Protolytic dissociation of cyano derivatives of naphthol, biphenyl and phenol in the excited state: A review

Journal of Molecular Structure 1099 (2015) 209e214

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Protolytic dissociation of cyano derivatives of naphthol, biphenyl and phenol in the excited state: A review Beata Szczepanik Institute of Chemistry, Jan Kochanowski University, Swietokrzyska 15G, 25-426 Kielce, Poland

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 March 2015 Received in revised form 14 May 2015 Accepted 19 May 2015 Available online 20 June 2015

The excited state proton transfer (ESPT) has been extensively studied for hydroxyarenes, phenols, naphthols, hydroxystilbenes, etc., which undergo large enhancement of acidity upon electronic excitation, thus classified as photoacids. The changes of acidic character in the excited state of cyanosubstituted derivatives of phenol, hydroxybiphenyl and naphthol are reviewed in this paper. The acidity constants pKa in the ground state (S0), pK*a in the first singlet excited state (S1) and the change of the acidity constant in the excited state DpKa for the discussed compounds are summarized and compared. The results of the acidity studies show, that the “electro-withdrawing” CN group in the molecules of naphthol, hydroxybiphenyl and phenol causes dramatic increase of their acidity in the excited state in comparison to the ground state. This effect is greatest for the cyanonaphthols (the doubly substituted CN derivatives are almost as strong as a mineral acid in the excited state), comparable for cyanobiphenyls, and smaller for phenol derivatives. The increase of acidity enables proton transfer to various organic solvents, and the investigation of ESPT can be extended to a variety of solvents besides water. The results of theoretical investigations were also presented and used for understanding the protolytic equilibria of cyano derivatives of naphthol, hydroxybiphenyl and phenol. © 2015 Elsevier B.V. All rights reserved.

Keywords: Protolytic dissociation Excited-state acidity Cyano derivatives of naphthols Hydroxybiphenyl and phenol

1. Introduction Proton transfer is an important elementary step in many chemical and biological processes [1,2]. The excited state proton transfer (ESPT) has been extensively studied for hydroxyarenes, phenols, naphthols, hydroxystilbenes, etc., which undergo large enhancement of acidity upon electronic excitation [1e3]. Proton dissociation and acidebase equilibria in the excited state are strongly affected by substituents introduced in the aromatic ring. The introduction into the aromatic ring of the hydroxyarenes molecules an electron acceptor group, such as cyano, nitro or sulphonate groups, changes their acidity in both ground and excited states [4e9]. An aqueous medium is required for ESPT to take place in the case of simple phenols and naphthols, but substitution of electron withdrawing groups has greatly enhanced the photoacidity and made ESPT possible even in non-aqueous solvents [10e15]. The isomeric naphthols and their derivatives are prototypical photoacids whose excited-state behavior have been the subject of numerous publications. Several derivatives of 1- and 2-

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naphthol which possess electron-withdrawing cyano- or fluoroalkanesulfonyl substituents at 5- or 6-position have greatly enhanced excited-state acidities in water and also function as photoacids in polar and non-aqueous solvents [4,5,9,15e17]. This property of cyanonaphtols with one or two cyano groups is used to name these compounds “super” photoacids. A number of theoretical and experimental investigations have been carried out to understand the protolytic equilibria and proton transfer dynamics by taking 1- and 2-naphthols as the prototype for hydroxyaromatic compounds [18e27]. The donor-acceptor derivatives of biphenyl with hydroxyl and cyano substituents also demonstrate a strong photoinduced increase of acidity, similarly to cyano-derivatives of 2-naphthol [28,29]. Wehry et al. [30], Schulman et al. [31], Kaneko et al. [32] and Granucci et al. [33] have investigated the prototropic equilibria of phenols in ground and excited states. They reported that an increase of the acidity of orto- and meta-cyano derivatives was about 6 order of magnitude in the excited state in comparison to the ground state in an aqueous solution. The study of the acidity changes for monocyano- and dicyano-phenols in alcohol and water have shown that 3,4-dicyanophenol is the strongest acid among the investigated cyano-substituted phenols [34]. In this paper, the results of the acidity studies for cyano

