Multiresponsive Photo-, Solvato-, Acido-, and Ionochromic Schiff Base

Article pubs.acs.org/JPCC

Multiresponsive Photo‑, Solvato‑, Acido‑, and Ionochromic Schiff Base Probe Arturo Jiménez-Sánchez,*,† Norberto Farfán,† and Rosa Santillan‡ †

Facultad de Química, Universidad Nacional Autónoma de México, México D.F. 04510, México Departamento de Química, Centro de Investigación y de Estudios Avanzados del IPN, Apdo. Postal 14740, México, D. F. 07000, México

‡

S Supporting Information *

ABSTRACT: Nonclassical protomeric tautomerism in Schiff bases have the advantage of controlling and differentiating specific interactions in the −CN− linkage since its interactions are not governed by keto−enol tautomerism. Here, we report about the optical properties of a Schiff base probe (P1). X-ray structure analysis evidenced the existence of an intramolecular hydrogen bond which is responsible of a photochromic-fluorescent behavior. The properties of P1 were investigated by UV−vis and fluorescence spectroscopy in solution and solid state. A positive solvatochromism resulting from specific interactions taking place in P1 was studied by three different solvent scales, namely Lippert−Mataga, Kamlet−Taft, and Catalán, finding consistent results. Moreover, a strong acidochromic behavior was detected and the pKa and pKa* values were determined, finding a photobasic character. Further, an ionochromic behavior was stablished. P1 exhibits a λ-ratiometric fluorescence response toward Sn(IV) giving a luminescence color change from blue to green, displaying also a chromogenic response. Finally, theoretical calculations were conducted to analyze the probe mechanism in terms of natural transition orbitals (NTOs) and spatial extent of charge transfer excitations. The present contribution focused on the factors determining the ability of a single Schiff base probe to present photo-, solvato-, acido-, and ionochromism.

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INTRODUCTION Organic multiresponsive molecules whose optical properties can be regulated by various chemical stimuli have attracted much attention due to their ability to signal different interactions.1,2 On the other hand, Schiff bases are common organic structures which can be easily synthesized through a one-step synthetic procedure.3,4 As a result, a vast library of Schiff bases has been developed for several applications, including optical data storage,5−7 molecular electronics and computing,8,9 molecular switches10,11 and sensors.12,13 However, most of these applications are based on the tautomeric preferences of the Schiff bases,14 and very few reports regarding specific interactions in nonclassical protomeric tautomerism15−17 which do not display the typical enolimine−ketoenamine tautomers have been reported. For example, specific interactions in the −CN− imine bond are highly valuable in the field of molecular assembly for micro- and nanostructure fabrication18 and multiresponsive assemblies.19−21 In this regard, single small molecules which present multiresponsive properties are highly desirable due to their simplicity to interact with the chemical environment.22 Further, compared to other covalent bonds such as the click chemistry linkers, the Schiff base structure provides extraordinary reversibility with changing pH values and the stability of these bonds varies with pH,23−25 © XXXX American Chemical Society

or even when changing the polarity and polarizability of the media.26−28 In addition, Schiff bases having a push−pull character bearing electron acceptor and donor groups connected by a conjugated bridge (D−π−A) can exhibit a solvatochromic behavior, which depends on the intramolecular charge transfer (ICT) character.29−32 Despite these characteristics, the photoacid or photobasic character present in D−π−A containing Schiff bases and their implications on the interaction with the chemical environment have not been well documented.33−35 In this work, we describe on the multiresponsive properties of a fluorescent probe (P1) mediated by (a) UV−vis light to give a reversible color change (photochromism) in the solid state with the subsequent fluorescence modulation; (b) solvent nature to modulate emission wavelength and Stokes shift (solvatochromism); (c) acid−base stimuli (acidochromism) to give a chromogenic change; and (d) metal-ion interactions, particularly a λ-ratiometric fluorescence response with Sn(IV) with the concomitant luminescence color change from blue to green, finding a high selectivity for this metal in aqueous Received: March 25, 2015 Revised: May 26, 2015

A

DOI: 10.1021/acs.jpcc.5b02884 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

the presence of a strong donor group which promotes the planar conformation and increases the quinoid character through an intramolecular charge transfer process from the NMe2 group to the Nimine atom. Regarding the crystal packing, P1 is not governed by close contacts, which is an important consideration for open packing,39 Figure 2. In fact, intramolecular interactions have

solutions and also a chromogenic response. Moreover, we demonstrate that the probe P1 displays both, chromogenic and “turn-off” fluorescent changes when interacting with Hg2+. The above-mentioned responses were fully characterized and analyzed by both experimental and theoretical means. Thus, we implemented the first systematic study on a Schiff base to demonstrate its multiresponsive properties, namely, we synthesized probe P1 and its analogue P2 (having no −N(Me)2 donor group), finding that all the mentioned responses result as a consequence of the −CN− imino group interactions, but the electronic arrangement of the molecule, a donor−π− acceptor type, is of capital importance to modulate the multiresponsive behavior.

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RESULTS AND DISCUSSION Synthesis and Characterization. Schiff base derivative P1 was prepared by condensation of 4-dimethylamino-transcinnamaldehyde with (1S,2R)-(−)-cis-1-amino-2-indanol in equimolar amounts under reflux of methanol for 2 h, Scheme 1. The solvent and water formed during the reaction were

Scheme 1. Synthesis of Compound P1 Figure 2. Crystal packing diagrams of P1 along the b axis represented in Spacefill; and O1−H···N2 to O1···H−N2 intramolecular hydrogen bond (green color).

