Understanding the Selective-Sensing Mechanism of Al3+ Cation by a

Article pubs.acs.org/JPCA

Cite This: J. Phys. Chem. A 2019, 123, 6970−6977

Understanding the Selective-Sensing Mechanism of Al3+ Cation by a Chemical Sensor Based on Schiff Base: A Theoretical Approach Manuel A. Treto-Suaŕ ez,† Yoan Hidalgo-Rosa,† Eduardo Schott,§,⊥ Ximena Zarate,*,∥,⊥ and Dayan Paé z-Hernań dez*,†,‡

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†

Doctorado en Fisicoquímica Molecular and ‡Center of Applied Nanosciences (CANS), Universidad Andres Bello, Ave. República #275, Santiago de Chile 8370146, Chile § Departamento de química inorgánica, UC Energy Research Center, Facultad de Química y de Farmacia, Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, Macul, Santiago 7820436, Chile ∥ Instituto de Ciencias Químicas Aplicadas, Theoretical and Computational Chemistry Center, Facultad de Ingeniería, Universidad Autónoma de Chile, Av. Pedro de Valdivia 425, Santiago 7500912, Chile ⊥ Millennium Nuclei on Catalytic Processes towards Sustainable Chemistry (CSC), Santiago 7810000, Chile S Supporting Information *

ABSTRACT: A methodology that allows us to explain the experimental behavior of a turn-on luminescent chemosensor is proposed and verified in 1[(1H-1,2,4-triazole-3-ylimino)-methyl]-naphthalene-2-ol] (L1), selective to Al3+ cations. This sensor increases its emission when interacting with ions upon excitation at 442 nm, which is denoted as the chelation-enhanced fluorescence effect. Photoinduced electron transfer is responsible for the fluorescence quenching in L1 at 335 nm, in Ni2+/L1 at 385 nm, and in Zn2+/ L1 at 378 nm. In Ni2+/L, ligand-to-metal charge transfer (LMCT), from the molecular orbital of the ligand to the Ni 3dx2 − y2 orbital, can contribute to the quenching of fluorescence. Based on oscillator strength, the highest luminescence intensity of L1 at 401 nm and that of Al3+/L1 at 494 nm in relation to the others is evidenced. The consideration of the relative energies of the excited states and the calculation of the rate and lifetime of the electron transfer deactivation are necessary to get a good description of the sensor.

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INTRODUCTION The development of chemical sensors as simple devices for the detection of analytes has significantly increased in recent years.1−3 This trend is given, because they are simple and lowcost instrumentation that allows to extend the application of an analytical method to different complex systems in situ, in vivo, and without the need of sample pretreatment.1,4,5 These properties allow to reduce the number of possible random and systematic errors. Also, they offer a versatile way to set up sites for the recognition of species of interest,6,7 focusing their applications in areas such as environmental control,3,7 development of molecular-logic devices,5 biomarkers, and clinical diagnosis,6,7 which lets us understand the cellular function and biochemical processes.7 A synthetic sensor or chemosensor is defined by IUPAC8 as “a device that transforms chemical information, ranging from the concentration of a specific sample component to total composition analysis, into an analytically useful signal. The chemical information can result from a chemical reaction or from a physical property of the system investigated”.8 Due to the two processes that take place in the chemical detection of analytes, that is, molecular recognition and signal transduction, there are generally three parts that constitute a chemosensor:4 the receptor, the active unit, and, in some cases, a linker. The receptor part is responsible for the interaction © 2019 American Chemical Society

with the analyte. The active unit is responsible for transforming the energy into a useful analytical signal. The linker can modify the geometry and set the electronic interaction between the other two components8 (Figure 1). Schreckenbach and Afaneh

Figure 1. Schematic representation of a chemosensor.

