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Article

Benzimidazole-Based N,O Boron Complexes as Deep Blue Solid-State Fluorophores

by
Patrícia A. A. M. Vaz
1,2,
João Rocha
2,
Artur M. S. Silva
1 and
Samuel Guieu
1,2,*
1
LAQV-REQUIMTE, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
2
CICECO, Aveiro Institute of Materials, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
*
Author to whom correspondence should be addressed.
Materials 2021, 14(15), 4298; https://doi.org/10.3390/ma14154298
Submission received: 25 June 2021 / Revised: 26 July 2021 / Accepted: 29 July 2021 / Published: 31 July 2021
(This article belongs to the Special Issue Organic Luminescent Materials)
Browse Figures
Versions Notes

Abstract

:
Benzimidazole-based boranils were designed and synthesized in order to assess the influence of halogen substituents on their optoelectronic properties. All compounds are photoluminescent in solution and solid state. Compared to the free ligands, the new boranils emit at a lower wavelength, by elimination of the excited-state intramolecular proton transfer observed with the ligands. In the solid state, some of the boranils exhibit a deep blue emission, presenting Commission Internationale de l’Éclairage (CIE) coordinates with an x-component of less than 0.16 and a y-component smaller than 0.04, highly desired values for the development of blue emitting materials.

Graphical Abstract

1. Introduction

Benzimidazole and other azole derivatives raise much interest due to their luminescence properties, and as promising candidates for the development of sensors and solar cells [1,2,3,4,5]. They are receiving increasing attention due to their non-linear optical properties [6,7] and excellent thermal stability [8,9]. Widely reported in the literature, these compounds are also of interest because of their intense emission via excited-state intramolecular proton transfer (ESIPT) [10,11,12]. Due to their optical properties, fully conjugated core, and possibility of functionalization of the aromatic ring, benzimidazole derivatives are privileged ligands to develop materials with bright blue emissions [7,13]. Another advantage of these compounds is that they have an excellent coordination ability with a wide range of transition metals, and may present interesting selectivity and sensitivity toward different metal cations [14].
Complexation with boron is widely used in the synthesis of luminescent dyes. Boron is a nontoxic alternative to transition metals, and boron complexes have been studied for their excellent photoluminescent properties [15,16,17,18,19]. The latter are cheaper than certain transition metals, and because of the strong covalent bonds with the ligand, they may allow the synthesis of more stable luminescent materials [20,21,22]. For example, difluoroboron complexation enabled thermally activated delayed fluorescence, which find interesting applications in organic light-emitting diodes [23]. Benzimidazoles bearing an ortho-hydroxy aromatic substituent are interesting fluorescent molecules (Figure 1) because of their large Stokes shifts due to their ESIPT [24,25]. This mechanism can be inhibited by binding a boron atom to the hydroxy group and the nitrogen atom of the azole unit: longer emission wavelengths are lost, resulting in a hypsochromic shift [26]. Derivatives of 2-(2-hydroxyphenyl)benzothiazole 1 and 2-(2-hydroxyphenyl)benzoxazole 2 are examples of compounds explored for blue emission [27,28,29]. Recently, boron complexes 3 incorporating imidazo[1,5-a]pyridine as a N,O-type ligand were reported [30], as well as tetraaryl substituted imidazole boron difluoride complexes 4 [31]. Examples of reported boron complexes based on bidentate 2-(2-pyridyl)imidazole 5, and N-alkylated 2-(2-hydroxyphenyl)benzimidazole 6, showed emissions in both solution and solid state [32,33].
Although imidazole-phenol complexes have been studied for application in electronic materials [34], the nitrogen atom not involved in the complexation usually bears a substituent, and if 2-(1H-benzo[d]imidazol-2-yl)phenol has been studied for boron complexation [35], to the best of our knowledge no studies of their photophysical properties have been performed.
The introduction of halogen atoms on the backbone of fluorophores is a strategy used to promote their phosphorescence. Indeed, the heavy atom effect can promote intersystem crossing between the excited singlet and triplet states. If a halogen bond is formed in the solid state, the effect can be even more pronounced, allowing the observation of phosphorescence in purely organic solids at room temperature [36]. This may find applications in the development of organic light emitting devices.
Here, we wish to report the synthesis and characterization of boron complexes 8 based on halogenated 2-(1H-benzo[d]imidazol-2-yl)phenol 7 (Figure 2), and the study of their optoelectronic properties. In the design of the fluorophores, the nitrogen not involved in the complexation was kept unsubstituted to minimize the conjugation of the backbone, in order to produce a blue emission. Halogen atoms have been introduced on one aromatic ring, in an attempt to promote intersystem crossing and therefore phosphorescent dyes.

