 0.04). The transition energies (in nm) were calculated at the TD-B3LYP/6-31G(d) level of theory. Table 2 Major Low-Energy Electronic Transitions for PI1 and PI2, and Their Anionic and Cationic Salts in Their Ground States (S0) and First Excited Singlet States (S1) Calculated at the TDB3LYP/6-31G(d) level of theory.a state S0 PI1 [PI1·F]−

S1 S0 S1

λcalc/ nm 360.20 419.77 530.51 429.87 491.53

fcalc 0.3092 0.5032 0.2075 0.5194 0.5141

assignment HOMO−1 → LUMO (73.9%) HOMO → LUMO+1 (98.4%) HOMO → LUMO (99.3%) HOMO → LUMO (98.8%) HOMO → LUMO (98.8%)


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365.62 0.4068 HOMO → LUMO+1 (61.0%) 422.19 0.3125 HOMO → LUMO+1 (73.5%) 354.01 0.5575 HOMO−2 → LUMO (75.8%) PI2 412.03 0.3184 HOMO → LUMO+1 (98.1%) S1 610.08 0.2729 HOMO → LUMO (99.4%) S0 443.54 0.2281 HOMO → LUMO (99.2%) [PI2·F]− S1 495.44 0.2678 HOMO → LUMO (89.4%) S0 375.94 0.3364 HOMO → LUMO (90.9%) [PI2·H]+ S1 471.76 0.5541 HOMO → LUMO (97.8%) a Singlet energies for vertical transitions calculated at the optimized S geometries. 1 [PI1·H]+

S0 S1 S0

CONCLUSIONS In summary, novel difunctionalized assembly systems (PI1 and PI2) based on pyrazinoindole moiety were prepared by the regioselective formal [3 + 3] cycloadditions of a diazoindolinimine with 2H-azirines, and fully characterized by single-crystal X-ray crystallography. These assemblies exhibit yellow (for PI1) and orange (for PI2) emissions that are associated with through-space ICT transitions between the triarylboryl and diethylanilino branched-functional moieties. The emissions in each assembly system can be reversibly switched to the blue-energy region, which is triggered by fluoride binding to the boryl center, or protonation of the anilino moiety. Moreover, fluoride binding to PI1 and PI2 results in tremendously enhanced ФPL values compared to those of the neutral assemblies. In particular, rapid, consecutive, and reversible processes involving binding to either fluoride or acid in both assemblies were demonstrated by 1H and 19F NMR spectroscopy. The clear observation of blue emissions from filter-paper pieces covered with these assemblies when immersed in aqueous acidic solutions strongly suggest that PI1 and PI2 are promising candidates for indicators that detect acid through emissive color changes. The computational studies reveal that ion binding results in alternating ICT transitions between the pyrazinoindolyl unit and each branched functional moiety, leading to widened bandgaps that result in blue-shifted emissive patterns. Relevant studies into the photophysical properties of expanded pyrazinoindoles are currently in progress. ACS Paragon Plus Environment


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Experimental Section General consideration. Commercial available reagents were used without purification. All reaction mixtures were stirred magnetically and were monitored by thin-layer chromatography using silica gel pre-coated glass plates, which were visualized with UV light and then, developed using either iodine or a solution of anisaldehyde. Flash column chromatography was carried out using silica gel (230-400 mesh). 1H NMR (400 MHz), 13C{1H} NMR (100 MHz), and 19F NMR (377 MHz) spectra were recorded on NMR spectrometer. Deuterated chloroform was used as the solvent and chemical shift values () are reported in parts per million relative to the residual signals of this solvent [δ 7.26 for 1H (chloroform-d), δ 3.62, 1.79 for 1H (THF-d8), δ 77.2 for 13C{1H} (chloroform-d) and δ 21.29, 26.19, 68.03 for 13C{1H} (THF-d8)]. Infrared spectra were recorded on FT-IR spectrometer as either a thin film pressed between two sodium chloride plates or as a solid suspended in a potassium bromide disk. High resolution mass spectra (HRMS) were obtained by electron impact (EI) ionization technique (magnetic sector - electric sector double focusing mass analyzer) from the KBSI (Korea Basic Science Institute Daegu Center). Melting points were determined in open capillary tube. Synthetic procedure of pyrazinoindole.5 To a test tube were added 3-diazoindolin-2-imines (1a) (2.0 equiv), azirines (1.0 equiv, 0.5 mmol), and [Rh2(oct)4]2 (4.0 mol %) in DCE (2.5 mL) under nitrogen atmosphere. The resulting mixture was stirred at 80 oC for 2 h under nitrogen. Then, Et3N (1.5 equiv) was added and the solution was additionally stirred at 80 °C for 2 h. The mixture was cooled to room temperature, filtered through a pad of Celite, and concentrated under reduced pressure. The residue was then purified by flash column chromatography to give pyrazinoindoles. 2-(4-Bromophenyl)-3-(4-iodophenyl)-5-methyl-5H-pyrazino[2,3-b]indole (3a): Yield: 189.0 mg (70%); Rf = 0.3 (DCM : Hexane = 2 : 1); Ivory solid; Melting point: 249-251 oC; 1H NMR (400 MHz, CDCl3)  8.40 (d, J = 7.8 Hz, 1H), 7.69-7.63 (m, 3H), 7.51-7.46 (m, 3H), 7.40-7.36 (m, 3H), 7.26 (d, J = 8.6 Hz, 2H), 3.98 (s, 3H);