been considered a fundamental factor for photochromism in the solid state since the photophysical properties are related to crystal packing.40 Here, the indanol fragment prevents close contact between molecules leading to an open-structure packing,41 which can allow a photochromic response. Moreover, the indanol moiety is clearly twisted out of the plane with respect to the same moiety of the other molecule in the unit cell (the centrosymmetric dimer) due to steric hindrance. Therefore, the size and shape of this fragment is suitable to create voids for the photoinduced motion, a favorable condition in photochromism. Finally, the presence of an intramolecular hydrogen bond in P1 promotes the photoinduced intramolecular proton transfer in the solid state. For this reason, we propose that the intramolecular proton transfer from O1 to N2 is through the O1−H···N2 [2.668(4) Å, 128.9°] channel forming the O1··· H−N2 mode. This interaction is in agreement with similar intramolecular O−H···N hydrogen bond interactions previously reported.42−44 Solid-State Characterization of P1: Photochromic Properties. The absorption and emission spectra in the solid state were taken from crystalline films fabricated by vapor deposition as described in the Experimental Section. The photochromic properties in the solid state were determined under continuous UV irradiation provided by a Hg/Xe lamp (Hamamatsu, LC6 Lightningcure, 200 W) equipped with a narrow band interference filter of 365 nm (Semrock Hg01 for λirr). The results indicate that the films of P1 exhibit an interesting photochromic behavior (color change from yellow to deep red). The absorption spectroscopy technique was employed to corroborate the change in color, spectra of the UV-irradiated film of P1 are shown in Figure 3a. Formation of a new red-shifted band was observed after 5−10 s of irradiation, the intensity of this band increased with the irradiation time, measured from 0 to 120 s. The new red-shifted band was

removed with a Dean−Stark trap to yield a solid, which was washed with a (9:1) n-hexane:ethyl acetate mixture. Complete spectroscopic data are provided in the Supporting Information. X-ray diffraction analysis of P1. Compound P1 crystallized in the monoclinic P21 space group containing two molecules per unit cell. The molecular structure consists of an electronic π-system from the dimethylamino-cinnamaldehyde derived fragment connected to the nitrogen position of the (1S,2R)-(−)-cis-1-amino-2-indanol moiety (Nimine), Figure 1.

Figure 1. ORTEP diagram of P1, thermal ellipsoids are drawn at 30% level for all atoms except H. Selected distance (Å) and angles (deg): N1−C1 = 1.383(4), C1−C2 = 1.398(4), C2−C3 = 1.372(4), C3−C4 = 1.398(4), C4−C5 = 1.457(3), C5−C6 = 1.309(3), C6−C7 = 1.453(5), C7−N2 = 1.274(4); C1−N1−C2 = 120.9(3), C3−C4−C5 = 122.5 (2), C5−C6−C7 = 123.2 (3), C7−N2−C8 = 117.7 (2).

The bond distance from N2 to C7 (CN) is 1.273(4) Å, in agreement with previous reported data for similar structures.36,37 Crystal structure information as well as bond lengths and angles are summarized in Table S1, Supporting Information. Further X-ray diffraction analysis showed that the main π-backbone has a semiplanar conformation, wherein the indanol moiety is out of the molecular plane (plane A, Figure S1, Supporting Information). The typical conformation of imine derivatives from cinnamaldehyde is non planar showing a deviation of 27° out of the phenyl group plane.38 In contrast, P1 showed a shorter deviation of 6.6°, attributed to B


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Figure 4. Absorption spectra taken after 50 s of laser ablation (in 18 intervals) into the particle aggregates of P1 in water: Triton-X (0.2 mM). Inset: Absorption change profiles at 375 and 475 nm.

absorption bands for the bulk sample. Contrary to the observations in the photochromic process, we observed an abrupt decrease in the band around 465 nm for the protonated imine with increasing ablation period, and the concomitant formation of the band around 390 nm. This suggests that the formation of aggregates hinders hydrogen interactions and thus the protonation ability of P1 due to the strong changes in its conformation. Solvatochromism. Table 1 summarizes the UV−vis absorption and emission data in 20 solvents. Analysis of these spectra revealed that P1 has large Stokes shifts values, varying from 1985 cm−1 in chloroform to 9650 cm−1 in methanol, which implies the lowest probability of having reabsorption of the fluorescence emission,48 and also this strong Stokes shiftsolvent dependence implies that electronic photoexcitation induces significant geometric changes in the excited state of the probe in a given medium. Then, with the aim to understand the effect of the media on the photophysical properties of P1, a solvatochromic analysis was performed. The experimental absorption and emission spectra of P1 in different solvents are shown in Figure 5. The position of the absorption band shows a nonmonotonic behavior with respect to the solvent polarity. However, in solvents like water, chloroform, methanol, ethanol and ethylene glycol, the probe P1 showed an additional absorption band around 470 nm. This new red-shifted band can be attributed to specific solvent−solute hydrogen bonding interactions. The effect of solvent interactions on the absorption and photoluminescence spectra of P1 was investigated in terms of the Lippert−Mataga approximation. This description is based on the analysis of the linear relationship between solvent polarity and Stokes shift obtained in a series of solvents. In particular, the Lippert theory considers that the absorption and emission spectral shifts are due to specific solvent effects such as hydrogen bond, charge transfer (CT) interactions, preferential solvation, acid−base effects and solvent−solute hydrogen bonding.50−52 The Lippert−Mataga equation relates Stokes shift and solvent polarity according to eq 1:

Figure 3. (a) UV−vis absorption and (b) Fluorescence emission spectra of P1 crystal powder under UV irradiation (365 nm) at room temperature.

attributed to the π → π* electronic transition of the protonated imine conjugated π-system due to intramolecular hydrogen transfer from the hydroxyl group to the imino nitrogen atom. In addition, the solid state fluorescence spectrum of P1 shows an emission band around 520 nm. The fluorescence characteristics of the film were investigated during the irradiation experiment, the emission spectra exhibit gradual decrements of intensity and it was practically quenched at 180 s, Figure 3b. The fluorescence quenching was attributed to the formation of the protonated imino nitrogen through an intramolecular O−H···N hydrogen bond channel. This is in agreement with the fact that hydrogen donor−acceptor characteristics of some molecules change in the excited state,45,46 due to intramolecular charge transfer properties (ICT), see the Theoretical Calculations section. We studied the photochromic back reaction finding that it takes place thermally when the solid sample was heated to ∼105 °C. It is worth mentioning that this back reaction by sample annealing is fairly common for protonation induced photochromism.47 In accordance with this, no photochemical reaction was observed even when we irradiated around the lowest-energy band (410−500 nm). To confirm that intramolecular hydrogen bond is responsible for the photochromic process, we prepared particle aggregates in water: Triton-X 0.2 mM by stirring with a previously irradiated powder sample. The absorption spectra were obtained before and during 50 s of laser ablation in 18 intervals, Figure 4. In the laser ablation technique the bulk aggregate is broken down to smaller particles by an intense laser beam, in this case a Nd:YAG laser was used, at 355 nm (30 mJ pulse−1 cm−2, 10 Hz repetition rate) observing a bathochromic shift in the

