Author's Accepted Manuscript

pH dependent fluorescence switching inSalicylideneaniline: ‘off-on-off’ operation con-trolled by surfactant micelles

Diganta Das, Kaku Dutta

PII: S0022-2313(13)00504-8DOI: http://dx.doi.org/10.1016/j.jlumin.2013.08.027Reference: LUMIN12106

To appear in: Journal of Luminescence

Received date: 15 March 2013Revised date: 8 August 2013Accepted date: 14 August 2013

Cite this article as: Diganta Das, Kaku Dutta, pH dependent fluorescenceswitching in Salicylideneaniline: ‘off-on-off’ operation controlled by surfac-tant micelles, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2013.08.027

This is a PDF file of an unedited manuscript that has been accepted forpublication. As a service to our customers we are providing this early version ofthe manuscript. The manuscript will undergo copyediting, typesetting, andreview of the resulting galley proof before it is published in its final citable form.Please note that during the production process errors may be discovered whichcould affect the content, and all legal disclaimers that apply to the journalpertain.

www.elsevier.com/locate/jlumin

pH dependent fluorescence switching in Salicylideneaniline:

‘off-on-off’ operation controlled by surfactant micelles

Abstract

Salaniline, a condensation product of salicylaldehyde and aniline, shows “off-on”

fluorescent behaviour with pH in 1:1 (v/v) CH3CN:H2O and when 3% (w/v) anionic

sodium dodecylsulphate (SDS) surfactant is present. The fluorescent switch behaviour

of salaniline forced to “off-on-off” type by 3% (w/v) neutral triton X-100 (TX-100)

and 3% (w/v) cationic cetyltrimethylammonium bromide (CTAB) surfactant. The

fluorescent “on” window is observed in the pH range 8.0 – 12.5 for TX-100 and 7.0 –

11.0 for CTAB. Different charge nature of the surfactants affects the

protonation/deprotonation behaviour of salaniline differently, hence the photoinduced

electron transfer (PET) processes and the fluorescent switch behaviour.

Abstract

The fluorescence response to pH of salicylideneaniline (SA) in 1:1 (v/v) CH3CN:H2O

and 3% (w/v) aqueous micellar mediums - negative sodium dodecylsulphate (SDS),

positive cetyltrimethyl ammonium bromide (CTAB) and neutral Triton X-100 (TX-

100) are reported. SA shows pH dependent fluorescent “off-on” behaviour in 1:1 (v/v)

CH3CN:H2O and 3% (w/v) SDS while “off-on-off” behaviour is observed in 3% (w/v)

TX-100 and CTAB. This different fluorescent switch behaviour is observed because

the dissimilar charge nature of the mediums together with pH could

stabilise/destabilise the intermediates formed during acid or base catalysed keno-enol

tautomerism in SA differently.

Keywords: salicylideneaniline. Keto-enol tautomerism.off–on–off. fluorescence. pH.

SDS. CTAB. TX-100.

1. Introduction

Designing of small fluorescent signalling molecules as sensors, switches, logic

gates and molecular level machines has gained great deal of attention in recent

times[1-3]. Emission properties of such molecules can be tuned as reported by the

effect of external modulators that affect the fluorescence of a molecule to be light [4],

redox potential [5] and metal ions [6]. pH is also one of the external modulators which

has received a lot of interest because of its high operability (simply controlled) [7–

10]. Most of the pH dependent systems are of either ‘‘on–off’’ or ‘‘off–on’’ mode

[7,8] hence, systems showing ‘‘off–on–off’’ or ‘‘on–off–on’’ mode is of importance

[9-10]. de Silva et al. [11] reported a ‘‘off–on–off’’ sodium ion molecular switch by

derivatizing anthracene with crown ether and anabasin. 9-Anthramethyl-bis(2-

picolyl)amine has been reported by the same group as an ‘‘off–on–off’’ proton switch

[12]. An ‘‘off–on–off’’ proton switch derived from natural products coumarine is also

known [13]. pH controlled ‘‘off–on–off’’switch based on copper mediated pyrene

fluorescein in polymer–micelle aggregated supramolecular system is reported [14]. A

dipod and a tetrapod of benzene have been found to show two perturbations in

fluorescence with Ag+ resulting in an ‘‘on–off–on’’switch [15]. Another ‘‘on–off–

on’’ switch operated in physiological pH reported is Eu(III)–Phenanthroline

supramolecular conjugate [16]. A large number of PET systems have been reported to

date [7] which depends on a molecular recognition event to inhibit or initiate

fluorescence.

