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Journal of Inclusion Phenomena and Macrocyclic Chemistry (2018) 90:51–60 https://doi.org/10.1007/s10847-017-0762-0

ORIGINAL ARTICLE

Spectral, electrochemical and computational investigations on the host–guest interaction of Coumarin-460 with p-sulfonatocalix[4]arene

Bosco Christin Maria Arputham Ashwin1 · Ramesh Kumar Chitumalla2 · Arulanandu Herculin Arun Baby1 · Joonkyung Jang2 · Paulpandian Muthu Mareeswaran1

Received: 6 June 2017 / Accepted: 1 November 2017 / Published online: 16 November 2017 © Springer Science+Business Media B.V., part of Springer Nature 2017

AbstractThe supramolecular host–guest investigation of Coumarin 460 (C460), a salient coumarin family dye molecule is studied with a noteworthy host molecule, p-sulfonatocalix[4]arene (p-SC4). The investigation is carried out by both experimental and theoretical approach. The binding affinity of C460 with p-SC4 is experimentally studied using absorption, emission, excited state lifetime and Cyclic Voltammetry methods. The binding constant is around 103 M− 1, which shows potent binding. The binding stoichiometry is 1:1. The binding orientations and binding energies are studied using computational simulations. The mode of binding is also established using NMR spectral techniques.

Keywords Calixarenes · Coumarin · Fluorescence · Cyclic voltammetry · Computational calculations

Introduction

The local environment in the vicinity of a fluorophore influ-ences the nature of fluorescence [1]. This aspect makes a fluorophore useful to study different physical aspects regarding that local environment. The change in the nature of fluorescence with respect to change in the local envi-ronment like polarity, viscosity in homogeneous systems; pore size, the pore volume in the heterogeneous systems; aggregation number, cavity size in the microheterogeneous systems explains dynamics in the local environment [1]. The change in fluorescence is an important parameter to

study the interaction of fluorescent dye with host systems [2]. The nature of cavity and nature of rim controls the entry of fluorescent guest within the cavity [3]. The host systems like cyclodextrins, cucurbiturils are rigid and the binding of fluorophore are controlled by the cavities, whereas in calix-arenes, the cavity is flexible. The flexible cavities easily accommodate the fluorophore irrespective of its size [4]. The facile rim modification renders the calixarenes comfortable to design target oriented host systems [5]. The ipso sulfona-tion of calixarenes results in sulfonatocalixarenes, which are the predominant category of water soluble calixarenes [6]. The fluorescent guest accommodation within the cavity of p-sulfonatocalixarenes (p-SC4) are well established aspect in host–guest chemistry [7–9].

Coumarins are important dye available in natural resources [10]. Coumarins have interesting optical properties and widely used as sensors, staining agent, and dye lasers [11, 12]. Cou-marins are classified into simple coumarins, furanocoumarins, pyronocoumarins and pyrone substituted coumarins [13]. The derivatives of coumarins have a variety of characters depends upon the substitutions present in the coumarin skeleton [14]. There are reports on coumarin derivatives to inhibit the carbon dioxide transport enzyme and anticancer activity [15, 16]. It is reported that the nervous activity is increased by coumarins [17, 18]. However, activities will differ depends on the substit-uents present coumarins. The optical properties of coumarins

Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10847-017-0762-0) contains supplementary material, which is available to authorized users.

* Joonkyung Jang jkjang@pusan.ac.kr

* Paulpandian Muthu Mareeswaran muthumareeswaran@gmail.com;

