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Modulation of the Excited-State Dynamics of 2,2′-Bipyridine-3,3′-diol in Crown Ethers: A Possible Way To Control the Morphology of a Glycine Fibril through Fluorescence Lifetime Imaging Microscopy Debasis Banik, Arpita Roy, Niloy Kundu, and Nilmoni Sarkar* Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, West Bengal, India S Supporting Information *

ABSTRACT: In this article, we have investigated the modulation of excited-state intramolecular double proton transfer (ESIDPT) dynamics of 2,2′-bipyridine-3,3′-diol (BP(OH)2) in two crown ethers (CEs), namely, 18-Crown-6 (18C6) and 15-Crown-5 (15C5). From steady-state UV−visible measurements, we have shown that there is no significant interaction between the dienol tautomeric form of BP(OH)2 and two CEs. However, in the presence of CEs, an additional emission band (∼415 nm) is generated along with the diketo tautomer band (∼465 nm). In time-resolved analysis, we have observed the generation of ∼260 ps rise component in the presence of 18C6. Therefore, by combining the results of steady-state and time-resolved emissions, we have proposed that the water-assisted ESIDPT route of BP(OH)2 generates a hydronium ion (H3O+) in the excited state. 18C6 binds nicely to this H3O+ ion. As a result, retarded ESIDPT dynamics is observed in 18C6. However, as 15C5 cannot bind H3O+ properly, no rise component is found. With the addition of potassium chloride (KCl), the contribution of the rise component decreases due to unavailability of free 18C6 cavity to capture the H3O+ ion generated in the excited state. Addition of calcium chloride (CaCl2) leads to complete removal of the rise component due to the inhibition of the water-assisted ESIDPT route. From wavelength-dependent behavior, we have observed that the rise component is present only at 465 nm in 18C6. We have also shown that the fibrillar morphology of glycine can be successfully probed through fluorescence lifetime imaging microscopy using BP(OH)2 as an imaging agent. Modulation of fibrillar morphology has been found in the presence of two CEs. The interaction of glycine fiber with CEs can be explained by lifetime distribution analysis.

1. INTRODUCTION Excited-state intramolecular proton transfer (ESIPT) processes are ubiquitous and have been extensively investigated for the last few decades. It is well documented that ESIPT reactions have direct applications in different types of biological processes,1−3 UV filters,4 white light-emitting diodes,5 photostabilizers,6 proton transfer (PT) lasers,7−9 and so on. Various groups have reported the utility of ESIPT probes in biological imaging.10 Among several ESIPT chromophores, 2,2′-bipyridine-3,3′-diol (BP(OH)2) has aroused special attention of the scientific communities as a molecular informer for probing real biological systems, for example, tautomerization in duplex DNA.11−13 BP(OH)2 is a planar molecule containing two aromatic residues in the crystalline state.14,15 It has two strong intramolecular hydrogen bonds in its dienol (DE) tautomeric form. Upon photoexcitation, excited-state intramolecular double proton transfer (ESIDPT) takes place to generate a diketo (DK) tautomer in the excited state.16 Femtosecond fluorescence upconversion and transient absorption experiments provide the mechanistic details of ESIDPT of BP(OH)2.17,18 One can control the PT mechanism by varying the selection of systems and the excitation and emission wavelengths.17−21 Marks et al. have shown that there is no significant effect of deuteration of solute on the ESIDPT dynamics of BP(OH)2 in protic and apropic solvents.22 © 2016 American Chemical Society

Excited-state proton transfer (ESPT) dynamics of BP(OH)2 and related compounds have been extensively investigated from experimental and theoretical points of view. 23−34 The absorption maximum of BP(OH)2 is centered at 340 nm due to the π−π* transition of its DE tautomeric form. In neat water, another absorption band appears between 400 and 450 nm due to the solvation of its DK tautomer in the ground state. The large Stokes-shifted fluorescence emission at 465 nm is responsible for the proton-transferred DK tautomer in the excited state. Due to the nonpolar nature of BP(OH)2, solvent polarity has slight control on the absorption and emission profiles of DE and DK tautomers.24,27 However, the ESIDPT dynamics is significantly affected by the hydrogen bonding ability of polar protic solvents. Because of this nice solvatochromism, BP(OH)2 has been extensively used as a probe to assess different chemical microenvironments.13,17,35−48 Plasser et al. have shown that the ESIDPT mechanism of BP(OH)2 follows a sequential pathway in the gas phase. They have found that the first step of the PT [i.e., DE to monoketo (MK)] is ultrafast (∼7 fs) in nature. Their results agree with the experimentally observed time scale of 10 ps for the second Received: July 27, 2016 Revised: October 6, 2016 Published: October 6, 2016 11247

