Phenanthroimidazole based systems due to the dramatic supramolecular properties, stability, ease of synthesis, a high extinction coefficient and tuneable photophysical properties have been explored as potential chromophores in a molecular-tweezers system for explosives [45Day, A.R.; Steck, E.A. Reactions of phenanthraquinone and retenequinone with aldehydes and ammonium acetate in acetic acid solution. J. Am. Chem. Soc., 1943, 65, 452-456.
[http://dx.doi.org/10.1021/ja01243a043] ], amine sensors [46Krebs, F.C.; Jørgensen, M. A new versatile receptor platform. J. Org. Chem., 2001, 66(18), 6169-6173.
[http://dx.doi.org/10.1021/jo015690j] [PMID: 11529747] , 47Bu, L.; Sawada, T.; Kuwahara, Y.; Shosenji, H.; Yoshida, K. Crystallographic structure and solid-state fluorescence enhancement behavior of a 2-(9-anthryl)phenanthroimidazole-type clathrate host upon inclusion of amine molecules. Dyes Pigm., 2003, 59, 43-52.
[http://dx.doi.org/10.1016/S0143-7208(03)00099-8] ], fluorophore in a super-radiant laser dye [48Krebs, F.C.; Lindvold, L.R.; Jorgensen, M.J. Superradiant properties of 4,4′-bis(1H-phenanthro[9,10-d]imidazol-2-yl)biphenyl and how a laser dye with exceptional stability can be obtained in only one synthetic step. Tetrahedron Lett., 2001, 42, 6753-6757.
[http://dx.doi.org/10.1016/S0040-4039(01)01330-2] ], fluorescent tag [49Noormofidi, N.; Slugovc, C. Acid/base-sensitive ring-opening metathesis polymers based on phenanthroimidazole dyes. Macromol. Chem. Phys., 2007, 208, 1093-1100.
[http://dx.doi.org/10.1002/macp.200700038] ], OLED materials [50Zhanga, Y.; Ng, T-W.; Lua, F.; Tonga, Q-X.; Lai, S-L.; Chanc, M-Y.; Kwong, H-L.; Lee, C-S. A pyrene-phenanthroimidazole derivative for non-doped blue organic light-emitting devices. Dyes Pigm., 2013, 98, 190-194.
[http://dx.doi.org/10.1016/j.dyepig.2013.01.022] -52Zhanga, X.; Linb, J.; Ouyanga, X.; Liua, Y.; Liub, X.; Gea, Z.J. Novel host materials based on phenanthroimidazole derivatives for highly efficient green phosphorescent OLEDs. Photochem. Photobiol., 2013, 268, 37-43.
[http://dx.doi.org/10.1016/j.jphotochem.2013.06.012] ] and chemosensors [53Zheng, K.; Lin, W.; Tan, L. A phenanthroimidazole-based fluorescent chemosensor for imaging hydrogen sulfide in living cells. Org. Biomol. Chem., 2012, 10(48), 9683-9688.
[http://dx.doi.org/10.1039/c2ob26956b] [PMID: 23147805] -56Song, K.C.; Choi, M.G.; Ryu, D.H.; Kim, K.N.; Chang, S-K. Ratiometric chemosensing of Mg2+ ions by a calix[4]arene diamide derivative. Tetrahedron Lett., 2007, 48, 5397-5400.
[http://dx.doi.org/10.1016/j.tetlet.2007.06.010] ]. The one-step synthesis of phenanthroimidazole derivatives has remarkably increased our own interest to involve a variety of aldehydes that may further extend the way for the introduction of other functionalities and to extend the conjugation for different purposes. The objective of designing a selective and sensitive dual anion responsive ratiometric probe has been achieved by developing a conjugated π-electron system containing specific ionophores, in which the electron rich phenanthroimidazole (the donor) and electron deficient dicyanovinyl (the acceptor) units are linked through a phenyl ring. The introduction of two distinct potential ionophores structurally, the -NH and dicyanovinyl units make sure DCPPI as an efficient dual anion sensing ICT probe which works on a push-pull mechanism of the donor and acceptor moieties in the present typical structural motif.

A simple three step synthetic route to afford DCPPI is shown in Scheme. (1). Phenanthrene was first oxidized [67Furniss, B. S.; Hannaford, A. J.; Smith, P.W.G.; Tatchell, A. R. Vogel’s Textbook of Practical Organic Cemistry, 5^th; Tearson Higher Education, 1996, pp. 1023-1024.] by K₂Cr₂O₇ in aqueous-acidic medium to obtain 9,10-phenanthroquinone 2 in good yield (76%). Compound 2 upon reaction [45Day, A.R.; Steck, E.A. Reactions of phenanthraquinone and retenequinone with aldehydes and ammonium acetate in acetic acid solution. J. Am. Chem. Soc., 1943, 65, 452-456.
[http://dx.doi.org/10.1021/ja01243a043] ] with different aromatic aldehydes in the presence of ammonium acetate (AcONH₄) gave derivatives 3a-f in 65%-80% yields. It is important to mention that 2 upon reaction with teraphthaldehyde gives mixture of products that were separated through column chromatography to obtain compound 3f as a major product (~85%) and bis-phenanthroimidazolyl derivative 4 as a minor by product. Further, in order to introduce second ionophore unit, aldehyde function of compound 3f was reacted with malononitrile in the presence of triethylamine (TEA) to get 2,2–dicyanovinyl phenanthroimidazole, DCPPI as an orange color solid in 85% yield. The introduction of dicyanovinyl function makes DCPPI a suitable substrate to work as Michael acceptor which can easily undergo nucleophilic addition reaction. As a proof of concept, potential probe DCPPI when treated with sodium cyanide in methanol gave tricyanophenanthroimidazolyl derivative, DCPPIA in good yield. The chemical structures were well characterized and data are given in supporting information (Fig. S1- S12).

