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Oct 7, 2013 - The origins of fluorescence quenching by Hg(II) ion chelation and fluorescence enhancement ... and the S1 → S0 transition becomes a ch...
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Role of Fluorophore−Metal Interaction in Photoinduced Electron Transfer (PET) Sensors: Time-Dependent Density Functional Theory (TDDFT) Study Hyunjung Lee, Robert D. Hancock, and Hee-Seung Lee* Department of Chemistry and Biochemistry, University of North Carolina Wilmington, Wilmington, North Carolina 28403, United States S Supporting Information *

ABSTRACT: The origins of fluorescence quenching by Hg(II) ion chelation and fluorescence enhancement by Zn(II) ion chelation to a PET sensor are investigated. Specifically, the fluorescence quenching and enhancing mechanisms associated with the ligand ADPA (N-(9-anthracenylmethyl)-N-(2-pyridinylmethyl)-2-pyridinemethanamine), protonated ADPA and metal bound (Zn(II) and Hg(II)) ADPA are studied via density functional theory (DFT) and time-dependent DFT (TDDFT) methods. The study found that a structural change in the excited state of ADPA induces reordering of the frontier molecular orbitals, and the S1 → S0 transition becomes a charge transfer transition from the fluorophore to the tertiary nitrogen of the dipicolylamine (DPA) unit, which is forbidden. Protonation on the tertiary amine or chelation of Zn(II) prevents such changes, and the HOMO−LUMO transition is contained within the fluorophore. Therefore, fluorescence is restored. The chelation of Hg(II), on the other hand, promotes extensive interaction between the Hg(II) ion and the fluorophore, which is reflected in the short Hg(II)−fluorophore distance (3.11 Å). A noticeable structural change upon the S0 → S1 transition is observed in the Hg(II)−ADPA system as well, where the resulting S1 → S0 transition becomes a charge transfer transition from mercury to the fluorophore and the fluorescence is thus quenched. Therefore, the present DFT/TDDFT calculations reproduce the fluorescence on−off behavior associated with the entire ADPA family of complexes, which illustrates that the combination of DFT and TDDFT calculations, including excited state geometry optimization, can be a valuable tool to uncover the detailed fluorescence sensing mechanisms.

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INTRODUCTION Development of fluorescent sensors for metal ions is of great interest due to environmental and health concerns. For example, metal ions such as Zn(II) and Cu(II) are believed to be involved in neurological diseases, such as Alzheimer’s or Parkinson’s disease, causing the pathology.1,2 Extreme toxicity of heavy metals such as mercury, and its consequence for human health, are also well documented.3 Thus, an extensive amount of work has been done on the effective sensing of metal ions, as summarized in recent reviews.4−8 Typical fluorescent sensors are made of two parts: the fluorophore and the ligand. Ligands contain heteroatoms that bind to metal ion, such as nitrogen and sulfur, but the fluorophores may be part of the ligand, or separated from the metal binding site by a linker. In other cases, heteroatoms are a part of the fluorophore, which bind directly to the metal ion being sensed. An example of the former is ADPA,9 which is studied in this work, and DQPMA10 is an example of the latter (see Figure 1 for key to ligand abbreviations). Depending on how the metal binding to the ligand affects the fluorescent intensity of the fluorophore, sensors can be regarded as either “turn-on” or “turn-off” sensors. For turn-on sensors, which is the more desirable of the two varieties, the fluorescence intensity increases in the presence of the metal ion, whereas the © XXXX American Chemical Society

Figure 1. Structures of ligands discussed in this paper.

Special Issue: Terry A. Miller Festschrift Received: July 4, 2013 Revised: September 10, 2013

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Figure 2. Schematic diagram illustrating the mechanism of PET effect (left) and CHEF effect (right).

produced by heavy metals with large ζ values is solely due to spin−orbit coupling. In fact, Czarnik16 suggested that fluorescence quenching in ligands with a tethered anthracenyl (ANT) group, such as L1 (Figure 1), is possible because the fluorophore undergoes covalent-like bonding with Hg(II) and forms a π-complex. Our recent study on the Hg(II)−ADPA complex17 showed that Hg(II) does form a covalent-like bond with the ANT group and the fluorescence intensity is directly related to the Hg(II)− ANT distance. However, the current state of knowledge on the detailed mechanism of fluorescence quenching caused by Hg(II) ion in ligands with tethered fluorophore, such as L1 and ADPA, is rather limited and mostly speculative. It is possible that the fluorophore−metal interaction is needed for L1 and ADPA to efficiently promote the spin−orbit effect. But, on the other hand, it is expected that significant covalent character in the interaction between the fluorophore and the metal ion causes a reordering of MOs within the singlet state, and changes the simple picture illustrated in Figure 2. Then, the fluorescence enhancement through metal binding (as in the Zn(II) complex) may be no longer available for the Hg(II) complex, even without the spin−orbit effect. Therefore, it would be interesting to see if the difference in the effect of Zn(II) and Hg(II) binding to the ligand like ADPA can be explained by the absorptions/emissions within the singlet states of the complex, without relying on the spin−orbit coupling effect. In the present work, we investigated the effect of metal ion binding on the fluorescence intensity for the ligand ADPA, N(9-anthracenylmethyl)-N-(2-pyridinylmethyl)-2-pyridinemethane, through a series of density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. In particular, we focus on how the electronic structures of the ground and first singlet excited state of ADPA are affected by metal binding. The fluorescent sensor ADPA was first designed to work as an off-on−off switch for sensing proton.9 Free ADPA ligand is very weakly fluorescent at high pH, but the fluorescence intensity increases as the pH is lowered and has a maximum at around pH = 6. A further decrease of pH diminishes the fluorescence intensity. When ADPA binds to Zn(II), a large CHEF effect is observed and the ligand works as a turn-on sensor for Zn(II).9,17 But, when it binds to Hg(II), a π-complex is formed between the ANT and Hg(II), and the fluorescence is quenched.17 Therefore, ADPA is an ideal system to investigate the role of fluorophore−metal interaction in fluorescence quenching. We will compare the mechanisms of the CHEF effect for Zn(II) and CHEQ effect for Hg(II). In addition, the electronic structures of bare and protonated ADPA are

