Carbazole- and Triphenylamine-Substituted Pyrimidines: Synthesis and Photophysical Properties

A series of pyrimidine derivatives bearing one, two or three triphenylamine/9-ethylcarbazole substituents has been synthesized by Suzuki cross-coupling reaction. All compounds showed absorption bands in the UV region and the emission of violet-blue light upon irradiation. Protonation led to quenching of the fluorescence, although some derivatives remained luminescent with the appearance of a new red-shifted band in the spectra. Accurate control of the amount of acid enabled white photoluminescence to be obtained both in solution and in solid state.

The emission properties of pyrimidine fluorophores are highly sensitive to the environment. As a consequence, a strong emission solvatochromism, which is typical of ICT chromophores [26][27][28][29], is observed: a polarity increase provides a significant bathochromic shift of the emission band [30][31][32][33]. On the other hand, protonation of the pyrimidine ring (pK a1 = 1.1) significantly enhances its electron-withdrawing character and leads to a red-shift of both absorption and emission bands. These shifts are generally accompanied by emission quenching, but in some cases, particularly with weak electron-donating groups are generally accompanied by emission quenching, but in some cases, particularly with weak electron-donating groups such as methoxy fragments, red-shifted emission with increased intensity is observed [32,34,35]. White photoluminescence can be obtained both in solution and in doped polystyrene films when the neutral and protonated forms are present in the appropriate ratio [34,35].
We report here the synthesis of a series of pyrimidine chromophores with one, two or three triphenylamine/9-ethylcarbazole substituents. The photophysical properties of the new materials were carefully studied and the luminescence behaviour in the presence of acid was evaluated. In some cases, the protonated pyrimidines remained emissive, which enabled white luminescence to be obtained by accurate control of the amount of acid.

Synthesis
Compounds 1 and 2 were obtained in moderate to good yield by Suzuki cross-coupling reaction from the corresponding chloropyrimidines and boronic acids according to a known methodology for similar pyrimidine structures (Scheme 1) [21][22][23][24]. All compounds were identified by 1 H and 13 C-NMR, and all previously unknown molecules were also characterized by high resolution mass spectroscopy (HRMS).

Photophysical Properties in Solution
The UV-Vis and photoluminescence (PL) spectroscopic data for compounds 1 and 2 in dichloromethane are presented in Table 1. The analyses were carried out using low concentration solutions (1.0-2.0 × 10 −5 M). Self-absorption effects were not observed under the conditions employed.
As representative examples, the spectra of compounds 1a-c are shown in Figure 1 (see Figure S1 in the Supporting Information for spectra of compounds 2a-c).