reported a structural classification of sensors into three classes based on their different analyte recognition pathways.9 Class I constitutes a sensor where the receptor and fluorophore are indistinguishable. In class II, the fluorophore and receptor are different and can be linked with or without a spacer or linker. Class III is sensors based on resonance energy transfer through the formation of an exciplex or excimer-forming probe. This energy transfer is distance-dependent and requires the presence of more than one fluorophore in the structure Received: April 11, 2019 Revised: July 16, 2019 Published: July 18, 2019 6970

DOI: 10.1021/acs.jpca.9b03366 J. Phys. Chem. A 2019, 123, 6970−6977

Article

The Journal of Physical Chemistry A

Figure 2. Schematic representation of the luminescence of (a) turn-on and (b) turn-off optical chemosensors.

energy transfer can happen through two mechanisms:19,22 The one described by Dexter (DET) that shows electronic exchange due to orbital overlap, which include short-range interactions,19 and the Coulombic mechanism that involves a long-range dipole−dipole interaction known as Fö rster resonance energy transfer (FRET).19,23 These mechanisms compete with other radiative and nonradiative deactivation processes happening from the excited states of systems,24 namely, photoinduced electron transfer (PET),18,25 that are answerable for the quenching of luminescence in many cases. Also, if the electron transfer involves intramolecular rotation, then it can result in a state called twisted intramolecular charge transfer (TICT).19,26 If the energies of the orbitals centered on the ligand and metal are similar, then charge-transfer bands are observed, and the direction of this electron transfer is determined by the relative energy levels of these orbitals.27,28 Thus, metal-to-ligand charge transfer (MLCT) happens through the electron transfer from molecular orbitals with a metal character to those with a ligand character.29,30 Also, ligand-to-metal charge transfer (LMCT) happens for the opposite direction, which is very common in metals with orbitals available for accepting electrons from the ligands.27−29 On the other hand, when the same molecule contains both donor and proton receptor groups in proximity, the proton can be transferred directly from the acid to the basic site in the excited state. This process is so-called intramolecular protons from excited state (ESIPT).31,32 Even though investigations focused on these processes have been reported, the selectivity of chemosensors to certain metal ions in terms of the sensing mechanisms has not been addressed in a rigorous way. The literature reports theoretical studies specifically based on energy analysis of the molecular orbitals in the ground state (S0) without considering the relative energies of the first excited state.9,33−35 This methodology can lead to erroneous results especially when there are significant changes in the geometry of both states. That is why, as the processes involved in the sensing depend on the electronic and molecular properties of the chemical species, our purpose is to perform a correct computational characterization of the sensor 1-[(1H-1,2,4-triazole-3-ylimino)methyl]-naphthalene-2-ol, selective to Al3+, synthesized by Zheng et al..36 Specifically, to elucidate the sensing mechanism toward a family of ions (Al3+, Zn2+, and Ni2+), through computational approaches suitable to these aims,33,37 the methodology proposed here will encourage the scientific community to carry out a more rational design of luminescent sensors to make the process of synthesis more efficient in terms of working time, consumption of reagents, and data collection and in the obtention of predetermined photophysical proper-