13C{1H}

NMR (100 MHz, CDCl3)  147.1, 144.5, 144.0,


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142.7, 139.4, 139.1, 137.6, 134.7, 132.2, 132.0, 131.7, 129.4, 122.4, 122.2, 121.2, 119.7, 109.6, 94.8, 27.7; IR (film): 2359, 2093, 1644, 1261, 1140, 1003, 547 cm-1; HRMS (EI) m/z: [M]+ Calcd for C23H15BrIN3 538.9494; Found 538.9497. 3-(4-Bromophenyl)-2-(4-iodophenyl)-5-methyl-5H-pyrazino[2,3-b]indole (3b): Yield: 181.0 mg (67%); Rf = 0.3 (DCM : Hexane = 2 : 1); Ivory solid; Melting point: 222-224 oC; 1H NMR (400 MHz, CDCl3)  8.39 (d, J = 7.7 Hz, 1H), 7.68-7.62 (m, 3H), 7.52-7.46 (m, 3H), 7.41-7.35 (m, 3H), 7.24 (d, J = 8.6 Hz, 2H), 3.98 (s, 3H);

13C{1H}

NMR (100 MHz, CDCl3)  147.0, 144.5, 144.1,

142.7, 139.7, 138.8, 137.6, 134.7, 132.2, 132.0, 131.6, 129.4, 123.0, 122.1, 121.2, 119.7, 109.6, 94.1, 27.7; IR (film): 2970, 2341, 1639, 1365, 1216, 1008, 537 cm-1; HRMS (EI) m/z: [M]+ Calcd for C23H15BrIN3 538.9494; Found 538.9496. Pd-catalyzed amination of pyrazinoindole derivatives.14 To an oven-dried Schlenk tube was added Pd(OAc)2 (1.0 mol %), CyJohnPhos (2.0 mol %), NaOt-Bu (1.0 equiv), pyrazinoindole derivatives (3a or 3b, 0.2 mmol, 1.0 equiv), and toluene under nitrogen atmosphere. The flask was capped with a rubber septum under nitrogen purge and the diethylamine (3.0 equiv) was added. The resulting mixture was stirred at 80 oC for 2 h. Then, the mixture was cooled to room temperature, filtered through a pad of Celite, and concentrated under reduced pressure. The residue was purified by flash column chromatography (DCM : Hexane = 2 : 1) to give amination product. 4-(2-(4-Bromophenyl)-5-methyl-5H-pyrazino[2,3-b]indol-3-yl)-N,N-diethylaniline (4a): Yield: 59.2 mg (60%); Rf = 0.2 (DCM : Hexane = 2 : 1); Yellow solid; Melting point: 149-151 oC; 1H NMR (400 MHz, CDCl3)  8.36 (d, J = 7.8 Hz, 1H), 7.61-7.58 (m, 1H), 7.48-7.46 (m, 5H), 7.40 (d, J = 8.9 Hz, 2H), 7.36-7.32 (m, 1H), 6.61 (d, J = 9.0 Hz, 2H), 3.98 (s, 3H), 3.38 (q, J = 7.1 Hz, 4H), 1.18 (t, J = 7.0 Hz, 6H);

13C{1H}

NMR (100 MHz, CDCl3)  149.0, 148.0, 144.7, 143.7, 142.2,

140.4, 133.1, 131.9, 131.6, 131.5, 128.5, 126.1, 121.72, 121.69, 120.8, 120.1, 111.3, 109.4, 44.5, 27.6, 12.8; IR (film): 2904, 2359, 1641, 1275, 1261, 1140, 749 cm-1; HRMS (EI) m/z: [M]+ Calcd for C27H25BrN4 484.1263; Found 484.1265.