Photophysical Properties in Solution
The UV-Vis and photoluminescence (PL) spectroscopic data for compounds 1 and 2 in dichloromethane are presented in Table 1. The analyses were carried out using low concentration solutions (1.0-2.0 × 10 −5 M). Self-absorption effects were not observed under the conditions employed.
As representative examples, the spectra of compounds 1a-c are shown in Figure 1 (see Figure S1 in the Supporting Information for spectra of compounds 2a-c).  Compounds 1, which contain triphenylamine substituents, exhibited red-shifted absorption and emission bands with respect to their 9-ethylcarbazole analogues 2. This finding indicates that the 9ethyl-9H-carbazol-3-yl fragment is a weaker electron-donating group than triphenylamine. Triphenylamine derivatives 1 also displayed higher fluorescence quantum yields (up to 0.86 for 1b). The 4,6-disubstituted pyrimidines 1b and 2b showed a red-shifted emission and an enhanced fluorescence quantum yield in comparison with their C2-monosubstituted analogues 1a and 2a. In contrast, the addition of a third substituent (2,4,6-triarylpyrimidines) led to a slight blue shift in the emission and a decrease in the quantum yield. A similar phenomenon has previously been observed in tristyrylpyrimidines and was attributed to a decrease in the electron-withdrawing character of the pyrimidine central core due to the C2 electron-donating substituent, which decreases the ICT along the C4 and C6 arms [32].
In an effort to gain further insights into the photophysical properties of these push-pull molecules, in particular to evaluate the ICT upon excitation, the emission behaviour of compounds 1 and 2 was studied in a variety of different aprotic solvents. The data obtained are summarized in Table 2. The position of the longest wavelength absorption maximum was not affected significantly but an increase in the solvent polarity, estimated by the Dimroth-Reichardt polarity parameters [46,47], resulted in a red-shifted emission (see Figure 2 for compound 1b and Figures S2-S6 in the Supporting Information for compounds 1a, 1c, and 2a-c). This bathochromic shift in the emission Compounds 1, which contain triphenylamine substituents, exhibited red-shifted absorption and emission bands with respect to their 9-ethylcarbazole analogues 2. This finding indicates that the 9-ethyl-9H-carbazol-3-yl fragment is a weaker electron-donating group than triphenylamine. Triphenylamine derivatives 1 also displayed higher fluorescence quantum yields (up to 0.86 for 1b). The 4,6-disubstituted pyrimidines 1b and 2b showed a red-shifted emission and an enhanced fluorescence quantum yield in comparison with their C2-monosubstituted analogues 1a and 2a.
In contrast, the addition of a third substituent (2,4,6-triarylpyrimidines) led to a slight blue shift in the emission and a decrease in the quantum yield. A similar phenomenon has previously been observed in tristyrylpyrimidines and was attributed to a decrease in the electron-withdrawing character of the pyrimidine central core due to the C2 electron-donating substituent, which decreases the ICT along the C4 and C6 arms [32].
In an effort to gain further insights into the photophysical properties of these push-pull molecules, in particular to evaluate the ICT upon excitation, the emission behaviour of compounds 1 and 2 was studied in a variety of different aprotic solvents. The data obtained are summarized in Table 2. The position of the longest wavelength absorption maximum was not affected significantly but an increase in the solvent polarity, estimated by the Dimroth-Reichardt polarity parameters [46,47], resulted in a red-shifted emission (see Figure 2 for compound 1b and Figures S2-S6 in the Supporting Information for compounds 1a, 1c, and 2a-c). This bathochromic shift in the emission band is consistent with stabilization of the highly polar emitting excited state by polar solvents. The solvatochromic shift of the emission band can be used to evaluate the ICT upon excitation. For all compounds, the emission maxima were plotted versus the Dimroth-Reichardt polarity parameter, and in all cases good linearity was found (see Figure S7 in the Supporting Information). The slopes of the corresponding regression lines indicate a stronger ICT for triphenylamine derivatives 1 with respect to their 9-ethylcarbazole analogues 2. In the triphenylamine series, the slope increased in the order 1a < 1c < 1b, thus indicating that the strongest ICT was obtained for the 4,6-disubstituted pyrimidine 1b. In a similar way, the slope increased in the order 2a < 2c < 2b for the 9-ethylcarbazole derivatives. band is consistent with stabilization of the highly polar emitting excited state by polar solvents. The solvatochromic shift of the emission band can be used to evaluate the ICT upon excitation. For all compounds, the emission maxima were plotted versus the Dimroth-Reichardt polarity parameter, and in all cases good linearity was found (see Figure S7 in the Supporting Information). The slopes of the corresponding regression lines indicate a stronger ICT for triphenylamine derivatives 1 with respect to their 9-ethylcarbazole analogues 2. In the triphenylamine series, the slope increased in the order 1a < 1c < 1b, thus indicating that the strongest ICT was obtained for the 4,6-disubstituted pyrimidine 1b. In a similar way, the slope increased in the order 2a < 2c < 2b for the 9-ethylcarbazole derivatives.  