Photophysical behavior and fluorescence quenching by halidesof quinidine dication: Steady state and time resolved study

Neeraj Kumar Joshi a, Neeraj Tewari a,1, Priyanka Arora a, Ranjana Rautela a, Sanjay Pant a,Hem Chandra Joshi b,n

a Photophysics Laboratory, Department of Physics, DSB Campus, Kumaun University, Nainital 263002, Uttarakhand, Indiab Institute for Plasma Research, Laser Diagnostics Division, Bhat, Near Indira Bridge, Gandhinagar 382428, Gujarat, India

a r t i c l e i n f o

Article history:Received 27 March 2014Received in revised form8 October 2014Accepted 20 October 2014Available online 29 October 2014

Keywords:FluorescenceQuenchingHalideGround state complex

a b s t r a c t

The fluorescence quenching of quinidine in acidified aqueous solution by various halides (Cl� , Br�

and I�) was studied using steady state and time resolved fluorescence techniques. The quenchingprocess was characterized by Stern–Volmer (S–V) plots. Possibility of conformers (one is not quenchedby halide and the other is quenched) is invoked to explain the observed results.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Inorganic halides are abundant in nature as minerals or assolvated halide ions by the dissolution of minerals or salts. Thedetermination of halide concentration in the environment is impor-tant for both monitoring excessive halide levels as well as monitor-ing halide deficiencies in natural resources [1–3].

Fluorescence quenching processes allow quantitative determina-tion of halides [4–10]. The determination of halide using fluores-cence quenching is a popular technique because of the highsensitivity that it can offer and the simplicity of quenching reactions(where only a small volume of sample is required, the reactions areusually non-destructive). Besides, the phenomenon of fluorescencequenching has found applicability in various studies [11–15,16–18].

A typical bimolecular quenching involves close contact betweenthe fluorescent species and the quencher and can be due to heavyatom effect, energy or electron transfer process. On the other handsome trivial processes such as attenuation of the emitted light by thefluorophore itself (self-absorption) or absorption from other speciesmay lead to decrease in the fluorescence intensity, which are notconsidered as quenching processes.

In earlier works excited state dynamical behavior of quinine,quinidine and related compounds has been reported in acidifiedaqueous solution at different pH values and at different tempera-tures as well as in polymers [9,10,19–26]. Quinidine has twoprotonable groups with pKa values of 4.2 and 7.9–8.8 [8]. However,in 1N H2SO4 we can only expect Qdþ þ [8].

Moreover, because of projected applications of such moleculesin sensors for halide ions and to understand various quenchingprocesses, a systematic study of fluorescence quenching is needed.In the past quenching by chloride ions in some cinchona alkaloidse.g. quinine sulfate and cinchonine was reported [9,10,19] but thedetailed mechanism regarding quenching was not traced out.Again due to the difference in stereo-structure, quenching studyof individual alkaloids becomes interesting. As quinine and quini-dine are enantiomers, it would be interesting to investigatewhether their structures have some role in affecting the quenchingbehavior.

To the best of our knowledge, fluorescence quenching ofquinidine by halides in acidified aqueous solution has not beenstudied yet. Hence in the present work we have undertaken adetailed study of its quenching by halides and various quenchingparameters have been estimated in order to understand the natureof the possible quenching mechanism. Possibility of conformers(one is not quenched by halide and the other is quenched) isinvoked to explain the observed results. The molecular structure ofquinidine is shown in Scheme 1.

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Journal of Luminescence

http://dx.doi.org/10.1016/j.jlumin.2014.10.0410022-2313/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ91 7923962056.E-mail address: hem_sup@yahoo.co.uk (H.C. Joshi).1 Present address: Lajpat Rai (P.G.) College, Sahibabad, India.

Journal of Luminescence 158 (2015) 412–416

2. Experimental section

2.1. Materials

Quinidine (obtained from Aldrich) of 98% purity was tested forits fluorescence purity by matching with reported fluorescencespectrum (to ensure that there is no contamination) and used assuch. All the solvents used were either of spectroscopic grades orwere checked for their fluorescence purity. Doubly distilled waterwas used in these experiments. The samples were prepared bydissolving appropriate concentration of quinidine in 1N H2SO4.NaCl, KBr and KI were used for the quenching study. Concentrationof H2SO4 has been kept the same in all the samples to rule out theeffect of the presence of SO4

�� ion.

