ESIPT based dual fluorescent sensor and concentration dependent reconfigurable boolean operators

Abstract

Probes 1 and 2 (CH₃CN–H₂O; 4 : 1) at pH 7.0 ± 0.1 (10 mM, HEPES) selectively bind with Zn²⁺ ions and give a new emission band at 490 nm. These can estimate 20 nM Zn²⁺ ions as the lowest detection limit. The determination of Zn²⁺ ions by probe 1 is not interfered by the presence of other metal ions viz. Na⁺, K⁺, Mg²⁺, Ca²⁺, Sr²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cd²⁺, Ag⁺, Hg²⁺. However, in the case of probe 2 Cu²⁺ causes only the fluorescence quenching. In acetonitrile, the addition of different concentrations of Cu²⁺ (2 μM, 5 μM, 10 μM) and a fixed amount of F⁻ (25 μM) to a solution of 2 (0.25 μM, CH₃CN) elaborates Cu²⁺ ion concentration dependent NOR, INH and AND Boolean operators with three distinct emission channels at 585, 515 and 400 nm, respectively.

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1. Introduction

Fluorescent detection systems are attracting a great deal of interest due to their capabilities to sense ¹ various chemically, environmentally, and biologically significant species and their potential applications as molecular switches/devices in information processing.² Diverse fluorescent-sensing approaches have been extensively investigated, including internal charge transfer³ (ICT), photoinduced electron transfer⁴ (PET), metal-ligand charge transfer⁵ (MLCT), excimer/exciplex formation etc. for getting cation and anion recognition. Amongst these fluorescence phenomena the excited state intramolecular proton transfer (ESIPT) has attained a special status due to vast applications as laser dyes,⁶ UV-photostabilizer,⁷ scintillators,⁸ membrane⁹ and protein probes¹⁰ and as a potential component for photoswitches and organic LEDs.¹¹ ESIPT usually takes place in the molecules bearing an H-bond donor group usually a phenolic/amino moiety associated with basic site (O, N) in the ground state through H-bond interactions.¹² This covalently attached proton in the electronically excited state migrates to a neighbouring hydrogen-bonded atom which is less than 2 Å away. The significant amount of excited state energy is dissipated in this process. The formed phototautomer emits light at a lower energy with an unusually large Stokes shift (range from 100 to 500 nm) and thermally equilibrates back to the ground state with the proton bound to its original atom.

Therefore, ESIPT molecules show two emission channels—one due to normal emission (enol form) from the first excited state and characterized by an emission band with small Stokes shift and the second due to emission from stabilized excited state (keto form) via ESIPT, characterized by the large Stokes shift. The ESIPT molecules have the advantage of wide range emission where different emission colours can be balanced by the addition of different analytes due to this dual emission.^13a At least in principle, the stimuli induced bathochromic or hypsochromic shift of these normal and ESIPT based emission channels can further open new emission channels and can thus provide an opportunity for simultaneous sensing of multiple analytes.^13b,13c

Here we report that the Zn²⁺ induced fluorescence signal by probes 1 and 2¹⁴ under aqueous conditions provides an opportunity for selective ratiometric sensing of 100 nM – 1.3 μM and 20–1000 nM of Zn²⁺ ions, respectively. Other metal ions, viz. Na⁺, K⁺, Mg²⁺, Ca²⁺, Sr²⁺, Fe³⁺, Cd²⁺, Co²⁺, Ni²⁺, Cd²⁺, Ag⁺, Hg²⁺ do not interfere with Zn²⁺ estimation. Probe 2 in the presence of Cu²⁺ ions undergoes the quenching of ESIPT emission due to a strong paramagnetic effect, but the presence of a fluoride ion with Cu²⁺ opens new emission channels for elaboration of boolean operators, viz. NOR, INH and AND, depending on Cu²⁺ concentration. The Boolean operators based on the concentration of protons are well documented.¹⁵ To the best of our knowledge, it is the first Cu²⁺ concentration dependent Boolean operator. The differential behaviour of probe 2 towards analytes has been corroborated using theoretical calculations.