The sensor motif consisting of imidazole units is expected to interact with anions through hydrogen bonding interaction and/or deprotonation [1Martínez-Máñez, R.; Sancenón, F. Fluorogenic and chromogenic chemosensors and reagents for anions. Chem. Rev., 2003, 103(11), 4419-4476.
[http://dx.doi.org/10.1021/cr010421e] [PMID: 14611267] -6Lakowicz, J.R. Principles of Fluorescence Spectroscopy, 3^rd ed; Springer: New York, 2006.
[http://dx.doi.org/10.1007/978-0-387-46312-4] , 15Xu, Z.; Chen, X.; Kim, H.N.; Yoon, J. Sensors for the optical detection of cyanide ion. Chem. Soc. Rev., 2010, 39(1), 127-137.
[http://dx.doi.org/10.1039/B907368J] [PMID: 20023843] -29Dong, M.; Peng, Y.; Dong, Y-M.; Tang, N.; Wang, Y-W. A selective, colorimetric, and fluorescent chemodosimeter for relay recognition of fluoride and cyanide anions based on 1,1′-binaphthyl scaffold. Org. Lett., 2012, 14(1), 130-133.
[http://dx.doi.org/10.1021/ol202926e] [PMID: 22149895] ]. Consequently, the change in intramolecular charge transfer (ICT) process will significantly affect the physicochemical behavior of the probe by push-pull mechanism [7Suksai, C.; Tuntulani, T. Chromogenic anion sensors. Chem. Soc. Rev., 2003, 32(4), 192-202.
[http://dx.doi.org/10.1039/b209598j] [PMID: 12875025] -12Wang, F.; Wang, L.; Chen, X.; Yoon, J. Recent progress in the development of fluorometric and colorimetric chemosensors for detection of cyanide ions. Chem. Soc. Rev., 2014, 43(13), 4312-4324.
[http://dx.doi.org/10.1039/c4cs00008k] [PMID: 24668230] , 19Gunnlaugsson, T.; Kruger, P.E.; Jensen, P.; Tierney, J.; Ali, H.D.; Hussey, G.M. Colorimetric “naked eye” sensing of anions in aqueous solution. J. Org. Chem., 2005, 70(26), 10875-10878.
[http://dx.doi.org/10.1021/jo0520487] [PMID: 16356013] -29Dong, M.; Peng, Y.; Dong, Y-M.; Tang, N.; Wang, Y-W. A selective, colorimetric, and fluorescent chemodosimeter for relay recognition of fluoride and cyanide anions based on 1,1′-binaphthyl scaffold. Org. Lett., 2012, 14(1), 130-133.
[http://dx.doi.org/10.1021/ol202926e] [PMID: 22149895] ] and the color changes make the sensing event more sensitive, reliable and visible to the naked-eye. To strengthen our hypothesis we first observed the optoelectronic behavior of synthesized compounds in solvents of different polarity and data are given in Table S1. The absorption and emission spectra of derivatives 3a-g and DCPPI showed good solvatochromic behavior in which the absorption maxima, relative fluorescence intensity, Stoke’s shift and color of the solution varied according to hydrogen bonding capabilities and polarities of solubilizing milieu [59Misra, A.; Shahid, M. Chromo and fluorogenic properties of some azo-phenol derivatives and recognition of Hg2+ ion in aqueous medium by enhanced fluorescence. J. Phys. Chem. C., 2010, 114, 16726-16739.
[http://dx.doi.org/10.1021/jp1049974] ] such as hexane, 1,4-dioxane, acetonitrile (MeCN), chloroform (CHCl₃), N,N’-dimethylformamide (DMF) and ethanol (EtOH) (Figs. 1 and S13, S13a). It is important to mention that hydrogen bonding between probe DCPPI and the solvents led to a bathochromic shift of the fluorescence maxima accompanied by the increase in the Stoke’s shift and molar absorbtivity. UV-vis absorption spectrum of DCPPI (5µM) in MeCN exhibited intramolecular charge transfer (ICT) band at 427 (ε = 2.99 x 10⁴ M^-1 cm^-1) while, the emission spectrum of DCPPI (1.5 μM) displayed emission at 614 nm (λ_ex. at 427 nm; Ф_DCPPI = 0.168; Stoke’s shift (∆ῡ)= 7132 cm^-1) (Fig. 2a, b).

