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Photo-substitution reactions of perylene red dyes
Journal of Luminescence 218 (2020) 116845
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Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Photo-substitution reactions of perylene red dyes Johan Lub a, b, *, Paul A. van Hal a, c, Erica M.G. Custers a, Hugo H. Knobel d, Rifat A.M. Hikmet a, e a
Philips Research, High Tech Campus 34, AE Eindhoven, 5656, the Netherlands Eindhoven University of Technology, P.O. Box 513, MB, 5600, Eindhoven, the Netherlands c ASML, Le Havre 78, SV Eindhoven, 5627, the Netherlands d Eurofins Materials Science, High Tech Campus 11, AE Eindhoven, 5656, the Netherlands e Signify, High Tech Campus 7, AE Eindhoven, 5656, the Netherlands b
A R T I C L E I N F O
A B S T R A C T
Keywords: Lumogen red 305 Photo stability Photo substitution Photo-oxidation Fluorescent compounds
Irradiation of Lumogen Red 305 (3) in ethyl acetate with intense blue LED light led to very slow bleaching, showing a high photochemical stability. However, irradiation in ethanol led to the formation of a low yield product in which one of the phenoxy groups is substituted by an acetoxy group, the oxidized form of an ethoxy group. This reaction product was only formed in the presence of blue light and oxygen. Adding a small amount of acetic acid to the solutions resulted in a much higher yield of the product, indicating a possible photo oxidation of ethanol to acetic acid during the process in this solvent. A similar dye (5) containing less phenoxy side groups than Lumogen Red 305 showed the same reaction although the product formed showed lower chemical stability. The photo-substitution of phenoxy groups by acetoxy groups is a new reaction. The results are used to discuss the structural effects on the stability of these type of fluorescent dyes under irradiation conditions, potential reaction mechanisms and the possibility of using the photochemical transformations for preparative purposes.
1. Introduction Derivatives of perylene can be used as color converting dyes in LED applications [1,2]. The original blue light of LED chips is converted to white light with the aid of a combination of yellow and red emitting dyes such as compound 1 (Lumogen Orange 240, see structure in Table 1) and compound 3 (Lumogen Red 305, see structure in Table 1), respectively. To avoid excessive heating of the dyes, which can led to thermal degradation and quenching, a remote phosphor configuration is preferred over directly applying the dyes on top of the LED chip [3]. Although both dye compounds exhibit high quantum efficiencies, compound 1 lacks photochemical stability, resulting in rapid bleaching. Other yellow dyes derived from perylene that are commercially avail able also show insufficient photochemical stability [2,4]. Therefore, a new class of yellow emitting dyes have been developed with increased lifetime and high quantum efficiency for application in LED lamps, especially in combination with the remote phosphor technology [2,4–6]. Compound 3 and its derivatives show sufficient photochemical stability to be applied as red phosphors in LED lamps [7]. In our previous publication [8], we studied the photochemical con version of compound 1 in solvents by irradiation with intense blue LED light. In ethyl acetate, a relatively inert solvent, bleaching of the
yellow/orange compound was observed. We were not able to detect degradation products by means of NMR or chromatographic techniques. In ethanolic solution, the compound turned red under irradiation con ditions that included the presence of oxygen. A moderate yield of de rivative 2 (see Table 1) was detected. One of the hydrogen atoms of the perylene moiety of 1 was replaced by an ethoxy group (R3 in Table 1). This shows the relative reactivity of this part of perylene derivatives under aerobic irradiation conditions. The role of oxygen was apparent from the fact that irradiation under anaerobic conditions, product 2 was not formed. Here, we focus on compound 3. Although this red compound is stable enough for regular application in LED lamps, it still lacks the stability necessary for high intensities and long lifetime. The photochemical stability in poly(methyl methacrylate) has been the subject of several studies [9–11], although none of these studies analyzed the degradation products derived from 3. This is mainly due to the fact that separation of degradation products from a polymeric matrix is difficult. However, a study in solvents also did not reveal degradation products from 3 [12]. The role of these perylene derivatives as photo catalysts is well docu mented [13,14]. Normally the assumption is oxidation or dehalogena tion reaction of the perylene that leads to subsequent oxidation of the solvent; however, the intermediate perylene degradation products have
* Corresponding author. Philips Research, High Tech Campus 34, AE Eindhoven, 5656, the Netherlands. E-mail address: [email protected] (J. Lub). https://doi.org/10.1016/j.jlumin.2019.116845 Received 24 July 2019; Received in revised form 26 September 2019; Accepted 26 October 2019 Available online 31 October 2019 0022-2313/© 2019 Elsevier B.V. All rights reserved.
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Journal of Luminescence 218 (2020) 116845
not been reported. Compared to compound 1, compound 3 lacks hydrogen atoms at R1, 2 R , R3 and R4: these are substituted by phenoxy groups that are responsible for the absorption red shift and possibly the higher photo chemical stability. To get more insight in the possibly different re activities of hydrogen substituted compound 1 and phenoxy substituted compound 3, we introduce compound 5 that contains both hydrogen and phenoxy groups in the central area of the perylene moiety and can therefore present a combination of the reactivities of compounds 1 and 3. In this publication, an attempt is made to characterize photo-reaction products derived from 3 and 5 after irradiation in solution.
performed to separate the reaction products were conducted at higher concentration in a vessel with a diameter of 44 mm (see section 2.3). Absorption and emission spectra were measured in ethyl acetate on a PerkinElmer Lambda 950 UV/Vis spectrometer and a PerkinElmer LS55 fluorescence spectrometer, respectively, under the same conditions as described in Ref. [8]. 1H, 13C and 2D 1H–13C gradient-enhanced HMBC NMR spectra were measured on samples dissolved in deuterated chlo roform using Bruker Avance 300 MHz and 600 MHz NMR spectrometers [8]. LC-MS experiments were performed on an Agilent 1200 series Liquid Chromatography system fitted with a, 2.1 mm ID x 100 mm l x 3.5 μm dp Eclipse plus C18 column (Agilent). The system was equipped with optical diode array detection and mass detection was done using Electrospray Ionization and a TOF-analyzer. Preparative column chro matography was performed on a Combiflash Companion apparatus (Teledyne Isco) employing pre-packed silica cartridges from Grace (USA). The Maldi-TOF mass spectra were recorded on an ABSciex 4800 Maldi-TOF/TOF Analyzer using 2,5-dihydroxybenzoic acid as matrix.
2. Experimental procedure 2.1. Materials 2,9-bis(2,6-diisopropylphenyl)-5,6,12,13-tetraphenoxyanthra [2,1,9-def:6,5,10-d’e’f’]diisoquinoline-1,3,8,10(2H, 9H)-tetraone (3, commercial number: F305) was a kind gift from Dr. M. Koenemann of BASF. 2,9-bis(2,6-diisopropylphenyl)-5,12-diphenoxyanthra[2,1,9def:6,5,10-d’e’f’]diisoquinoline-1,3,8,10(2H, 9H)-tetraone (5) was pre pared by Dr. S. Hamon (Syncom b.v. Groningen, The Netherlands) ac cording to a literature procedure [15]. All other chemicals and solvents were obtained from Sigma-Aldrich.
