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Synthesis and fluorescence properties of hexameric and octameric subphthalocyanines based cyclic phosphazenes
Dyes and Pigments 98 (2013) 442e449
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Synthesis and fluorescence properties of hexameric and octameric subphthalocyanines based cyclic phosphazenes Bünyemin Ços¸ut*, Serkan Yes¸ilot, Mahmut Durmus¸, Adem Kılıç Department of Chemistry, Gebze Institute of Technology, P.O. Box 141, Gebze 41400, Kocaeli, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 January 2013 Received in revised form 22 March 2013 Accepted 26 March 2013 Available online 8 April 2013
Hexameric and octameric subphthalocyanines bearing cyclophosphazene derivatives were synthesized by threating hexa(p-hydroxyphenoxy)cyclotriphosphazene and octa(p-hydroxyphenoxy)cyclotetraphosphazene with excess of boron subphthalocyanine chloride in toluene. The hexameric and octameric subphthalocyanines bearing cyclophosphazene derivatives were fully characterized by elemental analysis, ESI and MALDI mass spectrometry, FT-IR, 1H, 13C and 31P NMR spectroscopy. The photophysical properties of title compounds were investigated by means of UV/Vis absorption and fluorescence spectroscopies in dilute 1,4-dioxane solutions. The fluorescence quenching behavior of these complexes by 1,4-benzoquinone were also reported. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Cyclophosphazene Subphthalocyanines NMR Photophysical Fluorescence quantum yield Quenching
1. Introduction Phosphazenes comprise a broad class of molecules based on the repeating phosphorus and nitrogen atoms, including cyclic or linear oligomers and polymers [1,2]. The most noticeable characteristic of this type of compound is the associated synthetic versatility, which enables the introduction of almost any side group on phosphorus and allows the properties to be tailored by the selection of suitable functional groups. Thus, it allows for the preparation of various types of molecules and polymers for varied applications, including biomedical, membranes, catalytic properties, electrical conductivity, liquid crystal, fire-resistant materials, electrooptical and semiconducting materials and energy generation and storage [3e6]. There has been recently considerable interest in fluorescent compounds based on cyclic phosphazene cores [7,8] for development of hyperbranched multichromophoric materials. The construction of multi-chromophore molecular systems for materials science applications represents a quite attractive and challenging issue. Multi porphyrins [9e11] and phthalocyanines [12,13] have shown to be interesting scaffolds for the preparation of molecular and supramolecular architectures. Subphthalocyanines (SubPcs) are the
* Corresponding author. Tel.: þ90 262 6053140; fax: þ90 262 6053101. E-mail address: [email protected] (B. Ços¸ut). 0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2013.03.028
lowest homologs of the phthalocyanine family, bearing boron within their central cavity, they are of great interest due to their specific structure and physicochemical characteristics. The interest lies in their unique properties, such as intense fluorescence and nonlinear optical properties stemming from its bowl-shaped structure and 14-p electron aromatic conjugation system [14]. SubPc derivatives have only recently emerged as functional materials in a variety of applications including: organic electronic devices such as photovoltaic cells (OPVs) [15], chemical sensors [16], light-emitting diodes (OLEDs) [17], thin film transistors (OTFTs) [18] and supramolecular building blocks for molecular cages [19]. In addition, a novel approach for the synthesis of subphthalocyanine assemblies have been reported with the assembly of six subphthalocyanines substituted on a central hexaphenylbenzene ring [20]. Cyclic phosphazenes containing either a six-membered or eight-membered ring with alternate phosphorus and nitrogen atoms are the closest to the benzene motif and are attractive targets for the design of supramolecular systems. In our previous work we have described the synthesis of some phthalocyanine compounds substituted with cyclotriphosphazenyl-rings on the peripheral position and their photophysical and photochemical properties have also been investigated [21e23]. In this paper, we explored the preparation of dendrimeric subphthalocyanines using cyclic phosphazene cores which have many advantageous properties for the design of supramolecular systems such as stability i.e., they do not
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breakdown under the harsh chemical conditions used during synthesis and their optical and electronic properties can be tuned by substituted groups [24]. Hence we report here the synthesis and characterization of hexameric and octameric subphthalocyanines based on cyclic phosphazene cores (Fig. 1) and an investigation of their photophysical properties. 2. Experimental
analyzed. NMR spectra were recorded in THF-d8 or CDCl3 solutions on a Varian 500 MHz spectrometer. 2.3. The parameters for fluorescence quantum yields The fluorescence quantum yields of the samples 5 and 6 were determined in 1,4-dioxane by comparison with the fluorescence of unsubstituted subphthalocyanine (Std-SubPc) as a standard. Fluorescence quantum yields (FF) were calculated by the comparative method (Eq. (1)) [25],
2.1. Materials Hexachlorocyclotriphosphazene (trimer) and octachlorocyclotetraphosphazene (tetramer) (Otsuka Chemical Co. Ltd) were purified by fractional crystallization from n-hexane. Boron subphthalocyanine chloride was purchased from Aldrich. The deuterated solvents (CDCl3 and tetrahydrofuran-d8) for NMR spectroscopy, cyclohexene, ethanol, 4-benzyloxyphenol, Pd(OH)2, Cs2CO3, toluene, silica gel 60, tetrahydrofuran were obtained from Merck. 1,8,9-Anthracenetriol for MALDI matrix was obtained from Fluka. All other reagents and solvents were reagent grade quality and obtained from commercial suppliers. 2.2. Equipment Electronic absorption spectra were recorded with a Shimadzu 2101 UV spectrophotometer in the UVevisible region. Fluorescence emission spectra were recorded on a Varian Eclipse spectrofluorimeter using 1 cm pathlength cuvettes at room temperature. Mass spectra were acquired in linear modes with average of 50 shots on a Bruker Daltonics Microflex mass spectrometer (Bremen, Germany) equipped with a nitrogen UV-Laser operating at 337 nm. Many different MALDI matrices were tried to find an intense molecular ion peak and low fragmentation under the MALDI-MS conditions for these compounds. 1,8,9-Anthracenetriol MALDI matrix yielded the best MALDI-MS spectra. 1,8,9-Anthracenetriol (20 mg/mL in tetrahydrofuran) matrix for compounds 5 and 6 were prepared. MALDI samples were prepared by mixing compounds 5 or 6 (2 mg/ mL in tetrahydrofuran) with the matrix solution (1:10 v/v) in a 0.5 mL eppendorf micro tube. Finally 1 mL of this mixture was deposited on the sample plate, dried at room temperature and then
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FF ¼ FF ðStdÞ
F$AStd $n2 FStd $A$nStd 2
(1)
where FF(Std) is the fluorescence quantum yield of standard. Unsubstituted subphthalocyanine was employed as the standard (FF ¼ 0.25 in benzene) [26]. F and FStd are the areas under the fluorescence emission curves of samples (5 and 6) and the standard, respectively. A and AStd are the respective absorbance of the samples and standard at the excitation wavelengths. The refractive indices of the solvents were employed in calculating fluorescence quantum yields in different solvents. n and nnstd are the refractive indices of solvents used for the standard and samples (n1,4dioxane ¼ 1.422 and nbenzene ¼ 1.501). The concentration of the solutions at the excitation wavelength fixed at 1.25 109 M. 2.4. Fluorescence quenching studies by 1,4-benzoquinone (BQ) Fluorescence quenching experiments on the cyclophosphazene centered subphthalocyanine derivatives (5 and 6) were carried out by the addition of different concentrations of BQ to a fixed concentration of the samples, and the concentrations of BQ in the resulting mixtures were 0, 0.008, 0.016, 0.024, 0.032, 0.040 M. The fluorescence spectra of 5 and 6 at each BQ addition were recorded, and the changes in fluorescence intensity related to BQ addition were shown by the SterneVolmer (SeV) equation [27] (Eq. (2)):
I0 ¼ 1 þ KSV ½BQ I
(2)
where I0 and I are the fluorescence intensities of fluorophore in the absence and presence of quencher, respectively. [BQ] is the
Fig. 1. Structure of subphthalocyanine substituted hexameric (5) and octameric (6) cyclic phosphazenes.
