Zinc-mediated diastereoselective assembly of a trinuclear circular helicate

Abstract

Coordination of a triptycene-based ditopic Schiff base ligand (H₂L) with Zn²⁺ leads to the formation of a novel trinuclear circular species. A combination of ¹H NMR spectroscopy, infrared spectroscopy, mass spectrometry and elemental analysis allow the unambiguous characterization of H₂L and Zn₃L₃·8H₂O. DFT molecular modelling has been used to study the possible isomers of the cyclic trinuclear Zn(II) complex that results from the combination of conformational and optical isomers of the ligand. A NOESY experiment demonstrated that in the trinuclear circular zinc(II) helicate the three isomers of the ligand have an anti configuration of N,O-donor domains and an s-cis configuration of their imine bonds. Besides, a UV-VIS absorption and fluorescence emission study of H₂L and Zn₃L₃·8H₂O has been performed.

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Introduction

In recent years, iptycenes (9,10-dihydro-9,10-[1,2]benzenoanthracene derivatives) have attracted considerable research attention because of their application as porous materials.^1–7 Triptycene is the simplest member of the iptycene family, where three benzene rings are held together by a [2.2.2] bicyclic bridge. The rigid three-bladed geometry of triptycene generates a 120° angle between the arene rings and this is attributed to their inefficient packing in the solid state. As a result, void spaces thus generated in the clefts between the arenes^8,9 are responsible for the porosity in materials derived from triptycene. This characteristic structural feature of triptycenes has been also utilized in the design of potential fluorescent chemosensors with enhanced solubility in organic solvents, wherein the presence of bulky structural motif minimizes interchain π–π interactions.^10,11 This results in lesser extent of self-quenching and consequently fluorescence quantum yield of the triptycene derivative increases considerably.¹² In contrast, electron-deficient guest species can penetrate in host clefts and interact with triptycene derivatives leading to fluorescence quenching.^12–14

Recently, we have reported fluorescence quenching of triptycene-based polymers by host–guest interaction with fullerene¹⁵ and the application of triptycene based tripods as chemosensors for selective detection of metal ions in aqueous media.¹⁶ We have also reported the utility of organometallic acceptors containing the triptycene motif as building blocks in the construction of metallasupramolecular cages.¹⁷ Herein, we have extended our studies to explore the use of a triptycene-based Schiff base ligand (H₂L, Scheme 1) as a supramolecular synthon in coordination driven self-assembly processes towards the design of trinuclear circular helicates. These latter compounds are rather rare and their formation remains challenging, comprising the vast majority of ligands employed for the preparation of such species aromatic diimine derivatives.^18,19 Nevertheless, several circular helicates with ligands other than aromatic diimine derivatives have been recently published.²⁰ DFT molecular modelling has been used to determine the most probable diastereoisomer from all possible ones of the obtained cyclic trinuclear zinc(II) complex.

Scheme 1 The two possible enantiomers of the triptycene-based ditopic Schiff base ligand, obtained by condensation of 2,6-diaminotriptycene and 2-hydroxybenzaldehyde. The numbering scheme for NMR studies on (9R,10R)-H₂L and (9S,10S)-H₂L are shown.

According to the best of our knowledge, the use of triptycene-based ditopic Schiff base ligands for the preparation of complexes in which the metal ions define a triangle with the ligands wrapping over and under has been unexplored. We have paid attention to imines because they are able to stabilize many different metals in various oxidation states, controlling the performance of metals in a large variety of useful applications²¹ in biological, clinical, analytical and industrial fields, in addition to their important roles in catalysis and organic synthesis.

Results and discussion

Schiff base ditopic ligand

The investigation was initiated to design the helicand H₂L that can be able to adopt the adequate topology required for optimal binding to naked metal ions leading to trinuclear circular helicates. The extended conjugation between the rigid three-bladed triptycene hub and both salicylaldimine pendants should lead to an Y-shaped spatial arrangement of H₂L, with a ca. 120° angle between the two pendant arms. In light of the orientation of the two N,O-donor sets of H₂L, either to the same or to opposite sides, respect to the triptycene residue, its conformational isomers have been qualified as syn or anti. Besides, two configurations of the imine bond, s-cis and s-trans,²² are possible for the anti-isomer, as Fig. 1 shows. We speculated that the assembly of three naked metal ions and three anti-isomers of the bideprotonated ligand could lead to trinuclear circular helicates (M₃L₃), while the participation of syn conformers only can lead to circular, but non-helical, trinuclear species.