opposite is true for turn-off sensors. The underlying principle behind many turn-on sensors is the so-called CHEF (chelation enhanced fluorescence) effect. Briefly, turn-on sensors do not emit fluorescence (or are very weakly fluorescent) in the absence of metal ions because the fluorescence is quenched by the photoinduced electron transfer (PET) effect, where the lone pair electrons on the heteroatoms (e.g., amine group) are of higher energy than the HOMO of the fluorophore. Upon excitation of an electron from the HOMO to the LUMO of the fluorophore, these lone pair electrons drop down to the partially empty HOMO of fluorophore, which prevents the excited electron from returning to the fluorophore HOMO (fluorescence quenching). But, metal ion coordination to the quenching lone pair brings the energy of the lone pair state below that of the fluorophore HOMO, which will block the quenching and restore the fluorescence (CHEF). Such a CHEF effect is particularly pronounced for Zn(II) and Ca(II). See Figure 2 for the schematic energy level diagram involved in the PET and CHEF effects. However, some metal ions produce the opposite effect: the chelation enhanced quenching (CHEQ) effect, where coordination of the metal ion quenches the fluorescence. It has been often suggested that heavy metal ions such as Hg(II) and Pb(II) with high spin−orbit coupling constants (ζ) stabilize the triplet state and cause radiationless relaxation to the ground state with a long lifetime, effectively quenching the fluorescence.11,12 This is usually called the “heavy atom effect”, which promotes intersystem crossing between different spin states. However, it appears that some heavy metal ions do not fit into a such simple picture. For example, heavy metal ions such as La(III) and Lu(III) have very large ζ values but generate large CHEF effects with ligands such as PDA.13 It seems that the large CHEF effect produced by La(III) and Lu(III) is due to the fact that the metal−ligand bonding is too ionic.13 Therefore, the presence of heavy metal ion alone is not often sufficient to produce CHEQ effect. The role of the heavy atom effect was also questioned in a recent study of an excited state [Re(X)(CO)3(bpy)] complex, where the rate of triplet state formation increases in the order of X = Cl > Br > I,14 which is contradictory to the notion of a heavy atom effect. A recent DFT study of an Au complex also showed that the mere presence of a heavy atom does not necessarily promote intersystem crossing.15 The detailed electronic structure and the symmetry of molecular orbitals (MO) matter when it comes down to the effective spin−orbit coupling and subsequent singlet−triplet transition.15 As these findings suggest, it is not entirely clear whether the CHEQ effect B

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investigated for comparison. The off−on−off nature of the proton sensing mechanism of ADPA is compared with that of sensing Zn(II) and Hg(II). It appears that the origins of the CHEF effect associated with the Zn(II)−ADPA complex and the protonated ADPA (ADPAH+) are identical. But, as expected, the mechanism of fluorescence quenching involved in the doubly protonated ADPA (ADPAH2+) is rather different from what is observed for the Hg(II)−ADPA complex.

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METHODS

All DFT/TDDFT calculations reported in this work were carried out with the ab initio quantum chemistry package GAMESS.18 Geometry optimizations of the free and protonated ligand ADPA, as well as the metal coordinated ADPA were performed within the framework of Kohn−Sham DFT with the B3LYP19,20 exchange−correlation functional. The SV(P) basis set21 was used for the main group elements, whereas the Lanl2DZ22−24 effective core potentials (ECP) were employed for metals. To investigate the electronic properties of singlet excited states, time-dependent DFT (TDDFT) approach was applied. Excitation energies and oscillator strengths were obtained with the optimized ground state geometry using the same functional and basis sets used in the geometry optimizations. The geometry of the lowest singlet excited state was also optimized with the TDDFT approach and the emission frequency was estimated at the respective excited state geometry. All DFT/TDDFT calculations were performed in aqueous solution environment using the polarizable continuum model (PCM) as implemented in GAMESS. Metal containing systems have two explicit water molecules coordinated to the metal ion.

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Figure 3. (TOP) Optimized ground state geometries of (a) ADPA, (b) ADPAH+, and (c) ADPAH2+ obtained from the DFT/B3LYP/ SV(P) calculations with the PCM solvation. (Bottom) Optimized excited state (S1) geometries of (d) ADPA, (e) ADPAH+, and (f) ADPAH2+ obtained from the TDDFT calculations.

fluorophore. In addition, two pyridyl rings form more stable trans configuration with two pyridyl nitrogen atoms facing opposite directions, which is a typical behavior of flexible polypyridyl ligands in aqueous solution.28,29 The site of first protonation is believed to be the tertiary amine nitrogen, given the fact that the pKa value for the protonated tertiary amine (7.5) is much higher than that of pyridyl ring (4.4).9 The structural change upon protonation to the tertiary amine is rather significant. The most noticeable change is in the relative orientation of two pyridyl rings. The two pyridyl rings in ADPAH+ are now roughly on the same plane. Upon complexation with a proton, one of the pyridyl rings is rotated from the trans conformer to the cis conformer. The trans−cis transformation is typically seen in the complexation with a metal ion because the N-donors in the pyridyl rings are required to face the metal ion. It is interesting to see the same behavior when the proton is coordinated to the tertiary amine (rN−H = 1.05 Å) even though the nitrogen atoms in two pyridyl rings are not coordinated to the proton. The distance from the proton to the pyridyl nitrogen is quite long, 1.95 Å to the nitrogen closer to the ANT and 2.32 Å to the other. The bond lengths barely change upon protonation. The only possible exception is the distance between the central nitrogen and the neighboring carbons, in particular, C(1)−N(3), where it increases from 1.456 to 1.507 Å (Figure 1 and Figure S1, Supporting Information for key to atom labels). Unlike ADPAH+, the protonation sites for ADPAH2+ are not entirely clear. Following the pKa argument described above, it may sound reasonable to have one proton on the tertiary amine and another on the pyridyl ring. However, it is often observed that in poly protonated species, the protons will try to occupy basic sites as far as possible from each other, overriding greater basicity of more central basic groups such as the central nitrogen of ADPA.30 In fact, our DFT calculation showed that the ADPAH2+ structure with both protons on the pyridyl rings is more stable than the one with one proton on the central nitrogen and another on the pyridyl ring. However, preliminary DFT/TDDFT calculations indicated that the fluorescence sensing mechanisms associated with two possible ADPAH2+ structures are qualitatively very similar. Thus, we report the results only for the case where both protons are attached to the pyridyl rings in the present section. With both protons attached

