Tt

MF

a

ARRA

KHPOT

1

yshehsC

bab

a(ir[

sddtvt

1d

Spectrochimica Acta Part A 87 (2012) 61– 66

Contents lists available at SciVerse ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

j ourna l ho me page: www.elsev ier .com/ locate /saa

he positive halochromism of phenolate dyes in hydroxylic solutions ofetraalkylammonium cations

arcos Caroli Rezende ∗, Moisés Dominguez, Andrés Aracenaacultad de Química y Biología, Universidad de Santiago, Av. B.O’Higgins 3363, Santiago, Chile

r t i c l e i n f o

rticle history:eceived 20 September 2011eceived in revised form 7 November 2011

a b s t r a c t

By contrast with the negative halochromic behaviour shown by phenolate betaines in the presence ofalkaline and alkaline-earth cations, the addition of tetraalkylammonium salts to hydroxylic solutionsof these dyes generate bathochromic shifts of their charge-transfer band. This positive halochromic

ccepted 9 November 2011

eywords:alochromismhenolate betaine dyes

behaviour by organic cations was examined systematically and its origin rationalized by nonspecificchanges of the medium permittivity, and by specific dye–cation interactions in solution.

© 2011 Elsevier B.V. All rights reserved.

rganic cationsetraalkylammonium salts

. Introduction

Addition of alkaline or alkaline-earth cations to hydrox-lic solutions of a solvatochromic dye may cause significanthifts of its charge-transfer (CT) band. This phenomenon, termedalochromism, has been studied with dyes that exhibited in gen-ral a negative solvatochromic behaviour [1–5]. The observedalochromic shifts in these systems followed the same negativeolvatochromic pattern of the dye, with hypsochromic shifts of itsT-band with the increased addition of the inorganic salt.

Examples of a positive halochromism are less common, but haveeen observed with dyes that exhibit a positive solvatochromism:ddition of an alkaline salt to solutions of these dyes produced aathochromic shift of their CT-band [6–8].

The cationic halochromism is interpreted as arising from inter-ctions between the added cation and a hydrogen-bond-acceptingHBA) group in the dye, in the same way as hydrogen-bondnteractions between hydroxylic solvents and an HBA betaine areesponsible for its solvatochromic behaviour in these solvents9–11].

Inorganic cations, like hydroxylic solvents, act as electrophilicpecies in these interactions, and a general term may be coined toescribe these solvents and inorganic salts as electrophile-bond-onors (EBD) vis-à-vis solvatochromic dyes. The observations of

he first paragraph may thus be summarized by saying that the sol-atochromic and halochromic behaviour of a dye are qualitativelyhe same in the presence of EBD species.

∗ Corresponding author. Tel.: +56 2 7181035.E-mail address: marcos.caroli@usach.cl (M.C. Rezende).

386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2011.11.010

A different picture emerges when the addition of a tetraalky-lammonium cation to hydroxylic solutions of a solvatochromicdye is examined. Phenolate betaines exhibiting a negative solva-tochromic behaviour in hydroxylic solvents have their CT-bandshifted bathochromically (positive halochromism) upon additionof a tetraalkylammonium cation [12].

If these organic salts are also regarded as EBD species, then theconcluding remark of the previous paragraph is no longer true: thesolvatochromic and halochromic behaviour of a dye are not alwaysqualitatively the same in the presence of EBD species.

In the present communication we attempted to reconcile theseconflicting conclusions by examining in more detail the positivehalochromism of a phenolate betaine in different hydroxylic sol-vents in the presence of tetraalkylammonium cations. By varyingthe solvent and the ammonium cation, a general picture emerged,regarding dye–cation interactions in solution. In addition, thepresent communication also examined possible explanations forthe distinct behaviour of tetraalkylammonium salts as EBD species,when compared with inorganic salts.

2. Experimental

2.1. Preparation of dyes

Melting points were recorded with an Electrothermal apparatusand were not corrected. 1H NMR spectra were obtained with a 400-MHz Avance Bruker instrument, employing tetramethylsilane as

internal standard. UV–vis spectra were recorded with a Scinco S-4100 equipment.