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4-(3-(4-Bromophenyl)-5-methyl-5H-pyrazino[2,3-b]indol-2-yl)-N,N-diethylaniline (4b): Yield: 48.5 mg (50%); Rf = 0.3 (DCM : Hexane = 2 : 1); Yellow solid; Melting point: 151-153 oC; 1H NMR (400 MHz, CDCl3) 

8.41 (d, J = 7.8 Hz, 1H), 7.63-7.59 (m, 1H), 7.51-7.45 (m, 5H), 7.36-

7.33 (m, 3H), 6.63 (d, J = 8.9 Hz, 2H), 3.97 (s, 3H), 3.37 (q, J = 7.1 Hz, 4H), 1.17 (t, J = 7.0 Hz, 6H);

13C{1H}

NMR (100 MHz, CDCl3)  147.6, 146.5, 146.1, 144.0, 142.4, 139.9, 134.4, 132.0,

131.4, 131.3, 128.8, 126.7, 122.3, 122.1, 120.7, 120.0, 111.7, 109.4, 44.5, 27.7, 12.8; IR (film): 2359, 2083, 1867, 1639, 1274, 1261, 748, 562 cm-1; HRMS (EI) m/z: [M]+ Calcd for C27H25BrN4 484.1263; Found 484.1260. Dimesitylborylation of pyrazinoindole derivatives.15 PI1 or PI2 precursor (4a or 4b, 0.2 mmol, 1.0 equiv) was dissolved in THF (1.0 mL) and cooled to -78 oC using a dry ice bath. After 30 min at this temperature, n-BuLi (1.5 equiv, 1.6 M) was added dropwise at -78 oC, and the reaction mixture was stirred for 1 h. Then, a solution of dimesitylboron fluoride (1.5 equiv) in dry THF (1.0 mL) was added dropwise and the reaction mixture was stirred for 2 h and gradually warmed up to room temperature. Then, a small amount of water was added with stirring. The solution was extracted with diethyl ether and the organic phase was dried over anhydrous MgSO4 followed by rotary evaporation of diethyl ether. The crude product was purified by silica gel column chromatography (EtOAc : hexane = 1 : 15) to give the desired product. 4-(2-(4-(Dimesitylboryl)phenyl)-5-methyl-5H-pyrazino[2,3-b]indol-3-yl)-N,N-diethylaniline (PI1): Yield: 70.7 mg (54%); Rf = 0.3 (EtOAc : Hexane = 1 : 15); Yellow-green solid; Melting point: 269-271 oC; 1H NMR (400 MHz, CDCl3)  8.40 (d, J = 7.8 Hz, 1H), 7.61-7.53 (m, 3H), 7.507.48 (m, 3H), 7.36-7.32 (m, 3H), 6.81 (s, 4H), 6.51 (d, J = 9.0 Hz, 2H), 3.99 (s, 3H), 3.37 (q, J = 7.0 Hz, 4H), 2.31 (s, 6H), 2.06 (s, 12H), 1.18 (t, J = 7.0 Hz, 6H); 13C{1H} NMR (100 MHz, CDCl3)  149.5, 147.8, 145.0, 144.9, 144.8, 144.7, 142.2, 142.0, 141.0, 138.6, 136.4, 132.9, 131.7, 130.0, 128.4, 128.3, 126.3, 121.8, 120.7, 120.1, 111.0, 109.4, 44.5, 27.6, 23.7, 21.4, 12.8; IR (film): 2359, 2330, 2094, 1643, 1275, 1216, 748 cm-1; HRMS (EI) m/z: [M]+ Calcd for C45H47BN4 654.3894; Found 654.3892. ACS Paragon Plus Environment

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4-(3-(4-(Dimesitylboryl)phenyl)-5-methyl-5H-pyrazino[2,3-b]indol-2-yl)-N,N-diethylaniline (PI2): Yield: 61.5 mg (47%); Rf = 0.3 (EtOAc : Hexane = 1 : 15); Yellow solid; Melting point: 264266 oC; 1H NMR (400 MHz, CDCl3)  8.42 (d, J = 7.7 Hz, 1H), 7.62-7.55 (m, 3H), 7.48-7.45 (m, 3H), 7.35-7.27 (m, 3H), 6.81 (s, 4H), 6.54 (d, J = 8.9 Hz, 2H), 3.98 (s, 3H), 3.36 (q, J = 7.0 Hz, 4H), 2.30 (s, 6H), 2.05 (s, 12H), 1.17 (t, J = 7.0 Hz, 6H);

