Photophysical properties and excited state intramolecular protontransfer in 2-hydroxy-5-[(E)-(4-methoxyphenyl)diazenyl]benzoic acidin homogeneous solvents and micro-heterogeneous environments

Pynsakhiat Miki Gashnga a, T. Sanjoy Singh b, Tushar S. Basu Baul a, Sivaprasad Mitra a,n

a Centre for Advanced Studies, Department of Chemistry, North-Eastern Hill University, Shillong 793022, Meghalaya, Indiab Department of Chemistry, Assam University, Silchar 788011, Assam, India

a r t i c l e i n f o

Article history:Received 28 September 2013Received in revised form5 December 2013Accepted 7 December 2013Available online 14 December 2013

Keywords:Excited state intramolecular proton transferSpontaneous emissionCritical micelle concentrationBinding constantFluorescence decay

a b s t r a c t

A systematic study on the photophysical properties and excited state intramolecular proton transfer(ESIPT) behavior of 2-hydroxy-5-[(E)-(4-methoxyphenyl)diazenyl]benzoic acid, is reported using steady-state and time-resolved fluorescence spectroscopy in homogeneous solvents as well as in differentmicro-heterogeneous environments. Depending on the nature of intramolecular hydrogen bond (IHB),the salicylic acid derivative may exist in two different ground state conformers (I and II). Structure Ihaving IHB between the carbonyl oxygen and phenolic hydrogen can undergo ESIPT upon excitation asevidenced by largely Stokes-shifted fluorescence at �455 nm; whereas, normal fluorescence in the blueside of the spectrum (�410 nm) is due to the spontaneous emission from conformer II. The results inhomogeneous solvents were compared with those in bio-mimicking environments of β-cyclodextrin(CD) and surfactants. The intensity of the ESIPT fluorescence increases substantially upon encapsulationof the probe into the cyclodextrin as well as micellar nano-cavities. Detailed analysis of the spectroscopicdata indicates that the probe forms 1:1 complex with CD in aqueous medium. Binding constant of theprobe with the micelles as well as critical micelle concentration was obtained from the variation offluorescence intensity on increasing concentration of different surfactants in aqueous medium.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Proton translocation is one of the most frequently occurredprimary photoreactions in organic molecules and their complexes[1,2]. The study and understanding of such processes is veryimportant for chemistry as well as for biology, as they play a keyrole in the photosynthesis in plants and also for functioning ofvarious biological organisms [3,4]. Molecules undergoing excitedstate intramolecular proton transfer (ESIPT) also known to pos-sesses technological relevance in laser dye materials, in fluorescentsensors, molecular memory storage devices and polymer protec-tors [5–8]. Furthermore, the phenomenon of potential barrierpenetration or proton tunneling through multi-dimensionalpotential energy surface (PES) has an important role in manybranches of physics like quantum field theory, fission of atomicnuclei and solid state physics [9,10].

Since the first discovery of ESIPT in methyl salicylate (MS) [11],salicylic acid (SA) and its derivatives are among the most exten-sively studied molecules, both theoretically as well as by several

experimental techniques [12–15]. These molecules commonlyhave a strong intramolecular hydrogen bond (IHB) between thephenolic group and the neighboring carbonyl oxygen that act asproton donor and acceptor, respectively. The existing IHB strengthgets further enhanced upon excitation, due to the increased acid-base character of the two moieties adjacent to each other, and theresulting proton translocation comprise the main idea of ESIPT.The unusually large Stokes-shifted emission gives a typical signa-ture of ESIPT; however, emission from other rotamers can furthercomplicate the photophysical properties in these systems [16].

Effect of different substitution on the ESIPT of parent intramo-lecularly hydrogen bonded compound seems to be an active areaof research now-a-days. Because ESIPT is direct consequence ofincreased IHB strength on excitation, any electron donating orwithdrawing substituent, more particularly at the para position ofthe hydroxyl group, will have a pronounced effect. For example, itwas reported that the photochemistry of 2-(2-hydroxy-4-methox-yphenyl) benzoxazole (4-MHBO) is quite similar to that of theparent compound 2-(2-hydroxyphenyl) benzoxazole (HBO) andshows only large Stokes-shifted fluorescence emission from theproton transferred keto tautomer; however, 2-(2-hydroxy-3-meth-oxyphenyl) benzoxazole (3-MHBO) shows dual emission fromboth normal enol form as well as from the keto structure [17].