Salicylideneaniline and its congeners attracted attention because they are best

known organic compounds that exhibit thermochromism and photochromism in the

solid state [17-21]. The crystals of SAs have been classified into two types,

thermochromic (TC) and photochromic (PC) [22-23]. The thermochromism is due to

conversion of enol form of SA into cis-keto form while photochromism is due to

conversion of enol form of SA into trans-keto form of SA.

Recently we reported that different charge nature of aqueous surfactant

micelles could influence the pH dependent fluorescent switch behaviour of

salicylaldehyde-2,4-dinitrophenylhydrazone (S-2,4DNP) differently [24]. The charge

nature of the micelles could influence the stability of the protonated/deporotonated

states of (S-2,4DNP) and hence the photoinduced electron transfer processes and

subsequently the fluorescent switch behaviours.

In this paper we report that salicylideneaniline acts as pH dependent ‘‘off–on’’

fluorescent switch in 1:1 (v/v) CH3CN:H2O and 3% (w/v) SDS aqueous micellar

solution while “off-on-off” switch behaviour is observed in 3% (w/v) CTAB and TX-

100 aqueous micellar solutions.

2. Experimental

2.1. Apparatus

Fluorescence spectra were recorded in a Hitachi 2500 spectro-photometer

using a quartz cuvette at room temperature. pH values were measured using a Merck

digital pH meter. A 1H NMR spectra were recorded on a Bruker Ultrashield 300

spectrophotometer. Chemical shifts were expressed in ppm (CDCl3 was used as

solvent with TMS as internal standard) and coupling constants (J) in Hz. FTIR data

were measured as KBr pallet, using a Perkin Elmer spectrophotometer (RX1).

2.2. Reagents and chemicals

Salicylaldehyde and aniline were purchased from LOBA, Chemie; SDS, TX-

100, CTAB and acetonitrile (99.9%) were purchased from Merck, Germany. Water

used was from Milli-Qpurification system.

2.3. Synthesis of Sal-aniline(SA)

Salicylideneaniline (Structure shown in Scheme 1) was synthesized as per

reported procedure [25]. Salicylaldehyde was reacted with aniline in a round bottom

flask, containing 50 mL ethanol, fitted with a reflux condenser and a silica gel guard

tube. The mixture was refluxed for 30 min with continuous stirring, using magnetic

stirrer. The mixture was then cooled in ice-bath and kept over night where upon

crystalline precipitate of respective ligands separated out. The product was filtered

off, washed with ethanol and dried in vacuo over calcium chloride. Solid Pale green

product was obtained. Yield 80% and melting points 48˚C.

The product was characterised by IR and 1H-NMR spectra. The infrared

spectra of ligand showed characteristic strong bands in the region 3205-3450 cm-1

assigned to νO-H [26-27]. The peak at 1600 cm-1 in the spectra is attributable to ν(C=N)

[28]. The absorption peaks appeared around 1550 cm-1 are attributable to ν(C=C)

(aromatic). The stretching frequencies of νC-O (phenolic) and νC-N (amino) appeared at

1261-1294 cm-1 and 1350-1381 cm-1, respectively, in the spectra [28]. 1H NMR of

sal-aniline showed δH values at 1.6 (s) ppm (-OH); at 7-8 ppm (m) (-CH- of benzene

rings) and at 8.6 ppm (-CH-. 1 β C=N) [29] .

2.4. Preparations of solutions

All experiments were performed with freshly prepared solutions after keeping

for 24 h of storage at room temperature in order to ensure equilibrium. 10−4 M

solution of SA in 1:1 (v/v) CH3CN:H2O was used in the experiments. A portion of

this solution (20 mL) was placed in a small beaker with pH meter and a small

magnetic stir bar. The pH of the solution was changed by adding small amounts of

Hydrochloric acid or sodium hydroxide solutions while stirring and the fluorescence

was recorded at regular pH intervals.

3. Results and discussion

Scheme 1: Structure of SA in enol and keto form.