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concentration of the quencher and KSV is the SterneVolmer constant which was the product of the bimolecular quenching constant (kq) and the sF and was expressed in Eq. (3).
KSV ¼ kq $sF
(3)
The ratios of I0/I were calculated and plotted against [BQ] according to Eq. (2), and KSV was determined from the slope. 2.5. Synthesis The 2,2,4,4,6,6-hexakis(benzyloxyphenoxy)cyclotriphosphazatriene (1) and 2,2,4,4,6,6-hexakis (hydroxyphenoxy)cyclotriphosphazatriene (2) [28] were prepared and purified according to the literature procedure. 2.5.1. Synthesis of compound 3 A mixture of octachlorocyclotetraphosphazene (tetramer), N4P4Cl8 (0.69 g, 1.5 mmol), 4-benzyloxyphenol (2.64 g, 13.2 mmol), and Cs2CO3 (9.75 g, 30 mmol) was heated under reflux in tetrahydrofuran (75 mL) and stirred for 15 h. The reaction was filtered and the volatile materials were evaporated under vacuum and the product was purified by preparative TLC on silica gel using hexane: THF (1:1) as the eluent. The desired material was obtained as a white solid (yield: 2.07 g, 78%). (Found: C 70.45, H 5.03, N 3.12%,
C104H88N4O16P4 (1773.517) requires C 70.42, H 5.00, N 3.16%). IR ((ATR)): 1185 (br) (P]N); 959 cm1 (PeO); {1H}31P NMR (CDCl3) d ¼ 11.29 [s, 4P, N4P4 ring]. 1H NMR (CDCl3) d ¼ 7.20e7.29 (m, 40H; C6H5), 6.64 (“d”, J ¼ 7 Hz, 16H; OC6H4O), 6.77 (“d”, J ¼ 7 Hz, 16H; OC6H4O), 4.86 (s, 16H; OCH2).{1H}13C NMR (CDCl3) d ¼ 154.38,144.25, 135.88, 127.52, 126.90, 126.41, 120.88, 114.07 (12C; aromatic carbons); 69.33 (1C; OCH2)]. MS (MALDI) m/z (%): 1774.60 (100) [MþH]þ. 2.5.2. Synthesis of compound 4 To a solution of 2,2,4,4,6,6,8,8-oktakis(benzyloxyphenoxy) cyclotetraphosphazatetraene (3) (1.77 g, 1 mmol) in dry THF (15 mL) were added cyclohexene (9 mL), palladium hydroxide (20 wt% on carbon, 0.8 g), and ethanol (9 mL) solution. The mixture was heated under reflux for 6 h and filtered. The solvent was evaporated under vacuum and the product was purified by preparative TLC on silica gel using hexane: THF (1:1) as the eluent. The desired material was obtained as oily product (yield: 0.71 g, 68%). (Found: C 54.65, H 3.90, N 5.28%, C48H40N4O16P4 (1052.138) requires C 54.76, H 3.83, N 5.32%). IR ((ATR)): 3294 (br), (OeH); 1183 (br) (P]N); 949 cm1 (PeO); {1H}31P NMR ((CD3)2CO) d ¼ 10.84 [s, 4P, N4P4 ring]. 1H NMR ((CD3)2CO) d ¼ 8.38 (s, 8H, OC6H4OH), 6.56 (“d”, J ¼ 7 Hz, 16H; OC6H4O), 6.67 (“d”, J ¼ 7 Hz, 16H; OC6H4O). {1H}13C NMR((CD3)2CO) d ¼ 154.08, 144.31, 121.74, 115.46, (6C; aromatic carbons); MS (MALDI) m/z (%):1053.26 (100) [MþH]þ.
Scheme 1. Chemical structure and synthetic pathway of 1e6.