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Journal of Luminescence

0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jlumin.2013.12.017

n Corresponding author. Tel.: þ91 364 272 2634; fax: þ91 364 255 0486.E-mail addresses: smitra@nehu.ac.in, sivaprasadm@yahoo.com (S. Mitra).

Journal of Luminescence 148 (2014) 134–142

Substitution effect of ESIPT in SA was also reported by severalresearch groups [18–20]. It was concluded that substitution ofelectron donating groups in the para position definitely suppressesthe proton transfer in the excited state, as evidenced by mirrorimage relationship of fluorescence emission with the excitationspectra; however, substitution in the ortho position does notsubstantially alter the photochemical pathway.

Studies on organized micro-heterogeneous assemblies serve asa good miniature models for studying and mimicking importantphenomena in biological systems [21,22]. Among the studies onmicro-heterogeneous media, inclusion complexes are one of theinteresting subjects for many researchers which have beendemonstrated for a variety of chemical, biological and industrialapplications in drug delivery systems [23], nano-sized electronicdevices [24] and energy storage materials and devices [25].Surfactants have also attracted considerable attention for theirspecial properties and many applications in pharmaceutics, drugdelivery, emulsification, nanomaterial synthesis, and vesicle for-mation [26–28] etc. Surfactant aggregates are potential supramo-lecular host systems because of the presence of different types ofbinding sites under varying experimental conditions, such asconcentration, pH, and ionic strength of the surrounding medium[29–31]. The proton transfer dynamics in these confined systemsplays an important role in many biological processes.

Substituted polyaromatic aryl-azo compounds provide a richclass of colorants having diverse applications [32–34]. One of themost promising applications of diazo dyes is reported to be in thefield of potential sensitizers for photodynamic therapy (PDT)for the treatment of tumor and certain other diseases [35,36]due to their high nonradiative quantum yield (ϕnr) compared tothe fluorescence quantum yield (ϕf). Functionalized substitutedazo dyes like 2-hydroxy-5-[(E)-(aryl)diazenyl]benzoic acid are aclass of organic ligands having versatile synthetic, structural andbiological importance [37–45]. Recently, the synthesis, IR andNMR spectroscopic properties and molecular structure (X-raydiffraction) for 2-hydroxy-5-[(E)-(4-methoxyphenyl)diazenyl]ben-zoic acid (HMBA, see Chart 1 – for structure) was reported [46,47].The title compound, HMBA is basically planar molecule and theconformation about the diazo group is E, although there is a slighttwist about the diazenyl group. The carboxylic acid group iscoplanar with C2–C7 ring [O1–C1–C2–C3¼178.2(2)1], which facil-itates the formation of an intramolecular hydrogen bondingcontact, such that O3–O2 is 2.630(2) Å. Intermolecular hydrogenbonding interactions are also present [47]. Furthermore, thephotophysical properties of HMBA are expected to be interestingdue to the presence of a highly nonradiative ESIPT channel in

salicylic acid moiety. Reports on the solution phase absorptionspectroscopy of aryl-azo dyes are also available in recent litera-ture; however, the fluorescence properties of these ligands andtheir metal complexes are scantily studied [48,49]. In this paper,we report the photophysical properties of HMBA in homogeneoussolvents as well as in heterogeneous medium like cyclodextrinand surfactant by steady-state and time-resolved fluorescencespectroscopy.

2. Materials and experimental method

2.1. Materials

Reagents required for the synthesis of HMBA like para-anisi-dine, salicyclic acid (Lancaster), hydrochloric acid, sodium hydro-xide and sodium nitrite (Sisco Research Laboratories, India) wereused without further purification. The organic solvents used wereof spectroscopic grade (499.5%) as received from Aldrich Chemi-cal Company. The analytical grade type-II water, also used assolvent, was obtained from Elix 10 water purification system(Millipore India Pvt. Ltd.). Surfactants like sodium dodecyl sulfate(SDS), cetyltrimethylammonium bromide (CTAB) and triton X-100(TX-100) as well as β-cyclodextrin (CD) were obtained fromAldrich Chemical Company and used as received without furtherpurification. All experiments were carried out at room tempera-ture (293 K). The sample concentration (�10 μM) was low enoughto avoid any aggregation and kept constant during the variation ofsurfactants and/or CD concentrations.