2. Photophysical behaviour of 1 and 2 towards metal ions

2.1 Effect of metal ions on the absorbance properties of probes 1 and 2

A solution of 1 (20 μM, CH₃CN : H₂O 4 : 1, v : v), on addition of Cu²⁺ or Zn²⁺ ions showed a gradual increase in absorbance at λ_max 292, 346 and 415 nm with concomitant appearance of a new absorption band between 400–440 nm attributed to a visible color change from colourless to yellow (Fig. SI–1 and SI–2).†

The intensity of absorbance at λ_max 415 nm increased gradually with the increase in concentration of Cu²⁺ or Zn²⁺ ions and achieved a plateau after a certain concentration. These small but distinct gradual changes in the spectrum of 1, on addition of Cu²⁺ or Zn²⁺ ions, show the formation of 1 : 1 stoichiometric complexes with log β_Cu2+L = 4.03 ± 0.4 and log β_Zn2+L = 4.53 ± 0.4 using the Specfit 32 multianalysis programme.

In the case of probe 2 (10 μM) at pH 7.0 ± 0.1 (CH₃CN : HEPES buffer 4 : 1, v : v) on addition of Na⁺, K⁺, Mg²⁺, Ca²⁺, Sr²⁺, Fe²⁺, Co²⁺, Ni²⁺, Ag⁺, Hg²⁺ and Cd²⁺ metal ions did not cause any significant change in its absorption properties but on addition of Cu²⁺ and Zn²⁺ showed a distinct color change from colorless to yellow and the emergence of a new peak at λ_max 430 nm. The incremental addition of Zn²⁺ and Cu²⁺ ions to the solution of probe 2 (10 μM) at pH 7.0 ± 0.1 (10 mM HEPES in CH₃CN : H₂O 4 : 1, v : v) showed a decrease in the absorption intensity at λ_max 292, 315, 332 and 375 nm with concomitant increase in the absorption intensity at λ_max 430 nm with isobestic points at 400, 345 and 300 nm. The presence of isobestic points shows that different species are in equilibrium and there is no stepwise formation of the complexed species. The spectral fitting of these absorbance data shows the formation of 1 : 1 (2 : M²⁺) stoichiometric complexes with logβ_ML = 6.1 ± 0.6 for Zn²⁺ and logβ_ML = 5.7 ± 0.4 for Cu²⁺ complexes (Fig. 1 and 2). The difference in logβ values shows that 2 binds Zn²⁺ nearly 2.51 times more strongly than Cu²⁺ ions.

Fig. 1 Effect of the incremental addition of Zn²⁺ ions on the absorption spectrum of 2 (10 μM) at pH 7.0 ± 0.1 (10 mM HEPES in CH₃CN : H₂O 4 : 1, v : v). The inset shows spectral fitting of the absorbance data at 380 and 430 nm for chemosensor 2 on addition of different concentrations of Zn²⁺ ions (the points are experimental values, the line shows the curve fitting).

Fig. 2 The effect of an incremental addition of Cu²⁺ ions on the absorption spectrum of 2 (10 μM) at pH 7.0 ± 0.1 (10 mM HEPES in CH₃CN : H₂O 4 : 1, v : v). The inset shows spectral fitting of the absorbance data at 370 nm and 425 nm for chemosensor 2 on addition of different concentrations of Cu²⁺ ions (the points are experimental values, the line shows the curve fitting).

2.2 Effect of metal ions on the fluorescence properties of probes 1 and 2

The addition of different metal ions, viz. Na⁺, K⁺, Mg²⁺, Ca²⁺, Sr²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Cd²⁺, Ag⁺ and Hg²⁺, to the solution of fluorophore 1 (0.5 μM, λ_ex = 330 nm) at pH 7.0 ± 0.1 (CH₃CN : HEPES buffer 4 : 1, v : v) did not induce any significant change in its emission. However, on addition of Zn²⁺ to the solution of probe 1, the normal emission band at 390 nm underwent decrease in the emission intensity along with an increase in the intensity at 480 nm with an isobestic point at 400 nm and achieved a plateau with the addition of 5 μM (10 equiv.) of Zn²⁺ ions. The spectral fitting of these data shows the formation of ML (logβ_ML = 5.3 ± 0.03) complex (Fig. 3a, S3†). The ratio of emission intensities varies from 0.12–6 on gradual addition of Zn²⁺ ions, indicating a 50-fold emission ratio change. Probe 1 can be used for ratiometric estimation of Zn²⁺ ions¹⁶ between 100 nM – 6 μM and other metal ions do not interfere with the estimation of Zn²⁺ ions (Fig. 3b).