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2.5.3. Synthesis of compound 5 A mixture of 2,2,4,4,6,6-hexakis(hydroxyphenoxy)cyclotriphosphazatriene (2) (30 mg, 0.038 mmol) and Cl[B(SubPc)] (163 mg, 0.38 mmol) was refluxed in dry toluene (4 mL) for 24 h. The reaction mixture was refluxed with additional Cl[B(SubPc)] (82 mg, 0.19 mmol) for an additional 48 h. The reaction mixture filtered off and the volatile materials were evaporated under vacuum and the residue was purified by silica-gel column chromatography with n-hexane/THF (gradient from 4/1 to 3/1, v/v) as the eluent, The desired material was obtained as a purple solid (yield: 32 mg, 27%). (Found: C 68.65, H 3.09, N 17.28%, C180H96B6N39O12P3 (3154.796) requires C 68.53, H 3.07, N 17.32%). IR ((ATR)): 1187 (br) (P]N); 949 cm1 ((PeO); {1H}31P NMR (THF-d8) d ¼ 12.12 [s, 3P, N3P3 ring)]. 1H NMR (THF-d8) d ¼ 6.45 (“d”, J ¼ 6 Hz, 12H; OC6H4O), 6.54 (“d”, J ¼ 6 Hz, 12H; OC6H4O), 8.04e8.69 (m, 36H; SubPc-a), 7.63e7.78 (brd, 36H; SubPc-b). {1H}13C NMR(THF-d8) d ¼ 161.53, 154.44, 148.77, 143.51, 135.33, 130.61, 128.94, 127.19, 124.16, 121.75,118.92, 115.25; MS (MALDI) m/z (%): 3155.6 (100) [MþH]þ and peaks derived from the sequential loss of M- [B(SubPc)], M-2 [B(SubPc)], M-3[B(SubPc)] and M-4[B(SubPc)]. 2.5.4. Synthesis of compound 6 A mixture of 2,2,4,4,6,6,8,8-oktakis(hydroxyphenoxy)cyclotetraphosphazatetraene (4) (20 mg, 0.019 mmol) and Cl[B(SubPc)] (106 mg, 0.25 mmol) was refluxed in dry toluene (4 mL) for 24 h. The reaction mixture was refluxed with additional Cl[B(SubPc)] (52 mg, 0.12 mmol) for an additional 48 h. The reaction mixture filtered off and the volatile materials were evaporated under vacuum and the residue was purified by silica-gel column chromatography with n-hexane/THF (gradient from 4/1 to 3/1, v/v) as the eluent. The desired material was obtained as a purple solid (yield 18 mg, 23%). (Found: C 68.60, H 3.05, N 17.33%, C240H128B8N52O16P4 (4206.069) requires C 68.53, H 3.07, N 17.32%). IR (ATR) 1186 (br) (P]N); 953 cm1 (PeO); {1H}31P NMR (THF-d8) d ¼ 11.06 [s, 4P, N4P4 ring]. 1H NMR (THF-d8) d ¼ 6.42 (“d”, J ¼ 6.5 Hz, 16H; OC6H4O), 6.62 (“d”, J ¼ 6.5 Hz, 16H; OC6H4O), 8.42e8.69 (48H; SubPc-a), 7.55e7.76 (48H; SubPc-b). {1H}13C NMR(THF-d8) d ¼ 159.03, 154.08, 151.68, 141.96, 137.17, 130.93, 129.31, 127.83, 124.88, 121.73,119.14, 115.02; MS (MALDI) m/z (%):4207.068 (100) [MþH]þ and peaks derived from the sequential loss of M-[B(SubPc)], M-2[B(SubPc)], M-3[B(SubPc)] and M-4[B(SubPc)].
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at around 1180 cm1 (br) (P]N) and 950 cm1 (PeO) for a phosphazene ring as expected [29]. The molecular ion peak at m/z 3155.6 for 5 and at 4206.4 for 6 in the respective MALDI-TOF mass spectra confirmed the structures of the compounds. MALDI-TOF MS showed the parent peak and partial fragmentation peaks indicating that the phenoxy sites were completely substituted with SubPc (Fig. 2). Cyclophosphazene centered SubPc derivatives 5 and 6, MS revealed molecular ions as peaks, that a loss of the -[B(SubPc)] sites under the mass spectrometric conditions. The 31P NMR spectra of all derivatives were analyzed in two different spin systems as A3 or A4. The proton-decoupled 31P NMR spectra of compound 1, 2 and 5 showed an A3 spin system as expected and the resonances belonging to the phosphorous atoms were observed as only one signal at 10.05, 11.62 and 12.12 ppm, respectively. Furthermore, the proton-decoupled 31P NMR spectra of compounds 3, 4 and 6 displayed an A4 spin system as expected and the resonances belonging to phosphorous atoms were observed only as one signal at 11.29, 10.84 and 11.06 ppm, respectively. Two typical examples of 31P NMR spectra were shown in Fig. 3 for compounds 5 and 6. The 1H NMR spectra of compounds 5 and 6 exhibited of the a and b-phenyl protons of the subphthalocyanine side-arm from 8.8 to 7.3 ppm as expected [20] (Fig. 4). 3.2. Ground state electronic absorption and fluorescence properties The overall shape of the electronic absorption spectra of the subphthalocyanine is similar to that of the metallophthalocyanines, and so dominated by two intense bands, the Q-band observed at ca. 560 nm and the Soret (B) band in the UV region of the spectrum observed at around 300 nm. These bands are blue-shifted compared to those of metallophthalocyanine (the Q-band observed at ca. 700 nm and the Soret (B) band observed at ca. 350 nm) [30] counterparts due to the 14p conjugated system of subphthalocyanines compared to 18p conjugated systems for phthalocyanines.
3. Results and discussion 3.1. Synthesis and structural characterization The synthetic procedures for preparation of the compounds presented in this work are shown in Scheme 1. Compounds 1 and 2 were synthesized and purified according to the literature procedure [28]. Octachlorocyclotetraphosphazene was reacted with excess of 4-benzyloxyphenol in the presence of Cs2CO3 in THF to obtain compound 3. The 4-benzyloxyphenoxy units of compound 3 were converted to 4-hydroxyphenoxy group with cyclohexene in a mixture of THF/ethanol in the presence of Pd(OH)2 as catalyst to give compound 4. Compounds 5 and 6 were obtained from a nucleophilic displacement reactions of excess Cl[B(SubPc)] with 2 or 4 under an argon atmosphere in toluene. The progress of the reaction was monitored by TLC analysis and products were purified by silica-gel column chromatography with n-hexane/THF (gradient from 4/1 to 3/1, v/v) as the eluent. The new compounds were characterized by FT-IR, 1H, 13C and 31P NMR and matrix-assisted laser desorption/ionization time-of flight (MALDI-TOF) mass spectrometry. All the results were consistent with the predicted structures as shown in the experimental section. The FT-IR spectra of cyclic phosphazene compounds features peaks
Fig. 2. Positive ion and linear mode MALDI TOF-MS spectra of (A) 5 and (B) 6 were obtained in 1,8,9-anthracenetriol (20 mg/mL THF) as a MALDI matrix using nitrogen laser accumulating 50 laser shots.
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Fig. 3. Single peak obtained from 31P NMR (202.38 MHz, in THF-d8) as shown in (A) and (B) for compounds 5 and 6, respectively confirm full substitution of the cyclic phosphazene core.
The absorption properties of cyclophosphazene centered hexameric and octameric compounds (5 and 6) were studied in diluted solutions in 1,4-dioxane at room temperature, and both compounds gave intense Q bands at 562 nm in this solvent. Fig. 5 shows normalized absorption and fluorescence emission spectra of the compound 5 in 1,4-dioxane. The octameric compound (6) also exhibited similar spectra in this solvent. This figure exhibited good overlap between absorbance and emission spectra of these compounds which is very important for light emitting materials.