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B. Szczepanik / Journal of Molecular Structure 1099 (2015) 209e214

derivatives of 2-naphthol, biphenyl and phenol in the excited state are reviewed. The acidity constants pKa in the ground state (S0), pK*a in the first singlet excited state (S1) and the change of the acidity constant in the excited state DpKa for the discussed compounds are summarized and compared. The results of theoretical investigations were also presented and used for understanding the protolytic equilibria of cyano derivatives of naphthol, hydroxybiphenyl and phenol.

ring system) the acid form dominates in the ground state, and it is converted to an excited R*OH molecule by photoexcitation. The R*OH undergoes a reaction of ESPT to solvent according to the following scheme:

2. Methods 2.1. Acid-base equilibria The pKa of biphenyl cyano derivatives: 4-hydroxy-40 -cyanobiphenyl and 4-hydroxy-2,6-dimethyl-40 -cyanobiphenyl in the ground state was calculated using Eq. (1) [35].

pKa ¼ pH þ log½ðε  εA Þ=ðεHA  εÞ

(1)

where: ε is the sum of the molar extinction coefficients of the solution mixture containing the anion A and acid HA, εA is the molar extinction coefficient of the pure anion A, and εHA is the molar extinction coefficient of HA. Generally, the measurements of photoacids excited-state acidities (pKa*) are based on the analysis of fluorescence properties of the neutral hydroxyaromatic compound, HA, and its conjugate base, A. The HA in the excited state is a stronger acid than in the ground state if the absorption or emission spectrum of the conjugate base displays a bathochromic shift relative to that of the conjugate acid [36,37]. This thermodynamic cycle is described by }rster equation (Eq. (2)), where pKa* ¼ DG*a/2.3RT is the the Fo ground (excited)-state acidity constant and hn1(2) is the energy of the 0e0 electronic transition for the conjugate acid (base) [38]. A more useful treatment is to take hn1 and hn2 as the averages of the absorption and fluorescence transitions of each acid and base species (Eqs. (3) and (4)).

pKa* ¼ pKa  ðhn1  hn2 Þ=2:3RT ~A n 00 ¼

~HA n 00 ¼

~aA þ n ~fA n 2 ~aHA n

þ 2

~fHA n

(2)

(3)

(4)

The change of the acidity constant (DpKa) in the excited state was calculated using the Eq. (5):

DpKa ¼

 0; 625    Nhc  A ~00  n ~A ~HA ~HA n n ¼ 00  n 00 00 2; 3RT T

(5)

where: T is the temperature on the Kelvin, ~nA nHA 00 and ~ 00 are the energies (cm1) of the 0e0 transitions for the anion A and the neutral acid HA, ~naA and ~naHA are the wavenumbers of the absorption band maxima of the anion A and the neutral acid HA, ~nfA and ~nfHA are the wavenumbers of the fluorescence maxima of these species. Hydroxyarenes undergo a number of processes in addition to fluorescence and excited-state proton transfer (ESPT), such as proton-induced quenching and homolytic OH bond cleavage to produce radicals, studied most extensively for 1-naphthol and phenol derivatives. The presence of competing processes and incomplete excited-state equilibrium may lead to erroneous results in determination of pKa* by fluorescence titration [17]. In the case of hydroxyaryls (HA≡ROH, where R is an aromatic

The dissociation rate constant of R*OH is kd. The dissociation process generates the (R*O/Hþ) ion pair at their “contact” distance, a, from which they may associate with the rate constant ka. The separation of the partners over the distance r requires overcoming an attractive electrostatic potential V(r) ¼ (RD/r) (RD is the Debye (Onsager) distance, when r ¼ RD the Coulomb interaction equals to the thermal energy). Both excited species, R*OH and R*O, decay to their ground state in a few nanoseconds, by a combination of 0 radiative and nonradiative processes (rate constants k0 and k 0, respectively). Using this approach, which considered the diffusion equation generalized to the Debye-Smoluchowski equation (DSE), the overall acid constant (equilibrium dissociation constant) is expressed with Eq. (6) [8,17,39].