2. Materials and Methods

2.1. Materials and Methods

All reagents were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used without any further purification. The final compounds were purified by crystallization. 1H, 19F and 13C NMR spectra were recorded on Bruker 300 or 500 [300.13 MHz (1H), 282.41 (19F), 75.47 MHz (13C) or 500.13 MHz (1H)]. Unequivocal 13C assignments were made on the basis of 2D Heteronuclear Single Quantum Coherence (HSQC) (1H/13C) and Heteronuclear Multiple Bond Correlation (HMBC) experiments. DMSO-d6 was used as the solvent and tetramethylsilane (TMS) as the internal standard. Chemical shifts δ are reported in parts per million (ppm) relative to TMS (δ = 0), and the values of coupling constants (J) are given in Hertz (Hz). High-resolution mass spectra (HRMS-ESI+) were recorded on an LTQ Orbitrap™ XL hybrid mass spectrometer (Thermo Fischer Scientific, Bremen, Germany) controlled by LTQ Tune Plus 2.5.5 and Xcalibur 2.1.0. The capillary voltage of the electrospray ionization source (ESI) was set to 3.1 kV. Melting points were determined on a BUCHI Melting point apparatus (BÜCHI Labortechnik AG, Flawil, Switzerland) and are uncorrected. The ultraviolet-visible (UV-Vis) spectra in dimethyl sulfoxide solutions were obtained on a Shimadzu UV-2501 PC spectrophotometer (1 cm path length quartz cell, Shimatzu, Nakagyō-ku, Japan) and the UV-Vis absorption spectra of the solids were measured at room temperature on a JASCO V-560 instrument (JASCO Inc., Easton, MD, USA). The excitation and emission spectra, also in dimethyl sulfoxide solutions, were recorded on a Jobin Yvon FluoroMax-3 spectrofluorometer (Horiba, Lier, Belgium) and a JASCO spectrofluorometer (JASCO Inc., Easton, MD, USA). Fluorescence quantum yields φF were determined using fluorescein in 0.1 M NaOH water solution as a fluorescence standard. The absolute emission quantum yields in the solid state were measured at room temperature using a system (Quantaurus-QY Plus C13534, Hamamatsu, Shizuoka, Japan) with a 150 W xenon lamp coupled to a monochromator for wavelength discrimination, an integrating sphere as the sample chamber, and a multichannel analyzer for signal detection. The method is accurate to within 10%.