Scheme. (1) (i) K2Cr2O7/H2SO4/Δ (ii) R-CHO/AcONH4/AcOH/Δ (iii) Malononitrile/DCM/TEA /r.t. (iv) NaCN/MeOH/r.t.

Fig. (1) Normalized typical emission spectra and images (inset) of DCPPI in different medium.

It is important to mention that the non-covalent ionic recognition event of a receptor having amino and hydroxyl fragments as a potential ionophore are susceptible to interact with anions (like F^- and AcO^-) through hydrogen bonding interaction in solvents of variable polarity and are dependent on the basicity and structure aspects of the anions [15Xu, Z.; Chen, X.; Kim, H.N.; Yoon, J. Sensors for the optical detection of cyanide ion. Chem. Soc. Rev., 2010, 39(1), 127-137.
[http://dx.doi.org/10.1039/B907368J] [PMID: 20023843] -29Dong, M.; Peng, Y.; Dong, Y-M.; Tang, N.; Wang, Y-W. A selective, colorimetric, and fluorescent chemodosimeter for relay recognition of fluoride and cyanide anions based on 1,1′-binaphthyl scaffold. Org. Lett., 2012, 14(1), 130-133.
[http://dx.doi.org/10.1021/ol202926e] [PMID: 22149895] ]. Therefore, the protic polar solvents should be avoided in anion recognition studies to substantiate actual mode of interaction [68Hijji, Y.M.; Barare, B.; Kennedy, A.P.; Butcher, R. Synthesis and photophysical characterization of a Schiff base as anion sensor. Sens. Actuators B. Chem., 2009, 136, 297-302.
[http://dx.doi.org/10.1016/j.snb.2008.11.045] , 69Boiocchi, M.; Del Boca, L.; Gómez, D.E.; Fabbrizzi, L.; Licchelli, M.; Monzani, E. Nature of urea-fluoride interaction: incipient and definitive proton transfer. J. Am. Chem. Soc., 2004, 126(50), 16507-16514.
[http://dx.doi.org/10.1021/ja045936c] [PMID: 15600354] ]. While analyzing the photophysical behavior of different derivatives toward anion, we found that acetonitrile (MeCN) is a suitable aprotic polar solvent because of ease in dissolving a wide range of ionic and nonpolar species, and easily miscible with water to create a homogeneous medium. Thus, considering the sensitivity of anions we intended to observe photophysical behavior of probe DCPPI toward different anions in MeCN.

Notably, upon interaction with different class of anions (10 Equiv) such as, F^-, Cl^-, Br^-, I^-, NO₃^-, N₃^-, SO₄^2-, H₂PO₄^-, CO₃^2-, AcO^-, CN^-, and SCN^-considerable change in the electronic transition spectrum of DCPPI was observed in the presence of F^-, AcO^- and CN^- ions. The ICT band centered at 427 nm disappeared and a new transition band appeared at 545 nm with F^- (ε = 3.06 x 10⁴ M^-1 cm^-1) and AcO^- (ε = 2.11 x 10⁴ M^-1 cm^-1) ions, respectively. Similarly, upon interaction with CN^- a blue shift of ~102-63 nm was observed and a new broad band appeared at 325 (ε = 2.76 x 10⁴ M^-1 cm^-1) with shoulders at 364 (ε = 2.18 x 10⁴ M^-1 cm^-1), and 347 (ε = 2.15 x 10⁴ M^-1 cm^-1) nm (Fig. 2a). Moreover, upon interaction with different tested anions the emission intensity of DCPPI (at 614 nm; λ_ex.= 427 nm; 1 µM) showed partial fluorescence quenching (~ 46%) with AcO^- whereas, with F^-and CN^-emission band centered, at 614 nm disappeared completely and new emission bands appeared at 492 and 490 nm (blue shift, ~122 and 124 nm) respectively (Fig. 2b). The spectral pattern observed in the presence of F^- and AcO^- suggested about the probability of deprotonation and/or H-bonding with the ionophore, the –NH fragment of DCPPI. Similarly, upon interaction with CN^- the variation observed in the emission and absorption spectra suggested about the possibility of formation of new stable species in the medium. Additionally, colorimetric naked eye sensitive light yellow-green color solution of DCPPI changed to a light pink with F^- (stable for almost 24 hr) and AcO^- (stable for ~10 min) while with CN^- a transient red-pink color disappeared within second to become colorless. However, under UV light considerable fluorogenic response observed wherein, a stable fluorescent white-blue color of DCPPI appeared as light blue, immediately upon interaction with F^-, AcO^- and CN^- ions (Fig. 3). The other tested anions failed to exhibit any significant change in the absorption and emission spectra.

Fig. (2) (a) Absorption (5 µM) and (b) Emission spectra (1 µ M) of interaction of DCPPI with different anions (10 Equiv) in MeCN.

Fig. (3) Images show change in color of DCPPI upon interaction with different anions (a) normal and (b) under UV light.