Fig. 6 shows fluorescence emission spectra of the unsubstituted subphthalocyanine (SubPc) 5 and 6 in 1,4-dioxane. All of these compounds were excited at same wavelength (510 nm). Fluorescence emission peaks were observed at 572 nm for unsubstituted SubPc, 576 nm for the hexameric compound 5 and 583 nm for the octameric compound 6 in 1,4-dioxane. The emission bands of 5 and 6 are redshifted by 4 nm for compound 5 and 11 nm for compound 6, when compared to the corresponding unsubstituted subphthalocyanine in 1,4-dioxane at same concentration (C ¼ 2.5 109 M) (Fig. 6).
Fig. 4. 1H NMR spectra (500 MHz, in THF-d8) of compound 5 (A) and compound 6 (B) at 25 C.
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The effect of the concentration on the both intensity and wavelength of the emission spectra is shown in Fig. 7. When increasing the concentration, firstly the intensity of the emission band was also increased but after a critical concentration intensity was decreased due to the self-quenching of the molecules. Additionally, the wavelength of the emission was also increased from 574 nm (C ¼ 1.00 1010 M) to 606 nm (C ¼ 1.00 107 M) with an increase in the concentration of compounds. This feature could be attributed to increasing of the intermolecular non-covalent pep interactions among the molecules in the higher concentrated solutions. 3.3. Fluorescence quantum yields
Fig. 5. Electronic absorption and fluorescence emission spectra of compound 5 in 1,4dioxane. Excitation wavelength ¼ 510 nm.
The fluorescence quantum yields (FF) of the 5 and 6 were measured in 1,4-dioxane and unsubstituted subphthalocyanine was used as standard in benzene solution. The refractive indices of the solvents were used for calculations of FF. Both new compounds and the standard were excited at same wavelength (510 nm). The FF values of all these compounds were measured under the same conditions for comparison. The fluorescence quantum yields were found to be 0.178 for 5, and 0.156 for 6 in 1,4-dioxane. FF values of the cyclophosphazene centered compounds are lower than unsubstituted subphthalocyanine which used as standard. The trimer centered derivative (5) showed a slightly larger FF value than tetramer centered derivative (6) which could be due to greater self-quenching of 6 than 5. Increasing of the subphthalocyanine groups might be caused increasing of self-quenching of the cyclophosphazene centered molecules. However, the FF values of 5 and 6 are higher than hexameric hexaphenylbenzene centered subphthalocyanine counterpart which studied by Morisue and coworkers [20]. 3.4. Fluorescence quenching properties
Fig. 6. Fluorescence emission spectra of unsubstituted subphthalocyanine, hexameric compound 5 and octameric compound 6 in 1,4-dioxane. (C ¼ 1.25 109 M, Excitation wavelength ¼ 510 nm).
The fluorescence quenching behavior of 5 and 6 by 1,4benzoquinone (BQ) were also investigated in this work. In the presence of BQ as a quencher, an energy transfer occurs between the
Fig. 7. Fluorescence emission spectra of octameric compound 6 in 1,4-dioxane at different concentrations. Excitation wavelength ¼ 510 nm. (Insets: Emission intensity or wavelength versus concentration).
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Fig. 8. Fluorescence emission spectral changes 1,4-dioxane of compound 6 (C ¼ 7.50 109 M) by addition of different concentrations of BQ in toluene. [BQ] ¼ 0, 0.008, 0.016, 0.024, 0.032, 0.040 M and saturated with BQ.
by standard methods (elemental analysis, IR, 1H, 13C and 31P NMR spectroscopy, and mass spectra). The photophysical properties of 5 and 6 were investigated in 1,4-dioxane and compared with unsubstituted subphthalocyanine which is used as standard. The fluorescence emission bands of the newly synthesized compounds are shifted to higher wavelength region compared to emission band of unsubstituted subphthalocyanine. Subphthalocyanine which could be due to the non-covalent pep interactions between subphthalocyanine molecules on the hexameric or octameric compounds. The FF values of studied compounds (5 and 6) are lower than unsubstituted subphthalocyanine. However, these values are higher than the FF value (FF ¼ 0.067) of hexaphenylbenzene centered hexameric subphthalocyanine derivative in 1,4-dioxane as previously reported [20]. As a result, 5 and 6 could be good candidates for advanced materials such as light emitting electroluminescence devices due to the appropriate wavelength (blue-green region) and high fluorescence quantum yields values of these compounds. References
Fig. 9. SterneVolmer plots for BQ quenching of studied compounds in 1,4-dioxane (C ¼ 7.50 109 M). [BQ] ¼ 0, 0.008, 0.016, 0.024, 0.032, 0.040 M.
excited state of compounds (5 and 6) and the BQ. The fluorescence quenching of 5 and 6 by BQ in 1,4-dioxane obeyed the SterneVolmer kinetics. Fig. 8 exhibited the quenching of compound 6 by BQ in 1,4dioxane as an example for studied compounds. The slope of the plots shown at Fig. 9 gave the SterneVolmer constant (KSV) values. The KSV values were found 31.52 M1 for compound 5 and 40.09 M1 for compound 6 by quenching of BQ. Bimolecular quenching constant (kq) values were found to be 0.42 1013 M1.s1 for 5 and 11.42 1013 M1.s1 for 6 in 1,4-dioxane. The KSV and kq values of the cyclotetramer centered compound (6) are higher than cyclotrimer centered counterpart in 1,4-dioxane. The energy transfer between the excited cyclotetramer centered octameric derivative (6) and the BQ was more effective than between the excited cyclotrimer centered hexameric derivative 5. The kq values were of the order of 1013e1014 M1 s1, which exceed the proposed value of 1010 M1 s1 for diffusion-controlled (dynamic) quenching (according to the EinsteineSmoluchowski approximation at room temperature [31]). This, also, was an indication that the mechanism of BQ quenching by cyclophosphazene centered hexameric and octameric compounds (5 and 6) was not diffusion-controlled (i.e., not dynamic quenching, but static quenching). 4. Conclusion The synthesis and characterization of hexameric and octameric subphthalocyanine derivatives by using a cyclic phosphazene ring as a platform were reported. All compounds were fully characterized
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[27] Rose J. Advanced physico-chemical experiments. London: Sir Isaac Pitman & Sons Ltd; 1964. [28] Barbera J, Bardajı M, Jimenez J, Laguna A, Martınez MP, Oriol L, et al. Columnar mesomorphic organizations in cyclotriphosphazenes. J Am Chem Soc 2005;127:8994e9002. [29] Allcock HR. Phosphonitrilic compounds. II. Reactions of phosphonitrilic chlorides with cathechol and triethylamine. J Am Chem Soc 1964;86: 2591e5. [30] Stillman MJ, Nyokong T. In: Leznoff CC, Lever ABP, editors. Phthalocyanines: properties and applications, vol. 1. New York: VCH Publishers; 1989. [Chapter 3]. [31] Murov SL, Carmichael I, Hug GL. Handbook of photochemistry. 2nd ed. New York: M. Decker; 1993207.
