In Situ Monitoring of the Synthesis of a Pyranine-Substituted

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J. Phys. Chem. B 2007, 111, 478-484

In Situ Monitoring of the Synthesis of a Pyranine-Substituted Phthalonitrile Derivative via the Steady-State Fluorescence Technique Ali Gelir,*,† I4 smail Yılmaz,‡ and Yas¸ar Yılmaz† Departments of Physics and Chemistry, Istanbul Technical UniVersity, Maslak 34469, Istanbul, Turkey ReceiVed: July 14, 2006; In Final Form: October 2, 2006

A new approach based on fluorescence quenching of a chromophore, pyranine (8-hydroxypyrene-1,3,6trisulfonic acid, trisodium salt, POH), was established for monitoring the synthesis of the pyranine-substituted phthalonitrile derivative POPht. 4-Nitrophthalonitrile binds to the POH through nitro elimination, by means of a nucleophilic aromatic substitution reaction in a basic medium, and forms phthalonitrile (Pht) bearing deprotonated POH units, Pht + POH f POPht. This binding process results in a considerable blue shift (from 515 to 430 nm) in the fluorescence emission spectra of POH. Besides, the fluorescence intensities of unreacted pyranines and POPht molecules in the reacting mixture decrease as they are quenched by nitrite appearing as one of the byproducts. POH, therefore, acts as both an aromatic substituent for Pht and a fluorescence probe for monitoring the substitution reaction. Thus, the change in the fluorescence spectra introduces a novel method for in situ monitoring of the synthesis of the phthalonitrile derivative POPht. This new material may have a potential to synthesize the phthalocyanine having an enhanced fluorescence property and solubility in an aqua medium that would result in a desired candidate molecule in many applications of phthalocyanines.

Introduction Synthetic tetrapyrolic compounds such as phthalocyanines have been proposed as convenient molecular models for the study of the physicochemical properties of naturally occurring tetrapyrolic macrocycles including porphyrins.1 Owing to their increased stability, improved spectroscopic characteristics, diverse coordination properties, and architectural flexibility, phthalocyanines have surpassed porphyrins in a number of applications. The immense potential of phthalocyanines in diverse fields makes them one of the most highly studied macrocyclic and coordination compounds.2-5 The physical, chemical, and electronic properties of phthalocyanines can be fine-tuned via the addition of an appropriate substitution on the benzene ring of macrocycles.6-19 Therefore, the synthesis of substituted precursors is vital in the preparation of new phthalocyanine derivatives with improved properties and designed chemical structures. One of the most important steps for preparation of a substituted phthalocyanine is the nucleophilic aromatic substitution reaction of 3- or 4-nitrophthalonitrile precursors with alcohol, thiol, and amine derivatives as nucleophiles, leading to tetrasubstituted phthalocyanines.20 It is wellknown that the phthalocyanines bearing aromatic molecules (fluorescence probes) such as napthalene, pyrene, etc. as the peripheral groups can be synthesized. The interaction of a fluorescence probe with a molecule may be realized on the changes of the fluorescence characteristics of the aromatic molecule. First, it may be the binding process where aromatic molecules, such as pyrene, naphthalene, or their derivatives, bind chemically or physically to the molecules, thereby causing a shift in their fluorescence spectra or a change * To whom correspondence should be addressed. E-mail: gelira@ itu.edu.tr. Fax: +90 212 285 6386. † Department of Physics. ‡ Department of Chemistry.