DOI: 10.1021/acs.jpcb.6b07524 J. Phys. Chem. B 2016, 120, 11247−11255

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The Journal of Physical Chemistry B Scheme 1. Schematic Representation of the ESIDPT Process of BP(OH)2

PT step (Scheme 1).31 The sequential mechanism is also supported by the so-called nodal-plane model32,33 and is frequently preferred in ESIPT.34 Abou-Zied has proposed two different PT mechanisms of BP(OH)2 in aqueous solution.35 According to the first one, a dizwitterion tautomer is formed due to direct interaction of water molecules with each polar part of BP(OH)2. The second one indicates water-mediated PT of BP(OH)2. Iyer and Datta have reported a rise time in the time-resolved fluorescence of BP(OH)2 in a nafion membrane at a low level of hydration.41 According to their proposal, dianion (BPO22−) is generated in the excited state, which emits at 420 nm. The rise time is attributed to the generation of the ESPT state from BPO22−. De and Datta also obtain a rise time of BP(OH)2 in the lamellar structure of Aerosol-OT (AOT).42 Purkayastha group has studied the ESIDPT dynamics of BP(OH)2 in lipid vesicles and nanoporous gold nanoparticles.46,47 Hazra and co-workers have detected the rise component of BP(OH)2 in neat water and in β-cyclodextrin using an upconversion setup.48 However, they did not observe any rise component in the presence of cucurbit[7]uril (CB7). Crown ethers (CEs) are cyclic oligomers of ethylene oxide (−CH2CH2O−).49 Depending on the number of ethylene oxide units, various types of CEs are available, such as 12Crown-4 (12C4), 15-Crown-5 (15C5), 18-Crown-6 (18C6), etc. CEs are specifically used as a capping agent for alkali metal cations.50 In addition, they are well-known hosts in host−guest chemistry.51 In this article, our aim is to investigate the interaction of the ESIDPT chromophore BP(OH)2 with two CEs (15C5 and 18C6). In the presence of 18C6, the PT dynamics is retarded with the generation of a 260 ps rise component. With the addition of a monocationic salt (KCl), the contribution of the rise component decreases. Addition of a dicationic salt (CaCl2) leads to complete removal of the rise component. No rise component is observed in the presence of 15C5. Glycine is an amino acid that can form a fibrillar structure in aqueous solution.52 We have shown that the fibrillar morphology of glycine can be probed using BP(OH)2 as an imaging agent through fluorescence lifetime imaging microscopy (FLIM). We have also shown the modulation of the fibrillar structure in the presence of two CEs. To the best of our knowledge, this is the first report of retarded ESIDPT dynamics of BP(OH)2 in CEs along with its (BP(OH) 2 −CE system) application in controlling the fibrillar morphology of glycine through FLIM.

purchased from Sigma-Aldrich and used as received without further purification. Potassium chloride (KCl) and calcium chloride (CaCl2) are purchased from Merck. For experimental purposes, triple-distilled Milli-Q water is used to prepare all of the solutions of BP(OH)2. Chemical structures of all of the materials used for this experiment are shown in Schemes 1 and 2. Scheme 2. Chemical Structures of 18-Crown-6 (18C6) and 15-Crown-5 (15C5)

2.2. Instruments and Methods. A detailed description of the instruments and methods are given in the Supporting Information.

3. RESULTS AND DISCUSSION 3.1. Steady-State Absorption and Emission Studies. The steady-state UV−visible absorption spectra of BP(OH)2 in the presence of 18C6 and 15C5 are shown in Figures 1a,b, respectively. As reported earlier, we have observed two absorption bands (∼340 and ∼400−450 nm) of BP(OH)2 in aqueous solution.37 With the addition of 100 mM 18C6, no change is observed in the 340 nm absorption band. However, the absorbance between 400 and 450 nm shows a small decrease. This decrease indicates a deficit of free water molecules to stabilize the DK tautomer of BP(OH)2 in the ground state.43 Similarly, there is no change in the absorption profile of BP(OH)2 in the presence of 100 mM 15C5. In summary, absorption behavior of DE tautomer of BP(OH)2 remains almost unaffected in the presence of CEs. It has been reported that the cavity diameters of 15C5 and 18C6 are between 1.70−2.20 and 2.60−3.20 Å, respectively.53 On the other hand, the three axial radii of BP(OH)2 are 5.8, 3.4, and 1.9 Å, respectively.44 Therefore, the formation of BP(OH)2− CEs host−guest inclusion complexes is not possible due to the size mismatch between the host cavity and guest. This is the probable reason for the unaffected UV−visible absorption spectra of DE tautomer of BP(OH)2 in the presence of CEs. The steady-state fluorescence spectra of BP(OH)2 in the presence of two CEs are depicted in Figures 2a,b. In aqueous solution, BP(OH)2 emits at 465 nm (λex = 340 nm). With the

