Bis-arylsulfenyl- and bis-arylselanyl-benzo-2,1,3-thiadiazoles: synthesis and photophysical characterization

Abstract

Bis-arylsulfenyl- and bis-arylselanyl-benzo-2,1,3-thiadiazoles were synthesized in good yields by copper-catalysed cross-coupling reaction of arylthiols or diaryl diselenides with the commercially available 4,7-dibromobenzo[c][1,2,5]thiadiazole. The arylsulfenyl derivatives present absorptions in the visible region (∼420 nm) with molar absorptivity coefficient and radiative rate constant values ascribed to spin and symmetry allowed π–π* electronic transitions, with almost complete absence of solvatochromic effect. An emission located in the cyan green to green region (514–570 nm), with a large Stokes shift (90–146 nm) was observed, probably associated to the charge transfer character of the S₁ state. Theoretical calculations were also performed in order to study the geometry, charge distribution and photophysical properties of the molecules in their ground and excited electronic states. TD-DFT calculations were performed using the PBE1PBE and CAM-B3LYP functionals with cc-pVDZ basis set for geometrical optimisations in the S₀ and S₁ states and jun-cc-pVTZ basis set to obtain vertical transition energies and electronic properties. Solvent effects were included by IEF-PCM formalism using solvents with different dielectric constants. The computationally predicted transition energies calculated with CAM-B3LYP are in good agreement with the experimental results. No substantial solvatochromic effect was found in the absorption maxima, but in the emission from S₁ state a redshift was observed on increasing the solvent polarity. This fact, combined with higher dipole moment in the first excited state and some spatial separation of HOMO and LUMO orbitals could indicate an intramolecular charge transfer character of the S₁ state.

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Introduction

Organyl sulfides, represent special scaffolds usually found in various naturally occurring and biologically active compounds¹ as well as being employed as important intermediates in organic synthesis.² Thus, the development of efficient and new methodologies for the synthesis of these compounds has emerged in organic synthesis. Traditional methods for the formation of C–S bonds frequently require harsh reaction conditions² and recently considerable attention has been devoted to transition metal-catalysed protocols for this purpose.³ In this sense, copper-catalysed reactions using specific ligands and additives are probably the most important method for C–S bond formation. Copper-catalysed protocols have been widely studied using organic disulfides or thiols, as sulfur source, and a plethora of organyl sulfides, including sulfenyl heterocycles, was synthesized in high yields.⁴

In the context of heterocyclic compounds, benzo-2,1,3-thiadiazole (BTD) derivatives comprise an interesting class of molecules and possess interesting electro-optical properties.⁵ Their charge transport capability make them attractive candidates for organic light-emitting diodes (OLEDs).⁶ In addition, BTD derivatives have received much attention in recent years because of their use as fungicides, herbicides and antibacterials,⁷ and were employed as bioprobes for the analyses of numerous cell types.⁸

Recently, a range of arylsulfonyl-BTDs 6 was synthesized and used as pyruvate kinase M2 (PKM2) modulators for the treatment of cancer.⁹ In this study the authors described the synthesis of sulfenyl-benzo-2,1,3-thiadiazoles derivatives via sequential palladium-catalysed reactions and subsequent oxidation of sulfides to arylsulfonyl-BTDs 6 using H₂O₂ and AcOH (eqn (1), Scheme 1).⁹ Sulfenyl-benzo[1,2-c:4,5-c]bis([1,2,5]thiadiazole) derivatives 7 were synthesized by Yamashita and co-workers by reaction of benzo[1,2-c:4,5-c]bis([1,2,5]thiadiazole) with thiophenol in DMF at 80 °C under argon atmosphere (eqn (2), Scheme 1).¹⁰

Scheme 1 Previous works on synthesis of sulfur-containing benzo-2,1,3-thiadiazoles.

To the best of our knowledge, however, the direct synthesis and photophysical characterization of functionalized bis-arylsulfenyl-benzo-2,1,3-thiadiazoles has not been explored. In this context and in continuation of our interest in the synthesis of heterocycles bearing organochalcogen moieties, here we describe the direct copper-catalysed synthesis of arylsulfenyl-benzo-2,1,3-thiadiazoles by reaction of arylthiols with 4,7-dibromobenzo[c][1,2,5]thiadiazole 1. In addition, we dedicate our efforts to understand the photophysics of the synthesized molecules comparing experimental and theoretical properties predicted with TD-DFT calculations.

Experimental

General information

The reactions were monitored by TLC carried out on Merck silica gel (60 F254) by using UV light as visualizing agent and 5% vanillin in 10% H₂SO₄ and heat as developing agents. Baker silica gel (particle size 0.040–0.063 mm) was used for flash chromatography. Proton nuclear magnetic resonance spectra (¹H NMR) were obtained at 300 MHz on Bruker DPX 300 spectrometer. Spectra were recorded in CDCl₃ solutions. Chemical shifts are reported in ppm, referenced to tetramethylsilane (TMS) as the external reference. Coupling constants (J) are reported in hertz. Abbreviations to denote the multiplicity of a particular signal are s (singlet), d (doublet), dd (doublet of doublet) and m (multiplet). Carbon-13 nuclear magnetic resonance spectra (¹³C NMR) were obtained at 75 MHz on Bruker DPX 300 spectrometer. Chemical shifts are reported in ppm, referenced to the solvent peak of CDCl₃. Low-resolution mass spectra were obtained with a Shimadzu GC-MS-QP2010 mass spectrometer. High resolution mass spectra (HRMS) were recorded on a Bruker Micro TOF-QII spectrometer 10416. The solvents and reagents were used as received or purified using standard procedures. Spectroscopic grade solvents (Merck) were used for fluorescence and UV-Vis measurements. UV-Vis absorption spectra in solution were performed on a Shimadzu UV-2450 spectrophotometer at a concentration range of 10⁻⁴ to 10⁻⁵ M. Steady state fluorescence spectra were taken with a Shimadzu spectrofluorometer model RF-5301PC. The maximum absorption wavelength was used as excitation wavelength for fluorescence measurements. The quantum yield of fluorescence (Φ_F) was measured at 25 °C using spectroscopic grade solvents within solutions with absorbance intensity lower than 0.05 (optical dilute methodology). Coumarin 343 (Aldrich) in ethanol was used as fluorescence quantum yield standard (Φ_F = 0.63).¹¹ All measurements were performed at room temperature (25 °C).