RESULTS AND DISCUSSION

A. Off−On−Off Fluorescence Sensing Mechanism of Free and Protonated ADPA. The fluorescent sensor ADPA is unique in the sense that it can be used as an off-on−off proton switch.9 This is possible because two different proton receptors can work as an independent PET switch. The off− on−off switch based on PET such as ADPA has attracted a great deal of interest in the development of molecular logic gates.25−27 It has been shown that the completely deprotonated free ligand state (ADPA) is very weakly fluorescent due to PET, whereas the singly protonated ligand at the tertiary amine (ADPAH+) is strongly fluorescent.9,17 The fluorescence maximum occurs around pH = 6. A further decrease of pH protonates additional nitrogen atom (ADPAH2+), which is believed to induce a secondary PET process. Thus, ADPAH2+ becomes nonfluorescent.9 To better understand the metal coordinated ADPA systems, we first investigated the fluorescence quenching/enhancing mechanisms associated with the bare and protonated ADPA. We optimized the ground state geometries of ADPA, ADPAH +, and ADPAH2+ and the rendering of optimized ground state geometries are shown in Figure 3a−c. The key geometric parameters of all three species are available in the Supporting Information (Tables S1−S3). An interesting feature in the optimized structure of ADPA is that two pyridyl rings are not parallel to each other. One of the rings (left-hand side in Figure 3a) is rotated and it is at roughly a 45° angle from the plane of the other ring. Therefore, in bare ADPA, the pyridyl ring that is closer to the fluorophore (anthracenyl unit, ANT) is approximately at the right angle with respect to the C

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significant and, as such, artificial separation of donor and acceptor cannot properly represent the ANT−DPA systems. Because each of our calculations involves the entire donor− acceptor pair, we visually inspected the MOs around the HOMO/LUMO for the whole system and labeled each MO as “ANT” or “DPA”, depending on whether it is mostly localized on the ANT or the DPA unit. In Figure 4, the energy level diagram of each system has two columns. The MOs on the left belong to the ANT unit, whereas those on the right belong to the DPA unit. Some representative MOs near the HOMO/ LUMO are plotted in Figure 5. The calculated S0 → S1

to the pyridyl rings in ADPAH2+, two pyridyl rings are, again, roughly in parallel, and the structural change from ADPAH+ to ADPAH2+ is minimal. The only possible exception is the C(1)−N(3) distance, which decreases back to 1.470 from 1.507 Å upon the second protonation. With the optimized ground state geometries, we investigated the absorptions to the singlet excited states via TDDFT calculations. The excitation energies and oscillator strengths as well as the excited state compositions for the three ADPA systems are reported in Table 1. Orbital energy level diagrams Table 1. Electronic Excitation Energies (eV), Oscillator Strengths ( f), and Compositions of Low-Lying Singlet Excited States of ADPA, ADPAH+, and ADPAH2+ Obtained from the TDDFT/B3LYP/SV(P) Calculations with the PCM Solvationa system

transition

exct energy (eV)

ADPA ADPAH+ ADPAH2+

S0 → S1 S0 → S1 S0 → S1

3.11 3.08 2.51

0.1284 0.1159 0.0199

S0 → S2

2.63

0.0045

S0 → S3

3.07

0.0884

f

composition H H H H H H H

→ → → → → → →

L L L L+1 L L+1 L+2

% contribution 98.2 98.5 55.9 43.9 44.7 55.0 95.8

a

The TDDFT calculations were performed with the optimized ground state geometries.

near the HOMO/LUMO of these systems are also plotted in Figure 4. In DFT studies of PET sensors, the donor and acceptor MOs and the energy levels were often computed separately with an assumption that the interaction between the donor and acceptor is negligible.31,32 However, as will be shown later, especially for the Hg(II)−ADPA system, the interaction between the ANT unit and the dipicolylamine (DPA) unit is

Figure 5. Frontier molecular orbitals of the ground state (a) ADPA, (b) ADPAH+, and (c) ADPAH2+ obtained from the DFT/B3LYP/ SV(P) calculations with the PCM solvation.

excitation energies for ADPA and ADPAH+ are 399 and 403 nm (in H2O), respectively, which is close to the experimental

Figure 4. Energies of frontier molecular orbitals (MO) for ADPA, ADPAH+, and ADPAH2+ obtained from the TDDFT calculations with the optimized ground state geometries (Figure 3a−c). The MOs mainly localized on the anthracenyl unit are labeled as “ANT”, whereas those localized on the dipicolylamine unit are labeled as “DPA”. For ADPA, the orbital localized on N(3) (marked with #) is located below the HOMO of fluorophore (ANT). D

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Figure 6. Energies of frontier molecular orbitals (MO) for ADPA, ADPAH+, and ADPAH2+ obtained from the TDDFT calculations with the optimized excited state (S1) geometries (Figure 3d−f). The MOs mainly localized on the anthracenyl unit are labeled as “ANT”, whereas those localized on the dipicolylamine unit are labeled as “DPA”. For ADPA, the orbital localized on N(3) (marked with #) is now located above the HOMO of fluorophore.