pKa* ¼ log½ðkd expðRD =aÞÞ=ka 

(6)

where: a is the contact distance, kd and kr are the dissociation and recombination rate constants, RD is the Debye (Onsager) distance. 2.2. Dipole moments in the excited state The experimental dipole moments for biphenyl cyano derivatives: 4-hydroxy-40 -cyanobiphenyl and 4-hydroxy-2,6dimethyl-40 -cyanobiphenyl in the first excited singlet state were calculated on the basis of solvatochromic analysis using the solvent shifts of the fluorescence spectra with the ground state dipole moments obtained on the basis of the semi-empirical calculations (Eq. (7)) [40].

m ðm  mG Þ hc~vf ¼  E E Df þ const 2pε0 a3 (7) where :

ε1 1 n2  1  $ 2 Df ¼ 2ε þ 1 2 2n þ 1

In Eq. (1), ~vf e the maximum of the fluorescence band, ε0 e the dielectric constant in vacuum, mE, mG e the dipole moments of the solute in the excited and ground state, respectively, h and c e the Planck constant and the velocity of light, 3 e the static dielectric constant of the solvent and n e the refractive index of solvent. The Onsager cavity radius a is taken as 6 Å in analogy to similar biphenyls [41]. 2.3. Quantum chemical calculations Semiempirical AM1 calculations on gas phase 2-naphthol and its cyano derivatives (for both acid and basic forms) in their first 3 singlet states were performed using the AMPAC 6.55 package [7,42]. Electron density distributions and dipole moments of cyano hydroxybiphenyl molecules in their ground and excited states were calculated based on AM1 method calculations [29]. All AM1

B. Szczepanik / Journal of Molecular Structure 1099 (2015) 209e214

method calculations were carried out for ground states at the SCF level, and using additionally the CI method including singleelectron excitation (SECI) in the case of electron-excited states. Ab initio calculations in the ground and excited states of cyanophenols molecules were done using the Gaussian 98 program [43]. The distributions of electronic densities and dipole moments of neutral molecules in ground and first excited states were calculated in the base B3LYP/6-31 þ G(d). The optimalization of geometry in the ground state was obtained in the base B3LYP/6-31G(d) and in the excited state in the base RCIS/6-31G(d) [34].

3. Excited-state protolytic reactions of cyanonaphtols Tolbert et al. [4,5] and Pines et al. [6] studied the acidity of 5cyano-1-naphthol and 5,8-dicyano-1-naphthol in the excited state. These compounds show a remarkably increased photoacidity, and ESPT is observed in non-aqueous solvents such as alcohols and DMSO. But, cyano-1-naphthols exhibit very weak fluorescence and strong quenching in protic solvents. Cyano-substituted derivatives of 2-naphthol (5-, 6-, 7-, and 8-cyano-2-naphthol (5CN2, 6CN2, 7CN2, and 8CN2), as well as 5,8-dicyano-2-naphthol (DCN2)) are also strong photoacids and have better fluorescence properties [4,5,11]. A list of cyano-2-naphthols discussed in this review is presented in Scheme 1. The pKa value of N2 drops from 9.5 in the ground electronic state (S0) to 2.8 in its first excited singlet state (S1) in water. The introduction of electron-withdrawing CN group in N2 molecule makes these molecules more acidic than N2, slightly in the ground state, and more dramatically in the excited state. The CN substituents at C-5 and C-8 position are more effective at lowering pK*a for 2-naphthol than substituents at C-6 and C-7 position (Table 1). The doubly substituted CN derivative of N2 (DCN2) is the most acidic, almost as strong as a mineral acid, with a value of pKa* of 4.5 (Table 1). The N2 molecule is sufficiently acidic in the excited state to transfer a proton to water during its radiative lifetime, but not to pure organic solvents. The presence of CN groups in N2 enhances the photoacidity of these derivatives and they can undergo proton transfer to various organic solvents (alcohols, DMSO) [12e15]. The efficiency of excited state proton transfer from hydroxyarenes depends on (1) solvent polarity, (2) the solvent basicity, which is responsible for proton solvation, and (3) the solvent acidity, which stabilizes the excited anion [17]. A common explanation of strong acidity of hydroxyaryls in the excited state is intramolecular charge transfer (ICT) in the excited state of the acid, from the hydroxyl oxygen to the aromatic ring [38]. Although this effect exists in the acid form (HA), it is considerably larger for the base (A) [38,44,45]. Dipole moments of N2 and cyano derivatives discussed increase slightly with excitation for acid form of these molecules (8CN is an exception) and decrease