General procedure for the synthesis of bis-arylsulfenyl-benzo-2,1,3-tiadiazoles 3a–f

To a 5 mL round-bottomed flask containing the 7-dibromobenzo[c][1,2,5]thiadiazole 1 (0.5 mmol), the appropriated arylthiol 2a–f (1.0 mmol), CuO nanoparticles (CuO NPs) (20 mol%), KOH (2.0 mmol) and DMSO (1.5 mL) were added. The homogeneous reaction mixture was stirred at 80 °C for 24 hours under N₂ atmosphere. After this time, the solution was cooled to room temperature, diluted with ethyl acetate (20 mL), and washed with water (3 × 20 mL). The organic phase was separated, dried over MgSO₄ and concentrated under vacuum. The obtained products 3a–f were purified by chromatography on neutral alumina using a mixture of ethyl acetate/hexane (10 : 90) as the eluent.

4,7-Bis((4-methoxyphenyl)thio)benzo[c][1,2,5]thiadiazole (3a). Yield: 0.179 g (87%); orange oil. ¹H NMR (CDCl₃, 300 MHz): δ 7.34 (d, J = 8.8 Hz, 4H); 6.77 (d, J = 8.8 Hz, 4H); 6.57 (s, 2H); 3.67 (s, 6H). ¹³C NMR (CDCl₃, 75 MHz): δ 160.32, 152.21, 136.36, 129.66, 125.52, 120.67, 115.11, 55.18. MS (relative intensity) m/z: 412 (59), 273 (100), 207 (23), 139 (28), 96 (16), 77 (11). HRMS calcd for C₂₀H₁₇N₂O₂S₃ [M + H]⁺ 413.0446. Found: 413.0422.

4,7-Bis(phenylthio)benzo[c][1,2,5]thiadiazole (3b). Yield: 0.151 g (86%); yellow solid; mp 125–126 °C. ¹H NMR (CDCl₃, 300 MHz): δ 7.42–7.39 (m, 4H); 7.27–7.25 (m, 6H); 6.84 (s, 2H). ¹³C NMR (CDCl₃, 75 MHz): δ 152.89, 133.47, 131.41, 129.56, 128.91, 128.60, 127.64. MS (relative intensity) m/z: 352 (60), 243 (93), 207 (48), 109 (40), 77 (100), 51 (88). HRMS calcd for C₁₈H₁₃N₂S₃ [M + H]⁺ 353.0235. Found: 353.0238.

4,7-Bis(p-tolylthio)benzo[c][1,2,5]thiadiazole (3c). Yield: 0.158 g (83%); orange oil. ¹H NMR (CDCl₃, 300 MHz): δ 7.34 (d, J = 8.1 Hz, 4H); 7.12 (d, J = 8.1 Hz, 4H); 6.76 (s, 2H); 2.29 (s, 6H). RMN ¹³C (CDCl₃, 75 MHz): δ 152.77, 139.13, 134.14, 130.45, 129.35, 127.48, 126.84, 21.22. MS (relative intensity) m/z: 380 (14), 281 (25), 257 (25), 207 (100), 73 (20), 40 (41). HRMS calcd for C₂₀H₁₇N₂S₃ [M + H]⁺ 381.0548. Found: 381.0548.

4,7-Bis((4-chlorophenyl)thio)benzo[c][1,2,5]thiadiazole (3d). Yield: 0.168 g (80%); orange solid; mp 110–112 °C. ¹H NMR (CDCl₃, 300 MHz): δ 7.34 (d, J = 8.5 Hz, 4H); 7.25 (d, J = 8.5 Hz, 4H); 6.91 (s, 2H). ¹³C NMR (CDCl₃, 75 MHz): δ 152.89, 134.94, 134.58, 130.07, 129.81, 128.75, 127.97. MS (relative intensity) m/z: 420 (15), 277 (100), 242 (39), 207 (17), 108 (32), 75 (26). HRMS calcd for C₁₈H₁₁Cl₂N₂S₃ [M + H]⁺ 420.9455. Found: 420.9451.

4,7-Bis((4-fluorophenyl)thio)benzo[c][1,2,5]thiadiazole (3e). Yield: 0.173 g (89%); yellow solid; mp 132–134 °C. ¹H NMR (CDCl₃, 300 MHz): δ 7.44 (dd, J = 8.8 and 5.2 Hz, 4H); 7.00 (t, J = 8.8 Hz, 4H); 6.77 (s, 2H). ¹³C NMR (CDCl₃, 75 MHz): δ 163.14 (d, J = 249.7 Hz), 152.65, 136.17 (d, J = 8.4 Hz), 129.27, 126.93, 126.26 (d, J = 3.7 Hz), 116.87 (d, J = 22.0 Hz). MS (relative intensity) m/z: 388 (23), 281 (27), 261 (36), 207 (100), 73 (23). HRMS calcd for C₁₈H₁₁F₂N₂S₃ [M + H]⁺ 389.0046. Found: 389.0043.

4,7-Bis(naphthalen-1-ylthio)benzo[c][1,2,5]thiadiazole (3f). The yield: 0.203 g (90%); yellow solid; mp 104–106 °C. ¹H NMR (CDCl₃, 300 MHz): δ 7.92 (m, 2H); 7.71–7.63 (m, 6H); 7.41–7.37 (m, 6H); 6.85 (s, 2H). ¹³C NMR (CDCl₃, 75 MHz): δ 152.92, 133.70, 132.91, 132.81, 129.93, 129.26, 128.88, 128.59, 127.82, 127.65, 127.54, 126.83, 126.65. MS (relative intensity) m/z: 452 (2), 368 (11), 111 (25), 83 (23), 55 (100), 43 (93). HRMS calcd for C₂₆H₁₇N₂S₃ [M + H]⁺ 453.0548. Found: 453.0547.

General procedure for the synthesis of bis-arylselanyl-benzo-2,1,3-tiadiazoles 5a–d

To a 3 mL round-bottomed flask containing 7-dibromobenzo[c][1,2,5]thiadiazole 1 (0.5 mmol), an appropriated diaryl diselenide 4a–d (0.5 mmol), CuI (20 mol%), 1,10-phenanthroline (1,10-phen) (20 mol%), KOH (2.0 mmol) and DMSO (1.5 mL) were added. The homogeneous reaction mixture was stirred at 110 °C for 24 hours under N₂ atmosphere. After this time, the solution was cooled to room temperature, diluted with ethyl acetate (20 mL), and washed with water (3 × 20 mL). The organic phase was separated, dried over MgSO₄ and concentrated under vacuum. The obtained products 5a–d were purified by chromatography on neutral alumina using a mixture of ethyl acetate/hexane (10 : 90) as the eluent.