13C{1H}

NMR (100 MHz, CDCl3)  147.8,

147.4, 146.6, 145.2, 144.4, 143.9, 142.4, 142.0, 141.0, 138.7, 136.2, 134.4, 131.4, 130.0, 128.7, 128.3, 126.9, 122.1, 120.6, 120.0, 111.4, 109.4, 44.5, 27.8, 23.7, 21.4, 12.8; IR (film): 2359, 2089, 1640, 1459, 1275, 1260, 749 cm-1; HRMS (EI) m/z: [M]+ Calcd for C45H47BN4 654.3894; Found 654.3896. X-ray crystallography. Single crystals of PI1 and PI2 were coated with Paratone oil and mounted onto a glass capillary. The crystallographic measurements were performed on a Bruker D8QUEST CCD area detector diffractometer with a graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structure was solved by direct methods and all non-hydrogen atoms were subjected to anisotropic refinement by a full-matrix least-squares method on F2 by using the SHELXTL/PC package, resulting in X-ray crystallographic data of PI1 and PI2 in CIF format (CCDC 1848294−1848295). Hydrogen atoms were placed at their geometrically calculated positions and refined riding on the corresponding carbon atoms with isotropic thermal parameters. The detailed crystallographic data are given in Table S1 and S2 in the Supporting Information. UV/Vis absorption and photoluminescence (PL) measurements. The solution UV/Vis absorption and PL measurements were performed in degassed tetrahydrofuran with a 1.0 cm quartz cuvette (3.0  105 M). PL measurements were also carried out under various conditions, such as in cyclohexane, toluene, tetrahydrofuran, acetone, and dimethyl sulfoxide solution (3.0  105 M) and in film state (PMMA film doped with 5 wt% of PI1 and PI2, respectively) at ambient temperature. The absolute photoluminescence quantum yields (PLQY) of film samples were obtained at room temperature using a 3.2 inch integrating sphere (FM-sphere, HORIBA) equipped on a Fluoromax4P (HORIBA) spectrophotometer. To investigate the possibility of PI1 and PI2 as an emissive ACS Paragon Plus Environment


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indicator for acid, the following experimental were performed: To produce the filter paper pieces (Advantec-2, 1.5 × 1.5 cm) covered with PI1 or PI2, each filter paper piece is immersed into PI1 or PI2 solution in acetone (3.0  105 M) during 5 seconds, respectively and dried in the air for 12 hr. And then, each piece is immersed into the standard pH 1.0, 2.0, 3.0, and 5.0 aqueous solution during 2 seconds, respectively and dried in the air for 5 h. These PL measurements for these filter paper samples were also carried out at ambient temperature. Theoretical calculations. The optimized structure at ground and first excited state of PI1 and PI2 and their anionic or cationic adducts ([PI1·F]−, [PI1·H]+, [PI2·F]−, and [PI2·H]+) were obtained using the density functional theory (DFT) method with the B3LYP functional16,17 and 6-31G(d)18 basis sets. Time-dependent density functional theory (TD-DFT)19 measurements using the hybrid B3LYP functional (TD-B3LYP) were used to obtain the electronic transition energies which also included an account of electron correlation. All the calculations were performed in THF solution. Solvent effects were evaluated with the conductor-like polarizable continuum model (CPCM).20,21 All calculations were carried out using the GAUSSIAN 09 program.22 The GaussSum 3.0 was used to calculate the percent contribution of a group in a molecule to each molecular orbital.23


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. X-ray cryatallography data (PI1 and PI2), copies of the NMR spectra and spectroscopic and computational data for all products. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This paper is dedicated to Professor Hong-Seok Kim (Kyungpook National University) for his honorable retirement. This work was supported by Basic Research Laboratory (NRF2017R1A4A1015405 for K. M. Lee and P. H. Lee) and Creative Research Intiatives (NRF-20110018355 for P. H. Lee) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning. REFERENCES (1) (a) Comprehensive Heterocyclic Chemistry III; Katritzky, A. R.; Ramsden, C. A.; Scriven, E. F. V.; Taylor, R. J. K. Eds.; Pergamon: Oxford, 2008. (b) Heterocyclic Chemistry 5th ed; Wiley; Joule, J. A.; Mills, K. 2010. (c) Modern Heterocyclic Chemistry, Eds.; Wiley-VCH Verlag GmbH & Co. KGaA, 1st ed.; Alvarez-Builla, J.; Vaquero, J. J.; Barluenga, J. 2011. (2) (a) Yang, Z.; Mao, Z.; Xie, Z.; Zhang, Y.; Liu, S.; Zhao, J.; Xu, J.; Chi, Z.; Aldred, M. P. Recent Advances in Organic Thermally Activated Delayed Fluorescence Materials. Chem. Soc. Rev. 2017, 46, 915-1016. (b) Wex, B.; Kaafarani, B. R. Perspective on Carbazole-Based Organic Compounds as Emitters and Hosts in TADF Applications. J. Mater. Chem. C 2017, 5, 8622-8653. (c) Yang, X.; ACS Paragon Plus Environment


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