Anthracene Fluorescence Quenching by a Tetrakis (Ketocarboxamide) Cavitand

This is an electronic reprint of the original article. This reprint may differ from the original in pagination and typographic detail. Author(s): Title: Year: Version: Please cite the original version: All material supplied via JYX is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Anthracene Fluorescence Quenching by a Tetrakis (Ketocarboxamide) Cavitand Janosi, Tibor Zoltan; Korppi-Tommola, Jouko; Csok, Zsolt; Kollar, Laszlo; Myllyperkiö, Pasi; Erostyak, Janos Janosi, T. Z., Korppi-Tommola, J., Csok, Z., Kollar, L., Myllyperkiö, P., & Erostyak, J. (2014). Anthracene Fluorescence Quenching by a Tetrakis (Ketocarboxamide) Cavitand. Journal of spectroscopy, 2014, Article 708739. https://doi.org/10.1155/2014/708739 2014 Research Article Anthracene Fluorescence Quenching by a Tetrakis (Ketocarboxamide) Cavitand Tibor Zoltan Janosi,1,2 Jouko Korppi-Tommola,3Zsolt Csok,4,5 Laszlo Kollar,4,5 Pasi Myllyperkio,3and Janos Erostyak1,2 1Institute of Physics, University of Pecs, Hungary 2Szentagothai Research Centre, Spectroscopy Research Group, University of Pecs, Ifj´ us´ ag´ ut 6, H-7624 P´ ecs, Hungary 3Nanoscience Center, University of Jyvaskyla, Finland 4Department of Inorganic Chemistry, University of Pecs, Hungary 5P´ ecs Research Group for Selective Syntheses, Hungarian Academy of Sciences, Hungary Correspondence should be addressed to Tibor Zoltan Janosi; [email protected]tk.pte.hu Received 12 June 2014; Accepted 8 July 2014; Published 6 August 2014 Academic Editor: Renata Diniz Copyright © 2014 Tibor Zoltan Janosi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Quenching of both fluorescence lifetime and fluorescence intensity of anthracene was investigated in the presence of a newly derived tetrakis (ketocarboxamide) cavitand at various concentrations. Time-correlated single photon counting method was applied for the lifetime measurements. A clear correlation between the fluorescence lifetime of anthracene as a function of cavitand concentration in dimethylformamide solution was observed. The bimolecular collisional quenching constant was derived from the decrease of lifetime. Fluorescence intensity was measured in the emission wavelength region around 400 nm as a result of excitation at 280 nm. Effective quenching was observed in the presence of the cavitand. The obtained Stern-Volmer plot displayed upward curvature. The results did not follow even extended Stern-Volmer behavior, often used to describe deviations from static bimolecular quenching. To explain our results we adopted the Smoluchowski model and obtained a reasonable estimate for the molecular radius of the cavitand in solution. 1. Introduction Cavitands [1] are a class of molecules that contain a conformationally rigid cavity. As a result of their shape, the cavitands and the related bowl-shaped molecules have been used as hosts in host-guest complexes. They have become increasingly important in supramolecular chemistry and nanochemistry due to their potential in applications such as sensors, nanoreactors, and drug delivery systems [2–6]. As the donor-acceptor pair is formed, the charge distributions of the donor and the acceptor are modified and are different from those of the isolated molecules. Fluorescence spectroscopy—both static and time resolved—may be used to study such modifications and even bring evidence of donoracceptor pair formation. In the present study, the interaction between a tetrakis (ketocarboxamide) cavitand (1)(Figure 1) and anthracene was investigated