(which may be the same or not), arranged in such a way that an energy transfer between them can be established.9 Chemosensors can also be classified according to the nature of the signal emitted by the active unit, namely, electrochemical, potentiometric, magnetic, calorimetric, and optical.8 Optical sensors have been used for about 150 years,7 and the measurable properties are absorbance, transmittance, luminescence (fluorescence and phosphorescence), polarization of light, among others.7,8 In this type of sensors, the active unit is known as the fluorophore, and the receptor is known as the ionophore.9,10 It has been reported that these sensors are excellent for the detection of analytes as the luminescence can be generated, disappeared, or diminished upon the incorporation of the analyte, which makes the signal easily detectable with the naked eye.3,4,8 Metal cations are analytes of great interest not only for their role in many biochemical processes but also for their high toxicity that threatens human health.11 At certain concentrations, metals such as copper, potassium, iron, and zinc are very necessary for the proper functioning of organisms. However, when these concentrations are exceeded, they can be very harmful.7 On the other hand, metals such as aluminum, lead, nickel, cadmium, and mercury cause irreversible alterations in the health and ecosystems.7,11 Aluminum is one of the most interesting metals because it is the thirdmost abundant metal and displays high impact from anthropogenic actions.6,7 Al3+ is a toxic element that can cause many diseases such as osteoporosis, chronic renal failure, fibrosis, osteomalacia, amyotrophic lateral sclerosis, and Alzheimer’s and Parkinson’s diseases.6,11,12 For these reasons, the determination of Al3+ ions in situ and in vivo in complex systems is a topic imperative to address.1,4,5,13 In this sense, optical sensors have become an attractive alternative for the sensing of Al3+ cations,4,14 although they have shown good results in their detection and quantification, in most cases, they still show a lack of selectivity or interference due to the presence of other metal ions.15,16 The performance of optical sensors shows that they can present fluorescence quenching upon interaction with metal cations (turn-off sensors), where the underlying principle is the so-called CHEQ (chelation enhancement of the quenching emission) effect.17,18 In the case of turn-on sensors, they do not emit or are very weakly luminescent materials, and the luminescence is activated after recognition of the analyte. The underlying principle behind this latter type of sensors is the socalled CHEF (chelation-enhanced fluorescence) effect11,19 (see Figure 2). These effects are determined by a series of transduction mechanisms,10,20 that is, those that are based on energy transfer (ET)21 and those of charge transfer.7 The 6971


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Figure 3. (a) Molecular structure of L1. (b) Molecular structures of M/L1 with M = Al3+, Ni2+, or Zn2+.

Figure 4. S0 minima of L1 and their relative energies (ΔE, kcal/mol) to Min 1. Hydrogen bond lengths are presented in Å, and dihedral angles are in degrees.

ties as a deep knowledge on the sensing performance can be analyzed and predicted.

k rd(i → f ) =

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COMPUTATIONAL DETAILS We report a set of calculations that account the structure and optical properties of the chemical sensor 1-[(1H-1,2,4-triazole3-ylimino)-methyl]-naphthalene-2-ol labeled here as L1 and a family of complexes M/L1, where M is Al3+, Ni2+, or Zn2+ (Figure 3). A conformational search of L1 was carried out to find the global minimum through the Avogadro software.38 For this, all degrees of freedom were left without restriction. The optimized geometries were proven to be minima in the potential energy surface (PES) by calculating the vibrational frequencies. All the structures of the S0 and S1 states were optimized with the ORCA code at the DFT level using the B3LYP hybrid functional and def2-TZVPP basis set for all atoms.39 A timedependent DFT (TD-DFT) approach was employed to compute the vertical excitations using the Coulomb-attenuating method CAM-B3LYP functionals, which was developed to minimize deviations in charge-transfer excitation energies.40 The solvation effects were considered via a conductor-like screening model (CPCM) using the parameters of DMF as solvents (ε = 38.3 and refraction = 1.43).41 In all cases, the coordination number was completed including explicitly water molecules to accomplish the appropriate coordination for each metal ion in the study. For Al3+ ion, the hexacoordinated complex was studied, such was reported in the experimental data.36 The Ni2+ and Zn2+ are tetracoordinated complexes.29,42,43 The radiative rate (krd) and radiative lifetime (τrd) of the emission are determined by using the emission energy (Ei,j) and the transition dipole moment (μi, j) (eq 1).44,45

1 4e 2 = 3 4 (ΔEi , j)3 (μi , j )2 τrd 3c ℏ

(1)

The calculation of the rate of electron transfer (ket) and electron transfer lifetime (τet) provides a qualitative prediction of the type of charge-transfer process.33 These parameters can be quantified by Marcus theory45,46 using the reorganization energy (λ) and the total change in free energy (ΔG) between the two charge-transfer states (eq 2). 2 1 2π 1 (VF − CT)2 ket = e[−(λ +ΔG) /4λkBT ] = τet ℏ 4πλkBT (2) where VF−CT is the electron transfer coupling, which can be expressed as VF − CT =