2.2. Synthesis

2.2.1. General Procedure for the Synthesis of Ligands 7a–d

The general procedure is described here for 7a: salicylaldehyde (1.0 equiv., 1.0 mL, 9.6 mmol) and o-phenylenediamine (1.5 equiv., 1.6 g, 14.4 mmol) were mixed in methanol (2 mL). After everything dissolved, a red solid immediately formed. Acetic acid (20 mL) was added and the reaction mixture was stirred at room temperature for 4 h. The reaction was quenched by adding water (60 mL), the solid was collected by filtration and washed with methanol (3 × 20 mL). The solid obtained was purified by flash column chromatography over silica gel, eluent hexane/ethyl acetate (2:1 v/v), to afford the product 7a as off-white crystals after crystallization in chloroform.
2-(1H-benzo[d]imidazol-2-yl)phenol 7a [37]
The compound 7a was obtained as colorless microcrystals (1.25 g, 5.94 mmol, 62%). M.P.: 242–243 °C; 1H NMR (300 MHz, DMSO-d6) δ 13.17 (br s, 2H, OH and NH), 8.05 (dd, 3JH-H 7.8, 4JH-H 1.4 Hz, 1H, Harom), 7.66 (br s, 2H, Harom), 7.38 (ddd, 3JH-H 8.6, 3JH-H 7.1, 4JH-H 1.6 Hz, 1H, Harom), 7.30–7.27 (m, 2H, Harom), 7.05–6.99 (m, 2H, Harom). HRMS-ESI+ m/z for [C13H10N2O + H]+ calcd 211.0866, found 221.0868.
2-(1H-benzo[d]imidazol-2-yl)-4-chlorophenol 7b [37]
The compound 7b was obtained as colorless microcrystals (90 mg, 0.36 mmol, 60%). M.P.: 306–307 °C; 1H NMR (500 MHz, DMSO-d6) δ 13.27 (br s, 2H, OH and NH), 8.17 (d, 4JH-H 2.6 Hz, 1H, Harom), 7.68 (br s, 2H, Harom), 7.42 (dd, 3JH-H 8.8, 4JH-H 2.6 Hz, 1H, Harom), 7.31 (br s, 2H, Harom), 7.08 (d, 3JH-H 8.8, 1H, Harom). HRMS-ESI+ m/z for [C13H9ClN2O + H]+ calcd 245.0476, found 245.0479.
2-(1H-benzo[d]imidazol-2-yl)-4-bromophenol 7c
The compound 7c was obtained as a brown solid (90 mg, 0.32 mmol, 63%). M.P.: 311–312 °C; 1H NMR (300 MHz, DMSO-d6) δ 13.30 (br s, 2H, OH and NH), 8.29 (d, 4JH-H 2.5 Hz, 1H, Harom), 7.68 (dd, 3JH-H 5.9 Hz, 4JH-H 3.2 Hz, 2H, Harom), 7.52 (dd, 3JH-H 8.8, 4JH-H 2.5 Hz, 1H, Harom), 7.30 (dd, 3JH-H 5.9 Hz, 4JH-H 3.2 Hz, 2H, Harom), 7.02 (d, 3JH-H 8.8, 1H, Harom). 13C NMR (75 MHz, DMSO-d6) δ 157.2 (1C, C-O), 150.3 (1C, C = N), 134.0 (1C, CHarom), 128.5 (1C, CHarom), 123.2 (4C, CHarom), 119.6 (1C, CHarom), 114.7 (1C, C-Br), 112.4 (2C, Carom) 110.2 (1C, Carom). HRMS-ESI+ m/z for [C13H9BrN2O + H]+ calcd 288.9971, found 288.9975.
2-(1H-benzo[d]imidazol-2-yl)-4-iodophenol 7d
The compound 7d was obtained as a brown solid (95 mg, 0.2 mmol, 68%). M.P.: 254–255 °C; 1H NMR (300 MHz, DMSO-d6) δ 13.28 (br s, 2H, OH and NH), 8.42 (d, 4JH-H 2.2 Hz, 1H, Harom), 7.67 (br s, 2H, Harom), 7.65 (dd, 3JH-H 8.7, 4JH-H 2.2 Hz, 1H, Harom), 7.30 (dd, 3JH-H 6.1 Hz, 4JH-H 2.4 Hz, 2H, Harom), 6.89 (d, 3JH-H 8.7 Hz, 1H, Harom). 13C NMR (75 MHz, DMSO -d6) δ 157.7 (1C, C-O), 150.2 (1C, C = N), 139.7 (1C, CHarom), 134.2 (1C, CHarom), 123.1 (4C, CHarom), 119.9 (1C, CHarom), 115.3 (1C, C-I), 110.4 (2C, Carom), 80.9 (1C, Carom). HRMS-ESI+ m/z for [C13H9IN2O + H]+ calcd 336.9832, found 336.9833.