UV/Visible spectrum of salicylideneaniline (SA) in 1:1 (v/v) CH3CN:H2O (pH

7.6) showed peaks at λmax value 335 nm. Stability of SA in presence of water was

judged by recording UV/Visible spectra of SA at different added concentration of

water (10% to 90% w/v of H2O in CH3CN). No change in λmax value was observed

which confirm the stability of SA in the solutions. Attempts to have fluorescence

spectrum of SA on excitation by photons below 335 nm failed. However when SA

was excited with 420 nm photons showed fluorescence emission in the range 430 to

630 nm in 1:1 (v/v) CH3CN:H2O as well as in 3% (w/v) CTAB, TX-100 and SDS.

The emission λmax were found to be in between 490-520 nm depending on the

medium. It is reported that SA in enol form is fluorescent inactive but when irradiated

with light of near UV region transforms into keto form which is fluorescent active

[30]. Hence the observed fluorescence is due to the formation of keto form of SA by

photons of 410 nm wavelength.

Scheme 2: Mechanism showing the acid and base catalysed conversion of keto form

of SA into enol form of SA.

The fluorescence spectra of SA were recorded at different pH in 1:1 (v/v)

CH3CN:H2O (Fig. 1) as well as in 3% (w/v) CTAB, TX-100 and SDS. Fig. 2 to Fig. 5

shows the fluorescence intensity versus pH profile of SA in different medium. In 1:1

(v/v) CH3CN:H2O the fluorescence intensity of SA is zero till pH 6.0 and then starts

to increase, attain maximum value at pH 9.2 (Fig. 2). Further increase in pH showed a

decrease in fluorescent intensity by about 22% till pH 10.0 and remained same

thereafter. Thus in 1:1 (v/v) CH3CN:H2O fluorescence “off-on” behaviour is observed

for SA as pH is increased. Similar pH dependent “off-on” behaviour has been

observed when SA is in 3% (w/v) SDS (Fig. 3). The fluorescent intensity starts to

increase at pH 7.2 and attains maximum at pH 10.4, a decrease by 20% of the

maximum intensity was observed till pH 12.0. This pH dependent fluorescent

behaviour is found to be reversible that is similar behaviour is observed whether pH

was increased from low value to high value or vice versa.

When the medium is either 3% (w/v) TX-100 or CTAB the increasing pH

could induce “off-on-off” fluorescent behaviour for SA. In case of 3% (w/v) TX-100

the fluorescence intensity starts to increase when pH value became 8.0, the increase

continues till pH 10.4 and thereafter starts to decrease and attains minimum at pH

13.0 (Fig. 4). In 3% (w/v) CTAB the fluorescence intensity starts to increase when the

pH became 6.0 and attains maximum fluorescence intensity at pH 10.2. The intensity

starts to decrease on further increase in pH and attains minimum at pH 12.0 and

remains unaltered thereafter (Fig. 5). Hence in 3% (w/v) CTAB also SA shows pH

dependent fluorescence “off-on-off” behaviour. Table 1 summarises the different

combinations of pH values and mediums in order to put SA either in fluorescent “on”

or fluorescent “off” mode.

The different fluorescent switch behaviour of SA in different medium can be

explained based on the fact that keto (fluorescent) form of SA gets converted into enol

form (non fluorescent) both by acid catalysis and base catalysis. The mechanism of

conversion of keto form into enol form of SA has been shown in Scheme 2. At pH 6.0

and below the high H+ ion concentration is good enough to convert the keto form into

the enol form (Scheme 2) and therefore SA does not show fluorescence below pH 6.0

in any medium. In 3% (w/v) SDS and at higher pH, as SDS is negative it does not

favour the formation of the intermediate SAO- (Scheme 2) , due to electrostatic

reason, required for the base hydrolysis of keto into enol. Thus SA can remain in the

keto form even at higher pH and hence fluorescent “off-on” behaviour is observed.

The similar fluorescent “off-on” behaviour observed in 1:1 (v/v) CH3CN:H2O also

means that the intermediate is not favourable. In this case probably polarity of the

medium plays some role which is not clear to us.

In 3% (w/v) CTAB after pH 6.0 the H+ ion concentration is not good enough

for acid hydrolysis because the cationic CTAB opposes the formation of cationic

intermediate SAH+ (Scheme 2) and hence the fluorescence intensity starts to increase.