mareeswaran@alagappauniversity.ac.in

1 Department of Industrial Chemistry, Alagappa University, Karaikudi, Tamilnadu, India

2 Department of Nanoenergy Engineering, Pusan National University, Busan 46241, Republic of Korea

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are also tunable from blue to green (400–550 nm) with the substitution of electron accepting or withdrawing groups pre-sent in the core skeleton [19]. This is due to the intramolecular charge transfer. Therefore, these energy transfer experiments can be used as a spectroscopic ruler to calculate the distance between the fluorophores. The laser applications of coumarin dyes receives importance due to its optical tunablity [20]. The coumarin 460 has wide applications in dye lasers [21]. They are used in color producer in Eu3+/Tb3+ white light emitting diodes (WLEDs) [22]. The reports on change in optical prop-erties open various aspects of handling dyes as molecular encapsulation and nano-confinement using core–shell nano-particles [23]. Herein, we report the optical and electrochemi-cal studies of coumarin 460 (C460) with encapsulation of p-SC4. In addition to the experimental characterizations, we also have performed density functional theory (DFT) based quan-tum chemical calculations to study the interactions of C460 with p-SC4. The binding orientation and binding strength of the C460 with p-SC4 have been elucidated with the help of DFT simulations.

Experimental section

Materials

Coumarin 460 and t-butylcalix[4]arene are procured from Avra Synthesis and Sigma–Aldrich, respectively. A stock solu-tion of C460 is prepared in acetonitrile. Ultra-pure water (Mil-lipore) is used as a solvent throughout the study. The p-SC4 is synthesized using the reported porcedure [8, 24] and character-ized by 1H NMR spectral technique (Supporting Information).

Instruments

UV–Visible absorption spectrometric measurements and fluorescence emission measurements are performed by using SCHIMADZU UV-2401PC and JASCO FP-8200 Spectrofluo-rometer respectively at room temperature. Fluorescence decays are recorded using the time correlated single photon count-ing (TCSPC) method. Electrochemical studies are carried out using Auto lab electrochemical analyzer (GPES software). A classical three electrode cell assembly is used for the electro-chemical measurements. Cyclic Voltammetry measurements are carried out using glassy carbon electrode with diameter 3 mm as working electrode at proper potentials for each sam-ple with single cycle. The reference electrode is Ag/AgCl and platinum electrode as the counter electrode. All experiments are carried out at 30 ± 1 °C. The working electrode is polished well to a mirror with 0.05 μm alumina aqueous slurry, and washed with double distilled water before performing each experiment. The NMR spectral analysis is carried out using Bruker 500 MHz NMR Spectrometer. D2O:CD3CN (2:1)

mixture is used as a solver for 1H NMR titration and rotating frame nuclear overhauser effect spectroscopy (ROESY) tech-nique for equivalent mixture of C460 and p-SC4.

Binding constant and stoichiometry calculations using absorption spectral technique

For binding constant calculation, the concentration of cou-marin 460 is fixed as 1 × 10− 6 M, the concentration of the host, p-SC4 is varied from 1 × 10− 5 to 5 × 10− 4 M and the absorp-tion measurements are recorded for each sample. The binding constant value (Ka), of C460 with p-SC4 is evaluated with the aid of the Benesi–Hildebrand Eq. [25]. (Eq. 1)

Here, ∆A is the change in absorbance of the C460 on the addition of p-SC4. ∆ε is the difference in the molar extinction coefficient between the free C460 and C460–p-SC4 complex. The plot of 1/ΔA vs. 1/[p SC4] gives a straight line. By using the slope value of the line, the binding constant Ka is calcu-lated in solution.

Nonlinear regression analysis is an alternative and accurate approach to estimate binding constant by graphical method [26]. In the case of C460 and p-SC4, in which a 1:1 complex is suggested, the following equation is used [27] (Eq. 2).

The binding stoichiometry is evaluated by Job’s plot method. The concentration of C460 is varied from 1 × 10− 6 to 9 × 10− 6 M and the p-SC4 concentration is in the reverse order from 9 × 10− 6 to 1 × 10− 6 M. The plot of the mole fraction versus change in the absorbance Job’s plot. The free energy change, ΔG value also can be calculated from the binding con-stant, Ka value using the Eq. (3) [28],

Binding constant calculations using fluorescence spectral technique

The concentration of the C460 is fixed at 1 × 10− 6 M and the concentration of the p-SC4 is varied from 1 × 10− 5 to 5 × 10− 4 M. The emission spectrum of these mixtures are recorded in the absence and in the presence of p-SC4. The evaluation of the binding constant is based on the changes of fluorescence intensity with the change of p-SC4 concentration. The binding constant was calculated by using the modified Stern–Volmer equation, Eq. (4) [25, 29].