The phenolate 1 was generated in situ by addition ofdrops of a 1-M methanolic solution of tetramethylammonium

6 imica

ht[hwha

25a(osfiod31H7P7(

im

2

bB

2 M.C. Rezende et al. / Spectroch

ydroxyde to solutions of N-(4-hydroxyphenyl)-5-nitro-2-hiophenecarboxaldimine, prepared as described previously4], by reaction of 5-nitro-2-thiophenecarboxaldehyde with 4-ydroxyaniline. In a similar way, Reichardt’s ET(30) betaine 4as generated in situ from the corresponding N-(3,5-diphenyl-4-ydroxyphenyl)-2,4,6-triphenylpyridinium perchlorate, prepareds described previously [13].

N-(3,5-Diphenyl-4-hydroxyphenyl)-5-nitro--thiophenecarboxaldimine (3) A solution of-nitro-2-thiophenecarboxaldehyde (Aldrich) (1.6 g, 10 mmol)nd 3,5-diphenyl-4-hydroxyanilonium hydrochloride [13]2.8 g, 10 mmol) in acetonitrile (50 mL) to which five dropsf concentrated HCl were added, were refluxed for 2 h. Theolution was rotary evaporated and the dark residue puri-ed by flash chromatography on silica, employing a mixturef hexane–dichloromethane 1:1 as eluent, to give the N-(3,5-iphenyl-4-hydroxyphenyl)-5-nitro-2-thiophenecarboxaldimine

(1.5 g, 41% yield) in the form of reddish orange needles, mp75–177 ◦C. Analysis, found C 74.76; H, 4.44%; calculated for C23

16N2O3 C, 75.00; H, 4.35%. 1H NMR [CDCl3] ı 8.64 (s, 1H, CH N),.91 (d, 1 H, J = 4.2 Hz, Th–H ortho to NO2), 7.59 (4 H, d, J = 7.1 Hz,h′-H), 7.51 (4 H, t, J = 7.5 Hz, Ph′-H), 7.42 (4 H, t, J = 7.3 Hz, Ph′-H),.33 (1 H, d, J = 4.1 Hz, Th-H), 7.31(2H, s, Ph-H meta to OH), 5.521H, s, OH).

Solutions of the phenolate 2 were prepared in situ by treat-ng 3–5 mL solutions of phenol 3 (ca. 3 × 10−4 M) with 1–2 �L of

ethanolic 1-M tetrabutylammonium hydroxide (Aldrich).

.2. Theoretical calculations

The molecular structure of phenolate 1 and the tetra-utylammonium cation were optimized in gas-phase with the3LYP/6-31G(d) method in the Gaussian 03 package [14].

NO2

NH2

OH

RR

i) 5-Nitro-2-thiophenecarboxald

N+Ph

Ph

Ph

4

i,ii

Scheme 1. Halochromic dyes 1, 2 a

Acta Part A 87 (2012) 61– 66

Transition energies for the dye–cation pair were calculated withthe semiempirical ZINDO/S method implemented in the Gaussianpackage [14].

3. Results

3.1. Preparation and halochromism of the dyes

The phenolate dye 2 was prepared following a pro-cedure similar to that described previously [4] for thepreparation of its analogue 1. Condensation of 5-nitro-2-thiophenecarboxaldehyde with 4-amino-3,5-diphenylphenol [13]led to the formation of the N-(3,5-diphenyl-4-hydroxyphenyl)-5-nitro-2-thiophenecarboxaldimine 3 (Scheme 1). The phenolate2 was generated in situ by treating solutions of 3 with drops ofmethanolic tetrabutylammonium hydroxide.

The ET(30) dye 4 was prepared following a procedure describedpreviously [13].

The negative solvatochromism of compound 1 has beendescribed before [4]. Addition of increasing concentrations ofLiClO4 to solutions of 1 in ethanol and in DMSO elicited hyp-sochromic shifts of its CT-band, in agreement with a negativehalochromic behaviour [4].

The spectral behaviour of solutions of solvatochromic phe-nolates 1, 2 and 4 in four hydroxylic solvents (water, methanol,ethanol, n-propanol and n-butanol) were investigated at 25 ◦C,after addition of increasing concentrations of four tetraalky-lammonium salts (tetramethyl-, benzyltrimethyl-, tetrapropyl-and tetrabutyl-ammonium bromides). Thus, by varying the

solvent, the tetraalkylammonium cation and the structureof the halochromic phenolate, the positive halochromismcaused by these organic salts was investigated in a systematicway.

