Theoretical determination of the pKas of the 8-hydroxyquinoline-5-sulfonic acid: A DFT based approach

Chemical Physics Letters 472 (2009) 30–34

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Theoretical determination of the pKas of the 8-hydroxyquinoline-5-sulfonic acid: A DFT based approach Tangui Le Bahers, Carlo Adamo, Ilaria Ciofini * LECIME, Laboratoire d’Electrochimie, Chimie des Interfaces et Modélisation pour l’Energie, CNRS UMR-7575, Ecole Nationale Supérieure de Chimie de Paris – Chimie ParisTech, 11 rue P. et M. Curie, 75231 Paris Cedex 05, France

a r t i c l e

i n f o

Article history: Received 14 January 2009 In final form 27 February 2009 Available online 4 March 2009

a b s t r a c t The three acid dissociation constants (pKas) of the 8-hydroxyquinoline-5-sulfonic acid were computed using a computational protocol based on Density Functional Theory. A hybrid exchange correlation functional was applied and bulk solvent effects were treated within the framework of the Polarizable Continuum Model. Direct solute–solvent interactions were taken into account adding explicit water molecules. The computed pKas are in line with the experimental data and allow better defining the first pKa, confirmed to be negative. From the calculated pKas, ‘ab initio’ distribution diagrams of the relative concentration of the different species in solution as a function of pH were drawn. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction 8-Hydroxyquinoline (8-HQ) and its 5-sulfonic acid derivative (8-HQS, Fig. 1) are well-known bidentate chelating agents able to form extremely stable complexes with most transition and main groups metal cations [1–9]. These latter are extremely fluorescent and have widely been used for fluorimetric determination of metal cations since the ligand itself is weakly emissive [10]. Among these systems, the most studied is probably the Al(III)(8-HQ)3 complex due to its strong fluorescence both in solution and in solid state, which made it a good candidate for application as emissive material in the emerging display technology based on organic light emitting diodes (OLEDs) [11]. Substituting 8-HQ with 8-HQS allowed increasing the relatively low solubility of the complexes in polar and protic media. The poor fluorescence 8-HQ, firstly pointed out by Ballard and Edwards [12], was explained in term of a photo-tautomerisation reaction which quenches the fluorescence. Several experimental and theoretical studies were subsequently carried out to understand this phenomenon [8,13,14], the mechanism proposed for the photo-tautomerisation involving an excited state proton transfer (from the OH function to the N atom) so to produce the zwitterionic form of 8-HQ. Since spectral – absorption and emission – properties of 8-HQ derivatives can dependent of its protonation degree, an accurate knowledge of the acid dissociation constants (i.e. the pKas) of these species both at the ground and the excited state could be of great help in understanding their overall photochemical behavior as a function of the pH. * Corresponding author. E-mail address: ilaria-ciofi[email protected] (I. Ciofini). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.02.072

The different acid forms involved and the equilibriums that can take place in aqueous solution in the case of 8-HQS are schematically depicted in Fig. 1. From an experimental point of view, a number of papers focused on the determination of ground and excited state pKas of hydroxoquinoline derivatives can be found in literature [15–19]. At the ground state, it is quite unambiguously granted that for 8-HQ the first dissociation constant, corresponding to the deprotonation of the NH+ group, is 5.13 and the second, corresponding to the deprotonation of the OH group, is 9.89 [17]. The situation is less clear for 8-HQS for which different values are proposed for the first dissociation constant (related to the deprotonation of sulfonic acid group) mainly due to the accuracy of the methods of determination applied. In particular, experimental values range from a positive – but smaller than 2 [18] – to a negative pKa value [19]. With the aim of clarifying the distribution of the different forms of 8-HQS in water solution as a function of the pH, the three acid dissociation constants, corresponding to the deprotonation of the  + sulfonic acid ðSO3 H=SO 3 Þ, hydroxyl (OH/O ) and quinoline (NH / N) groups respectively, were computed using an ab initio approach. In particular, Density Functional Theory in conjunction with a Polarizable Continuum Model (hereafter PCM) to treat bulk solvent effects was applied to compute the different pKas. Two explicit water molecules, solvating the OH and N functional groups, were added to simulate direct solute–solvent interactions at the same level of theory. The possible co-existence of enolic and zwitterion anionic forms (8-HQS-A and 8-HQS-Z respectively, Fig. 1) was also considered. Using the computed pKas, a distribution diagram of the species as a function of pH could be derived, allowing to rule out the presence of the fully protonated form of 8-HQS (8HQS-C Fig. 1) in solution and defining the range of pH of the existence of all the other forms. Furthermore, it was possible to