in the fluorescence intensity.21,22 The second process may be the change in the spectra or intensity of the aromatic molecules due to dynamic or static interactions with the molecules concerned, or with byproducts formed during a reaction.22,23 Third, the fluorescence characteristic of a fluorescence-probebonded molecule may be changed due to the interaction of this molecule with byproducts formed during a reaction. Thus, the change in the emission spectra of a fluorescence probe during a synthesis process may be related to the stages of a reaction. A recent study21 shows that the fluorescence spectrum of pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid, trisodium salt, POH), the molecular structure of which is shown in Figure 1a, shifts to lower wavelengths (from 515 to 406 nm) when it is bonded chemically to a vinyl group of the growing acrylamide polymer chains over a OH group. At the same time, three SO3groups on the probe molecule interact with protonated amide groups of macroradicals and thus cause a gradual red shift in the maximum of the short-wavelength peak between 406 and 430 nm.21 It is also well-known from the literature that POH is a highly pH sensitive fluorescence probe.24-28 When the pH of the environment is less than the pKa* of POH, the excited POH, (POH)*, decays to its ground state by fluorescence emission at about 445 nm. When the pH of the environment is higher than the pKa* of POH, it dissociates to (PO)* upon deprotonation and dissipates its energy by fluorescence emission at about 515 nm. Here, we propose that the fluorescence properties of these aromatic peripheral groups may be used for in situ monitoring of the synthesis of the phthalonitrile (Pht) derivatives, the most important precursor of the phthalocyanines. Although there exists a great deal of information about phthalocyanine precursors in the literature, there is no technique or method given so far for in situ monitoring of the reaction stages of the synthesis of Pht derivatives.

10.1021/jp064462w CCC: $37.00 © 2007 American Chemical Society Published on Web 12/16/2006

Synthesis of Pyranine-Substituted Phthalonitrile

J. Phys. Chem. B, Vol. 111, No. 2, 2007 479 Merck) as the initiator, and 6.5 × 10-5 mol of N,N′-methylenebisacrylamide (BIS) as the cross-linker in 10 mL of pure water. The samples were deoxygenated by bubbling nitrogen for 10 min, and gelation was performed in the thermally regulated cell holder (temperature was adjusted to 70 °C) of the spectrophotometer by adding 10 µL of N,N,N′,N′-tetramethylethylenediamine (TEMED; Merck). The fluorescence spectrum of the POPth added to the gel solution in trace amount was examined during the gelation process. Results and Discussion

Figure 1. Molecular structure (a) and emission spectra of POH (b) in DMSO (I), in water of pH 0.5 (II), and in water of pH 9 (III).

In this work, a new approach based on the fluorescence quenching of POPht due to nitrite formed as a byproduct during the reaction is introduced for in situ monitoring of the formation of a substituted Pht derivative, POPht. Experimental Section Chemicals and Reagents. All reagents and solvents were of reagent grade quality obtained from a commercial supplier. Dimethyl sulfoxide (DMSO) was freshly distilled after being dried over alumina. 4-Nitrophthalonitrile was prepared according to the procedure available in the literature.29 Instrumentation. The steady-state fluorescence (SSF) measurements were done by using a Perkin-Elmer LS50 luminescence spectrophotometer at the 90° position and with 10 nm slit widths. Hellma model quartz cuvettes with a 10 mm light path were used for these measurements. 1H NMR spectra were recorded on a Bruker 250 MHz spectrometer. The infrared (IR) spectra were recorded as KBr pellets on a Mattison 1000 spectrophotometer. Synthesis of Phthalonitrile Derivative POPht. To a threeneck round-bottom flask charged with 0.95 mmol (500 mg) of POH was added 5 mL of anhydrous DMSO, and then the resulting solution was stirred for 75 min under a nitrogen atmosphere at 45 °C. Then 1.2 mmol of Na2CO3 (121.3 mg) was added to the solution. Ten hours later, 1 mmol (165.1 mg) of 4-nitrophthalonitrile was added to the solution, and the reaction was maintained under the same conditions for 3 days. The reaction mixture was cooled to room temperature and then poured onto the solution of dichloromethane to give a precipitate which was filtered and washed with dichloromethane. This compound was highly soluble in water, methanol, and DMSO. Yield: 0.52 g (82%). Anal. Calcd for C24H9Na3O10S3N2 (Mw ) 650.51): C, 44.31; H, 1.40; N, 4.31. Found: C, 44.22; H, 1.43; N, 4.19. IR (KBr, νmax/cm-1): 2210 (-CtN), 1620, 1506, 1210 (O-S-O), 1050 (C-O-C). 1H NMR (d6-DMSO): δ (ppm) ) 7.45-7.70 (m, 3H, ArH), 7.90-8.33 and 9.10-9.35 (m, 6H, PrH). UV/vis (DMSO): λmax ) 420, 375, 360. Preparation of the Samples for Quenching Study. A 20 mL stock solution of 1 M NaNO2 with 10-4 M POH was prepared in DMSO. A total of 11 samples with different NaNO2 concentrations, ranging from 0.1 to 40 mM, were prepared by diluting the stock solution. Synthesis of the AAm Gel with POPth. A stock solution was prepared by dissolving 1 mol of AAm (acrylamide) as monomers, 7 × 10-5 mol of ammonium persulfate (APS;