in order to test the ability of (1)totransport molecules selectively. (1) was synthesized according to the procedure we have described before [7]. In short, using piperidine and the corresponding tetraiodocavitand under high-pressure (90 bar CO pressure) palladium-catalyzed aminocarbonylation conditions resulted in excellent chemoselectivities towards this tetrasubstituted, “double-carbonylated” compound. The selective and highly variable functionalization of the basic deepened cavitand skeleton is a prerequisite for any further applications including molecular recognition and selective transport processes [8]. It was estimated that the relatively small circumference of anthracene would allow it to penetrate into the host cavity. Fluorescence properties of anthracene in several solvents [9– 11] have been studied hence forming a solid background for observations of noncollisional—that is, due to complex Hindawi Publishing Corporation Journal of Spectroscopy Volume 2014, Article ID 708739, 8 pages http://dx.doi.org/10.1155/2014/708739 2Journal of Spectroscopy formation—quenching behavior in solutions where guest and the host were present at varying relative concentrations. Since there are several molecular interactions such as vibronic coupling, energy transfer, and conformational changes that may serve as a source of fluorescence quenching, care was takeninanalyzingthequenchingresultswithtwodifferent models, using the classical Stern-Volmer plots as well as the Smoluchowski-type quenching model [12]. 2. Material and Methods The tetrakis (ketocarboxamide) cavitand (1) was synthesized as previously described [7]. Its molecular structure is presented in Figure 1. Both anthracene and 1weredissolvedinspectroscopic grade N,N-dimethylformamide (DMF) delivered by Sigma- Aldrich (product number: 154814). The concentration of anthracene solution was kept constantat1mM.Theconcentrationsof1were10,20,40,60,80, 100, 150, and 200 𝜇M. The ultraviolet and visible absorption were measured in a 1 mm quartz cuvette. A fluorescence spectrometer (PerkinElmer LS 55) with xenon lamp was used to obtain fluorescence spectra. Time-correlated single photon counting (TCSPC) method was used to obtain the fluorescence lifetimes of the solutions. TCSPC measurements were performed with a commercial PicoQuant HydraHarp 400 TCSPC data acquisition system. PicoQuant PLS290 laser source with ∼900 ps pulse duration served as an excitation source. The lifetimes were estimated using least squares fitting technique with one-exponential decay model. 3. Results and Discussion 3.1. Absorption. Figure 2 shows the measured absorption spectra of pure 1(dotted line, 𝑐 = 200𝜇M) and pure anthracene (solid line, 𝑐=1mM). The dashed lines on Figure 2 represent the absorption of the solution of anthracene and cavitand in various concentrations. Itturnedoutthatthetotalabsorptionofthesample was a linear combination of the absorbances of the two constituents. This means that, in solution, the two molecules do have only a weak interaction with each other or even no interaction. In complexation one would expect visible shifts in the spectra. 3.2. Fluorescence Lifetime. Figure 3 shows the fluorescence lifetimes of anthracene in the mixtures as a function of the concentration of the cavitand quencher (1). The excitation wavelength was 290 nm, while the emission was detected at 404 nm. There is a remarkable and nearly linear decrease in the lifetime as the function of the concentration of 1.Itis worth noting that the scale of 𝑦-axisstartsfrom3.70,sothe change—even at the highest concentration—is less than 4%. Shorter fluorescence lifetimes were observed in the presence of 1that indicates some kind of dynamic quenching effect between the anthracene and 1. In order to gain more information from this data, we created the Stern-Volmer plot (Figure 4). 