2.2.2. General Procedure for the Synthesis of Complexes 8a–d

The typical synthetic procedure of benzimidazole-based N,O-chelated boron complexes is described as following for 8a: BF3·OEt2 (3 equiv., 0.06 mL, 0.6 mmol) was added dropwise at room temperature to a stirred mixture of ligand 7a (0.2 mmol) in anhydrous tetrahydrofuran (2 mL), and the reaction mixture was stirred overnight. The solvent was removed under reduced pressure and the solid obtained was washed with anhydrous diethyl ether several times to obtain the complex.
6,6-difluoro-6,12-dihydrobenzo[e]benzo[4,5]imidazo[1,2-c][1,3,2]oxazaborinin-7-ium-6-uide 8a [35]
The compound 8a was obtained as a white solid (62 mg, 0.2 mmol, 100%). M.P.: 348–350 °C. 1H NMR (300 MHz, DMSO-d6) δ 14.65 (b s, 1H, NH), 8.05 (dd, 3JH-H 8.1 Hz, 4JH-H 1.5 Hz, 1H, Harom), 7.85 (dd, 3JH-H 6.2 Hz, 4JH-H 2.4 Hz, 2H, Harom), 7.59 (ddd, 3JH-H 7.8 Hz, 3JH-H 6.4 Hz, 4JH-H 1.3 Hz, 1H, Harom), 7.55 (dd, 3JH-H 6.2 Hz, 4JH-H 2.4 Hz, 2H, Harom), 7.21 (dd, 3JH-H 9.0 Hz, 4JH-H 0.7 Hz, 1H, Harom), 7.15 (ddd, 3JH-H 7.5 Hz, 3JH-H 6.2 Hz, 4JH-H 0.9 Hz, 1H, Harom). 13C NMR (75 MHz, DMSO d-6) δ 157.1 (1C, C-O), 146.8 (1C, C = N), 135.2 (1C, CHarom), 131.2 (2C, Carom), 129.3 (1C, CHarom), 125.9 (2C, CHarom), 120.0 (1C, CHarom), 117.3 (1C, CHarom), 114.1 (2C, CHarom), 109.2 (1C, Carom).19F NMR (282 MHz, DMSO-d6) δ -132.2 (dd, J 27.7 Hz, J 10.7 Hz, 2F). HRMS-ESI+ m/z for [C13H9BF2N2O + H] + calcd 259.0854, found 259.0853.
2-Chloro-6,6-difluoro-6,12-dihydrobenzo[e]benzo[4,5]imidazo[1,2-c][1,3,2]oxazaborinin-7-ium-6-uide 8b
The compound 8b was obtained as a white solid (29 mg, 0.099 mmol, 46%), M.P.: 308–309 °C. 1H NMR (300 MHz, DMSO-d6) δ 14.71 (s, 1H, NH), 8.21 (d, 4JH-H 2.6 Hz, 1H, Harom), 7.85–7.78 (m, 2H, Harom), 7.61 (dd, 3JH-H 8.9 Hz, 4JH-H 2.6 Hz, 1H, Harom), 7.49–7.57 (m, 2H, Harom), 7.16 (d, 3JH-H 8.9 Hz, 1H, Harom). 13C NMR (75 MHz, DMSO d-6) δ 155.3 (1C, C-O), 146.1 (1C, C = N), 134.5 (1C, CHarom), 132.2 (2C, Carom), 125.8 (1C, CHarom), 125.2 (1C, CHarom), 124.9 (1C, CHarom), 123.3 (1C, C-Cl), 121.2 (1C, CHarom), 115.1 (1C, CHarom), 113.4 (1C, CHarom), 110.0 (1C, Carom). 19F NMR (282 MHz, DMSO-d6) δ -132.1 (dd, J 25.4 Hz, J 8.7 Hz, 2F). HRMS-ESI+ m/z for [C13H8B37ClF2N2O + H] + calcd 293,0465, found 293,0466.
2-Bromo-6,6-difluoro-6,12-dihydrobenzo[e]benzo[4,5]imidazo[1,2-c][1,3,2]oxazaborinin-7-ium-6-uide 8c
The compound 8c was obtained as a white solid (34 mg, 0.10 mmol, 81%). M.P.: 395–396 °C. 1H NMR (300 MHz, DMSO-d6) δ 14.75 (s, 1H, NH), 8.34 (d, 4JH-H 2.5 Hz, 1H, Harom), 7.78–784 (m, 2H, Harom), 7.72 (dd, 3JH-H 8.9 Hz, 4JH-H 2.5 Hz, 1H, Harom), 7.49–7.58 (m, 2H, Harom), 7.10 (d, 3JH-H 8.9 Hz, 1H, Harom). 13C NMR (75 MHz, DMSO d-6) δ 155.6 (1C, C-O), 146.0 (1C, C = N), 137.2 (1C, CHarom), 132.3 (2C, Carom), 127.8 (1C, CHarom), 125.8 (1C, CHarom), 125.2 (1C, Charom), 123.3 (1C, C-Br), 121.6 (1C, CHarom), 115.1 (1C, CHarom), 113.4 (1C, CHarom), 110.7 (1C, Carom). 19F NMR (282 MHz, DMSO-d6) δ -132.1 (dd, J 25.4 Hz, J 8.7 Hz, 2F). MS-ESI+ m/z for [C13H8BBrFN2O] + 317.0 (79Br), 319.0 (81Br). HRMS-ESI- m/z for [C13H8BBrF2N2O-H]-calcd 334.9808, found 334.9827.
6,6-difluoro-2-iodo-6,12-dihydrobenzo[e]benzo[4,5]imidazo[1,2-c][1,3,2]oxazaborinin-7-ium-6-uide 8d
The compound 8d was obtained as a brown solid (0.200 g, 0.5 mmol, 79%). M.P.: 379–380 °C. 1H NMR (300 MHz, DMSO-d6) δ 14.68 (s, 1H, NH), 8.48 (d, 4JH-H 2.2 Hz, 1H, Harom), 7.86 (dd, 3JH-H 8.7 Hz, 4JH-H 2.2 Hz, 1H, Harom), 7.78–7.81 (m, 2H, Harom), 7.49–7.58 (m, 2H, Harom), 6.97 (d, 3JH-H 8.7, 1H, Harom). 13C NMR (75 MHz, DMSO-d6) δ 156.1 (1C, C-O), 142.8 (1C, C = N), 142.8 (1C, CHarom), 133.6 (1C, CHarom), 123.2 (1C, Carom), 132.0 (1C, Carom), 121.7 (1C, CHarom), 125.8 (1C, CHarom), 125.2 (1C, CHarom), 115.1 (1C, CHarom), 113.3 (1C, CHarom), 108.2 (1C, Carom), 81.6 (1C, C-I). 19F NMR (282 MHz, DMSO-d6) δ -132.0 (dd, J 25.4 Hz, J 8.7 Hz, 2F). HRMS-ESI+ m/z for [C13H8BF2IN2O + Na]+, calcd 406,9640, found 406,9639.