ΔE|μF⃗ − CT | (ΔμF⃗ − CT )2 + 4 |μF⃗ − CT |2

(3)

where μ⃗F − CT is the transition dipole moment among the local excited fluorophore state and the charge-transfer state and ΔμF−CT is the difference between the permanent moments of the S1 or T1 in the generalized Mulliken−Hush scheme within the two charge-transfer states.45,47 Interaction energy is studied in M/L1 through the Morokuma−Ziegler decomposition scheme. The interaction energy was determined by applying a fragment approximation to the molecular structure and the decomposition of the interaction energy (ΔEInt) between the two fragments that constitute the entire system: M (metal with the coordination sphere) and L1. The ΔEInt between the fragments can be decomposed in the electrostatic interaction (ΔEElec), Pauli repulsion (ΔEPauli), dispersion energy (ΔEDis), and orbital terms (ΔEOrb)48,49 (eq 4). 6972


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The Journal of Physical Chemistry A ΔE Int = ΔE Pauli + ΔE Elec + ΔE Dis + ΔEOrb

(4)

The ΔEPauli has a destabilizing character and represents the energy change due to the antisymmetrization of the fragment wave function as a consequence of the interaction.48,49 The remaining three terms corresponds to the classical electrostatic interaction (ΔEElec), the dispersive contribution (ΔEDis), and the effect of the orbital interaction and polarization effects (ΔEOrb). All these terms have a stabilizing contribution to the interaction energy. Particularly, ΔEOrb was analyzed through the natural orbital of chemical valence (NOCV) methodology proposed by Mitoraj.48

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Figure 5. Optimized structures in the S0 and S1 states for L1 and Mn/ L1 (Al3+/L, Ni2+/L, and Zn2+/L).

RESULTS AND DISCUSSION A structural analysis of L1 has been carried out by the exploration of the dihedral angles and the hydrogen bond distance. The global minimum corresponds to Min 1, which shows a planar geometry. Besides, nine local minima (Min 2 to 10) were located on the PES (see Figure 4 and Figure S1, Supporting Information). The main geometrical differences are the rotation of the dihedral angle formed by the planes that contain the rings, which varies from 179.95° to −1.99°. This rotation produces the rupture of the hydrogen bond in the Min 4 to Min 10; this fact makes the energy rise until 20.36 kcal/ mol for Min 10 compared to Min 1. The formation of the intramolecular hydrogen bond (O−H···N) has an important effect on the stability of geometries since its rupture implies an increase in energy by the rotation of the triazole group. This trend has been also observed for the other local minima represented in Figure S1, Supporting Information. The experimental UV−vis absorption spectrum of the systems displayed three absorption bands at 267, 333, and 380 nm. When L1 is excited at 288 nm, an extensive emission band can be observed at 348 nm, while it shows weak emission upon excitation at 442 nm. However, the addition of Al3+ ions produces a change in absorbance at 435 nm and an increase in fluorescence intensity at 466 nm, which suggests a CHEF effect. This change is manifested with Al3+ ions even in the presence of other metal ions.36 In order to rationalize this behavior, we start by the optimization of the structures in the S0 and S1 states. These optimized structures were the input data to calculate the excitations that constitute the UV−vis absorption and emission spectra. In all systems, a prominent structural change between the S0 and S1 states is not observed. However, when L1 interacts with the metal ions, the intramolecular hydrogen bond (O−H···N) disappears, and the rigidity of the structure increases (see Figure 5). In Al3+/L1, it is possible to observe the hexacoordination that is reported experimentally.36 For Ni2+/L1 and Zn2+/L1, the tetracoordination is showed (Figure 5). The root-mean-square deviation (RMSD) supplies a percentage of the differences among the two structures based on the comparison of their atomic positions. According to this parameter between the S0 and S1 states of the free and coordinated sensors, around ∼0.6%, the differences in the molecular geometry are consistent with the small redshift that the systems show. With the purpose of explaining the CHEF and CHEQ effects on the systems (L1 and M/L1), TD-DFT calculations for these optimized structures were performed. This computational study allowed to reproduce the three absorption bands observed experimentally for L1 at wavelengths of ∼363, 335, and 253 nm (these transitions are tabulated in Table 1 and

