Facile Synthesis of Asymmetric aza-Boron Dipyrromethene Analogues Bearing Quinoxaline Moiety

An asymmetric aza-BODIPY analogue bearing quinoxaline moiety was synthesized via a titanium tetrachloride-mediated Schiff-base-forming reaction of 6,7-dimethyl-1,4-dihydroquinoxaline-2,3-dione and benzo[d]thiazol-2-amine. This novel aza-BODIPY analogue forms a complementary hydrogen-bonded dimer due to the quinoxaline moiety in the crystal structure. It also shows intense absorption and fluorescence, with fluorescence quantum yields close to unity. The electrochemical measurements and the DFT calculations revealed the presence of the low-lying HOMO, which benefits their potential applications as an electron-transporting material.

Recently, we have reported a Schiff-base-forming reaction of azaarylamines and diketopyrrolopyrrole, in which the lactam moieties of diketopyrrolopyrrole were successfully converted to aza-BODIPY structures in the presence of titanium tetrachloride (TiCl 4 ) and triethylamine (NEt 3 ) [40,41].A series of asymmetric aza-BODIPYs were also synthesized from monolactams based on the TiCl 4 -based method.The asymmetric compounds exhibited moderately high fluorescence quantum yields of 0.13-0.27(Chart 1, 1a,b) [39].Lu et al. revealed that 1,8-naphthalenimide, isoindole-1,3-dione, and their derivatives can also be converted to asymmetric aza-BODIPYs in simple ethanol reflux with azaarylamines, followed by boron complexation (Chart 1, 3a,b) [29].The aza-BODIPYs thus synthesized, which have a ketone moiety, showed intense fluorescence in solution and moderate solidstate emission (Φ F = 0.005-0.22)due to the asymmetric structures.Jiao et al. reported the synthesis of similar asymmetric aza-BODIPYs from isoindole-1,3-dione based on the TiCl 4mediated method (Chart 1, 2a-c) [28].In contrast to conventional symmetric BODIPYs, these asymmetric aza-BODIPYs exhibited large Stoke shifts and high photostability.The above-mentioned studies indicate that the Schiff-base-forming reaction is a versatile method of synthesizing asymmetric aza-BODIPYs with unique optical and electronic properties.also be converted to asymmetric aza-BODIPYs in simple ethanol reflux w mines, followed by boron complexation (Chart 1, 3a,b) [29].The aza-BOD thesized, which have a ketone moiety, showed intense fluorescence in solu erate solid-state emission (ФF = 0.005-0.22)due to the asymmetric structu reported the synthesis of similar asymmetric aza-BODIPYs from isoindole-1 on the TiCl4-mediated method (Chart 1, 2a-c) [28].In contrast to conventio BODIPYs, these asymmetric aza-BODIPYs exhibited large Stoke shifts and h bility.The above-mentioned studies indicate that the Schiff-base-forming re satile method of synthesizing asymmetric aza-BODIPYs with unique op tronic properties.

Chart 1. Structures of asymmetric aza-BODIPYs synthesized via a Schiff-base-formi
To broaden the scope of the TiCl4-mediated Schiff-base-forming reactio thesis of asymmetric aza-BODIPYs, herein, we investigated the synthesis o metric aza-BODIPYs (4a and 4b) bearing a quinoxaline moiety.The targe were synthesized from 6,7-dimethyl-1,4-dihydroquinoxaline-2,3-dione and zothiazole.These asymmetric aza-BODIPYs showed intense absorption an in solution.In the crystal structure of 4a, a complementary hydrogen-bond the J-type arrangement of the dimer were observed in the packing diagram
In the crystal structure of 4a, a complementary hydrogen-bonded dimer and the J-type arrangement of the dimer were observed in the packing diagram.