SN R

R

O-

R = H (1)R= Ph (2)

ehyde; ii) Bu 4N+OH -

O-

Ph

Ph

nd 4 employed in this work.

M.C. Rezende et al. / Spectrochimica Acta Part A 87 (2012) 61– 66 63

0.0 0.4 0.8 1.2 1.6-2.5

-2.0

-1.5

-1.0

-0.5

0.0

H2O

MeOH

EtOH

BuOH

ΔΔ

E T / k

cal.m

ol-1

[BzEt3NBr] / M

0.0 0.4 0.8 1.2 1.6-6

-5

-4

-3

-2

-1

0

H2O

MeOH

EtOH

BuOH

E T / k

cal.m

ol-1

[Bu4NBr] / M

(a)

(b)

Fig. 1. Halochromic shifts �ET of the absorption energy of dye 1 (ca. 3 × 10−4 M,1 kcal mol−1 = 4.18 kJ mol−1) by the addition of increasing concentrations of (a) ben-zm

3

bt

dswcho

tnwptss

3

ct

0.0 0.4 0.8 1.2 1.6-6

-5

-4

-3

-2

-1

0

Bu4N+

Pr4N+

BzEt3N+

Me4N+

ΔE T /

kca

l.mol

-1

[salt] / M

−4

formation of an ionic pair between the dye phenolate and the posi-tively charged tetrabutylammonium cation. The pair association isfacilitated in the case of the unhindered phenolate group of 1 and

0.0 0.4 0.8 1.2 1.6

-5

-4

-3

-2

-1

0

Dye 2

Dye 1

Dye 4

ΔE T /

kca

l.mol

-1

[Bu4N+] / M

yltriethylammonium bromide and (b) tetrabutylammonium bromide, in water,ethanol, ethanol and n-butanol.

.2. Effect of the solvent

The halochromic shifts of compound 1 in the presence ofenzyltriethyl- and tetrabutyl-ammonium bromides varied withhe hydroxylic solvent, as can be seen in Fig. 1.

The effect of the solvent on the halochromic shifts is clearlyiscernible in the graphs. Firstly, the halochromic shifts for bothalts increased in the order n-BuOH < EtOH < MeOH < H2O. In otherords, the perturbation introduced by the addition of the organic

ations to the hydroxylic medium and expressed by the resultingalochromic shifts of the dye increased with the solvent polarity,r with its hydrogen-bond-donor strength.

Secondly, the sensitivity of the solvent to the perturba-ion by the tetraalkylammonium salt increased in the order-BuOH < EtOH < MeOH < H2O. This perturbation was not apparenthen the spectra of relatively dilute solutions of the salt in lessolar media (n-butanol, ethanol) were recorded. By contrast, addi-ion of even small amounts of the organic salt to solutions of 1 in thetrongest HBD solvent (water) resulted in discernible halochromichifts.

.3. Effect of the salt structure

The halochromic shifts of dye 1 in water, with increasing con-entrations of added tetramethyl-, benzyltriethyl-, tetrapropyl- oretrabutyl-ammonium bromide, are shown in Fig. 2.

Fig. 2. Halochromic shifts �ET of the absorption energy of dye 1 (ca. 3 × 10 M,1 kcal mol−1 = 4.18 kJ mol−1) in water by the addition of increasing concentrationsof tetramethyl-, benzyltriethyl-, tetrapropyl- or tetrabutyl-ammonium bromide.

Halochromic shifts increase in the order tetramethyl-< benzyltriethyl- < tetrapropyl- < tetrabutyl-ammonium. This orderreflects the increasing hydrophobicity of the N-alkyl chain in thesesalts. This is in line with the observation made above, that saltperturbations to the aqueous medium are mostly evident for thosesalts that depart mostly from the polar nature of water.

3.4. Effect of the halochromic dye

A comparison of the halochromic response of dye 1 with ortho-diphenyl substituted phenolates 2 and 4 under the same conditionsmay shed light on the possible existence of specific dye–cationinteractions in solution.

Fig. 3 compares the halochromic shifts of the three in methanol,in the presence of increasing concentrations of added tetrabuty-lammonium bromide.