2.3. 2,9-bis(2,6-diisopropylphenyl)-1,3,8,10-tetraoxo-6,12,13triphenoxy-1,2,3,8,9,10-hexahydroanthra[2,1,9-def:6,5,10-d’e’f’] diisoquinolin-5-yl acetate (4) A solution of 10 mg of 2,9-bis(2,6-diisopropylphenyl)-5,6,12,13-tet raphenoxyanthra[2,1,9-def:6,5,10-d’e’f’]diisoquinoline-1,3,8,10(2H, 9H)-tetraone (3) in 25 ml of a mixture of acetic acid/ethyl acetate 2:98 (v/v) was irradiated in a tube of 44 mm diameter for 8 h. The solution was subsequently extracted with 15 ml of a 0.8 M aqueous sodium hydrogen carbonate solution and 15 ml of brine. After drying over magnesium sulfate and evaporation, 2.0 mg the product 4 (21% yield) was obtained after column chromatography (SiO2, CH2Cl2). 1H NMR (300 MHz, δ in ppm, J in Hz): 8.50 (s, 1H), 8.37 (s, 1H), 8.32 (s, 1H), 8.27 (s, 1H), 7.46 (t, J ¼ 8.0, 2H), 7.25–7.35 (m, 10H), 7.08–714 (m,
2.2. Equipment For irradiation, an LED lighting device generating 450 nm light with an intensity of 4.1 W/cm2 at 12 mm distance of the irradiation tubes was used [8]. The irradiation experiments in the presence of air and anaer obic were performed as described in earlier work [8]. The experiments Table 1 Structures and mass of the perylene derivatives discussed in this publication.
Compound number
1 2 3
R 1 ¼ R4
R2
R3
M þ H (calculated)
H H
H H
H CH3CH2O
a
4
a
1079.4
1045.4
5
H
895.4
6
H
861.4
7
H
HO
a: see ref. 8. 2
819.4
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Journal of Luminescence 218 (2020) 116845
3H), 6.93–6.97 (m, 6H), 2.71 (m, J ¼ 6.2, 4H), 2.49 (s, 3H), 1.14 (d, J ¼ 6.2, 12H), 1.13 (d, J ¼ 6.2, 12H). 13C NMR (75 MHz, δ in ppm, * ¼ CH, # ¼ CH3) δ 168.81, 163.10, 156.76, 156.69*, 155.95, 155.04, 148.62, 145.70, 130.22*, 130.13*, 129.68*, 125.97*, 124.92*, 124.07*, 123.10, 122.45*, 121.88, 121.25*, 120.71*, 120.46, 120.02, 119.94*, 118.53*, 116.35, 29.17*, 24.07#, 24.04#, 21.80#. MALDI: M þ H ¼ 1045.48. UV–Vis: λmax (ethyl acetate) ¼ 556 nm, ε ¼ 46500 L mol 1 cm 1 and 521 nm ε ¼ 31000 L mol 1 cm 1. Photo luminescence: λmax (ethyl acetate) ¼ 586 nm.
that the incorporation of ethanol is accompanied by an oxidation reac tion on the perylene derivative or that ethanol is photo-oxidized to acetic acid before incorporation. The fact that no products were detected under anaerobic conditions point to the hypothesis that ethanol is oxidized to acetic acid before reaction with 3, which acts as a photo catalyst. Oxidations of solvents like ethanol by compound 3 are known under these irradiation conditions [13] that may explain the formation of acetic acid from ethanol in the presence of oxygen. One of the possible structures that can be formed is product 4 (see Table 1) in which one of the phenoxy groups is substituted for an acetoxy group. A summary of these reactions is outlined in Fig. 1. The proposed reaction sequence of Fig. 1 implies that a photo chemical reaction with acetic acid could also lead to possible structure 4. To test this possibility, the irradiation experiment was performed with 2% acetic acid added to the ethanolic and ethyl acetate solutions. The irradiations resulted in much larger spectral changes than without the acetic acid, as shown in Fig. 2. LC-MS results showed that the same compound is formed as in ethanol, but in a much higher yield. The result found in a mixture of acetic acid and ethyl acetate shows that ethanol is not necessary for these reactions. Irradiation under anaerobic conditions gave the same results, so oxygen is not required for this reaction as proposed in Fig. 1. Although the same product is formed in acetic acid as in ethanol, which points to a mechanisms as outlined in Fig. 1, it is also possible that both processes run differently. Thus, it is still possible that the reaction in ethanol initially leads to an oxidized form of compound 3 that after reaction with ethanol forms derivative 4. This could be the subject of further study. From the spectrum of Fig. 2 it can be concluded that during the irradiation in the presence of acetic acid a new red compound is formed with a blue shifted absorption compared to that of initial compound 3. In order to determine the structure of the compound formed, the reaction was performed with a larger quantity of 3 in a more concentrated so lution. Under these conditions, LC-MS results revealed that the conver sion was lower than in the initial diluted solutions, but enough material was generated to separate the reaction product by column chromatog raphy. NMR data of the separated product indeed shows the loss of one of the phenoxy side groups and the incorporation of an acetoxy group with typical signals in 1H NMR at 2.58 ppm and in 13C NMR at 21.8 ppm due to the CH3 group and in 13C NMR at 168.8 ppm due to the acetoxy carbonyl atom. The original singlet due to H1, H2, H3 and H4 (see Table 1) observed in the 1H NMR spectrum of compound 3 is now split into 4 singlets (see Fig. 3) as a result of the incorporation of the acetoxy group. Fig. 3 also shows the NMR 2D C–H 3 bands coupling spectrum of the product. The figure shows that these four singlets couple via three bands with the carbonyl carbon atoms of the imide groups observed at 163 ppm in the 13C NMR spectrum. This means that all four hydrogens H1–H4 are still in place and the acetoxy group has indeed replaced the phenoxy group at R3 (see Table 1). Thus, photo substitution of one of the phenoxy groups by an acetoxy group had occurred in compound 3 to form compound 4. Further investigation of the LC-MS results did not suggest products in which a second phenoxy group was replaced by an acetoxy group. This can be explained by a change in reactivity of product 4 compared to compound 3. However, a more likely explanation is that the acetoxy bond is more reactive than the original phenoxy bonds since the acetate anion or acetic acid is a better leaving group than the phenolate anion or phenol. This would mean that upon photo-substitution on product 4 by acetic acid, the acetoxy group is replaced, which is effectively no reac tion. The initial replacement of phenoxy by acetoxy is then explained by the fact that during the reaction acetic acid is in large excess to com pound 3 and thus also in large excess of the leaving phenol group. The conversion of 3 is rather clean, i.e. without or only with small amounts of by-products, making it a possibly interesting synthetic pro cess for generating “asymmetric substituted” perylene derivatives. However, the long irradiation times suggest relatively low quantum yields which is a drawback for such a process. The synthetic options can
2.4. 2,9-bis(2,6-diisopropylphenyl)-1,3,8,10-tetraoxo-12-phenoxy1,2,3,8,9,10-hexahydroanthra[2,1,9-def:6,5,10-d’e’f’]diisoquinolin-5-yl acetate (6) This irradiation experiment was performed in the same way as described in section 2.3 starting from 2,9-bis(2,6-diisopropylphenyl)5,12-diphenoxyanthra[2,1,9-def:6,5,10-d’e’f’]diisoquinoline-1,3,8,10 (2H, 9H)-tetraone (5). 4.8 mg of product 6 (50% yield) was obtained. 1H NMR (300 MHz, δ in ppm, J in Hz): 9.60 (d, J ¼ 8.2, 1H), 9.21 (d, J ¼ 8.2, 1H), 8.74 (d, J ¼ 8.2, 1 h), 8.72 (d, J ¼ 8.2, 1H), 8.52 (s, 1H), 8.41 (s, 1H), 7.43–7.51 (m, 5H), 7.35 (d, J ¼ 7.7, 2H), 7.33 (d, J ¼ 7.7, 2H), 7.16 (d, J ¼ 8.2, 2H), 2.73 (m, J ¼ 6.2, 4H), 2.58 (s, 3H), 1.18 (d, J ¼ 6.2, 12H), 1.16 (d, J ¼ 6.2, 6H), 1.15 (d, J ¼ 6.2, 6H). 13C NMR (75 MHz, δ in ppm, * ¼ CH, # ¼ CH3) δ 168.67, 163.40, 163.29, 162.82, 162.65, 155.49, 154.93, 147.88, 145.61, 145.57, 133.85, 133.11, 131.92*, 130.63*, 130.27, 130.23, 130.18*, 129.77*, 129.56, 129.31, 129.07*, 128.47*, 127.72*, 127.55, 127.23, 126.04, 125.33*, 125.18*, 124.32, 124.25, 124.15*, 124.10*, 123.86, 122.99, 122.38, 119.19*, 28.12*, 24.03#, 24.00#, 21.78#. MALDI: M þ H ¼ 861.34. UV–Vis: λmax (ethyl acetate) ¼ 519 nm, ε ¼ 45400 L mol 1 cm 1 and 490 nm ε ¼ 30700 L mol 1 cm 1. Photoluminescence: λmax (ethyl acetate) ¼ 552 nm. 3. Results and discussion 3.1. Irradiation of compound (3) A solution of 3 in ethyl acetate was irradiated at 70 � C for 8 h. A few percent of bleaching of the dye was observed by UV–vis spectroscopy. LC-MS analyses of the irradiated sample did not show any new com pounds that absorb in the visible region or show separate peaks with well-defined mass in the UV region, so no information was obtained of products possibly formed during irradiation. The quantity of degrada tion products was probably too low, too unstable, and/or didn’t contribute to the absorption spectrum. After irradiation of 3 in ethanol or n-propanol, effects similar to those in ethyl acetate, namely a few percent of bleaching was observed. However, in these solvents a small amount of a new compound absorbing in the red region was detected by LC-MS measurements. Irradiation in ethanol under vacuum (instead of air) resulted in a small bleaching effect, similar to that in ethyl acetate, but no products were detected by LC-MS. An ethanolic solution of compound 3 kept at 70 � C for 8 h did not show any change in the absorbance spectra or in LC-MS traces when the sample was not illuminated. Therefore, it is concluded that for formation of the product in ethanol, both oxygen and light are simultaneously required. Irradiation of 3 in n-propanol resulted in a compound with a mass 14 amu higher than the product formed in ethanol, as shown by LC-MS experiments: from this we conclude that the alcohols are incorporated in the perylene derivative during these reactions. The mass of the product detected by LC-MS from the reaction in ethanol is 1045.4 (M þ H), which is lower than that of the starting compound 3 namely: 1079.4 (M þ H). This means that a fraction of the original structure of 3 is split off. Stoichiometrically this could mean that one of the four phenoxy groups is replaced by an acetoxy group (or a propionoxy group in the case of irradiation in propanol). This means 3