Synthesis and Characterization
The Schiff-base-forming reaction of 6,7-dimethyl-1,4-dihydroquinoxaline-2,3-dione and benzo[d]thiazol-2-amine in the presence of TiCl 4 and NEt 3 and subsequent boron complexation provided the target asymmetric aza-BODIPYs, 4a and 4b (Scheme 1).The aza-BODIPY 4a was obtained in a higher yield of 42% than 4b (27%) despite its lower solubility than 4b.We characterized 4a and 4b by NMR spectroscopy (Figures S1-S4).The structure of 4a was unambiguously elucidated by X-ray crystallography.In a unit cell, an acetonitrile molecule was also found as a solvent molecule.The single crystal of 4a was obtained via the slow diffusion of acetonitrile into a chloroform solution of 4a (Figure 1).The aza-BODIPY 4a features a highly coplanar structure in the crystal structure.The C9-O1 bond length of 4a (1.228(3) Å) is a typical C=O bond length (ca.1.22 Å).The longer bond distance of B1-N3 (1.577(4) Å) than that of B1-N1 (1.543(4) Å) implies the asymmetric structure of 4a.Similar trends have been reported for the B-N bonds in other asymmetric amido/imino BF2 complexes [28,29,31,42,43].In the unit cell, two molecules were stacked with each other to form π-π stackings (Figure S5).The amide unit of the quinoxaline moiety formed a hydrogen bond with the neighboring molecule with N-H⋯O distance of 2.85 Å (Figure 2).The hydrogen bonds mean that the molecules are nearly parallel to each other.In the molecular packing diagram, hydrogen-bonded dimers are slip-stacked in a J-type arrangement.The interplanar distances between the mean planes of the next neighbors are 3.16 and 3.38 Å (Figure 2).The above results indicate that hydrogen bonding is critical for influencing molecular packing and determining the crystal structure.The structure of 4a was unambiguously elucidated by X-ray crystallography.In a unit cell, an acetonitrile molecule was also found as a solvent molecule.The single crystal of 4a was obtained via the slow diffusion of acetonitrile into a chloroform solution of 4a (Figure 1).The aza-BODIPY 4a features a highly coplanar structure in the crystal structure.The C9-O1 bond length of 4a (1.228(3) Å) is a typical C=O bond length (ca.1.22 Å).The longer bond distance of B1-N3 (1.577(4) Å) than that of B1-N1 (1.543(4) Å) implies the asymmetric structure of 4a.Similar trends have been reported for the B-N bonds in other asymmetric amido/imino BF 2 complexes [28,29,31,42,43].In the unit cell, two molecules were stacked with each other to form π-π stackings (Figure S5).The amide unit of the quinoxaline moiety formed a hydrogen bond with the neighboring molecule with N-H• • • O distance of 2.85 Å (Figure 2).The hydrogen bonds mean that the molecules are nearly parallel to each other.In the molecular packing diagram, hydrogen-bonded dimers are slip-stacked in a J-type arrangement.The interplanar distances between the mean planes of the next neighbors are 3.16 and 3.38 Å (Figure 2).The above results indicate that hydrogen bonding is critical for influencing molecular packing and determining the crystal structure.The structure of 4a was unambiguously elucidated by X-ray crystallography.In a unit cell, an acetonitrile molecule was also found as a solvent molecule.The single crystal of 4a was obtained via the slow diffusion of acetonitrile into a chloroform solution of 4a (Figure 1).The aza-BODIPY 4a features a highly coplanar structure in the crystal structure.The C9-O1 bond length of 4a (1.228(3) Å) is a typical C=O bond length (ca.1.22 Å).The longer bond distance of B1-N3 (1.577(4) Å) than that of B1-N1 (1.543(4) Å) implies the asymmetric structure of 4a.Similar trends have been reported for the B-N bonds in other asymmetric amido/imino BF2 complexes [28,29,31,42,43].In the unit cell, two molecules were stacked with each other to form π-π stackings (Figure S5).The amide unit of the quinoxaline moiety formed a hydrogen bond with the neighboring molecule with N-H⋯O distance of 2.85 Å (Figure 2).The hydrogen bonds mean that the molecules are nearly parallel to each other.In the molecular packing diagram, hydrogen-bonded dimers are slip-stacked in a J-type arrangement.The interplanar distances between the mean planes of the next neighbors are 3.16 and 3.38 Å (Figure 2).The above results indicate that hydrogen bonding is critical for influencing molecular packing and determining the crystal structure.To give a deep insight into the hydrogen-bonding interactions in solution, temperature-dependent chemical shifts of the 1 H NMR spectra of 4a were investigated in 1,1,2,2- The structure of 4a was unambiguously elucidated by X-ray crystallography.I unit cell, an acetonitrile molecule was also found as a solvent molecule.The single crys of 4a was obtained via the slow diffusion of acetonitrile into a chloroform solution of (Figure 1).The aza-BODIPY 4a features a highly coplanar structure in the crystal structu The C9-O1 bond length of 4a (1.228(3) Å) is a typical C=O bond length (ca.1.22 Å).T longer bond distance of B1-N3 (1.577(4) Å) than that of B1-N1 (1.543(4) Å) implies asymmetric structure of 4a.Similar trends have been reported for the B-N bonds in ot asymmetric amido/imino BF2 complexes [28,29,31,42,43].In the unit cell, two molecu were stacked with each other to form π-π stackings (Figure S5).The amide unit of quinoxaline moiety formed a hydrogen bond with the neighboring molecule with H⋯O distance of 2.85 Å (Figure 2).The hydrogen bonds mean that the molecules nearly parallel to each other.In the molecular packing diagram, hydrogen-bonded dim are slip-stacked in a J-type arrangement.The interplanar distances between the me planes of the next neighbors are 3.16 and 3.38 Å (Figure 2).The above results indicate t hydrogen bonding is critical for influencing molecular packing and determining the cr tal structure.