absorption maximum (370 nm in methanol). 34 These absorptions are exclusively HOMO → LUMO type transitions within the fluorophore (Table 1). In case of ADPAH2+, the oscillator strengths for the absorption to S1 and S2 states are very small ( f = 0.0199 and 0.0045, respectively), and the S1 and S2 states can be considered as dark states. The first allowed transition for ADPAH2+ is S0 → S3, which is a transition within the ANT unit. It is often a common practice to explain the PET mechanism of a given donor−acceptor system based on the possible absorption pathways using the orbital energy level diagram, such as Figure 4, obtained from the TDDFT calculations with the optimized ground state geometry.32,33,35,36 In a typical PET process, the lone pair orbital of a nearby amine group (N(3) in ADPA case) should be of higher energy than the HOMO of fluorophore. Upon excitation from the HOMO of fluorophore to a higher state, an electron is transferred from the lone pair to the partially empty HOMO of fluorophore, which prevents the excited electron from returning to the ground state (fluorescence quenching). This type of PET mechanism is often called a-PET (acceptor-excited PET, Figure 2).37,38 For the most part, Figure 4 allows us to justify the off−on nature of fluorescent sensing mechanism associated with the ADPA/ ADPAH+ systems. Upon protonating N(3), the energy levels drop in general with respect to those of ADPA, but the lone pair orbital localized at N(3) goes down much further (out of the scale in Figure 4). As a consequence, the electron transfer from the donor nitrogen to the fluorophore should be blocked after the excitation and the fluorescence is restored in ADPAH +. Therefore, ADPA can be used as a turn-on proton sensor. Note that the empty states localized on the DPA unit are found well above the LUMO, and therefore, relaxation of excited electron to the DPA unit is not allowed either. Adding a second proton to the nitrogen atom in the DPA unit significantly changes the energy levels associated with the DPA unit. The energy level diagrams of ADPAH+ and ADPAH2+ shown in Figure 4 indicate that the second

protonation has hardly any impact on the energy levels associated with the ANT unit, but it pulls down the MOs localized on the DPA unit considerably. In fact, the LUMO and LUMO+1 of ADPAH2+ system do not belong to the ANT unit anymore, but it is now localized on the pyridyl rings (see Figure 5c for LUMO). As a result, the first allowed transition is the S0 → S3 transition (HOMO → LUMO+2), which is an excitation within the ANT unit. The S1 and S2 states are dark states and the nature of S0 → S1 and S0 → S2 transitions are charge transfer from the ANT to the DPA unit. The excited electrons from the S0 → S3 transition are then quickly relaxed via internal conversion to the S1 state.39 However, the emission to the S0 state is blocked due to the charge transfer nature of the S1 → S0 transition and the ADPAH2+ system is nonfluorescent. This type of quenching mechanism is usually called d-PET (donorexcited PET),37,38,40,41 as oppose to the a-PET mechanism mentioned above. In contrast to the ADPAH+ and ADPAH2+ systems, the fluorescence quenching mechanism associated with ADPA itself is not immediately clear. The discussion given above for the sensing mechanism of ADPAH+ implies that the a-PET should be responsible for the fluorescence quenching associated with ADPA. However, according to the energy level diagram shown in Figure 4, the MO localized on the donor nitrogen (N(3)) is actually located below the HOMO of ANT unit, although the energy difference between two MOs is small. Thus, it seems that the lone pair electron of N(3) cannot drop down to the empty fluorophore HOMO after the excitation and the a-PET mechanism appears to be not feasible. For ADPA, d-PET type quenching is not possible either because the empty MOs localized on the DPA unit have much higher energies than the LUMO of ANT unit. However, it should be noted that the MOs shown in Figure 4 belong to the ground state. Excitation of the fluorophore can initiate the reordering (sometimes significant) of these MOs during the relaxation process in the excited state, along with the change in structure. Electronic relaxation and accompanying structural change may put the E

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HOMO of ANT below the donor nitrogen level so that the electron transfer from the donor nitrogen to the ANT HOMO is possible (a-PET mechanism, Figure 2). To resolve this issue without speculation, we optimized the geometry of ADPA on the excited electronic state (S1) using the TDDFT method with the same functional and basis set as the ground state calculations. For completeness, we computed the excited state geometries of ADPAH+ and ADPAH2+ as well. The optimized excited state geometries of ADPA, ADPAH +, and ADPAH2+ are shown in Figure 3b and the key geometric parameters are listed in the Supporting Information (Tables S4−S6). For ADPAH+ and ADPAH2+, the difference in structure between the ground and the excited state is quite small. The only noticeable change is found in the dihedral angles of the ADPAH2+ system, where the difference can be as large as 30°. On the other hand, unlike the protonated systems, there is a drastic change in the structure of the bare ligand ADPA as the system is relaxed on the excited singlet state. One of the pyridyl rings that is closer to the ANT unit is rotated upon relaxation and two pyridyl rings become almost perpendicular to each other. A more important change in the structure of ADPA when it is excited to the S1 state is the geometry around the tertiary nitrogen, N(3). In the ground state, N(3) has sp3 character where the sum of three angles, C− N−C, around N(3) is 336.78°. However, upon excitation to S1, the geometry around N(3) becomes completely flat and the sum of three C−N−C angles is now 359.99°, which implies that N(3) is sp2 hybridized. Such structural change will increase the energy of lone pair electrons on N(3). The emissive properties of three ADPA systems were then investigated using the TDDFT method with the optimized excited state geometry. According to the Kasha’s rule,39,42 the emission from the excited fluorophore is exclusively from the lowest singlet excited state (S1). Any excitation to higher electronic state will lead to a fast relaxation to the S1 state before the possible emission to the ground state (S0). Therefore, the TDDFT study with the optimized excited singlet state geometry provides us with the information about the corresponding emission process. In other words, the TDDFT calculations were performed with an assumption that the S1 → S0 transition (emission) can be considered as the reverse of S0 → S1 transition obtained with the excited state geometry. In Figure 6 are shown the energy level diagrams thus obtained from the TDDFT calculation with the excited state geometries. The HOMO/LUMO of three ADPA systems with the optimized excited state geometry are also plotted in Figure 7. The transition energies and oscillator strengths as well as the composition of each transition are summarized in Table 2 for three ADPA systems. As suspected, the lone pair orbital on N(3) of ADPA is now located above the HOMO of ANT, which allows the transfer of an electron from N(3) to the partially empty HOMO of ANT during the relaxation process. Then, the emission process involves a charge transfer from the LUMO of ANT to N(3), but this transition is forbidden, as indicated by a small oscillator strength (Table 2). This forbidden transition is represented by a dashed line in Figure 6. We note that a similar fluorescence quenching mechanism that involves a reordering of frontier orbitals on the excited state was first reported by Theodorakopoulos and co-workers43 for a system with a tertiary amine linker attached to a pyrene fluorophore. It is interesting to see that a similar structural change around the tertiary nitrogen upon excitation to S1 was reported in ref 43. In the case of

Figure 7. (a) HOMO and (b) LUMO of ADPA (left), ADPAH+ (middle), and ADPAH2+ (right) obtained with the optimized excited state (S1) geometries (Figure 3d−f).