211

Table 1 The acidity constants pKa in the ground state (S0), pK*a in the first singlet excited state (S1) and the change of the acidity constant in the excited state DpKa for the cya}rster values were calculated using Eqs. (1) and (5)) nonaphtols discussed (DSE and Fo [7,8,17]. Compound

pKa

DCN2 5CN 6CN 7CN 8CN N2

7.80 8.75 8.40 8.75 8.35 9.50

pK*a

DpKa (Fo} rster)

} rster Fo

DSE

4.50 0.75 0.40 0.20 0.75 2.80

e 0.75 0.37 1.30 0.40 e

12.3 9.5 8.8 8.95 9.1 6.7

more dramatically in the excited state for A (Table 2) [7]. However, the dipole moment presents only an average measure for the charge density distribution. Additional support for the ICT effect is obtained from the calculation of the Mulliken charge on the oxygen atom, as well as the CeO bond lengths [7]. The correlation of the Mulliken charges on the oxygen atom with the corresponding experimental solution-phase pKa and pK*a values confirms that the charge is small and varies only slightly with pKa for HA. For A the charge is large and varies more clearly [7,8]. The CeO bond length for N2 in the ground state is around 1.37 Å for the acid, shortening to 1.26 Å in the anion. With increasing ground state acidity for cyano derivatives, the CeO bonds in both form were shorten. Upon the excitation of anionic derivatives (to S1), this bond further shortens, indicating an enhanced double-bond character with increasing ICT effect, which is expected to play a dominant role in anion stabilization [7,8]. These results allow to draw the following conclusion: intramolecular charge migration from the hydroxyl oxygen to the aromatic ring occurs upon excitation of N2 and its cyano derivatives and the dominance of this effect for the anion plays a major role in increasing photoacidity [7,8]. 4. Excited-state protolytic dissociation of cyanobiphenyls Biphenyl cyano derivatives with a phenyl ring with a hydroxy group as a donor part: 4-hydroxy-40 -cyano-biphenyl (HCB) and 4hydroxy-2,6-dimethyl-40 -cyanobiphenyl (DMHCB) (Scheme 2) demonstrate a strong photoinduced increase of acidity in the excited state [28,29]. In the case of DMHCB, the change of the acidity (DpKa) could not be estimated in an aqueous solution, due to the fluorescence of DMHCB anion quenching. The acidity constants in the excited state were estimated in butanol solution containing sodium butanolate [28]. The values of pKa (ground state), pK*a (excited state) acidity constants and DpKa for HCB and DMHCB are collected in Table 3 (DpKa was determined on the basis of the €rster cycle for aqueous and butanol solutions (Eqs. (1)e(4)) [28]. Fo In the ground state, HCB and DMHCB are very weak acids with

Table 2 Calculated (AM1, 16 CI orbitals) dipole moments (m0, ground state, and m1, excited state, D) for acid and base forms of N2 and its cyano derivatives [7]. Molecule

Acid

m0

m1

m0

m1

N2 5CN 6CN 7CN 8CN DCN2

1.0a 4.0 3.7 2.7 2.7 0.7

1.2b 4.3 3.9 3.2 2.3 2.0

5.9 5.2 3.2 5.0 6.5 5.4

1.9 2.4 1.0 3.2 3.8 1.3

a

Scheme 1. Chemical structure of the discussed cyanonaphtols.

b

Estimated experimental value is 1.5 D. Estimated experimental value is 2.0 D.

Base

212

B. Szczepanik / Journal of Molecular Structure 1099 (2015) 209e214 Table 4 Ground and excited state dipole moments [D] experimental and calculated using the AM1 method (for vacuum) for HCB and DMHCB (a calculated dipole moment values in excited states with a planar structure (MICT) and with a twisted structure (TICT)) [28,29].