Photophysical Properties
The photophysical properties of 4a and 4b were measured in chloroform solution and film (Figure 4).The details are summarized in Table 1.The aza-BODIPY 4a exhibits absorption with distinctive vibronic structures comprising the less intense 0-0 band at 449 nm and the 0-1 vibronic band at 423 nm as a main band in the visible region.A similar absorption spectral profile with the intense 0-1 vibronic band at 460 nm is also observed for 4b.The molar absorption coefficients are 1.24 × 10 4 M -1 cm -1 and 1.97 × 10 4 M -1 cm -1 for 4a and 4b in tetrahydrofuran solution, respectively (Figure S8).Despite the aza-BODIPYlike structure, the absorption spectra of 4a and 4b exhibit blueshifts from those of the conventional BODIPY 5 with maximum wavelength (λmax) at 500 nm and aza-BODIPY 6 with λmax at 713 nm [44,45] (Chart 2).As detailed in the following section, the DFT calculations revealed that the blueshifts can be ascribed to the different frontier molecular orbital (MO) distribution patterns from those of regular BODIPYs.

Photophysical Properties
The photophysical properties of 4a and 4b were measured in chloroform solution and film (Figure 4).The details are summarized in Table 1.The aza-BODIPY 4a exhibits absorption with distinctive vibronic structures comprising the less intense 0-0 band at 449 nm and the 0-1 vibronic band at 423 nm as a main band in the visible region.A similar absorption spectral profile with the intense 0-1 vibronic band at 460 nm is also observed for 4b.The molar absorption coefficients are 1.24 × 10 4 M −1 cm −1 and 1.97 × 10 4 M −1 cm −1 for 4a and 4b in tetrahydrofuran solution, respectively (Figure S8).Despite the aza-BODIPYlike structure, the absorption spectra of 4a and 4b exhibit blueshifts from those of the conventional BODIPY 5 with maximum wavelength (λ max ) at 500 nm and aza-BODIPY 6 with λ max at 713 nm [44,45] (Chart 2).As detailed in the following section, the DFT calculations revealed that the blueshifts can be ascribed to the different frontier molecular orbital (MO) distribution patterns from those of regular BODIPYs.The aza-BODIPYs 4a and 4b display intense fluorescence at 458 and 475 nm wi small Stokes shifts of 438 and 687 cm −1 , respectively, as a mirror image to the absorpti spectra.The ФF values are almost unity for 4a (0.93) and 0.72 for 4b.The slightly smal ФF value of 4b is mainly due to the structural flexibility arising from the long alkoxy cha which enhances the nonradiative decay processes.
In contrast to the reported asymmetric aza-BODIPYs such as 1a and 1b, which exhi moderate quantum yields in the film state (0.27 for 1a and 0.22 for 1b), the fluorescence 4a and 4b is virtually quenched in the film state, exhibiting ФF values as law as ca.0.0 0.04.In the film state, the 0-0 emission band disappeared due to self-absorption.The s nificant overlap of the absorption and emission due to the small Stokes shifts causes t self-absorption quenching in the solid state.