Table 2. Electronic Excitation Energies (eV), Oscillator Strengths (f), and Compositions of S0 → S1 Transitions Obtained from the TDDFT/B3LYP/SV(P) Calculations with the PCM Solvationa system

transition

exct energy (eV)

ADPA ADPAH+ ADPAH2+

S0 → S1 S0 → S1 S0 → S1

1.97 2.67 1.75

f

composition

% contribution

0.0025 0.1142 0.0075

H→L H→L H→L

99.6 99.5 99.7

a

The TDDFT calculations were performed with the optimized singlet excited state geometries of ADPA, ADPAH+, and ADPAH2+.

ADPAH+ and ADPAH2+, the structural change upon excitation from the S0 to S1 state is very small. As a result, the energy level diagrams shown in Figure 6 are quite similar to those in Figure 4, except that the HOMO−LUMO gap is reduced. Thus, for ADPAH+, the emission process simply involves the excited electron dropping back to the HOMO of ANT. This transition is an allowed transition (solid arrow in Figure 6) with a large oscillator strength, and therefore, ADPAH+ should be fluorescent. The computed emission wavelength is 465 nm, which is comparable to the experimental emission maximum of 430 nm.17 As we discussed above, the dPET mechanism is expected to play a role in the fluorescence quenching for ADPAH2+. The electrons excited to the LUMO +2 state, which is the lowest unoccupied state that belongs to the ANT unit, relax to the LUMO (localized on DPA) through internal conversion. But the S1 → S0 transition is forbidden due to the charge transfer character. This is clearly seen in Figure 6, Figure 7, and Table 2, where the S1 → S0 transition involves a charge transfer from one of the DPA rings to the ANT unit. The oscillator strength of this transition is again very small, although it is 3 times larger than that of ADPA system. In summary, the standard a-PET and d-PET mechanisms are responsible for the PET process associated with ADPA and ADPAH2+, respectively. However, as shown in the case of ADPA, care must be taken because the energy ordering of molecular orbitals obtained from the ground state geometry may appear to be inconsistent with the simple picture described in Figure 2. The excited state properties of the given sensor should be computed to correctly identify the entire PET process, which is often neglected in the DFT studies of fluorescent sensors. B. Effect of Metal Ion Chelation. As demonstrated in the recent study by us and others,9,17,34 fluorescent sensor ADPA shows an increase in fluorescence intensity in the presence of F

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Zn(II) ion (chelation enhanced fluorescence (CHEF) effect), but the chelation of Hg(II) quenches fluorescence, producing a chelation enhanced quenching (CHEQ) effect. For the metal ions producing the CHEF effect, such as Zn(II), it has been shown that metal chelation lowers the energy of lone pair electrons (donor) below the HOMO of fluorophore, preventing the PET effect. Therefore, the fluorescence is restored. However, the detailed mechanism associated with the CHEQ effect caused by chelation of Hg(II) is not entirely clear. It has often been argued that metal ions with high atomic number (thus larger spin−orbit coupling constant, ζ) are responsible for the CHEQ effect through strong spin−orbit interaction. However, as suggested in our recent work17 and by Czarnik,16 for a ligand like ADPA and L1, a π-complex formation might be necessary between the mercury(II) and the fluorophore that is tethered to the ligand for the fluorescence quenching. The strong interaction between Hg(II) and the fluorophore can certainly have significant impact on the simple PET picture described in Figure 1, but the specific role of Hg(II)−ANT interaction is not clear at this point. Here, we present the result of our DFT/TDDFT study on the metal chelated ADPA and provide a possible explanation for the difference in fluorescence intensity between Zn(II) and Hg(II) ion chelations to ADPA. Our focus here is to understand the effect of Hg(II)−fluorophore interaction to the singlet states (ground and excited states) and rationalize how it can prevent the fluorescence restoration process, which is supposed to be in effect upon metal binding as in the case of Zn(II)−ADPA complex. Thus, spin−orbit coupling is not considered in the present work. We first optimized the ground and excited state structures of the ADPA complexes with either Zn(II) and Hg(II) coordinated. Note that two explicit water molecules are coordinated to the metal ions, one at the equatorial and another at the axial position. For the ground state Hg(II) complex, the optimum geometry search started from the X-ray crystal structure17 of (ADPA)(HgCl2) with two chlorine atoms replaced by water molecules. For the Zn(II) complex, two different orientations of anthracenyl fluorophore were investigated: one with the same orientation as in the Hg(II) complex and another with the anthracenyl group turned away from Zn(II). The energy difference between two optimized Zn(II) complexes with different fluorophore orientation was found to be insignificant. Subsequent TDDFT calculations also showed that the mechanism of CHEF effect for the Zn(II) complex does not depend on the orientation of fluorophore. Thus, to make a direct comparison between the Zn(II) and Hg(II) complexes, we report the result of Zn(II) complex with the same fluorophore orientation as in the Hg(II) complex. The structures of metal chelated ADPA in its ground electronic state are shown in Figure 8a, and the key geometric parameters are listed in the Supporting Information (Tables S7 and S8). The preliminary results on the ground state structure of Hg(II)−ADPA complex were reported in our previous work as well.17 To make sure that the formations of metal−ligand systems under investigation were energetically favorable, we computed the binding energies of the ADPA(ZnII)(H2O)2 and ADPA(HgII)(H2O)2 systems in the PCM environment. Our calculations show that the binding energy of Zn complex is 8.29 kcal/mol, whereas that of Hg complex is 9.08 kcal/mol. Experimentally, the same trend was found with 50% MeOH/ H2O solution,17 although the difference in formation constants

Figure 8. Optimized (a) ground state and (b) excited state (S1) geometry of ADPA(ZnII)(H2O)2 (left) and ADPA(HgII)(H2O)2 (right). Geometry optimizations were done with the same basis set and functional as the calculations for ADPA and protonated ADPA (Figure 3).

