Supramolecularly Assisted Modulation of Optical Properties of

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Supramolecularly Assisted Modulation of Optical Properties of BODIPY-Benzimidazole Conjugates Shrikant S. Thakare, Goutam Chakraborty, Parvathi Krishnakumar, Alok K. Ray, Dilip Kumar Maity, Haridas Pal, and Nagaiyan Sekar J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b08429 • Publication Date (Web): 07 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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The Journal of Physical Chemistry

Supramolecularly Assisted Modulation of Optical Properties of BODIPY-Benzimidazole Conjugates Shrikant S. Thakare a, Goutam Chakraborty b, Parvathi Krishnakumar c, Alok K. Ray b, Dilip K. Maity c, Haridas Pal* d, Nagayan Sekar* a a

Institute of Chemical Technology, Mumbai-400019, bLaser and Plasma Technology Division

BARC, Mumbai-400085, cHomi Bhabha National Institute, Anushaktinagar, Mumbai-400094, d

Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai-400085.

ABSTRACT: Supramolecular host-guest interaction of neutral and cationic (protonated) forms of two BODIPY-benzimidazole (mono-benzimidazole and di-benzimidazole) conjugate dyes with the macrocyclic host Cucurbit[7]uril (CB7) has been investigated using photophysical and DFT studies. Expectedly, cationic forms of the dyes show exceptionally stronger binding than that of the neutral forms with CB7, which can be ascribed to strong ion−dipole interaction between the positive charge of the dye and highly polarizable carbonyl portals of the host. The formation of dye-host inclusion complexes is supported by the significant changes in the photophysical properties and longer rotational relaxation times of the dye in presence of the CB7. Job’s plot studies indicate formation of 1:1 inclusion complex for the mono and 1:2 inclusion complex for the di-benzimidazole BODIPY dyes. Quantum chemical calculations are in good agreement with the inferences outlined from photophysical measurements. Findings in

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the studied dye-CB7 systems are of direct relevance to the applications like drug delivery, aqueous dye lasers, sensors, and so on.

1. INTRODUCTION The intricacies of host−guest inclusion complex formation and the consequent modulations in the photophysical properties of the chromophoric guest molecules have become the spotlight in supramolecular chemistry1,2. These modulations in the guest behavior find their useful applications in the field of chemistry3,4, materials5,6, biology7 and medicalsciences8,9. The noncovalent interactions like hydrophobic, van der Waals, hydrogen-bonding, electrostatic (i.e. iondipole or dipole-dipole) interactions are the most essential parameters to construct the supramolecular entities involving host-guest systems10. As these bonding interactions are relatively weak, the supramolecular assemblies show complete reversibility in solution and respond to the presence of various external stimuli11–13. This important property acts as a very useful tool in designing stimuli responsive functional materials for various applications. The dye molecules show very significant and noteworthy changes in their photophysical and other physicochemical properties on interaction with the macrocyclic cavities. These important changes include enhancement in the fluorescence14, increase in the photochemical stability15,16, improved water solubility2 and so on. In many chromophores, their acid−base equilibria can be modulated by the supramolecular hosts and thereby causes the shift in the pKa value of the dye17. This supramolecular pKa shift is the result of the changing microscopic environment over the chromophore on its inclusion into the host cavity which leads to either increase or decrease in the acidity/basicity of the dye depending upon its chemical structure. The macrocyclic hosts like cyclodextrins18,19, calixarenes18,19, and cucurbiturils20,21 are well studied for the supramolecularly assisted pKa shift for the encapsulated guest molecules. However, the Cucurbit[n]urils (CBn) are


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extensively studied recently due to their large influence on the acid-base equilibrium of the guest dyes. Among macrocyclic molecules, the CBn family is well established hosts in the supramolecular chemistry due to their unique chemical constitution, rigid hydrophobic cavity, highly polar carbonyl portals, availability with various cavity sizes and adaptability to form strong inclusion complexes with a variety of guest molecules2. CBn are highly symmetrical pumpkin-shaped macrocyclic molecules formed by the linking of glycoluril monomer units in a cyclic manner by a pair of methylene bridges22. The important CBn homologues with varying cavity and portal sizes are CB5, CB6, CB7, CB8, and CB10hosts, consisting of 5, 6, 7, 8, and 10 glycoluril monomer units, respectively22. All CBn members possess a non-polar cavity for the hydrophobic interaction with the encapsulated hydrophobic residues of the guest and the highly polarizable carbonyl portals that can provide strong ion−dipole or charge−dipole interaction for the encapsulated guest molecule. Thus guest molecules with cationic charge or intra-molecular charge transfer (ICT) character interact very strongly with various CBn hosts due to the involvement of the Coulombic interaction provided by highly polarizable portals of the host cavities. These interactions help to build exceptionally stable inclusion complexes and accordingly make the CBn hosts as the preferred choice in host-guest studies over the other conventional hosts1,2. Among the CBn homologues, CB7 has been extensively studied in the host-guest chemistry involving a chromophoric guest molecule. This is mainly due to its reasonably good water solubility and its appropriate cavity dimensions well suited to encapsulate most of the conventional chromophoric molecules1,2. CB7 shows exceptionally strong binding affinity for cationic as well as neutral guest molecules due to ion−dipole, charge-dipole and hydrophobic