Fig. 1 Capped sticks (left) and spacefill (right) representations of anti-s-cis (top), syn (centre) and anti-s-trans (bottom) conformational isomers of H₂L obtained by DFT energy-minimisation. Only (R,R) isomers are represented for simplicity. Black arrows are indicating the orientation of the N,O-donor sets for chelation (syn or anti). Configurations of the C=N bond, s-cis and s-trans, are highlighted in green. Relative energy (ΔE, ΔH and ΔG in kcal mol⁻¹) for the three conformers studied for H₂L (anti-s-trans, anti-s-cis and syn) from DFT energy-minimisations is shown at bottom.

Condensation reaction of 2-hydroxybenzaldehyde and 2,6-diaminotriptycene (2 : 1 molar ratio) occurs efficiently in ethanol solution, leading to the triptycene-based Schiff base ditopic ligand H₂L. Although we have not been able to obtain single crystals adequate for X-ray diffraction studies. H₂L has been isolated and unambiguously characterized by elemental analysis, mass spectrometry, infrared and NMR spectroscopies. Two-dimensional proton–proton (COSY) in combination with proton–carbon (HMQC and HMBC) correlation spectra have been used to achieve a full assignment of NMR signals.

To gain insight into the structure of the conformational isomer of H₂L in solution, we have undertaken a 1D NOE experiment (Fig. 2) in combination with molecular modelling (Fig. 1). The iminic proton H-17, resonating at δ 8.89 (selective band), showed correlations with the proton signals at δ 7.58 (H-21, 3.8%), δ 7.54 (H-1, 3.6%) and δ 7.03 ppm (H-7, 3.5%). These data are only consistent with one of the three modelled structures, the syn conformer of H₂L. This shows H-17⋯H-21, H-17⋯H-1 and H-17⋯H-7 distances of about 2.37, 2.10, and 2.13 Å, respectively.

Fig. 2 1D ¹H NMR spectrum (in black) and selective 1D NOE spectrum (in red) of H₂L in dmso-d₆. The signal at 8.89 ppm for selective NOE irradiation with a shape pulse was omitted for clarity.

With respect to the energy differences between the three calculated conformational isomers of H₂L, DFT studies indicate that the syn-isomer has the most favourable ΔG (Fig. 1), but in practice, all they are isoenergetic. Furthermore, all of them are kinetically accessible, since the transition states for their interconversion are very low, giving rise to a rather flat potential energy surface. As a consequence, H₂L can easily adopt an adequate spatial arrangement to behave as helicand, with both anti conformations that can lead to neutral trinuclear circular helicates. Furthermore, the syn conformer would lead to trinuclear circular species, but non-helical.

Cyclic trinuclear Zn(II) complex

To explore the application of this triptycene-based N,O-donor ligand as a potential building block for constructing novel supramolecular architectures, we studied the reaction of L²⁻ with naked Zn²⁺ ions. This self-assembly reaction yielded the cyclic trinuclear complex Zn₃L₃·8H₂O, which has been isolated and unambiguously characterized by elemental analysis, mass spectrometry (Fig. 3), infrared and NMR spectroscopies (ESI†). TLC was used to ensure purity of the compound.

Fig. 3 Comparison of experimental (in grey) and theoretically calculated (in red) peak at m/z about 1667 for Zn₃L₃·8H₂O.

Mass-spectrometric analysis was used to confirm the M₃L₃ composition of the neutral complex obtained via self-assembly of three units of the metal acceptor (Zn²⁺) and three units of the helicand triptycene based ditopic ligand (L²⁻). The only possible shape that can be assigned to the M₃L₃ composition of the complex is that of a neutral discrete cyclic trinuclear species. CHNO microanalysis supported the formation of the octahydrated metal complex with stoichiometry in equimolar ratio (M : L). The infrared stretching frequencies of C=N and C–O groups are consistent with coordination of both imine-N and phenol-O atoms to the metal ions.^23,24 The ¹H NMR spectrum of Zn₃L₃·8H₂O showed the absence of phenol protons, which is indicative of double deprotonation of the ligand (L²⁻), and subsequent coordination to Zn²⁺ metal centres. Besides, a typical high-field chemical shift (about 0.4 ppm) of the imino protons of the arms in 2 and 6-positions clearly shows the participation of the azomethine groups in the coordination. Two-dimensional proton–proton (COSY) in combination with proton–carbon (HMQC and HMBC) correlation spectra have been used to achieve a full assignment of NMR signals.