Photophysical measurements were also performed on a 10 −2 M solution of camphorsulfonic acid (CSA) in dichloromethane. The results are summarized in Table 3. As one would expect, a bathochromic shift of the charge transfer absorption band was observed for all compounds due to the protonation of the pyrimidine ring [45,48]. This was associated with a dramatic quenching of the fluorescence for the triphenylamine derivatives. As a consequence, emission bands could not be identified for compounds 1a and 1c, whereas for 1b a low intensity emission in the red region was detected (λmax = 651 nm, ΦF < 0.01). It is worth noting that the protonated form of 1b was more emissive in chloroform solution (λabs = 493 nm, λem = 625 nm, ΦF = 0.11). In contrast, the 9-ethylcarbazole derivatives 2b and 2c remained fluorescent and they emitted green-yellow light with high emission quantum yields (ΦF = 0.63 and 0.45, respectively). The decay lifetimes (τ) were determined to be 3.6 ns and 4.2 ns (τ values for the neutral molecules were 1.8 ns and 1.7 ns, respectively). Surprisingly, 2a was not emissive in acidified dichloromethane. The effect of protonation was studied in a more detailed way by titration of solutions of compounds 1b, 2b and 2c with CSA. The changes observed in the UV-vis and emission spectra for 2b are illustrated in Figures 3 and 4, respectively (see the Supporting Information for data for compounds 1b and 2c). The absorption spectra showed the Photophysical measurements were also performed on a 10 −2 M solution of camphorsulfonic acid (CSA) in dichloromethane. The results are summarized in Table 3. As one would expect, a bathochromic shift of the charge transfer absorption band was observed for all compounds due to the protonation of the pyrimidine ring [45,48]. This was associated with a dramatic quenching of the fluorescence for the triphenylamine derivatives. As a consequence, emission bands could not be identified for compounds 1a and 1c, whereas for 1b a low intensity emission in the red region was detected (λ max = 651 nm, Φ F < 0.01). It is worth noting that the protonated form of 1b was more emissive in chloroform solution (λ abs = 493 nm, λ em = 625 nm, Φ F = 0.11). In contrast, the 9-ethylcarbazole derivatives 2b and 2c remained fluorescent and they emitted green-yellow light with high emission quantum yields (Φ F = 0.63 and 0.45, respectively). The decay lifetimes (τ) were determined to be 3.6 ns and 4.2 ns (τ values for the neutral molecules were 1.8 ns and 1.7 ns, respectively). Surprisingly, 2a was not emissive in acidified dichloromethane. The effect of protonation was studied in a more detailed way by titration of solutions of compounds 1b, 2b and 2c with CSA. The changes observed in the UV-vis and emission spectra for 2b are illustrated in Figures 3 and 4, respectively (see the Supporting Information for data for compounds 1b and 2c). The absorption spectra showed the progressive disappearance of the charge transfer absorption band of the neutral form, whereas a red-shifted charge transfer absorption band for the protonated form progressively appeared. The presence of an isosbestic point is evident and this is characteristic of an equilibrium between two species (Figure 3). The same trend was observed in the emission spectra: the addition of CSA led to the progressive disappearance of the emission band of the neutral form and this was associated with the enhancement of a new red-shifted band corresponding to the emission of the protonated form with an isoemissive point (Figure 4). progressive disappearance of the charge transfer absorption band of the neutral form, whereas a redshifted charge transfer absorption band for the protonated form progressively appeared. The presence of an isosbestic point is evident and this is characteristic of an equilibrium between two species (Figure 3). The same trend was observed in the emission spectra: the addition of CSA led to the progressive disappearance of the emission band of the neutral form and this was associated with the enhancement of a new red-shifted band corresponding to the emission of the protonated form with an isoemissive point (Figure 4).    progressive disappearance of the charge transfer absorption band of the neutral form, whereas a redshifted charge transfer absorption band for the protonated form progressively appeared. The presence of an isosbestic point is evident and this is characteristic of an equilibrium between two species (Figure 3). The same trend was observed in the emission spectra: the addition of CSA led to the progressive disappearance of the emission band of the neutral form and this was associated with the enhancement of a new red-shifted band corresponding to the emission of the protonated form with an isoemissive point (Figure 4).    The coexistence of both neutral and protonated species with complementary emitting colors in the solution enabled white light emission to be achieved under UV-irradiation. Thus, compound 2c emitted violet light at λ max = 398 nm and this turned to green-yellow at λ max = 552 nm upon protonation. Excitation at 370 nm led to the observation of white light after the addition of 45 equivalents of CSA to a 1.25 × 10 −5 M solution of 2c in dichloromethane ( Figure 5). The same phenomenon was also observed for compounds 1b and 2b (Figures S11 and S12 of the Supporting Information, respectively). For 1b and 2c, the calculated CIE chromaticity coordinates (Table 4) were close to those of pure white light (0.33, 0.33).
Molecules 2018, 23, x 6 of 12 The coexistence of both neutral and protonated species with complementary emitting colors in the solution enabled white light emission to be achieved under UV-irradiation. Thus, compound 2c emitted violet light at λmax = 398 nm and this turned to green-yellow at λmax = 552 nm upon protonation. Excitation at 370 nm led to the observation of white light after the addition of 45 equivalents of CSA to a 1.25 × 10 −5 M solution of 2c in dichloromethane ( Figure 5). The same phenomenon was also observed for compounds 1b and 2b (Figures S11 and S12 of the Supporting Information, respectively). For 1b and 2c, the calculated CIE chromaticity coordinates (Table 4) were close to those of pure white light (0.33, 0.33).