2.2. Instruments and data analysis

The IR spectra were measured on a Perkin-Elmer L 120-000Aspectrometer with KBr pellets in the range 4000–400 cm�1. 1HNuclear magnetic resonance spectra were recorded on a BrukerDPX-400 MHz spectrometer with chemical shifts reported asppm (in DMSO-d6, tetramethylsilane as internal standard).Steady-state absorption spectra were recorded on a Perkin-Elmermodel Lambda25 absorption spectrophotometer. Fluorescencespectra were recorded in a Hitachi model F-4500 spectrofluori-meter and all the spectra were corrected for the instrumentresponse function. Quartz cuvettes of 10 mm optical path lengthreceived from Perkin-Elmer, USA (part no. B0831009) and Hellma,Germany (type 111-QS) were used for measuring absorption andfluorescence spectra, respectively. In both fluorescence emissionand excitation spectra measurements, 5 nm bandpass was used in

Chart 1. Different possible conformers of 2-hydroxy-5-[(E)-(4-methoxyphenyl)diazenyl]benzoic acid (HMBA). Atom numbering for describing the structural properties isalso shown.

P.M. Gashnga et al. / Journal of Luminescence 148 (2014) 134–142 135

the excitation and emission side. Fluorescence quantum yields (ϕf)in different solvents were calculated by comparing the totalfluorescence intensity under the whole spectral range with thatof a similar system, 4-methyl-2,6-diformyl phenol (MFOH, ϕf¼0.1)as described before [50], using the following equation.

ϕif ¼ϕs

fFi

Fs1�10�As

1�10�Ai

ni

ns

� �2

ð1Þ

The fluorescence decay curves in different solvent medium wereobtained using time-correlated single photo counting (TCSPC)technique. The excitation was done at 400 nm obtained by focus-ing the output (800 nm, 2 MHz repetition rate) of a cavity dumpedTi:Sa laser (Cascade, KM Labs Inc. USA) on 1 mm BBO crystal. Thedetection system for TCSPC measurements was composed of amonochromator (Japan Spectroscopic, CT-10), a microchannel –

plate photomultiplier (Hamamatsu, MCP 2809U), a constant frac-tion discriminator (Tennelec TC454) and time-to-amplitude con-verter (Tennelec TC864). However, the fluorescence decay curvesin presence of different surfactants and CD were obtained usingLED based time-correlated single photon counting (TCSPC) systemobtained from Photon Technology International (PTI). The excita-tion was done at 365 nm. The instrument response function (IRF)was obtained by using a dilute colloidal suspension of dried non-dairy coffee whitener.

The fluorescence decay curves were analyzed by non-linearleast-square iterative convolution method based on Lavenberg–Marquardt algorithm [51] and expressed as a sum of exponentials

IðtÞ ¼∑iαiexpð�t=τiÞ ð2Þ

where, αi is the amplitude of the ith component associated withfluorescence lifetime τi such that ∑αi¼1. The reliability of fittingwas checked by numerical value of reduced chi-square (χ2) andalso by visual inspection of residual distribution in the wholefitting range.

3. Results and discussion

3.1. Synthesis and characterization of the 2-hydroxy-5-[(E)-(4-methoxyphenyl)diazenyl]benzoic acid (HMBA)

The compound 2-hydroxy-5-[(E)-(aryldiazenyl)] benzoic acid(Chart 1) was synthesized by diazonium-coupling between para-anisidine and salicyclic acid in alkaline medium under ice-coldconditions described previously [46]. The product was recrystallized

from methanol (m. p.: 209–210 1C) and the purity was established bychromatographic techniques. IR and 1H NMR spectroscopic datareported in Ref. [46] correspond well within the experimental errors.