2. EXPERIMENTAL SECTION 2.1. Materials. 2,2′-Bipyridine-3,3′-diol [BP(OH)2], 18Crown-6 (18C6), 15-Crown-5 (15C5), and glycine are 11248


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Figure 1. Steady-state UV−vis absorption spectra of BP(OH)2 in crown ether concentrations of 0 and 100 mM: (a) 18-Crown-6 (b) 15-Crown-5.

Figure 2. Steady-state fluorescence spectra of BP(OH)2 as a function of increasing CE concentration (0−100 mM): (a) 18-Crown-6 and (b) 15Crown-5. (Inset shows the appearance of the 415 nm band as a function of CE concentration.)

addition of 18C6 and 15C5, the fluorescence intensity of the 465 nm band is increased along with the generation of a new band at 415 nm (inset of Figures 2a,b). It was reported that the dianion of BP(OH)2 (BPO22−) emits at 420 nm.41 Therefore, the appearance of the band at 415 nm indicates the formation of a dianion in the excited state in the presence of CEs. Note that Iyer et al. and Ghosh et al. found the formation of a dianion in a nafion membrane and lipid vesicles, respectively.41,46 This observation indicates modulation of the excitedstate dynamics of BP(OH)2 in the presence of CEs, although there is no significant interaction in the ground state. Therefore, time-resolved measurements are necessary to obtain a clear idea about the excited-state dynamics of BP(OH)2 in two CEs. 3.2. Time-Resolved Emission Study. Time-resolved emission decays of BP(OH)2 with gradual addition of 18C6 and 15C5 are shown in Figures 3a,b, respectively. All emission decays are collected at 465 nm by exciting the samples at 375 nm. Information regarding the time-resolved fluorescence decay analysis is tabulated in Table 1. τ1 and τ2 are the two lifetime components of BP(OH)2, and a1 and a2 are the relative contributions of two lifetime components. As reported earlier, we have observed two lifetime components (∼0.56 and 5.58 ns) of BP(OH)2 in neat water.35 The fast one (∼0.56 ps) is responsible for the proton-transferred DK tautomer. The longer component appears due to water-assisted generation of the DK tautomer. With gradual addition of 18C6 (from 0 to 100 mM), the fast lifetime component is increased from 0.56 to

0.87 ns. Interestingly, we have observed the generation of a rise component having lifetime ∼260 ps from BP(OH)2 with 35 mM of 18C6. The contribution of the rise component is increased from 1 to 26% on going from 35 to 100 mM 18C6. The rise component is clearly observed in Figure 3c. Bhattacharyya and co-workers have studied the effect of salt on the ultrafast PT dynamics of 8-hydroxypyranine-1,3,6trisulfonate (HPTS) in niosome.54 With the addition of sodium chloride (NaCl), the rate of ESPT is significantly retarded. They have mentioned that the added salt creates an ionic atmosphere around the deprotonated form of HPTS. As a result, the Coulombic force between the ejected proton and deprotonated HPTS decreases. This phenomenon is called electrolytic screening. As a result, the diffusion coefficient of the ejected proton decreases and retardation of ESPT is observed. The decrease in diffusion coefficient of the ejected proton of HPTS in water as a function of salt concentration is also reported by Huppert and co-workers.55 In our study, similar screening is observed due to the encapsulation of the ejected proton in the form of a hydronium ion (H3O+) (as discussed in the next paragraph) within 18C6. As a consequence, the diffusion coefficient of the proton decreases drastically, and we have observed retardation of the ESPT rate (260 ps rise component in 18C6 vs 35 ps in water48). Therefore, the 260 ps rise component is completely consistent with the diffusion rate in water. On the other hand, we have not found any rise component of BP(OH)2 in the presence of 15C5 (Figures 3b,d). With increasing addition of 15C5, the decay pattern 11249