Scheme 2. Chemical structure of 4-hydroxy-40 -cyano-biphenyl (HCB) and 4-hydroxy2,6-dimethyl-40 -cyanobiphenyl (DMHCB).

pKa ~ 10. The acidity of HCB strongly changes in the excited state resulting in pK*a ¼ 0.5 in an aqueous solution (Table 3). The values of DpKa calculated for both compounds in alcoholic solutions are equal 10.5 for HCB and 13.0 for DMHCB (Table 3). The charge transfer in the excited state in molecules of biphenyl derivatives containing a hydroxyl group causes so high an increase in the acidity of those compounds that they may be included in the group of “super” photoacids, along with cyano derivatives of 2-naphthol. The change in the pKa values (DpKa) for HCB in aqueous solution. (9.9) is larger than this value for N2 (DpKa ¼ 6.7), and the corresponding values for the monocyano-naphthols are also slightly smaller (DpKa ¼ 8.8 for 6CN to 9.5 for 5CN, Table 1). Although the ground state pKa value of HCB (9.4, Table 3) is somewhat larger than that of the cyanonaphtols (8.4e7.8, Table 1) (the biphenyls are less acidic), HCB is similarly acidic in the excited state (pK*a ¼ 0.5, Table 3) in an aqueous solution as compared to the monocyano-naphthol superphotoacids (pK*a ¼ 0.2 to 0.75, Table 1). In the case of DMHCB, the DpKa value in 1-butanol is 2.5 units more negative than that for HCB (see Table 3). The values of pKa measured in non-aqueous solutions cannot be compared directly to the measurements in aqueous solution as the tend to overestimated the increase of photoacidity [46]. The difference between DpKa values for HCB equals 0.6 units for changing from water to 1-butanol. Assuming the same solvent induced acidity enhancement in 1-butanol to apply for DMHCB, DpKa equals 12.4 in water instead of 13.0 in 1-butanol. This substantial increase of the acidity of the sterically hindered compound is comparable with the most acidic dicyano-naphthol DCN2 (DpKa ¼ 12.3, Table 1). The explanation of the nature of the HCB and DMHCB emitting state (a mesomerically stabilized less polar intramolecular charge transfer (MICT) state of near planar geometry, or a more polar one, with strongly decoupled intramolecular CT resembling a twisted intramolecular charge transfer (TICT) state) is based on the results of the quantum-chemical calculations and a strong solvatochromism of fluorescence for HCB and DMHCB. The decrease of the excess charges within the OH group and increase of a negative charge on the cyano group of HCB and DMHCB molecules in the excited state and the increase of the values of the excited state dipole moment (8.6 D for HCB, and 9.8 D in low-polar solvents and 27 D in solvents more polar than 1-chlorobutane for DMHCB, Table 4) confirmed a TICT character of the excited state, especially for compound with a sterical hindrance [29]. The comparison of the length of the CeO bonds in HCB and DMHCB molecules and its anions in the ground

Compound

m0 [D]a

m1 [D]a

m1 [D]b

HCB DMHCB

3.7 3.2

5.45 (MICT) 5.36 (MICT) 17.27 (TICT)

8.6 (9.8)c 27.0

a

From AM1 calculations. Experimental values. c Values in brackets for the solvents less polar than Df ¼ 0.2, the other value for the polarity range above Df ¼ 0.2. b

Table 5 The dissociation constants pKa in the ground state (S0) and the changes of acidity constant DpKa in the first singlet excited state (S1) of the discussed cyanophenols in }rster cycle, Eqs. (1)e(4)). water and methanol (calculated from the Fo Compound

pKa (water)

DpKa (water)

DpKa (methanol)

PhOH oCNP mCNP pCNP DCNP

9.8 7.0 8.3 7.7 6.5

5.8 6.5 6.1 4.4 7.4

e 7.5 6.9 1.2 8.5

[30,44] [31] [31] [31] [34]

[30,44] [31] [31] [31] [34]