Fig. 3 (a) Effect of incremental addition of Zn²⁺ on the fluorescence spectrum. (b) Plot of the fluorescence intensity ratio between 480 and 390 nm (FI₄₈₀/FI₃₉₀) vs. [Zn²⁺] ions (inset shows a ratiometric emission response of 1 to different metals ions 0.5 μM) of fluorophore 1 at pH 7.0 ± 0.1 (10 mM HEPES in CH₃CN : H₂O 4 : 1, v : v).

Probe 2 (0.1 μM, λ_ex 330) at pH 7.0 ± 0.1 (10 mM HEPES in CH₃CN : H₂O 4 : 1, v : v) on addition of Zn²⁺ showed the formation of a new emission band at 480 nm. The addition of other metal ions viz. Na⁺, K⁺, Mg²⁺, Ca²⁺, Sr²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Cd²⁺, Ag⁺ and Hg²⁺ did not cause any change in the fluorescence of probe 2 except for Cu²⁺ which led to the fluorescence quenching of the ESIPT band.

On gradual addition of Zn²⁺ ions to the solution of fluorophore 2 (0.1 μM, λ_ex 330) at pH 7.0 ± 0.1 (10 mM HEPES in CH₃CN : H₂O 4 : 1, v : v), the emission intensity at λ_max 585 nm was quenched with concomitant formation of a hypsochromically shifted (Δλ = 105 nm) new emission band at 480 nm which achieved a plateau with the addition of 1 equiv. of Zn²⁺ ions. The spectral fitting of these data shows the formation of ML with logβML = 6.2 + 0.4 (Fig. 4a). The ratio of emission intensities varies from 0.19–7.75 on gradual additions of Zn²⁺ ions, indicating a 41-fold change in the emission ratio. Probe 2 can be used for estimation of Zn²⁺ ions between 20–1000 nM and other metal ions do not interfere in the estimation of Zn²⁺ ions (Fig. 4b).

Fig. 4 (a) Effect of incremental addition of Zn²⁺ on the fluorescence spectrum (the inset shows spectral fitting of the fluorescence data at 480 and 585 nm). (b) Plot of fluorescence intensity ratio (FI₄₈₀/FI₅₈₅) vs. [Zn²⁺] ions (the inset shows the ratiometric emission response of 2 to different metals ions).

On gradual addition of Cu²⁺ ions to the solution of fluorophore 2 (0.1 μM, λ_ex 330) at pH 7.0 ± 0.1 (10 mM HEPES in CH₃CN : H₂O 4 : 1, v : v), the emission intensity at λ_max 585 was quenched. In order to monitor the proper quenching effect on addition of Cu²⁺ ions, the initial concentration of probe 2 was taken as 1 μM. So, the addition of Cu²⁺ ions (10 μM) to the solution of probe 2 (1 μM, λ_ex 330) caused the 82% quenching of the probe and achieved a plateau with 10 μM of Cu²⁺ ions. The spectral fitting of these data shows the formation of ML complex with logβ_ML = 5.0 ± 0.25 (Fig. 5).

Fig. 5 Effect of incremental addition of Cu²⁺ on the fluorescence spectrum of fluorophore 2 (1 μM) at pH 7.0 ± 0.1 (10 mM HEPES in CH₃CN : H₂O 4 : 1, v : v). The inset shows spectral fitting of the fluorescence data at 585 nm for chemosensor 2 on addition of different concentrations of Cu²⁺ ions (the points are experimental values, the line shows the curve fitting).

Thus, probe 1 shows a nearly 50 times increase in the ratio of fluorescence I₃₉₀/I₄₈₀ and can be used to estimate 0.1 μM to 6 μM of Zn²⁺ whereas probe 2 shows a nearly 41 times increase in ratio of intensity at 585 and 480 nm (I₄₈₀/I₅₈₅) and can be used to estimate 20 nM to 1000 nM of Zn²⁺.