Before following the synthesis process of POPht in terms of the change in the fluorescence spectra of the probe molecule POH, we examine the spectroscopic properties of POH for different conditions. This is necessary for interpreting the experimental data that were collected during the synthesis of POPht. Figure 1b shows the emission spectra of 10-4 M POH in DMSO and in water at different pH values. The excited POH, (POH)*, may decay to its ground state by fluorescence at 445 nm, or it may dissociate to (PO)* upon deprotonation, depending on the pH of the solution.30 The excited-state anion (PO)* may dissipate its energy by fluorescence emission at 515 nm when the pH is greater than 5 (peak III in Figure 1b), where the hydrogen ion concentration is insufficient to reprotonate (PO)* within its lifetime. The observable net result of POH upon excitation is, therefore, the formation of (PO)- and H+, when pH > pKa* + 2, where pKa* is the excited-state pKa of POH26 as previously discussed in detail in the literature.21,22,28,30 Therefore, peak II in Figure 1b (445 nm peak) corresponds to protonated POH molecules in very high acidic conditions. Peak I in Figure 1b represents the emission spectra of POH in DMSO in which deprotonation of the -OH group is not possible. Another important process which affects the emission spectra of POH is the binding process. When POH binds to any radical (R) over a -OH group and forms POR, the emission about 415 nm is observed.21,22,28 For example, typical fluorescence spectra of POH at different stages of the free radical polymerization of AAm are shown in Figure 2a. First, a new peak appears around 406 nm when the polymerization is initiated. Then, the intensity of the new peak (short-wavelength peak) increases and shifts simultaneously to some higher wavelengths (to 427 nm at the end of the polymerization) as the intensity of the 515 nm peak (long-wavelength peak) decreases during the course of polymerization as seen in Figure 2a. Here it should be noted that this blue shift is not due to the pH effect since the maximum of the fluorescence spectra of POH in pure water below pH 1 (445 nm peak given in Figure 1b) is clearly different from the shortwavelength peak (the peak that appears at 406 nm only if the polymerization is initiated and shifts gradually to 427 nm during the polymerization). Figure 2b presents these gradual red shifts in the shortwavelength peak, between 406 and 427 nm, as a function of the polymerization time for different AAm concentrations. This shift takes shorter times when the AAm concentration is increased. It was clearly shown21 that this gradual red shift is due to the binding of SO3- groups electrostatically to the PAAm chains over protonated amide groups. In brief, Figures 1 and 2 together show that the origins of the blue shifts in the emission spectra of POH are clearly different from one another; the shift from 515 to 406 nm is due to binding over OH, while the shift from 515 to 445 nm is due to the pH effect. Here, we introduce the synthesis procedure for the Pht derivative POPht. The synthesis procedure for the Pht derivative

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Figure 2. (a) Typical fluorescence spectra of POH at different stages of the free radical cross-linking copolymerization of 2 M AAm with BIS. (b) Shift in the maxima of the short-wavelength peak of the emission spectra of POH (from 406 to 427 nm) during the polymerization (0.3 M) and gelation (2 M) of PAAm.