3.2. Steady-state and time-resolved fluorescence in homogeneousmedia

The UV–vis absorption spectra (Fig. 1) of HMBA consist of asingle broad band in 300–400 nm region with a maximumcentered on �350 nm. The large value of molar extinction coeffi-cient indicates the absorption to be originated mainly due to π–π*

transition of the benzene ring. The fluorescence emission spectrashow the main peak at �410 nm with a shoulder at around450 nm. The representative fluorescence emission spectral profilesof HMBA in some solvents are shown in Fig. 2, whereas, all thespectral behaviors are listed in Table 1. Although the datapresented in Table 1 looks apparently similar for different solventsystems, the spectral shape and relative intensity of the twoemission bands differ significantly depending on the solvent(Fig. 2). Also, the fluorescence emission shows little broadeningin polar protic solvents like alcohol and water in comparison withnon-polar and/or polar aprotic solvents. The fluorescence quantumyield (ϕf) values, given in Table 1, are fairly low and could beexplained by the presence of strong non-radiative decay channelalong the IHB, similar to those reported in the literature typicallyfor several other o-hydroxy carbonyl compounds. Nevertheless,the ϕf values of HMBA in nonpolar solvents are found to becomparatively higher than in polar protic solvents. The lowquantum yield values in polar protic solvents is mainly due tothe formation of other non-radiative decay channels, which areactive via intermolecular hydrogen bonding interactions with thesolvents [52,53]. From the quantum yield data, it can be inter-preted that nonpolar solvents have different influence from that ofthe polar protic solvents on the ESIPT states. As shown in Fig. 2(b),the fluorescence emission spectra of HMBA can be reproducedwith two independent Gaussian type of functions having max-imum at ca. 410 nm and 450 nm, with a Stokes-shift of 3387 and5550 cm�1, respectively. The origin of these fluorescence bandscan be attributed to specific species undergoing different excitedstate photophysics as shown in Scheme 1. The intramolecularlyhydrogen bonded salicylic acid derivatives can have two differentground state enol conformers, depicted as IE and IIE in Chart 1.The ground state energy difference between these two structuresis quite low as found to be about only 4–5 kcal mol�1 for differentsalicylic acid derivatives [19]; and therefore, both these two

Fig. 1. (a) Steady state absorption (open circles), fluorescence emission (solid squares, λexc¼360 nm) and fluorescence excitation (solid triangles, λmon¼410 nm) spectra ofHMBA in hexane. (b) Excitation wavelength dependence of the emission (solid points) and excitation (open points) spectra of HMBA in carbon tetrachloride.

P.M. Gashnga et al. / Journal of Luminescence 148 (2014) 134–142136

structures can coexist in solution. Structure IE possesses IHBbetween the carbonyl oxygen and phenolic hydrogen atoms.Therefore, it can undergo intramolecular proton translocationupon excitation (pro-ESIPT); whereas, structure IIE should follownormal fluorescence as a deactivation channel because of theabsence of suitable geometry for ESIPT to occur (non-ESIPT). So,the largely Stokes-shifted (�5550 cm�1) fluorescence is believedto be originated from the excited keto form (IK) of structure IE dueto the ESIPT and the fluorescence in the blue side of the spectrum(�410 nm) is due to the spontaneous emission from conformerIIE. However, it is to be noted here that the energy parameters forthe inter-conversion of several species is neither known in theground state nor in the excited state. Therefore, the representationin Scheme 1 can only be considered as a qualitative description ofthe photophysical behavior of different conformers of HMBAdiscussed earlier.

The presence of two ground state conformers of HMBA (IE andIIE) and their corresponding emission at �450 and �410 nm,respectively is further confirmed from the excitation wavelengthvariation study. Representative results for HMBA in CCl4 are shownin Fig. 1b. It is seen that while at λexc¼365 nm, the ESIPT emission(450 nm) appears as a clear shoulder in the emission band alongwith the main fluorescence (410 nm) from the non-ESIPT struc-ture, excitation at the blue side (λexc¼300 nm) gives the highenergy emission almost exclusively. In contrast, excitation at thered-end (λexc¼400 nm, for example) gives only the ESIPT emis-sion. This observation clearly indicates the presence of twodifferent emissive species. However, both the excitation spectrumcorresponding to these emissions appear in 355–360 nm region

and further confirm that these two species are almost identical inenergy as discussed before.

Fluorescence decay behavior of HMBA in different representa-tive solvent systems was monitored by picosecond time-correlatedsingle photon counting method. Surprisingly, even for a simplemolecule like HMBA, the fluorescence decays are far from singleexponential in nature. Representative fluorescence decay profilesof HMBA in CCl4 and acetonitrile are shown in Fig. 3. Visualinspection of the weighted residuals (shown in the upper panel)and reduced chi-square values indicate the necessity two to threeexponentials to properly fit the experimentally obtained datapoints in all the cases. It is often difficult to mechanistically assignthe various components of the multi-exponential decay. Particu-larly, for HMBA, there are large degrees of conformational flex-ibility, both in ground and excited state, to give a distribution ofdifferent structures with varying fluorescence decay time. Insteadof giving too much importance to individual decay components,we define the average lifetime ⟨τ⟩ of the fluorophore in solutionusing Eq. (3) to discuss the fluorescence decay behavior.