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T. Le Bahers et al. / Chemical Physics Letters 472 (2009) 30–34

SO3-

B

C3 C2

C4 N

+

H2

C5

C10

C7

C9

C6

C8

D

N

SO3-

SO3H

SO3-

OH

A 8-HQS-A N

+

H

OH 1

C

N

SO3-

OH

E

8-HQS-N

8-HQS-C

O 8-HQS-A2

N

+

H O 8-HQS-Z Fig. 1. Different acidic forms and dissociation reactions considered for 8-HQS.

estimate the relative ground state stability and concentration of the zwitterionic (8-HQS-Z) and anion – and enolic – form (8HQS-A) supposed to be involved in the quenching of fluorescence at the excited state [12–14]. 2. Methods All calculations were carried out using the GAUSSIAN code [20]. A hybrid Hartree Fock/Density Functional model, referred to as PBE0, was used [21]. The PBE0 was obtained by casting the PBE exchange and correlation functional [22] in a hybrid DFT/HF scheme, where the HF/DFT exchange ratio is fixed a priori to 1/4 [23]. Structural optimizations and subsequent frequency calculations were performed using an all-electron Pople double zeta basis set including one polarization function on heavier atoms (6-31G(d)). The molecular structure of each compound was fully optimized in absence of symmetry constrains. Bulk solvent effects were included using the Polarizable Continuum Model (PCM) of Tomasi and co-workers [24]. More specifically, the Conductor-like PCM model as implemented in GAUSSIAN (CPCM) [25,26], was applied and water was considered as solvent. Default (UFF) radii were used for structural optimisations while solvation energies were computed using UAHF radii (see below). pKas are related to the free energy variation of a generic deprotonation reaction of an acid species in aqueous solution þ ðAHaq ¼ A aq þ Haq Þ by the relation:

pK a ¼

DGaq 2:303RT

From a theoretical point of view, pKa can be computed relying on a thermodynamic cycle (such as the Born–Haber cycle schematically depicted in Scheme 1) and evaluating gas phase free energies and solvation contributions of all species but H+ at different level of the-

ΔGgas AHgas

X N ¼ ½Hþ

A-gas

-ΔGAH,solv

+

ΔGA-,solv

AHaq

ory. Proton free energy in gas phase (GH+,gas) and its corresponding solvation energy (DGH+,solv) are better taken from experiments to avoid problems related to the existence of solvated protons species. In this Letter, a GH+,gas = 6.28 kcal/mol and DGH+,solv = 263.98 kcal/mol were used as previously reported in literature. A –RT ln (24.46) term is added to take into account the change in standard concentrations going from the gas phase (1 atm) to aqueous solution (1 M = 1 mol/l). At DFT level, Saracino et al. [27] defined a cheap and efficient computational protocol to compute the pKa (relying on the Born– Haber cycle depicted in Scheme 1) where the solvation energies are computed at Hartree Fock level on structure optimized in solution at CPCM/DFT level using UAHF radii to build up the solute cavity. In such a way it is implicitly assumed that the variation in the vibrational contribution to DG between the gas phase and the solution is negligible. In the original application of this approach [27] – here referred as SIB (Saracino-Improta-Barone) – no explicit solvent molecules are added. Such approach has been extensively used both for ground and excited pKa calculations [28] and it usually provides constants with an error between 1 and 2 units [29–31]. Note that an error of 1 pKa unit corresponds to a variation of only 1.4 kcal/mol for DGaq, a value very close to the ±1 kcal/mol error bar, generally considered as a challenging accuracy for theoretical calculations. Alternatively one could also compute the vibrational contribution to the free energy in solution by performing frequency calculations for AH and A in water using a PCM approach (DGAH,aq and DGA-,aq) and add the DGH+,aq value derived from experiments, that is GH+,gas + DGH+,solv = 270.26 kcal/mol. This approach will be referred to as DIR (Direct) in the following. Using the computed dissociation constants and considering the different equilibrium depicted in Fig. 1, the fraction of each species (x) present in solution as a function of the pH was then computed using the following relations:

A-aq

+

H+gas ΔGH+,solv H+aq

ΔGaq Scheme 1. Representation of the Born–Haber thermodynamic cycle used for pKa calculations.

KA

1 KC KB KDKB þ 1 þ ½Hþ þ ½Hþ þ ½Hþ 2

½Hþ KA KB XA ¼ XN ½Hþ KC XZ ¼ XN ½Hþ KEKD X A2 ¼ X N ½H þ 2 XC ¼ XN

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T. Le Bahers et al. / Chemical Physics Letters 472 (2009) 30–34

of including the direct solvation effects that could be important to define the relative stability of the different species. For both models, the pKas related to all reactions depicted in Fig. 1 were computed using both the SIB and the DIR methods, as described in the computational details.

Table 1 Main structural parameters (distances in Å) computed for 8-HQS in PCM or in PCM + 2 water molecules (PCM + 2w). For labeling refer to Fig. 1. 8-HQS-C

8-HQS-N

8-HQS-Acis

8-HQS-Z

8-HQS-A2

PCM O–H1 O–C8 C9–C8 C9–N N–H2

0.970 1.335 1.422 1.361 1.019

0.969 1.345 1.417 1.363 1.018

0.982 1.343 1.428 1.355 –

– 1.262 1.460 1.350 1.028

– 1.259 1.482 1.354 –

PCM + 2w O–H1 O–C8 C9–C8 C9–N N–H2

1.089 1.325 1.429 1.362 1.056

1.082 1.337 1.423 1.364 1.051

1.001 1.343 1.435 1.364 –

– 1.283 1.452 1.368 1.048

– 1.283 1.467 1.358 –

3.1. Structural and energetic features of stable species The structure, together with the corresponding main structural parameters, of the different acidic form optimized in solution both in absence and in presence of (two) explicit water molecules are reported in Table 1 and in Fig. 2, respectively. For cationic (8-HQS-C) and neutral (8-HQS-N) species the most stable conformation of the hydroxyl group (i.e. the trans, the OH group pointing toward the solvent) is reported, while in the case of the anionic form (8-HQS-A) the cis conformation (allowing the formation of an intra-molecular H bond) is found to be the most stable. Indeed, for 8-HQS-A the stabilization of the cis with respect to the trans form (5.2 kcal/mol in PCM) is reduced in presence of explicit water molecules allowing an efficient net of intermolecular H bonds. In this latter case, the cis 8-HQS-A is found to be only 0.5 kcal/mol more stable of the corresponding trans isomer. From a purely structural point of view, both in presence and in absence of explicit solvent molecules a shortening of the C–O and C–S bonds upon deprotonation of the hydroxyl and sulfonic acid groups, respectively, is observed. Generally, the internal structural parameters of the aromatic rings are not affected neither by the protonation degree of the systems nor by the presence of explicit solvent molecules. On the other hand, as expected all bonds comprising atoms involved in either intra-molecular or intermolecular H bonds are significantly affected. In particular, the hydroxyl OH and enolic C–OH bonds are systematically underestimated in absence of explicit water molecules while the contrary holds for the pseudo-ketonic C–O bonds in the case of 8-HQS-Z and 8HQS-A2. Indeed, the largest difference computed between the model without explicit solvent molecules and that containing