We have explored the possible isomers of the cyclic trinuclear Zn(II) complex using DFT molecular modelling. Fig. 4 shows the six most stable diastereoisomers that have been considered for the theoretical calculations. Other combinations were dismissed because it was not possible the assembly of cyclic trinuclear complexes. Four of the six possible diastereoisomers are circular helicates, as they result from the assembly of three naked metal ions and three anti conformers (as s-cis or s-trans forms) of the racemic ligand. One might note that helicity is not possible when the trinuclear complex contains syn conformers of the ligand. The handedness for these four helicates is determined by the configuration of the C=N bond. Thus, the Δ helicate with three anti-s-cis conformers contains three (R,R) ligands, while the Δ helicate with three anti-s-trans conformers contains three (S,S) ligands. A change from s-cis to s-trans or vice versa, forces a change of chirality as well. Therefore, helicity for these trinuclear circular species is conditioned by both conformation and chirality.

Fig. 4 Six possible diastereoisomers of the cyclic trinuclear zinc(II) complex (four circular helicates and two circular non-helical species) obtained by DFT energy-minimization. Only Δ isomers of helicates have been considered and minimized for simplicity. To make it easier to see, schematic representations have been included.

Fig. 5 shows the relative energy (from DFT energy-minimisations in phase gas) for the six possible isomers studied in the cyclic trinuclear zinc(II) complex formation. These studies indicate that the most favourable ΔG value corresponds to the circular non-helical species that combines one R,R-anti-s-trans form and two R,R-syn forms of the ligand. In contrast, the less favourable ΔG value corresponds to the circular helicate that combines two R,R-anti-s-cis forms and one S,S-anti-s-trans form of the ligand. However, the energy difference between the species having the least favourable and the most favourable ΔG values is less than 3 kcal mol⁻¹, and therefore anyone of six isomers can be expected in solution.

Fig. 5 Relative energy (from DFT energy-minimisations) for the six possible isomers studied in the cyclic trinuclear zinc(II) complex formation.

For the complete characterization of the cyclic trinuclear zinc(II) complex formed, its three-dimensional structure in solution was investigated using a 2D NOESY experiment (Fig. 6) in combination with molecular modelling (Fig. 4). This allows us to deduce which of the six isomers of the cyclic trinuclear zinc(II) complex is in solution (Fig. 6). The iminic proton H-17, resonating at δ 8.98 ppm, showed correlations with H-21 (at δ 7.53 ppm) and H-1/H-5 (at δ 7.71 ppm), which in the modelled structure are separated by about 2.2 Å. These data are only consistent with one of the six possibilities, the circular helicate that combines three anti-s-cis forms of the ligand. This result reflects the free rotation around the carbon–nitrogen single bond of the ligand, which in free form adopted a syn relative disposition, but coordinated is adopting the anti-s-cis form. Thus, the isomer experimentally observed in solution results from the zinc-assisted diastereoselective assembly of three units of the anti-s-cis helicand H₂L, as (R,R)- or (S,S)-enantiomers.

Fig. 6 NOESY spectrum of Zn₃L₃·8H₂O. Cross peaks corresponding to the correlation of H-17 with H-1/H-5 and H-21 is highlighted.

Optical studies

A study on the optical properties of H₂L and Zn₃L₃·8H₂O showed that both compounds have yellow fluorescence emission (near to 570 nm under UV excitation, λ_exc = 350 nm), although with a significant difference in their intensities, as zinc complexation to H₂L leads to significant fluorescence quenching. Thus, the noise observed in the fluorescence spectrum of Zn₃L₃·8H₂O (Fig. 7) is apparently caused by its low quantum yield.

Fig. 7 Emission (red line) and excitation (blue line) spectra of Zn₃L₃·8H₂O (in THF, 3.2 × 10⁻⁵ M). The absorption spectrum is also shown (black line).

The excitation spectrum is overlapped with the absorption spectrum of Zn₃L₃·8H₂O in its band of lower energy. Due to the strong absorption of the solvent used (THF), it was not possible to record, in a trustworthy way, the excitation spectrum beyond 280 nm. Anyway, the good agreement between the absorption and excitation spectra demonstrates the presence of an only fluorescent species in the ground electronic state. The fluorescence emission spectrum shows a large Stokes shift, about 12 500 cm⁻¹, that corresponds to significant reorganization energy of ca. 6250 cm⁻¹. This implies a strong intramolecular structural reorganization in the excited electronic state or conversion from the Franck–Condon state to a dark state. Reorganization of the weakly polar solvent THF could also have a minor contribution to the full Stokes shift.