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Journal of Luminescence 218 (2020) 116845
Fig. 1. Proposed process for the formation of 4 from 3 and ethanol under aerobic irradiation conditions.
Fig. 2. UV–Vis spectrum of 3 in acetic acid/ethyl acetate 2:98 v/v before (red curve) and after 8 h of irradiation (blue curve).
Fig. 4. Photoluminescence spectra of 3 (red) and 4 (blue) in ethyl acetate. Dotted lines represent excitation spectra and uninterrupted solid lines represent emission spectra.
while in the latter case a phenoxy group was replaced by an acetoxy group which stoichiometrically is an oxidized ethoxy group. In order to obtain more insight into these reactions and reactivities of the perylene derivatives, it was decided to investigate the photochemical behavior in ethanol of perylene derivative 5, which contains two hydrogen atoms R1 ¼ H and R4 ¼ H, similar to unsubstituted perylene derivative 1, and two phenoxy groups R2 and R3, similar to tetra-substituted derivative 3. The replacement of one of these hydrogens by an ethoxy group and/or replacement of one of the phenoxy groups by an acetoxy group may occur. Solutions of 5 in ethyl acetate, ethanol or n-propanol were irradiated at 70 � C for 8 h. In all cases, considerably more bleaching of the dye was observed than with the tetra substituted compound 3. This points to the unique high stability of 3 towards blue light already observed by com parison of 3 to other perylene derivatives [4,8]. Even though the bleaching in 5 was much higher than in 3, no defined compounds could be observed with the LC-MS method. This was also observed after irra diation of compound 1 in ethyl acetate as reported previously [8]. LC-MS revealed the formation of a small amount of a product after irradiation in ethanol. Irradiation in ethanol under vacuum instead of air resulted in bleaching, similar to that in ethyl acetate, but under these anaerobic condition no products were detected by LC-MS. Thus oxygen is needed for the formation of the detected product. Furthermore, a nonilluminated ethanolic solution of compound 5 kept at 70 � C for 8 h did not show any change in the spectra or in LC-MS traces. Thus, for the formation of the product in ethanol, both oxygen and light are needed, just as was the case with compound 3. The mass detected of the product is 819.4 (M þ H). If one the hydrogen atoms, R1 or R4, had been substituted by an ethoxy group as in the case of formation of compound 2 from 1, a mass of 939.4 (M þ H) would be expected. If the phenoxy group R3 had been replaced by an acetoxy group as in the case of for mation of compound 4 from 3, a mass of 861.4 (M þ H) would be expected. The transformation in n-propanol resulted in the same compound obtained in ethanol with a mass of 819.4 (M þ H). The absence of a mass difference of 14 in ethanol and propanol points to the conclusion that the alcohols are not incorporated in the detected product. To find out whether a reaction of 5 with acetic acid occurs similar to
Fig. 3. Part of the 2D 1H–13C HMBC spectrum in deuterated chloroform of reaction product 4 formed out of 3 by irradiation in an acetic acid-ethyl ace tate mixture.
be the subject of future studies. Fig. 4 shows the photoluminescence spectra of the starting com pound 3 and product 4 in ethyl acetate. The excitation spectra exhibits the same shape as the absorption spectra in ethyl acetate. The blue shift previously observed in the UV–vis spectrum during the irradiation (see Fig. 2) is also apparent when comparing the excitation and the emission spectra of compounds 3 and 4. Thus, the replacement of a phenoxy group by an acetoxy group in these perylene derivatives leads to a blue shift in absorption and emission. 3.2. Irradiation of compound (5) The photo-substitution reactions of ethanol on unsubstituted per ylene derivative 1 [8] and on tetra-substituted derivative 3 occurred only with simultaneous oxidation by oxygen. However, the type of re actions and products differ. In the first case, the hydrogen atom H ¼ R3 was replaced by an ethoxy group in an oxidative coupling reaction, 4
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Journal of Luminescence 218 (2020) 116845
the reaction with compound 3, the irradiation experiment was per formed with 2% acetic acid in ethyl acetate. Irradiation resulted in a blue shift of the absorption spectrum, shown in Fig. 5. LC-MS results revealed that the same compound was detected as in ethanol, but in much higher yield. In order to determine the structure of the photoproduct, a more concentrated solution was irradiated and separation was performed by column chromatography. NMR data of the separated product showed the loss of one of the phenoxy groups and the incor poration of an acetoxy group with typical signals in 1H NMR at 2.58 ppm and in 13C NMR at 21.8 ppm due to the CH3 group and in 13C NMR at 168.7 ppm due to the acetoxy carbonyl atom. These spectral properties are very similar to those of acetoxy derivative 4. Furthermore, the mass of the pure product measured by MALDI-TOF is 861.4 (M þ H), which points to a product where one of the phenoxy groups had indeed been substituted by an acetoxy group; for example compound 6. This would mean that the conversion of 5 probably occurs similarly to the conver sion of 3 with acetic acid. Fig. 6 shows the NMR 2D C–H 3 bands coupling spectrum of the product. The 4 carbonyl signals of the imide group observed around 163 ppm in the 13C NMR spectrum couple with 4 hydrogen atoms in the 1H NMR spectrum. These signals can be assigned to atoms H1, H2, H3 and H4. Thus, these four atoms were not replaced by an acetoxy group in the reaction. With the knowledge obtained of the conversion of 3 to 4, it is obvious that one of the phenoxy groups of 5 is substituted by an acetoxy group. Thus, 6 is obtained from compound 5. The type of reaction of compound 5 in ethanol resembles the type of reaction with compound 3 and not with compound 1. No other reaction products could be detected by LC-MS. The reactivity in ethanol of the carbon atom that contains the phenoxy group (R3) of 5 is much higher than with the hydrogen atoms at R1 or R4. This is probably also the case for compound 6. As explained before for compound 4, the acetoxy group is more reactive than the phenoxy group. This means in the case of compound 6 that substitution of the acetoxy group by acetoxy itself is also preferred over substitution of the remaining phenoxy group of 6. When compound 6 was subjected to an LC-MS measurement, only the peak with mass 819.4 (M þ H) was observed. This peak can be explained by the formation of hydrolysis product 7 (see Fig. 7). This is probably due to the acidic-aqueous conditions used in the LC-MS system. It explains the detection of 7 by the LC-MS analysis of the reaction product after irradiation of 5 in ethanol and propanol. Such a hydrolysis product was not detected in compound 4. The difference in hydrolytic reactivity between compound 4 and 6 can be explained by the acetoxy group (R3) in 4 being strongly sterically impeded by the neighboring phenoxy group (R1), while this group is absent in this position in compound 6. The effect of steric impedance will probably also play a role in the good (photo-)chemical stability of
Fig. 6. Part of the 2D 1H–13C HMBC spectrum in deuterated chloroform of reaction product 6 formed out of 5 by irradiation in an acetic acid-ethyl ace tate mixture.