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interactions. The effect of CB7 on the pKa shift of the guests and its utilization in pharmaceutics has been recently reviewed1,8. Besides this, CB7 is also known for its applicability in sensing, molecular logic gates, dye lasers, supramolecular catalysis, drug delivery, nanocapsules, and others1,2. So the host−guest studies involving CB7 macrocycle have gained much attention which can be seen through increased number of research publications in recent years. Boron-dipyromethane (BODIPY) dyes are very well known and extensively studied fluorophoric system. These molecules show sharp absorption and emission bands with extremely large molar absorption coefficients and high fluorescence quantum yields23–25. Researcher have taken great efforts to explore these dyes in various applications such as polarity sensors26,27, metal sensors28–30, dye lasers31,32, molecular logic gates33, biological probes34 and so on. The studies on the interactions of the BODIPY dyes with supramolecular hosts are very rare35,36 and thus we have selected these molecules to study the modulations in their photophysical properties on inclusion into the supramolecular host cavity. In the present study, we have investigated the interaction of two BODIPY dyes, namely, monobenzimidazole-BODIPY (dye 1) and dibenzimidazole-BODIPY (dye 2) with the CB7 host (Chart 1). Both the dyes were synthesized as per the reported synthetic procedure in our lab and characterized by IR, NMR and HRMS analysis (details are given in Sections S1 and S2 and in Figures S1-S6 in the supporting information, SI). While dye 1 is recently reported in the literature, to the best of our knowledge, dye 2 is a newly synthesized novel fluorophore in the present work37. Chemical structures of the two dyes and the schematic shape of the CB7 cavity are shown in Chart 1, along with the dimensional parameters of the guest dyes and the host cavity for a quick visualization.


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Chart 1. Molecular structures of the BODIPY dyes under study and the important molecular dimensions of the dyes and the CB7 host are shown.

2. MATERIALS AND METHODS The dyes under investigation were synthesized independently in the present study. Highly pure CB7 sample was procured from Sigma-Aldrich and used without further purification. Nanopure water used for the experiment was obtained from a Millipore Elix 3/A10 water purification system (conductivity less than 0.1 µS cm−1). Due to the inherent solubility of the dyes in water, Host−guest interaction studies were carried out in 3% methanolic aqueous solutions following the changes in the absorption and fluorescence characteristics of the dyes with addition of the gradually increasing concentration of the host, while maintaining the total dye concentration constant. The dye concentration was kept reasonably low (3 μM) throughout the experiment to


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avoid the aggregation of the free dyes in the solution. Measurements in the present study for different prototropic forms of the dyes were carried out at suitably selected pH conditions (pH 2, 7 and 8) depending on the acid-base characteristics of the dyes at ambient temperature (∼25 °C). Ground-state absorption spectra were recorded in a quartz optical cell (path length 1 cm) using a Thermo-Scientific UV−visible spectrophotometer. Steady-state (SS) fluorescence measurements were carried out in a quartz cuvette (1 cm) using a Horiba fluoromax-4 spectrofluorimeter. For the fluorescence measurements the samples were excited at a suitable wavelength in the longer wavelength absorption band of the dyes. Time-resolved (TR) fluorescence measurements were carried out using a time-correlated single-photon-counting (TCSPC) spectrometer obtained from Horiba Jobin Yvon IBH (U.K.). In these measurements, a 445 nm diode laser (pulse width ∼100 ps, repetition rate 1 MHz) was used as the excitation source, and the fluorescence decays were recorded using a MCP-PMT based detection module (IBH, U.K.). A light scatterer (suspension of TiO2 particles in water) was used to measure the instrument response function (IRF) of the present setup. The full width at half-maximum (FWHM) of a typical IRF is found to be about 110 ps. 3. RESULTS AND DISCUSSION 3.1. Steady-state absorption. In aqueous solution, around neutral pH conditions, the dye 1 showed a broad absorption band centered at 510 nm (cf. Figure S7 A), whereas the dye 2 displayed a broad absorption band with maximum at 520 nm (cf. Figure S8 A). The absorption maxima of dye 2 was significantly red shifted in comparison to that of dye 1, due to the presence of two benzimidazole donor groups in the former. Both of these dyes are extremely acid responsive and show large enhancements in their absorbance and fluorescence intensities in acidic solution. Considering this fact, we have obtained the pKa values of dye 1 and dye 2 in their