2.2. Instrumentation

Steady state absorption spectra, at room temperature, wererecorded by a dual beam JASCO V-550 spectrophotometer. Theexcitation and emission spectra were recorded by using a JASCO FP– 777 spectrofluorometer and the data were analyzed by relatedsoftware. The samples were excited in frontal geometry to rule outany inner filter effect. Fluorescence decay times were recordedwith the help of a Edinburgh – 199-time domain spectrometer andanalyzed by TCC – 900 software. The excitation source was athyratron-gated hydrogen filled nanosecond flash lamp. Lampprofile was measured at the excitation wavelength using a Ludoxscatterer. The pulse width was about 1.5 ns with repetition rate of30 kHz. A time correlated single photon counting (TCSPC) techni-que was used to collect the decay curves and the resolution of thesystem was about 200 ps. The number of counts in the peakchannel was at least 10,000. Time-resolved fluorescence decaycurves were analyzed by deconvoluting the observed decay withthe instrument response function (IRF) to obtain the intensitydecay function represented as a sum of discrete exponentials; I (α,t)¼Σiαiexp(�t/τi), where I (t) is the fluorescence intensity at timet and αi is the amplitude of the ith life time such that ∑i αi¼1. Themean lifetime, τm¼Σαiτi, gives the information on the averagefluorescence yield of the system.

3. Results and discussion

Steady state absorption, fluorescence and decay curves of Qdþ þ

were recorded at 298 K in the absence and in the presence of halideions (Cl� , Br� and I�). Qdþ þ exhibits absorption maximum at�355 nm. The emission spectra (Fig. 1) for Qdþ þ show red-shift in

emission maximum upon excitation on the red edge of the absorp-tion spectrum, i.e., edge excitation red shift (EERS). Similar trend isobserved in the presence of the halides.

In the presence of halide ions no change on the absorptionmaximum as well as on the shape of absorption spectrum ispresent. Quenching in the fluorescence intensity is observed in thepresence of halide ions. Fluorescence quenching by Cl� ions isshown in Fig. 2. Further, no other emission is developed. Theseobservations suggest that the fluorophore–quencher interactiondoes not change the shape of the fluorescence spectra. Hence,formation of any emissive exciplex can also be discarded. It is to benoted that the excitation spectra show successive red-shifts whilemonitored towards the red side of the emission even in theabsence of the halides and hence cannot be attributed to exciplexformation.

The Stern–Volmer relationship [27] establishes the correlationof intensity changes with the quencher concentration [Q] asfollows:

I0=I¼ 1þðKSV þKgÞ Q½ �þKSVKg Q½ �2 or

I0=I–1� �

= Q½ � ¼ ðKSV þKgÞþKSVKg Q½ � ð1Þ

where KSV (¼kqτ0) and Kg are the dynamic/(S–V) quenchingconstant and ground state association constant of the complex,respectively. kq is the bimolecular quenching constant and τ0

Scheme 1. Molecular structure of quinidine.

Fig. 1. Emission spectra (showing EERS) of Qdþ þ for different excitation wave-lengths: (a) 350 nm, (b) 370 nm and (c) 380 nm.

Fig. 2. Emission spectra of Qdþ þ in the presence of various concentrations of Cl� .

N.K. Joshi et al. / Journal of Luminescence 158 (2015) 412–416 413

corresponds to the decay time in the absence of the quencher. I0corresponds to the intensity in the absence of the quencher and I isthe intensity in the presence of quencher.

If one notices decrease in the fluorescence intensity but not inthe decay time (only static quenching, i.e., KSV¼0), or if the decaytime does not change in the proportion to the change in fluores-cence intensity, this indicates the contribution of static quenching.In the case of quenching being static, a fluorophore can form anon-luminescent association complex with quencher. The unbo-und fluorophore exhibits its intrinsic lifetime.

The other limiting case of that is the only dynamic quenching(in the absence of static quenching, i.e., Kg¼0). Here the effect ofquenching competes with the emission in the time scale and isdetermined by the diffusion of the quencher in the medium and itscollisions with the excited probe molecule. In this case, the relativechange in the intensity strictly follows the corresponding changein the fluorescence decay time. In such case Eq. (1) reduces asfollows:

I0=I¼ τ0=τ¼ 1þKSV Q½ � ¼ 1þkqτ0 Q½ � ð2ÞAll the terms in Eq. (2) has same meaning as explained for Eq. (1).τ is the decay time in the presence of quencher. If the dynamic andstatic quenching occurs simultaneously, then Eq. (1) describes aquadratic dependence with the quencher concentration and con-sequently there is a positive deviation in the graph I0/I versus [Q].