the highly substituted red compound 3 compared to other perylene derivatives such as 1 and 5. If the conversion of 5 to 6 in ethanol indeed occurs according to Fig. 7, it is remarkable that only acetic acid reacts, even though a large excess of ethanol is present that could potentially also react (as observed during the photochemical reaction with 1 [8]). This may be explained by protonation of the photo-excited perylene derivative by acetic acid, leaving a rather nucleophilic acetate anion for substitution. The synthetic options of the photo substitution reaction with acetic acid may also be of interest. Apart from the fact that the (near) absence of by-products leads to a rather clean process to form an “asymmetric substituted” perylene derivative, the relative ease of hydrolysis to compound 7 opens the way to make more complex structures using the reactivity of the hydroxyl group (R3). Fig. 8 shows the photoluminescence spectra of the starting com pound 5 and product 6 in ethyl acetate. The excitation spectra exhibits the same shape as the absorption spectra. The blue shift observed in the UV–vis spectrum during the irradiation (see Fig. 5) is also apparent from both the excitation and the emission spectra of the pure compounds 5 and 6. A blue shift was also observed when one of the phenoxy groups of compound 3 was replaced by an acetoxy group to form compound 4. Thus, the replacement of a phenoxy group by an acetoxy group in these type of perylene derivatives leads to a blue shift in absorption and emission. A possible explanation for this effect may be the more electron withdrawing properties of the acetoxy group compared to the phenoxy group, resulting in a lower energy ground state (HOMO) while this effect is less pronounced in the excited state (LUMO). 4. Conclusions The photochemical conversion of the perylene derivatives 1, 3 and 5 in ethyl acetate leading to bleaching is not yet understood. Analyses of the mixtures after irradiation did not reveal products in which the per ylene chromophore had reacted forming bleached products. The various products obtained by irradiation in ethanol of these derivatives revealed that at least one of the sites at R1, R2, R3 and R4 are reactive. The photo-substitution reactions of 3 and 5 with acetic acid lead to the same product as the reactions in ethanol, albeit in low yield, sug gesting a mechanism in which ethanol is oxidized to small amounts of acetic acid during the irradiation in the presence of oxygen. The fact that only acetate groups react while a large excess of ethanol is present may be explained by protonation via acetic acid of the excited state of the perylene derivative leaving a rather nucleophilic acetate anion for substitution. However, alternative mechanisms are not excluded.
Fig. 5. UV–Vis spectrum of 5 in acetic acid/ethyl acetate 2:98 v/v before irradiation (red curve) and after 8 h of irradiation (blue curve). 5
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Journal of Luminescence 218 (2020) 116845
Fig. 7. Proposed process for the formation of 6 from 5 and ethanol under aerobic irradiation conditions, and 7 from 6 under acidic LC-MS conditions.
of the highly substituted red compound 3 compared to other less substituted perylene derivatives such as 1 and 5. Declaration of competing interestCOI The authors declare that there is no conflict of interest. Acknowledgments Branco Kusters and Dani€elle Beelen (both Philips) are thanked for their assistance with the experiments and analyses. Dr. M.G. Debye (Eindhoven University of Technology) is kindly acknowledged for careful reading of the manuscript and suggestions to improve its readability. References [1] P. Acuna, S. Leyre, J. Audenaert, Y. Meuret, G. Deconinck, P. Hanselaer, Opt. Express 22 (2014) A1079. [2] J. Lub, S.L.J. Hamon, P.A. van Hal, T.J. Visser, J.M. Jansen, R.A.M. Hikmet, Dyes Pigments 149 (2018) 662. [3] R.A.M. Hikmet, J.C. Kriege, J.F.M. Cillessen, R.T. Wegh, P.J.C. van der Wel and R. M. A Driessens, WO 2012001645 (2012). [4] J. Lub, R.A.M. Hikmet and D. Veldman, US 20160017219 (2016). [5] J. Lub, R.A.M. Hikmet and D. Veldman, WO 2015062916 (2015). [6] J. Lub, P.A. van Hal and R.A.M. Hikmet, US 2016062657 (2016). [7] J. Lub, , R.A.M. Hikmet and T. van Bommel, US 20150372240 (2015). [8] J. Lub, P.A. van Hal, R. Smits, L. Malassenet, J. Pikkemaat, R.A.M. Hikmet, J. Lumin. 207 (2019) 585. [9] L. Luis Cerd� an, A. Costela, G. Dur� an-Sampedro, I. García-Moreno, M. Calle, M. Juan-y-Seva, J. de Abajob, G.A. Turnbull, J. Mater. Chem. 22 (2012) 8938. [10] G. Griffini, L. Brambilla, M. Levi, M. Del Zoppo, S. Turri, Sol. Energy Mater. Sol. Cells 111 (2013) 41. [11] N. Tanaka, Barashkov, N.J. Heath, W.N. Sisk, Appl. Opt. 45 (2006) 3846. [12] A.A. Earp, T. Rawling, Jim B. Franklin, G.B. Smith, Dyes Pigments 84 (2010) 59. [13] H.-X. Gong, Z. Cao, M.-H. Li, S.-. u Liao, M.-J. Lin, Org. Chem. Front. 5 (2018) 2296. [14] I. Ghosh, T. Ghosh, J.I. Bardagi, B. K€ onig, Science 346 (2014) 725–728. [15] J. Choi, C. Sakong, J. Choi, C. Yoon, J. Kim, Dyes Pigments 90 (2011) 82.
Fig. 8. Photoluminescence spectra of 5 (red) and 6 (blue) in ethyl acetate. Dotted lines represent excitation spectra and uninterrupted solid lines represent emission spectra.