Photophysical Properties in Solid State
Filter paper pieces covered with 2b and 2c in the absence and presence of different amounts of CSA were prepared by immersion of the filter paper into a dichloromethane solution of the appropriate compound (1 wt% doped on polystyrene). The samples were dried in air. The fluorescence spectra of the samples were acquired and emission maxima in the violet region at λmax = 405 nm and 406 nm, respectively, were determined in the absence of acid. The intensities of these bands gradually decreased on increasing the amount of CSA, whereas a distinctly novel enhanced emission appeared in the green-yellow region (λmax = 544 nm for 2b and λmax = 554 nm for 2c). After careful tuning of the number of equivalents of CSA, white light was observed due to the simultaneous emission from both the neutral and protonated compounds. These significant emission changes were easily followed by the naked eye under UV irradiation (Figures 7 and S13 in the Supporting Information). Energy transfer between neutral and protonated molecules has been suggested for related systems [49].

Photophysical Properties in Solid State
Filter paper pieces covered with 2b and 2c in the absence and presence of different amounts of CSA were prepared by immersion of the filter paper into a dichloromethane solution of the appropriate compound (1 wt% doped on polystyrene). The samples were dried in air. The fluorescence spectra of the samples were acquired and emission maxima in the violet region at λ max = 405 nm and 406 nm, respectively, were determined in the absence of acid. The intensities of these bands gradually decreased on increasing the amount of CSA, whereas a distinctly novel enhanced emission appeared in the green-yellow region (λ max = 544 nm for 2b and λ max = 554 nm for 2c). After careful tuning of the number of equivalents of CSA, white light was observed due to the simultaneous emission from both the neutral and protonated compounds. These significant emission changes were easily followed by the naked eye under UV irradiation ( Figure 6 and Figure S13 in the Supporting Information). Energy transfer between neutral and protonated molecules has been suggested for related systems [49].

General Information
All solvents were reagent grade for synthesis and spectroscopic grade for photophysical measurements. The starting materials were purchased from Sigma-Aldrich (St Louis, MO, USA) or Alfa Aesar (Haverhill, MA, USA) and were used without further purification. For air-and moisturesensitive reactions, all glassware was flame-dried and cooled under nitrogen. NMR spectra were recorded in CDCl3 on a Bruker Avance 300 spectrometer ( 1 H at 300 MHz and 13 C at 75 MHz, Billerica, Ma, USA). The chemical shifts δ are reported in ppm and are referenced to the residual protons of the deuterated solvent or carbon nuclei ( 1 H, δ = 7.27 ppm; 13 C, δ = 77.0 ppm). The coupling constants J are given in Hz. In the 1 H-NMR spectra, the following abbreviations are used to describe the peak patterns: s (singlet), d (doublet), dd (doublet of doublets), t (triplet), m (multiplet). In the 13 C-NMR spectra, the nature of the carbons (C, CH, CH2 or CH3) was determined by performing a JMOD experiment. Melting points (°C) were measured on a Kofler hot-stage with a precision of 2 degrees (±2 °C). High-resolution mass analyses were performed at the 'Centre Régional de Mesures Physiques de l'Ouest' (CRMPO, Université de Rennes 1, Rennes, France) using a Bruker MicroTOF-Q II apparatus. Analytical thin layer chromatography (TLC) was performed on 60 F254 silica gel plates (Merck, Darmstadt, Germany), and compounds were visualized by irradiation with UV light at 254 and 365 nm. Flash chromatography was performed using silica SI 60 (60-200 mesh ASTM, Acros, Waltham, MA, USA). UV-visible and fluorescence spectroscopy studies were conducted on a Spex Fluoromax-3 spectrophotometer (Jobin-Yvon Horiba, Kyoto, Japan). Compounds were excited at their absorption maxima (band of lowest energy) to record the emission spectra. All solutions were measured with optical densities below 0.1. Fluorescence quantum yields (±10%) were determined relative to 9,10-bis(phenylethynyl)anthracene in cyclohexane (ΦF = 1.00). Stokes shifts were calculated considering the lowest energetic absorption band.