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aqueous solutions using absorption measurements following the variations in the absorbance at absorption maxima with changing pH of the solutions; the pKa values were estimated to be 4.03 and 3.70 for dye 1 and dye 2, respectively. From these pKa values, it is evident that both the dyes exist mostly in their protonated (cationic) form at pH ~2 and in the neutral form in the pH range of about 7-8. However, since the pKa values for both the dyes undergo significant upward shifts, as will be discussed in section 3.2, and since the pKa values of the CB7 bound dyes are found to be about 6.2 for dye 1 and about 4.9 for dye 2 (cf. section 3.2), therefore we carried out the interaction studies of CB7 with the present dyes at pH 2 (for the cation form for both the dyes), at pH 8 (for neutral form of dye 1) and at pH 7 (for neutral form of dye 2) in their aqueous solutions, keeping the concentration of the dyes constant throughout the experiments.

Figure 1. Absorption spectra A of dye 1 at pH 8 and B of dye 2 at pH 7, in aqueous solutions in the absence and in presence of different concentrations of CB7 host. The concentrations of both Dye 1 and Dye 2 were 3µM.

At pH 8, with gradual addition of CB7 (up to 100µM), the absorption maximum of dye 1 is shifted to a shorter wavelength, from 510 to 502 nm, accompanied with a decreased optical density or absorbance (Figure 1A). Similar observation was noticed in the case of dye 2 at pH 7,


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where its absorption maximum shifted from 520 to 513 nm, along with a decrease in the absorbance (Figure 1B). These observations indicate significant interactions of CB7 with the studied chromophoric dyes. In the previous studies of the CB7 host with the coumarin derivatives having benzimidazole subunits it was revealed that the benzimidazole moiety actually undergoes encapsulation by the CB7 cavity17. We expect a similar kind of binding interaction for the present dyes with the CB7 host. As expected, interaction of the benzimidazole group will be more pronounced for the cationic forms of the studied dyes at the acidic pH condition where imidazole nitrogen exists in the protonated state. The observations made from the absorption studies at pH 2 for the two dyes in the absence and presence of different CB7 concentrations are shown in Figures 2A and B, respectively. As shown in Figure 2A, at pH 2, the dye 1 displays its narrow absorption band with peak centered at 493 nm, which is assigned to the dye with protonated benzimidazole unit. With the gradual addition of CB7, absorption maximum is shifted to a longer wavelength and finally appears at around 497 nm. This spectral shift is also accompanied with an increase in the optical density. These changes in the absorption characteristics can be ascribed to the dye-CB7 hostguest complex formation. Similar results were also obtained in the case of dye 2. It shows the absorption maximum at 507 nm in the absence of CB7 at pH 2, which is finally shifted to 512 nm in the presence of high concentration of CB7 host along with a small increase in the absorbance values (cf. Figure 2B). The interesting point to be mentioned here is that, while the interaction of CB7 with the neutral forms of the studied dyes causes a reduction in the optical density or absorbance (cf. Figure 1), suggesting a decrease in the oscillator strength for the absorption process, the interaction with the cationic forms leads to an increase in the absorbance (cf. Figure 2), suggesting a contrasting increase in the oscillator strength for the absorption


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process. Though a decrease in the oscillator strength for neutral forms of the studied dyes on binding to CB7 can be rationalized in terms of the lower polarizability inside the CB7 cavity,38-39 as discussed in Note S5 of the SI, the reason for the increase in the oscillator strength for the cationic forms of the dyes on binding to the CB7 host is not very clear to us. However, it is possible that inclusion into the CB7 cavity brings in a better planarity for the chromophoric moieties in the cationic forms of the dyes, leading to a better charge delocalization in the molecules and hence causing an increase in the transition dipole moment for the electronic transition consequently increasing the oscillator strengths for the absorption process (cf. Note S5, SI). Such an inference is in fact supported by the results from the quantum chemical calculations, as discussed in section 3.8.

Figure 2. Absorption spectra of (A) dye 1 and (B) dye 2 in aqueous solutions at pH 2 in the absence and in presence of different concentrations of CB7 host. The concentrations of dye 1 and dye 2 were 3μM.