It is seen that, under the same conditions, the unsubstituted phe-nolate 1 is more sensitive to the addition of the organic cation thanthe ortho-diphenyl derivatives 2 and 4. This observation suggeststhe existence of specific dye–cation interactions in solution, and the

Fig. 3. Halochromic shifts �ET of the absorption energy of dye 1 (ca. 3 × 10−4 M,1 kcal mol−1 = 4.18 kJ mol−1), 2 and 4 in methanol, by the addition of increasingconcentrations of tetrabutylammonium bromide.

64 M.C. Rezende et al. / Spectrochimica

Fig. 4. Structure of the dye–cation pair formed between 1 and Bu4N+. The oxygenanF

mt

4

4

bi

ptha

emmss

l

4

sibt

nst(stmd

ε

Ta(

ocfsst

nd nitrogen atoms that define the dO–N distance are indicated, together with theireighbouring carbon atoms C1 and C2, that define the dihedral angle C1–O–N–C2.or the sake of clarity, the hydrogen atoms of the butyl chains were omitted.

ade more difficult when the charged oxygen atom is shielded byhe ortho-phenyl substituents in 2 or 4.

. Discussion

.1. Rationalization of the observed trends

The effects of the solvent, of the salt or the dye structure maye rationalized by invoking nonspecific or specific solute–solvent

nteractions.In the first case, if the solvent is regarded as a continuum, the

osition of the charge-transfer band of the dye should vary withhe medium dielectric constant. Addition of an organic salt to aydroxylic solvent has the effect of changing this dielectric constantnd therefore, of changing the �max value of the CT-band of the dye.

Specific solute–solvent interactions may also be invoked toxplain the trends presented in Figs. 1–4. In salt solutions, the dyeay be solvated by the solvent and by the added tetraalkylam-onium cation, in a competitive process that results in different

pectral responses, as the solvent is gradually expelled from theolvation layer of the dye by the increased salt concentration.

The two interpretations will be addressed in detail below, fol-owed by a conclusion that harmonizes them.

.2. Nonspecific interactions of the medium with the dye

In an approach related with the interpretation of salt effects onolvatochromic shifts based on a continuum model [15], the pos-tive halochromism induced by tetraalkylammonium cations maye assigned to a decrease of the solvent dielectric constant, when aetraalkylammonium salt is added to a hydroxylic solvent [16–18].

The complex dielectric constant of aqueous tetraalkylammo-ium bromides was measured by Wen and Kaatze [16]. For 1-M saltolutions, the low frequency dielectric constant ε(0) decreased withhe increased alkyl chain length in the order Me4N+ (66.35) > Et4N+

62.21) > Pr4N+ (56.67) > Bu4N+ (51.33). The static dielectric con-tant of aqueous solutions of tetraalkylammonium bromides, forhe normal methyl-pentyl series, could be expressed as a polyno-

ial of the salt concentration c [17], approximated to the linearependence (1):

r = εr(0) − �εc (1)

he slope �ε, called the dielectric decrement, increased with thelkyl chain length in the order Me (4.6) < Et (12.5) < Pr (14.4) < Bu22.1) < Pe (27.1 dm3 mol−1) [18].

The nonspecific decrease of the medium permittivity of aque-us solutions of tetraalkylammonium salts with the increased saltoncentration may explain the trends of Fig. 2. However, the dif-

erent halochromic responses of dye 1 compared with the moreterically hindered phenolates 2 and 4 (Fig. 3) cannot be explainedolely by invoking nonspecific medium effects, because changes inhe medium permittivity should affect all dyes in the same way. A

Acta Part A 87 (2012) 61– 66

more detailed picture of the effect of the added organic cation to thedye interactions with the aqueous medium is therefore required.