Here, I0 is the fluorescence intensity of guest molecule (C460) in the absence of host molecule (p-SC4), I is the fluo-rescence intensity in the presence of various concentrations

(1)1∕ΔA = 1∕KaΔε [p-SC4] + 1∕Δε[C460]

(2)A =A0 + A∞Ka[p-SC4]

1 + Ka[p-SC4]

(3)ΔG = −RT lnKa

(4)log[(

I0 − I)

∕I]

= n log[

p − SC4]

+ logKa

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of p-SC4, Ka is the binding constant and n is the stoichio-metric ratio.

The quenching constant, kq value also calculated by using the Stern–Volmer equation, Eq. (5) [30].

where, τ is the excited state lifetime of C460.

Excited state lifetime titration

The concentration of C460 is fixed at 1 × 10− 6 M and the concentration of p-SC4 is varied of four addition (1 × 10− 6, 2 × 10− 6, 3 × 10− 6 M) the excited state lifetime is measured for these concentrations.

Cyclic voltammetry studies

Coumarin 460 and p-SC4 are prepared in 1 × 10− 3 M concen-tration separately. A 5 ml solution of C460 is taken in a cell setup and cyclic voltammetry (CV) is performed. Then 1 ml of p-SC4 is added and sonicated for 15 min. This procedure is continued for maximum 5 ml of p-SC4. The above proce-dure is repeated by fixing 5 ml of p-SC4 and introducing 1 ml addition of coumarin 460 up to 5 ml. The CV spectral changes also useful to prove evidently the formation of the inclusion complex between C460 and p-SC4 in solution phase.

Computational details

All the DFT simulations reported in this paper were per-formed using the Gaussian 09 quantum chemical program [31]. The GaussView program has been used for the visuali-zation. The optimized geometries were subjected to vibra-tional frequency analysis to ensure no imaginary frequen-cies on the potential energy surface. M06-2X [32], a hybrid meta-GGA based Minnesota exchange–correlation functional (54% Hartree–Fock exchange) was used in combination with 6-31G(d) basis set. The functional M06-2X has been used successfully to study the non-covalent interactions. The geometry of p-SC4 has been retrieved from the available crystal structure. To study the interactions between C460 and p-SC4, we have modeled two systems in which the C460 is placed horizontally and vertically inside the cavity of p-SC4.

Results and discussion

The host guest interaction of C460 with p-SC4 success-fully investigated by absorption, emission, NMR spectral and electrochemical techniques. The structures of p-SC4 and C460 are shown in Chart 1.

(5)I0∕I = 1 + kq �[

p − SC4]

Absorption spectral studies

The absorption spectra of p-SC4 and C460 are shown in Figs. S1, S2 respectively. The absorption maximum of p-SC4 is around 210 and 280 nm. The C460 have the absorp-tion maximum at 380 nm. The p-SC4 is not having any absorption at this wavelength. Therefore, the absorption around 380 nm is suitable for calculating binding constant. The concentration of C460 is fixed, the concentration of p-SC4 is varied and the absorption spectra are recorded. The absorption titration is shown in Fig. 1.

The absorption of peak at 380 nm is increased upon increasing the concentration of p-SC4. This observa-tion emphasizes the presence of considerable interaction

Chart 1 Structures of p-SC4 and C460

Fig. 1 Absorption spectral titration of C460 (1 × 10− 6 M) in the absence and presence of p-SC4 (1 × 10− 5 to 5 × 10− 4 M)

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between p-SC4 and C460. The binding constant is calcu-lated using the absorption at 380 nm using Benesi–Hilde-brand method. The Benesi–Hildebrand equation (Eq. 1) and details of the calculation are given in the experimental section. The Benesi–Hildebrand plot is shown in Fig. S3. The calculated binding constant is 1.0 × 103 M− 1 (Table 1). Furthermore a nonlinear regression plot (Fig. 2) also con-structed to emphasis the binding constant. The estimated Ka value for the formation of the 1:1 C460–p-SC4 com-plex calculated from nonlinear regression plot by rearrang-ing Eq. 2 is 1.1 × 103 M− 1 (Table 1). Both the linear and nonlinear approaches result in similar binding constant values and shows good binding of p-SC4 with C460. The ΔG value calculated using the binding constant values and Eq. 3 are − 17.40 and − 17.64 kJ mol− 1 which confirms the binding is spontaneous.