From the decay data, KSV values can be estimated correctly. Therelation, I0/[I�1]/[Q]¼(KSVþKg)þKSVKg [Q] gives Kg after puttingthe KSV value obtained from the decay time data.

The S–V plot obtained by using the experimentally determinedvalues of I0 and I is found to be linear in case of Cl� and Br� ions. Atypical intensity S–V plot of for Cl� is shown in Fig. 3; however,positive deviation from linearity in case of I� ion was observed(Fig. 4).

The linear relation of S–V plot (using intensity) in case of Cl�

and Br� ions indicates that decrease in the fluorescence intensityis either due to only static or only dynamic quenching. In order toascertain the mechanism of quenching by Cl� and Br� ions,transient experiments with increasing concentration of Cl� andBr� ions are carried out. Decay data are collected at λex¼355 nmand λem¼450 nm. Fluorescence decay data were found to be bestfitted with two exponential function with decay time τ1�5.5 nsand τ2�20.8 ns (Fig. 5). Such type of behavior is typical in quinineand related alkaloids [19,20]. Further, on increasing the concentra-tion of Cl� and Br� ions, the decay time of shorter component (τ1)remains almost constant while the longer component (τ2) isshortened. Quenching in the fluorescence decay time by Cl� ionsis shown in Fig. 6 and decay data are summarized in Table 1. Wealso monitored the decay data at three different emission wave-lengths (Table 2).

It is evident that the shorter decay component does not showappreciable change with the addition of Cl� (Table 2). This is incontrast with the quenching behavior observed in quinine sulfate

Fig. 3. Plot of (Io/I) versus [Q] for Cl� .

Fig. 4. Plot of (Io /I) versus [Q] for I� .

Fig. 5. Decay profiles and corresponding residuals for Qdþ þ in the presence of Cl�

at 0 M (a), 10�3 M (b), 2�10�3 M (c), 4�10�3 M (d), 6�10�3 M (e), 8�10�3 M(f), and 10�2 M (g). [λex¼355 nm and λem¼450 nm.]

N.K. Joshi et al. / Journal of Luminescence 158 (2015) 412–416414

(QSþ þ) where both the decay components show a decreasealthough the decrease is rather small in case of the shortercomponent [19] and a rise time is observed for longer emissionwavelengths. In Qdþ þ , the shorter component does not change withincrease in halide concentration whereas the longer component

decreases considerably. In earlier works [20], it was found that Qdþ þ

showed two decay components in water at the shorter wavelengthside whereas an exponential decay was observed for longer emissionwavelengths. The results were explained on the basis of continuousmodel for solvent reorientation relaxation and a two state model wasfound to be inadequate [20].

However, the present study reveals that the presence of twodecay components should come from the emission from twoconfigurations. This can be explained by observed a small shiftin excitation spectra when monitored across the emission profileand a double exponential decay for shorter wavelengths. Thedifference in the photophysical behavior when compared toquinine sulfate cation (QSþ þ) may be due to structural differencein these two compounds as discussed earlier [8,28–30]. In earlierworks, in case of quinine dication, an intramolecular bond hasbeen suggested which, however, is absent in quinidine [8]. Briefly,difference in the quenching behavior in Qdþ þ when compared toQSþ þ can be attributed to their difference in structures. A largegeometrical change can result in such behavior [31]. Clustering ofwater molecule around the probe can result in large geometricalchanges and hence may reflect in the observed behavior [32].

Further, a plot between τ0/τ versus [Cl�] is shown Fig. 6. In caseof both ions (Cl� and Br�) the value of slope obtained by eitherS–V plot for intensity ratio or from S–V plot for decay time ratio isalmost the same (Figs. 3 and 5). It implies that quenching behaviorof Qdþ þdue to either by Cl� or Br� follows Eq. (2); hence thequenching is dynamic. Dynamic quenching constants (KSV) deter-mined from Fig. 6 are 67 M�1, and value of kq is 3.2�109 M�1 s�1

for Cl� (Table 3).Further, a plot of I0/I versus [I�] is shown in Fig. 4 which

exhibits a positive deviation. Fig. 4 suggests that the quenching isnot purely dynamic and hence it may be due to the simultaneouspresence of dynamic and static quenching, which can be attributedto the ground-state complex formation [33,34]. To explore thequenching behavior, experimental data for intensity are fitted toEq. (1), and using least-square fit procedure, KSV and Kg have beendetermined. Fig. 7 shows the plot of (I0/I�1)/[Q] versus [Q], whichis straight line with intercept (KSVþKg) and slope (KSVKg). This plotis linear with correlation coefficient nearly equal to unity. The

Fig. 6. Plot of (τo /τ) versus [Q] for Cl� .