Δν ̅ = υA̅ − υF̅ = C

(μe − μg )2 2 (Δf ) +C hc a3

(1)


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The Journal of Physical Chemistry C Table 1. Photophysical Properties of Compound P1 in Different Solvents.a solvent

E

n

Δf

λa (nm)

va̅ (cm‑1)

λe (nm)

ve̅ (cm‑1)

va̅ − ve̅ (cm‑1)

cyclohexane dioxane toluene diethyl ether MTBE chloroform BAc EAc THF DCM octanol i-PrOH acetone EtOH MeOH ACN DMF EG DMSO water

2.02 2.22 2.38 4.27 4.50 4.81 5.07 6.081 7.52 8.93 10.30 20.18 21.01 24.65 33.00 36.64 36.70 41.40 46.68 80.40

1.4235 1.4224 1.4961 1.3526 1.3690 1.4459 1.3940 1.3723 1.4050 1.4242 1.4295 1.3776 1.3588 1.3611 1.3288 1.3442 1.4305 1.4320 1.4793 1.3325

0 0.021 0.013 0.165 0.1161 0.148 0.1163 0.201 0.210 0.217 0.225 0.277 0.285 0.289 0.308 0.305 0.274 0.276 0.263 0.279

356 360 365 358 356 473 362 360 364 368 368 364 362 365 366 362 365 471 364 472

28090 27778 27397 27933 28090 21142 27624 27778 27472 27174 27174 27472 27624 27397 27320 27624 27397 21230 27472 21186

402 420 415 419 418 522 431 440 445 443 448 453 452 456 566 459 461 530 465 523

24876 23809 24096 23866 23923 19157 23202 22727 22472 22573 22321 22075 22124 21930 17668 21786 21692 18868 21505 19120

3214 3969 3301 4067 4167 1985 4422 5051 5000 4601 4853 5397 5500 5467 9650 5838 5705 2362 5967 2066

a

Key: Dielectric constant (ε), refractive index (n);49 orientation polarizability (Δf); absorption wavelength (λa); absorption wavenumber (va̅ ); emission wavelength (λe); emission wavenumber (ve̅ ). Solvent notation: MTBE (methyl tert-butyl ether), THF (tetahydrofuran), DCM (dichloromethane), i-PrOH (2-propanol), BAc (butyl acetate), EAc (ethyl acetate), EtOH (ethanol), MeOH (methanol), ACN (acetonitrile), DMF (dimethylformamide), EG (ethylene glycol), DMSO (dimethyl sulfoxide).

where vA̅ and vF̅ are the wavenumbers (in cm−1) of the absorption and emission, respectively, h is Planck’s constant, c is the speed of light, a is the Onsager radius, and Δμ = μe − μg is the dipole moment difference between the ground and excited states. The solvent polarity Δf is defined in terms of the dielectric constant (ε) and the refractive index (n) of the solvent as (eq 2). ⎛ ε−1 n2 − 1 ⎞ Δf = f (ε) − f (n2) = ⎜ − 2 ⎟ ⎝ 2ε + 1 2n + 1 ⎠

intramolecular charge transfer, preferential solvation and/or acid−base effects, are present. More recently, an empirical methodology to explain experimental solvatochromic properties of chromophores from theoretical solvent characteristics was developed by Catalán.55 It is well-known that the relationship between solvent effects and spectral shifts can be denoted by a multilinear equation.56 The new mathematical treatment of solvent effects introduced by Catalán is based on four empirical and independent solvent scales.

(2)

It can be seen in Figure 5 that the absorption maximum does not display significant shifts in nonpolar aprotic and polar aprotic solvents. In contrast, the photoluminescence spectra evidence a strong red-shifting in the emission bands as the polarity increases, which suggests that the dipole moment is larger in the excited-state than in the ground-state. In order to have experimental evidence of the intramolecular charge transfer (ICT) process taking place in P1, the Δμ values were calculated from the Lippert−Mataga plots. The graph of Stokes shift as a function of solvent polarity shows three regions: the nonpolar (0.0−0.15), the polar-aprotic (0.2−0.3) and the polar-protic (0.2−0.31), Figure S2. Thus, from the slope in the polar-aprotic region a Δμ of 13.5 D (slope =10102 ± 1035, R2 = 0.83) was estimated using an Onsager radius of 5.6 Å for P1 estimated by density functional theory (DFT). The Δμ value calculated for P1 is comparable to the values reported in the literature (3−20 D) for other solvatochromic fluorophores.53,54 Additionally, the nonpolar region of the Lippert−Mataga plot exhibits a clear nonlinear trend. This result is due to the fact that in this region, solvation is less effective and there is no good reorientation of nonpolar solvent dipoles around the fluorophore. The overall nonlinear trend suggests that solvent polarity is not the only factor affecting the Stokes shifts, such that specific solvent effects including hydrogen bond and

y = y0 + aSA SA + bSBSB + cSPSP + dSdPSdP

(3)

