N , N -Dimethyl-4-amino-2,1,3-benzothiadiazole: Synthesis and Luminescent Solvatochromism †

: N , N -Dimethyl-4-amino-2,1,3-benzothiadiazole (BTD NMe2 ) was synthesized from the commercially available 2,1,3-benzothiadiazole (BTD) by nitration in a sulfonitric mixture, followed by a reduction of the nitro group and subsequent methylation with iodomethane. BTD NMe2 was fully characterized by means of nuclear magnetic resonance (NMR) and infrared spectroscopy. The solutions of BTD NMe2 in common organic solvents revealed to be appreciably luminescent in the visible range. The electronic transitions related to the absorption and emission properties were associated with the HOMO–LUMO energy gap by means of electrochemical measurements and DFT calculations. Finally, BTD NMe2 was successfully used to prepare luminescent-doped poly(methyl methacrylate) samples.

Despite the fact that 4-amino-2,1,3-benzothiadiazole was deeply investigated both as a free compound [26] and a possible ligand [27][28][29][30][31][32][33] for the preparation of transition metal complexes, N,N-dimethyl-4-amino-2,1,3-benzothiadiazole (BTD NMe2 ) is much less studied.The only preparation reported in literature dates back to 1976 [34], but the compound was only poorly characterized.Herein, we report an alternative synthesis and the characterization of BTD NMe2 , with a particular interest in the photophysical properties of the compound.The possible application of BTD NMe2 as a dopant for polymeric materials was also explored.