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Synthesis and fluorescence properties of hexameric and octameric subphthalocyanines based cyclic phosphazenes Bünyemin Ços¸ut*, Serkan Yes¸ilot, Mahmut Durmus¸, Adem Kılıç Department of Chemistry, Gebze Institute of Technology, P.O. Box 141, Gebze 41400, Kocaeli, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 January 2013 Received in revised form 22 March 2013 Accepted 26 March 2013 Available online 8 April 2013
Hexameric and octameric subphthalocyanines bearing cyclophosphazene derivatives were synthesized by threating hexa(p-hydroxyphenoxy)cyclotriphosphazene and octa(p-hydroxyphenoxy)cyclotetraphosphazene with excess of boron subphthalocyanine chloride in toluene. The hexameric and octameric subphthalocyanines bearing cyclophosphazene derivatives were fully characterized by elemental analysis, ESI and MALDI mass spectrometry, FT-IR, 1H, 13C and 31P NMR spectroscopy. The photophysical properties of title compounds were investigated by means of UV/Vis absorption and fluorescence spectroscopies in dilute 1,4-dioxane solutions. The fluorescence quenching behavior of these complexes by 1,4-benzoquinone were also reported. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Cyclophosphazene Subphthalocyanines NMR Photophysical Fluorescence quantum yield Quenching
1. Introduction Phosphazenes comprise a broad class of molecules based on the repeating phosphorus and nitrogen atoms, including cyclic or linear oligomers and polymers [1,2]. The most noticeable characteristic of this type of compound is the associated synthetic versatility, which enables the introduction of almost any side group on phosphorus and allows the properties to be tailored by the selection of suitable functional groups. Thus, it allows for the preparation of various types of molecules and polymers for varied applications, including biomedical, membranes, catalytic properties, electrical conductivity, liquid crystal, fire-resistant materials, electrooptical and semiconducting materials and energy generation and storage [3e6]. There has been recently considerable interest in fluorescent compounds based on cyclic phosphazene cores [7,8] for development of hyperbranched multichromophoric materials. The construction of multi-chromophore molecular systems for materials science applications represents a quite attractive and challenging issue. Multi porphyrins [9e11] and phthalocyanines [12,13] have shown to be interesting scaffolds for the preparation of molecular and supramolecular architectures. Subphthalocyanines (SubPcs) are the
* Corresponding author. Tel.: þ90 262 6053140; fax: þ90 262 6053101. E-mail address: [email protected] (B. Ços¸ut). 0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2013.03.028
lowest homologs of the phthalocyanine family, bearing boron within their central cavity, they are of great interest due to their specific structure and physicochemical characteristics. The interest lies in their unique properties, such as intense fluorescence and nonlinear optical properties stemming from its bowl-shaped structure and 14-p electron aromatic conjugation system [14]. SubPc derivatives have only recently emerged as functional materials in a variety of applications including: organic electronic devices such as photovoltaic cells (OPVs) [15], chemical sensors [16], light-emitting diodes (OLEDs) [17], thin film transistors (OTFTs) [18] and supramolecular building blocks for molecular cages [19]. In addition, a novel approach for the synthesis of subphthalocyanine assemblies have been reported with the assembly of six subphthalocyanines substituted on a central hexaphenylbenzene ring [20]. Cyclic phosphazenes containing either a six-membered or eight-membered ring with alternate phosphorus and nitrogen atoms are the closest to the benzene motif and are attractive targets for the design of supramolecular systems. In our previous work we have described the synthesis of some phthalocyanine compounds substituted with cyclotriphosphazenyl-rings on the peripheral position and their photophysical and photochemical properties have also been investigated [21e23]. In this paper, we explored the preparation of dendrimeric subphthalocyanines using cyclic phosphazene cores which have many advantageous properties for the design of supramolecular systems such as stability i.e., they do not
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breakdown under the harsh chemical conditions used during synthesis and their optical and electronic properties can be tuned by substituted groups [24]. Hence we report here the synthesis and characterization of hexameric and octameric subphthalocyanines based on cyclic phosphazene cores (Fig. 1) and an investigation of their photophysical properties. 2. Experimental
analyzed. NMR spectra were recorded in THF-d8 or CDCl3 solutions on a Varian 500 MHz spectrometer. 2.3. The parameters for fluorescence quantum yields The fluorescence quantum yields of the samples 5 and 6 were determined in 1,4-dioxane by comparison with the fluorescence of unsubstituted subphthalocyanine (Std-SubPc) as a standard. Fluorescence quantum yields (FF) were calculated by the comparative method (Eq. (1)) [25],
2.1. Materials Hexachlorocyclotriphosphazene (trimer) and octachlorocyclotetraphosphazene (tetramer) (Otsuka Chemical Co. Ltd) were purified by fractional crystallization from n-hexane. Boron subphthalocyanine chloride was purchased from Aldrich. The deuterated solvents (CDCl3 and tetrahydrofuran-d8) for NMR spectroscopy, cyclohexene, ethanol, 4-benzyloxyphenol, Pd(OH)2, Cs2CO3, toluene, silica gel 60, tetrahydrofuran were obtained from Merck. 1,8,9-Anthracenetriol for MALDI matrix was obtained from Fluka. All other reagents and solvents were reagent grade quality and obtained from commercial suppliers. 2.2. Equipment Electronic absorption spectra were recorded with a Shimadzu 2101 UV spectrophotometer in the UVevisible region. Fluorescence emission spectra were recorded on a Varian Eclipse spectrofluorimeter using 1 cm pathlength cuvettes at room temperature. Mass spectra were acquired in linear modes with average of 50 shots on a Bruker Daltonics Microflex mass spectrometer (Bremen, Germany) equipped with a nitrogen UV-Laser operating at 337 nm. Many different MALDI matrices were tried to find an intense molecular ion peak and low fragmentation under the MALDI-MS conditions for these compounds. 1,8,9-Anthracenetriol MALDI matrix yielded the best MALDI-MS spectra. 1,8,9-Anthracenetriol (20 mg/mL in tetrahydrofuran) matrix for compounds 5 and 6 were prepared. MALDI samples were prepared by mixing compounds 5 or 6 (2 mg/ mL in tetrahydrofuran) with the matrix solution (1:10 v/v) in a 0.5 mL eppendorf micro tube. Finally 1 mL of this mixture was deposited on the sample plate, dried at room temperature and then
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FF ¼ FF ðStdÞ
F$AStd $n2 FStd $A$nStd 2
(1)
where FF(Std) is the fluorescence quantum yield of standard. Unsubstituted subphthalocyanine was employed as the standard (FF ¼ 0.25 in benzene) [26]. F and FStd are the areas under the fluorescence emission curves of samples (5 and 6) and the standard, respectively. A and AStd are the respective absorbance of the samples and standard at the excitation wavelengths. The refractive indices of the solvents were employed in calculating fluorescence quantum yields in different solvents. n and nnstd are the refractive indices of solvents used for the standard and samples (n1,4dioxane ¼ 1.422 and nbenzene ¼ 1.501). The concentration of the solutions at the excitation wavelength fixed at 1.25 109 M. 2.4. Fluorescence quenching studies by 1,4-benzoquinone (BQ) Fluorescence quenching experiments on the cyclophosphazene centered subphthalocyanine derivatives (5 and 6) were carried out by the addition of different concentrations of BQ to a fixed concentration of the samples, and the concentrations of BQ in the resulting mixtures were 0, 0.008, 0.016, 0.024, 0.032, 0.040 M. The fluorescence spectra of 5 and 6 at each BQ addition were recorded, and the changes in fluorescence intensity related to BQ addition were shown by the SterneVolmer (SeV) equation [27] (Eq. (2)):
I0 ¼ 1 þ KSV ½BQ I
(2)
where I0 and I are the fluorescence intensities of fluorophore in the absence and presence of quencher, respectively. [BQ] is the
Fig. 1. Structure of subphthalocyanine substituted hexameric (5) and octameric (6) cyclic phosphazenes.