4,7-Bis(phenylselanyl)benzo[c][1,2,5]thiadiazole (5a). Yield: 0.175 g (78%); dark yellow solid; mp 129–131 °C. ¹H NMR (CDCl₃, 300 MHz): δ 7.54–7.50 (m, 4H); 7.25–7.22 (m, 6H); 6.86 (s, 2H). RMN ¹³C (CDCl₃, 75 MHz): δ 153.47, 135.73, 135.36, 129.90, 129.48, 127.05, 125.08. MS (relative intensity) m/z: 448 (9), 291 (25), 207 (14), 77 (79), 51 (51), 40 (100). HRMS calcd for C₁₈H₁₃N₂SSe₂ [M + H]⁺ 448.9126. Found: 448.9107.

4,7-Bis(p-tolylselanyl)benzo[c][1,2,5]thiadiazole (5b). Yield: 0.164 g (69%); yellow solid; mp 101–103 °C. ¹H NMR (CDCl₃, 300 MHz): δ 7.53 (d, J = 8.1 Hz, 4H); 7.15 (d, J = 8.1 Hz, 4H); 6.89 (s, 2H); 2.36 (s, 6H). ¹³C NMR (CDCl₃, 75 MHz): δ 153.50, 139.12, 136.04, 130.56, 129.43, 125.41, 123.22, 21.24. MS (relative intensity) m/z: 476 (5), 305 (12), 111 (17), 81 (57), 69 (100), 43 (88). HRMS calcd for C₂₀H₁₇N₂SSe₂ [M + H]⁺ 476.9439. Found: 476.9392.

4,7-Bis((4-chlorophenyl)selanyl)benzo[c][1,2,5]thiadiazole (5c). Yield: 0.188 g (73%); orange solid; mp 87–89 °C. ¹H NMR (CDCl₃, 300 MHz): δ 7.47 (d, J = 8.4 Hz, 4H); 7.23 (d, J = 8.4 Hz, 4H); 6.93 (s, 2H). ¹³C NMR (CDCl₃, 75 MHz): δ 153.26, 136.53, 134.96, 130.00, 129.67, 125.15, 124.73. MS (relative intensity) m/z: 516 (8), 325 (25), 290 (5), 81 (63), 69 (100), 41 (33). HRMS calcd for C₁₈H₁₁Cl₂N₂SSe₂ [M + H]⁺ 516.8339. Found: 516.8318.

4,7-Bis((4-fluorophenyl)selanyl)benzo[c][1,2,5]thiadiazole (5d). Yield: 0.189 g (78%); orange solid; mp 151–153 °C. ¹H NMR (CDCl₃, 300 MHz): δ 7.62 (dd, J = 8.8 and 5.4 Hz, 4H); 7.03 (t, J = 8.8 Hz, 4H); 6.89 (s, 2H). ¹³C NMR (CDCl₃, 75 MHz): δ 163.39 (d, J = 249.8 Hz), 141.03, 138.06 (d, J = 8.1 Hz), 129.72, 125.48, 121.87 (d, J = 3.6 Hz), 117.03 (d, J = 22.0 Hz). MS (relative intensity) m/z: 484 (39), 324 (40), 309 (100), 229 (30), 83 (35), 69 (24). HRMS calcd for C₁₈H₁₁F₂N₂SSe₂ [M + H]⁺ 484.8937. Found: 484.8906.

Theoretical calculations

DFT and TDDFT calculations were used to provide information about the electronic structure of 3a–f and 5a–d. Among the functionals, PBE1PBE¹² has provided good agreement with experimental results and has been one of the most used functionals to calculate 2,1,3-benzothiadiazole (BTD) derivatives.¹³ Thus this functional was initially choose to calculate the electronic spectrum of the selected structures. The ground electronic state and first excited state equilibrium geometries were obtained by optimising the geometry of each species by using PBE1PBE functional together with cc-pVDZ basis set. Subsequently, a single-point energy and population evaluation was performed using the same functional, but changing the basis function to the jun-cc-pVTZ basis set to obtain the vertical absorptions and emissions.¹⁴ The jun-basis set, named calendar basis set, is recommended by Truhlar et al. as a better default option than aug-basis sets because of its performance versus cost. The calendar basis set are constructed by removing diffuse functions from the aug basis sets. The Jun-cc-pV*Z basis sets remove the diffuse function from H and He and also removes the highest angular momentum diffuse function from all other atoms, from aug-cc-pV*Z. All equilibrium geometries were confirmed by vibrational analysis and have no imaginary frequencies. Solvent effects were included in all cases by the Integral Equations Formalism of the Polarisable Continuum Model (IEF-PCM).¹⁵ The solvents considered at PBE1PBE level were hexane, toluene, dichloromethane, ethanol and N,N-dimethylformamide. In addition, the ground and first excited state were also optimised and have their electronic transitions calculated using CAM-B3LYP¹⁶ functional with the same basis set previously mentioned. With this functional, the solvent effects were considered using hexane, toluene, 1,4-dioxane and dichloromethane as solvents. The main difference between CAM-B3LYP and PBE1PBE is the long-range corrections considered by the first one, which provides a more accurate description of charge-transfer excited states.^16,17 Thus, this functional was chosen because of its good results in molecules with donor–acceptor design containing the benzothiadiazole group.^18,19 The ground and excited state electrostatic potential surfaces and dipole moment were obtained by population analysis using ChelpG formalism (charges from electrostatic potential using a grid based method)²⁰ and self-consistent field density and CI density respectively, at the corresponding optimised geometries. The calculations were carried out using Gaussian 09 Package.²¹

Results and discussion

Synthesis

Initially, 4,7-dibromobenzo[c][1,2,5]thiadiazole 1 and 4-methoxythiophenol 2a were selected as a model system to optimize the reaction conditions. Thus, in a first attempt, a mixture of substrates 1 (0.5 mmol) and 2a (1.0 mmol) were stirred in DMSO at 110 °C in N₂ atmosphere for 24 h, using CuO NPs (5 mol%) as a catalyst and KOH (2.0 eq.) as base.²² Under these reaction conditions, only traces of the desired product 3a were observed (Table 1, entry 1). When the same reaction was performed using 4.0 eq. of KOH, the desired product 3a was obtained, however in low yield (Table 1, entry 2). Fortunately, when the amount of catalyst was increased from 5 to 20 mol%, moderate to good yields of product 3a were obtained (Table 1, entries 3 and 4). A good result was achieved when we performed this reaction at 80 °C using 20 mol% of CuI and 4.0 eq. of KOH, with the product 3a being obtained in 86% yield (Table 1, entry 5). When the reaction was performed using 4.0 eq. of KOH without CuO NPs, the desired product 3a was not formed (Table 1, entry 6). A decrease in the yield of 3a was observed when this reaction was performed using the catalytic system CuI (20 mol%)²³ and 1,10-phen (20 mol%) in reactions carried out at 80 and 110 °C (Table 1, entries 7 and 8).