A study of the stability of the fluorescence signal of an acetonitrile solution of H₂L in alkaline medium shows that its intensity decreases vs. time (Fig. 8). An NMR monitoring of a CD₃CN solution of H₂L in the presence of NMe₄OH during 3 h has thrown some light on the observed instability of the fluorescence intensity. H₂L without UV excitation remains unchanged for about 2 h and 30 min, but after 3 h small signals of 2-hydroxybenzaldehyde and 2,6-diaminotriptycene appear in the ¹H NMR spectrum as result of the Schiff base hydrolysis, which can be promoted by UV light.²⁵

Fig. 8 Variation of the fluorescence emission spectrum with time for λ = 525 nm at λ_exc = 350 nm of an acetonitrile solution of H₂L in the presence of NMe₄OH (a), and after addition of Zn²⁺ (50 μL, 1 g L⁻¹) and SDBS (100 μL, 1.5 mM) (b).

To retard the hydrolysis of H₂L in alkaline medium under UV light, we have studied the use of the following surfactants: sodium dodecylbenzenesulfonate (SDBS), sodium dodecylsulfate (SDS) and saponin at concentration levels of 1.5, 1.1 and 0.9 mM, respectively.^26–28 The best results were found for SDBS added after the addition of Zn²⁺ (coefficient of variation about 1.5%). In these conditions fluorescence intensity of H₂L was stable during at least 60 min (Fig. 8).

Fluorescence emission was measured at different concentration levels of Zn²⁺ (0–50 μg L⁻¹) to know if there is a correlation between analytical signal and concentration of Zn²⁺ added to a solution of H₂L in alkaline medium. The reproducibility within each level was acceptable (coefficients of variation of fluorescence intensities were generally below 10% for each measurement), but no correlation between analytical signal and concentration of Zn²⁺ was found.

Then, we have studied whether the UV-vis spectroscopy data (absorbance) correlate with the concentration of Zn²⁺ (0–2.4 μg L⁻¹) added to a solution of H₂L in alkaline medium (Fig. 9). The obtained results have shown coefficients of variation in the range 0.1–0.3%. The limits of detection (LOD) and quantification (LOQ) obtained at 348 nm were 4.2 and 14.1 μg L⁻¹, respectively.

Fig. 9 Variation of the UV-vis absorbance spectrum with the addition of Zn²⁺ at 267 and 348 nm.

Conclusions

The new triptycene-based ditopic Schiff base ligand H₂L was synthesized and satisfactorily characterized. H₂L is able to adopt the adequate spatial arrangement required for optimal binding to naked Zn²⁺ ions leading to a trinuclear circular helicate, or a trinuclear circular non-helical species. The assembly of three naked metal ions and three anti conformers of the ligand would result in a helicate, while the assembly of three metal ions and three syn conformers of H₂L would lead to a non-helical species.

A novel trinuclear circular species has been isolated and unambiguously characterized by elemental analysis, mass spectrometry, infrared and NMR spectroscopies. We have synthesized the circular helicate Zn₃L₃·8H₂O, which results from the zinc-assisted diastereoselective assembly of three units of the anti-s-cis helicand H₂L, as (R,R)- or (S,S)-enantiomers.

H₂L and Zn₃L₃·8H₂O have yellow fluorescence emission, although with a significant difference in their intensities, as zinc complexation to H₂L leads to significant fluorescence quenching. We have demonstrated that absorbance at 267 and 348 nm correlate with the concentration of Zn²⁺ added to a solution of L²⁻.