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concentration of the quencher and KSV is the SterneVolmer constant which was the product of the bimolecular quenching constant (kq) and the sF and was expressed in Eq. (3).
KSV ¼ kq $sF
(3)
The ratios of I0/I were calculated and plotted against [BQ] according to Eq. (2), and KSV was determined from the slope. 2.5. Synthesis The 2,2,4,4,6,6-hexakis(benzyloxyphenoxy)cyclotriphosphazatriene (1) and 2,2,4,4,6,6-hexakis (hydroxyphenoxy)cyclotriphosphazatriene (2) [28] were prepared and purified according to the literature procedure. 2.5.1. Synthesis of compound 3 A mixture of octachlorocyclotetraphosphazene (tetramer), N4P4Cl8 (0.69 g, 1.5 mmol), 4-benzyloxyphenol (2.64 g, 13.2 mmol), and Cs2CO3 (9.75 g, 30 mmol) was heated under reflux in tetrahydrofuran (75 mL) and stirred for 15 h. The reaction was filtered and the volatile materials were evaporated under vacuum and the product was purified by preparative TLC on silica gel using hexane: THF (1:1) as the eluent. The desired material was obtained as a white solid (yield: 2.07 g, 78%). (Found: C 70.45, H 5.03, N 3.12%,
C104H88N4O16P4 (1773.517) requires C 70.42, H 5.00, N 3.16%). IR ((ATR)): 1185 (br) (P]N); 959 cm1 (PeO); {1H}31P NMR (CDCl3) d ¼ 11.29 [s, 4P, N4P4 ring]. 1H NMR (CDCl3) d ¼ 7.20e7.29 (m, 40H; C6H5), 6.64 (“d”, J ¼ 7 Hz, 16H; OC6H4O), 6.77 (“d”, J ¼ 7 Hz, 16H; OC6H4O), 4.86 (s, 16H; OCH2).{1H}13C NMR (CDCl3) d ¼ 154.38,144.25, 135.88, 127.52, 126.90, 126.41, 120.88, 114.07 (12C; aromatic carbons); 69.33 (1C; OCH2)]. MS (MALDI) m/z (%): 1774.60 (100) [MþH]þ. 2.5.2. Synthesis of compound 4 To a solution of 2,2,4,4,6,6,8,8-oktakis(benzyloxyphenoxy) cyclotetraphosphazatetraene (3) (1.77 g, 1 mmol) in dry THF (15 mL) were added cyclohexene (9 mL), palladium hydroxide (20 wt% on carbon, 0.8 g), and ethanol (9 mL) solution. The mixture was heated under reflux for 6 h and filtered. The solvent was evaporated under vacuum and the product was purified by preparative TLC on silica gel using hexane: THF (1:1) as the eluent. The desired material was obtained as oily product (yield: 0.71 g, 68%). (Found: C 54.65, H 3.90, N 5.28%, C48H40N4O16P4 (1052.138) requires C 54.76, H 3.83, N 5.32%). IR ((ATR)): 3294 (br), (OeH); 1183 (br) (P]N); 949 cm1 (PeO); {1H}31P NMR ((CD3)2CO) d ¼ 10.84 [s, 4P, N4P4 ring]. 1H NMR ((CD3)2CO) d ¼ 8.38 (s, 8H, OC6H4OH), 6.56 (“d”, J ¼ 7 Hz, 16H; OC6H4O), 6.67 (“d”, J ¼ 7 Hz, 16H; OC6H4O). {1H}13C NMR((CD3)2CO) d ¼ 154.08, 144.31, 121.74, 115.46, (6C; aromatic carbons); MS (MALDI) m/z (%):1053.26 (100) [MþH]þ.
Scheme 1. Chemical structure and synthetic pathway of 1e6.