⟨τ⟩¼∑iαiτi ð3Þ

The average fluorescence decay times, ⟨τ⟩ of HMBA in some of thesolvents like hexane, CCl4 and acetonitrile were reported and foundout be 67.5 ps, 1.16 ns and 36.1 ps respectively. As also evident fromthe decay profiles given in Fig. 3, the calculated ⟨τ⟩ value of HMBA inCCl4 is unexpectedly large in comparison with both the non-polarhexane as well as polar acetonitrile solutions. It is difficult toascertain the reason behind the discrepancy at this point as we lackthe detailed picosecond time-resolved fluorescence behavior of the

Fig. 2. (a) Fluorescence emission spectra of HMBA in different solvents. (b) Reconstruction of fluorescence emission of HMBA in acetonitrile with two Gaussian function.

Table 1Steady state spectral properties of HMBA in different homogeneous solvents.

Solvents λabsa (nm) λfl

b (nm) ΔνSSc (cm�1) ϕfd (10�3)

Cyclohexane 355 398, 445 (s) 3043, 5697 5.7Hexane 353 397, 447 (s) 3139, 5957 6.51,4-Dioxane 358 398, 445 (s) 2807, 5461 2.3Acetonitrile 360 414, 458 (s) 3623, 5943 5.8Chloroform 355 398, 442 (s) 3043, 5544 5.6Carbon tetrachloride 355 412, 445 (s) 3897, 5697 5.3Methanol 358 403, 442 (s) 3119, 5308 4.4Water 360 408, 448 (s) 3267, 5456 3.2

a Absorption maxima.b Fluorescence maxima.c Stokes shift.d Fluorescence yield.

IEIIE

IK

(IE)*(IIE)*

(IK)*

Abs

orpt

ion

Abs

orpt

ionE

mission

(410 nm)

Em

ission(450 nm

)

Scheme 1. Photophysical properties of HMBA in the ground and excited state. Theconformers are designated as specified in Chart 1.

P.M. Gashnga et al. / Journal of Luminescence 148 (2014) 134–142 137

probe in a variety of pure solvents. A possible hypothesis may be theinteraction of the excited fluorophore with the so called electrondeficient CCl4 and formation of a fluorescing complex, which isrelatively stable and decays in longer time span. Nevertheless,calculation of radiative (κr) and total non-radiative (Σκnr) decayparameters using the quantum yield (ϕf) of fluorescence data givenin Table 1 from Eq. (4) reveals that the non-radiative decay parameteris always about two orders of magnitude higher than the radiativecounterpart in all the solvents.

κr ¼ϕ=⟨τ⟩; ∑κnr ¼ ð1�ϕÞ=⟨τ⟩ ð4Þ

It is to be noted here that the ESIPT reaction is greatly affectedby the hydrogen bonding capability of the solvent. In proticsolvents, the intramolecular hydrogen bonded ring in the fluor-ophore molecule might break to form an intermolecular hydrogenbond between the fluorophore and the solvent molecule similar tothe conversion of structure IE to IIE given in Chart 1. This reducesthe formation of the proton transferred photo-tautomer (IK),which decreases the tautomer emission and increases the normalemission. However, in polar aprotic solvents, the tautomer emis-sion is predominant, and the normal emission is weak. Further-more, the non-radiative decay rate in hydrogen bonding polarsolvents are higher compared to the nonpolar solvents. For thesame reason, the radiative decay rate constant shows the reverseorder. This effect is also prominent in the measured quantum yield(ϕf) data (Table 1); where, higher fluorescence quantum yields areobserved for nonpolar solvents and are lower in the case of polarprotic solvents.