The details of the Job’s plot experiment is given in the experimental section. The Job’s plot is shown in Fig. 3. The peak at 0.5 mol fraction confirms the 1:1 binding of p-SC4 with C460.

Fluorescence spectral studies

The fluorescence spectra of C460 is given in Fig. S4 excited at 380 nm and the fluorescence maximum is around 470 nm.

The p-SC4 does not possess fluorescent characteristics. Therefore, the fluorescence technique is used to study the interaction of p-SC4 with C460. The concentration of C460 is fixed, the concentration of p-SC4 is varied and the emis-sion spectra are recorded for all samples (Fig. 4). The experi-mental details are given elaborately in the experimental sec-tion. The quenching in the fluorescence intensity of C460 at 470 nm is observed upon increasing the concentration of p-SC4. The binding constant value is calculated using Modified Stern–Volmer equation (Eq. 4). The modified Stern–Volmer plot is given in Fig. S5. The binding constant for p-SC4 with C460 is 1.7 × 103 M− 1, which shows an effi-cient binding of p-SC4 with C460. The ΔG is calculated using Eq. 3 is − 18.76 kJ mol− 1. The values are collected in Table 1. The negative ΔG value of fluorescence study also confirms the spontaneous binding of p-SC4 with C460.

Table 1 Binding constant and binding energy of C460 with p-SC4 using absorption and emission techniques

Binding constant (M− 1) ΔG (kJ mol− 1)

Absorption (1.0 ± 0.06) × 103 (linear fit) − 17.40 ± 0.14(1.1 ± 0.02) × 103 (nonlinear fit) − 17.64 ± 0.09

Fluorescence (1.7 ± 0.02) × 103 − 18.76 ± 0.22

Fig. 2 Nonlinear regression plot for binding constant calculation

Fig. 3 Job’s plot for stoichiometric ratio calculation

Fig. 4 Emission spectral titration of C460 (1 × 10− 6 M) in the absence and presence of p-SC4 (1 × 10− 5 to 5 × 10− 4 M) (excited at 380 nm)

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The quenching constant is calculated using Stern–Volmer equation (Eq. 5). The Stern–Volmer plot is given in Fig. S6. The quenching constant value for C460 with p-SC4 is 3.5 × 1011 M− 1s− 1 and the Stern–Volmer constant, Ksv is 392.03. The efficient quenching constant confirms the pos-sibility of strong interaction between p-SC4 and C460. The quenching of fluorescence by means of encapsulation of p-SC4 is established previously [7, 28, 29, 33–35]. The elec-tron transfer from amine nitrogen to aromatic moiety, which is the reason for fluorescence of the molecule, is inhibited due to encapsulation. It has been proposed that this effect arises due to the hydrophobic nature of p-SC4 cavity [36].

Excited state lifetime studies

The excited state lifetime is measured using TCSPC method. The experimental details are given in the experimental sec-tion. The concentration of C460 is fixed, the concentration p-SC4 is varied and the excited state lifetime is measured for the samples. The excited state lifetime for C460 is 1.093 ns. The increasing the concentration of p-SC4 results in a decrease in the excited state lifetime. However, the decrease in the lifetime is moderate and decreases up to 1.045 ns. The decay in the lifetime are shown in the Fig. 5 and the lifetime values are collected in Table 2. This trend of decrease in life-time values is akin to the emission spectral titration, where there is a decrease in the fluorescence intensity.

Electrochemical studies

The CV studies are employed to study the interaction of p-SC4 with C460 in the solution state. Both the CV titra-tions are shown in Fig. 6. Figure 6a shows the CV of C460 in the absence and presence of p-SC4. There is no oxida-tion peak for C460 and a reduction peak at − 0.6706 V.