Table S1, Supporting Information). The difference with the experimental values, around 30 nm, is within the error range commonly reported for TD-DFT.50,51 The transitions correspond to π → π* from the highest occupied molecular orbital (HOMO (H)) to the lowest unoccupied molecular orbital (LUMO (L)) at 363 nm. The second (at 335 nm) and the third (at 253 nm) bands are characterized as H−1 → L and H−1 → L+1 transition, respectively (see Table 1 and Table S1). When L1 interacts with the metal ions, the redshift (∼100 nm) of the absorption bands is patterned (see Table 1 and Table S1, Supporting Information). The transitions in M/L1 correspond to π → π* from the H → L, H−1 → L, and H−2 → L, except for Ni2+/L1 that presents transitions H → L (dNi−π*), H−1 → L+1, and H−2 → L+1 (see Table 1 and Table S1). As all systems show either the presence or absence of PET in these absorption bands, the possible mechanisms that produce the CHEF and CHEQ effects manifested in the systems cannot be clearly explained only with this analysis. Briggs and Besley33 showed that it can lead to erroneous results if one only based the analysis on the energies of the molecular orbitals in the S0 state without considering the relative energies and structures of S1 or T1. Additionally, a correct analysis requires that the order and topology of the molecular orbitals of S1 with respect to S0 remain unchanged after the optimization process. Considering the energies in the S1 optimized state and TDDFT calculations allows us to determine the bands of emission for systems and analyze the strength of oscillator (f) as a measure of the intensity. Based on f, the highest luminescence intensity of L1 at 401 nm and that of Al3+/L1 at 494 nm in relation to the other systems are evidenced (see Table 2 and Table S2). The emission times (τrad) and the radiative rate (krad) are inside the value range of fluorescence, lower than 10−6 and 106 s−1, respectively52 (see Table 2 and Table S2). These results agree with the reported experimental results, but they do not explain the low luminescence intensity of L1 when excited at 393 nm (442 nm, experimental).36 In this sense, we have made the calculations of the electron transfer rate (ket) and electron transfer lifetime (τet) in the intramolecular charge-transfer process. These parameters deliver a qualitative prediction from competition of the PET process with the radiative emission.33 The ket and τet show that, when L1 is excited at 335 nm, the charge transfer is faster than when it is excited at 253 nm. In addition to this marked difference, it is found that charge transfer is faster than nonradiative deactivation in L1 at 335 nm and, on the adversely, much slower when it is excited at 253 nm (see Table 6973


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Table 1. Singlet → Singlet Absorption Data in L1 and M/L1 Considering the Solvent Effect (DMF; ε = 38.3 and refraction = 1.43)a B3LYP L1

Al3+/L1

Ni2+/L1

Zn2+/L1

CAM-B3LYP

λa

F

active MOs

A

λa

f

active MOs

A

363 335 253 393 378 290 456 385 366 392 378 294

0.435 0.106 0.208 0.327 0.094 0.285 0.031 0.194 0.141 0.362 0.103 0.304

H−L H−1−L H−L+1 H−L H−1−L H−2−L H−L H−1−L+1 H−2−L+1 H−L H−1−L H−2−L

π−π* π−π* π−π* π−π* π−π* π−π* dNi−π* π−π* π−π* π−π* π−π* π−π*

325 287 229 345 318 256 454 346 307 346 320 241

0.517 0.171 0.386 0.419 0.207 0.118 0.021 0.102 0.256 0.461 0.201 0.117

H−L H−1−L H−L+1 H−L H−1−L H−L+1 H−L H−1−L+1 H−2−L+1 H−L H−1−L H−2−L

π−π* π−π* π−π* π−π* π−π* π−π* dNi−π* π−π* π−π* π−π* π−π* π−π*

systems

λa is the theoretical absorption wavelength (nm), A is the assignment of transitions, and f is the oscillator strength, H (HOMO) and L (LUMO).