Furthermore, to our own interest we attempted to understand the optical behavior of DCPPI toward anions in partial aqueous medium. This is because in aqueous or semiaqueous medium, salvation energy [57Sluys, W.G. The solubility rules: Why are all acetates soluble? Chem. Educ., 2001, 78, 111.
[http://dx.doi.org/10.1021/ed078p111] , 58Marcus, Y. Thermodynamics of solvation of ions. J. Chem. Soc., Faraday Trans., 1991, 87, 2995-2999.
[http://dx.doi.org/10.1039/FT9918702995] ] for CN^- remains low (ΔH_hyd = -67 kJ/mol) in comparison to other anions (for example ΔH_hyd = -505, -363, -336, -295, -375, -260, kJ/mol for F^-, Cl^-, Br^-, I^-, AcO^-, and H₂PO₄^- respectively in H₂O/MeCN) and would enhance selectivity of DCPPI for CN^-.The absorption spectrum of DCPPI (5 µM) in H₂O-MeCN (50%) showed ICT band at 423 (ε = 2.26 x 10⁴ M^-1 cm^-1) nm and exhibited emission toward high wavelength, at 634 nm (at λex. = 423 nm). Interestingly, when we examined the probe in the presence of different tested anions (20 Equiv) DCPPI showed high selectivity for CN^- in which ICT band centered, at 423 nm disappeared and new broad band appeared at 363 (ε = 1.70 x 10⁴ M^-1 cm^-1), 345 (ε = 1.50 x 10⁴ M^-1 cm^-1) and 325 nm (ε = 1.64 x 10⁴ M^-1 cm^-1) (Fig. 4a). Similarly, the emission spectrum of DCPPI (1 µM) displayed significant fluorescence quenching (at 634 nm) only with CN^- and a new emission band appeared at 514 nm (blue shift, ~120 nm) (Fig. 4b). The significant blue shift in the emission behavior of DCPPI could be attributed to a possible Michael type nucleophilic addition reaction, leading to the formation of a Michael adduct in the medium. However, rest of the anions failed to exhibit any significant change in the transition spectra. Interference studies have been performed by the addition of excess of competitive anions (40 Equiv) to a probable solution of DCPPI+CN^-. The observed insignificant change in the absorption and emission spectra of DCPPI+CN^- (Fig. S14) make sure high selective affinity of DCPPI for cyanide anion.

Fig. (4) (a) Absorption (5µM) and (b) Emission spectra (1µM) of DCPPI upon interaction with different anions (20 Equiv) in H2O-MeCN (50 %).

The binding affinities of DCPPI toward tested anions have been examined through the absorption and emission titration experiments in acetonitrile (Fig. 5). Upon a sequential addition of F^- (0-9 Equiv) the ICT band centered, at 427 nm reduced gradually and a new absorption band appeared at 545 nm, ratiometrically. The formation of an isosbestic point at 470 nm suggested the existence of more than one species in the medium (Fig. 5). Similarly, the emission titration experiment was performed by the addition of F^- (0-30 Equiv.) ion to a solution of DCPPI. The emission intensity of DCPPI centered, at 614 nm diminished gradually and a new emission band with enhanced relative fluorescence intensity appeared at 492 nm, ratiometrically (Fig. 5). A careful examination of emission titration spectra suggested that upon addition of ~10 Equiv of F^- initially, the relative emission intensity, at 614 nm quenched significantly with decrease in quantum yield (~65%; Ф_DCPPI+F^-= 0.058). Further, addition of F^- ions (10 to 30 Equiv.) led to significant increase in emission intensity of a new emission band, at 492 nm, and the net quantum yield enhanced by ~74% (Ф_DCPPI+F^-= 0.223). The observed significant change in the emission spectra suggested about the deprotonation of N–H fragment of DCPPI. Consequently, ICT increases due to charge propagation form phenanthroimidazolyl unit to electron deficient dicyanovinyl fragment. Job’s plots analysis revealed a 1:1 stoichiometry for a probe-fluoride interaction, consistently. The binding constants based on absorption and emission titration spectral data have been estimated through Benesi-Hildebrand (B-H) method [70Benesi, H.A.; Hildebrand, J.H. A spectrophotometric investigation of the interaction of iodine with aromatic hydrocarbons. J. Am. Chem. Soc., 1949, 71, 2703-2707.
[http://dx.doi.org/10.1021/ja01176a030] , 71Connors, K.A. Binding constant, 1^st ed; Wiley: New York, 1987. ], and were found to be K_ass(abs) = 2.96 x 10⁴ /M and K_ass(em) = 3.82 x 10⁴ / M respectively . (Figs. 5, S16a, and Table 1).

Similarly, the absorption titration spectra of DCPPI, revealed almost similar spectral pattern in the presence of AcO^- (0–7 Equiv) ions in which, ICT band (at 427 nm) disappeared and a new band appeared at 545 nm, concomitantly (Fig. S15a, b). Moreover, the emission spectra upon addition of AcO^-(0-35 Equiv.) displayed ratiometric behavior in which the emission intensity (at 614 nm) diminished completely, along with decrease in quantum yield by ~58% (Ф_DCPPI+AcO-= 0.07) and a new emission band appeared at 486 nm (Ф_DCPPI+AcO- = 0.083). The formation of isosbestic and isoemissive points at 475 and 548 nm, respectively, further suggested about the presence of more than one species in the medium. Job’s plots analysis revealed a 2:1 equilibrium for an interaction between DCPPI and AcO^- for which binding constant has been estimated using emission titration data and were found to be 1.48 x 10² /M (Fig. S15c, S15d, Table 1).