General Procedure for Suzuki Cross-coupling Reactions
A stirred mixture of the chloropyrimidine derivative (1 mmol), arylboronic acid (1.2 mmol per chlorine atom), Pd(PPh3)4 (0.05 mmol per chlorine atom), 1 M aqueous sodium carbonate (1.2 mmol, 1.2 mL per chlorine atom), and ethanol (1.5 mL) in degassed toluene (15 mL) was heated at reflux under nitrogen for 15 h in a Schlenk tube. The reaction mixture was cooled, filtered, and dissolved

General Information
All solvents were reagent grade for synthesis and spectroscopic grade for photophysical measurements. The starting materials were purchased from Sigma-Aldrich (St Louis, MO, USA) or Alfa Aesar (Haverhill, MA, USA) and were used without further purification. For air-and moisture-sensitive reactions, all glassware was flame-dried and cooled under nitrogen. NMR spectra were recorded in CDCl 3 on a Bruker Avance 300 spectrometer ( 1 H at 300 MHz and 13 C at 75 MHz, Billerica, MA, USA). The chemical shifts δ are reported in ppm and are referenced to the residual protons of the deuterated solvent or carbon nuclei ( 1 H, δ = 7.27 ppm; 13 C, δ = 77.0 ppm). The coupling constants J are given in Hz. In the 1 H-NMR spectra, the following abbreviations are used to describe the peak patterns: s (singlet), d (doublet), dd (doublet of doublets), t (triplet), m (multiplet). In the 13 C-NMR spectra, the nature of the carbons (C, CH, CH 2 or CH 3 ) was determined by performing a JMOD experiment. Melting points ( • C) were measured on a Kofler hot-stage with a precision of 2 degrees (±2 • C). High-resolution mass analyses were performed at the 'Centre Régional de Mesures Physiques de l'Ouest' (CRMPO, Université de Rennes 1, Rennes, France) using a Bruker MicroTOF-Q II apparatus. Analytical thin layer chromatography (TLC) was performed on 60 F254 silica gel plates (Merck, Darmstadt, Germany), and compounds were visualized by irradiation with UV light at 254 and 365 nm. Flash chromatography was performed using silica SI 60 (60-200 mesh ASTM, Acros, Waltham, MA, USA). UV-visible and fluorescence spectroscopy studies were conducted on a Spex Fluoromax-3 spectrophotometer (Jobin-Yvon Horiba, Kyoto, Japan). Compounds were excited at their absorption maxima (band of lowest energy) to record the emission spectra. All solutions were measured with optical densities below 0.1. Fluorescence quantum yields (±10%) were determined relative to 9,10-bis(phenylethynyl)anthracene in cyclohexane (Φ F = 1.00). Stokes shifts were calculated considering the lowest energetic absorption band.

General Procedure for Suzuki Cross-coupling Reactions
A stirred mixture of the chloropyrimidine derivative (1 mmol), arylboronic acid (1.2 mmol per chlorine atom), Pd(PPh 3 ) 4 (0.05 mmol per chlorine atom), 1 M aqueous sodium carbonate (1.2 mmol, 1.2 mL per chlorine atom), and ethanol (1.5 mL) in degassed toluene (15 mL) was heated at reflux under nitrogen for 15 h in a Schlenk tube. The reaction mixture was cooled, filtered, and dissolved with a mixture of AcOEt and water 1:1 (50 mL) and the organic layer was separated. The aqueous layer was extracted with AcOEt (2 × 25 mL). The combined organic extracts were dried with MgSO 4 and the solvents were evaporated. emission band and the appearance of a new red-shifted complementary emitting band. Thus, white photoluminescence could be obtained by controlled protonation. White emission was also achieved in solid state.

Supplementary Materials:
The following material is available online: Figure S1: absorption and emission spectra of compounds 2a-c in dichloromethane solution; Figures S2-S6: emission spectra of 1a, 1c, and 2a-c in different aprotic solvents; Figure S7: emission maxima as a function of the Dimroth-Reichardt polarity parameter E T (30) for compounds 1 and 2; Figures S8-S10: changes in the absorption and emission spectra of a solution of 1b and 2b upon addition of CSA; Figures S11 and S12: changes in the color of a solution of 1b and 2b after the addition of CSA; Figure S13: fluorescence spectra and changes in the color of 2b in solid state after the addition of CSA; Figures S14-S23: 1 H, 13