a

Table 2. Singlet → Singlet Emission Data of L1 and M/L1 Considering the Solvent Effect (DMF; ε = 38.3 and refraction = 1.43)a B3LYP systems

λe

f

L1 Al3+/L1 Ni2+/L1 Zn2+/L1

401 494 409 456

0.401 0.189 0.002 0.049

CAM-B3LYP τ

krd × × × ×

4.2 0.13 19 2.0

108 108 108 108

2.4 79 5.2 5.0

× × × ×

10−9 10−9 10−9 10−9

A

λe

f

π−π* π−π* dNi−π* π−π*

355 395 421 326

0.502 0.323 0.001 0.142

τ

krd 4.4 0.35 37 44

× × × ×

108 108 108 108

2.3 29 2.7 2.3

× × × ×

A 10−9 10−9 10−9 10−9

π−π* π−π* dNi−π* π−π*

λe is the theoretical emission wavelength (nm), A is the assignment of transitions, f is the oscillator strength, H (HOMO) and L (LUMO), krd is the emission radiative rate (s−1), and τ is the emission-radiative lifetime (s). a

3 and Table S3). This same difference is shown by Al3+/L1 with respect to Ni2+/L1 and Zn2+/L1. The electron transfer is

molecular orbitals of the ligand to the Ni 3dx2 − y2 orbital, can contribute to the more quenching of fluorescence (Figure 6). The LMCT is responsible for the quenching of luminescence in many systems of nickel complexes30,42,43,51 by the partial occupation of its 3d orbitals that allow to accept electrons from the ligand. Complexes of Ni, which are the derivatives of Met (Leu, Gly, Asp, and Glu) azurin mutants developed by Gray et al.,53 the tridentate deprotonated pyridine bis-amide ligand developed by Patra and Mukherjee,30 and the [Ni(SeSchCl)(H2O)Cl] complex by Sari et al.54 reveal the transitions of LMCT (see Figure 6). The CHEF effect in Al3+/L1 at λa = 393 nm is caused by the inhibition of PET in L1, and the radiative deactivation is faster than the potential PET at 378 nm. These results are in agreement with the experimental study made by Zheng et al..36 According to the analysis of the Morokuma−Ziegler decomposition, the electron transfer is established from the ligand to the metal through the σ interaction (see Δρ in Table 4). The Al3+/L1 system shows a greater interaction of the metal with the electron pairs responsible for the PET in the sensor according to its highest ΔEInt. More illustrative than the total interaction energy is the value of ΔEOrb, which represents how strong is the coordination of the lone pair of the nitrogen atom with the 3d metal orbitals. In Table 4, it is possible to appreciate that Al3+/L1 has the largest value and, in consequence, the less PET possibility.

Table 3. Parameters of the Markus Theory Obtained for the Intramolecular Charge Transfer Considering the Solvent Effect (DMF; ε = 38.3 and refraction = 1.43)a systems

ket

L1335 Al3+/L1378 Ni2+/L1385 Zn2+/L1378

× × × ×

2.0 1.1 5.0 4.0

τet 12

10 106 1011 1011

5.0 5.9 2.0 2.5

× × × ×

−13

10 10−7 10−12 10−12

λet

ΔG

2.84 1.58 1.82 1.71

0.07 0.07 0.53 0.54

ket is the rate of electron transfer (s−1), τet is the electron transfer lifetime (s), λet is the reorganization energy (eV), ΔG (eV) is the total change in free energy (ΔG) between the two charge-transfer states, and L1335 is the free sensor with the band at 335 nm. a