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Figure 3. Time-resolved fluorescence decays of BP(OH)2 with increasing concentration of (a) 18C6 and (b) 15C5. (c) Time-resolved rise of BP(OH)2 in the presence of 100 mM 18C6. (d) Comparison of BP(OH)2 in 100 mM 18C6 and 15C5.

changes from biexponential to single exponential. Therefore, it is important to provide an interpretation for this rise component of BP(OH)2 in the presence of 18C6. Abou-Zied reported that water-mediated PT is one of the ESIDPT routes of BP(OH)2.35 After excitation, two hydroxyl groups of BP(OH)2 lose two protons to the nearby water molecules. Before protonation of the imine group in the excited state, each proton is solvated by the water network formed by three water molecules.13 During this solvation process, H3O+ is generated in the excited state. It is reported that H3O+ nicely fits inside the 18C6 cavity like a potassium ion and its stability constant decreases with a decreasing as well as increasing number of oxygen atoms in the CE.56,57 As a result, due to the formation of the 18C6−H3O+ adduct (Scheme 3), proton in the form of H3O+ is trapped inside the cavity of 18C6. In this situation, the imine group has to wait for the free proton for some time. Because of this, the ESIDPT process of BP(OH)2 becomes delayed in the presence of 18C6 compared to that in neat water (35 ps growth component in neat water48). This is the probable reason for the rise component in the presence of 18C6. As the stability constant of the 15C5−H3O+ adduct is less than that of the 18C6−H3O+ adduct, 15C5 cannot accommodate H3O+ within its cavity. Due to this, no rise component is found in the presence of 15C5. 3.3. Effect of KCl Addition. We have investigated the effect of KCl (up to 63 mM) on 18C6−BP(OH)2 and 15C5− BP(OH)2 systems (Table 2). With the addition of KCl, the contribution of the rise component is decreased from 26% (in

Table 1. Fluorescence Lifetimes of BP(OH)2 with Increasing 15-Crown-5 (15C5) and 18-Crown-6 (18C6) Concentrationa BP(OH)2 in CE (mM) 0.0 12 24 45 65 83 100 6 12 24 35 45 55 75 100

τ1/nsb (a1) 15C5 0.56 0.60 0.64 0.73 0.79 0.85 0.90 18C6 0.66 0.67 0.71 0.74 0.76 0.78 0.82 0.87

τ2/nsb (a2)

(0.99) (0.99) (0.99) (1.00) (1.00) (1.00) (1.00)

5.58 (0.01) 1.81 (0.01) 1.30 (0.01)

(0.99) (0.99) (0.99) (1.01) (1.02) (1.17) (1.17) (1.26)

2.38 2.06 1.66 0.26 0.26 0.26 0.26 0.26

(0.01) (0.01) (0.01) (−0.01) (−0.02) (−0.17) (−0.17) (−0.26)

τ1 and τ2 are the two lifetime components of BP(OH)2, a1 and a2 are the relative contributions of two lifetime components, λex = 375 nm, and λem = 465 nm. bExperimental error ±5%. a