Experimental

Synthesis and characterization of the compounds

H₂L. A solution of 2,6-diaminotriptycene (100 mg, 0.352 mmol) and 2-hydroxybenzaldehyde (0.1 mL, 0.703 mmol) in ethanol (10 mL) with 4–5 drops of acetic acid was stirred at room temperature for overnight. Reaction progress was monitored by TLC. Then, it was obtained a yellow solid, which was washed with methanol to get the desired product. Further purification was not required. Yield: above 90%. ¹H NMR (400 MHz, dmso-d₆), ppm 13.05 (s, 2H, 2×OH), 8.89 (s, 2H, 2×H-17), 7.57 (d, 2H, J = 7.7, J = 1.7, 2×H-21), 7.53 (d, 2H, J = 2.1, H-1 and H-5), 7.49 (d, 2H, J = 7.8, H-4 and H-8), 7.44 (d, 2H, J = 8.6, J = 2.1, H-13 and H-16), 7.35 (t, 2H, J = 7.5, J = 1.7, 2×H-19), 7.03 (d, 2H, J = 7.8, J = 2.1, H-3 and H-7), 7.00 (d, 2H, J = 8.5, J = 2.3, H-14 and H-15), 6.92 (t, 2H, J = 7.5, J = 0.9, 2×H-20), 6.90 (d, 2H, J = 8.3, 2×H-18), 5.71 (s, 2H, H-9 and H-10). ¹³C NMR (125 MHz, dmso-d₆): δ/ppm 163.5 (2×C-17), 160.6 (2×C-18a), 147.0 (2×C-10a), 145.6 (C-2, C-6), 145.2 (C-11, C-12), 144.2 (2×C-8a), 133.6 (2×C-19), 132.9 (2×C-21), 125.7 (C-14, C-15), 124.9 (C-4, C-8), 124.2 (C-13, C-16), 119.7 (2×C-17a), 119.6 (2×C-20), 118.8 (C-3, C-7), 117.1 (C-1, C-5), 117.0 (2×C-18), 52.6 (C-9, C-10). IR (ATR, ν/cm⁻¹): 3441 (OH_phenol), 1623, 1609 (C=N), 1190 (C–O). MS (MALDI-TOF⁺) m/z (%): 493.7 (100) [H₂L + H]⁺. UV-vis (acetonitrile, nm) λ_max = 267, 348. Elemental analysis: C, 82.6; H, 4.6; N, 5.6%. Calcd for C₃₄H₂₄N₂O₂ (492.0) C, 82.9; H, 4.9; N, 5.7%.

Zn₃L₃·8H₂O. To an acetonitrile (10 mL) suspension of H₂L (37 mg, 0.075 mmol), 1.5 mL of a 0.1 M methanol solution of NMe₄OH·5H₂O (27.5 mg, 0.15 mmol) were added. The resultant yellow solution was mixed with a methanol (2.5 mL) solution of Zn(ClO₄)₂·6H₂O (28.5 mg, 0.075 mmol). The mixture was stirred at room temperature for 30 min and concentrated under vacuum. The yellow solid that precipitated was filtered off, washed with 15 mL of methanol during 30 min and dried in air. TLC was used to ensure purity of the compound. Alternatively, Zn₃L₃·8H₂O was obtained from a methanol solution (2 mL) of H₂L (37 mg, 0.075 mmol) and Zn(OAc)₂·H₂O (30 mg, 0.075 mmol), which was heated under reflux for 2 h. Yield: 21 mg, 50%. ¹H NMR (400 MHz, dmso-d₆), ppm 8.94 (s, 2H, 2×H-17), 7.67 (d, 2H, J = 2.0, H-1 and H-5), 7.49 (d, 2H, J = 8.1, J = 1.5, 2×H-21), 7.40 (d, 2H, J = 8.5, J = 2.0, H-13 and H-16), 7.31 (t, 2H, J = 8.6, J = 1.6, 2×H-19), 7.22 (d, 2H, J = 7.9, H-4 and H-8), 7.00 (d, 2H, J = 8.5, J = 2.2, H-14 and H-15), 6.66 (d, 2H, J = 7.7, J = 2.1, H-3 and H-7), 6.65 (d, 2H, J = 8.0, 2×H-18), 6.63 (t, 2H, J = 7.2, J = 0.9, 2×H-20), 5.52 (s, 2H, H-9 and H-10). ¹³C NMR (100 MHz, dmso-d₆): δ/ppm 171.32 (C-18a); 169.35 (2×C17); 147.15 (C-4a + C-8a); 144.59 (C-9a + C-10a); 144.33 (C-11 + C-12); 143.85 (C-2 + C-6); 137.95 (2×C21); 136.42 (2×C19); 125.84 (C-14 + C-15); 124.96 (C-4 + C-8); 124.36 (C-13 + C-16); 122.98 (C-20); 120.02 (C-3 + C-7); 119.16 (C-17a); 115.24 (C-18); 114.84 (C-1 + C-5); 52.44 (C-9 + C-10). IR (KBr, ν/cm⁻¹): 3435 (OH_water), 1607 (C=N), 1184 (C–O). MS (MALDI-TOF⁺) m/z (%): 1667.1 (36%) [Zn₃L₃ + H]⁺, 1605.2 (7%) [Zn₂L(H₂L)₂ + H]⁺, 1111.1 (36%) [Zn₂L₂ + H]⁺, 1048.1 (100%) [Zn(H₂L)₂ + H]⁺. UV-vis (THF, nm) λ_max = 270, 350. Elemental analysis: C, 67.7; H, 4.5; N, 4.6; O, 12.0%. Calcd for C₁₀₂H₈₂N₆O₁₄Zn₃ (1809.2) C, 67.7; H, 3.5; N, 4.6; O, 12.4%.