Here SA, SB, SP, and SdP are the solvent acidity, basicity, polarizability and dipolarity properties, respectively. The coefficients aSA, bSB, cSP, and dSdP represent the contribution of each type of interactions. Then, a Catalán solvent analysis was carried out in order to understand the solvent parameters that affect the photophysical properties (va̅ bs, ve̅ m, and Δv,) ̅ in probe P1. The {SA, SB, SP, SdP} parameters for each solvent are taken from ref55. The regression coefficients y0, aSA, bSB, cSP, and dSdP, standard errors and the multilinear correlation coefficient, r, are presented in Table 2. In the case of va̅ bs, a good multilinear fit of 0.905 was obtained. Because of the strong proton transfer emission promoted by chloroform, this solvent was excluded just for the multilinear regression analysis. The analysis indicates that the major factors contributing to the solvatochromic changes in va̅ bs are the acidity and polarizability of the solvent, since the coefficient values aSA and cSP are relatively large and have smaller standard errors, in particular the data analysis indicates that solvent acidity is the dominant effect. However, in the particular case of va̅ bs, the SP parameter is in fact an influencing factor, because the multilinear regression without considering SP gives a very low r-value according to eq 3, namely, 0.697 for {SA,SB,SdP} variables, D


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where α, β, and π* are the acidity, basicity, and polarity/ polarizability solvent parameters, respectively. Table 3 shows the obtained multilinear regression coefficients and their standard errors. Thus, it can be noticed that Kamlet−Taft and Catalán analysis are in overall agreement, and solvent acidity is the dominant parameter. In addition, comparison of Tables 2 and 3 reveals that correlation coefficients are better for Catalán scale which highlights the importance of solvent polarizability. Kamlet−Taft analysis also reveals important charge transfer properties in P1 from the larger value of p(ve̅ m) compared to p(va̅ bs). It is worth mentioning that in accordance to the three solvatochromic analyses, solvent acidity is dominant, but this hydrogen bond donating ability becomes more important in the excited state (parameters aSA and α for ve̅ m). From the above considerations, we realized that the relationship of va̅ bs vs the high frequency polarizability function squared f(n2) = (n2 − 1)/(2n2 + 1), from the Lippert−Mataga relation, shows a linear trend (r = 0.975, Figure S2, Supporting Information) when excluding chloroform, which in accordance with the empirical scales of Catalán and Kamlet−Taft, indicates the existence of a relation between the absorption properties and the polarizability of the solvent. From the solvatochromic analysis, we propose the specific interaction scheme which considers polarity, polarizability, hydrogen bond donor and acceptor interactions of the media for P1, Scheme 2. Acidochromism. Since solvatochromic analysis for P1 revealed that solvent acidity is the most important parameter influencing the spectroscopic properties in solution, its sensitivity to the acidity of the media was first evaluated by titration in acetonitrile solution with an HCl: water mixture (1:99, v/v). The absorption spectra for the titration (Figure 6) shows that as the acidity increases a red-shifted band at 476 nm is formed and the intensity of the absorption band at 356 nm decreases. Thus, the obtained spectra pass through three clear isosbestic points which indicate an equilibrium between two different species. This new band is in agreement with that observed in water, chloroform, MeOH, EtOH and ethylene glycol and also with the band detected in the thin film experiment after irradiation. Furthermore, the titration process of compound P1 was investigated by using fluorescence spectroscopy. The photoluminescence spectrum (Figure 6) shows that an increase in acidity promotes the fluorescence quenching of P1 in solution. A plausible explanation of these spectral observations is the protonation of the imine nitrogen atom which disturbs the ICT process and promotes the nonradiative deactivation pathway; see the Theoretical Calculations. The protonation at low acid concentration of the N(Me)2 group having quinoid character is discarded due to its lower basicity,57 see below. Moreover, to further study the acid−base properties of P1, we estimated its pKa values. The dissociation constants were determined by the potentiometry technique. Therefore, we

Figure 5. UV−vis absorption (above) and photoluminescence (below) spectra for P1 in different solvents. Nonpolar to polar solvents: cyclohexane, dioxane, toluene, methyl tert-butyl ether, diethyl ether, butyl acetate, ethyl acetate, tetrahydrofuran, dichloromethane, noctanol, 2-propanol, acetone, acetonitrile, dimethylformamide, and dimethyl sulfoxide.

indicating that the solvent polarizability factor should not be neglected. In the case of ve̅ m and Δv ̅ the largest values also were aSA, cSP and dSdP accompanied by the smallest standard errors as in the case of vabs, indicating that the predominant factor influencing ̅ the solvatochromic changes in ve̅ m and Δv ̅ is the solvent acidity, although polarizability and dipolarity cannot be neglected as well. Similarly, we used the Kamlet−Taft empirical scale to verify the results obtained by Lippert−Mataga and Catalán analysis. Although Kamlet−Taft analysis describe the polarity/polarizability parameters in the same term, its reference point is cyclohexane, whereas Catalán analysis refers to the gas phase. Then we used eq 4: y = y0 + aα + bβ + pπ *

(4)

Table 2. Estimated Coefficients y0, aSA, bSB, cSP, and dSdP for va̅ bs, ve̅ m, and Δv ̅ (in cm−1) and Multiple Correlation Coefficient (r) for Regression Analysis of Compound P1 According to the Catalán Solvent Parameters {SA, SB, SP, SdP} Listed in Table S2 observable

y0 (cm‑1)

aSA

bSB

cSP

dSdP

r

va̅ bs ve̅ m Δv ̅

29429 ± 436 27038 ± 1007 2390 ± 1120

−709 ± 150 −3043 ± 352 −2335 ± 397

−35 ± 168 −293 ± 387 257 ± 431

−2327 ± 436 −3127 ± 1467 799 ± 1633

−246 ± 162 −2820 ± 375 2573 ± 431

0.905 0.976 0.957

E


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Table 3. Estimated coefficients y0, a, b, and p for va̅ bs, ve̅ m, and Δv ̅ (in cm−1) and Multiple Correlation Coefficients (r) for Regression Analysis of Compound P1 According to the Catalán Solvent Parameters {α, β, and π*} Listed in Table S3 observable

y0 (cm−1)

a

b

p

r

va̅ bs ve̅ m Δv ̅

27970 ± 107 25155 ± 718 2815 ± 740

−462 ± 96 −2535 ± 646 −2073 ± 667

−98 ± 151 −594 ± 1009 692 ± 1041

−637 ± 144 −3214 ± 960 2577 ± 991

0.902 0.881 0.862

Scheme 2. Possible Specific Interactions for P1, Where α, β, and π* Stand for Acidity, Basicity and Polarity/Polarizability Parameters in the Kamlet−Taft Solvent Scale and SA, SB, SdP, and SP Stand for Acidity, Basicity, Polarity, and Polarizability in the Catalán Scale

Figure 7. Concentration distribution curves of P1 (5 mM) with NaCl 50 mM at 25 °C. The molar fraction are plotted using potentiometric constants. P1−H2 (diprotic), P1−H (monopritic or imine-protonated specie), P1 (neutral), and P1−H−1 (anionic) species.