between two complexes was much larger than what is predicted from the binding energies. Overall, the structures of two complexes are very similar, with the metal ion roughly on the same plane as the DPA. However, the distances between Zn and the coordinating nitrogen atoms of DPA are considerably shorter than those of the Hg(II) complex. This is understandable because Zn(II) has a smaller van der Waals radius. The average Zn−N distances is 2.17 Å, whereas, for the Hg(II)−ADPA complex, the average Hg−N distance is 2.52 Å. For the same reason, the metal−oxygen (water) distances are much shorter in the Zn(II)−ADPA complex (2.10 Å vs 2.44 Å). But, it should be noted that the distance between Hg(II) and ANT is not longer, but slightly shorter, than the distance between Zn(II) and ANT. The distance between the metal and the nearest carbon atom in the ANT group is 3.17 Å for the Zn(II) complex, but it is 3.11 Å for the Hg(II) complex. It turns out that a shorter than expected Hg(II)−ANT distance has a critical impact on the electronic structure of the Hg(II)−ADPA complex. A shorter Hg(II)−ANT distance and a larger van der Waals radius of Hg(II) implies that the fluorophore experiences much more extensive interaction with Hg(II) than with Zn(II), which brings the energies of the MOs associated with Hg(II) to the lower energy side considerably. As shown in Figure 9a, LUMO and LUMO+1 states of the Hg(II)−ADPA complex have significant contribution from mercury. It appears that the LUMO and LUMO+1 states are

Figure 9. Frontier molecular orbitals of ADPA(HgII)(H2O)2 for the optimized (a) ground state and (b) excited state (S1) geometries. From the left to the right in (a) HOMO, LUMO, and LUMO+1 are plotted, whereas HOMO and LUMO are plotted in (b). G

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Table 3. Electronic Excitation Energies (eV), Oscillator Strengths (f), and Compositions of Low-Lying Singlet Excited States of ADPA(ZnII)(H2O)2 and ADPA(HgII)(H2O)2 Obtained from the TDDFT/B3LYP/SV(P) Calculations with the PCM Solvationa system II

ADPA(Zn )(H2O)2 ADPA(HgII)(H2O)2

a

transition

exct energy(eV)

f

S0 → S1 S0 → S1

2.96 2.82

0.0969 0.0727

S0 → S2

3.07

0.0568

composition H H H H H

→ → → → →

L L L+1 L L+1

% contribution 97.5 87.8 11.6 11.2 86.5

The TDDFT calculations were performed with the optimized ground state geometries.

Table 4. Electronic Excitation Energies (eV), Oscillator Strengths (f), and Compositions of S0 → S1 Transitions Obtained from the TDDFT/B3LYP/SV(P) Calculations with the PCM Solvationa system II

ADPA(Zn )(H2O)2 ADPA(HgII)(H2O)2 a

transition

exct energy (eV)

f

composition

% contribution

S0 → S1 S0 → S1

2.54 0.72

0.0927 0.0016

H→L H→L

98.9 99.7

The TDDFT calculations were performed with the optimized singlet excited state geometries of ADPA(ZnII)(H2O)2 and ADPA(HgII)(H2O)2.

Figure 10. Energies of frontier molecular orbitals for (a) ADPA(ZnII)(H2O)2 and (b) ADPA(HgII)(H2O)2. The diagram on the left for each complex is obtained for the optimized ground state geometry (absorption), whereas the one on the right is for the optimized excited state (S1) geometry (emission). For absorption, the arrows indicate the transition between the ground state and the low-lying singlet states. Emission is allowed for ADPA(ZnII)(H2O)2, but not for ADPA(HgII)(H2O)2 (indicated by a dashed arrow). Two orbitals marked with an asterisk contain significant contribution from the mercury (Figure 9).

the result of mixing between the LUMO of fluorophore and the MOs localized on Hg(II). In the case of Zn(II)−ADPA complex, the MOs associated with Zn(II) are not found anywhere near the HOMO/LUMO of the system. The HOMO and LUMO of the ground state Zn(II)−ADPA complex are localized only on the fluorophore (not shown). In fact, TDDFT calculations revealed the clear difference between the two systems in the low-lying excited states. As shown in Table 3, for the Hg(II)−ADPA complex, the S0 → S1 excitation and the S0 → S2 excitation have similar oscillator strengths and excitation energies. The former is predominantly HOMO → LUMO transition and the later mostly HOMO → LUMO+1 transition. Both transitions have some charge transfer character from the ANT to Hg(II). In case of the Zn(II)−ADPA complex, only the S0 → S1 excitation is allowed, which is the HOMO → LUMO transition localized within the fluorophore. However, the difference in absorption between the Zn(II)− ADPA and Hg(II)−ADPA systems alone does not clearly

explain the fact that the Zn(II)−ADPA complex shows a strong CHEF effect, whereas the Hg(II)−ADPA system shows the opposite. To understand the difference in emission intensity between the two systems, we optimized the structures of Hg(II)−ADPA and Zn(II)−ADPA systems in their lowest singlet excited state using the TDDFT approach. The structures of metal chelated ADPA in their excited singlet state are shown in Figure 8b, and the key geometric parameters are listed in the Supporting Information (Tables S9 and S10). For the Zn(II)− ADPA complex, there is very little difference in structure between the ground and excited singlet state. The average Zn(II)−N distance stays almost the same and the Zn(II)−ANT distance is reduced slightly to 3.06 Å. Only the dihedral angles change noticeably, but the difference is less than 4°. On the other hand, a significant structural change is observed for the Hg(II)−ADPA complex when the system is relaxed in the singlet excited state. The most noticeable change is the fact that Hg(II) moves away from the fluorophore (by 1.1 Å), H