Although the relatively long irradiation times in acetic acid/ethyl acetate mixtures were needed to convert compounds 3 and 5 into ace toxy derivatives 4 and 6, respectively, pointing to a low quantum yield, the rather clean conversion can be of interest for synthetic purposes. Therefore, further optimization is needed to use this photo-substitution reaction for preparative purposes. This will be a topic for forthcoming studies as well as the use of hydrolyzed products such as 7 for derivatization. The high hydrolytic reactivity of compound 4 compared to 6 can be explained by effect of sterically impedance of the phenoxy groups in 6. This steric effect may play a role in the good (photo-) chemical stability
6
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Photo-substitution reactions of perylene red dyes Johan Lub a, b, *, Paul A. van Hal a, c, Erica M.G. Custers a, Hugo H. Knobel d, Rifat A.M. Hikmet a, e a
Philips Research, High Tech Campus 34, AE Eindhoven, 5656, the Netherlands Eindhoven University of Technology, P.O. Box 513, MB, 5600, Eindhoven, the Netherlands c ASML, Le Havre 78, SV Eindhoven, 5627, the Netherlands d Eurofins Materials Science, High Tech Campus 11, AE Eindhoven, 5656, the Netherlands e Signify, High Tech Campus 7, AE Eindhoven, 5656, the Netherlands b
A R T I C L E I N F O
A B S T R A C T
Keywords: Lumogen red 305 Photo stability Photo substitution Photo-oxidation Fluorescent compounds
Irradiation of Lumogen Red 305 (3) in ethyl acetate with intense blue LED light led to very slow bleaching, showing a high photochemical stability. However, irradiation in ethanol led to the formation of a low yield product in which one of the phenoxy groups is substituted by an acetoxy group, the oxidized form of an ethoxy group. This reaction product was only formed in the presence of blue light and oxygen. Adding a small amount of acetic acid to the solutions resulted in a much higher yield of the product, indicating a possible photo oxidation of ethanol to acetic acid during the process in this solvent. A similar dye (5) containing less phenoxy side groups than Lumogen Red 305 showed the same reaction although the product formed showed lower chemical stability. The photo-substitution of phenoxy groups by acetoxy groups is a new reaction. The results are used to discuss the structural effects on the stability of these type of fluorescent dyes under irradiation conditions, potential reaction mechanisms and the possibility of using the photochemical transformations for preparative purposes.
1. Introduction Derivatives of perylene can be used as color converting dyes in LED applications [1,2]. The original blue light of LED chips is converted to white light with the aid of a combination of yellow and red emitting dyes such as compound 1 (Lumogen Orange 240, see structure in Table 1) and compound 3 (Lumogen Red 305, see structure in Table 1), respectively. To avoid excessive heating of the dyes, which can led to thermal degradation and quenching, a remote phosphor configuration is preferred over directly applying the dyes on top of the LED chip [3]. Although both dye compounds exhibit high quantum efficiencies, compound 1 lacks photochemical stability, resulting in rapid bleaching. Other yellow dyes derived from perylene that are commercially avail able also show insufficient photochemical stability [2,4]. Therefore, a new class of yellow emitting dyes have been developed with increased lifetime and high quantum efficiency for application in LED lamps, especially in combination with the remote phosphor technology [2,4–6]. Compound 3 and its derivatives show sufficient photochemical stability to be applied as red phosphors in LED lamps [7]. In our previous publication [8], we studied the photochemical con version of compound 1 in solvents by irradiation with intense blue LED light. In ethyl acetate, a relatively inert solvent, bleaching of the
yellow/orange compound was observed. We were not able to detect degradation products by means of NMR or chromatographic techniques. In ethanolic solution, the compound turned red under irradiation con ditions that included the presence of oxygen. A moderate yield of de rivative 2 (see Table 1) was detected. One of the hydrogen atoms of the perylene moiety of 1 was replaced by an ethoxy group (R3 in Table 1). This shows the relative reactivity of this part of perylene derivatives under aerobic irradiation conditions. The role of oxygen was apparent from the fact that irradiation under anaerobic conditions, product 2 was not formed. Here, we focus on compound 3. Although this red compound is stable enough for regular application in LED lamps, it still lacks the stability necessary for high intensities and long lifetime. The photochemical stability in poly(methyl methacrylate) has been the subject of several studies [9–11], although none of these studies analyzed the degradation products derived from 3. This is mainly due to the fact that separation of degradation products from a polymeric matrix is difficult. However, a study in solvents also did not reveal degradation products from 3 [12]. The role of these perylene derivatives as photo catalysts is well docu mented [13,14]. Normally the assumption is oxidation or dehalogena tion reaction of the perylene that leads to subsequent oxidation of the solvent; however, the intermediate perylene degradation products have
* Corresponding author. Philips Research, High Tech Campus 34, AE Eindhoven, 5656, the Netherlands. E-mail address: [email protected] (J. Lub). https://doi.org/10.1016/j.jlumin.2019.116845 Received 24 July 2019; Received in revised form 26 September 2019; Accepted 26 October 2019 Available online 31 October 2019 0022-2313/© 2019 Elsevier B.V. All rights reserved.
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not been reported. Compared to compound 1, compound 3 lacks hydrogen atoms at R1, 2 R , R3 and R4: these are substituted by phenoxy groups that are responsible for the absorption red shift and possibly the higher photo chemical stability. To get more insight in the possibly different re activities of hydrogen substituted compound 1 and phenoxy substituted compound 3, we introduce compound 5 that contains both hydrogen and phenoxy groups in the central area of the perylene moiety and can therefore present a combination of the reactivities of compounds 1 and 3. In this publication, an attempt is made to characterize photo-reaction products derived from 3 and 5 after irradiation in solution.
performed to separate the reaction products were conducted at higher concentration in a vessel with a diameter of 44 mm (see section 2.3). Absorption and emission spectra were measured in ethyl acetate on a PerkinElmer Lambda 950 UV/Vis spectrometer and a PerkinElmer LS55 fluorescence spectrometer, respectively, under the same conditions as described in Ref. [8]. 1H, 13C and 2D 1H–13C gradient-enhanced HMBC NMR spectra were measured on samples dissolved in deuterated chlo roform using Bruker Avance 300 MHz and 600 MHz NMR spectrometers [8]. LC-MS experiments were performed on an Agilent 1200 series Liquid Chromatography system fitted with a, 2.1 mm ID x 100 mm l x 3.5 μm dp Eclipse plus C18 column (Agilent). The system was equipped with optical diode array detection and mass detection was done using Electrospray Ionization and a TOF-analyzer. Preparative column chro matography was performed on a Combiflash Companion apparatus (Teledyne Isco) employing pre-packed silica cartridges from Grace (USA). The Maldi-TOF mass spectra were recorded on an ABSciex 4800 Maldi-TOF/TOF Analyzer using 2,5-dihydroxybenzoic acid as matrix.
2. Experimental procedure 2.1. Materials 2,9-bis(2,6-diisopropylphenyl)-5,6,12,13-tetraphenoxyanthra [2,1,9-def:6,5,10-d’e’f’]diisoquinoline-1,3,8,10(2H, 9H)-tetraone (3, commercial number: F305) was a kind gift from Dr. M. Koenemann of BASF. 2,9-bis(2,6-diisopropylphenyl)-5,12-diphenoxyanthra[2,1,9def:6,5,10-d’e’f’]diisoquinoline-1,3,8,10(2H, 9H)-tetraone (5) was pre pared by Dr. S. Hamon (Syncom b.v. Groningen, The Netherlands) ac cording to a literature procedure [15]. All other chemicals and solvents were obtained from Sigma-Aldrich.