Gelir et al.

Figure 3. Change in the fluorescence spectra of POH before (a) and after (b) the addition of Na2CO3.

SCHEME 1 : Schematic Representation of the Reaction of Pthalonitrile-Pyranine Binding, POH + Pht f POPht

Figure 4. Change in the fluorescence intensity from the 415 nm (open circles) and 520 nm (filled circles) peaks during the dissolving time of Na2CO3 in DMSO with POH.

containing a POH unit (POH + Pht f POPht) was initiated by a base-catalyzed nucleophilic aromatic nitro displacement of 4-nitrophthalonitrile with POH in DMSO. Scheme 1 shows the schematic representation of the suggested reaction, POH f POPht. First, the synthesis was started by dissolving POH in DMSO. The fluorescence spectra of this solution were recorded during 75 min, before the addition of Na2CO3 and 4-nitrophthalonitrile. During this time interval, no change in the emission spectra of the POH was observed. Then, Na2CO3 alone was added to the solution. After addition of Na2CO3, the emission spectra changed considerably as seen in Figure 3, where the peak at 430 nm was split into two peaks, 430 and 520 nm. The splitting process lasted 2 h. This split in the emission peak is probably due to the change in the pH of the solution, because the dissolution of Na2CO3

causes a considerable increase in the pH of the solution. As the pH of the solution is increased, the deprotonation rate and, thus, the probability of emission from the (PO)* (emission at 520 nm) state of the pyranine increases, as discussed in the above paragraphs. The change in the maximum intensities corresponding to 430 and 520 nm is plotted in Figure 4 versus the splitting time. An additional experiment to determine the solubility of Na2CO3 in DMSO without POH was performed using the atomic absorption technique (AAT) to be able to explain the changes in the emission spectra as a result of dissolved Na2CO3. The concentration of sodium ions (Na+) measured versus dissolving time is plotted in Figure 5, where the Na+ concentration changes almost linearly with two different slopes with time, which does not fit Figure 4.

Synthesis of Pyranine-Substituted Phthalonitrile

Figure 5. Variation of the sodium concentration during the dissolution of Na2CO3 in DMSO without POH by using the AAT.

Figure 6. Change in the fluorescence intensities corresponding to the maxima of the 415 nm (filled cicles) and 515 nm (open circles) peaks during the formation of POPht suggested in Scheme 1.

Comparison of Figures 4 and 5 shows the fact that the solution including POH accelerates the dissolution rate of Na2CO3. This may be due to the decrease in the effective concentration of Na+ ions in the solution including POH, considering that POH favors replacement of the H+ ion with a Na+ ion. Having finished the splitting process, 4-nitrophthalonitrile was added to the solution (POH and Na2CO3 dissolved previously in DMSO), and the reaction was kept under a nitrogen atmosphere at 45 °C for 3 days. During this time the maxima of the intensities of both 430 and 520 nm peaks changed. These changes are plotted in Figure 6 as a function of reaction time. As seen in the figure, the 520 nm peak intensity decreases during the reaction time, but the 430 nm peak intensity first increases up to a certain time and then decreases almost to zero at the end of the reaction. Here, it should be noted that there exist some significant variations in the intensities, especially in the

J. Phys. Chem. B, Vol. 111, No. 2, 2007 481

Figure 7. Stern-Volmer plot for the quenching of POH.