On the other hand, the pKa values were also obtained by spectrophotometric technique to corroborate the assignment of the protonation events of individual functional groups (Nimine and Namine),60 see Figure S3, Supporting Information. The pKa values were in good agreement with potentiometric titrations, being in fact that Nimine is the first protonation with a pKa = 6.28 ± 0.02 (pKb = 7.72) and Namine is the second protonation, pKa = 4.31 ± 0.06 (pKb = 9.69). In this regard, Benassi et al. recently described a series of acidochromic dimethylamino− azinium iodides,61 in comparison with the protonation of the dimethylamino donor group of P1 (pKa = 4.31), they determined a lower pKa value of 3.10 for the protonation of a dimethylaminostyryl derivative. This difference comes from the fact that these azidinium salt species have a greater extent of charge separation, leading to a decrease in the dissociation constant. Moreover, a higher pKa value of 5.15 was reported for N,N′-dimethylaniline,62 the donor moiety of P1. With the aim to investigate the photoinduced charge transfer influence on the basicity of the probe, we obtained the pKa* values by fluorimetric titration. The spectra were taken in pure water, using hexadecyltrimethylammonium bromide (HTAB, 5 mM) and CHES buffer at pH 9 to have the nonprotonated specie. For the pKa* determination, the indirect method proposed by Förster63,64 and Weller,65−67 was employed. This method introduces some approximations since it is based on the knowledge of the pKa and the measurement of the absorption (and emission if required) transition energies of the protonated and nonprotonated forms. Then, it does not consider possible reversibility of the proton-transfer process, neglecting the difference in the vibrational states between the electronic ground and excited states. Therefore, the value of pKa* can be found with the aid of Förster−Weller cycle, eq 5.

Figure 6. HCl titration of compound P1. Top: Absorption spectra showing an isosbestic point. Bottom: Emission spectra showing the fluorescence quenching.

used the pKa values derived from potentiometry to calculate the pH distribution of molar fractions of individual diprotic, monoprotic, neutral and anionic forms of P1 (Figure 7). The pKa value for the deprotonation of P1 resulted to be 11.03 ± 0.02, while the first protonation, on the Nimine atom, is 6.36 ± 0.01 (pKb = 7.64) and the second protonation, on the Namine atom, is 4.27 ± 0.05 (pKb = 9.73), where the later pKa value is in agreement with other N,N′-dimethylaniline derivatives and Schiff bases.58,59 F


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The Journal of Physical Chemistry C ΔpK a = pK a − pK a* = ≈

Nh Δυ ̅ ln 10 RT

N [ΔE F(AH) − ΔE F(A−)] ln 10 RT

Figure 10) shows that a single electronic transition involves an important electron reorganization having a CT character from the amine to the imine group with a density depletion in the N(Me2) amino group, but a density excess occurring on the imine moiety, thus conferring a photobasic character70 to the Nimine atom. Ionochromism. The ionochromic behavior of Schiff bases and the interaction with a wide range of metal ions have been well documented.71,72 Versatile mechanisms have been developed, most of them include the tautomeric modulation of the enolimine−ketoenamine equilibrium.14 However, few works describing water-soluble fluorescent−colorimetric Schiff base metal ion sensors with λ-ratiometric response exist.73,74 In view of the variety of responses of P1 with distinct chemical stimuli to the imine linkage, the interaction of Schiff base P1 with several metal ions was studied. Then, the receptor site of P1 toward metal ions can be composed by the Nimino atom and the hydroxyl group of the indanol moiety. The UV−vis and fluorescence spectra of P1 in the presence of SnPh22+ [Sn(Cl2)Ph2], Cd2+, Co2+, Cu2+, Fe2+, Hg2+, Ni2+, Pb2+, Ca2+, K+, Li+, Fe3+, and Zn2+ ions were studied, observing important changes only for SnPh22+ and Hg2+ ions. In the course of spectrophotometric titrations of P1 with SnPh22+, the spectra present two nonequivalent isosbestic points located at 392 and 440 nm, indicating two different species, Figure 9a. On the other hand, fluorescence titrations resulted in the formation of a new red-shifted band, giving rise to the so-called λratiometric fluorescence response.75 The fluorescence spectra show also two isosbestic points located at 480 and 520 nm, indicating two different species as well. We observed a strong luminescence color change from blue (free P1) to green (P1 + SnPh22+), Figure 9a. These results are of great importance since λ-ratiometric fluorescence response represents a convenient alternative to detect an analyte quantitatively when the probe is fluorescent per se, avoiding a “turn-off response”. Moreover, since λratiometric detection is based on intensity ratios at two wavelength values, its primary advantage is the use of a simple instrumentation to provide a self-referenced ratiometric measurement that is not possible with single-channel recording of fluorescence intensity. In order to have more insight in the binding stoichiometry of the formed complexes between P1 and SnPh22+ ion, we determined the composition and stability constants from spectrophotometric and potentiometric data. Then, we stablished the following set of equilibria,

(5)

where N is Avogadro’s number and ΔEF is Förster energy gap, which is the free energy gap separating the ground state and the stable (thermodynamic) energy level of the species involved in the proton transfer in their electronic excited states. The spectroscopic energy difference between the nonprotonated and protonated forms was estimated from an analysis of both the normalized absorption and fluorescence spectra of the two species, Figure 8.