[34] [34] [34] [34]

state and excited states with a planar structure (MICT) and with a twisted structure (TICT) showed that they significantly shortened, especially in the TICT state. These structural changes of excited cyano derivatives of biphenyl containing an OH group, resulting from strong charge transfer from the donor subunit onto acceptor part of those molecules, are associated with a strong increase in acidity of these compounds [28,29]. 5. Excited-state acidity of cyanophenols The pKa of phenol (PhOH) in the ground state is equal 9.8, dropping to pK*a ¼ 4.0 in S1 state, near 6 orders of magnitude (Table 5) [30,47]. The values of pKa and the changes of this value in the excited state (DpKa) for mono- and disubstituted cyanophenols (Scheme 3) in an aqueous solution and in methanol were presented in Table 5. The acidity of o-cyanophenol (oCNP), m-cyanophenol (mCNP) and 3,4-dicyanophenol (DCNP) in an aqueous solution increases more strongly in comparison to the PhOH molecule in the excited state resulting in DpKa ¼ 6.5, 6.1 and 7.4, respectively [31,33]. The values of DpKa calculated in methanol are 8.5 for DCNP, 7.5 for oCNP and 6.9 for mCNP. Only in the case of pcyanophenol (pCNP) a change in photoacidity both in aqueous and methanol solutions is considerably smaller than for PhOH and other cyanophenols: DpKa ¼ 4.4 [31] and 1.2 [34], respectively (Table 5). Photoacidity enhancement obtained from measurements, €rster cycle in methanol, is larger than that for water based on the Fo solutions in the case of DCNP, oCNP and mCNP by about 1 unit (Table 5). For pCNP is observed higher photoacidity enhancement in water solution than in methanol (above 3 units) (Table 5). In the S1 state, the acidity of discussed compounds increases both in

Table 3 }rster values were calculated using Eqs. The acidity constants pKa in the ground state (S0), pK*a in the first singlet excited state (S1) and DpKa of the cyanobiphenyls discussed (Fo (1)e(4)) [28]. Compound

HCB DMHCB

Aqueous solution

Butanol/sodium butanolate

pKa

pK*a

DpKa

DpKa

9.4 10.1

0.5

9.9 e

10.5 13.0

B. Szczepanik / Journal of Molecular Structure 1099 (2015) 209e214

Scheme 3. List of the discussed cyanophenols.

alcohol and aqueous solutions in the same order: para < meta < orto < 3,4-di. The order of the acidity increase in the S1 state suggests that the inductive effect of cyano substituents is more important in the case of cyanophenols than the resonance effect [31]. The introduction of cyano groups into a phenol molecule increases the acidity of the OH group so substantially that in alcohol solutions the phenol derivative with two cyano groups and monocyanophenols undergo the proton dissociation and form anions already in the ground state. The strongest effect is observed for dicyano- and o-derivatives and the weakest for p-derivative in correlation with pKa values of these compounds [34]. The presence of cyano groups modifies the charge distribution in cyanophenol molecules in the ground state in comparison to phenol and a migration of the negative charge from the oxygen atom to the aromatic ring is proceeded [31]. As a result, the acidity of the hydroxyl group increases so considerably that the anion formation is possible in the ground state in methanol solution containing only the traces of water [34]. Upon electronic excitation, all investigated compounds undergo a similar modification of the charge distribution on the substituents. The decrease of charge density on the hydroxyl group in the excited state corresponds to the direction of the increase in the acidity of investigated phenols [31,34]. A change in dipole moments upon excitation (the highest increase for dicyanoderivative of phenol and the weak decrease for p-isomer) also correlates closely with the DpKa of these compounds (Table 6) [34]. 6. Conclusions The excited-state acidity of cyano derivatives of 2-naphthol, biphenyl and phenol have been discussed in the present article. It is evident that the “electro-withdrawing” CN group makes the molecules of these derivatives much more acidic than 2-naphthol or phenol, slightly in the ground state and more dramatically in the excited state. This effect is greatest for the cyanonaphthols (the doubly substituted CN derivatives are almost as strong as a mineral acid in the excited state [4,5]), comparable for cyanobiphenyls (excited-state acidity of HCB is comparable with 5-, 8-cyano-2naphthol), and smaller for phenol derivatives. In an aqueous solution, an increase in the acidity of orto- and meta-cyano derivatives of phenol in the excited state in comparison to the ground state is comparable, about 6 order of magnitude. The acidity changes for

Table 6 Ground state dipole moments [D] (m0) and excited state dipole moments [D] (m1) obtained on the basis of ab initio calculations for cyanophenols molecules [34]. Compound

m0 [D]

m1 [D]

DCNP oCNP mCNP pCNP

8.47 3.63 5.99 5.24

10.84 5.08 9.19 4.73

213

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