3.3. Spectroscopy of aqueous HMBA in heterogeneous media

3.3.1. In cyclodextrinComplexation behavior of aqueous HMBA was monitored in

presence of β-cyclodextrin (CD) both by steady-state and time-resolved fluorescence spectroscopy. It has been observed that theabsorption intensity increases with increase in the CD concentra-tion without much change in the spectral peak position. Thisincrease in the absorption peak position can be interpreted due togreater solubilization of the probe in the presence on cyclodextrin.However, the ESIPT fluorescence shows regular increase in inten-sity, though without any significant shift in spectral position

(Fig. 4). The change in fluorescence intensity is due to the bindingof the probe inside the CD cavity. The encapsulation facilitates theESIPT process by shielding the probe from the aqueous environ-ment. The stoichiometric ratio and apparent binding constant forHMBA/CD complex can be determined by analyzing the changes influorescence emission with CD concentration. The equilibriumreaction for 1:1 binding between HMBA and CD can be written as

HMBAþCD⟷K

HMBA : CD ð5ÞThe equilibrium constant for the above reaction is

K ¼ ½HMBA : CD�eq½HMBA�eq½CD�eq

ð6Þ

If the initial concentration of the probe is represented by [HMBA]0and in the condition of [CD]⪢[HMBA]0, the above equation can bereduced to

K ¼ ½HMBA : CD�eqð½HMBA�0�½HMBA : CD�eqÞ½CD�

ð7Þ

At any instance, the observed fluorescence intensity (I) is the sumof the fluorescence intensities from the free and bound HMBA,respectively. Under this condition, one can write

I¼ I0½HMBA�eq½HMBA�0

þ Iα½HMBA : CD�eq

½HMBA�0ð8Þ

Where, I0 and Iα are the fluorescence intensities of the free andfully complexed HMBA, respectively. Since, [HMBA]¼[HMBA]eqþ[HMBA:CD]eq, from Eq. (8) we can further write

½HMBA : CD�eq½HMBA�0

¼ I� I0Iα� I0

ð9Þ

From Eqs. (7) and (9), the modified form of Benesi–Hilderbrand(BH) relation [24] can be written as

1I� I0

¼ 1Iα� I0

þ 1KðIα� I0Þ

1½CD� ð10Þ

Therefore, for an 1:1 complex formation, the double reciprocal plotof 1/(I� I0) against 1/[CD] should give a straight line; from theslope and intercept of which, the equilibrium constant (K) can becalculated. The representative linear fitting using Eq. (10) is shownin the inset of Fig. 4 and confirm that HMBA indeed forms 1:1

Fig. 3. Time resolved fluorescence decay (scattered points) and simulated curves(solid line) of HMBA in CCl4 and acetonitrile. The instrument response function(IRF) is also shown. The upper panels show the distribution of weighted residualsand other statistical parameters for two and three exponential fitting of theexperimental data points in CCl4.

Fig. 4. Variation in ESIPT fluorescence intensity of aqueous solution of HMBA inpresence of β-cyclodextrin. The concentration of β-cyclodextrin ([β-CD]/mM¼0.0,0.77, 3.1, 4.6, 6.2, 7.9, and 9.5) increases along the arrow direction. Inset shows thelinear BH plot for 1:1 HMBA/β-CD complex.

P.M. Gashnga et al. / Journal of Luminescence 148 (2014) 134–142138

complex with CD with the calculated value for binding constantK�112.7 M�1.

Fluorescence lifetime of HMBAwas measured in presence of CDand observed to be multi-exponential decay similar to that inhomogeneous solvents discussed above. The representative datafor the ESIPT fluorescence encapsulated in CD environment isshown in Fig. 7. The average decay time calculated using Eq. (3)increases substantially in presence of cyclodextrin while comparedwith the homogeneous medium. The large increase in averagedecay time in CD environment indicates a different microenviron-ment of the probes on encapsulation into the CD cavities. Thedecay parameters along with the fluorescence quantum yield (ϕf),the radiative (κr) and total nonradiative (∑κnr) decay rate con-stants of HMBA with varying CD concentration are listed inTables 2 and 3. The increase in fluorescence quantum yield (ϕf)and substantial decrease in ∑κnr in the CD environment points tothe restricted motion of the fluorophore inside the cavities.