Table 1 Optimization of reaction conditions^a

Entry	KOH (eq.)	CuO NPs (mol%)	Temperature (°C)	Yield of 3a^b (%)
a Reactions are performed in the presence of compound 1 (0.5 mmol) and 2a (1.0 mmol), in DMSO (1.5 mL), for 24 h under N2 atmosphere.b Yields are given for isolated product.c Reaction was performed using CuI (20 mol%) and 1,10-phenanthroline (20 mol%) as catalytic system.
1	2.0	5	110	Traces
2	4.0	5	110	25
3	4.0	10	110	43
4	4.0	20	110	75
5	4.0	20	80	87
6	4.0	—	80	—
7	4.0	20^c	80	46
8	4.0	20^c	110	55

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In an optimised reaction, 4,7-dibromobenzo[c][1,2,5] thiadiazole 1 (0.5 mmol), 4-methoxythiophenol 2a (1.0 mmol), CuO NPs (20 mol%) and KOH (4.0 eq.) were dissolved in DMSO (1.5 mL) and after that the mixture was stirred at 80 °C for 24 h under N₂ atmosphere, affording the correspondent 4,7-bis((4-methoxyphenyl)thio)benzo[c][1,2,5]thiadiazole 3a in 87% yield. Then, the possibility to synthesize the selenium analogues of compound 3a was also studied. Interest in the chemistry and application of different selenium functionalized compounds as potential pharmaceuticals,²⁴ new materials,²⁵ fluorescent molecules²⁶ and ionic liquids²⁷ has expanded over the last years. Thus, a mixture of 4,7-dibromobenzo[c][1,2,5]thiadiazole 1 (0.5 mmol), diphenyl diselenide 4a (0.5 mmol), CuO NPs (20 mol%), KOH (4.0 eq.) and DMSO (1.5 mL) was stirred at 80 °C for 24 h under N₂ atmosphere, and the desired product 5a was obtained only in 33% yield (Scheme 2, Condition 1). However, when the reaction of compounds 1 and 4a was performed using the catalytic system CuI (20 mol%) and 1,10-phen (20 mol%), KOH (4.0 eq.) as base, DMSO (1.5 mL) as solvent at 110 °C in N₂ atmosphere for 24 h, selenylation product 5a was achieved in 78% yield (Scheme 2, Condition 2).

Scheme 2 Synthesis of compound 5a. Condition 1: CuO NPs (20 mol%), KOH (4.0 eq.), DMSO (1.5 mL), 80 °C, N₂, 24 h (33%). Condition 2: CuI (20 mol%), 1,10-phen (20 mol%), KOH (4.0 eq.), DMSO (1.5 mL), 110 °C, N₂, 24 h (78%).

To extend the scope of our synthetic methodology, the possibility to perform these reactions of benzo[c][1,2,5]thiadiazole 1 with arylthiols 2a–f or diaryl diselenides 4a–d was investigated under our two developed methodologies and the results are presented in Table 2.

Table 2 Synthesis of different 4,7-bis(arylthio)- and 4,7-bis(arylselanyl)- benzo[c][1,2,5]thiadiazoles^a

Entry	Product (yield)^b	Entry	Product (yield)^b
a Condition A: reactions are performed with compound 1 (0.5 mmol), arylthiols 2a–f (1.0 mmol), CuO NPs (20 mol%) and KOH (4 eq.) in DMSO (1.5 mL) at 80 °C for 24 h under N2 atmosphere. Condition B: reactions are performed with compound 1 (0.5 mmol), diaryl diselenides 4a–d (0.5 mmol), CuI (20 mol%), 1,10-phen (20 mol%) and KOH (4 eq.) in DMSO (1.5 mL) at 110 °C for 24 h under N2 atmosphere.b Yields are given for isolated product.
1		6
2		7
3		8
4		9
5		10

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Firstly, we employed a range of arylthiols 2a–f and our results reveal that the reactions are not sensitive to the electronic effect of the aromatic ring in the arylthiols (Table 2, entries 1–6). For example, arylthiols containing electron-donating (OMe, Me), electron-withdrawing (Cl, F) and electron-neutral group at the aromatic ring, gave good yields of the desired sulfenyl-benzo[c][1,2,5]thiadiazoles 3a–e (Table 2, entries 1–5). In addition, reaction performed with α-naphthylmercaptan 2f furnished the respective product 3f in 90% yield (Table 2, entry 6). It is worth mentioning that it was also evaluated the reactivity of benzo[c][1,2,5]thiadiazole 1 with other diaryl diselenides 4a–d under the Condition 2 described in Scheme 1. Different diaryl diselenides containing EDG and EWG were reacted with compound 1, affording the respective selanyl-benzo[c][1,2,5]thiadiazoles 5a–d in moderated to good yields (Table 2, entries 7–10). The possible mechanism for these cross-coupling reactions using CuO NPs or CuI involving aryl halides and thiols/dichalcogenides has been well explored and described in some studies.^3a,22,23

Photophysical characterisation

The absorption spectra of the bis-arylsulfenyl- (3a–f) and bis-arylselanyl-benzo-2,1,3-tiadiazoles (5a–d) are shown in Fig. 1 using hexane and dichloromethane as solvents. The additional data in 1,4-dioxane and toluene presented similar results and are presented in the ESI.† The relevant data from UV-Vis absorption spectroscopy are presented in Table 3, where the solvents are presented in order of increasing dipole moment. The UV-V is spectra allowed the obtention of the pure radiactive lifetimes (τ⁰) and the radiactive rate constant (k⁰_e) using the Strickler–Berg relations:²⁸

Fig. 1 UV-Vis absorption spectra in solution of the bis-arylsulfenyl- (3b–e) and bis-arylselanyl-benzo-2,1,3-tiadiazoles (5a–d) in hexane (top) and dichloromethane (bottom).