The probes 1 and 2 contain an intramolecular hydrogen bond between the phenolic O–H and N– of the benzimidazole ring that undergoes ESIPT and yields normal and ESIPT emissions. The coordination of Zn²⁺ removes the phenolic protons which disrupts the ESIPT emission at 585 nm and exhibit increase in the fluorescence emission intensity at 480–490 nm designated to the ICT normal emission mechanism. Zn²⁺ has closed shell d-orbitals so that the energy or the charge transfer processes cannot take place. However, in probe 2, the co-operativity of two benzimidazole units enhances coordination with paramagnetic Cu²⁺ and completely quenches the fluorescence emission at 585 nm. The quenching as a result of addition of Cu²⁺ suggests that an excitation energy or charge transfer may have occurred from 2 to the open shell d-orbital of Cu²⁺ exhibiting a very fast and efficient nonradiative decay^16e of excited states of probe 2. This differential behaviour of the two metals has further been evaluated using theoretical calculations.

2.3 Reconfigureable logic functions

The reconfiguration of the emission channel by changing the inputs has found applications in elaboration of multiple logic gates within a single molecule. However, reconfiguring the emission channels and thus logic gates by altering the concentration of the inputs are unprecedented. As reported earlier, probe 2 (CH₃CN) selectively binds with the fluoride ions^14a amongst other anions, viz. . Cl⁻, Br⁻, I⁻, NO₃⁻, CH₃COO⁻, HSO₄⁻, H₂PO₄⁻ and ClO₄⁻, and gave a new emission band at 515 nm. Probe 2 in combination with altering the concentration of the inputs (viz. metal ions and anions) exhibits Boolean operations that can be reconfigured into different combinations of logic gates. The appearance of different emission channels at distinctly different wavelengths permits them to be read simultaneously.

Probe 2 (0.5 μM, CH₃CN) on addition of fluoride ions at different concentrations shows “ON-OFF” (λ_max 585 nm, [F⁻] = 2 μM) and “ON-OFF-ON” (λ_max 585 to 515 nm, [F⁻] = 25 μM) process and on addition of Cu²⁺ (2 μM) shows “ON-OFF” phenomenon at 585 nm is observed. Therefore, addition of 2 μM fluoride ions provides only NOT gate at 585 nm which on increasing the concentration of fluoride ions to 25 μM is reconfigured to a YES and NOT combination with respective emission channels at 515 and 585 nm. These F⁻ ion-based outputs from probe 2 are further modulated by adding different amounts of Cu²⁺ ions, and they enable the elaboration of NOR, INHIBIT and AND Boolean operations (Fig. 6a). The emission maxima of probe 2 (0.5 μM, CH₃CN) at 585 nm is completely quenched by the addition of either Cu²⁺ (2 μM) or F⁻ (25 μM) ions and also in the presence of both it remains in the OFF state. This state provides the opportunity for elaboration of NOR logic function (Fig. 6b).

Fig. 6 (A) Fluorescence conditions (a) 2 (0.5 μM, CH₃CN only; (b) 2 + Cu²⁺ (2 μM); (c) 2 + F⁻ (25 μM); (d) 2 + Cu²⁺ (10 μM) + F⁻ (25 μM). (B) The bar diagram for the elaboration of NOR logic gate at λ_em 585 nm shows threshold values and its inset shows the truth table and notation for the elaboration of NOR logic gate.

The emission of 2 in the presence of F⁻ ions (25 μM) appears at 515 nm and is inhibited by the addition of 5 μM Cu²⁺. The emission at 515 nm is in the ON state only in the presence of F⁻ ions and in all other circumstances is in OFF state. So, emission at 515 nm can be used for the elaboration of INHIBIT logic operation (Fig. 7a). Further, on increasing the concentration of Cu²⁺ to 10 μM in the presence of 25 μm of F⁻ ions, the emergence of a new emission band at 400 nm takes place. This emission channel does not open when either Cu²⁺ or F⁻ is present individually, but the simultaneous presence of both inputs opens an emission channel at 400 nm. This situation provides the opportunity for elaboration of AND Boolean logic function (Fig. 7b).

Fig. 7 The bar diagram for the elaboration of (a) INHIBIT logic gate at λ_em 515 nm; (b) AND logic gate at λ_em 400 nm and the horizontal line shows the threshold values. The inset shows the truth table and a notation for the elaboration of respective logic gates.