The anionic intermediate SAO- (Scheme 2) formed during base hydrolysis is highly

favourable in case of cationic CTAB and hence keto form gets converted into enol

form at higher pH. Thus in 3% (w/v) CTAB fluorescent “off-on-off” behaviour was

observed. In 3% (w/v) TX-100 the base hydrolysis is possible as being neutral TX-

100 does not oppose the formation of the intermediate SAO- (Scheme 2) and therefore

fluorescent “off-on-off” behaviour is observed.

Since CTAB is positively charged it can destabilise the intermediate SAH+

required for acid hydrolysis more and can stabilise the intermediate SAO- required for

base hydrolysis more than that by TX-100. Hence in 3% (w/v) CTAB, compared to

that in 3% (w/v) TX-100, the acid hydrolysis stops at relatively lower pH while base

hydrolysis starts at relatively lower pH. This is reflected by the fact the fluorescent

“on” window is towards lower pH value in 3% (w/v) CTAB (6.0 to 12.0) than in 3%

(w/v) TX-100 (8.0 to 13.0).

4. Conclusion

Salicylideneaniline shows pH dependent fluorescent ‘‘off–on–off’’ behaviour

in neutral 3% (w/v) TX-100 and cationic 3% (w/v) CTAB. But in anionic 3% (w/v)

SDS and 1:1 (v/v) CH3CN:H2O its fluorescent behaviour is restricted to ‘‘off–on’’

type by pH. This different fluorescent switch behaviour is because of the ability of the

different charge nature of the micelles in controlling the acid and base catalysed

conversion of fluorescent keto form into non fluorescent enol form of SA.

Acknowledgements

The authors thank DST, New Delhi and UGC, New Delhi for financial support to the

department.

References

[1] A.P. de Silva, K.C. Loo, B. Amorelli, S.L. Pathirana, M. Nyakirang’ani, M.

Dharmasena, S. Demarais, B. Dorcley, P. Pullay, A. Salih, J. Mater. Chem. 15 (2005)

2791-2795.

[2] Y. Shiraishi, R. Miyamot, T. Hirai, Tetrahedron Lett. 48 (2007) 6660-6664.

[3] A.P. de Silva, N.D. Mc Clenaghan, Chem. Eur. J. 10 (2004) 574-586; F.M.

Raymo, Adv. Mater. 14 (2002) 401-414; G.J. Brown, A.P. de Silva, S. Pagliari,

Chem. Commun. (2002) 2461-2463; A.P. de Silva, H.Q.N. Gunaratne, T.

Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T. Rademacher, T.E. Rice, Chem. Rev.

97 (1997) 1515-1566; L. Fabbrizzi, M. Licchelli, P. Pallavicini, Acc. Chem. Res. 32

(1999) 846-853; A.P. de Silva, D.B. Fox, T.S. Moody, S.M. Weir, Trends Biotechnol.

19 (2001) 29-34.

[4] L. Gobbi, P. Seiler, F. Diederich, Angew. Chem. Int. Ed. 38 (1999) 674-678; A.

Beyeler, P. Belser, L. DeCola, Angew. Chem. Int. Ed. 36 (1997) 2779-2781; F.M.

Raymo, M.J. Tomasulo, Phys. Chem. A 109 (2005) 7343-7352.

[5] L. Fabbrizzi, M. Licchelli, S. Mascheroni, A. Poggi, D. Sacchi, M. Zema, Inorg.

Chem. 41 (2002) 6129-6136; G. De Santis, L. Fabbrizzi, M. Licchelli, N. Sardone,

A.H. Velders, Chem. Eur. J. 2 (1996) 1243-1250; G. Zhang, D. Zhang, X. Guo, D.

Zhu, Org. Lett. 6 (2004) 1209-1212.

[6] A.P.de Silva, H.Q.N. Gunaratne, C.P. McCoy, J. Am.Chem.Soc.119 (1997) 7891-

7892; S.A. de Silva, B. Amorelli, D.C. Isidor, K.C. Loo, K.E. Crooker, Y.E. Pena,

Chem. Commun. 13 (2002) 1360-1361; R.-H. Yang, W.-H. Chan, A.W.M. Lee, P.-F.