The impact of pH values on the stability of 4a was also examined (Figure S9).T absorption and fluorescence spectral profiles of 4a in acetonitrile do not change in t range from pH = 2 to pH = 11.At a higher pH than pH = 12, both absorption and fluore cence spectra showed redshifts, and the fluorescence spectra became broad and structur less.These spectral changes are probably due to the deprotonation of the amide unit

[V] [eV] [nm]
[nm] 4a 1.20 -1.21 -3.66 2.63 -6.29 423 487 4b 1.17 The aza-BODIPYs 4a and 4b display intense fluorescence at 458 and 475 n small Stokes shifts of 438 and 687 cm −1 , respectively, as a mirror image to the abs spectra.The ФF values are almost unity for 4a (0.93) and 0.72 for 4b.The slightly ФF value of 4b is mainly due to the structural flexibility arising from the long alkox which enhances the nonradiative decay processes.
In contrast to the reported asymmetric aza-BODIPYs such as 1a and 1b, which moderate quantum yields in the film state (0.27 for 1a and 0.22 for 1b), the fluores 4a and 4b is virtually quenched in the film state, exhibiting ФF values as law as c 0.04.In the film state, the 0-0 emission band disappeared due to self-absorption.nificant overlap of the absorption and emission due to the small Stokes shifts ca self-absorption quenching in the solid state.
The impact of pH values on the stability of 4a was also examined (Figure absorption and fluorescence spectral profiles of 4a in acetonitrile do not chang range from pH = 2 to pH = 11.At a higher pH than pH = 12, both absorption and cence spectra showed redshifts, and the fluorescence spectra became broad and st less.These spectral changes are probably due to the deprotonation of the amide The aza-BODIPYs 4a and 4b display intense fluorescence at 458 and 475 nm with small Stokes shifts of 438 and 687 cm −1 , respectively, as a mirror image to the absorption spectra.The Φ F values are almost unity for 4a (0.93) and 0.72 for 4b.The slightly smaller Φ F value of 4b is mainly due to the structural flexibility arising from the long alkoxy chain, which enhances the nonradiative decay processes.
In contrast to the reported asymmetric aza-BODIPYs such as 1a and 1b, which exhibit moderate quantum yields in the film state (0.27 for 1a and 0.22 for 1b), the fluorescence of 4a and 4b is virtually quenched in the film state, exhibiting Φ F values as law as ca.0.03-0.04.In the film state, the 0-0 emission band disappeared due to self-absorption.The significant overlap of the absorption and emission due to the small Stokes shifts causes the self-absorption quenching in the solid state.
The impact of pH values on the stability of 4a was also examined (Figure S9).The absorption and fluorescence spectral profiles of 4a in acetonitrile do not change in the range from pH = 2 to pH = 11.At a higher pH than pH = 12, both absorption and fluorescence spectra showed redshifts, and the fluorescence spectra became broad and structureless.These spectral changes are probably due to the deprotonation of the amide unit of the quinoxaline moiety.The observed high stability of 4a over the wide range of pH benefits its use under physiological conditions.