Table 3 Relevant photophysical data of the UV-Vis spectra of the bis-arylsulfenyl- (3a–f) and bis-arylselanyl-benzo-2,1,3-tiadiazoles (5a–d), where λ_abs is the absorption maxima (nm), ε is the molar absorptivity (M⁻¹ cm⁻¹), f_e is the calculated oscillator strength, k⁰_e is the calculated radiactive rate constant (10⁸ s⁻¹) and τ⁰ is the inherent emission lifetime (ns)

Dye	Solvent	λabs	ε	fe	k^0e	τ^0
3a	Hexane	427	3183	0.064	0.35	28.41
	Toluene	435	3479	0.071	0.38	26.53
	1,4-Dioxane	424	3173	0.067	0.37	27.00
	Dichloromethane	432	2418	0.051	0.27	36.39
3b	Hexane	416	6445	0.131	0.76	13.16
	Toluene	420	5698	0.117	0.66	15.10
	1,4-Dioxane	415	6101	0.131	0.76	13.16
	Dichloromethane	418	5113	0.098	0.56	17.80
3c	Hexane	423	6662	0.133	0.74	13.47
	Toluene	426	6927	0.140	0.77	12.98
	1,4-Dioxane	423	6445	0.136	0.76	13.14
	Dichloromethane	425	5303	0.109	0.60	16.53
3d	Hexane	412	4406	0.090	0.53	18.87
	Toluene	417	4878	0.101	0.58	17.23
	1,4-Dioxane	417	5540	0.118	0.68	14.75
	Dichloromethane	414	4023	0.083	0.48	20.63
3e	Hexane	415	4457	0.089	0.52	19.39
	Toluene	421	4746	0.094	0.53	18.91
	1,4-Dioxane	417	4511	0.095	0.55	18.34
	Dichloromethane	416	3479	0.073	0.42	23.73
3f	Hexane	422	9063	0.188	1.06	9.46
	Toluene	430	9754	0.205	1.11	9.04
	1,4-Dioxane	422	9500	0.203	1.14	8.75
	Dichloromethane	420	8500	0.186	1.05	9.48
5a	Hexane	416	1646	0.032	0.19	53.38
	Toluene	429	1703	0.037	0.20	50.24
	1,4-Dioxane	423	1537	0.032	0.18	55.44
	Dichloromethane	421	1323	0.029	0.16	61.25
5b	Hexane	421	7586	0.149	0.84	11.93
	Toluene	434	7511	0.142	0.75	13.25
	1,4-Dioxane	419	7308	0.151	0.86	11.64
	Dichloromethane	420	5942	0.124	0.70	14.22
5c	Hexane	422	3296	0.063	0.35	28.34
	Toluene	428	3608	0.067	0.37	27.24
	1,4-Dioxane	422	3533	0.071	0.40	24.96
	Dichloromethane	423	2740	0.057	0.32	31.48
5d	Hexane	427	5167	0.100	0.55	18.27
	Toluene	429	5033	0.092	0.50	19.92
	1,4-Dioxane	424	4743	0.098	0.54	18.39
	Dichloromethane	424	3877	0.081	0.45	22.19

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In this equation v̄₀ is the wavenumber (energy in 1/λ units) of the maximum of the absorption band and the integral is the area under the absorption curve from a plot of the molar absorptivity coefficient ε (M⁻¹ cm⁻¹) vs. wavenumber v̄ (cm⁻¹), related to a single electron oscillator and the pure radiative lifetime τ⁰ is defined as 1/k⁰_e.²⁹ The oscillator strength (f_e) can also be calculated from eqn (2):

The bis-arylsulfenyl- (3a–f) and bis-arylselanyl-benzo-2,1,3-tiadiazoles (5a–d) present absorption maxima located around 420 and 424 nm, respectively. Additionally, the intense absorption bands observed around 310 nm can be associated to the absorption of the benzo-2,1,3-tiadiazole core. The molar absorptivity coefficient ε values, as well as the calculated radiative rate constant (k⁰_e) for all compounds indicate that spin and symmetry allowed electronic transitions, which could be related to ¹ππ* transitions. From Fig. 1 it can be observed that any significative change takes place on the absorption maxima location associated with the different organic groups present in the benzo-2,1,3-thiadiazoles moiety. The same behavior was observed when sulfur atom was changed to selenium. In this way, changes on the substituent in the benzo-2,1,3-tiadiazoles core from bis-arylsulfenyl- to bis-arylselanyl do not affect the photophysics in the ground state of these compounds.

Additionally, a very small solvatochromic effect could be observed in the ground state (Δλ_abs from 3 to 10 nm) for the bis-arylsulfenyl compounds 3a–f, indicating an almost absent intramolecular charge transfer state (see ESI†).

The bis-arylselanyl derivatives 5a–d present more significative solvatochromism in the ground state (Δλ_abs from 5 to 15 nm), probably due to the better electron delocalization provided by the selenium atom. It could also be observed that changes in the chalcogen atom – sulfur to selenium – shifts the absorption maxima to longer wavelengths, also indicating that the electrons are in the ground state held tighter in the molecular structure of the sulfur derivative than in its selenium analogs.³⁰

The studied compounds presented non constant inherent emission lifetimes τ⁰. In a general way, the inherent lifetimes directly excited into S₀ → S₁ electronic transition increases to the arylselanyl derivatives in despite of the sulfur analogues, probably indicating to the selenium derivatives a reduction in the non-radiative relaxation due to more restricted molecular motion. These results indicate an efficient non-radiative energy transfer to compounds 5a–d, which can be related to lower calculated fluorescence quantum yields (Table 4).

Table 4 Relevant photophysical data of the steady-state fluorescence spectra of the bis-arylsulfenyl- (3a–f) and bis-arylselanyl-benzo-2,1,3-tiadiazoles (5a–d), where λ_em is the emission maxima (nm), Δλ_ST is the Stokes shift (nm) and Φ_F is the fluorescence quantum yield (%)