Thus, by using three different concentrations of Cu²⁺ in the presence of 25 μM of fluoride ions and 0.5 μM of probe 2, three different emission channels of logic gates are activated. At 2 μM [Cu²⁺] only NOR gate exists at 585 nm; at 5 μM [Cu²⁺] both NOR (585 nm) and INHIBIT (515 nm) Boolean operations become active and at 10 μM [Cu²⁺] along with NOR (585 nm) and INH (515 nm) operations, an additional emission channel at 400 nm representing AND logic operation emerges (Fig. 8). It is noteworthy that the elaborations of three logic operations are independent of the inputs (Cu²⁺ and F⁻ ions) sequence.

Fig. 8 Presence of different logic functions on addition of different concentrations of Cu²⁺ ions to the solution of 2 in CH₃CN at F⁻ (25 μM).

Thus, probe 2 with large differentials of the emission maxima with differential concentration of F⁻ and Cu²⁺ ions provides opportunities for configuration of NOR, INH and AND Boolean operations at three distinctly different emission channels to be read at 585, 515 and 400 nm. In order to reconfigure the arithmetic functions, the reset capacity of the system was investigated. For cycling and reconfiguration of the logic functions, it is important to have all operations executed in the same cell. Here, NOR, INHIBIT and AND logic functions, achieved using Cu²⁺ and F⁻ ions as inputs, are reset by addition of aqueous EDTA solution (solvates fluoride ions and binds Cu²⁺) and again restore the system to the initial state.

3. Theoretical calculations

Probes 1 and 2 have two Lewis base sites—imidazole imine nitrogen and phenolic oxygen for interaction with Cu²⁺ and Zn²⁺. These Lewis base sites can interact as such with metal ions and can also cause deprotonation at one of the binding sites. In order to provide insight to the differential mode of the complexation of probe 2 towards Cu²⁺, Zn²⁺ and 2, the Cu²⁺ complex in combination with fluoride (which led to formation of AND gate), the TD-DFT calculations¹⁷ were conducted at the B3LYP/6-31G (d, p) level of theory on probe 2 and its complexes with Cu²⁺, Zn²⁺ and Cu²⁺.F⁻ ions. TD-DFT vertical excitation energies were calculated by using the B3LYP functional in combination with either 6-31G (d,p) for probe 2 or LAN2DZ basis set for complexes of probe 2 with metal ions.

Excited state optimization calculation shows that in probe 2 upon excitation to an excited state (HOMO→LUMO), due to ICT from the naphthol ring to the benzimidazole ring, there occurs a change in the electron density that increases the acidity of the acidic centre and the basicity of the basic centre. This leads to migration of the proton to give a phototautomer at 585 nm in probe 2. In 2.Zn²⁺ complex, ESIPT is disrupted due to deprotonation of the hydroxyl proton which causes increase in the HOMO→LUMO energy gap and planarity between the two rings as compared to the free probe 2. Excitation energy calculations indicate the highest oscillator strength is prominently characterized by HOMO→LUMO, the electron density of HOMO is mainly localized on the naphthol ring while the electron density of LUMO is localized on the benzimidazole ring, which results in complete ICT from the naphthol ring (donor) to the benzimidazole ring (acceptor) with enhanced electron withdrawing ability due to the presence of Zn²⁺ ions which exhibit its emission at 480 nm theoretically. In case of the 2.Cu²⁺ complex, the optimized structure shows that Cu²⁺ does not induce complete deprotonation, but rather transfers the hydroxyl proton of the naphthol ring to benzimidazole imine nitrogen. The frontier orbital calculations show that the electronic transistion is characterized by HOMO-1→LUMO + 1 where the electron density in both cases is spread over the entire structure but with an opposite charge, resulting in partial ICT due to the paramagnetic effect of Cu²⁺. The calculated frontier molecular orbitals of the 2.Cu²⁺.F⁻ complex show that there is no ESIPT. The transistions are predominately characterized by HOMO-1→LUMO at excited state geometry. HOMO-1 is localized on both the naphthol and benzimidazole rings and after excitation the electron is excited from the occupied π into an unoccupied π* orbital, giving excitation emission which produces the normal emission band at 400 nm and elaborates the “AND” logic function (Fig. 9 and Table 1).

Fig. 9 Theoretically optimized structures and molecular orbital diagram for probe 2 and their metal combinations.