Materials and Methods
Commercial solvents (Merck) were purified following standard methods [35].2,1,3-Benzothiadiazole and the other reagents were Aldrich products, which were used as ) was a TCI Chemicals product.4-Nitro-2,1,3-benzothiadiazole (BTD NO2 ) was synthesized by modifying a reported procedure [36].An amount of 24 mL of H 2 SO 4 98% and 8 mL of HNO 3 70% were mixed in a flask and frozen with a nitrogen bath.2,1,3-Benzothiadiazole (2.000 g, 14.7 mmol) was added, then the reaction was allowed to warm up at room temperature and stirred for three hours.The reaction mixture was then cooled with an ice bath, and water (15 mL) was slowly added.Subsequently, a solution containing about 18.0 g of NaOH in 40 mL of water was added within an hour.After removal from the ice bath, NaHCO 3 was added in small amounts until a neutral pH was reached.The product was extracted with 2 × 40 mL of dichloromethane, and the organic fraction was washed with water (2 × 20 mL), dried over Na 2 SO 4, and evaporated under reduced pressure to yield a reddish solid (yield: 95%).
The characterization data agree with those reported for the same product obtained with different synthetic routes [37].The reduction of 4-nitro-2,1,3-benzothiadiazole (BTD NO2 ) to afford the corresponding 4-amino-2,1,3-benzothiadiazole (BTD NH2 ) was carried out following a reported procedure [38], with slight modifications.To a solution containing 2.000 g of BTD NO2 (11.4 mmol) in 50 mL of ethanol, 9.208 g of FeSO 4 •7H 2 O (34.2 mmol), 4.878 g of ammonium chloride (91.2 mmol), 9 mL of water and 2.243 g of zinc dust (34.2 mmol) were added under vigorous stirring.The mixture was heated at 50 • C for three hours and, after cooling at room temperature, it was cleared by filtration on celite.The solid was washed with 3 × 10 mL of ethanol.The solution thus obtained was evaporated under reduced pressure.The crude product was dissolved in 40 mL of ethyl acetate, and 30 mL of a 25% aqueous solution of NH 4 Cl was added.The organic layer was extracted and washed with water (2 × 20 mL) and with 30 mL of a saturated aqueous solution of NaHCO 3 .The organic fraction was then dried over Na 2 SO 4 and concentrated under reduced pressure.The product was precipitated with isohexane and dried in vacuo (yield: 45%).The characterization data were in agreement with the data reported for the same product prepared with different synthetic routes [28].
Elemental analyses (C, H, N, S) were carried out using an Elementar Unicube microanalyzer.Infrared (IR) spectra were registered using a Perkin-Elmer SpectrumOne spectrophotometer between 4000 and 400 cm −1 using KBr disks.Mono-and bidimensional nuclear magnetic resonance (NMR) spectra were collected employing Bruker Avance 300 and Avance 400 instruments operating respectively at 300.13 MHz and 400.13 MHz of 1 H resonance.The 1 H and 13 C NMR spectra referred to the partially non-deuterated fraction of the solvent, itself referred to as tetramethylsilane.
The absorption spectra were collected in the range 235-700 nm employing a Perkin-Elmer Lambda 40 spectrophotometer.Photoluminescence emission (PL) spectra were registered at room temperature using a Horiba Jobin Yvon Fluorolog-3 spectrofluorometer.A continuous wave xenon arc lamp was used as the source, and the excitation wavelength was selected using a double Czerny-Turner monochromator.Suitable long pass filters were placed in front of the acquisition systems.The detector was composed of a single monochromator iHR320 and a photomultiplier tube Hamamatsu R928.Fluorescence quantum yields Φ f of 5•×10 −5 M solutions were calculated using 5 × 10 −5 M anthracene in ethanol as standard based on Equation (1) [39], where Φ f,std is the quantum yield of anthracene in ethanol (0.27), F and F std are, respectively, the areas under the fluorescence emission bands of the sample and the standard, A and A std are respectively the absorbance values of sample and standard at the excitation wavelength, n is the refractive index of the solvent used for the sample and n std is the refractive index of ethanol.
Electrochemical measurements were carried out on dry acetonitrile solutions of BTD NMe2 containing LiClO 4 as a supporting electrolyte and ferrocene (Fc) as an internal reference.The instrument used was an eDAQ ET014-199 potentiostat, connected to eDAQ 1 mm glassy carbon disk working electrode, eDAQ 1.6 mm diameter Pt/Ti counter-electrode, and a Pt wire as a reference.Fc/Fc + couple was used as the internal standard, and all of the measurements were carried out at room temperature under an argon atmosphere.
Synthesis of N,N-dimethyl-4-amino-2,1,3-benzothiadiazole, BTD NMe2 : The N-methylation of 4-amino-2,1,3-benzothiadiazole (BTD NH2 ) was carried out by modifying a reported procedure [48].An amount of 0.350 g of BTD NH2 (2.3 mmol) were dissolved in 15 mL of N,N-dimethylformamide (DMF), then 3.179 g of K 2 CO 3 (23.0mmol) and 1.4 mL of CH 3 I (23.0 mmol) were added under stirring.The mixture was heated at 75 • C for twelve hours.After cooling at room temperature, 50 mL of water were added, and the product was extracted with 2 × 80 mL of ethyl acetate.The organic fraction was washed with 100 mL of cold water, dried over Na 2 SO 4 , and evaporated under reduced pressure.The product was dissolved in 30 mL of pentane, the solution was purified by filtration, and the solvent was removed under nitrogen flow to afford a red oil (yield: 50%).Characterization of Di N,N-Dimethyl-4-amino-2,1,3-benzothiadiazole.
Anal  3 J HH = 5.9 Hz, BTD), 3.29 (s, 6H, Me). 13  Synthesis of BTD NMe2 @PMMA: An amount of 0.250 g of PMMA was dissolved in 6 mL of dichloromethane under slow stirring, then a solution containing 0.010 g of BTD NMe2 in 4 mL of dichloromethane was added.The solution was transferred in a cylindrical polyethylene holder (1 cm diameter) and allowed to evaporate at room temperature.The polymeric film thus obtained was finally kept overnight under 10 −2 torr vacuum to remove traces of solvent.Characterization of BTD NMe2 @PMMA.