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2.5.3. Synthesis of compound 5 A mixture of 2,2,4,4,6,6-hexakis(hydroxyphenoxy)cyclotriphosphazatriene (2) (30 mg, 0.038 mmol) and Cl[B(SubPc)] (163 mg, 0.38 mmol) was refluxed in dry toluene (4 mL) for 24 h. The reaction mixture was refluxed with additional Cl[B(SubPc)] (82 mg, 0.19 mmol) for an additional 48 h. The reaction mixture filtered off and the volatile materials were evaporated under vacuum and the residue was purified by silica-gel column chromatography with n-hexane/THF (gradient from 4/1 to 3/1, v/v) as the eluent, The desired material was obtained as a purple solid (yield: 32 mg, 27%). (Found: C 68.65, H 3.09, N 17.28%, C180H96B6N39O12P3 (3154.796) requires C 68.53, H 3.07, N 17.32%). IR ((ATR)): 1187 (br) (P]N); 949 cm1 ((PeO); {1H}31P NMR (THF-d8) d ¼ 12.12 [s, 3P, N3P3 ring)]. 1H NMR (THF-d8) d ¼ 6.45 (“d”, J ¼ 6 Hz, 12H; OC6H4O), 6.54 (“d”, J ¼ 6 Hz, 12H; OC6H4O), 8.04e8.69 (m, 36H; SubPc-a), 7.63e7.78 (brd, 36H; SubPc-b). {1H}13C NMR(THF-d8) d ¼ 161.53, 154.44, 148.77, 143.51, 135.33, 130.61, 128.94, 127.19, 124.16, 121.75,118.92, 115.25; MS (MALDI) m/z (%): 3155.6 (100) [MþH]þ and peaks derived from the sequential loss of M- [B(SubPc)], M-2 [B(SubPc)], M-3[B(SubPc)] and M-4[B(SubPc)]. 2.5.4. Synthesis of compound 6 A mixture of 2,2,4,4,6,6,8,8-oktakis(hydroxyphenoxy)cyclotetraphosphazatetraene (4) (20 mg, 0.019 mmol) and Cl[B(SubPc)] (106 mg, 0.25 mmol) was refluxed in dry toluene (4 mL) for 24 h. The reaction mixture was refluxed with additional Cl[B(SubPc)] (52 mg, 0.12 mmol) for an additional 48 h. The reaction mixture filtered off and the volatile materials were evaporated under vacuum and the residue was purified by silica-gel column chromatography with n-hexane/THF (gradient from 4/1 to 3/1, v/v) as the eluent. The desired material was obtained as a purple solid (yield 18 mg, 23%). (Found: C 68.60, H 3.05, N 17.33%, C240H128B8N52O16P4 (4206.069) requires C 68.53, H 3.07, N 17.32%). IR (ATR) 1186 (br) (P]N); 953 cm1 (PeO); {1H}31P NMR (THF-d8) d ¼ 11.06 [s, 4P, N4P4 ring]. 1H NMR (THF-d8) d ¼ 6.42 (“d”, J ¼ 6.5 Hz, 16H; OC6H4O), 6.62 (“d”, J ¼ 6.5 Hz, 16H; OC6H4O), 8.42e8.69 (48H; SubPc-a), 7.55e7.76 (48H; SubPc-b). {1H}13C NMR(THF-d8) d ¼ 159.03, 154.08, 151.68, 141.96, 137.17, 130.93, 129.31, 127.83, 124.88, 121.73,119.14, 115.02; MS (MALDI) m/z (%):4207.068 (100) [MþH]þ and peaks derived from the sequential loss of M-[B(SubPc)], M-2[B(SubPc)], M-3[B(SubPc)] and M-4[B(SubPc)].
445
at around 1180 cm1 (br) (P]N) and 950 cm1 (PeO) for a phosphazene ring as expected [29]. The molecular ion peak at m/z 3155.6 for 5 and at 4206.4 for 6 in the respective MALDI-TOF mass spectra confirmed the structures of the compounds. MALDI-TOF MS showed the parent peak and partial fragmentation peaks indicating that the phenoxy sites were completely substituted with SubPc (Fig. 2). Cyclophosphazene centered SubPc derivatives 5 and 6, MS revealed molecular ions as peaks, that a loss of the -[B(SubPc)] sites under the mass spectrometric conditions. The 31P NMR spectra of all derivatives were analyzed in two different spin systems as A3 or A4. The proton-decoupled 31P NMR spectra of compound 1, 2 and 5 showed an A3 spin system as expected and the resonances belonging to the phosphorous atoms were observed as only one signal at 10.05, 11.62 and 12.12 ppm, respectively. Furthermore, the proton-decoupled 31P NMR spectra of compounds 3, 4 and 6 displayed an A4 spin system as expected and the resonances belonging to phosphorous atoms were observed only as one signal at 11.29, 10.84 and 11.06 ppm, respectively. Two typical examples of 31P NMR spectra were shown in Fig. 3 for compounds 5 and 6. The 1H NMR spectra of compounds 5 and 6 exhibited of the a and b-phenyl protons of the subphthalocyanine side-arm from 8.8 to 7.3 ppm as expected [20] (Fig. 4). 3.2. Ground state electronic absorption and fluorescence properties The overall shape of the electronic absorption spectra of the subphthalocyanine is similar to that of the metallophthalocyanines, and so dominated by two intense bands, the Q-band observed at ca. 560 nm and the Soret (B) band in the UV region of the spectrum observed at around 300 nm. These bands are blue-shifted compared to those of metallophthalocyanine (the Q-band observed at ca. 700 nm and the Soret (B) band observed at ca. 350 nm) [30] counterparts due to the 14p conjugated system of subphthalocyanines compared to 18p conjugated systems for phthalocyanines.
3. Results and discussion 3.1. Synthesis and structural characterization The synthetic procedures for preparation of the compounds presented in this work are shown in Scheme 1. Compounds 1 and 2 were synthesized and purified according to the literature procedure [28]. Octachlorocyclotetraphosphazene was reacted with excess of 4-benzyloxyphenol in the presence of Cs2CO3 in THF to obtain compound 3. The 4-benzyloxyphenoxy units of compound 3 were converted to 4-hydroxyphenoxy group with cyclohexene in a mixture of THF/ethanol in the presence of Pd(OH)2 as catalyst to give compound 4. Compounds 5 and 6 were obtained from a nucleophilic displacement reactions of excess Cl[B(SubPc)] with 2 or 4 under an argon atmosphere in toluene. The progress of the reaction was monitored by TLC analysis and products were purified by silica-gel column chromatography with n-hexane/THF (gradient from 4/1 to 3/1, v/v) as the eluent. The new compounds were characterized by FT-IR, 1H, 13C and 31P NMR and matrix-assisted laser desorption/ionization time-of flight (MALDI-TOF) mass spectrometry. All the results were consistent with the predicted structures as shown in the experimental section. The FT-IR spectra of cyclic phosphazene compounds features peaks
Fig. 2. Positive ion and linear mode MALDI TOF-MS spectra of (A) 5 and (B) 6 were obtained in 1,8,9-anthracenetriol (20 mg/mL THF) as a MALDI matrix using nitrogen laser accumulating 50 laser shots.
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Fig. 3. Single peak obtained from 31P NMR (202.38 MHz, in THF-d8) as shown in (A) and (B) for compounds 5 and 6, respectively confirm full substitution of the cyclic phosphazene core.