Table 1 Comparison of TD-DFT data with the observed absorption and emission data

	λ abs (nm)		f	ε	transistion^c	λ em(nm)
	calcd	obsd				calcd	obsd
a Oscillator strength. b Molar absorptivity. c H = HOMO and L = LUMO. d Not determined.
2	350	330	1.28	48000	H > L	580	585
2.Zn^2+	420	430	0.68	21000	H > L	500	480
2.Cu^2+	430	425	0.52	13000	H-1 > L + 1	nd^d	nd
2.Cu^2+.F^−	324	330	0.92	12500	H-1 > L	430	400

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4. Conclusion

Thus, we have developed fluorescent probes for the selective estimation of Zn²⁺ and Cu²⁺ ions and a ratiometric sensing of Zn²⁺ ions. The flurophore 2 can be used to estimate 20 nM to 1000 nM of Zn²⁺ which is the lowest detection limit for the estimation of Zn²⁺ ions by a ratiometric approach. The fluorophore has been used as a Cu²⁺ concentration dependent Boolean logic operator which provides an opportunity to reconfigure NOR, INH and AND Boolean operations at three distinctly different emission channels, to be read at 585, 515 and 400 nm.

5. Experimental

Photophysical studies

UV-Vis spectroscopy experiments were carried out on a Shimadzu UV-1601 or PC Shimadzu UV-2400 machines using a slit width of 1.0 nm and 1 cm path length matched quartz cells. The fluorescence experiments were performed on Shimadzu RF-1501 fluorescence spectrophotometer using a slit width of 10 nm and a 1 cm path length quartz cell. All absorption and emission scans were saved as ACS II files and further processed in Excel™ to produce all graphs shown. Log β values for anions and metal ion complexation have been determined by analyzing the respective spectral data using an iterative method on multiwavelength data, using the global analysis programme Specfit 32. The stock solutions of fluorophores 1 and 2 (1 mM) were prepared in DMSO. An appropriate amount of aliquot was transfered to the measuring flask and the solution was diluted with doubly distilled acetonitrile to get the desired solution of the fluorophore. For metal ion titrations, the solutions of chemosensors 1 and 2 were prepared in CH₃CN–H₂O (4 : 1) containing 10 mM HEPES buffer. The solutions of metal ions were prepared in distilled water. The solutions of metal ions were added directly into a cuvette containing a solution of 1/2. Each time the solution was allowed to stand for 3 min before recording the fluorescence spectrum.

General procedure for elaboration of the logic operations

Fluorescence spectra were recorded on a Shimadzu RF1501 spectrofluorophotmeter with a 1 cm quartz cell at 25 ± 1 °C. For performing the studies, the solution of fluorophores, tetrabutylammonium fluoride (TBA F) and copper perchlorate (Cu(ClO₄)₂) were prepared in doubly distilled acetonitrile. The solution containing fluorophore 2 was taken in a quartz cell and their fluorescence spectra were recorded. The addition of a different concentration of TBA F and Cu(ClO₄)₂ was carried out with a micropipette in aliquots of 1.5–3.0 μl in the same cell and each time the solution was allowed to stand for 3 min before recording the fluorescence spectrum.

Computational details

To optimize the ground-state geometry of probe 2, B3LYP functional in conjunction with the 6-31G (d,p) basis set has been employed. The B3LYP functional uses Becke’s three-parameter exchange functional together with a function provided by Lee Yang Parr (LYP). The excited-state geometry of probe 2 was optimized at the TD-B3LYP/6-31G (d,p) and CIS/6-31G(d,p) level of theories. With the optimized ground- and excited-state geometries, the absorption and emission spectrum of probe 2 and its combination with copper and fluoride ions were calculated using the TD-DFT method with B3LYP, functionals and the 6-31G (d,p) basis set. The ground- and excited-state optimization and spectral calculations were carried out in acetonitrile medium using a polarized continuum model (PCM) to compare with the available experimental results.

Acknowledgements

We thank DST, New Delhi (ref No. SR/FT/CS-02/2011) for financial assistance and CDRI Lucknow for FAB mass spectra.

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Footnote

† Electronic Supplementary Information (ESI) available: Absorption spectra and spectral curve fitting data. See DOI: 10.1039/c2ra21170j/

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