3. Results and Discussion

3.1. Synthesis

The synthetic procedure for the synthesis of fluoroborates complexes 8a–d is described in Scheme 1.
Because benzimidazole rings are important building blocks for therapeutic drugs and optoelectronic materials, several strategies for their synthesis are reported in the literature [38]. The most common procedures call for the cyclocondensation of o-phenylenediamine derivatives with carboxylic acids under strongly acidic conditions, or with aldehydes under oxidative conditions. Although several oxidative and catalytic reagents have been employed in the synthesis of benzimidazoles from aldehydes [39], some of these methods have certain disadvantages, such as requiring large quantities of reagent, the high cost of the catalysts, prolonged reaction times, occurrence of side reactions, the harsh reaction conditions used, strong oxidizing nature of the reagents, or the use of toxic metal salts. For the synthesis of ligands 7a–d we have chosen the classical cyclocondensation of o-phenylenediamine with the corresponding salicylaldehydes 9a–d under oxidative conditions, using the atmospheric oxygen as the oxidant, and promoting the cyclization reaction by adding acetic acid. The reaction starts with the formation of a red solid (the intermediate Schiff base), which disappears after the addition of acetic acid. The reaction solution then became strongly photoluminescent, indicating the formation of the benzimidazole ring.
The synthesis of the fluoroborates complexes 8b–d followed the conditions described for 8a. Because this compound was obtained with a good yield, the conditions were maintained for the other complexes. Briefly, the ligands 7a–d were dissolved in THF and three equivalents of boron trifluoride diethyl etherate were added at room temperature. The complexes were obtained as off-white solids after evaporation of the reaction mixture under reduced pressure and after several wash cycles with diethyl ether. The instability of the complexes, which are sensitive to hydrolysis, made their isolation difficult, and their purification should be done as quickly as possible. Attempts to purify the complexes using silica gel column chromatography were unsuccessful, and a mixture of the complex and the ligand was systematically recovered. Additionally, the hydrolysis of the complexes was also observed during NMR characterization, if the spectrum was not recorded just after the preparation of the solution. All compounds were fully characterized (see Supplementary Materials).