The absorption properties of cyclophosphazene centered hexameric and octameric compounds (5 and 6) were studied in diluted solutions in 1,4-dioxane at room temperature, and both compounds gave intense Q bands at 562 nm in this solvent. Fig. 5 shows normalized absorption and fluorescence emission spectra of the compound 5 in 1,4-dioxane. The octameric compound (6) also exhibited similar spectra in this solvent. This figure exhibited good overlap between absorbance and emission spectra of these compounds which is very important for light emitting materials.
Fig. 6 shows fluorescence emission spectra of the unsubstituted subphthalocyanine (SubPc) 5 and 6 in 1,4-dioxane. All of these compounds were excited at same wavelength (510 nm). Fluorescence emission peaks were observed at 572 nm for unsubstituted SubPc, 576 nm for the hexameric compound 5 and 583 nm for the octameric compound 6 in 1,4-dioxane. The emission bands of 5 and 6 are redshifted by 4 nm for compound 5 and 11 nm for compound 6, when compared to the corresponding unsubstituted subphthalocyanine in 1,4-dioxane at same concentration (C ¼ 2.5 109 M) (Fig. 6).
Fig. 4. 1H NMR spectra (500 MHz, in THF-d8) of compound 5 (A) and compound 6 (B) at 25 C.
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The effect of the concentration on the both intensity and wavelength of the emission spectra is shown in Fig. 7. When increasing the concentration, firstly the intensity of the emission band was also increased but after a critical concentration intensity was decreased due to the self-quenching of the molecules. Additionally, the wavelength of the emission was also increased from 574 nm (C ¼ 1.00 1010 M) to 606 nm (C ¼ 1.00 107 M) with an increase in the concentration of compounds. This feature could be attributed to increasing of the intermolecular non-covalent pep interactions among the molecules in the higher concentrated solutions. 3.3. Fluorescence quantum yields
Fig. 5. Electronic absorption and fluorescence emission spectra of compound 5 in 1,4dioxane. Excitation wavelength ¼ 510 nm.
The fluorescence quantum yields (FF) of the 5 and 6 were measured in 1,4-dioxane and unsubstituted subphthalocyanine was used as standard in benzene solution. The refractive indices of the solvents were used for calculations of FF. Both new compounds and the standard were excited at same wavelength (510 nm). The FF values of all these compounds were measured under the same conditions for comparison. The fluorescence quantum yields were found to be 0.178 for 5, and 0.156 for 6 in 1,4-dioxane. FF values of the cyclophosphazene centered compounds are lower than unsubstituted subphthalocyanine which used as standard. The trimer centered derivative (5) showed a slightly larger FF value than tetramer centered derivative (6) which could be due to greater self-quenching of 6 than 5. Increasing of the subphthalocyanine groups might be caused increasing of self-quenching of the cyclophosphazene centered molecules. However, the FF values of 5 and 6 are higher than hexameric hexaphenylbenzene centered subphthalocyanine counterpart which studied by Morisue and coworkers [20]. 3.4. Fluorescence quenching properties
Fig. 6. Fluorescence emission spectra of unsubstituted subphthalocyanine, hexameric compound 5 and octameric compound 6 in 1,4-dioxane. (C ¼ 1.25 109 M, Excitation wavelength ¼ 510 nm).
The fluorescence quenching behavior of 5 and 6 by 1,4benzoquinone (BQ) were also investigated in this work. In the presence of BQ as a quencher, an energy transfer occurs between the
Fig. 7. Fluorescence emission spectra of octameric compound 6 in 1,4-dioxane at different concentrations. Excitation wavelength ¼ 510 nm. (Insets: Emission intensity or wavelength versus concentration).
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Fig. 8. Fluorescence emission spectral changes 1,4-dioxane of compound 6 (C ¼ 7.50 109 M) by addition of different concentrations of BQ in toluene. [BQ] ¼ 0, 0.008, 0.016, 0.024, 0.032, 0.040 M and saturated with BQ.
by standard methods (elemental analysis, IR, 1H, 13C and 31P NMR spectroscopy, and mass spectra). The photophysical properties of 5 and 6 were investigated in 1,4-dioxane and compared with unsubstituted subphthalocyanine which is used as standard. The fluorescence emission bands of the newly synthesized compounds are shifted to higher wavelength region compared to emission band of unsubstituted subphthalocyanine. Subphthalocyanine which could be due to the non-covalent pep interactions between subphthalocyanine molecules on the hexameric or octameric compounds. The FF values of studied compounds (5 and 6) are lower than unsubstituted subphthalocyanine. However, these values are higher than the FF value (FF ¼ 0.067) of hexaphenylbenzene centered hexameric subphthalocyanine derivative in 1,4-dioxane as previously reported [20]. As a result, 5 and 6 could be good candidates for advanced materials such as light emitting electroluminescence devices due to the appropriate wavelength (blue-green region) and high fluorescence quantum yields values of these compounds. References
Fig. 9. SterneVolmer plots for BQ quenching of studied compounds in 1,4-dioxane (C ¼ 7.50 109 M). [BQ] ¼ 0, 0.008, 0.016, 0.024, 0.032, 0.040 M.
excited state of compounds (5 and 6) and the BQ. The fluorescence quenching of 5 and 6 by BQ in 1,4-dioxane obeyed the SterneVolmer kinetics. Fig. 8 exhibited the quenching of compound 6 by BQ in 1,4dioxane as an example for studied compounds. The slope of the plots shown at Fig. 9 gave the SterneVolmer constant (KSV) values. The KSV values were found 31.52 M1 for compound 5 and 40.09 M1 for compound 6 by quenching of BQ. Bimolecular quenching constant (kq) values were found to be 0.42 1013 M1.s1 for 5 and 11.42 1013 M1.s1 for 6 in 1,4-dioxane. The KSV and kq values of the cyclotetramer centered compound (6) are higher than cyclotrimer centered counterpart in 1,4-dioxane. The energy transfer between the excited cyclotetramer centered octameric derivative (6) and the BQ was more effective than between the excited cyclotrimer centered hexameric derivative 5. The kq values were of the order of 1013e1014 M1 s1, which exceed the proposed value of 1010 M1 s1 for diffusion-controlled (dynamic) quenching (according to the EinsteineSmoluchowski approximation at room temperature [31]). This, also, was an indication that the mechanism of BQ quenching by cyclophosphazene centered hexameric and octameric compounds (5 and 6) was not diffusion-controlled (i.e., not dynamic quenching, but static quenching). 4. Conclusion The synthesis and characterization of hexameric and octameric subphthalocyanine derivatives by using a cyclic phosphazene ring as a platform were reported. All compounds were fully characterized
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