A well-established fact regarding the solvation of aqueoustetraalkylammonium salts is their solvent structural effect [16].In water, tetraalkylammonium cations are structure-makers, theaverage number of hydrogen-bonds in aqueous solutions of theirsalts are significantly increased [17]. The hydration numbers for dif-ferent R4N+ cations may vary significantly from author to author,but they all agree that the ordered solvation increases with thealkyl chain length [18]. The large size of the hydrated R4N+ cationaffects the remaining bulk water structure, by formation of anice-like arrangement of H2O molecules around the organic cationor its ion-pairs [18]. This effect has been described as a waterstructure-enforced ion-pair formation between large ionic species,that maximizes hydrogen-bond interactions and minimizes struc-ture breaking [19]. Thus, the addition of an R4N+X− salt to anaqueous solution of a halochromic phenolate should deplete itssolvation shell of water molecules. This scavenging effect of watermolecules by the organic salt should be greater for those unhin-dered phenolates, capable of interacting with a larger number ofwater molecules. Depletion of solvating water molecules in theirvicinity should be more strongly felt than in the case of more hin-dered phenolates.

An interpretation of the effect of the hydroxylic solvent on thehalochromic shifts of 1, compared in Fig. 1, is rendered more diffi-cult by the fact that the behaviour of tetraalkylammonium salts innon-aqueous solvents is much less understood than in water, andmany questions regarding their solvation in these solvents remainstill unanswered [18]. The positive standard entropies of transferfor Et4N+, Pr4N+ and Bu4N+, from water to methanol or ethanol,suggest that the ice-like cages built in water around these cationsare nearly or totally absent in alcohols [18]. Thus, the effect, bythe added organic cation, of depleting the solvation shell of thedye of solvent molecules, should be smaller in alcohols, decreasingwith the increased solvent hydrophobicity. This expected trend isverified by the curves of Fig. 1.

It is interesting to observe that nonspecific effects, related withvariations of the ionic strength of the salt solutions, had beeninvoked before to explain the hypsochromic shifts of the CT band ofphenolate betaine dyes in acetonitrile, upon addition of tetrabuty-lammonium iodide [20,21]. Specific interactions were ruled out bythe authors because of the hindered approach of the bulky cation tothe phenolate oxygen, enclosed by a crown-ether structure. Theseobservations apply to an aprotic solvent like acetonitrile, much lesspolar than water or small alcohols. This tentative explanation, how-ever reasonable, awaits more detailed investigations. It is hopedthat recent [22,23] and future investigations on the behaviour oftetraalkylammonium salts in non-polar media will shed light onthe halochromic response of phenolate betaine dyes in these sol-vents, a systematic investigation that is beyond the scope of thepresent communication.

4.3. Specific dye–cation interactions

An alternative picture for rationalizing the positivehalochromism induced by organic salts was derived from dynam-ics simulations of dye–cation interactions. This approach wasapplied successfully to the interpretation of the negative cationichalochromism of phenolate betaines, induced by the addition ofinorganic salts [5].

Following this approach, a dye–cation pair was generatedbetween phenolate 1 and Bu4N+ (Fig. 4). Next, the distance dO–N

between the charged oxygen of 1 and the cationic nitrogen wasvaried, and for each distance, the corresponding S0 → S1 tran-sition energy of 1 was calculated. Assuming that the variabledistance between the dye and the tetrabutylammonium cation

M.C. Rezende et al. / Spectrochimica Acta Part A 87 (2012) 61– 66 65

0 2 4 6 8 10 12 14 16 18-300

-250

-200

-150

-100

-50

0Δ

E tota

l / k

cal.m

ol-1

dO-N / Angstrom

FF

ciba

istof

dvTO

iBcc

gtpa(fdar

tgZtoa

ltoroavc

0 40 80 120 160 200

-40

0

40

80

120

ΔE to

tal /

kca

l.mol

-1

Dh/ degree

Fig. 6. Variation of the calculated total energy �Etotal of the dye–cation system of

tend to increase their S0 → S1 transition energies, causing neg-ative halochromic shifts in their spectra. This effect is similarto the one elicited by HBD solvents, which, through hydrogen-bonds of variable strength with the charged oxygen of these

0.00 0.03 0.06 0.09 0.12 0.15

0

5

10

15

20

ΔE T /

kca

l.mol

-1

dO-N-3 / (Angstrom)-3

ig. 5. Variation of the calculated total energy �Etotal of the dye–cation system ofig. 5, when the O–N distance dO–N was varied from 2 to 16 A.

orresponded to a variable concentration of the added salt, and thatts concentration was proportional to dO–N

−3 [5], the halochromicehaviour of dye 1 in the presence of increasing concentrations of

Bu4N+ cation could be simulated theoretically.The dye and the tetrabutylammonium cation structures were

nitially optimized by the hybrid B3LYP/6-31G(d) method. The twotructures were then coupled, so that the phenolate group of 1 fit-ed into the cavity formed by the charged nitrogen atom and threef the butyl chains. The resulting enforced ion-pair structure [19],ormed inside an ice-like solvating cage, is shown in Fig. 4.