Xia, H.-K. Zhang, K. Li, J. Am. Chem. Soc. 125 (2003) 2884-2885; B. Bodenant, F.

Fages, M.-H. Delville, J. Am. Chem. Soc. 120 (1998) 7511-7519; Y. Shiraishi, Y.

Tokitoh, T. Hirai, Chem. Commun. 58 (2005) 5316-5318; R.T. Bronson, M. Montalti,

L. Prodi, N. Zaccheroni, R.D. Lamb, N. K. Dalley, R.M. Izatt, J.S. Bradshaw, P.B.

Savage, Tetrahedron 60 (2004) 11139-11144.

[7] E.U. Akkaya, M.E. Huston, A.W. Czarnik, J. Am. Chem. Soc. 112 (1990) 3590-

3593; T. Gunnlaugsson, T.C. Lee, R. Parkesh, Tetrahedron 60 (2004) 11239-11249;

A.P. de Silva, D.B. Fox, A.J.M. Huxley, T.S. Moody, Coord. Chem. Rev. 205 (2000)

41-57; A.J. Bryan, A.P. de Silva, S.A. de Silva, R.A.D.D. Rupasinghe, K.R.A.S.

Sandanayake, Biosensors 4 (1989) 169-179; Y. Shiraishi, Y. Kohno, T. Hirai, J.

Phys. Chem. B 109 (2005) 19139-19147; G. Nishimura, Y. Shiraishi, T. Hirai, Chem.

Commun. 52 (2005) 5313-5315.

[8] Y. Shiraishi, Y. Tokitoh, G. Nishimura, T. Hirai, Org. Lett. 7 (2005) 2611-2644;

Y. Shiraishi, Y. Tokitoh, T. Hirai, Org. Lett. 8 (2006) 3841-3844.

[9] P.Pallavicini, V. Amendola, C. Massera, E. Mundum, A. Taglietti, Chem.

Commun. (2002) 2452-2453; V. Amendola, L. Fabbrizzi, C. Mangano, H. Miller, P.

Pallavicini, A. Parotti, A. Taglietti, Angew. Chem. Int. Ed. 41 (2002) 2553-2556.

[10] A.P. de Silva, H.Q.N. Gunaratne, C.P.McCoy, Chem. Commun.(1996) 2399-

2400; S.A. de Silva, A. Zavaleta, D.E. Baron, Tetrahedron Lett. 38 (1997) 2237-2240;

A. Bencini, A. Bianchi, C. Lodeiro, A. Masotti, A.J. Parola, F. Pina, J. Seixasde Melo,

B. Valtancoli, Chem. Commun. (2000) 1639-1640; M.T. Albelda, M.A. Bernardo,

P.Dı/az, E.Garcı/a-Espan\widetilde a, J. Seixasde Melo, F. Pina, C. Soriano, S. V.

Luis, Chem. Commun. (2001) 1520-1521; L. Fabbrizzi, F. Gatti, P. Pallavicini, L.

Parodi, New J. Chem. 22 (1998) 1403-1407; J.F. Callan, A. de Silva, N.D.

McClenaghan, Chem. Commun. (2004) 2048-2049; S.A. de Silva, K.C. Loo, B.

Amorelli, S.L. Pathirana, M. Nyakirang’ani, M. Dharmasena, S. Demarais, B.

Dorcley, P. Pullay, Y.A. Salih, J. Mater. Chem. 15 (2005) 2791-2795; Y. Diaz-

Fernandez, F. Foti, C. Mangano, P. Pallavicini, S. Patroni, A. Perez-Gramatges, S.

Rodorigez-Calvo, Chem. Eur. J. 12 (2006) 921-930.

[11] S.A. de Silva, B. Amorelli, D.C. Isidor, K.C. Loo, K.E. Crooker, Y.E. Pena,

Chem. Commun. (2002) 1360-1361.

[12] S.A. de Silva, A. Zavaleta, D.E. Baron, O. Allam, E.V. Isidor, N. Kashimura,

J.M. Percapio, Tetrahedron Lett. 38 (1997) 2237-2240.

[13] S.A. de Silva, K.C. Loo, B. Amorelli, S.L. Pathirana, M.Nyakirang’ani, M.