3.2. Photophysical Properties

The absorption and emission spectra of benzimidazoles 7a–d were recorded in anhydrous DMSO solution and are shown in Figure 3. DMSO was selected to maximize the solubility of both the free ligands and the boron complexes. The solutions were freshly prepared and used immediately, to prevent the hydrolysis of the complexes. The photophysical properties of compounds 7a–d are summarized in Table 1.
The ligand 7a exhibits a maximum absorption peak at 333 nm, which is ascribed to the π → π * transition (Figure 3). For the halogenated derivatives 7b–d, the maximum absorption wavelength is slightly red shifted, by ca. 10 nm. This small shift can be related to the presence of electron withdrawing groups (–Cl, –Br and I) that decreases the Highest Occupied Molecular Orbital-Lowest Unoccupied Molecular Orbital (HOMO-LUMO) energy gap of substituted derivatives [40,41]. In these kind of compounds, ESIPT is strongly influenced by the nature and position of the substituents, which adjust the strength of the hydrogen bond [42,43,44].
All compounds are fluorescent in DMSO solution and show large Stokes’ shifts of ca. 125 nm, characteristic of fluorophores presenting ESIPT [43,45,46]. The maximum emission wavelength of 7b–d is also slightly red shifted (5–10 nm) moving down the halogen series (Figure 3). The quantum yields are, however, very different, and for 7a and 7b they are very good, 93% and 95%, respectively. The quenching observed for 7c (quantum yield 33%) and 7d (quantum yield 4%) results from the expected internal heavy atom effect [47], which is stronger with the iodine than with the bromine substituents. All ligands 7a–d exhibit a bright blue fluorescence under irradiation at 254 or 365 nm (Table 2).
The absorption and emission spectra of boron complexes 8a–d in anhydrous DMSO are displayed in Figure 3. Their maximum absorption energy is also ascribed to the π → π * transition [48]. The emission and absorption spectra of the complexes are mirror images and display a small Stokes shift between 16 and 26 nm, indicating the absence of major internal conversions.
The ESIPT (Figure 4) witnesses the transfer of the proton from the hydroxy to the imidazole-N moiety through a pre-existing hydrogen bond, and is responsible for the large Stokes’ shift observed for the ligands. The complexation of boron supresses this phenomenon, as the hydrogen atom is no longer present and, as a consequence, the emission profile is dramatically altered. Here, the halogen atoms have a more significant effect on the emission properties: the quantum yields of 8b–d decrease significantly relatively to 8a. The small Stokes’ shifts and the spectral overlap may lead to the self-quenching of fluorescence for compounds 8a–d, and the halogen atoms may promote the intersystem crossing, leading to a quenching of the fluorescence instead of the expected promotion of phosphorescence.
Figure 5 depicts the emission spectra of complexes and ligands in the solid state and in DMSO solution. The emissions of the complexes are mirror images of their absorption, with structured emission bands, which is not the case for the ligands. This has been observed before for similar fluorophores [32]. While the solid state and solution emission spectra of the ligands present the same profile, the complexes do not. The emission maxima of the latter are red shifted in the solid state relatively to solution. This indicates possible intermolecular interactions in the solid state, such as π···π stacking, which may be responsible for both the red shift and the quenching observed.
All compounds exhibit a blue emission in solution and solid state (Figure 6 and Figure 7). In solution, 8b, with a moderate quantum yield of 26%, exhibits a deep blue emission with CIE coordinates of (0.16, 0.02) with a y-component smaller than 0.04, that is highly desirable and follows the specifications of the European Broadcast Union (EBU) television (0.15, 0.06) [49,50]. In the solid state, all complexes emit a blue color under a UV lamp (Table 2). Both 8a (0.16, 0.04) and 8b (0.16, 0.08) present a deep blue color in the solid state.