The incremental additions of p-SC4 has not developed any new oxidation and reduction peaks. The reduction peak of C460 shifted upwards and deformed while introduc-ing p-SC4. However, the change in current intensity of the curves at final potential is observed. This observation out-comes the presence of interaction of p-SC4 on C460. On the other hand, while changing the concentration of C460 with constant p-SC4 concentration considerable changes are observed (Fig. 6b). The anodic peak potential is increased from 0.8606 to 1.0630 V and cathodic peak potential also increased from − 0.6140 to − 0.8337 V. The anodic peak current, as well as cathodic peak current has increased upon incremental additions of C460. The peak potential and cur-rent values are collected in Table 3.

This observation is due to the disruption of p-SC4 adsorp-tion on electrode surface due to encapsulation and the increase of quinonoid form in the solution. The development of a new reduction peak at − 0.065 V with the increment of C460 concentration supports the possibility of ground state complex formation.

Computational studies

To get deeper insights into the interaction strength and inter-acting orientations between C460 and p-SC4, we have per-formed the quantum chemical calculations. In the two mod-eled systems (Fig. 7), the C460 is oriented vertically in the first one and horizontally in the second one. The calculated interaction energies were corrected for basis set superpo-sition error (BSSE) using the counterpoise procedure for accurate estimation [37]. The calculated interaction energies (BSSE corrected) of C460 with p-SC4 are − 16.81 (vertical) and − 17.92 kcal/mol (horizontal) (Table 4). Here, the nega-tive sign of the interaction energies indicates the stabiliza-tion upon complexation between C460 and p-SC4. Though the interaction energy for the horizontally oriented C460 is higher than that of vertically oriented one, the difference is moderate (1.11 kcal/mol). The higher interaction energy for horizontal orientation can be attributed to the interactions from multiple sides, whereas in vertical orientation the inter-actions are limited. From the optimized geometries of the complexes, it has been observed that the C460 is interacting with all the four sulfonate groups of p-SC4 in its horizontal orientation. On the other hand, in the vertical orientation

Fig. 5 Changes in the excited state lifetime of C460 in presence of p-SC4

Table 2 Excited state lifetime of C460 (1 × 10− 6 M) in the presence of various concentrations of p-SC4

p-SC4(M)

Lifetime, τ(ns)

0 1.0931 × 10− 6 1.0552 × 10− 6 1.0503 × 10− 6 1.045

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of C460, it is spatially close to only two sulfonate groups of the p-SC4.

To study the charge transfer phenomenon, we carried out population analysis on optimized (more stable i.e., model two) geometry of the host–guest complex. The electron

density (Fig. 8) in HOMO of the complex is completely located over C460 and it is transferred to p-SC4 in LUMO. The transfer of electron density from C460 to p-SC4 shows the charge transfer phenomenon upon complexation.

Fig. 6 Cyclic voltammetry of a free C460 (black) and change in oxidation and reduction potentials by introducing p-SC4 versus Ag/AgCl b free p-SC4 (black) and change in oxidation and reduction potentials by introducing C460 versus Ag/AgCl. The scan rate is 100 mV/s

Table 3 Changes in anodic and cathodic peaks of pSC4 by increasing the concentration of C460

Concentration percent-age of C460 (%)

EpA

(V)IpA

(μA)EpC

(V)IpC

(μA)New peak

EpC

(V)IpC

(μA)

0 0.8606 37.51 − 0.614 − 26.96 – –16.6 0.9387 55.48 − 0.6653 − 25.87 − 0.0695 − 11.3728.5 0.9900 61.65 − 0.7043 − 25.48 − 0.0671 − 11.9837.5 1.0220 66.07 − 0.7507 − 25.27 − 0.0658 − 13.0644.4 1.0630 71.90 − 0.7849 − 24.71 − 0.0647 − 13.1250 – – − 0.8337 − 23.78 − 0.0622 − 14.18

Fig. 7 The optimized geom-etries of the complex p-SC4-C460 obtained at M06-2X/6-31G(d) level of the theory. The horizontally (a) and vertically (b) oriented C460 in p-SC4

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NMR studies

The structure of C460 and p-SC4 with labelling of protons are given in Fig. S7 for easy interpretation. The individual