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complex in water (Table S1). It is reported that metal ion binding can inhibit the PT process. 58,59 Due to this complexation, the water-assisted PT route of BP(OH)2 becomes blocked. As a result, there is no chance of H3O+ formation in the presence of CaCl2. Therefore, the rise component disappears. Formation of H3O+ ion and the availability of free 18C6 cavity are responsible for the generation of the rise component. On using KCl and CaCl2, these two factors are affected. As a result, we have found reduction and complete removal of the rise component in the presence of KCl and CaCl2, respectively. 3.5. Wavelength-Dependent Study. The generation of the rise component is an indication of a two-step process. Generally, the contribution of the rise component increases as the fluorescence collection wavelength increases. Datta and coworkers found a similar trend in BP(OH)2 in nafion membrane.41 However, in the lamellar-structure AOT, they observed an opposite trend, that is, the contribution of the rise component decreases with increasing collection wavelength.42 The rise component is observed at intermediate emission wavelengths (470−510 nm) and not in the red edge (550 nm) of the fluorescence spectrum. They have mentioned that as the DK form emits at shorter wavelengths than the monoenol tautomeric form, the rise component is only present at intermediate wavelengths. We have investigated the wavelength-dependent behavior of BP(OH)2 in 18C6 and 15C5. For this purpose, we have collected time-resolved emission decays of BP(OH)2 (in 100 mM 18C6 and 15C5) at 465, 500, and 530 nm (Figure S2). Interestingly, we have observed that the rise component is only present at 465 nm in the presence of 18C6 (Table S2). With increasing collection wavelength, the disappearance of the rise component is noted. For both CEs, the decay becomes faster with increasing collection wavelength. As the DK tautomer (emits at 465 nm) is generated from the dianion (emits at 415 nm), we have observed the rise component at 465 nm. However, the disappearance of the rise component at the red end of 465 nm is an indication of complex dynamics occurring in this system. 3.6. Temperature-Dependent Study. We have investigated the effect of temperature on the 18C6−BP(OH)2 and 15C5−BP(OH)2 systems. For this purpose, we have collected time-resolved emission decays of BP(OH)2 in the presence of each CE from 283 to 333 K at a regular interval of 10 K (Figure S3). Interestingly, we have observed that at a low temperature, that is, at 283 K, the lifetime of the rise component becomes 350 ps (∼260 ps rise component is found at 298 K). The rise component appears between 283 and 303 K in the presence of 18C6 (Table S3). At higher temperatures (323 and 333 K), a long component of ∼2.5 ns appears with a very small contribution (1−2%). On the other hand, we have not found any rise component in the presence of 15C5 within the studied temperature range. 3.7. Modulation of Fibrillar Morphology of Glycine by CEs. Perween et al. have shown that glycine forms a fibrillar structure in aqueous solution.52 Very recently, Shaham-Niv et al. have reported that the glycine fiber is associated with several genetic disorders, such as nonketotic hyperglycinemia, Dglyceria aciduria, and iminoglycinuria.60 Therefore, detection and disruption of glycine fiber are important from the medical point of view. First, we have shown that the fibrillar morphology of glycine can be successfully probed using BP(OH)2 as an imaging agent through the FLIM technique.

Scheme 3. Structure of the 18-Crown-6−Hydronium Ion (H3O+) Complex

Table 2. Time-Resolved Decay Parameters of BP(OH)2 in 100 mM 18C6 and 15C5 with Increasing Addition of KCla BP(OH)2 in 100 mM CE

concentration of KCl (mM)

18C6

0.00 17 29 63 0.00 17 29 63

15C5

τ1/nsb (a1) 0.26 0.26 0.26 0.26 0.90 0.91 0.93 0.95

(−0.26) (−0.18) (−0.10) (−0.08) (1.00) (1.00) (1.00) (1.00)

τ2/nsb (a2) 0.87 0.87 0.86 0.87

(1.26) (1.18) (1.10) (1.08)

χ2 1.05 1.15 1.18 1.12 1.08 1.05 1.13 0.98

χ represents the goodness of fit between experimental and fitted data. bExperimental error ±5%. a 2

the absence of KCl) to 8% (in the presence of 63 mM KCl) in 18C6. However, the lifetime of the long component remains almost unaffected. On the other hand, in the presence of KCl in 15C5, little increase in average lifetime is observed. It is well known that 18C6 specifically binds to the potassium ion (K+).53 As a result, in the presence of KCl the availability of free 18C6 to capture the H3O+ ion generated in the excited state decreases. Due to this, the contribution of the rise component decreases in the presence of KCl. 3.4. Effect of CaCl2 Addition. To compare the results for the K+ (monocationic) ion with those for a dicationic salt, we have investigated the effect of CaCl2 on the 18C6−BP(OH)2 system. With the addition of CaCl2 (60 mM), we have observed the disappearance of the rise component of BP(OH)2 in the 18C6 system (Table 3). To obtain an idea about the Table 3. Time-Resolved Decay Parameters of BP(OH)2 in 100 mM 18C6 in the Presence of CaCl2 system BP(OH)2 100 mM 18C6 a

concentration of CaCl2 (mM)

τ1/nsa (a1)

τ2/nsa (a2)

χ2

0.00 60

0.26 (−0.26) 0.96 (0.44)

0.87 (1.26) 5.86 (0.56)

1.03 1.02

Experimental error ±5%.

disappearance of the rise component, we have collected timeresolved emission decay of BP(OH)2 (in the absence of 18C6) in the presence of CaCl2 in neat water (Figure S1 in the Supporting Information). With the addition of 60 mM CaCl2, a long lifetime component (∼6 ns) is generated, and this is probably due to the strongly hydrogen-bonded BP(OH)2−Ca2+ 11251


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Figure 4. (a) Fluorescence intensity and (b) lifetime images of the glycine fibril. (Scale bar is 25 μm.)