3.2. Supramolecularly assisted pKa shift. The present dyes are the bichromophoric molecules constituting of BODIPY and benzimidazole moieties. The lone pair of electrons on the N-atom


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of the benzimidazole moiety makes these dyes sensitive towards the pH of the solution and also to the polarity of the medium. It is well known that the encapsulation of guest molecule in the host cavity changes the acid-base properties of the prototropic dyes and consequently causes a shift in the pKa values of the guest chromophores. Such a pKa shift is well explained for coumarine dyes, C7 and C30, containing benzimidazole and N-methylbenzimidazole units, respectively, connected to a 7-N,N-diethylaminocoumarinmoiety17. The pH dependent spectrometric titrations were carried out in the absence and presence of CB7 for the present chromophores following absorption studies and the pH titration curves are shown in Figure 3A and B for dye 1 and 2, respectively. The pKa values estimated from the sigmoidal titration curves

are 4.03 and 3.70for dye 1 and dye 2, respectively, in the absence of CB7. In presence of CB7 these values shifted to 6.20 and 4.68, showing pKa shifts of 2.17 and 0.98unitsfrom the respective pKa values for the free dyes.

Figure 3. The pH titration curves for (A) dye 1 and (B) dye 2, both in the absence and in presence of CB7 host, as obtained following the absorbance changes for the two dyes at 493 and 507 nm, respectively. Concentrations of dye 1 and dye 2 were 6 μM.


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3.3. Steady-state fluorescence. The fluorescence measurements for dye 1 and dye 2 were carried out at different pH conditions. We have selected suitable pH conditions based on the pKa values of the dyes in the absence and presence of CB7 host (cf. section 3.2) to study the interactions of the individual species (protonated or neutral form) with the CB7 host. For these measurements, the CB7 solutions were prepared at the required pH in nanopure water to ensure that spectral changes are not due to the residual acids inherently present with the CB7 sample. Around the neutral pH conditions (pH 8 for dye 1 and 7 for dye 2), both the dyes were almost non-fluorescent and showed extremely weak emission bands. Thus, in dilute aqueous solution at pH 8, dye 1 gave a very weak emission band at around 514 nm attributed to LE emission. On increasing the dye concentration, two other concentration dependent emission bands evolved at lower energy region, one at around 575 nm and other around 674 nm, assigned to the aggregation of the dye (cf. S7B in SI). To be mentioned that such behavior is well reported in the literature with the increasing the concentration of number of dyes and are justifiably assigned to the aggregation of the dyes at higher concentrations.40-41. In presence of CB7, dye 1 showed a huge enhancement in the emission intensity for the LE band, but it required a very high concentration of the host (hundreds of μM) to saturate the emission intensity (cf. Section 3.5 and the related plot showing CB7 concentration dependent changes in the fluorescence intensity). At pH 7, dye 2 gave a very weak LE band centered at around 527 nm and another weak and concentration dependent band at 618 nm which is attributed due to the aggregation of the dye (cf. S8 B in SI). On addition of CB7, small red shift and huge enhancement in the fluorescence intensity was observed for the LE band at around 535 nm. (cf. Figure 4B). Similar to dye 1, the dye 2 also required a very high concentration of CB7 to attain saturation for the fluorescence


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intensity (cf. Section 3.5 and the related plot showing CB7 concentration dependent changes in the fluorescence intensity).

Figure 4. Steady-state fluorescence spectra (A) of dye 1at pH 8 and (B) of dye 2 at pH 7 in aqueous solution in the absence and in presence of different CB7 concentration. The concentrations of dye 1 and dye 2 were 3µM and the samples were excited at 480 nm and 485 nm in the two respective cases.

CB7 have gained enormous attention in the supramolecular chemistry due to its strength to bind cationic species through ion-dipole interaction. It always prefers cationic guests compared to the neutral ones.1,8,20,21,42-44. Considering this fact, we have carried out fluorescence measurements in acidic solution at pH 2 to ensure the availability of cationic(protonated)dye species for interaction with CB7. In these solutions, both the dyes exclusively exist in their protonated form as revealed from the pKa measurements (cf. section 3.2). In dilute aqueous solution at pH 2, the dye 1 displayed very high intensity emission band centered at around 512 nm, which on addition of CB7 gave a bathochromic shift of about 3 nm with new peak position at about 515 nm (cf. Figure 5 A). The intensity changes though not very large, but attained


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saturation with the addition of much lower (

Supramolecularly Assisted Modulation of Optical Properties of

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