Figure 8. Normalized absorption (dashed line) and emission (solid line) spectra of the basic and monoprotonated species of P1 in Ncyclohexyl-2-aminoethanesulfonic acid (CHES) buffered water at pH = 8.2 and 3-(N-morpholino)propansulfonic acid (MOPS) buffered water at pH = 6.1.

Thus, according to the Förster−Weller theory, when the protonated species absorb at higher energy than the basic one, an acidity increase in the excited state (pKa* < pKa) is obtained, the so-called photoacidity effect. Interestingly, in the case of probe P1 the Nimine monoprotonated specie absorbs at lower energy than the neutral one, highlighting a photobasicity effect, Figure 8. Then, considering the 0−0 energy of the first transition estimated by the crossing of the normalized absorption and emission spectra of the two species, in water buffered at pH = 8.2 (nonprotonated specie) and 6.1 (monoprotonated specie), we obtained a pKa*= 8.92 ± 0.05 for the Nimine protonation. The pKa and pKa* values for the Nimine protonation were computed by TD-DFT using the Born−Haber method for the pKa,68,69 and the Förster−Weller method63−67 for the pKa* determination based on the knowledge of the pKa values. In order to determine the ΔGsolv the water solvent was modeled by both IEF-PCM and by an implicit (IEF-PCM)−explicit solvent model (IE). In the IE approach three water molecules were included in order to model explicit interactions where its positions were fully optimized as well (Figure S10, Supporting Information). The computed IEF-PCM values were significantly different than those experimentally observed, i.e., pKa = 8.4 and pKa* = 8.9. However, with the IE solvent model the pKa values are in good agreement with the experimental ones, obtaining a pKa = 6.8 and pKa* = 7.9. Furthermore, inspection of the hole−electron pairs at the HOMO − LUMO levels of P1 (see Theoretical Calculation,

L + M ⇄ ML(log β110 = 12.3 ± 0.2) L + M ⇄ ML(OH) + H+(log β111 = 8.2 ± 0.3) 2L + M ⇄ ML 2(log β120 = 23.7 ± 0.5)

where L is the deprotonated P1 specie, M is SnPh22+, and log βpqr are mean values for three independent titrations. Then, when the in situ concentration of L and M reaches 2 × 10−5 M, the fraction of the ML2 and ML complexes are 99.9 and 0.01, respectively. Moreover, after saturation of M the fraction of the complexes reaches a 98:2 ratio, see the titration profiles in Figure S4, Supporting Information. In addition, the in situ prepared ML and ML2 Sn-complexes were also monitored by 1 H NMR spectroscopy (Figure S5, Supporting Information) and electrospray ionization mass spectrometry (ESI-MS, Figure S6, Supporting Information). It was observed that the binding G


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Figure 9. UV−Vis absorption and fluorescence titrations of 4 × 10−5 M P1 with (a) SnPh22+ (0 to 5.6 × 10−4 M), insets show the SnPh22+ ion concentration vs absorbance profile and the fluorescence intensity patterns at 457 and 528 nm with the corresponding luminescence color change; (b) Hg2+ (0−16 μM); and (c) fluorescence intensity difference at 530 nm for 1 equiv of P1 in the presence of 4 equiv of different metals (front bars) and competition with 2 equiv of SnPh22+ ions (back bars).

nonfluorescent (static quenching). 76 According to the potentiometric data, (Figure S8, Supporting Information) the obtained log β values for the protonated complex [Hg(P1− H)+], nonprotonated [Hg(P1)] complex and deprotonated complex [Hg(P1)−] are 8.82 ± 0.2, 9.99 ± 0.1 and 3.26 ± 0.3, respectively. Again, the in situ prepared [Hg(P1)] was monitored by 1H NMR spectroscopy (Figure S5, Supporting Information) and ESI-MS (Figure S6, Supporting Information). Similarly to SnPh22+, a downfield shift for the imine proton (Δδ = 8.26−8.33) was observed, also the hydroxyl proton signal disappears and the shifting in H-8 and H-9 signals were observed. The mass spectrometry data show a mass peak at m/ z = 543.1120 for the [Hg(Cl)P1]+ ion. Finally, Figure 9c shows the competition experiments. It is to be noted that no metal-ion interference is observed among the tested metals, even between Hg2+ and SnPh22+ ions a clear green fluorescence was observed. These results highlight the selectivity of P1 to recognize the organotin(IV) complex in the presence of 4-fold excess of other metals.

of SnPh2Cl2 ion mainly affected the imine proton, which shows a downfield shift (Δδ = 8.26−8.46) while the hydroxyl proton signal disappears, also a change of the aliphatic indanol proton signals (H-8 and H-9) were observed. This suggests that the metal coordination is taking place with the Nimine and the Ohydroxyl atoms. The mass spectrometry data reveal a mass peak at m/z = 842.2407 for the [Sn(PhCl)P12]+ ion, i.e. 0.5 equiv of SnPh22+. However, when the 1:1 concentration was reached, a mass peak at m/z = 669.0834 appeared which was assumed to be the [Sn(Ph2Cl)P1]+ ion. In the case of the interaction between P1 and Hg2+, the spectrophotometric, fluorimetric, and potentiometric titrations also reveal important changes, although the binding stoichiometry resulted to be trivial (Figure 9b). Indeed, the nonlinear curve fitting for the UV−vis and fluorescence spectra gives a simple 1:1 complex stoichiometry, with a concomitant color change to red and the simultaneous loss of fluorescence (“turnoff response”). A quencher vs P1 concentration profile (Stern− Volmer plot) at pH = 7 resulted to be nonlinear (Figure S7), which indicates the formation of a ground-state complex that is H