1H NMR spectra of C460 and p-SC4 in D2O and CD3CN are shown in Figs. S8 and S9. The binding modes can be determined using 1NMR techniques and ROESY spectral techniques [38–40]. The 1H NMR spectra of C460 in the

Table 4 Simulated complexation energies and dipole moments of horizontally and vertically oriented C460 with p-SC4 obtained at M06-2X/6-31G(d) level of the theory

p-SC4-C460 BSSE uncorrected(kcal/mol)

BSSE corrected(kcal/mol)

BSSE energykcal/mol

Dipole moment(D)

Horizontal − 27.89 − 17.92 9.97 12.61Vertical − 25.81 − 16.81 9.00 7.78

Fig. 8 The electron density distribution in frontier molecu-lar orbitals of the p-SC4-C460 complex obtained from compu-tational simulations

Fig. 9 1H NMR spectral titration of C460 alone (1:0), mixture of C460:p-SC4 in 1:0.5, 1:1, 1:2 and 1:3 ratios in D2O/CD3CN mixture. The sol-vent peak is excluded for clarity

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absence and presence of p-SC4 with various stoichiometry are measured in D2O/CD3CN solvent mixture. The Fig. 9 shows the 1H NMR titration of C460 with p-SC4. All C460 protons exhibit visible up field shifts upon complexation due to the ring current effect of the aromatic nuclei of p-SC4, which suggests that the C460 is included into the cavity of p-SC4 [41]. Table 5 shows the comparative chemical shift of free C460 and 1:1 C460:p-SC4 mixture. This results are similar with the binding modes by computational calcula-tions (Fig. 7).

The two dimensional NMR technique, ROESY (Fig. 10) is further used to confirm the binding mode of C460 with

p-SC4 in D2O/CD3CN. The contours of Sa, the (–CH2–) methylene bridge proton in lower rim of p-SC4 correlates with the aromatic protons C5 and C6 of C460. Similarly, the contours of Sb, the –OH proton of p-SC4 also correlates with the protons belonging to the aromatic moiety of C460 C5, C6 and C7. These correlations reveal that the whole C460 molecule is incorporated inside the cavity. Both the 1H NMR titration and ROESY results emphasized the cavity binding of C460 inside the aromatic cavity of p-SC4.

Conclusion

The absorption and emission spectral studies show that there is an efficient interaction in between C460 and p-SC4. The Job’s plot emphasized the 1:1 binding of C460 with p-SC4. The binding constant values calculated by linear and non-linear regression approach are around 103 M− 1 shows potent binding. The negative ΔG value confirms the spontaneous binding of C460 with p-SC4. The fluorescence spectral stud-ies also asserted the binding with binding constant value around 103 M− 1 along with negative ΔG value of C460 with p-SC4. The decrease in the excited state lifetime also sup-ported the trend of fluorescence spectral studies. The electro-chemical studies also evidently confirm the binding of C460

Table 5 Chemical shift values of the 1H NMR titration of C460 with p-SC4 in D2O/CD3CN

Proton Chemical shift, δ (ppm) Change in ppm

C460 C460:p-SC4 (1:1)

C1 1.60 1.40 0.20C2 2.75 2.59 0.16C3 3.85 3.67 0.18C4 6.35 6.18 0.17C5 6.95 6.76 0.19C6 7.13 6.97 0.16C7 7.90 7.74 0.16

Fig. 10 ROESY spectrum of 1:1 mixture of C460 and p-SC4 in D2O/CD3CN

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with p-SC4 at quinonoid moiety. With the help of theoretical stimulations, the binding orientation and binding energies are thoroughly studied. Furthermore the 1H and ROESY NMR studies explained the cavity binding of C460 which is coherent with the results obtained from other experimental and theoretical approaches.

Acknowledgements We acknowledge the financial support of Depart-ment of science and Technology (DST INSPIRE) [Project Number—IFA14/CH-147], India. B. M. Ashwin thanks to Alagappa University for providing AURF Fellowship for his research work. This work was supported by the Korea Research Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2016H1D3A1936765).

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