Fig. (5) (a) Absorption (5µM) and (b) emission (1.5µM) titration spectra of DCPPI upon addition of (0-9 Equiv) and (0-30 Equiv) of F- anions, respectively in MeCN. (c) Job’s plot and Benesi-Hildebrand plots obtained from emission spectral data.

Further understanding the binding behavior of cyanide both absorption and emission titration experiments was performed. The typical spectral changes that occur for DCPPI as a function of CN^- are shown in Fig. (6). As can be seen in Fig. (6), the intensity of ICT band (at 427 nm) decreased upon a gradual addition of 0–2 Equiv. of CN^- and a new band appeared at 545 nm (ε = 1.25 x 10⁴ M^-1cm^-1). However, further addition of CN^-(2 to 5.0 Equiv) decreased the absorption of new band, (at 545 nm) and high energy band appeared at 331-361 nm, progressively and the color of solution disappeared. More interestingly, the emission spectrum of DCPPI upon addition of 0-4 Equiv. of CN^- showed significant fluorescence quenching in which relative emission intensity (centered, at 614 nm) decreased, ~ 90% (Fig. 6) along with decrease in quantum yield by ~60% (Ф_DCPPI+CN-= 0.069). A further addition of CN^- (4.5-14 Equiv) led to successive fluorescence enhancement at 490 nm and the quantum yield increased by 50% (~2 fold; Ф_{DCPPI + CN}-= 0.14).

Table 1

Estimated data for probe DCPPI.

The color of solution became light blue with saturation in emission spectral profile (Fig. 6 and 3). It is interesting to mention that interaction of DCPPI with CN^- led to the existence of two successive reaction equilibria in the medium. The stoichiometry for the possible mode of interaction between DCPPI and CN^- has been realized. Job’s plot analysis revealed a 1:1 equilibrium at low concentration of CN^-. However, as the concentration of CN^- increased equilibrium changed to 1:2 from 1:1. The binding constants for a 1:2 equilibrium have been estimated through the nonlinear fittings of titration data and were found to be K_ass(abs) = 1.5 x 10¹⁰/M² and K_ass(em) = 4.06 x 10¹⁰/M² respectively (Figs. 6, S16b, and Table 1). Thus, the significant change in the physicochemical properties of DCPPI clearly suggested about the interaction of CN^- through different channels and in two successive steps. First, ICT get restricted, due to the formation of a Michael adduct, DCPPI-CN^- and fluorescence quenching observed at 614 nm. Secondly, the interaction of CN^- with second ionophore, the –NH fragment of resultant adduct, DCPPI-CN^- led to enhance ICT due to an increase in charge density on the phenanthroimidazolyl unit, that finally led to a significant fluorescence enhancement (at 490 nm) [19Gunnlaugsson, T.; Kruger, P.E.; Jensen, P.; Tierney, J.; Ali, H.D.; Hussey, G.M. Colorimetric “naked eye” sensing of anions in aqueous solution. J. Org. Chem., 2005, 70(26), 10875-10878.
[http://dx.doi.org/10.1021/jo0520487] [PMID: 16356013] -29Dong, M.; Peng, Y.; Dong, Y-M.; Tang, N.; Wang, Y-W. A selective, colorimetric, and fluorescent chemodosimeter for relay recognition of fluoride and cyanide anions based on 1,1′-binaphthyl scaffold. Org. Lett., 2012, 14(1), 130-133.
[http://dx.doi.org/10.1021/ol202926e] [PMID: 22149895] , 50Zhanga, Y.; Ng, T-W.; Lua, F.; Tonga, Q-X.; Lai, S-L.; Chanc, M-Y.; Kwong, H-L.; Lee, C-S. A pyrene-phenanthroimidazole derivative for non-doped blue organic light-emitting devices. Dyes Pigm., 2013, 98, 190-194.
[http://dx.doi.org/10.1016/j.dyepig.2013.01.022] -58Marcus, Y. Thermodynamics of solvation of ions. J. Chem. Soc., Faraday Trans., 1991, 87, 2995-2999.
[http://dx.doi.org/10.1039/FT9918702995] ].

Fig. (6) (a) UV-vis absorption (5µM) and (b) emission titration spectra (1.5µM) of DCPPI upon addition of 0-5 Equiv and 0-14 Equiv of CN- in MeCN. (c) Job’s plot and Benesi-Hildebrand plots obtained from emission spectral data.