Physical measurements

H₂L and its complexes were characterized by a combination of elemental analysis, infrared spectroscopy, mass spectrometry and ¹H NMR spectroscopy (see ESI†).

Elemental analyses of C, H, N and O were performed on a Thermo Finnigan, FLASH EA 1112 series analyzer. IR spectra were recorded as KBr pellets on a FT-IR Mattson Instruments 202Q spectrophotometer in the range 4000–600 cm⁻¹. MALDI-TOF mass spectra were recorded on a Bruker Ultraflex III TOF/TOF using methanol as solvent and DCTB as matrix. ¹H NMR spectra were recorded on a Bruker DPX-250 spectrometer using DMSO-d₆ as solvent.

Computational details

The calculations were carried out at the DFT-B3LYP level with the Gaussian 09 program.²⁹ The geometries were fully optimized by the gradient technique with the following basis: for Zn, the LANL2DZ basis set; for C, H, N and O were described by the standard polarized 6-31G* basis set. The nature of the optimized structures was assessed through a frequency calculation and the changes of Gibbs free reaction energies (ΔG values) were obtained by taking into account zero-point energies, thermal motion, and entropy contribution at standard conditions (temperature of 298.15 K, pressure of 1 atm).

Although H₂L displays two chiral centres at C-9 and C-10, and consequently, there are (R,R) and (S,S) enantiomers, only the (R,R) enantiomer was considered for simplicity. It was assumed that the azomethine group presents an E-configuration, since this is typically stabilized by an intramolecular O_phenol–H⋯N_imine bonding.³⁰ Therefore, for all these conformers, the imine proton (H-17) is always in close proximity to H-21. One might note that in the anti-s-cis conformer, one of the iminic H atoms is close to H-1, while the other one is near to H-5, while in the case of the anti-s-trans conformer the iminic H atoms are close to H-3 and H-7, respectively. In the third case, the syn conformer, the closest protons are H-1 and H-7. These latter couplings are those experimentally observed in the selective NOE spectrum of H₂L.

Optical studies

We have observed two intense absorption bands for H₂L and Zn₃L₃·8H₂O at wavelengths about 270 and 350 nm, using the UV-visible spectroscopy. Thus, we have done excitation at wavelength 350 nm for fluorescence spectroscopic analysis of H₂L and Zn₃L₃·8H₂O. In order to study the fluorescence emission of H₂L in alkaline medium used to the obtaining of Zn₃L₃·8H₂O, we have prepared an acetonitrile solution of H₂L in the presence of NMe₄OH.

Both fluorescence emission and UV-vis absorption of H₂L in alkaline medium has been studied dissolving 37 mg of H₂L in 7.5 mL acetonitrile with 1.6 mL of a methanol solution of tetramethylammonium hydroxide (0.1 M). Since the absorbance was very high, it was necessary use 20 μl of the solution of H₂L in 2 mL of MeOH to perform the UV-VIS studies. Instrumental parameters of both UV-vis absorption and fluorescence emission measurements are shown in ESI.† The limits of detection (LOD) and quantification (LOQ) were calculated according to LOD = 3SD/m and LOQ = 10SD/m, were SD is the standard deviation of eleven measurements of a blank and m is the slope of the calibration graph. Coefficients of variation were calculated from the averages and standard deviations as follows: 100 × SD/average.

Absorption spectra were performed using a Varian Cary 3 UV-vis spectrophotometer. Fluorescence emission and excitation spectra were performed using a SPEX Fluorolog-2 spectrofluorometer. The photometric correction were obtained using the Gardecki and Maroncelli method,³¹ giving spectral deviations of less than 5% in the range 300–750 nm. All the spectra have been corrected by the base line of the solvent. Results correspond to the mean value of five spectra measured independently and consecutively. Fluorescence spectra have been represented as a fluorescence quantum distribution (FQD) vs. wavenumber.

Acknowledgements

N. D. thanks the Indian Institute of Technology (IIT) Patna, for financial support. S. B. thanks IIT Patna for an Institute Research Fellowship.

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Footnote

† Electronic supplementary information (ESI) available: Spectroscopic studies on the compounds. See DOI: 10.1039/c6ra01783e

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