first stage of the reaction, besides the apparent trend of smoothing changes. Two main behaviors can be deduced from Figure 6. (i) The intensity of the emission at the 430 nm peak increases as the amount of POPht increases; thus, the intensity of the 520 nm peak decreases. (ii) Both intensities corresponding to 430 and 520 nm peaks are quenched by means of nitrite produced as a byproduct upon POPht formation. A detailed explanation will be given in the following paragraphs. When Na2CO3 is added, at the first step of the synthesis, to the DMSO including POH, PONa is formed in equilibrium with POH as expected. In the second step, i.e., when Pht is added to the solution including both POH and PONa, Pht molecules bind to PONa to form POPht, by eliminating NaNO2 as mentioned in Scheme 1. The fact that the amount of PONa is decreased in this way shifts the equilibrium. That is, the Na2CO3 molecules that are available in the reacting sample restart to dissolve and cause the POH molecules remaining in the solution at time t to be deprotonated, thus resulting in formation of additional PONa molecules. Regeneration of these additional PONa molecules may cause an increase in pH, to some extent, resulting in an increase in the intensity of the 520 nm peak and a decrease in the 430 nm peak. It is expected that this process should be impressive especially for the initial stages of the reaction due to the availability of Na2CO3 and POH molecules. As seen from the inset of Figure 6, these alterations are opposite each other (when the 430 nm peak increases, the 520 nm peak decreases and vice versa), and more effective in the initial stage of the reaction as is expected. Here, we discuss the quenching effect of the nitrite on the fluorescence intensities of POH and POPht. We observed that the nitrite ion formed as a byproduct is a good quencher for both POH and POPht. This point was proved with an additional experiment as explained below. There are different studies on the detection of the nitrite by using the quenching property on different aromatic molecules in different solutions.31-35 Here, we examined the quenching effect of the nitrite on the POH by changing the concentration of nitrite between 0.1 and 40 mM in DMSO with a 10-4 M fixed POH concentration and measuring the fluorescence intensity of the POH for each sample. In Figure 7, the ratio of the fluorescence intensities of POH with (I) and without (I0)

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quencher (nitrite), I0/I, versus nitrite concentration is presented in the range of 0-40 mM. Fitting the experimental data in Figure 7 to the relation

I0 τ 0 ) ) 1 + KSV[Q] I τ

(1)

gives a Stern-Volmer coefficient, KSV, around 0.0417 M-1. In the above relation, [Q] is the concentration of the quencher and τ and τ0 are the lifetimes of the excited state with and without quencher, respectively. Using ∼5 ns for the lifetime of pyranine,24 the kq value was calculated to be around 0.008 × 109 L mol-1 s-1. This value is in the low diffusion limit. The fact that kq is in the low diffusion limit is due to the repulsive interactions between SO3- groups on pyranine and NO2- molecules, as has been shown with a similar work done on the quenching of tryptophan.36 It seems that the similar mechanism in ref 36 may be responsible for the quenching of pyranine due to the attack of NO2- ions on the aromatic rings of pyranine. Moreover, the quenching of pyranine may be more favorable as compared to that of tryptophan since it includes four benzene rings and thus has a bigger impact parameter. Here it should be noted that although the intensity ratio is quenched linearly with increased nitrite content as shown in Figure 7, the fluorescence intensities during the synthesis vary with changing rates in exponential-like forms as seen in Figure 6. When Figures 6 and 7 are interpreted together, these points can be explained as follows. During the reaction both the product formation and fluorescence quenching of the product and other units, including pure POH, modified pyranine PONa, and the product POPht, proceed simultaneously. The 520 nm peak decreases since these two effects operate in the same direction, decreasing the number of POH molecules and quenching of them by nitrite. However, the 430 nm peak increases due to the product formation, Pht + POH f POPht, but it decreases as the concentration of byproduct, nitrite (as a quencher), increases; that is, the two mechanisms operate in opposite directions. Therefore, the intensity of the 430 nm peak first increases up to a certain time and then decreases as seen in Figure 6. Before that certain time product formation is dominant, but after that time the quenching effect due to the increased nitrite surmounts the intensity due to product formation. The linear behavior of I0/I with respect to the quencher concentration is not sufficient to decide certainly the type of quenching mechanism, dynamic or static. As mentioned above, we expect that the quenching mechanism should be dynamic. But some additional measurements must be done to determine the exact nature of the quenching mechanism,26,27 such as the fluorescence lifetime, viscosity, and temperature dependence of the fluorescence emission. This point is out of the scope of this pape and may be considered in a future work. Now we introduce an additional experiment performed to be able to verify the desired product (POPht). We carried out crosslinking copolymerization of AAm-BIS including POPht in a small amount and monitored the fluorescence spectrum of POPht during the gelation process. The change in the fluorescence spectra during the gelation process is given in Figure 8. No shift in the emission spectra of POPht was observed during the gelation. Only the intensity of the 430 nm peak increased due to the increasing viscosity of the gelling system. This behavior is completely different from the behavior of the usual AAm gelation process with POH as given in Figure