Moreover, since the changes in optoelectronic behavior of probe in the presence of different anions so rapid it was difficult to obtain a precise kinetic plot. We tried to obtain kinetic plot by the addition of CN^- (~20 Equiv) to a solution of DCPPI and measured the emission spectra at regular time interval. A time versus relative fluorescence intensity plot has indicated about the completion of reaction and the changes in photophysical properties of the probe approximately within ~ 10 min, wherein the intensity of emission band, centered at 614 nm decrease gradually while the intensity of a new emission band (appeared at ~490 nm) enhances ratiometrically (Fig. S17).

The limit of detection (LOD) for F^- and CN^- ions has been estimated through the fluorescence spectroscopic method, as reported previously [59Misra, A.; Shahid, M. Chromo and fluorogenic properties of some azo-phenol derivatives and recognition of Hg2+ ion in aqueous medium by enhanced fluorescence. J. Phys. Chem. C., 2010, 114, 16726-16739.
[http://dx.doi.org/10.1021/jp1049974] -66Srivastava, P.; Ali, R.; Razi, S.S.; Shahid, M.; Patnaik, S.; Misra, A. A simple blue fluorescent probe to detect Hg²⁺ in semiaqueous environment by intramolecular charge transfer mechanism. Tetrahedron Lett., 2013, 54, 3688-3693.
[http://dx.doi.org/10.1016/j.tetlet.2013.05.014] ]. A calibration curve was obtained by measuring the fluorescence spectra of different concentration (from 5.2 μM to 0.8 μM) of DCPPI. An approximately straight calibration curve suggested about a linear correlation between relative fluorescence intensities and concentrations of the probe and gave standard deviation (σ) 0.30 as a function of probe concentrations (Fig. S18a). To estimate calibration sensitivity (m) fluorescence plots between Δ (I – I₀) (where I₀ and I illustrate the emission intensities of DCPPI in the presence and absence of F^- and CN^-ions respectively) and concentration of both the anions (F^-/CN^-) were obtained in the aforementioned range (Fig. S18b, S18c). The slope of fluorescence curves gave calibration sensitivity (m) 6.03 and 4.27 for F^- and CN^-, respectively. Now, equation (4) has been utilized to obtain the limit of detection (LOD) for F^- and CN^- ions as 149 nM (2.83 ppb) and 210 nM (5.5 ppb), respectively (Table 1). Thus, the DCPPI can be applied to detect F^- and CN^- in nM range which might fulfill the requirement of a potential fluorescent chemosensor and chemodosimeter to sense anions in drinking water as well as in biosensing [72Yang, Y.; Zhao, Q.; Feng, W.; Li, F. Luminescent chemodosimeters for bioimaging. Chem. Rev., 2013, 113(1), 192-270.
[http://dx.doi.org/10.1021/cr2004103] [PMID: 22702347] ].

To gain further insight into the actual nature of interaction ¹H NMR titration experiments were performed. The ¹H NMR spectrum of DCPPI (2.1 x 10^-2 M; DMSO-d₆) (Fig. 7) showed doublet(s) at δ 8.495-8.467(J = 8.4 Hz), 8.152-8.124 (J = 8.4 Hz), 8.863-8.842 (J = 6.3 Hz) and 8.577-8.552 (J = 7.5 Hz) ppm corresponding to Ha, Hb, Hc, and Hf resonances, respectively. A multiplet appeared at δ 7.76-7.66 ppm may be assigned to resonances of Hd and He protons (Figs. 7, S5) while resonances appeared at δ 13.713 and at δ 8.53 ppm, respectively are attributable to -NH and H_V protons of imidazolyl and dicyanovinyl units. Upon a sequential addition of F^- ions (0-1.0 Equiv) to a solution of DCPPI the -NH resonance initially broadened and disappeared. The chemical shifts corresponding to resonances of Hc, Hb and Hd, He protons shifted upfield and appeared at δ 8.84 (Δδ = 0.023 ppm), 8.06 (Δδ = 0.086 ppm), and 7.69 (Δδ = 0.063 ppm) ppm. Similarly, significant downfield shifts corresponding to Hf (8.664; Δδ = 0.087 ppm), H_V (8.573; Δδ = 0.043 ppm) and Ha (8.637; Δδ = 0.142 ppm) proton resonances were also observed (Figs. S19- S22). Thus, the observed noticeable change particularly, at the –NH proton of DCPPI suggested about the deprotonation of the N–H fragment in the presence of F^- [19Gunnlaugsson, T.; Kruger, P.E.; Jensen, P.; Tierney, J.; Ali, H.D.; Hussey, G.M. Colorimetric “naked eye” sensing of anions in aqueous solution. J. Org. Chem., 2005, 70(26), 10875-10878.
[http://dx.doi.org/10.1021/jo0520487] [PMID: 16356013] -29Dong, M.; Peng, Y.; Dong, Y-M.; Tang, N.; Wang, Y-W. A selective, colorimetric, and fluorescent chemodosimeter for relay recognition of fluoride and cyanide anions based on 1,1′-binaphthyl scaffold. Org. Lett., 2012, 14(1), 130-133.
[http://dx.doi.org/10.1021/ol202926e] [PMID: 22149895] ].

Fig. (7) Stacked partial 1H NMR spectra of DCPPI (2.1 x 10-2 M) upon addition of F- ions (0-1.0 Equiv) in DMSO-d6.