2.3. 2,9-bis(2,6-diisopropylphenyl)-1,3,8,10-tetraoxo-6,12,13triphenoxy-1,2,3,8,9,10-hexahydroanthra[2,1,9-def:6,5,10-d’e’f’] diisoquinolin-5-yl acetate (4) A solution of 10 mg of 2,9-bis(2,6-diisopropylphenyl)-5,6,12,13-tet raphenoxyanthra[2,1,9-def:6,5,10-d’e’f’]diisoquinoline-1,3,8,10(2H, 9H)-tetraone (3) in 25 ml of a mixture of acetic acid/ethyl acetate 2:98 (v/v) was irradiated in a tube of 44 mm diameter for 8 h. The solution was subsequently extracted with 15 ml of a 0.8 M aqueous sodium hydrogen carbonate solution and 15 ml of brine. After drying over magnesium sulfate and evaporation, 2.0 mg the product 4 (21% yield) was obtained after column chromatography (SiO2, CH2Cl2). 1H NMR (300 MHz, δ in ppm, J in Hz): 8.50 (s, 1H), 8.37 (s, 1H), 8.32 (s, 1H), 8.27 (s, 1H), 7.46 (t, J ¼ 8.0, 2H), 7.25–7.35 (m, 10H), 7.08–714 (m,
2.2. Equipment For irradiation, an LED lighting device generating 450 nm light with an intensity of 4.1 W/cm2 at 12 mm distance of the irradiation tubes was used [8]. The irradiation experiments in the presence of air and anaer obic were performed as described in earlier work [8]. The experiments Table 1 Structures and mass of the perylene derivatives discussed in this publication.
Compound number
1 2 3
R 1 ¼ R4
R2
R3
M þ H (calculated)
H H
H H
H CH3CH2O
a
4
a
1079.4
1045.4
5
H
895.4
6
H
861.4
7
H
HO
a: see ref. 8. 2
819.4
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3H), 6.93–6.97 (m, 6H), 2.71 (m, J ¼ 6.2, 4H), 2.49 (s, 3H), 1.14 (d, J ¼ 6.2, 12H), 1.13 (d, J ¼ 6.2, 12H). 13C NMR (75 MHz, δ in ppm, * ¼ CH, # ¼ CH3) δ 168.81, 163.10, 156.76, 156.69*, 155.95, 155.04, 148.62, 145.70, 130.22*, 130.13*, 129.68*, 125.97*, 124.92*, 124.07*, 123.10, 122.45*, 121.88, 121.25*, 120.71*, 120.46, 120.02, 119.94*, 118.53*, 116.35, 29.17*, 24.07#, 24.04#, 21.80#. MALDI: M þ H ¼ 1045.48. UV–Vis: λmax (ethyl acetate) ¼ 556 nm, ε ¼ 46500 L mol 1 cm 1 and 521 nm ε ¼ 31000 L mol 1 cm 1. Photo luminescence: λmax (ethyl acetate) ¼ 586 nm.
that the incorporation of ethanol is accompanied by an oxidation reac tion on the perylene derivative or that ethanol is photo-oxidized to acetic acid before incorporation. The fact that no products were detected under anaerobic conditions point to the hypothesis that ethanol is oxidized to acetic acid before reaction with 3, which acts as a photo catalyst. Oxidations of solvents like ethanol by compound 3 are known under these irradiation conditions [13] that may explain the formation of acetic acid from ethanol in the presence of oxygen. One of the possible structures that can be formed is product 4 (see Table 1) in which one of the phenoxy groups is substituted for an acetoxy group. A summary of these reactions is outlined in Fig. 1. The proposed reaction sequence of Fig. 1 implies that a photo chemical reaction with acetic acid could also lead to possible structure 4. To test this possibility, the irradiation experiment was performed with 2% acetic acid added to the ethanolic and ethyl acetate solutions. The irradiations resulted in much larger spectral changes than without the acetic acid, as shown in Fig. 2. LC-MS results showed that the same compound is formed as in ethanol, but in a much higher yield. The result found in a mixture of acetic acid and ethyl acetate shows that ethanol is not necessary for these reactions. Irradiation under anaerobic conditions gave the same results, so oxygen is not required for this reaction as proposed in Fig. 1. Although the same product is formed in acetic acid as in ethanol, which points to a mechanisms as outlined in Fig. 1, it is also possible that both processes run differently. Thus, it is still possible that the reaction in ethanol initially leads to an oxidized form of compound 3 that after reaction with ethanol forms derivative 4. This could be the subject of further study. From the spectrum of Fig. 2 it can be concluded that during the irradiation in the presence of acetic acid a new red compound is formed with a blue shifted absorption compared to that of initial compound 3. In order to determine the structure of the compound formed, the reaction was performed with a larger quantity of 3 in a more concentrated so lution. Under these conditions, LC-MS results revealed that the conver sion was lower than in the initial diluted solutions, but enough material was generated to separate the reaction product by column chromatog raphy. NMR data of the separated product indeed shows the loss of one of the phenoxy side groups and the incorporation of an acetoxy group with typical signals in 1H NMR at 2.58 ppm and in 13C NMR at 21.8 ppm due to the CH3 group and in 13C NMR at 168.8 ppm due to the acetoxy carbonyl atom. The original singlet due to H1, H2, H3 and H4 (see Table 1) observed in the 1H NMR spectrum of compound 3 is now split into 4 singlets (see Fig. 3) as a result of the incorporation of the acetoxy group. Fig. 3 also shows the NMR 2D C–H 3 bands coupling spectrum of the product. The figure shows that these four singlets couple via three bands with the carbonyl carbon atoms of the imide groups observed at 163 ppm in the 13C NMR spectrum. This means that all four hydrogens H1–H4 are still in place and the acetoxy group has indeed replaced the phenoxy group at R3 (see Table 1). Thus, photo substitution of one of the phenoxy groups by an acetoxy group had occurred in compound 3 to form compound 4. Further investigation of the LC-MS results did not suggest products in which a second phenoxy group was replaced by an acetoxy group. This can be explained by a change in reactivity of product 4 compared to compound 3. However, a more likely explanation is that the acetoxy bond is more reactive than the original phenoxy bonds since the acetate anion or acetic acid is a better leaving group than the phenolate anion or phenol. This would mean that upon photo-substitution on product 4 by acetic acid, the acetoxy group is replaced, which is effectively no reac tion. The initial replacement of phenoxy by acetoxy is then explained by the fact that during the reaction acetic acid is in large excess to com pound 3 and thus also in large excess of the leaving phenol group. The conversion of 3 is rather clean, i.e. without or only with small amounts of by-products, making it a possibly interesting synthetic pro cess for generating “asymmetric substituted” perylene derivatives. However, the long irradiation times suggest relatively low quantum yields which is a drawback for such a process. The synthetic options can
2.4. 2,9-bis(2,6-diisopropylphenyl)-1,3,8,10-tetraoxo-12-phenoxy1,2,3,8,9,10-hexahydroanthra[2,1,9-def:6,5,10-d’e’f’]diisoquinolin-5-yl acetate (6) This irradiation experiment was performed in the same way as described in section 2.3 starting from 2,9-bis(2,6-diisopropylphenyl)5,12-diphenoxyanthra[2,1,9-def:6,5,10-d’e’f’]diisoquinoline-1,3,8,10 (2H, 9H)-tetraone (5). 4.8 mg of product 6 (50% yield) was obtained. 1H NMR (300 MHz, δ in ppm, J in Hz): 9.60 (d, J ¼ 8.2, 1H), 9.21 (d, J ¼ 8.2, 1H), 8.74 (d, J ¼ 8.2, 1 h), 8.72 (d, J ¼ 8.2, 1H), 8.52 (s, 1H), 8.41 (s, 1H), 7.43–7.51 (m, 5H), 7.35 (d, J ¼ 7.7, 2H), 7.33 (d, J ¼ 7.7, 2H), 7.16 (d, J ¼ 8.2, 2H), 2.73 (m, J ¼ 6.2, 4H), 2.58 (s, 3H), 1.18 (d, J ¼ 6.2, 12H), 1.16 (d, J ¼ 6.2, 6H), 1.15 (d, J ¼ 6.2, 6H). 13C NMR (75 MHz, δ in ppm, * ¼ CH, # ¼ CH3) δ 168.67, 163.40, 163.29, 162.82, 162.65, 155.49, 154.93, 147.88, 145.61, 145.57, 133.85, 133.11, 131.92*, 130.63*, 130.27, 130.23, 130.18*, 129.77*, 129.56, 129.31, 129.07*, 128.47*, 127.72*, 127.55, 127.23, 126.04, 125.33*, 125.18*, 124.32, 124.25, 124.15*, 124.10*, 123.86, 122.99, 122.38, 119.19*, 28.12*, 24.03#, 24.00#, 21.78#. MALDI: M þ H ¼ 861.34. UV–Vis: λmax (ethyl acetate) ¼ 519 nm, ε ¼ 45400 L mol 1 cm 1 and 490 nm ε ¼ 30700 L mol 1 cm 1. Photoluminescence: λmax (ethyl acetate) ¼ 552 nm. 3. Results and discussion 3.1. Irradiation of compound (3) A solution of 3 in ethyl acetate was irradiated at 70 � C for 8 h. A few percent of bleaching of the dye was observed by UV–vis spectroscopy. LC-MS analyses of the irradiated sample did not show any new com pounds that absorb in the visible region or show separate peaks with well-defined mass in the UV region, so no information was obtained of products possibly formed during irradiation. The quantity of degrada tion products was probably too low, too unstable, and/or didn’t contribute to the absorption spectrum. After irradiation of 3 in ethanol or n-propanol, effects similar to those in ethyl acetate, namely a few percent of bleaching was observed. However, in these solvents a small amount of a new compound absorbing in the red region was detected by LC-MS measurements. Irradiation in ethanol under vacuum (instead of air) resulted in a small bleaching effect, similar to that in ethyl acetate, but no products were detected by LC-MS. An ethanolic solution of compound 3 kept at 70 � C for 8 h did not show any change in the absorbance spectra or in LC-MS traces when the sample was not illuminated. Therefore, it is concluded that for formation of the product in ethanol, both oxygen and light are simultaneously required. Irradiation of 3 in n-propanol resulted in a compound with a mass 14 amu higher than the product formed in ethanol, as shown by LC-MS experiments: from this we conclude that the alcohols are incorporated in the perylene derivative during these reactions. The mass of the product detected by LC-MS from the reaction in ethanol is 1045.4 (M þ H), which is lower than that of the starting compound 3 namely: 1079.4 (M þ H). This means that a fraction of the original structure of 3 is split off. Stoichiometrically this could mean that one of the four phenoxy groups is replaced by an acetoxy group (or a propionoxy group in the case of irradiation in propanol). This means 3