3.3. Collisional Quenching. When an additional constituent is added into a solution of a fluorescent species normally fluorescence quenching occurs. This is seen as reduction of fluorescence intensity and shortening of fluorescence lifetime. One of known quenching mechanisms is collisional quenching that not only reduces the spectral intensity of fluorescence but also shortens the lifetime. Collisional quenching of the fluorescence lifetime is described by the Stern-Volmer equation [13]: 𝜏0 𝜏−1=𝑘𝐶𝜏0𝑄=𝐾𝐶𝑄, (1) where Qis the quencher’s concentration, 𝜏0and 𝜏are, respectively, the fluorescence lifetimes in the absence and inthepresenceofquencher,and𝑘𝐶is the bimolecular quenching constant. The bimolecular quenching constant reflects the accessibility of the quencher to the excited state molecules. Plotting (𝜏0/𝜏−1)as a function of the concentration yields alinearplotwithaslopeequalto𝐾𝐶. The linear fitting on the data of the present study results in the following constants: 𝐾𝐶is 208±9M−1 while the bimolecular quenching constant is 𝑘𝐶=5.37±0.23⋅1010 M−1 s−1. Pure collisional quenching of anthracene in different solvents by various quenchers has been previously reported [14,15]. The value of 𝐾𝐶was earlier measured [14]intheconcentration range of 10 and 30M−1 in n-heptane, n-hexane, and CCl4in the presence of fullerene. In another work [15]𝐾𝐶was obtained in the range of 2–106 M−1. Here the solvents were toluene, methanol, and diethylene glycol, while triethylamine and 4-butylaniline were used as quencher. Collisional frequency (𝑓) of a quencher with the fluorophore may be described by the following equation: 𝑓=𝑘0⋅𝑄, (2) where 𝑘0is the diffusion-controlled bimolecular rate constant, [𝑘0]=M−1 s−1. This constant can be obtained from the Smoluchowski equation [16,17] that describes the diffusion of molecules that arelargerthanthesolventmolecules.Thisisthecaseforthe anthracene and 1when dissolved in DMF. Consider 𝑘0=4𝜋𝑁 𝐴𝑅𝐷, (3) where 𝑁𝐴is the Avogadro number (𝑁𝐴=6⋅1023 mol−1), 𝑅 is the collision radius, and 𝐷denotes the sum of the diffusion coefficients of cavitand 1(𝐷𝐶) and anthracene (𝐷𝐴). It is usually assumed that the collision radius is equal to the sum of molecular radii [18]oftheanthracene(𝑅𝐴)and1(𝑅𝐶). We get 𝑘0=4𝜋𝑁 𝐴(𝑅𝐴+𝑅𝐶)(𝐷𝐴+𝐷𝐶).(4) Diffusion coefficients can be calculated by using the Stokes-Einstein equation [19]: 𝐷= 𝑘𝑇 6𝜋𝜂𝑅,(5) Journal of Spectroscopy 3 O OO O O OOO HH HH OOO O CO C O N C C O O NC O N CO C O N CO H2C CH3 CH2CH2 H2C CH3CH3 CH3 Figure 1: Structure of 1. 2.5 2.0 0.5 0.0 1.5 1.0 Absorbance 𝜆(nm) 260 280 300 320 340 360 380 400 INC 1mM anthracene 200 𝜇M tetrakis (ketocarboxamide) cavitand 1mM anthracene +10–200 𝜇M tetrakis (ketocarboxamide) cavitand Figure 2: Absorption spectrum of anthracene (1 mM), 1(200 𝜇M) and anthracene in the presence of different concentrations of 1(the arrow indicates increasing concentration of 1). where 𝑘 = 1.38⋅10−23 J/K is the Boltzmann’s constant, 𝑅is the radius of the molecule, 𝜂=0.92Pa⋅sistheviscosityofthe DMF, and 𝑇=293K is the absolute temperature. With these constants we get the following equation for 𝑘0: 𝑘0=1.758⋅109M−1 s−1 (2+ 𝑅𝐴 𝑅𝐶+𝑅𝐶 𝑅𝐴). (6) The diameter of the anthracene molecule may be approximatedbyassumingittohavethesamevolumeasitsvander Walls volume 𝑅𝐴=0.4nm [20]. From a molecular modeling calculation