Figure 5. Modulation of the fiber morphology of glycine in the presence of 18C6 (a, b) and 15C5 (c, d). [Fluorescence intensity images: (a) and (c), lifetime images: (b) and (d).] (Scale bar is 25 μm.)

Figures 4a,b represent the fluorescence intensity and lifetime images of the glycine fibril. Fibrous morphology (of the order of micrometers) has been observed from the FLIM images. We have maintained the concentration of glycine at 12 mM. We have employed 18C6 and 15C5 (both at 12 mM concentration) to modulate the fibrillar morphology of glycine (Figures 5a−d). In the presence of 18C6, we have observed the generation of flake-like morphology. Note that Singh et al. have

found a similar morphology when D-phenylalanine inhibits the fiber formation of L-phenylalanine.61 Therefore, 18C6 arrests the fibril formation of glycine and gives rise to flakes (Figures 5a,b). On the other hand, 15C5 cannot arrest the fibril formation (Figures 5c,d). Modulation of the fibrillar morphology of glycine by CEs can be nicely explained by lifetime distribution analysis obtained from the respective images. Lifetime distribution plots of 11252


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have also shown that the fibrillar morphology of glycine can be modulated by CEs. The interaction between the glycine fiber and CEs can be corroborated by lifetime distribution analysis.

BP(OH)2 in glycine fibers and in the presence of each CE are shown in Figure 6. BP(OH)2 exhibits a lifetime distribution

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b07524. Detailed experimental section, time-resolved emission parameters of BP(OH)2 in water in the presence of CaCl2, wavelength-dependent behavior, and temperature-dependent behaviors (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-3222283332. Fax: 91-3222-255303. Notes

The authors declare no competing financial interest.

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Figure 6. Lifetime distribution of BP(OH)2 inside the glycine fibril and in the presence of 18C6 and 15C5.

ACKNOWLEDGMENTS N.S. is grateful to SERB, Department of Science and Technology (DST), and the Council of Scientific and Industrial Research (CSIR), Government of India, for generous research grants. D.B. and N.K. are grateful to IIT Kharagpur for research fellowship. A.R. is grateful to CSIR for research fellowship.

between 1650 and 2250 ps with a maximum at around 2100 ps in neat fibril. In the presence of 18C6, we have observed a completely different lifetime distribution between 1000 and 1650 ps, with a maximum at around 1400 ps. This indicates an environment different from fibril. On the other hand, in the presence of 15C5, the lifetime distribution is similar to that in neat fiber. Additionally, a new population appears at around 1600 ps. Therefore, the interaction of 18C6 with the glycine fibril is completely different from that of 15C5. It is reported that CEs have an affinity toward the ammonium ion (NH3+) of amino acid.62,63 On binding with the −NH3+ group, 18C6 inhibits the possibility of hydrogen bonding and salt-bridged polar interactions between neighboring glycine molecules and thus restricts the fiber formation.64 However, 15C5 may not properly bind to NH3+ like H3O+. As a result, 15C5 cannot affect the morphology of the glycine fiber.

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REFERENCES

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4. CONCLUSIONS In conclusion, we have investigated the interaction of an ESIDPT chromophore BP(OH)2 with two CEs (18C6 and 15C5). Steady-state absorption measurements indicate that the behavior of DE tautomer of BP(OH)2 remains almost unaffected in the presence of CEs. In neat water, BP(OH)2 emits at ∼465 nm. With gradual addition of 18C6 and 15C5, the fluorescence intensity of the 465 nm band increases along with the generation of a new band at ∼415 nm. This new band indicates the water-assisted ESIDPT route of BP(OH)2. During this process, H3O+ is generated in the excited state. H3O+ nicely fits inside the 18C6 cavity, which results in the formation of an 18C6−H3O+ adduct. Due to this, the ESIDPT dynamics of BP(OH)2 becomes delayed with the generation of a rise component having lifetime ∼260 ps. On the other hand, no rise component is found in 15C5. In the presence of KCl, due to the unavailability of a free 18C6 cavity, the contribution of the rise component decreases. Addition of CaCl2 leads to complete removal of the rise component. Interestingly, the rise component appears only at an emission wavelength of 465 nm. By using BP(OH)2 as an imaging agent through FLIM we 11253


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diol in Crown Ethers - ACS Publications - American Chemical Society

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