3.3.2. In different surfactant mediumSolubilization behavior of HMBA was also monitored in anionic

(SDS), cationic (CTAB) and non-ionic (TX-100) surfactants. Onaddition of surfactants, the absorption spectra do not showsignificant change; only 3–4 nm red shift is observed along withslight increase in molar extinction coefficient relative to theaqueous medium. However, the fluorescence peak intensityincreases drastically with gradual increase in the concentrationof the surfactants which indicates the passage of the moleculesfrom highly polar aqueous medium to relatively nonpolar micellar

environment and also rationalized on the basis of the binding ofthe probe in a less polar site within the micellar aggregates ascompared to pure aqueous phase. At lower surfactant concentra-tion, the change in fluorescence response is not that high. How-ever, after a certain micellar concentration, the fluorescenceintensity shows a marked change. This sharp break point isconventionally assigned to critical micelle concentration (cmc).Typical plots of fluorescence intensity variation with surfactantconcentrations are shown in the inset of Fig. 5 where the inflectionpoint gives the critical micelle concentration of the surfactants.The corresponding values for SDS, CTAB and TX-100 were foundout to be 8.1 mM, 0.92 mM and 0.31 mM, respectively. Theseestimated cmc values obtained from experimental data are inquite good agreement with the literature values [54–56].

The binding of a probe to micelles were also estimated usingthe following equilibrium [57]:

SaþDm⟷KS Sm ð11Þ

Where, Sa and Sm denote the substrate concentrations expressed asmolarities in terms of total volume of solution in the aqueousphase and in the micellar pseudo-phase.

The equilibrium constant for the process (11), often termed asbinding constant, is given by

KS ¼½Sm�

½Sa�½Dm�ð12Þ

Considering the aggregation number of the micelle to be constant, thetotal substrate concentration (St) and total detergent concentration (Dt)can be written as [Sa]þ[Sm] and [Sm]þ[Dm]þcmc, respectively.

Fig. 5. ESIPT fluorescence of aqueous solution of HMBA in presence of SDS (a), CTAB (b) and TX-100 (c). The concentrations (in mM) of the surfactants are [SDS]¼0.0, 0.8, 4.5,9.3, 15.8, 27.3, 35.5; [CTAB]¼0.0, 0.22, 0.65, 0.91, 1.21, 1.5 and [TX-100]¼0.0, 0.15, 0.2, 0.3, 0.4, 0.5. The inset shows intensity variation at 445 nm in each case.

P.M. Gashnga et al. / Journal of Luminescence 148 (2014) 134–142 139

Fig. 6. Variation of (F�F0)/(Fm�F) with total surfactant concentration [Dt] for SDS (a) and TX-100 (b). The binding constant, Ks was calculated from the slope of the straightline using Eq. (17).

Fig. 7. ESIPT fldecay profile (scattered points) and simulated data (solid lines) of HMBA monitored at λemm¼445 nm in 9.5 mM β-cyclodextrin (a) and 35.5 mM SDS (b). Theupper panels show the distribution of weighted residuals for three exponential fit in each case.

Table 2Fluorescence quantum yield (ϕf), decay time (τ), mean fluorescence decay time ⟨τ⟩, radiative (κr) and non-radiative (κnr) decay constants of HMBA monitored at fluorescenceemission (λemm¼410 nm) in bulk water, β-CD and different micellar media.

Medium ϕf (10�3) Fluorescence decay time (τ) ⟨τ⟩ (ns) χ2 κr (107 s�1) κnr (109 s�1)

τ1 (ns) (α1) τ2 (ns) (α2) τ3 (ns) (α3)

Water 3.2 0.09 (0.81) 2.52 (0.08) 16.19 (0.11) 2.05 1.2 0.15 0.47β-CD [9.5 mM] 19.69 3.87 (0.06) 0.08 (0.67) 54.55 (0.27) 15.01 1.1 0.13 0.06SDS [35.5 mM] 8.67 0.10 (0.60) 0.11 (0.28) 19.48 (0.12) 2.43 1.2 0.35 0.41CTAB [2.5 mM] 5.23 0.11 (0.82) 1.78 (0.05) 17.56 (0.13) 2.46 1.2 0.21 0.40TX-100 [0.6 mM] 9.89 0.09 (0.76) 3.13 (0.10) 22.57 (0.14) 3.54 1.1 0.28 0.27

Table 3Fluorescence quantum yield (ϕf), decay time (τ), mean fluorescence decay time ⟨τ⟩, radiative (κr) and non-radiative (κnr) decay constants of HMBA monitored at fluorescenceemission (λemm¼445 nm) in bulk water, β-CD and different micellar media.