Electrochemical Properties
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements were performed in dichloromethane containing 0.1 M tetrabutylammonium perchlorate (TBAP) as a supporting electrolyte (Figure 5).Table 1 summarizes the redox potentials and experimental LUMO and HOMO energy levels estimated from the onset of the first reduction waves and the optical band gaps and LUMO values, respectively.The estimated LUMO (−3.66 eV for 4a and −3.63 eV for 4b) and HOMO (−6.29 eV for 4a and −6.16 eV for 4b) values are significantly lower than those of the widely used electron-transporting material, Alq 3 (aluminum tris(8-hydroxyquinolinnate)) (−3.0 eV for LUMO and −5.7 eV for HOMO), implying their potential applications for such a purpose in organic lightemitting diodes.
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measu were performed in dichloromethane containing 0.1 M tetrabutylammonium pe (TBAP) as a supporting electrolyte (Figure 5).Table 1 summarizes the redox p and experimental LUMO and HOMO energy levels estimated from the onset o reduction waves and the optical band gaps and LUMO values, respectively.The e LUMO (−3.66 eV for 4a and −3.63 eV for 4b) and HOMO (−6.29 eV for 4a and −6.4b) values are significantly lower than those of the widely used electron-transpo terial, Alq3 (aluminum tris(8-hydroxyquinolinnate)) (−3.0 eV for LUMO and −5 HOMO), implying their potential applications for such a purpose in organic light diodes.

Theoretical Calculations
Density functional theory (DFT) and time-dependent (TD)DFT calculati model structure of 4 at the B3LYP/6-31G(d) level were conducted to give a detaile into the electronic structures (Figures 6 and 7).The TDDFT calculations assign absorption band as a HOMO-LUMO transition (Table 2).The TDDFT calcula vealed the wavelength of the S1 excited state at 400 nm with the oscillator strengt The S1 state mainly comprises the HOMO-to-LUMO transition.Although the t energy is slightly overestimated, the TDDFT calculation reproduced the observed tion spectrum.Because of the absence of an s-indacene-like conjugated system in ture of 4, the HOMO and LUMO distribution patterns of 4 are different from

Theoretical Calculations
Density functional theory (DFT) and time-dependent (TD)DFT calculations on a model structure of 4 at the B3LYP/6-31G(d) level were conducted to give a detailed insight into the electronic structures (Figures 6 and 7).The TDDFT calculations assign the main absorption band as a HOMO-LUMO transition (Table 2).The TDDFT calculations revealed the wavelength of the S1 excited state at 400 nm with the oscillator strength of 0.46.The S1 state mainly comprises the HOMO-to-LUMO transition.Although the transition energy is slightly overestimated, the TDDFT calculation reproduced the observed absorption spectrum.Because of the absence of an s-indacene-like conjugated system in the structure of 4, the HOMO and LUMO distribution patterns of 4 are different from those of regular aza-BODIPYs, exhibiting larger MO coefficients of the HOMO on the meso-nitrogen atom compared with the LUMO.Therefore, the HOMO is more stabilized than the LUMO by the electron-deficient nitrogen atom at the meso-position.The observed blueshifts of the absorption spectrum of 4 can be explained by the resultant wide HOMO-LUMO gap.
regular aza-BODIPYs, exhibiting larger MO coefficients of the HOMO on the meso-nitro gen atom compared with the LUMO.Therefore, the HOMO is more stabilized than th LUMO by the electron-deficient nitrogen atom at the meso-position.The observe blueshifts of the absorption spectrum of 4 can be explained by the resultant wide HOMO LUMO gap.

Instrumentation and Measurements
Electronic absorption spectra were recorded on a JASCO V-770 spectrophotomete Fluorescence spectra were recorded on an SPEX Fluorolog-3-NIR spectrometer (HORIBA regular aza-BODIPYs, exhibiting larger MO coefficients of the HOMO on the meso-ni gen atom compared with the LUMO.Therefore, the HOMO is more stabilized than LUMO by the electron-deficient nitrogen atom at the meso-position.The obser blueshifts of the absorption spectrum of 4 can be explained by the resultant wide HOM LUMO gap.