Dye	Solvent	λem	ΔλST	ΦF
3a	Hexane	526	99	0.369
	Toluene	554	119	0.323
	1,4-Dioxane	558	134	0.264
	Dichloromethane	570	138	0.199
3b	Hexane	515	99	0.260
	Toluene	541	121	0.055
	1,4-Dioxane	554	139	0.023
	Dichloromethane	561	146	0.058
3c	Hexane	521	98	0.367
	Toluene	547	121	0.340
	1,4-Dioxane	551	128	0.207
	Dichloromethane	566	141	0.213
3d	Hexane	516	104	0.329
	Toluene	535	118	0.297
	1,4-Dioxane	542	125	0.203
	Dichloromethane	555	141	0.167
3e	Hexane	512	97	0.373
	Toluene	537	116	0.338
	1,4-Dioxane	545	128	0.243
	Dichloromethane	557	141	0.242
3f	Hexane	519	97	0.131
	Toluene	546	116	0.250
	1,4-Dioxane	553	131	0.147
	Dichloromethane	564	144	0.129
5a	Hexane	517	101	0.096
	Toluene	546	116	0.057
	1,4-Dioxane	552	129	0.036
	Dichloromethane	564	143	0.072
5b	Hexane	514	93	0.101
	Toluene	550	116	0.060
	1,4-Dioxane	550	131	0.031
	Dichloromethane	559	139	0.061
5c	Hexane	512	90	0.099
	Toluene	542	114	0.079
	1,4-Dioxane	550	128	0.054
	Dichloromethane	560	137	0.068
5d	Hexane	518	91	0.094
	Toluene	544	115	0.070
	1,4-Dioxane	554	130	0.031
	Dichloromethane	562	138	0.089

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The normalised fluorescence emission spectra of the bis-arylsulfenyl- (3a–f) and bis-arylselanyl-benzo-2,1,3-tiadiazoles (5a–d) are presented in Fig. 2. The emission curves were obtained exciting the compounds at the absorption maxima wavelength. The relevant data from fluorescence emissions are summarised in Table 4, where the solvents are presented in order of increasing dipole moment.

Fig. 2 Fluorescence emission spectra in solution of the bis-arylsulfenyl- (3b–e) and bis-arylselanyl-benzo-2,1,3-tiadiazoles (5a–d) in hexane (top) and dichloromethane (bottom). The absorption maxima were used as excitation wavelengths.

The bis-arylsulfenyl- and bis-arylselanyl-benzo-2,1,3-tiadiazoles present emission in the cyan green to green regions (∼540 nm). As already observed in the ground state, the bis-arylsulfenyl derivatives 3a–f present comparable location to the emission maxima, which indicates that the different organic moieties in the benzo-2,1,3-tiadiazole core do not play a fundamental role on the excited state of these compounds. A similar behavior was observed to the compounds (5a–d). However, a significative solvatochromic effect was observed in the excited state (Δλ_em from 39 to 45 nm) for the bis-arylsulfenyl compounds 3a–f. The bis-arylselanyl derivatives 5a–d also presented a significative solvatochromism in the excited state (Δλ_em from 44 to 48 nm), where increasing the dipole moment of the solvent (hexane to dichloromethane), the emission maxima redshifts (Fig. 3).

Fig. 3 Normalised fluorescence emission spectra in solution of the derivatives 3b (left) and 5a (right). The absorption maxima were used as excitation wavelengths.

This behavior can be understood when a relaxed excited state is more polar than the ground state (i.e., μ_e > μ_g). In this case, the stronger the interaction between the solute and the solvent, the lower the energy of the excited state, the larger the redshift of the emission band.^31,32 As already observed to chalcogen compounds, this results can probably also be attributed to the nature of the ¹ππ* electronic transition, indicating that these compounds allow better electronic delocalization in the excited state.³² The large Stokes shift, as well as the solvathochromic effect in the excited state indicates that these compounds presents charge separation, such as an intramolecular charge transfer character in the excited state (ICT state). Moreover, the bis-arylsulfenyl derivatives 3a–f presented higher fluorescence quantum yields if compared to the selanyl analogues 5a–d, indicating that the presence of the selenium atom seems to be an efficient nonradiactive deactivation channel in these structures.

Fig. 4 depicts the compound 3f in solution under UV radiation, where a positive solvatochromism from green to yellow region, could be observed by varying the polarity of the solvent. A similar behavior could be observed with the other derivatives.

Fig. 4 Picture of the solutions of compound 3f in hexane (left), toluene (middle) and dichloromethane (right) under UV radiation (365 nm).

Theoretical calculations

The main structural characteristic of the bis-arylsulphenyl- and bis-aryselanyl-benzo-2,1,3-thiadiazoles is the loss of planarity. The planar structures are not stable, which means they are not a minimum at the surface potential energy. The lowest energy conformations present the groups attached to the 4- and 7-positions of BTD core almost perpendicular to the core π system. The conformational analysis for the 3b structure can be seen in Fig. 5. The conformation A has a lower energy than all remaining conformations found to the 3b molecule (B to D) at PBE1PBE/jun-cc-pVTZ//PBE1PBE/cc-pVDZ level of theory, in the gas phase. Considering the A conformation found to 3b, the remaining structures were optimised (3a–f and 5a–f).

Fig. 5 Conformational analysis for the 3b structure at PBE1PBE/jun-cc-pVTZ//PBE1PBE/cc-pVDZ level of theory, in the gas phase. Energies (E) in Hartree and ΔE (energy difference in relation to the more stable conformation, A) in kcal mol⁻¹.

The most stable conformations for the ground and excited state obtained using CAM-B3LYP and hexane as solvent are represented in Fig. 6. The most relevant calculated structural parameters obtained with both functionals and all solvents are provided in the ESI together with the optimised structures at PBE1PBE level (Table ESI1†). Due to the fact that PBE1PBE did not describe well the systems (see discussion below), all geometric parameters are discussed based on CAM-B3LYP functional.

Fig. 6 Molecular geometries for 3a–f (left) and 5a–d (right) series at ground state (on the left) and first excited state (on the right) calculated at CAM-B3LYP/cc-pVDZ level in hexane.

In the ground state, all structures presents similar geometries, with symmetry near to C_2v, independent of the R substituent attached to phenyl ring or if it is S or Se. Due to the symmetry, some angles and bond lengths are the same in both sides of the molecule and are not explicitly presented (Table ESI1†). Both S₀ and S₁ states have a similar geometry, indicating a high stability of those structures. This reflects a small geometrical reorganization in both transitions. The main structural difference between S₀ and S₁ states is the change in the dihedral angle between phenyl rings and the benzothiadiazole core (d1). In the ground state, d1 is close to 91 degrees, except in the 3f structure where the dihedral angle is close to 82 degrees. Moving to the excited state, this dihedral angle becomes to be equal to 66–70 degrees for 3a–d and 82–87 degrees for 5a–c. Although in the ground state there is no significant difference between the geometric parameters obtained with both functionals, using PBE1PBE the values obtained for d1 in the first excited state are smaller (50–57 degrees in the structures 3a–e, ∼78 in the 3f and 57–61 in the 5a–d). Another important change from S₀ to S₁ is the bond length between N–S2 (r3), that increase in the excited state, while the C7–S17 (r1) decreases. In fact, in the ground state all the structural parameters are very close to each other, but in excited state the dependence with R group attached to the phenyl rings and sulfur/selenium are reflected mainly on the dihedral angle d1. Moreover, the change in the dihedral angle d1 is the main structural difference calculated by the two functionals. No significant structural changes were observed when the solvent is changed.