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SUMMARY AND CONCLUSION In the present work, we have carried out extensive DFT/ TDDFT calculations to understand the nature of the Hg(II)induced fluorescence quenching process in a PET sensor, ADPA. A particular emphasis was made to distinguish the effects of Zn(II) and Hg(II) chelation, where Zn(II) shows a pronounced chelate enhanced fluorescence (CHEF) effect as opposed to the chelate enhanced quenching (CHEQ) effect produced by Hg(II). We also compared the CHEF/CHEQ mechanisms associated with the metal bound ADPA systems to those of protonated ADPA systems to build a consistent picture within the entire family of ADPA complexes. Our DFT/TDDFT calculations showed that the free ADPA and the doubly protonated ADPAH2+ systems quench fluorescence through an a-PET (acceptor excited) and d-PET (donor excited) mechanism, respectively. It should be noted, however, that the energy ordering of ADPA frontier molecular orbitals obtained from the optimized ground state geometry does not support the a-PET mechanism (Figure 2) by itself, given that the orbital localized on the tertiary amine (N(3)) is found below the HOMO of the fluorophore. It turns out that ADPA undergoes a significant structural change in the excited S1 state and the orbital localized on N(3) eventually becomes located above the HOMO of fluorophore in the excited state, which subsequently allows a-PET process. Therefore, it is important to investigate the structure of the excited state as well when PET sensing mechanisms are studied. In contrast to the ADPA and ADPAH2+ systems, singly protonated ADPA, ADPAH+, shows a pronounced CHEF effect. This is because the protonation on the tertiary amine lowers the energy of lone pair state below the HOMO of the fluorophore, which blocks the PET process. It was found that the CHEF effect associated with the Zn(II)−ADPA complex follows the same mechanism as the fluorescence enhancement in the ADPAH+ system. The orbital localized on N(3) is found below the HOMO of the fluorophore in both the ground state and the excited state. In fact, very little structural change is observed in the excited states of the Zn(II)−ADPA and ADPAH+ systems. The HOMO− LUMO excitation and emission are contained exclusively within the fluorophore and the fluorescence is restored, which is consistent with experimental observation. Experimentally, the CHEF effect induced by Zn(II) is much more pronounced than that of ADPAH+, but the calculations show similar oscillator strengths for both systems, which indicates the limitation of the present TDDFT/B3LYP/SV(P) approach. When Hg(II) binds to ADPA, there seems to be a significant interaction between the fluorophore and Hg(II) as shown in the unusually short fluorophore−Hg(II) distance. It is also reflected in the fact that the LUMO and LUMO+1 states of the Hg(II)−ADPA system have large contributions from the metal, which is not seen in the Zn(II)−ADPA system. Excitation from the HOMO to both of these states is allowed, although the oscillator strengths are smaller than what was obtained for the HOMO−LUMO excitation in the Zn(II)−ADPA system. Interestingly, the excited state Hg(II)−ADPA has a much longer fluorophore−Hg(II) distance and the LUMO is exclusively localized on mercury. Therefore, the S1 → S0 transition (emission) is now forbidden, as indicated by the negligible oscillator strength for the HOMO−LUMO transition. When the absorption/emission pathways associated with the Zn(II)−ADPA and Hg(II)−ADPA systems are compared,

which causes the two pyridyl rings to buckle toward the ANT unit. The distances from Hg(II) to the coordinating nitrogen atoms increase substantially (by 0.3−0.9 Å). The impact of such a structural change on the characteristics of frontier orbitals is drastic. As shown in Figure 9b, the LUMO is now entirely localized on mercury. Therefore, the emission process (S1 → S0) for the Hg(II)−ADPA complex involves charge transfer from Hg(II) to ANT, which is not expected to be allowed. To confirm this prediction, TDDFT calculations were performed with the optimized excited state geometry and the results are reported in Table 4. For both the Zn(II) and Hg(II) complexes, the S0 → S1 transition is exclusively a HOMO → LUMO transition, but only the Zn complex has substantial magnitude of oscillator strength. For the Hg(II)−ANT complex, the HOMO → LUMO transition is a charge transfer process, which is not allowed, as indicated by a small oscillator strength. Therefore, the emission from the Hg(II)−ADPA complex will be blocked as well. In fact, the oscillator strength shown in Table 4 for the Hg(II)−ADPA complex is even smaller than that of bare ADPA ligand (Table 2). This implies that the fluorescence intensity will be further reduced when a mercury ion binds to the ligand, which already shows diminished fluorescence intensity due to the PET effect. The present results from the TDDFT calculations are consistent with the previous fluorescence study,17 which shows a drop in the fluorescence intensity by 50% upon Hg(II) chelation to ADPA. In Figure 10 is shown the energy level diagrams for the frontier orbitals of the ground and the excited state Hg(II)− ADPA and Zn(II)−ADPA complexes. For the Zn(II)−ADPA complex, there is little difference in the orbital energy level diagram between the ground state and the excited state. Because the HOMO and LUMO are localized on the fluorophore, the excitation is local, and the excited electrons simply drop to the partially empty HOMO during the emission process. Thus, the chelation of Zn(II) to ADPA restores the fluorescence. It should be noted that such a mechanism of fluorescence restoration is similar to what is observed for the ADPAH+ system (Figures 4 and 6). In both systems, the energy of nitrogen lone pair drops below that of the HOMO of the fluorophore due to the chelation by metal or protonation, which prevents the fluorescence quenching via a PET process. On the other hand, the CHEQ effect by the chelation of Hg(II) works differently than the typical PET mechanisms, such as the a-PET and d-PET mechanisms discussed in the previous sections. The key element in the fluorescence quenching by Hg(II) appears to be the ability of Hg(II) to interact with the fluorophore directly, which brings the MOs associated with Hg(II) to near the HOMO/LUMO of fluorophore. This allows the interaction between the fluorophore LUMO and the MOs from Hg, which result in two nearby MOs (LUMO and LUMO +1, indicated by * in Figure 10) having roughly equal contributions from the ANT and Hg(II). The excitation to LUMO (S0 → S1) and LUMO+1 (S0 → S2) are both allowed with decent oscillator strengths, although the transitions have some charge transfer character. However, the excited electrons do not simply return to the ground state. Instead, excited electrons are going through a fast relaxation, along with the substantial structural change. In the optimized S1 state, the excited electron is localized on Hg(II) (Figure 9b). Therefore, S1 → S0 is a charge transfer transition and nonradiative, which is indicated by the dashed arrow in Figure 10. I