slower than the radiative deactivation in Al3+/L1 and much faster for the other systems under study. These results clear up the mechanisms that establish the CHEF and CHEQ effects in the systems. It is worth mentioning that these results are observed when the solvent effects are included in the calculations (see Table 3 and Table S3). The effects of the metal ion on the emission properties of L1 are summarized in Figure 6, where the absorption and emission processes are labeled as black and blue, respectively. Based on the results presented above, it is possible to propose the following sensing mechanism. In L1, Ni2+/L1, and Zn2+/ L1, the fluorescence is quenched because of a PET mechanism, faster than the radiative deactivation of the excited state (see Table 3), implying an electron transfer from the HOMO to HOMO-1 orbital. In the case of Ni2+/L, the LMCT, from the

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CONCLUSIONS As the study of the behavior of these systems represents a significant dare, in this paper, we proposed a methodology that allows us to accurately explain the behavior of a selective 6974


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Figure 6. Molecular orbital diagram based on the S0 (absorption) and S1 (emission) states of the L1, Al3+/L1, Ni2+/L1, and Zn2+/L1 systems. λa is the theoretical wavelength of absorption (black), λe is the theoretical wavelength of emission (blue), and f is the oscillator strength.

In this regard, it was found that PET is responsible for the fluorescence quenching in L1 at 335 nm, in Ni2+/L1 at 385 nm, and in Zn2+/L1 at 378 nm. In Ni2+/L, LMCT from the molecular orbitals of the ligand to the Ni 3dx2 − y2 orbital can be attributed to the quenching of fluorescence. Based on f, the highest luminescence intensity of L1 at 401 nm and that of Al3+/L1 at 494 nm in relation to the others are showed. The emission times (τrad) and the radiative rate (krad) are inside the value range of fluorescence, lower than 10−6 and 106 s−1, respectively. These emissions are given by the impossibility that the PET mechanism is manifested. According to the Morokuma−Ziegler decomposition analysis, the electron transfer is established from the ligand to the metal through σ type interaction and with the greater interaction of Al3+ with the electron pairs responsible for PET in the sensor. These results agree with the reported experimental results, but they do not explain the low luminescence intensity of L1 when excited at 442 nm. In this sense, the ket and τet show that, when L1 is excited at 335 nm, the charge transfer is faster than when it is excited at 253 nm. In addition to this marked difference, it is found that charge transfer is faster than nonradiative deactivation in L1 at 335 nm and, on the aversely, much slower when it is excited at 253 nm. The consideration of the relative

Table 4. Morokuma−Ziegler Energy Decomposition Analysis for Systemsa

All values are in kcal mol−1. ρ is the contours of the NOCV deformation density for M/L1, and k is the contribution of the interaction to the total orbital interaction.

a

sensor toward Al3+ ions. This study has enabled to rationalize the mechanisms of selective sensing of Al3+ ions in the L1 system through computational tools and quantum chemistry. 6975


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energies of excited states and the calculation of the rate and lifetime of the electron transfer deactivation are necessary to get a good description of the sensor. The methodology used in this study allows us to explain the experimental behavior of sensors. So, it is possible to use it for proposing new systems with better luminescence properties and selectivity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.9b03366. All calculations in the gas phase and the conformational analysis (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.Z.). *E-mail: [email protected] (D.P.-H.). ORCID

Ximena Zarate: 0000-0001-7400-6649 Dayan Páez-Hernández: 0000-0003-2747-9982 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Ph.D. Program in Molecular Physical Chemistry from Universidad Andres Bello, a subsidy of the DAD-UNAB, FONDECYT 1180017, and FONDECYT 1180565, and Millennium Science Initiative of the Ministry of Economy, Development and Tourism-Chile grant Nuclei on Catalytic Processes toward Sustainable Chemistry (CSC).

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REFERENCES

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Understanding the Selective-Sensing Mechanism of Al3+ Cation by a

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