Similarly, the ¹HNMR titration spectra (Fig. 8) of DCPPI illustrated significant changes in the presence of CN^- ions. Upon addition of 0.25 to 0.5 Equiv of CN^- chemical shift corresponding to H_V proton at (8.53 ppm) get reduced and a new signal (H_V’) appeared in the high field region, at (4.687 ppm). Further after the addition of 1.0 Equiv of CN^- H_V signal disappeared completely and H_V’ signal dominated. Simultaneously, after the addition of 0.5 Equiv of CN^- the -NH proton signal at (13.713 ppm) broadened without any significant shift and disappeared completely after the addition of 0.75 Equiv of CN^-. The other observed change in the ¹H NMR signals was almost consistent to changes occurred with F^- ions. Moreover, the resonances attributed to Hc, Hd and He protons shifted upfield to appear at (8.83 (Δδ = 0.03 ppm) and (7.715 (Δδ = 0.045 ppm) ppm while Hb, Hf and Ha protons shifted downfield to appear at (8.302(Δδ = 0.15 ppm) and 8.628(Δδ = 0.05 ppm) ppm, respectively (Fig. 8, S23- S25). Thus, clearly suggested about the dual mode of interaction wherein, first Michael adduct was formed due to the nucleophile addition of cyanide at the dicyanovinyl fragment. The new entity so formed then further establish an interaction with -NH fragment of phenanthroimidazolyl was established [26Kumari, N.; Jha, S.; Bhattacharya, S. Colorimetric probes based on anthraimidazolediones for selective sensing of fluoride and cyanide ion via intramolecular charge transfer. J. Org. Chem., 2011, 76(20), 8215-8222.
[http://dx.doi.org/10.1021/jo201290a] [PMID: 21892827] , 35Mo, H-J.; Shen, Y.; Ye, B-H. Selective recognition of cyanide anion via formation of multipoint NH and phenyl CH hydrogen bonding with acyclic ruthenium bipyridine imidazole receptors in water. Inorg. Chem., 2012, 51(13), 7174-7184.
[http://dx.doi.org/10.1021/ic300217v] [PMID: 22716094] ] through the -N…CN…H type of hydrogen bonding leading to typical deprotonation of the N–H function [19Gunnlaugsson, T.; Kruger, P.E.; Jensen, P.; Tierney, J.; Ali, H.D.; Hussey, G.M. Colorimetric “naked eye” sensing of anions in aqueous solution. J. Org. Chem., 2005, 70(26), 10875-10878.
[http://dx.doi.org/10.1021/jo0520487] [PMID: 16356013] -29Dong, M.; Peng, Y.; Dong, Y-M.; Tang, N.; Wang, Y-W. A selective, colorimetric, and fluorescent chemodosimeter for relay recognition of fluoride and cyanide anions based on 1,1′-binaphthyl scaffold. Org. Lett., 2012, 14(1), 130-133.
[http://dx.doi.org/10.1021/ol202926e] [PMID: 22149895] ].

Further, to prove the interaction of DCPPI with cyanide, ¹³C NMR spectra were analyzed. In the absence of cyanide ¹³C NMR spectrum of DCPPI showed resonances at δ 160.24, 148.67, 147.389, 137.730, 113.56 and 112.33, and 80.98 ppm attributable to C3, C8, C7, C4, C1 and C2 carbons, respectively (Fig. S6). Upon the addition of 1.5 Equiv of CN^- the C3, C2 and C7 carbon signals shifted considerably in the upfield region and appeared at δ 30.344, 25.473 and 143.33 ppm, respectively while the C8 signal shifted downfield to appear at 148.792 ppm. Moreover, the carbon resonances of the phenyl ring showed upfield shift and was in the range of ∆δ 0.15 to 3 ppm (Fig. S25). The significant upfield shift in the resonances of C2 and C3 carbons clearly supported the formation of a Michael types adduct, DCPPI-CN^- in the medium. Simultaneously, the downfield shift occurred in the resonances of C8 carbon also suggested about the interaction of CN^- with –NH fragment of the imidazolyl unit through the hydrogen bonding [19Gunnlaugsson, T.; Kruger, P.E.; Jensen, P.; Tierney, J.; Ali, H.D.; Hussey, G.M. Colorimetric “naked eye” sensing of anions in aqueous solution. J. Org. Chem., 2005, 70(26), 10875-10878.
[http://dx.doi.org/10.1021/jo0520487] [PMID: 16356013] -29Dong, M.; Peng, Y.; Dong, Y-M.; Tang, N.; Wang, Y-W. A selective, colorimetric, and fluorescent chemodosimeter for relay recognition of fluoride and cyanide anions based on 1,1′-binaphthyl scaffold. Org. Lett., 2012, 14(1), 130-133.
[http://dx.doi.org/10.1021/ol202926e] [PMID: 22149895] ].

Fig. (8) Stacked partial 1H NMR spectra of DCPPI (2.1 x 10-2 M) upon addition of CN- ions (0-0.75 Equiv.) in DMSO-d6.