Table 1Decay parameters of qunidine in 1N–H2SO4 (10�4 M). (λex¼355 nm, λem¼450 nm.)

Quencher [Q] M τ1 (ns) aα1 τ2 (ns) aα2bχ2 (τ0/τ)1 (τ0/τ)2

Cl� 0 5.3 3.2 20.8 96.8 1.02 1.00 1.001�10�3 5.2 4.3 19.4 95.7 1.12 1.01 1.072�10�3 5.2 5.4 18.2 94.6 1.10 1.01 1.144�10�3 5.3 6.3 16.4 93.7 1.13 1.00 1.276�10�3 5.3 7.1 14.9 92.9 1.08 1.00 1.408�10�3 5.3 7.8 13.5 92.2 1.11 1.00 1.531�10�2 5.3 8.3 12.3 91.7 1.04 1.00 1.68

Br� 0 5.3 3.2 20.8 97.8 1.02 1.00 1.001�10�3 5.3 4.6 18.3 95.4 1.11 1.00 1.145�10�3 5.3 6.1 13.1 93.9 1.13 1.00 1.601�10�2 5.4 7.2 9.7 92.8 1.07 1.00 2.155�10�2 5.3 8.2 3.1 91.8 1.16 1.00 7.121�10�1 5.3 9.4 1.6 90.6 1.15 1.01 13.75

I� 0 5.3 3.2 20.8 96.2 1.02 1.00 1.001�10�3 5.3 5.4 18.3 94.6 1.18 1.00 1.145�10�3 5.3 6.8 12.3 93.2 1.07 1.00 1.701�10�2 5.4 8.3 8.7 91.7 1.13 1.00 2.415�10�2 5.3 9.6 2.6 90.4 1.02 1.00 8.131�10�1 5.3 10.6 1.4 89.4 1.14 1.01 14.50

a α are relative amplitudes.b χ are for two exponential fits respectively.

Table 2Decay parameters for various emission wavelengths in the presence of Cl�

(λe¼355 nm).

[Cl�] M λem¼390 nm λem¼450 nm λem¼500 nm

τ1 (α1) τ2 (α2) τ1 (α1) τ2 (α2) τ(α)

0 3.2 19.1 5.3 20.8 20.6 (100)(6.4) (93.7) (3.2) (96.8)

4�10�3 3.2 16.0 5.3 17.0 17.0 (100)(7.2) (92.8) (3.60) (96.4)

6�10�3 3.1 14.5 5.3 14.7 14.6 (100)(8.6) (91.4) (5.1) (94.9)

8�10�3 3.2 13.2 5.3 13.5 13.5 (100)(9.5) (90.5) (6.2) (93.8)

α are relative amplitudes.Decay times (τ) are in ns.

Table 3Quenching parameters determined from Eq. (2).

[Q] KSV (M�1) kq (M�1 s�1)

Cl� 67 3.2�109

Br� 129 6.2�109

Fig. 7. Plot of [(Io/I)�1]/[Q] versus [Q] for I� .

N.K. Joshi et al. / Journal of Luminescence 158 (2015) 412–416 415

value of KSV is determined from the decay data (Eq. (2)) and afterputting the values of KSV, Kg is calculated and are given in Table 4.

The likely mechanism for quenching should be an electrontransfer process [35–37] depending on the ionic strength [36].Quenching of fluorescence through the electron transfer processhas been suggested for similar systems [38–40].

4. Conclusions

Summarizing fluorescence quenching of quinidine in acidifiedaqueous solution in the presence of halide ions has been studied.Various quenching parameters have been estimated in order tounderstand the nature of the possible quenching mechanism. Emis-sion from various conformers is suggested for the bi-exponentialdecay behavior.

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Table 4Quenching parameters determined from Eq. (1).

[Q] KSV (M�1) kq (M�1 s�1) Kg (M�1)

I� 122.25 5.82�109 43.10

N.K. Joshi et al. / Journal of Luminescence 158 (2015) 412–416416