In order to search for the optimum conformation of theye–cation pair, the total energy of the system was computed forarious O–N distances, starting from a minimum distance of 2 A.he resulting variation of this energy (�Etotal), plotted against the–N distance dO–N, is shown in Fig. 5.

The plot of Fig. 5 showed that dye 1 was affected by the prox-mity of the tetrabutylammonium cation within a range of 2–4 A.eyond this distance, the total energy of the system was practi-ally constant, indicating a negligible interaction between the pair,orresponding to the negligible salt effects in dilute solutions.

In order to minimize steric repulsions between the phenolateroup of 1 and the butyl chains of the tetrabutylammonium cation,he dihedral angle C1–O–N–C2 formed by the oxygen atom of thehenolate ring and its neighbouring carbon atom (C1), and the Ntom of the ammonium cation and its neighbouring carbon atomC2), was next varied (see Fig. 4), and the corresponding total energyor each conformation computed. The obtained plot, for an O–Nistance dO–N of 4 A, is shown in Fig. 6. A minimum was found for

dihedral angle Dh of 135◦, which was adopted as the optimumelative orientation for the dye–cation pair.

The transition energy S0 → S1 of dye 1 as its distance dO–N fromhe tetrabutylammonium cation, in its optimum orientation, wasradually reduced from 4 A to 2 A, was then estimated with theINDO/S method. The obtained transition energies �ET were plot-ed against dO–N

−3, following the assumption made previously [5]f the proportionality between the salt concentration in solutionnd this theoretical parameter.

Assuming a cation concentration (10−3–10−2 M) that was mucharger than that of the phenolate betaine (ca. 10−4 M), we envisagedhe cationic media as very large cubic reticulates with the verticesccupied by the nitrogen atoms of tetraalkylammonium cations. Aelatively small number of these atoms are replaced by phenolate

xygen atoms. In this purely geometric arrangement, for an aver-ge O–N distance dO–N, the number of cations present in a cubicolume V of side a is given by N = a3/dO–N

3 and the molar cationoncentration by [R4N+] = N/NA = a3/NAdO–N

3, where NA is the

Fig. 5, with an O–N distance of 4 A, when the dihedral angle Dh is varied. The dihedralangle Dh, defined in Fig. 4 as C1–O–N–C2, is formed by the oxygen and the nitrogenatoms, and their neighbouring carbon atoms of the ion-pair.

Avogadro number. This simple relationship allows us to write thatthe tetraalkylammonium concentration is proportional to dO–N

−3,[R4N+] ∝ dO–N

−3, so that the variable dO–N−3 could be employed in

our theoretical plots as an equivalent of the salt concentration.The plot of the transition energy differences �ET against dO–N

−3

is shown in Fig. 7.It is seen that the S0 → S1 transition energy of the dye dimin-

ishes as its distance from the tetrabutylammonium cation isreduced, or as dO–N

−3 increases. In other words, bathochromicshifts of the S0 → S1 transition band of 1 are theoreticallypredicted, as the organic salt concentration increases. Thispositive halochromic behaviour of 1, elicited by the tetra-butylammonium cation, is in agreement with the experimentalobservations, depicted in Figs. 1–4. This behaviour is theopposite to that observed in solutions of phenolate dyes inthe presence of alkaline and alkaline-earth cations [1–3], repro-duced theoretically by the same model [5]. Inorganic cations, byinteraction with the charged phenolate oxygen of these dyes,

Fig. 7. Variation of the calculated energy �ET for the S0 → S1 transition of dye 1,estimated with the ZINDO/S method, as its distance dO–N from the tetrabutylammo-nium cation, in its optimum orientation, was gradually reduced from 4 to 2 A. Thetetrabutylammonium salt concentration is assumed proportional to dO–N

−3.