Figure 8. Change in the fluorescence spectra of POPht during the cross-linking copolymerization of AAm-BIS. The maximum intensity increases with time.

Figure 9. IR spectra of POH (a) and POPht (b).

2, discussed in the third paragraph of this section and in the literature.21,22 This gelation experiment with POPht indicates that there is no considerable amount of POH in the solution and that the products (POPht) can never give off the radical (Pht) in water; all the intensity comes from the excited POPht (POPht*), which emits at 430 nm. This observation clearly indicates that the reaction proposed in Scheme 1 occurred in a good yield. Figure 9 shows the IR spectrum of POPht. As seen in this figure, the characteristic band at 2238 cm-1 represents the CtN vibrations in POPht and the broad peak at 3300 cm-1 corresponding to OH vibrations disappeared after formation of the ether derivative. The characteristic vibration peaks of ether and sulfonyl groups on POH appeared at 1050 and 1200 cm-1, respectively. In the 1H NMR analysis of the compound in

Synthesis of Pyranine-Substituted Phthalonitrile

J. Phys. Chem. B, Vol. 111, No. 2, 2007 483 Acknowledgment. The support of the Research Fund of the Technical University of Istanbul and State Planning Organization (DPT, Project No. 90177) is gratefully acknowledged. This work has been supported, in part, by the Turkish Academy of Sciences in the framework of the Young Scientist Award Program (Grant EA-TU ¨ BA- GEB_P/2001-1-1). Y.Y. and A.G. thank TUBITAK for financial support in the frame of Grant TBAG-HD/14 (105T029). We thank Mustafa O ¨ zcan and So¨nmez Arslan for their contributions. References and Notes

Figure 10. 1H NMR spectra of POH (a) and POPht (b).

deuteriated DMSO, the molecule showed a characteristic chemical shift at 7.45-9.35 ppm belonging to deshielded aromatic protons. The characteristic OH peak of POH at 10.80 ppm disappeared on formation of POPht as can be seen in Figure 10. Conclusion In this study we showed that the formation POPht by means of a nucleophilic aromatic substitution reaction can be followed in real time, by using the fluorescence quenching of POPht by nitrite appearing as a byproduct during the synthesis. The synthesized unit (POPht) could be used as a peripheral unit of the phthalocyanine having an enhanced fluorescence property and solubility in an aqua medium. Phthalocyanines are used for many applications, among which are dyes and photoconducting agents in photocopying devices,37 chemical sensors,38 photosensors,39 electrochromism agents,40 liquid crystals,41 nonlinear optical materials,42 and photovoltaic cells.43 It can be expected that new kinds of aromatics having high quantum yield side groups will increase the usage of the phthalocyanines in the related fields of application. For example, for polymer-based photovoltaic cells POPht may play a crucial role. Since POPht is able to bind to some kinds of polymers via SO3- side units during the polymerization, it can be possible to prepare POPht-templated polymeric gels. Thus, it may be possible to adjust the electrical properties of photovoltaic cells with the swelling degree of the polymeric gels. Phthalocyanine formation can be performed by a cyclotetramerization reaction through CN groups of POPht. Our future plan is to synthesize phthalocyanine from these pyranine-bonded phthalonitriles (POPht) and to observe the activities of the phthalocyanines by using the fluorescence characteristics of them. The same technique can also be applied for monitoring all kinds of reactions in which one of the byproducts is a quencher for the fluorescence molecules included in the reaction vessel in trace amounts.

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