4. Conclusions

Boron complexes based on halogenated 2-(1H-benzo[d]imidazol-2-yl)phenol were synthesized and characterized. All the halogenated benzimidazole complexes exhibit a blue emission in both DMSO solution and solid state, with moderate quantum yields, demonstrating that keeping the conjugation minimal is an adequate strategy to tune the emission color. On the other hand, the introduction of halogen substituents lowered their quantum yields, and did not promote their phosphorescence in the solid state, making these compounds less promising for electroluminescence. Unfortunately, the instability of the complexes, which are sensitive to hydrolysis, may prevent their applications as emissive materials. Nevertheless, some of them present a deep blue emission, making them interesting starting points for the development of emissive materials.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/ma14154298/s1: NMR spectra of all new compounds; absorption, emission and excitation spectra in solution; excitation and emission spectra in solid state.

Author Contributions

Conceptualization, S.G. and P.A.A.M.V.; investigation, P.A.A.M.V.; writing—original draft preparation, P.A.A.M.V.; writing—review and editing, S.G., J.R. and A.M.S.S.; supervision, S.G.; funding acquisition, J.R. and A.M.S.S. All authors have read and agreed to the published version of the manuscript.

Funding

Thanks are due to University of Aveiro, FCT/MEC, Centro 2020 and Portugal 2020, the COMPETE program, and the European Union (FEDER program) via the financial support to the LAQV-REQUIMTE (UIDB/50006/2020), to the CICECO-Aveiro Institute of Materials (UID/CTM/50011/2019, UIDB/50011/2020 & UIDP/50011/2020), financed by national funds through the FCT/MCTES, to the Portuguese NMR Network, and to the PAGE project “Protein aggregation across the lifespan” (CENTRO-01-0145-FEDER-000003). SG is supported by national funds (OE), through FCT, I.P., in the scope of the framework contract foreseen in the numbers 4, 5, and 6 of the article 23, of the Decree-Law 57/2016, of August 29, changed by Law 57/2017, of July 19. PV thanks the FCT for a doctoral grant (SFRH/BD/99809/2014).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