Instrumentation and Measurements
Electronic absorption spectra were recorded on a JASCO V-770 spectrophotome Fluorescence spectra were recorded on an SPEX Fluorolog-3-NIR spectrometer (HORI

Instrumentation and Measurements
Electronic absorption spectra were recorded on a JASCO V-770 spectrophotometer.Fluorescence spectra were recorded on an SPEX Fluorolog-3-NIR spectrometer (HORIBA) with an NIR-PMT R5509 photomultiplier tube (Hamamatsu).Absolute fluorescence quantum yields were measured using a Hamamatsu Photonics C9920-03G calibrated integrating sphere system with self-absorption correction.The thin films for solid fluorescent measurement were fabricated via spin-coating a solution of 4a and 4b in CHCl 3 , respectively. 1 H NMR spectra were recorded on a JEOL JNM-ECX500 spectrometer (operating at 495 MHz for 1 H) using a residual solvent as an internal reference for 1 H (δ = 7.26 ppm for CDCl 3 ).CV and DPV measurements were conducted in a CH 2 Cl 2 solution containing 0.1 M TBAP and 0.1 M samples with a scan rate of 0.1 V s −1 under a nitrogen atmosphere.A glassy carbon electrode and a platinum wire were used as the working and counter electrodes, respectively.A saturated calomel electrode (SCE) was used as a reference electrode.The voltammogram display follows the IUPAC convention.Preparative separations were performed using silica gel column chromatography (KANTO Silica Gel 60 N, spherical, neutral, 40-50 mm).All reagents and solvents used for reactions were of commercial reagent grade and were used without further purification unless noted otherwise.All solvents used in optical measurements were of commercial spectroscopic grade.

Crystallographic Data Collection and Structure Refinement
Suitable crystals of 4a for X-ray analysis were obtained from the vapor diffusion of acetonitrile into a chloroform solution of 4a.Data collection was carried out at −173 • C on a Rigaku Saturn724 diffractometer with MoKα radiation.The structure was solved via a direct method (SHELXT) and refined using a full-matrix least-squares technique (SHELXL).CCDC 2299504 contains the supplementary crystallographic data for this paper.These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif (accessed on 1 November 2023), or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44-1223-336033.

Computational Methods
The Gaussian 16, Revision A.03 [46] software package was used to carry out DFT and TDDFT calculations at the B3LYP/6-31G(d) level of theory.Structural optimizations were performed on model compounds.

Figure 2 .Scheme 1 .
Figure 2. Molecular packing diagram of 4a.Hydrogen bonds are shown as blue dashed lines.The interplanar distances between the mean planes of molecules excluding the two fluorine atoms are shown.

Figure 2 .
Figure 2. Molecular packing diagram of 4a.Hydrogen bonds are shown as blue dashed lines.The interplanar distances between the mean planes of molecules excluding the two fluorine atoms are shown.

Figure 2 .
Figure 2. Molecular packing diagram of 4a.Hydrogen bonds are shown as blue dashed lines.interplanar distances between the mean planes of molecules excluding the two fluorine atoms shown.

Figure 2 .
Figure 2. Molecular packing diagram of 4a.Hydrogen bonds are shown as blue dashed lines.The interplanar distances between the mean planes of molecules excluding the two fluorine atoms are shown.To give a deep insight into the hydrogen-bonding interactions in solution, temperaturedependent chemical shifts of the 1 H NMR spectra of 4a were investigated in 1,1,2,2tetrachloroethane-d 2 .As shown in Figure3, the downfield shift of the NH proton signal

Table 1 .
Summary of optical and electrochemical properties of 4a and 4b.
a Determined by DPV (0.1 M TBAP in CH 2 Cl 2 as a supporting electrolyte at a scan rate of 100 mV s −1 ).b E LUMO = −(E red onset + 4.8) [eV].c Estimated from absorption spectra.d E HOMO = E LUMO − E g [eV].