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The Journal of Physical Chemistry C Effect of −N(Me)2 in Compound P1. To confirm the existence of the specific protonation in Nimine atom and the effect of the N(Me)2 donor group, we synthesized compound P2, the analogue of P1 without the N(Me)2 group. The results show that P2 does not exhibit solvatochromic behavior when the spectra were taken in solvents such as ethanol (286 nm), 1,4-dioxane (287 nm) and diethyl ether (285 nm); however, in chloroform the protonated imine shows a new red-shifted band at 332 nm similar to the one observed in P1 (Figure S9, Supporting Information). Also, we performed a dilute HCl titration experiment for P2 in acetonitrile (exhibiting no redshifted band) by adding the same HCl concentration. The spectra passed again through an isosbestic point, in this case at 300 nm, due to the protonated and nonprotonated species (Figure S9, Supporting Information). This result supports the idea that the red-shifted band for P1 is due to the protonated imine specie. On the other hand, the N(Me)2 donor group contributes significantly to the photophysical and photochemical properties of P1 since neither fluorescence nor solvatochromism are observed in P2. Further, the −N(Me)2 rotation, giving rise to a twisted-intramolecular charge transfer state (TICT)77 was discarded as a reason for the dual-emission in P1, because of the quinoid character present in this fragment and its consequent rigidity as recently confirmed by Chevreux et al.78 and Murudkar et al.79 for dual-emissive and ultrafast rotating (N,N-dimethylamino)phenyl-derived probes. Theoretical Calculations. The optimized P1, P1−H+, P1− Hg, and P1-SnPh structures are shown in Figure S10 (Supporting Information) which were eventually corroborated by a frequency analysis finding no imaginary points for the structures. We first focused the analysis on the electron density characteristics upon excitation of the receptor P1 by means of the recently proposed spatial extent index80,81 at a PBE0/G31+G(d,p)/IEF-PCM (Water) level of theory. The obtained fraction of electron charge transferred upon excitation was qCT = 0.47 at a DCT = 5.13 Å spatial distance from the donor to the acceptor centroid, giving a large CT dipole moment value of 11.52 D. The H index defined as half of the sum of the centroid axis along the Donor − Acceptor direction is 1.98, which resulted to be 3.15 Å lower than the CT excitation length, this means no overlap between donor and acceptor centroids, which makes the CT redistribution highly efficient, Figure S11 shows the graphical representation of DCT centroids of charge C+(r)/ C−(r). Further inspection of electron density difference, (Figure 10) shows that the electronic transition involves a density increase on the imino nitrogen, which confers a photobasicity to this fragment on the excited state. Interestingly, a density depletion is observed for the amino nitrogen atom which disfavors the protonation of this group. On the other hand, although TD-DFT provides a good benchmark in the determination of spectroscopic properties due to the accurate description of ground and excited potential energy surfaces, in most of the cases, conventional TD-DFT results in a description of an excited state in terms of several single electronic excitations from an occupied to a virtual orbital. Fortunately, the various contributions to the electronic excitation can be clarified by a natural transition orbital (NTO) analysis,82 which provides a compact orbital representation of the electronic transition through a single configuration of a hole and electron interaction. Consequently, the photoinduced electron transfer process is not depicted by a simple change in the elementary molecular orbital occupancy, but in a hole− electron distribution. Then, to have a better understanding of

Figure 10. (a) Electron density differences between electronic ground and first excited state for P1 computed at PBE0/6-31+G(d,p)/IEFPCM-water (positive and negative variations of density are represented in purple and turquoise blue, respectively) and the corresponding (b) HOMO and (c) LUMO energy levels.

the multiresponse mechanism of probe P1, the representative transition energy diagram for the HOMO−LUMO orbitals was obtained, Figure 11. In general, the calculated NTO distributions show that the electronic transitions within P1 are dominated by an ICT character due to the HOMO− LUMO levels for the whole system, assigned to a π → π* electronic transition. Although the H+ and Hg2+ ions have little influence on the P1−H+ and P1−Hg species, the ICT character in these systems is maintained, and a decrease in the transition energy of the HOMO−LUMO vertical excitation arises with the HOMO level located ca. 0.2 eV above the HOMO level of P1, therefore, the ICT process prohibits the radiative deactivation (fluorescence). In the case of the SnPh22+ complex (phenyl rings are not shown for clarity), the lowest-energy excitation band corresponding to the HOMO−LUMO levels has a strong influence of the metal and the ICT character is very weak. However, the HOMO level is situated 0.31 eV below the corresponding HOMO level of P1 and the lower energy radiative deactivation is allowed. The NTO coefficients (w) representing the extent to which the electronic excitation can be written as a single excitation are 0.99, 0.99, 0.99, 0.97, and 0.96 for P1, [P1-H]+, [Hg(P1)]2+, [Sn(P1)]2+ and P2, respectively. It is worth mentioning that this dual-emission based λ-ratiometric sensing of metals is not common, since most of the reported ratiometric sensors are based on excitedstate proton transfer process (ESPT).83−85

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CONCLUSION Through an integrated experimental and computational approach, the results demonstrate that P1 is a very sensitive and versatile probe. In the implemented strategy, we compared the spectroscopic properties between P1 and its analogue P2 having no dimethylamino donor group. The optical properties of P1 respond to different stimuli. Particularly, it can be concluded that As a consequence of an intramolecular hydrogen bond channel, P1 behaves as a photochromic molecule in the solid state, changing from yellow to deep red in a few seconds when irradiating at 365 nm. The fluorescence is simultaneously quenched due to the Nimine protonation. The reverse reaction I


Article


Figure 11. Natural transition orbitals and transition energy diagram for (from left to right): P1, [P1-H]+, [Hg(P1)]2+, [Sn(P1)]2+, and P2 illustrating the nature of the lowest-energy transition, computed at PBE0/6-31+G(d,p)/IEF-PCM-Water. Singlet transition energies S0−S1 for [Sn(P1)]2+ correspond to the lowest transition energy.