Rute AS Ferreira (phantom-g, CICECO, Department of Physics, University of Aveiro) is acknowledged for the solid state quantum yield measurements.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Materials 14 04298 g001 550
Figure 1. Azole-based organoboron compounds suitable for optoelectronic applications.
Figure 1. Azole-based organoboron compounds suitable for optoelectronic applications.
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Materials 14 04298 g002 550
Figure 2. Halogenated boron complexes 8 based on 2-(1H-benzo[d]imidazol-2-yl)phenol 7 studied in this work.
Figure 2. Halogenated boron complexes 8 based on 2-(1H-benzo[d]imidazol-2-yl)phenol 7 studied in this work.
Materials 14 04298 g002
Materials 14 04298 sch001 550
Scheme 1. Synthetic procedure for the synthesis of fluoroborates complexes 8a–d.
Scheme 1. Synthetic procedure for the synthesis of fluoroborates complexes 8a–d.
Materials 14 04298 sch001
Materials 14 04298 g003 550
Figure 3. Absorption (full lines) and emission spectra (dotted lines) of compounds 7a–d and compounds 8a–d, in DMSO (at 1 × 10−5 mol L−1).
Figure 3. Absorption (full lines) and emission spectra (dotted lines) of compounds 7a–d and compounds 8a–d, in DMSO (at 1 × 10−5 mol L−1).
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Materials 14 04298 g004 550
Figure 4. ESIPT fluorescence of benzimidazol derivatives.
Figure 4. ESIPT fluorescence of benzimidazol derivatives.
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Materials 14 04298 g005 550
Figure 5. Emission spectra of ligands (red) and complexes (blue), in DMSO solutions (1.0 × 10−5 M, full line) and solid state (dotted line).
Figure 5. Emission spectra of ligands (red) and complexes (blue), in DMSO solutions (1.0 × 10−5 M, full line) and solid state (dotted line).
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Materials 14 04298 g006 550
Figure 6. The CIE coordinates for the compounds 7a–d and 8a–d in DMSO solution, showing the deep-blue color of 8b.
Figure 6. The CIE coordinates for the compounds 7a–d and 8a–d in DMSO solution, showing the deep-blue color of 8b.
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Materials 14 04298 g007 550
Figure 7. The CIE coordinates for the compounds 7a–d and 8a–d in solid state, showing the deep-blue color of 8a–b.
Figure 7. The CIE coordinates for the compounds 7a–d and 8a–d in solid state, showing the deep-blue color of 8a–b.
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Table
Table 1. Optical properties of compounds 7a–d and 8a–d.
Table 1. Optical properties of compounds 7a–d and 8a–d.
Dye λabs (nm) 1ε (M−1 cm−1) 2λem (nm) 3 δλst φf 4 (%) Solvent
7a 333 65,600 460 127 93 DMSO
325 - 455 130 39 Solid
7b 341 76,500 466 125 95 DMSO
325 - 466 141 18 Solid
7c 341 55,700 467 126 33 DMSO
325 - 466 141 5 Solid
7d 343 46,800 470 127 4 DMSO
325 - 500 175 <1 Solid
8a 336 55,300 360 24 75 DMSO
325 - 380 55 12 Solid
8b 344 83,900 370 26 26 DMSO
325 - 420 95 2 Solid
8c 345 82,300 361 16 1 DMSO
325 - 395 70 2 Solid
8d 345 91,300 370 25 <1 DMSO
325 - 500 175 <1 Solid
1 Absorption maximum. 2 Molar absorption coefficients. 3 Fluorescence maximum. 4 In solution: determined by comparison with fluorescein (φf = 0.90 in water with NaOH 0.1 mol L−1) [39] at room temperature; in the solid state: measured using an integrating sphere, the excitation wavelength was 375 nm.
Table
Table 2. Photographs of the ligands 7a–d and fluoroborates complexes 8a–d in solid state under UV lamp.
Table 2. Photographs of the ligands 7a–d and fluoroborates complexes 8a–d in solid state under UV lamp.
CompoundDay Light254 nm365 nm
7a Materials 14 04298 i001 Materials 14 04298 i002 Materials 14 04298 i003
7b Materials 14 04298 i004 Materials 14 04298 i005 Materials 14 04298 i006
7c Materials 14 04298 i007 Materials 14 04298 i008 Materials 14 04298 i009
7d Materials 14 04298 i010 Materials 14 04298 i011 Materials 14 04298 i012
8a Materials 14 04298 i013 Materials 14 04298 i014 Materials 14 04298 i015
8b Materials 14 04298 i016 Materials 14 04298 i017 Materials 14 04298 i018
8c Materials 14 04298 i019 Materials 14 04298 i020 Materials 14 04298 i021
8d Materials 14 04298 i022 Materials 14 04298 i023 Materials 14 04298 i024
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Vaz, P.A.A.M.; Rocha, J.; Silva, A.M.S.; Guieu, S. Benzimidazole-Based N,O Boron Complexes as Deep Blue Solid-State Fluorophores. Materials 2021, 14, 4298. https://doi.org/10.3390/ma14154298

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Vaz PAAM, Rocha J, Silva AMS, Guieu S. Benzimidazole-Based N,O Boron Complexes as Deep Blue Solid-State Fluorophores. Materials. 2021; 14(15):4298. https://doi.org/10.3390/ma14154298

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Vaz, Patrícia A. A. M., João Rocha, Artur M. S. Silva, and Samuel Guieu. 2021. "Benzimidazole-Based N,O Boron Complexes as Deep Blue Solid-State Fluorophores" Materials 14, no. 15: 4298. https://doi.org/10.3390/ma14154298

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