It is worth to mention that both F^- and CN^- ions have enough basicity to act as proton abstractors. When both the anions come close to the N–H fragment (acidic in nature) of phenanthroimidazolyl unit hydrogen bonding interaction occurs in the early transition state, which leads to a partial proton transfer from the probe to anions. Later on, the net increase in charge density, due to deprotonation of NH unit, causes shielding effect and leads to a significant change in the photophysical behavior of the probe along with change in the color of solution. Additionally, 1:1 and 1:2 stoichiometries evidenced different modes of interaction between DCPPI and F^- and CN^- ions, respectively. The photophysical and NMR spectral behavior clearly suggested that CN^- first reacted with dicyanovinyl function of DCPPI to form a Michael type adduct, DCPPI-CN^-, and restrict the ICT. Consequently, the emission intensity of the probe, centered at 614 nm decreased significantly due to hindrance in relay of CT from electron rich phenanthroimidazolyl ring to electron deficient/withdrawing dicyanovinyl unit of DCPPI. The same rationale has been taken into account to describe the changes in the electronic transition spectra of DCPPI. Therefore, DCPPI can also be termed as a chemodosimeter to detect cyanide anion.

Moreover, upon increasing the concentration of CN^- to a solution of DCPPI a new emission band with enhanced fluorescence intensity appeared at 490 nm due to hydrogen bonding interaction and/or deprotonation between CN^- and –NH fragment. Interestingly, almost similar pattern was observed when F^- interacted with DCPPI. Thus, on the basis of changes in optical properties of the DCPPI it is worth to infer that both anions are capable to induce deprotonation of N–H fragment [19Gunnlaugsson, T.; Kruger, P.E.; Jensen, P.; Tierney, J.; Ali, H.D.; Hussey, G.M. Colorimetric “naked eye” sensing of anions in aqueous solution. J. Org. Chem., 2005, 70(26), 10875-10878.
[http://dx.doi.org/10.1021/jo0520487] [PMID: 16356013] -29Dong, M.; Peng, Y.; Dong, Y-M.; Tang, N.; Wang, Y-W. A selective, colorimetric, and fluorescent chemodosimeter for relay recognition of fluoride and cyanide anions based on 1,1′-binaphthyl scaffold. Org. Lett., 2012, 14(1), 130-133.
[http://dx.doi.org/10.1021/ol202926e] [PMID: 22149895] ]. However, in case of cyanide the resultant increase of charge density remained delocalized to the phenanthroimidazolyl unit and the enhanced ICT mainly centered at the newly formed anionic species. Thus, the net CT gets restricted and could not propagate to reach up to cyano terminal. As a consequence, ICT became restricted and led to a considerable blue shift in the absorption and emission bands along with enhancement in relatively emission intensity. A plausible mechanism for interaction of DCPPI with both the anions has been shown in Scheme (2).

Scheme. (2) Proposed mechanism of interaction for DCPPI in the presence of F- and CN-anions.

Furthermore, variation in the optical behavior of DCPPI has also been correlated theoretically by density functional theory (DFT) calculation method. The DFT optimization using B3LYP/6-31G* level [73Frisch, M.J.; Truncks, G.W.; Schlegel, H.B. GAUSSIAN 03, Revision E.01; Gaussian Inc.: Wallingford, CT, 2007. ] revealed a planer structure of DCPPI with extended π-conjugation between phenanthroimidazolyl and dicyanovinyl units. The imidazole ring of DCPPI shows bond distances ~1.32 and 1.38 Å with an angle 111⁰ for N-C-N- unit. The bond length between dicyanovinyl group and phenyl ring appeared around 1.44 to 1.39 Å with an angle 131⁰. Due to deprotonation of the -NH unit by F^- ion the bond length of imidazolyl ring changed to 1.36 Å with an angle 116⁰. Thus, suggesting about the delocalization of charge on the imidazolyl ring. Whereas, upon interaction with CN^- bond length between phenyl and dicyanovinyl unit increased to 1.54-1.52 Å with an angle of 115 ⁰ possibly due to the formation of DCPPI-CN^- adduct while, the imidazolyl ring remained unaffected (Fig. 9).

The molecular orbital pictures indicate that the HOMO is localized on phenanthroimidazolyl unit while LUMO on DCPPI thus, the possibility for ICT between phenanthroimidazolyl to dicyanovinyl unit is reasonable to understand (Fig. 9). After -NH deprotonation by F^- the HOMO and LUMO reorganized to localize over entire molecule. Consequently, a decrease in the energy gap between HOMO and LUMO corroborated well with the enhanced ICT and fluorescence. Similarly, as DCPPI-CN^- adduct is formed the HOMO and LUMO become localized and delocalized at dicyano unit and on entire adduct, respectively. As a consequence, ICT between the phenanthroimidazolyl and dicyanovinyl units got restricted because of hindrance in extended π-conjugation and led to a blue shift in emission spectra which is well supported by the increase in energy gap between the HOMO and LUMO.

Fig. (9) (a) DFT optimized structures and (b) Molecular Orbitals for DCPPI-F- and DCPPI-CN-.