can be thermally driven in the solid state or by laser ablation in a particle suspension. P1 displays a noticeable solvatochromic effect, changing from 3214 cm−1 in cyclohexane to 9580 cm−1 in methanol. Here, three different emission states can be clearly identified, a local emission (apolar−aprotic solvents), charge transfer emission (apolar to polar solvents) and proton transfer emission (polarprotic solvents). Thus, a dipole moment difference of 13.5 D was determined by a Lippert−Mataga analysis. According to the Catalán and Kamlet−Taft solvent scales, the solvatochromic properties of P1 are influenced by the solvent polarizability and acidity parameters, where the latter is the dominant effect. Because of the Nimine protonation, P1 shows a strong acidochromic effect with the formation of the red-shifted band at 476 nm. The pKa and pKa* values were determined by both, experimental and theoretical means. In addition, the species distribution diagram was determined for the P1−H2 (diprotic), P1−H (monopritic or imine-protonated specie), P1 (neutral) and P1−H−1 (anionic specie). Thus, the ionochromic− fluorescent properties of P1 can be applied in a pH between ∼6.5 and 11.0, while the Nimine protonated specie is present between 6.36 and 4.27, under which the second protonation on the Namine atom is achieved. Probe P1 exhibits an interesting fluorescent-ionochromic behavior toward Hg2+ and Sn2+. In the case of Hg2+ a classic 1:1 stoichiometry was determined with a log β value of 9.99 ± 0.1 for the [Hg(P1)] complex, giving rise to an static quenching effect and the simultaneous change in color can be observed by the naked eye. However, in the case of Sn2+, we found a λratiometric response in the fluorescence spectra. The blue fluorescence of P1 can be tuned to green when forming the in situ [Sn(P1)] complex. In addition, the [Sn(P1)] and [Sn(P1)2] stoichiometry are present during the course of the metal ion titrations, being the 1:2 the predominant relation. More importantly, the λ-ratiometric response toward SnPh22+

of a single probe (P1) is highly promising for the fast and reliable quantitative detection of this metal, since simple changes in fluorescence intensity are of low analytical importance due to the absence of internal reference. The dimethylamino donor group results of capital importance, enhancing not only the signal intensity and sensitivity, but also allowing a fluorescence response. The acid−base strength on the ground (pKa) and excited state (pKa*) estimated theoretically were in good agreement with the experiment. Both approaches provided for a photobasic character of P1. In addition, the density index calculation together with the NTO analysis for P1, [P1−H]+, [Hg(P1)]2+, [Sn(P1)]2+, and P2 revealed a significant decrease in the transition energy of the HOMO−LUMO vertical excitation which in the particular case of Sn2+ allows the fluorescence emission of P1, while in H+ and Hg2+ do not. Finally, these results demonstrate that the multiresponsive scenario of probe P1 is a promising approach in the achievement of distinct applications in nonclassical protomeric Schiff bases only by modulating solvent effect and appropriate pH in solution, and hydrogen bond interactions in the solidstate.

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EXPERIMENTAL SECTION Materials and Physical Measurements. Starting materials and solvents were commercially available and used without further purification. Infrared spectra were obtained on a FTIR Varian Spectrometer ATR. Melting points were measured on an Electrothermal 9200 apparatus. 1H and 13C NMR spectra, homonuclear 1H−1H COSY and heteronuclear 1H−13C HETCOR correlation spectroscopy were recorded on a JEOL eclipse ECA +500 spectrometer. Chemical shifts (ppm) are relative to (CH3)4Si for 1H and 13C. High resolution mass spectra were obtained with an Agilent G1969A spectrometer. Electronic absorption spectra were obtained using a PerkiJ


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The Journal of Physical Chemistry C Notes

nElmer LAMBDA 2S UV/vis Spectrophotometer. Solid state absorption spectra were obtained with a PerkinElmer Lambda 900 UV/vis/NIR Spectrometer. The solution and solid state emission spectra were measured on a Varian Cary Eclipse fluorescence spectrometer. Solid crystalline film samples of P1 were fabricated by vacuum deposition placed into slide glasses, the temperature was monitored to avoid the thermal decomposition.86 X-ray Diffraction Analysis and Data Collection. Single crystals of compound P1 suitable for X-ray diffraction were obtained by slow evaporation from a saturated chloroform solution at room temperature. The crystal data were recorded on an Enraf Nonius Kappa-CCD (λMoKα= 0.71073 Å, graphite monochromator, T = 293 K). Each crystal was mounted on conventional MicroLoops. All reflection data set were corrected for Lorentz and polarization effects. The first structure solution was obtained using the SIR2004 program and then the SHELXL-97 programs were applied for refinement and output data. All software manipulations were done under the WinGX environment program set. Molecular perspectives were drawn under SHELXTL-XP,87 drawing application, and Mercury Crystal Structure Visualization software.88 Some intra- and intermolecular interaction were analyzed using OLEX289 and Platon90 software packages. All heavier atoms were found by Fourier map difference and refined anisotropically. The hydrogen atoms were geometrically modeled except for those involved in the hydrogen bonds which were found on the difference Fourier map. The crystal features and data collection are summarized in Table S1. The molecular structure of P1 is shown in Figure 1. Computational Details. Molecular geometry optimizations were obtained by density functional theory (DFT) as performed in the Gaussian 09 code.91 For pKa calculations zero point vibrational energies (ZPVE) were considered to account for thermal and entropic effects. The intramolecular charge transfer (ICT) properties of P1 were first analyzed by using TD-DFT with polarizable continuum model by using the integral equation formalism (for water).92,93 Hybrid functionals such as PBE0 have been found to be very accurate for CT parameters and excited states in charge transfer molecular systems.94 Then, we used the PBE0/6-31+G(d)/IEF-PCM level of theory for the ligand and, effective core potentials with a LANL2DZ basis set for Hg2+ and SnPh2+.95 Single electronic excitation by natural transition orbital (NTO) analysis were carried out at the same level of theory.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of CONACyT and PAPIIT 214513 and also supercomputing Xiuhcoatl cluster administration of Cinvestav and Mistli cluster of UNAM for the computational resources.

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ASSOCIATED CONTENT

S Supporting Information *

XRD profiles for P1 (CCDC Reference No. 1055809), the synthesis and characterization, crystal data, Catalán solvent parameters, Kamlet−Taft solvent parameters, Lippert plots, UV−vis spectra for the solvatochromic and HCl:ACN titration studies for compound P2, HyperQuad analysis of species composition, and TD-DFT calculations for spatial extent in charge-transfer excitations, and complete ref10. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02884.

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*(A.J.-S.) E-mail: [email protected]. K


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