It is worth mentioning that, when computing the distribution diagrams using these formulas, the possible direct isomerisation between the 8-HQS-A and 8-HQS-Z forms was neglected. 3. Results and discussion 8-HQS represents a peculiar polyprotic acid since, due to the spatial proximity of two of the acidic functions (the OH and NH+ groups) different inter- or intra-molecular H bond patterns can be foreseen in water as function of the species involved. In particular, the presence (or absence) of specific H bonds with the surrounding water molecules can significantly affect the stability of the different species, thus altering the computed pKa values, in particular those involving the zwitterionic and enolic forms (8-HQS-Z and 8-HQS-A, respectively). For these reasons, two different models were considered: the first constituted by the 8-HQS molecule in bulk PCM and the second constituted by a cluster of 8-HQS and two water molecules solvating the OH/O and NH+/N groups, respectively, in bulk PCM. This latter represents the simplest way

2.328 (2.202) 1.665

1.106 2.264

1.800

2.465 (2.013)

1.686

2.124

1.632

1.905

1.706

1.694

1.759

2.081 1.696

1.899 1.646

2.415 (2.092)

2.253 (2.218)

1.774

1.713

2.400

Fig. 2. Optimized molecular structures (in PCM) for 8-HQS-C, 8-HQS-N, 8-HQS-Z, 8-HQS-A (cis and trans conformers), and 8-HQS-A2 in presence or absence (in brackets) of two water molecules. Relevant H bond distances (in Å) are reported.

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T. Le Bahers et al. / Chemical Physics Letters 472 (2009) 30–34

opposite sign and larger thus leasing the over-stabilization of the zwitterion. Qualitatively the order of all other computed pKas values is in agreement with the experimental data (i.e. pKB < pKC < pKE < pKD) although the absolute values are very different.

1.0

0.8

0.6

8-HQS-N 8-HQS-A 8-HQS-Z 8-HQS-A2

Xx

two water molecules is still relatively small, of only 0.024 Å in the case of the OC8 bond of the 8-HQS-A2. The presence of explicit water molecules influences also the stability of the zwitterionic form (8-HQS-Z) with respect to the enolic (8-HQS-A) one. This latter is always find to be more stable but the relative stability is reduced from 7.6 kcal/mol, in absence of explicit waters, to 2.1 kcal/mol, in presence of explicit solvent molecule. Notably, no explicit water molecule was added around the ðSO3 H=SO 3 Þ group. Indeed, we believe a ‘simple’ PCM approach (averaging the direct solute–solvent contribution) to be more reliable than a model including only few water molecules – not representative of a full first solvation shell – on a frozen conformation. In summary, we can reasonable state that while internal parameters are hardly affect by explicit solvent molecules, those related to polar groups are significantly changed by the presence of an explicit solvent. As a consequence we can expect that other properties such as, for instance, UV–vis transitions of pp* character centred on the aromatic rings will be correctly reproduced even when a simple Polarizable Continuum Model [32] is used, while all properties related to the protic groups, such as pKas, should be better treated using an explicit solvent model.

0.4

0.2

3.2. Computed pKas and distribution diagrams 0.0 0

2

4

6

8

10

12

14

8

10

12

14

10

12

14

pH 1.0

0.8

8-HQS-N 8-HQS-A 8-HQS-Z 8-HQS-A2

Xx

0.6

0.4

0.2

0.0 0

2

4

6

pH 1.0

0.8

0.6

Xx

The pKas values computed applying both the SIB and the DIR methods are reported in Table 2 in comparison with the available experimental data (that is 8-HQ and 8-HQS). It is also worth to mention that the pKa value seems difficult to assess experimentally and that different values ranging from 2 to negative values can be found in literature depending on the technique used to determine them (i.e. potentiometric vs. spectrophotometric). First of all, we can remark that, independently of the solvent model and computational approaches used, pKA is always predicted to be strongly negative, thus ruling out the existence of the cationic species, even at very low pH, in water. This finding is somehow underlying the increased acidity of the sulfonic acid group within 8-HQS, its standard pKa being around 2. It is also worth to stress that the SIB method yields values that are significantly higher with respect to the DIR method. From the neutral species, that is computed to be the most stable at low pH, two possible deprotonation pathways can be envisaged leading to the formation of the enolic form, 8-HQS-A (pKB) or the zwitterionic form, 8-HQS-Z (pKC). For all methods but the SIB in absence of explicit solvent molecules, the latter is found to be significantly larger than pKB (from 5 to 1.4 pH units). These results are in agreement with the experimental data, showing the preferential formation of 8-HQS-A in aqueous solution at medium pH (between pH 5 and 10). The peculiar result obtained using the SIB approach in absence of explicit water molecules can be attributed to an overstabilization of the zwitterionic form in the solution due the higher solvation energy computed for this charge separated form. In fact, while in gas phase the enolic form is still computed to be the most stable, the difference in solvation energies of the two forms is of