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Fig. 1. Proposed process for the formation of 4 from 3 and ethanol under aerobic irradiation conditions.
Fig. 2. UV–Vis spectrum of 3 in acetic acid/ethyl acetate 2:98 v/v before (red curve) and after 8 h of irradiation (blue curve).
Fig. 4. Photoluminescence spectra of 3 (red) and 4 (blue) in ethyl acetate. Dotted lines represent excitation spectra and uninterrupted solid lines represent emission spectra.
while in the latter case a phenoxy group was replaced by an acetoxy group which stoichiometrically is an oxidized ethoxy group. In order to obtain more insight into these reactions and reactivities of the perylene derivatives, it was decided to investigate the photochemical behavior in ethanol of perylene derivative 5, which contains two hydrogen atoms R1 ¼ H and R4 ¼ H, similar to unsubstituted perylene derivative 1, and two phenoxy groups R2 and R3, similar to tetra-substituted derivative 3. The replacement of one of these hydrogens by an ethoxy group and/or replacement of one of the phenoxy groups by an acetoxy group may occur. Solutions of 5 in ethyl acetate, ethanol or n-propanol were irradiated at 70 � C for 8 h. In all cases, considerably more bleaching of the dye was observed than with the tetra substituted compound 3. This points to the unique high stability of 3 towards blue light already observed by com parison of 3 to other perylene derivatives [4,8]. Even though the bleaching in 5 was much higher than in 3, no defined compounds could be observed with the LC-MS method. This was also observed after irra diation of compound 1 in ethyl acetate as reported previously [8]. LC-MS revealed the formation of a small amount of a product after irradiation in ethanol. Irradiation in ethanol under vacuum instead of air resulted in bleaching, similar to that in ethyl acetate, but under these anaerobic condition no products were detected by LC-MS. Thus oxygen is needed for the formation of the detected product. Furthermore, a nonilluminated ethanolic solution of compound 5 kept at 70 � C for 8 h did not show any change in the spectra or in LC-MS traces. Thus, for the formation of the product in ethanol, both oxygen and light are needed, just as was the case with compound 3. The mass detected of the product is 819.4 (M þ H). If one the hydrogen atoms, R1 or R4, had been substituted by an ethoxy group as in the case of formation of compound 2 from 1, a mass of 939.4 (M þ H) would be expected. If the phenoxy group R3 had been replaced by an acetoxy group as in the case of for mation of compound 4 from 3, a mass of 861.4 (M þ H) would be expected. The transformation in n-propanol resulted in the same compound obtained in ethanol with a mass of 819.4 (M þ H). The absence of a mass difference of 14 in ethanol and propanol points to the conclusion that the alcohols are not incorporated in the detected product. To find out whether a reaction of 5 with acetic acid occurs similar to
Fig. 3. Part of the 2D 1H–13C HMBC spectrum in deuterated chloroform of reaction product 4 formed out of 3 by irradiation in an acetic acid-ethyl ace tate mixture.
be the subject of future studies. Fig. 4 shows the photoluminescence spectra of the starting com pound 3 and product 4 in ethyl acetate. The excitation spectra exhibits the same shape as the absorption spectra in ethyl acetate. The blue shift previously observed in the UV–vis spectrum during the irradiation (see Fig. 2) is also apparent when comparing the excitation and the emission spectra of compounds 3 and 4. Thus, the replacement of a phenoxy group by an acetoxy group in these perylene derivatives leads to a blue shift in absorption and emission. 3.2. Irradiation of compound (5) The photo-substitution reactions of ethanol on unsubstituted per ylene derivative 1 [8] and on tetra-substituted derivative 3 occurred only with simultaneous oxidation by oxygen. However, the type of re actions and products differ. In the first case, the hydrogen atom H ¼ R3 was replaced by an ethoxy group in an oxidative coupling reaction, 4
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the reaction with compound 3, the irradiation experiment was per formed with 2% acetic acid in ethyl acetate. Irradiation resulted in a blue shift of the absorption spectrum, shown in Fig. 5. LC-MS results revealed that the same compound was detected as in ethanol, but in much higher yield. In order to determine the structure of the photoproduct, a more concentrated solution was irradiated and separation was performed by column chromatography. NMR data of the separated product showed the loss of one of the phenoxy groups and the incor poration of an acetoxy group with typical signals in 1H NMR at 2.58 ppm and in 13C NMR at 21.8 ppm due to the CH3 group and in 13C NMR at 168.7 ppm due to the acetoxy carbonyl atom. These spectral properties are very similar to those of acetoxy derivative 4. Furthermore, the mass of the pure product measured by MALDI-TOF is 861.4 (M þ H), which points to a product where one of the phenoxy groups had indeed been substituted by an acetoxy group; for example compound 6. This would mean that the conversion of 5 probably occurs similarly to the conver sion of 3 with acetic acid. Fig. 6 shows the NMR 2D C–H 3 bands coupling spectrum of the product. The 4 carbonyl signals of the imide group observed around 163 ppm in the 13C NMR spectrum couple with 4 hydrogen atoms in the 1H NMR spectrum. These signals can be assigned to atoms H1, H2, H3 and H4. Thus, these four atoms were not replaced by an acetoxy group in the reaction. With the knowledge obtained of the conversion of 3 to 4, it is obvious that one of the phenoxy groups of 5 is substituted by an acetoxy group. Thus, 6 is obtained from compound 5. The type of reaction of compound 5 in ethanol resembles the type of reaction with compound 3 and not with compound 1. No other reaction products could be detected by LC-MS. The reactivity in ethanol of the carbon atom that contains the phenoxy group (R3) of 5 is much higher than with the hydrogen atoms at R1 or R4. This is probably also the case for compound 6. As explained before for compound 4, the acetoxy group is more reactive than the phenoxy group. This means in the case of compound 6 that substitution of the acetoxy group by acetoxy itself is also preferred over substitution of the remaining phenoxy group of 6. When compound 6 was subjected to an LC-MS measurement, only the peak with mass 819.4 (M þ H) was observed. This peak can be explained by the formation of hydrolysis product 7 (see Fig. 7). This is probably due to the acidic-aqueous conditions used in the LC-MS system. It explains the detection of 7 by the LC-MS analysis of the reaction product after irradiation of 5 in ethanol and propanol. Such a hydrolysis product was not detected in compound 4. The difference in hydrolytic reactivity between compound 4 and 6 can be explained by the acetoxy group (R3) in 4 being strongly sterically impeded by the neighboring phenoxy group (R1), while this group is absent in this position in compound 6. The effect of steric impedance will probably also play a role in the good (photo-)chemical stability of
Fig. 6. Part of the 2D 1H–13C HMBC spectrum in deuterated chloroform of reaction product 6 formed out of 5 by irradiation in an acetic acid-ethyl ace tate mixture.