Results and Discussion
The synthetic route here proposed for BTD NMe2 starts with the nitration of the 2,1,3benzothiadiazole heterocycle, followed by reduction of the nitro group and subsequent methylation with methyl iodide, as depicted in Scheme 1.As observable from the 1 H, 13 C { 1 H} and 1 H- 13 C HSQC NMR reported in Figure 1, the methyl groups are associated with a singlet at 3.29 ppm ( 13   The compound was isolated as a dark red oil that exhibited intriguing luminescent properties once dissolved in common organic solvents (see Figure 2).The pure oil itself did not display appreciable emissions probably because of concentration quenching.The evident solvatochromism was investigated considering four solvents characterized by different dielectric constants ε (n-hexane, dichloromethane, acetone, and acetonitrile).The absorption and emission spectra are shown in Figure 2. The selected properties of the solvents, including the orientation polarizability Δf (see Equation ( 2)), are summarized in Table 1.The table also reports the absorption and emission maxima of BTD NMe2 , Stokes shifts ῦA-ῦF and Φf values, calculated accordingly to Equation (1).The compound was isolated as a dark red oil that exhibited intriguing luminescent properties once dissolved in common organic solvents (see Figure 2).The pure oil itself did not display appreciable emissions probably because of concentration quenching.The evident solvatochromism was investigated considering four solvents characterized by different dielectric constants ε (n-hexane, dichloromethane, acetone, and acetonitrile).The absorption and emission spectra are shown in Figure 2. The selected properties of the solvents, including the orientation polarizability ∆f (see Equation ( 2)), are summarized in Table 1.The table also reports the absorption and emission maxima of BTD NMe2 , Stokes shifts ṽA -ṽF and Φ f values, calculated accordingly to Equation (1).As presented in Figure 2 and Table 1, the solvents characterized by higher ε values determine a bathochromic shift of the emission maxima in solution together with an increase in the Stokes shift, which varies from 4559 cm −1 in hexane to 7448 cm −1 in acetonitrile (see Table 1).The greatest variations occur on changing the solvent from hexane to dichloromethane, with a shift of the emission maximum from 526 to 604 nm and a consequent increase of the Stokes shift from 4559 cm −1 to 6613 cm −1 .The CIE 1931 chromaticity coordinates are reported in the diagram in Figure 3, where the change of emission is observable from yellowish-green to reddish-orange on increasing the ε value.The colour purity of the emission of BTD NMe2 in hexane is 0.79, while it is almost unitary for the other solvents.As presented in Figure 2 and Table 1, the solvents characterized by higher ε values determine a bathochromic shift of the emission maxima in solution together with an increase in the Stokes shift, which varies from 4559 cm −1 in hexane to 7448 cm −1 in acetonitrile (see Table 1).The greatest variations occur on changing the solvent from hexane to dichloromethane, with a shift of the emission maximum from 526 to 604 nm and a consequent increase of the Stokes shift from 4559 cm −1 to 6613 cm −1 .The CIE 1931 chromaticity coordinates are reported in the diagram in Figure 3, where the change of emission is observable from yellowish-green to reddish-orange on increasing the ε value.The colour purity of the emission of BTD NMe2 in hexane is 0.79, while it is almost unitary for the other solvents.The increase of the dielectric constant also causes a decrease in the fluorescence quantum yield, from 52% (hexane) to 16% (acetonitrile), probably attributable to the relative increase of non-radiative decay because of the red-shift of the emission.
As observable in Figure 4, the Stokes shift ῦA-ῦF increases roughly linearly with the orientation polarizability Δf (Pearson's coefficient R = 0.99), accordingly to the Lippert-Mataga equation (Equation ( 3)) [50,51].h is Planck's constant, c is the speed of light in a The increase of the dielectric constant also causes a decrease in the fluorescence quantum yield, from 52% (hexane) to 16% (acetonitrile), probably attributable to the relative increase of non-radiative decay because of the red-shift of the emission.
As observable in Figure 4, the Stokes shift ṽA -ṽF increases roughly linearly with the orientation polarizability ∆f (Pearson's coefficient R = 0.99), accordingly to the Lippert-Mataga equation (Equation ( 3)) [50,51].h is Planck's constant, c is the speed of light in a vacuum, a s is the radius of the cavity in which the molecule resides, and µ e and µ g are the dipole moments of the excited and ground state, respectively.The plot in Figure 4 confirms that the solvatochromic effect is related to specific solute-solvent interactions that involve the polarization [52].
Chem.Proc.2022, 8, 87 7 of 10 The radius obtained from the C-PCM/DFT optimization of the structure of BTD NMe2 is 3.92 Å.Based on Equation (3), the increase of dipole moment from the ground to the excited state is about 7 D.
The luminescent properties of BTD NMe2 were maintained after encapsulation in the PMMA matrix.The emission falls in the orange region of the CIE diagram with unitary colour purity, as observable in the CIE 1931 chromaticity diagram and the picture reported as an inset in Figure 3.
The photoluminescent properties were justified by means of electrochemical measurements and DFT calculations.As observable from the square wave voltammetry reported in Figure 5, the HOMO-LUMO gap can be estimated at around 2.5 eV, considering the irreversible oxidation and reduction processes.Such an outcome is in agreement with the onset of the absorption spectrum using acetonitrile as solvent.The TD-DFT calculations confirm that the lowest energy transition occurs between HOMO and LUMO, which are the π and π* frontier molecular orbitals mostly localized on the benzothiadiazole skeleton, with a contribution from both the molecular orbitals of the N,N-dimethylamino moiety (Figure 5).The radius obtained from the C-PCM/DFT optimization of the structure of BTD NMe2 is 3.92 Å.Based on Equation (3), the increase of dipole moment from the ground to the excited state is about 7 D.
The luminescent properties of BTD NMe2 were maintained after encapsulation in the PMMA matrix.The emission falls in the orange region of the CIE diagram with unitary colour purity, as observable in the CIE 1931 chromaticity diagram and the picture reported as an inset in Figure 3.
The photoluminescent properties were justified by means of electrochemical measurements and DFT calculations.As observable from the square wave voltammetry reported in Figure 5, the HOMO-LUMO gap can be estimated at around 2.5 eV, considering the irreversible oxidation and reduction processes.Such an outcome is in agreement with the onset of the absorption spectrum using acetonitrile as solvent.The TD-DFT calculations confirm that the lowest energy transition occurs between HOMO and LUMO, which are the π and π* frontier molecular orbitals mostly localized on the benzothiadiazole skeleton, with a contribution from both the molecular orbitals of the N,N-dimethylamino moiety (Figure 5).