we estimate the radius of the cavitand molecule as 3nm. With the parameters presented above the estimated value of the diffusion-controlled bimolecular rate constant becomes 1.5⋅1010 M−1 s−1. The measured bimolecular constant (5.37⋅1010 M−1 s−1)is of the same order of magnitude as the theoretically estimated value, but clearly higher. Since the experimental rate constant 4Journal of Spectroscopy 3.90 3.85 3.80 3.75 3.70 𝜏(ns) 050 100 150 200 Tetrakis (ketocarboxamide) cavitand concentration (𝜇M) 𝜆EXC :290nm 𝜆EM :404nm Figure 3: Anthracene fluorescence lifetime in the presence of different concentrations of 1. is much higher than the constant estimated from diffusion properties, some additional quenching mechanisms must be present [21]. 3.4. Fluorescence Intensity. Also the intensity dependence of the anthracene fluorescence in presence of 1was studied. The measured fluorescence intensities were corrected in order to eliminate the effect of apparent quenching, which is the decline of excitation intensity due the absorption of 1at the excitation wavelength. This effect was compensated by the following consideration. The sample is characterized by the concentration (𝑐)and the molar extinction coefficient (𝜀)inacuvettewithanoptical path length of 𝑑.Theinitialintensityofthelightis𝐼0. According to the Beer-Lambert law, the intensity of light as a function of optical path (𝑥)is 𝐼𝑥=𝐼 010−𝜀⋅𝑐⋅𝑥.(7) We get the effective excitation intensity (𝐹eff)bycalculating the integral of the intensity along the optical path length: 𝐹eff =∫𝑑 0𝐼010−𝜀⋅𝑐⋅𝑥𝑑𝑥 =−𝐼0 𝜀⋅𝑐⋅ln 10[10−𝜀⋅𝑐⋅𝑥]𝑑 0 =−𝐼0 𝜀⋅𝑐⋅ln 10(10−𝜀⋅𝑐⋅𝑑 −10−𝜀⋅𝑐⋅0) =𝐹 eff =−𝐼0 𝜀⋅𝑐⋅ln 10(10−𝜀⋅𝑐⋅𝑑 −1) 𝐹eff =𝐼 01−10−𝜀⋅𝑐⋅𝑑 𝜀⋅𝑐⋅ln 10. (8) The theoretical excitation intensity for a totally transparent solution would be 𝐹𝑡=𝐼 0⋅𝑑. (9) The fluorescence excitation intensity correction factor is defined as 𝑓= 𝐹𝑡 𝐹eff =𝜀⋅𝑐⋅𝑑⋅ln 10 1−10−𝜀⋅𝑐⋅𝑑 .(10) For more details, see [22,23]. In the present study reference fluorescence intensity (𝐼0) was first measured without the quencher and then at different quencher concentrations. The corrected emission spectraofanthraceneattwoexcitationwavelengths(280nm and360nm)havebeenplottedinFigure 5.Therewasno observable change in the fluorescence intensity when the anthracene was excited at 360 nm (upper-right corner of Figure 5). On the main graph, it can be seen that the emission induced by 280 nm light was strongly diminished in the presence of 1.Theshapeofthespectrawasnotalteredby the quenching; that is, the intensity decrease was wavelengthindependent. To create the Stern-Volmer plot, the measured 𝐼and 𝐼0 values at the peak intensity at 402 nm were used. If we assume that the fluorophore is quenched both by thecollisionandbythecomplexformationwiththesame quencher, the modified Stern-Volmer equation [12] describes the fractional fluorescence remaining (𝐼/𝐼0). Consider 𝐼0 𝐼=(1+𝐾𝐶𝑄)(1+𝐾𝑆𝑄),(11) where Qis the quencher’s concentration and 𝐾𝐶and 𝐾𝑆are the collisional and static quenching constants, respectively. Rearranging (11)weget (𝐼0/𝐼−1) 𝑄=(𝐾𝐶+𝐾𝑆)+(𝐾𝐶⋅𝐾𝑆)𝑄. (12) As it was mentioned earlier, the collisional quenching appears in both the fluorescence lifetime and the fluorescence intensityandcausesasamedecreaseinbothofthem. Combining (1)and(11) we can separate the contribution of the static part from the total quenching: 𝐼0 𝐼=𝜏0 𝜏(1+𝐾𝑆𝑄). (13) Figure 6 shows the Stern-Volmer plots for the quenching of the anthracene fluorescence intensity in the presence of 1. The open symbols indicate the total quenching, that is, 𝐼0/𝐼−1, while the filled symbols represent the quenching