The lack of planarity in 2,1,3-benzothiadiazole derivative may also play a role in the long wavelength absorption bands. Decreasing conjugation is known for increasing the HOMO–LUMO gap of conjugated molecules.¹⁸ The calculated absorption and emission wavelengths are shown in Table 5 together with their respective oscillator strength and dipole moments.

Table 5 Calculated photophysical data of the bis-arylsulfenyl- (3a–f) and bis-arylselanyl-benzo-2,1,3-thiadiazoles (5a–d) with CAM-B3LYP/jun-cc-pVTZ//CAM-B3LYP/cc-pVDZ. The λ_abs is the absorption maxima (nm), λ_em is the emission maxima, f_e is the oscillator strength and μ is the dipole moment (D) for molecules in their respective S₀ and S₁ electronic states

Dye	Solvent	S0			S1
		λabs	fe	μ	λem	fe	μ
3a	Hexane	413.43	0.187	7.7	547.71	0.170	13.0
	1,4-Dioxane	413.97	0.190	7.9	550.25	0.177	13.3
	Toluene	414.60	0.195	8.0	551.42	0.180	13.4
	CH2Cl2	414.03	0.187	8.9	564.03	0.217	15.1
3b	Hexane	410.96	0.184	5.6	541.05	0.166	11.3
	1,4-Dioxane	411.58	0.187	5.8	544.10	0.173	11.3
	Toluene	412.23	0.192	5.8	544.74	0.176	11.4
	CH2Cl2	411.83	0.184	6.6	558.72	0.216	12.9
3c	Hexane	413.11	0.186	6.8	545.19	0.168	12.3
	1,4-Dioxane	413.68	0.189	6.9	547.80	0.175	12.6
	Toluene	414.32	0.194	7.0	548.91	0.178	12.7
	CH2Cl2	413.82	0.186	7.9	562.85	0.216	14.3
3d	Hexane	406.24	0.193	1.8	534.79	0.180	7.3
	1,4-Dioxane	406.86	0.196	1.9	537.14	0.187	7.5
	Toluene	407.50	0.201	1.9	538.16	0.189	7.6
	CH2Cl2	407.55	0.192	2.3	551.84	0.226	8.7
3e	Hexane	407.27	0.186	2.3	530.07	0.159	7.9
	1,4-Dioxane	407.93	0.189	2.3	532.67	0.166	8.2
	Toluene	408.59	0.194	2.4	533.79	0.169	8.3
	CH2Cl2	408.71	0.185	2.9	549.02	0.208	9.4
3f	Hexane	411.69	0.190	5.5	536.81	0.161	10.9
	1,4-Dioxane	411.91	0.195	5.6	539.20	0.168	11.6
	Toluene	412.49	0.200	5.7	540.22	0.171	11.7
	CH2Cl2	411.63	0.193	6.5	553.04	0.210	13.3
5a	Hexane	419.06	0.188	5.3	543.59	0.164	10.7
	1,4-Dioxane	419.43	0.192	5.5	545.82	0.172	11.0
	Toluene	420.01	0.197	5.5	546.78	0.175	10.6
	CH2Cl2	418.69	0.189	6.2	559.55	0.220	12.5
5b	Hexane	420.96	0.190	6.5	546.93	0.165	12.0
	1,4-Dioxane	421.89	0.198	6.7	550.11	0.176	12.4
	Toluene	421.31	0.193	6.7	549.15	0.173	12.3
	CH2Cl2	420.45	0.190	7.6	562.50	0.22	13.9
5c	Hexane	414.53	0.196	1.4	536.27	0.171	6.9
	1,4-Dioxane	414.91	0.199	1.4	538.49	0.178	7.1
	Toluene	415.49	0.204	1.4	539.45	0.182	7.2
	CH2Cl2	414.67	0.196	1.8	552.55	0.225	8.2
5d	Hexane	415.46	0.191	1.9	537.60	0.166	7.3
	1,4-Dioxane	415.87	0.194	1.9	539.82	0.173	7.5
	Toluene	416.47	0.199	2.0	540.88	0.177	7.6
	CH2Cl2	415.70	0.190	2.4	554.42	0.221	8.7

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It is possible to see that, in contrast to the other works, the PBE1PBE functional did not describe well λ_abs and λ_em of these systems, probably because the chalcogen atoms. This functional predicted a higher λ_abs and an even higher λ_em compared to the experimental data, mainly for the 5a–d structures (see Table ESI2†). On the other hand, the theoretical results calculated using CAM-B3LYP functional are very close to the experimental data, mainly in the ground state, validating the rationality of our method and basis set chosen. With this functional, the calculated absorption maxima obtained for the bis-aryl-sulfenyl-benzo-2,1,3-thiadiazoles (3a–f) is located around 410 nm and for the bis-aryl-selenyl-benzo-2,1,3-thiadiazoles (5a–d) the absorption maxima is located around 417 nm. The weak oscillator strengths presented are also associated with the lack of planarity since the more planar conformation, the higher oscillator strength.

No significant change takes place on the λ_abs related to different organic groups attached to the benzo-2,1,3-thiadiazole core. The same behavior was observed when sulfur atom was changed by selenium, in accordance with experimental data, although with Se the absorption maxima is shifted to longer wavelength. In addition, a quite similar absorption wavelength was obtained for all solvents, indicating the absence of the solvatochromic effect.

The emission maxima wavelength showed for 3a–e compounds are 530–564 nm, 536–553 nm for 3f and 536–562 nm for 5a–d. In the same way as in the ground state, a small influence of the substituent group attached to the benzene ring on the emission spectra was found. However, the presence of the halogen atom at this position slightly decreases both the emission and absorption wavelength. This could be explained by the inductive effect, i.e. simple electrostatic effects which arise from changes in electron distribution. It is known that the substitution of fluorine in positions close to a chromophore group, but not conjugated with it, leads to short wavelength shifts of the absorption bands of the more highly excited states due to the inductive effect of the fluorine atom.³³ The relative magnitude of this effect differs in the ground and excited states and depends upon the degree of excitation. When there are a large overlap between the upper orbitals and the substituent group, energy changes in the excited orbitals could be produced, which make the band appear to behave anomalously on substitution.³³

In contrast to the almost absent solvent dependence in absorption spectra, the fluorescence emission spectra displays a clear solvent dependence, with a bathochromic shift in the emission wavelength maxima when solvent polarity is increased (from hexane to dichloromethane around 18 nm in 3a–e and around 16 nm in 3f and 5a–d). This reflects the larger change in dipole moment from S₀ to S₁. Since the molecules are more polar in the excited state, more polar solvents will stabilize more the S₁ than S₀, resulting in emission at lower energies, as the solvent polarity is increased. In addition, the oscillator strengths also are increased from hexane to dichloromethane in excited state. Absorption spectrum is less sensitive to solvent polarity because the molecule is exposed to the same local environmental in the ground and excited states.³⁴ The representative HOMO and LUMO molecular orbitals are shown in Fig. 7 for 3b and 5a structures.