Conclusions
N,N-Dimethyl-4-amino-2,1,3-benzothiadiazole (BTD NMe2 ) was prepared from 2,1,3benzothiadiazole in a three-step synthetic path that involved nitration, subsequent reduction and methylation.The compound was fully characterized by means of nuclear magnetic resonance (NMR) and infrared spectroscopy.The compound was revealed to be highly fluorescent and characterized by a noticeable solvatochromism.The emission features rationalized based on electrochemical measurements and DFT calculations were maintained once embedded in the poly(methyl methacrylate).The photoluminescence properties exhibited by the BTD NMe2 make it a suitable candidate for advanced technology applications, and further functionalizations are currently under investigation.

Figure 2 .
Figure 2. Absorption (left) and emission (right) spectra of 5 × 10 −5 M solutions of BTD NMe2 in different solvents recorded at room temperature.Inset: picture of the solutions under UV light (λexcitation = 365 nm).

Figure 2 .
Figure 2. Absorption (left) and emission (right) spectra of 5 × 10 −5 M solutions of BTD NMe2 in different solvents recorded at room temperature.Inset: picture of the solutions under UV light (λ excitation = 365 nm).

Table 1 .
Fluorescence data of BTD NMe2 in different solvents.

Table 1 .
Fluorescence data of BTD NMe2 in different solvents.