0.4 Table 2 Computed and experimental pKas of 8-HQS (refer to Fig. 1 for labeling of the different equilibriums). 8-HQS

pKA pKB pKC pKD pKE a

Exp.a 8-HQ/8-HQS

8-HQS2W

SIB

DIR

SIB

DIR

4.9 4.9 4.5 15.5 16.0

9.5 3.0 2.5 18.6 13.0

3.4 8.1 9.8 13.7 12.0

8.2 2.9 4.5 12.8 11.2

From Refs. [17,19], respectively.

–/<0 5.14/4.09 6.6/ 9.88/ 8.42/8.66

8-HQS-N 8-HQS-A 8-HQS-Z 8-HQS-A2

0.2

0.0 0

2

4

6

8

pH Fig. 3. Experimental (top) and computed distribution diagrams using the SIB (middle) and DIR (bottom) approaches and two explicit water molecules. The unknown experimental pKA value was set to 2.

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T. Le Bahers et al. / Chemical Physics Letters 472 (2009) 30–34

The values computed in absence of explicit water molecules both using the SIB and the DIR methods give the largest variations with respect to the experimental data. This results is somehow expected since the model used –absence of explicit solvent molecules – is far to be realistic to describe the competition between intra and inter molecular H bonds. In the case of the model containing two water molecules, both the discrepancy between the experimental and computed values is between 3 and 4 pH units. In particular, the SIB method overestimates all pKas values, the error increasing for the pKas involving anionic forms (i.e. pKD and pKE). This seems to be the case also for the DIR method only when the doubly negatively charged species (8-HQS-A2) is involved. In general, the DIR method gives values that are slightly closer to the experimental values, the largest variation being 3 pH units for pKD. Indeed, the difference between the acidic dissociation behavior predicted using the SIB and DIR methods is clearly evidence by comparison of the computed distribution diagrams with the experimental one, reported in Fig. 3. Experimentally three species are expected to be present in solution: 8-HQS-N at low pH (pH < 6), 8-HQS-A at intermediate pH (6 < pH < 11) and 8-HQS-A2 at basic pH > 11. Using the pKas computed with the SIB approach, only two species are predicted to be stable in water solution: 8-HQSN for pH lower than 8 and 8-HQS-A for higher pH, since the third deprotonation is computed to be very unfavourable. On the other hand, the DIR approach predicts, at least qualitatively, the correct existence of three species in the pH range 0–14, although the 8HQS-N species is predicted to be more acidic and the 8-HQS-A species slightly more basic. Noteworthy all theoretical approaches predicts a small fraction of zwitterions present in solution at intermediate pH, in agreement with the distribution obtained using the experimental pKas. 3.3. Conclusions The 8-HQS acid has been chosen as a very simple yet challenging system to test different approaches for the computation of ground state pKa in polyprotic acids. An additional difficulty is related to the existence of neighbouring protonation sites that can yield concurrent intra- and inter-molecular H bond networks. As a consequence, standard procedures to compute pKa for monoprotic acids based on a representation of the solvent as a continuum dielectric (such as the SIB method) can be hardly applied in such case. A method explicitly including the first coordination sphere seems therefore mandatory. The results obtained using the latter

method (here defined as DIR) are only in semi-quantitative agreement with the available experimental data, but they allow to fully simulate the distribution diagram of the different acidic forms of 8HQS in solution. Acknowledgments The IDRIS (French national computer center) is gratefully acknowledged for computer time allocation within the Project 082115. Drs. T. Yoshida, T. Pauporté and P.P. Lainé are acknowledged for stimulating discussions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]

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