6 imica

dssb

cottht

5

icvsthd

cpipoolt

otdaoh

fBhlootoaa

odBmtotott[

[[

[[[

[[[

[[

[[

[[[

(2005) 1013–1017.[25] R.D. Skwierczynski, K.A. Connors, J. Chem. Soc. Perkin Trans. 2 (1994) 467–472.

6 M.C. Rezende et al. / Spectroch

yes, cause hypsochromic shifts of their solvatochromic band. Asuggested in the introduction, inorganic salts, like hydroxylicolvents, may be classified by a more general term, as electrophile-ond donors, or EBD species.

On the contrary, tetraalkylammonium, and other organications, affect the medium-dependent S0 → S1 transition energyf these dyes in a different way. By forming an ion-pair withhe phenolate dye, they shield its charged oxygen atom fromhe hydroxylic solvent, replacing its solvation shell by a moreydrophobic microenvironment, provided by the alkyl chains ofhe ammonium cation.

. Conclusions

The positive halochromism of phenolate dyes 1, 2 and 4n hydroxylic solvents, in the presence of tetraalkylammoniumations was examined in a systematic way, by varying the sol-ent, the organic cation and the dye structure. Positive halochromichifts increased with the HBD strength of the solvent and withhe hydrophobic nature of the tetraalkylammonium salt. Stericallyindered phenolate dyes exhibited smaller shifts than their unhin-ered analogues.

These trends were interpreted by means of nonspecific and spe-ific effects. In the first case, the known reduction of the solventermittivity of aqueous solutions of tetraalkylammonium salts was

nvoked to explain the positive halochromism of these dyes in theresence of organic salts. A more detailed explanation for thesebservations in water was based on the structure-forming abilityf these cations, and the formation of ice-like cages around thearge tetraalkylammonium ion, which had the effect of depletinghe solvating shell of the dye of water molecules.

Specific dye–cation interactions were also mimicked by meansf a simple model of a dye–cation pair separated by a variable dis-ance dO–N. The calculated energy of the S0 → S1 transition of theye decreased with the increased value of dO–N

−3, a parameterssumed proportional to the cation concentration [5]. These the-retical results were thus in agreement with the observed positivealochromism by these organic cations.

The interpretations based on nonspecific effects do not departrom the conclusions based on specific dye–cation interactions.oth approaches arrive at the same conclusion: addition ofydrophobic tetraalkylammonium cations to solutions of pheno-

ate dyes in hydroxylic solvents tends to deplete the solvating shellf the dye of its solvent molecules. This depletion may be the resultf an increased order of the solvating hydroxylic molecules aroundhe large, hydrophobic cation or its ion-pairs; or it may be the resultf replacing the hydrophilic solvating shell of the phenolate dye by

more hydrophobic microenvironment, provided by the cationiclkyl chains.

Our results and interpretations are in agreement with a previ-us study on the halochromism of aqueous solutions of the ET(30)ye 4 and its more hydrophilic analogue ET(8) in the presence ofu4NBr [24]. The authors applied the two-step solvent-exchangeodel of Skwierczynski and Connors [25] to the aqueous salt solu-

ions, treating the organic salt as a “co-solvent”, in an extensionf a previous study of binary solvent mixtures that had employedhe same dyes [26]. Interestingly, the idea of treating salt solutions

f hydroxylic solvents as binary solvent mixtures with an elec-rolyte “co-solvent” had been employed by us in the past to studyhe polarity of inorganic salt solutions with solvatochromic dyes12,27].

[

[

Acta Part A 87 (2012) 61– 66

In agreement with the conclusions of the present paper, theauthors invoked nonspecific and specific effects (described in gen-eral terms as a “specific solvation” of the dyes by Bu4NBr) torationalize the positive halochromism of the ET(30) and ET(8) dyesin aqueous tetrabutylammonium bromide solutions [24]. In thepresent work, a more detailed view of this specific solvation wasdeveloped, by simulating the dye–cation interactions. The forma-tion of these complexes, postulated as a consequence of the formalequilibria of the solvent-exchange model [25], acquired a more tan-gible reality, being corroborated by their tendency to elicit the samepositive halochromism observed experimentally.

Acknowledgements

This work was financed by Fondecyt project 1100022 and Coni-cyt project 24090025. We are grateful to Conicyt for grants to M.D.and A.A.

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