Dharmasena, S. Demarais, B. Dorcley, P. Pullay,Y. A. Salih, J. Mater. Chem. 15

(2005) 2791-2795.

[14] P. Bandyopadhyay, A.K. Ghosh, Phys. Chem. B 113 (2009) 13462-13464.

[15] P. Singh, S. Kumar, Tetrahedron Lett. 47 (2006) 109-112.

[16] T. Gunnlaugsson, J.P. Leonard, K. Senechal, A.J. Harte, J. Am. Chem. Soc. 125

(2003) 12062-12063.

[17] J.H. Day, Chem. Rev. 63 (1963) 65-80.

[18] K. Nassau, The Physics and Chemistry of Color, 2nd ed.; John Wiley and Sons:

New York, 2001.

[19] A. Samat, Lokshin, V. In Organic Photochromic and Thermochromic

Compounds; J.C. Crano, R.J. Guglielmetti, Eds.; Kluwer Academic: New York, 1999;

Vol. 2, pp 415-466.

[20] P. Bamfield, Chromic Phenomena; Royal Society of Chemistry: Cambridge,

U.K., 2001.

[21] H. Du¨rr, H. Bouas-Laurent, Eds. Photochromism. Molecules and Systems, rev.

ed.; Elsevier, Amsterdam, 2003. (b) Brown, G. H., Ed. Photochromism; Wiley: New

York, 1971.

[22] M.D. Cohen, G.M. Schmidt, J. Phys. Chem. 66 (1962) 2442-2445.

[23] M.D. Cohen, G.M. Schmidt, S.J. Flavian, J. Chem. Soc. (1964) 2041- 2051.

[24] P. Goswami, D.K. Das, J. of Luminescence 131 (2011) 760–763.

[25] D.A. Chowdhury, M.N. Uddin, F. Hoque, CMU. J. Nat. Sci. 10(2) (2011) 261-

268.

[26] D.A. Chowdhury, M.N. Uddin, M.A.H. Sarker, Chiang Mai J. Sci. 35(3) (2008)

483–494.

[27] I. Yilmaz, H. Temel, H. Alp, Polyhedron 27 (2008)125–132.

[28] A.A. Soliman, W. Linert, Thermochimica Acta 338 (1999) 67–75.

[29] W. Kamp, Organic Spectroscopy, Palgrave, Third Edition, New York, 1991.

[30] J Harada, T Fujiwara, K Ogawa J. Am. Chem. Soc. 2007, 129, 16216-16221

Figure Captions

Fig. 1 Fluorescence spectra of SA at different pH in 1:1 (v/v) CH3CN:H2O when

excited at wavelength 420 nm, the emission peak is observed at 500 nm.

Fig. 2 Changes in fluorescence intensity I of SA as a function of pH in 1:1 (v/v)

CH3CN: H2O.

Fig. 3 Changes in fluorescence intensity I of salaniline as a function of pH in 3%

(w/v) SDS.

Fig. 4 Changes in fluorescence intensity I of SA as a function of pH in 3% (w/v) TX-

100.

Fig. 5 Changes in fluorescence intensity I of SA as a function of pH in 3% (w/v)

CTAB.

Table 1 Different combinations of pH and medium on SA to switch it in either ‘‘on’’

or ‘‘off’’ mode.

pH CH3CN:H2O SDS TX-100 CTAB

6.0 off off off off

8.0 on off off on

10.0 on on on on

12.0 on on off off

Fluo

resc

ence

In

tens

ity

0

10

20

30

40

50

430 480 530 580 630

wavelength (nm) Fig.1

0

10

20

30

40

50

2 4 6 8 10 12

pH

I

Fig. 2

0

40

80

120

160

2 4 6 8 10 12

pH

I

2

Fig. 3

0

10

20

30

40

50

60

2 4 6 8 10 12 14

pH

I

Fig. 4

0

125

250

375

500

2 4 6 8 10 12 14pH

I

Fig. 5

Highlights

• Salicylideneaniline act as pH dependent “off-on” fluorescent switch in 1:1 (v/v) CH3CN:H2O.

• In surfactant micelles TX-100 (neutral) and CTAB (positive) makes the fluorescent switch “off-on-off” type while in

• In surfactant micelle SDS (negative) the pH dependent fluorescent switch is “off-on” type.