Fig. 7 Representative HOMO and LUMO molecular orbitals obtained for 3b (above) and 5a (below) at CAM-B3LYP level using hexane as solvent.

No significant differences were observed between HOMO and LUMO surface contour for the remaining structures or even when solvent is changed. Both HOMO and LUMO are π type, but the LUMO orbital is mostly centered on the BTD ring as a whole, whereas the HOMO is mainly located on the phenyl portion of the BTD ring (donor) and extend the conjugation over the chalcogen atom attached to it. This feature is important to understand de ICT contribution to the excited state. The vertical absorption from S₀ → S₁ is a ¹ππ* electronic transition from HOMO to LUMO orbitals, according to the molar absorptivity coefficient values and radiative rate constant obtained experimentally. Although changing the chalcogen atom also does not affect appreciably the HOMO and LUMO, one interesting feature of these plots is the small lobes centred on sulfur atom in the LUMO that is not present changing the chalcogen to Se.

The large Stokes shift and dipole moment in the first excited state together with the solvatochromic effect in emission energies and a observed spatial separation of the HOMO and LUMO orbitals are strong evidences to ICT character of the S₁ electronic state.^35,36 The intensity and position of the intramolecular charge transfer bands depends on the nature of the structure and substituents influencing the donor and acceptor character. According to the literature, planar π-conjugated moiety linked with distorted π-substituents associated with well separated HOMO and LUMO distributions are a clear indication of an efficient stabilizing ICT.³⁷

In the systems studied by us, although there is a π-conjugation of the chalcogen atoms with the donor part of the molecule, the π-extension is not so efficient since the RC₆H₄ unit is not conjugated with the 2,1,3-benzothiadiazole ring. In addition, since the LUMO orbital is delocalized over all 2,1,3-benzothiadiazole core, the charge separation is affected. This feature could decreases the band intensity related to an intramolecular charge transfer. Further details about the less energetic electronic transitions and molecular orbitals involved on it can be found in the ESI (Tables ESI3, ESI4 and Fig. ESI1–4†).

Additionally, one more intense band than the first one is presented in all theoretical calculations. This band is observed around 278 nm, 281 nm and 273 nm, for 3a–e, 3f and 5a–d structures, respective and could be related to the local excitation (LE) ¹ππ* at the 2,1,3-benzothiadoazole core. This band involves a transition between an occupied molecular orbital of the lower energy than HOMO, with π symmetry and fully spread over at 2,1,3-benzothiadiazole core, to LUMO. Increasing the solvent polarity, this band becomes less intense.

In Fig. 8, the electronic changes to which structures 3a–f and 5a–d are subjected when excitation occurs can be observed.

Fig. 8 Electrostatic potential surfaces of 3a–f and 5a–d structures in the ground electronic state (S₀) and first electronic excited state (S₁). The highest electron density potential is represented in red and the lowest electron density is represented in blue.

In each case, significant displacement of charge and increase in the magnitude of the dipole moment takes place upon excitation (see Table 5). The dipole moment is considerably higher in excited states than in ground states. With the increase in dielectric constant of solvent, the dipole moment also increase, as result of the higher stabilization of molecules when solvent is more polar, particularly at the excited state. The main effect of the halogen atom (bonded at para position) on the dipole moment is its significant decrease with respect to the other structures in S₀ and S₁. Additionally, the sulfur analogues present the higher dipole moment in the ground and excited state. It can be seen that upon excitation to S₁ state, the electrostatic potential over the thiophene unit increases, particularly over the sulfur atom, indicating an increase of the local electronic density. At the same time, it can be seen a decrease in electronic density over the side benzene ring when –Cl or –F are attached to it. No significant differences can be seen changing the chalcogen atom or changing the solvent.

Conclusions

In summary, new classes of benzo-2,1,3-thiadiazoles derivatives containing an organochalcogen moiety in their structures were synthesized and photophysically characterized. These compounds were synthesized in good yields by copper-catalysed cross-coupling reaction of arylthiols or diaryl diselenides with 4,7-dibromobenzo[c][1,2,5]thiadiazole. The obtained compounds present absorptions in the visible region with molar absorptivity coefficient and radiative rate constant values ascribed to spin and symmetry allowed π–π* electronic transitions. An emission located in the cyan green to green regions with a large Stokes shift (90–146 nm) was observed, probably due to intramolecular charge transfer state. DFT and TD-DFT calculations were performed and showed to be in full accordance with experimental data, at CAM-B3LYP level. The non-planarity of the 3a–f and 5a–d molecules increases the HOMO/LUMO gap, decreases the oscillator strength and shifts the absorption spectra to the smaller wavelength. The lowest excited state can be ascribed to be an intramolecular charge-transfer, but its intensity is decreased by the non-planar architecture of these systems, leading to a less efficient HOMO/LUMO separation. The vertical electronic transitions of all the molecules in lower energy absorption bands are a π → π* type. Changing the chalcogen atom from S to Se, there are no significant differences in the absorption and emission spectra, but λ_abs are slightly redshifts while λ_em are slightly blueshifts. Increasing the solvent polarity does not affect the maximum absorption wavelength but redshift the maximum emission wavelength.

Acknowledgements

We are grateful for the financial support and scholarships from the Brazilian agencies CNPq (400150/2014-0 and 447595/2014-8), FAPERGS (PRONEM 11/2024-9) and CAPES and Instituto Nacional de Inovação em Diagnósticos para a Saúde Pública (INDI-Saúde). Theoretical calculations carried out with the support of Centro Nacional de Supercomputação (CESUP), Universidade Federal do Rio Grande do Sul (UFRGS).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Spectroscopic characterization of the compounds and additional results of the theoretical calculations. See DOI: 10.1039/c6ra04157d

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