Medium ϕf (10�3) Fluorescence decay time (τ) ⟨τ⟩ (ns) χ2 κr (107 s�1) κnr (109 s�1)

τ1 (ns) (α1) τ2 (ns) (α2) τ3 (ns) (α3)

Water 3.2 0.26 (0.31) 0.06 (0.36) 3.5 (0.33) 1.2 1.2 0.26 0.82β-CD [9.5 mM] 19.69 2.74 (0.17) 0.09 (0.58) 20.5 (0.25) 5.6 1.1 0.35 0.17SDS [35.5 mM] 8.67 2.54 (0.15) 0.07 (0.64) 17.4 (0.21) 4.1 1.1 0.21 0.24CTAB [2.5 mM] 5.23 0.24 (0.62) 2.9 (0.24) 4.6 (0.14) 1.5 1.2 0.35 0.66TX-100 [0.6 mM] 9.89 0.27 (0.35) 1.6 (0.18) 5.7 (0.47) 3.1 1.2 0.32 0.32

P.M. Gashnga et al. / Journal of Luminescence 148 (2014) 134–142140

The fraction of micellar associated substrate is defined as

f ¼ ½Sm�½St �

ð13Þ

Then from Eq. (12), one obtains

f1� f

¼ KS ½Dt ��½St �f� ��KScmc ð14Þ

Under the condition of [Sm]⪡[Dt] and [Dt]⪢cmc, the above equationcan be approximated as

f1� f

¼ KS½Dt � ð15Þ

Experimentally, f can be calculated by steady state fluorescenceexperiments in presence and absence of micellar systems asfollows:

f ¼ F�F0Fm�F0

ð16Þ

Where, F, F0 and Fm are the area under the whole fluorescenceemission spectra of the probe in surfactant, water and in fullymicellized condition, respectively. Substituting the value of f inEq. (15), one can write

F�F0Fm�F

¼ KS½Dt� ð17Þ

A plot of (F�F0)/(Fm�F) vs. [Dt] gives a straight line, the slope ofwhich gives the value of the binding constant, Ks. The correspond-ing values for SDS, CTAB and TX-100 were found out to be(11270.4) M�1, (34070.5) M�1 and (390070.3) M�1 respec-tively and some representative plots are shown in Fig. 6.

Excited state lifetime of a fluorophore in a micellar solutionserves as a sensitive parameter for exploring the local environ-ment around the fluorophore which also contributes to the under-standing of different interactions between the probe and themicelle [58,59]. On the basis of this, time-resolved fluorescencestudies were performed on HMBA at fully micellized condition inall different surfactants namely SDS, CTAB and TX-100 and thecalculated values are reported in Tables 2 and 3. Here, in fullymicellized condition, a three exponential function was needed tofit the experimental data. The representative data for the ESIPTfluorescence under fully micellized with SDS is shown in Fig. 7.The corresponding parameters are listed in Tables 2 and 3. It isinteresting to note that the mean fluorescence decay time inmicellar media are much larger than the corresponding values inaqueous medium. The increase in quantum yield (ϕf) and sub-stantial decrease in κnr in the micellar environment points to therestricted motion of the fluorophore inside the micellar cavities.The appreciable difference of the corresponding parameters infully micellized state and pure water rules out the possibility of thelocation of the probe in the bulk phase.

4. Conclusion

The intramolecularly hydrogen bonded salicylic acid derivativescan have two different ground state conformers which can undergoexcited state intramolecular proton transfer (ESIPT) and normalfluorescence as a deactivation channel which is due to the sponta-neous emission. Fluorescence properties of HMBA were found to bemodified in presence of surfactants and β-cyclodextrin when com-pared with homogeneous solvents. The encapsulation and binding ofthe probe into the β-cyclodextrin cavity and micellar systems weremonitored with fluorescence peak maximum as well as increase inmean fluorescence decay time. Detailed analysis of the spectroscopicdata indicates that the probe forms 1:1 complex with β-CD inaqueous medium. Binding constant of the probe with micelles as

well as critical micelle concentration was obtained from the variationof fluorescence intensity on increasing concentration of differentsurfactants in the aqueous medium.

Acknowledgments

Thanks are due to the Department of Science & Technology(DST), Govt. of India for supporting the Chemistry Department,NEHU through FIST. Financial support through UGC-BSR ResearchStart-Up-Grant Project no. F.20-1/2012 (BSR)/20-1(2)/2012(BSR)from University Grants Commission (UGC), Government of India isgratefully acknowledged by TSS.

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