the highly substituted red compound 3 compared to other perylene derivatives such as 1 and 5. If the conversion of 5 to 6 in ethanol indeed occurs according to Fig. 7, it is remarkable that only acetic acid reacts, even though a large excess of ethanol is present that could potentially also react (as observed during the photochemical reaction with 1 [8]). This may be explained by protonation of the photo-excited perylene derivative by acetic acid, leaving a rather nucleophilic acetate anion for substitution. The synthetic options of the photo substitution reaction with acetic acid may also be of interest. Apart from the fact that the (near) absence of by-products leads to a rather clean process to form an “asymmetric substituted” perylene derivative, the relative ease of hydrolysis to compound 7 opens the way to make more complex structures using the reactivity of the hydroxyl group (R3). Fig. 8 shows the photoluminescence spectra of the starting com pound 5 and product 6 in ethyl acetate. The excitation spectra exhibits the same shape as the absorption spectra. The blue shift observed in the UV–vis spectrum during the irradiation (see Fig. 5) is also apparent from both the excitation and the emission spectra of the pure compounds 5 and 6. A blue shift was also observed when one of the phenoxy groups of compound 3 was replaced by an acetoxy group to form compound 4. Thus, the replacement of a phenoxy group by an acetoxy group in these type of perylene derivatives leads to a blue shift in absorption and emission. A possible explanation for this effect may be the more electron withdrawing properties of the acetoxy group compared to the phenoxy group, resulting in a lower energy ground state (HOMO) while this effect is less pronounced in the excited state (LUMO). 4. Conclusions The photochemical conversion of the perylene derivatives 1, 3 and 5 in ethyl acetate leading to bleaching is not yet understood. Analyses of the mixtures after irradiation did not reveal products in which the per ylene chromophore had reacted forming bleached products. The various products obtained by irradiation in ethanol of these derivatives revealed that at least one of the sites at R1, R2, R3 and R4 are reactive. The photo-substitution reactions of 3 and 5 with acetic acid lead to the same product as the reactions in ethanol, albeit in low yield, sug gesting a mechanism in which ethanol is oxidized to small amounts of acetic acid during the irradiation in the presence of oxygen. The fact that only acetate groups react while a large excess of ethanol is present may be explained by protonation via acetic acid of the excited state of the perylene derivative leaving a rather nucleophilic acetate anion for substitution. However, alternative mechanisms are not excluded.
Fig. 5. UV–Vis spectrum of 5 in acetic acid/ethyl acetate 2:98 v/v before irradiation (red curve) and after 8 h of irradiation (blue curve). 5
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Fig. 7. Proposed process for the formation of 6 from 5 and ethanol under aerobic irradiation conditions, and 7 from 6 under acidic LC-MS conditions.
of the highly substituted red compound 3 compared to other less substituted perylene derivatives such as 1 and 5. Declaration of competing interestCOI The authors declare that there is no conflict of interest. Acknowledgments Branco Kusters and Dani€elle Beelen (both Philips) are thanked for their assistance with the experiments and analyses. Dr. M.G. Debye (Eindhoven University of Technology) is kindly acknowledged for careful reading of the manuscript and suggestions to improve its readability. References [1] P. Acuna, S. Leyre, J. Audenaert, Y. Meuret, G. Deconinck, P. Hanselaer, Opt. Express 22 (2014) A1079. [2] J. Lub, S.L.J. Hamon, P.A. van Hal, T.J. Visser, J.M. Jansen, R.A.M. Hikmet, Dyes Pigments 149 (2018) 662. [3] R.A.M. Hikmet, J.C. Kriege, J.F.M. Cillessen, R.T. Wegh, P.J.C. van der Wel and R. M. A Driessens, WO 2012001645 (2012). [4] J. Lub, R.A.M. Hikmet and D. Veldman, US 20160017219 (2016). [5] J. Lub, R.A.M. Hikmet and D. Veldman, WO 2015062916 (2015). [6] J. Lub, P.A. van Hal and R.A.M. Hikmet, US 2016062657 (2016). [7] J. Lub, , R.A.M. Hikmet and T. van Bommel, US 20150372240 (2015). [8] J. Lub, P.A. van Hal, R. Smits, L. Malassenet, J. Pikkemaat, R.A.M. Hikmet, J. Lumin. 207 (2019) 585. [9] L. Luis Cerd� an, A. Costela, G. Dur� an-Sampedro, I. García-Moreno, M. Calle, M. Juan-y-Seva, J. de Abajob, G.A. Turnbull, J. Mater. Chem. 22 (2012) 8938. [10] G. Griffini, L. Brambilla, M. Levi, M. Del Zoppo, S. Turri, Sol. Energy Mater. Sol. Cells 111 (2013) 41. [11] N. Tanaka, Barashkov, N.J. Heath, W.N. Sisk, Appl. Opt. 45 (2006) 3846. [12] A.A. Earp, T. Rawling, Jim B. Franklin, G.B. Smith, Dyes Pigments 84 (2010) 59. [13] H.-X. Gong, Z. Cao, M.-H. Li, S.-. u Liao, M.-J. Lin, Org. Chem. Front. 5 (2018) 2296. [14] I. Ghosh, T. Ghosh, J.I. Bardagi, B. K€ onig, Science 346 (2014) 725–728. [15] J. Choi, C. Sakong, J. Choi, C. Yoon, J. Kim, Dyes Pigments 90 (2011) 82.
Fig. 8. Photoluminescence spectra of 5 (red) and 6 (blue) in ethyl acetate. Dotted lines represent excitation spectra and uninterrupted solid lines represent emission spectra.
Although the relatively long irradiation times in acetic acid/ethyl acetate mixtures were needed to convert compounds 3 and 5 into ace toxy derivatives 4 and 6, respectively, pointing to a low quantum yield, the rather clean conversion can be of interest for synthetic purposes. Therefore, further optimization is needed to use this photo-substitution reaction for preparative purposes. This will be a topic for forthcoming studies as well as the use of hydrolyzed products such as 7 for derivatization. The high hydrolytic reactivity of compound 4 compared to 6 can be explained by effect of sterically impedance of the phenoxy groups in 6. This steric effect may play a role in the good (photo-) chemical stability
6