due to complex formation according to (13), (𝐼0/𝐼)(𝜏/𝜏0)−1. A positive deviation from the linearity can be observed; theplotremainsconcavetowardthe𝑌-axis. This upward curvature of the apparent quenching shows that (11)isnot applicable for the analysis of this quenching. Furthermore, if we plot the (𝐼0/𝐼−1)/𝑄as the function of cavitand’s concentration, the resulted curve is not a straight line as it should be according to (12). These facts prove that modified Stern-Volmer equation (11) does not describe the observed quenching process properly. Journal of Spectroscopy 5 0.04 0.03 0.02 0.01 0.00 050 100 150 200 Tetrakis (ketocarboxamide) cavitand concentration (𝜇M) (𝜏0/𝜏) − 1 R2= 0.982 (𝜏0/𝜏) − 1 = 2.08 ∗ 10−4c/𝜇M Figure 4: Stern-Volmer plot for the fluorescence lifetime of anthracene. 1mM anthracene 1mM anthracene and 10–200 𝜇M tetrakis 340 360 380 400 420 440 460 480 500 𝜆(nm) 0 100 200 300 400 500 600 700 800 Fluorescence intensity (AU) 0 50 100 150 200 𝜆(nm) 380 400 420 440 460 480 500 1mM anthracene 1mM anthracene and 10–200 𝜇M tetra 𝜆EXC = 280 nm Fluorescence intensity (AU) (amide) cavitand (ketocarboxamide) cavitand 𝜆EXC = 360 nm Figure 5: Corrected fluorescence spectra of anthracene in the absence and in the presence of different concentrations (10–200 𝜇M) of the cavitand quencher (1). The excitation wavelength was 𝜆EXC = 280nm for the main and 𝜆EXC =360nm in the graph on the upperright corner. 3.5. Sphere of Action Quenching. For further analysis, we used thesphereofactionquenchingmodel[24]. This model is basedonsimpleandintuitiveassumptiononthemolecular level of the process. The instantaneous quenching takes place if the quencher and the fluorophore are in contact or very close to each other at the moment when the fluorescent molecule happens to be excited. In these encounters only a fraction of the excited fluorophore molecules is quenched by the collision. We assume random distribution of the quencher and the fluorophore at the moment of excitation. If the solutions are diluted, then we can use Poisson distribution to describe the probability of the quencher and the fluorophore molecules located close enough to each other for the quenching to occur. It is important to highlight that RET is not involved in the quenching processes, as there is no remarkable overlap between the fluorescence spectra of quenched and the absorption spectra of quencher. If 𝑊is the fraction of the excited state molecules quenched by some dynamic effect, then 𝐼0 𝐼=1+𝐾SV𝑄 𝑊,(14) where 𝑄is the quencher’s concentration and 𝐾SV is the Stern- Volmer quenching constants. The probability of 𝑛quencher molecules being located in volume 𝑉is precisely described by the binomial distribution. If the number of quencher molecules is sufficiently large and the average number of quencher molecules in the investigated volume is low enough, then the Poisson distribution is a good approximation of the binomial distribution. According to the Poisson distribution, the probability of 𝑛quencher molecules located in volume 𝑉is 𝑃(𝑛,𝑉)=𝜆𝑛 𝑉 𝑛!𝑒−𝜆𝑉,(15) where 𝜆𝑉is the average number of quenchers in volume 𝑉: 𝜆𝑉=𝑄⋅𝑉⋅𝑁𝐴.(16) We use the Poisson distribution since the present case represents a low 𝜆𝑉value. 𝜆𝑉is often expressed in percent; for example, in our case 𝜆𝑉=4% would mean that there is one cavitand molecule in volume 𝑉surrounded by 25 anthracene molecules. The probability that there is no quencher near a fluorophore is 𝑃(0,𝑉)=𝑒−𝜆𝑉.(17) This probability is equal to the fraction 𝑊of the excited fluorophore molecules that are quenched by collisions. Consider 𝑊=𝑒−𝜆𝑉.