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(6) Rurack, K. Flipping the Light Switch ‘ON’ − The Design of Sensor Molecules that Show Cation-Induced Fluorescence Enhancement with Heavy and Transition Metal Ions. Spectrochim. Acta Part A 2001, 57, 2161−2195. (7) Noland, E. M.; Lippard, S. J. Tools and Tactics for the Optical Detection of Mercuric Ion. Chem. Rev. 2008, 108, 3443−3480. (8) Formica, M.; Fusi, V.; Giorgi, L.; Micheloni, M. New Fluorescent Chemosensors for Metal Ions in Solution. Coord. Chem. Rev. 2012, 256, 170−192. (9) de Silva, S. A.; Zavaleta, A.; Baron, D. E.; Allam, O.; Isidor, E. V.; Kashimura, N.; Percarpio, J. M. A Fluorescent Photoinduced Electron Transfer Sensor for Cations with an Off-On-Off Proton Switch. Tetrahedron Lett. 1997, 38, 2237−2240. (10) Gan, W.; Jones, S. B.; Reibenspies, J. H.; Hancock, R. D. A Fluorescent Ligand Rationally Designed to be Selective for Zinc(II) over Larger Metal Ions. The Structures of the Zinc(II) and Cadmium(II) Complexes for N,N-bis(2-methlyquinoline)-2-(2aminoethyl)pyridine. Inorg. Chim. Acta 2005, 358, 3958−3966. (11) McClure, D. S. Triplet-Singlet Transitions in Organic Molecules. Lifetime Measurements of the Triplet State. J. Chem. Phys. 1949, 17, 905−913. (12) Solovyov, K. N.; Borisevich, E. A. Intramolecular Heavy-Atom Effect in the Photophysics of Organic Molecules. Physics-Uspekhi 2005, 48, 231−253. (13) Williams, N. J.; Dean, N. E.; Vanderveer, D. G.; Luckay, R. C.; Hancock, R. D. Strong Metal Ion Size Based Selectivity of the Highly Preorganized Ligand PDA (1,10-Phenanthroline-2,9-dicarboxylic Acid) with Trivalent Metal Ions. A Crystallographic, Fluorometric, and Thermodynamic Study. Inorg. Chem. 2009, 48, 7853−7863. (14) Cannizzo, A.; Blanco-Rodriguez, A. M.; Hahhas, A. E.; Sebera, J.; Zalis, S.; Vlcek, A., Jr.; Chergui, M. Femtosecond Fluorescence and Intersystem Crossing in Rhenium(I) Carbonyl-bipyridine Complexes. J. Am. Chem. Soc. 2008, 130, 8967−8974. (15) Tong, G. S. M.; Chow, P. K.; Che, C-.M. Where is the HeavyAtom Effect? Role of the Central Ligand in Tetragold(I) Ethynyl Complexes. Angew. Chem., Int. Ed. 2010, 49, 9206−9209. (16) Yoon, J.; Ohler, N. E.; Vance, D. H.; Aumiller, W. D.; Czarnik, A. W. A Fluorescent Chemosensor Signalling Only Hg(II) and Cu(II) in Water. Tetrahedron Lett. 1997, 38, 3845−3848. (17) Lee, H.; Lee, H-.S.; Reibenspies, J. H.; Hancock, R. D. Mechanism of “Turn-on” Fluorescent Sensors for Mercury(II) in Solution and Its Implications for Ligand Design. Inorg. Chem. 2012, 51, 10904−10915. (18) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. General Atomic and Molecular Electronic Structure System. J. Comput. Chem. 1993, 14, 1347−1363. (19) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (20) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A 1988, 38, 3098−3100. (21) Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (22) Hay, P. J.; Wadt, W. R. Ab initio Effective Core Potentials for Molecular Calculations. Potentials for the Transition Metal Atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270−283. (23) Wadt, W. R.; Hay, P. J. Ab initio Effective Core Potentials for Molecular Calculations. Potentials for Main Group Elements Na to Bi. J. Chem. Phys. 1985, 82, 284−298. (24) Hay, P. J.; Wadt, W. R. Ab initio Effective Core Potentials for Molecular Calculations. Potentials for K to Au Including the Outermost Core Orbitals. J. Chem. Phys. 1985, 82, 299−310.

it is reasonable to believe that the fluorophore−Hg(II) interaction plays an important role in the underlying CHEQ effect observed in the Hg(II)−ADPA system. It should be mentioned, however, that the present DFT/TDDFT calculations do not include the spin−orbit coupling effect. Therefore, the so-called “heavy atom effect” cannot be ruled out completely as a possible cause of the CHEQ effect associated with the Hg(II). However, the present study demonstrates that the heavy atom effect should not be taken for granted, as typically seen in the literature, because the CHEQ effect can be explained with singlet states only as shown in the present study with the Hg(II)−ADPA system. Finally, we note in passing that the TDDFT calculations with typical hybrid functionals often yield spurious low-lying charge transfer excited states, which can contaminate other states. However, a recent TDDFT/B3LYP study with the SV(P) basis on the PET sensor Zinpyr showed that the inclusion of the solvent effect can remove such nonphysical behavior and produce a correct result that is in qualitative agreement with experiment.44 It is possible that our TDDFT/B3LYP/SV(P) calculations still underestimate the charge transfer excited state energies. But, given the fact that the fluorescence on−off behavior is reproduced in all five systems studied in this work, the charge transfer state problem associated with the TDDFT/ B3LYP/SV(P) approach, if any, should not be significant enough to produce qualitatively different outcomes, at least for the ADPA family of complexes studied in this work .

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ASSOCIATED CONTENT

S Supporting Information *

The key geometric parameters for the ground and singlet excited state of all the species discussed in the text (ADPA, ADPAH+, ADPAH2+, ADPA(ZnII)(H2O)2, and ADPA(HgII)(H2O)2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*H.-S. Lee: e-mail, [email protected]; phone, 910-962-2439; fax, 910-962-3013. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the University of North Carolina Wilmington and the Department of Energy (Grant no. DEFG07-07ID14896) for generous support for this work.

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REFERENCES

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K

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