(18) 6Journal of Spectroscopy Fluorescence intensity quenching 0.5 0.4 0.3 0.2 0.0 050 100 150 200 Tetrakis (ketocarboxamide) cavitand concentration (𝜇M) R2= 0.95 Apparent static quenching [= (I0/I)(𝜏/𝜏0)−1] Total quenching [= (I0/I) − 1] Linear fit for apparent static quenching 0.1 Figure 6: Stern-Volmer plot for the total and for the apparent static quenching of anthracene by 1. From (14)and(18), we get 𝐼0 𝐼=(1+𝐾SV𝑄)𝑒𝑄⋅𝑉⋅𝑁𝐴.(19) If 𝜆𝑉is small, that is, 𝑄and 𝑉are small enough, we can use linear approximation: 𝑊=𝑒−𝜆𝑉≈1−𝜆𝑉.(20) Now (14)mayberewritten[25]: (1−𝐼/𝐼0) 𝑄=𝐾SV 𝐼 𝐼0+1−𝑊 𝑄=𝐾SV 𝐼 𝐼0+𝑉⋅𝑁𝐴.(21) Thus 𝑉canbederivedfromtheanalysisofthe(1 − (𝐼/𝐼0))/𝑄 plot (Figure 6) as the function of 𝐼/𝐼0,andit represents the active volume surrounding the quencher. The action radius can be calculated from the active volume (𝑉) as follows: 𝑉=4 3𝑟3𝜋. (22) The fitting (Figure 7)resultedin258±68M−1 for the value of 𝑉⋅𝑁𝐴; thus the calculated active volume is 43±11.3⋅10−23 l, whiletheestimatedactionradiusis4.68±0.4nm. This value is slightly higher than the sum of diameter of anthracene and 1and is likely the distance limit for the instantaneous quenching. It was previously mentioned that the use of Poisson distribution is limited by the value of 𝜆𝑉.Itisusually recommended to use Poisson distribution below the 𝜆𝑉value of 8%. In our case, the value of 𝜆𝑉at the highest quencher concentrationis5%.Thisisitsmaximumbecause𝜆𝑉is changing linearly with the quencher’s concentration. It means Table 1: Comparison of the constants derived from the sphere of action quenching model (𝐾SV is the Stern-Volmer constant and 𝑅is the action radius) [10,11]. Quencher 𝐾SV (M−1)𝑅(nm) Aniline 12–38 1.2–3.8 Allyl 2,4-dinitrophenyl ether 766–1410 0.61–0.72 11782 ±78 4.68 ±0.4 that our system fits this restriction, and the use of Poisson distribution is appropriate. The 𝑅2of fitting is 0.986, which means that the applied sphere of action quenching model describes the observed change in fluorescence intensity properly. Positive deviation from the Stern-Volmer plot at the anthracene quenching has been previously observed [9–11]. In two of these studies, aniline and allyl 2,4-dinitrophenyl ether induced the anthracene fluorescence quenching in various solvents, and the effects were described by the sphere of action quenching model. The calculated constants are compared in Table 1. The higher value of the action radius (4.68 ±0.4 nm) is in accordance with the bigger molecule size. The relatively high Stern-Volmer constant indicates that 1is able to effectively quench the anthracene fluorescence. 4. Conclusions In summary, it has been shown that a newly derived tetrakis (ketocarboxamide) cavitand causes simultaneous dynamic and prompt quenching of anthracene fluorescence. The dynamic quenching was explained by the collision mechanism, while the Smoluchowski model of sphere of action quenching was applied to describe the contribution of the prompt effect. The derived constants are in good agreement Journal of Spectroscopy 7 2400 2200 2000 1800 1600 1400 1200 1000 0.7 0.8 0.9 1.0 R2= 0.986 I/I0 (1 − I/I0)/Q = 1782 M−1 ∗I/I 0+258M−1 (1 − (I/I0))/Q (M−1) Figure 7: Plot of (1−(𝐼/𝐼0))/𝑄against 𝐼/𝐼0at different concentrations of 1. with the size of molecules, and they correlate well with the values previously reported in the literature for other quenchers. The measured quenching indicates weak molecular level interaction between the anthracene and the cavitand derivative 1;however,itdoesnotproveunambiguouslythe complex formation. 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