Luminol fluorescence quenching by triethyl amine and non-linear nopr. 38A...¢  2016. 7. 20.¢  luminol

  • View

  • Download

Embed Size (px)

Text of Luminol fluorescence quenching by triethyl amine and non-linear nopr. 38A...¢  2016. 7....

  • Indian Journal of Chemistry Vo1.38A, August 1 999, pp. 760-767

    Luminol fluorescence quenching by triethyl amine and non-linear Stern-Volmer

    plot : Solvent effect

    o Guha, S Mitra, R Das & S Mukherjee* Department of Physical Chemistry,

    Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700 032, India

    Received 12 March 1 999; revised 21 May 1 999

    The interaction of 3-aminophthalhydrazide (Iuminol) with a quencher, triethyl amine (TEA) has been studied employing steady-state and nanosecond time resolved emission spectroscopy. The bimolecular rate constant for fluorescence quenching in water is found to be about two orders of magnitude higher than that obtained in 1 ,4- dioxane (DIO). The Stern-Volmer (S-V) plot for fluorescence quenching exhibits positive deviation from linearity when water is used as solvent. On the other hand, the S-V plot shows negative deviation from linearity when DIO is used as a solvent. The rate constants obtained in ethanol, dimethyl sulphoxide (DMSO) and DIO are found to be similar to the rate constant for diffusional process. The possible interpretation of the quenching mechanism and nature of deviation from S-V plots have been discussed in relation to the strength of interaction between the colliding species. It is shown that the rate of luminol fluorescence quenching is dependent on the nature of the solvent, being particularly fast in aqueous medium.

    It is wel l-known that certain quenching reactions lead to curved Stern-Volmer (S-V) plots. Both positive and negative curvatures from l inearity have been observed and a number of explanations for this have been proposed 1-4,30-35 . A lthough a number of studies on fluorescence quenching have been reported, very few studies have been made on the variation of curvature of the S-V plot and quenching rate constant (kq) with the nature of the solvent. The effect of solvent on the rate of bimolecular quenching and photoprocesses involving excited molecule has received l ittle attention so far.

    In some of the works on quenching reaction it is shown that polarity of the solvent medium can be expected to play an important role in the mechanism of fluorescence quench ing reaction5•6. Recently, it has been shown that the rate of fluorescence quenching of aromatic hydrocarbons by olefins is dependent upon the solvent, being particularly fast in aqueous medium. It is suggested that th is type of solvent effect is due to solvophobic interactions or stabi l ization of charge transfer interaction, and it is generally associated with the occurrence of polar structures along the reaction coord inates7-9 . Recently, in the case of dimethoxynaphthalene fluorophore1

    0 , it has been

    shown that the value of quenching rate constant (kq) is dependent upon the nature of the solvent.

    Reactions i nvolving luminol participation are widely used for analytical purposes and also for studying the mechanism of transformation of chemical bond energy into electronic excitation energy of molecules. For the last few years, we are engaged in studying the excited state photophysics of

    luminol in different solvent medial l , 12 . In this paper we report our observations on the interaction of luminol with triethyl amine (TEA). It is shown that the quenching rate constants are mainly dependent on the solvent and the process is considerably faster in aqueous medium. A study has been initiated here involving quenching of luminol fluorescence by TEA to show the solvent-dependent deviation from Stern­ Volmer (S-V) plots.

    Materials and Methods

    The sample of luminol (98%) was obtained from Fluka AG and used as received. Luminol is sparingly soluble in pure water (- 1 0-5 M), and the concentration of luminol was maintained at that l imit. Triethyl amine (TEA, from E. Merck) was used as received. Solvent ethanol, dimethyl sulphoxide (OM SO) and I , 4-dioxane (DIO) (spectroscopic grade from E. Merck) were dried and disti l led before use. Triply disti l led water was used for solution preparation .

  • Ie)




    1.0 (a)


    J U

    , <

    t '.4


    440 om A

    - !


    J G



    510 480 440 DID A

    2 3 4 l/(TEA] x lOs mol dm-l

    76 1

    Fig. I-Absorption (a) and emission (b : water, c : dioxane) spectra for luminol-TEA interaction. [Iuminol] = 4.5 x 1 O-5M; range of [TEA](a : 0 � 6) = 0 - 3 .8 x l O·sM; (b : 0 � 9) = 0- 4.6 x l O·sM ; (c : 0 � 7) = 0 - 3 .0 x 1 0· IM ; (d) plot of I /(Ac-Ar) vs. I /[TEA] where Ar = absorbance of free donor (luminol) ,Ab = absorbance of the hydrogen bonded complex, Ac = absorbance of a solution of donor where c is the concentration of TEA. '0' stands for the corresponding number of the curve where [TEA]=O.


    The electronic absorption spectra were recorded on a JASeO (Model 7850) UV NIS spectrophotometer. Fluorescence emission spectra were recorded on a Perkin-Elmer MPF 44B fluorimeter. Fluorescence l ifetimes were measured with a SP-70 nanosecond spectrometer (Appl ied Photophysics Ltd. ,UK) using a pulsed n itrogen lamp based on the time-correlated single photon counting technique as described earlier' 3, ' 5 . Different solutions were prepared keeping the luminol concentration fixed in each case and varying the quencher [TEA],The temperature was maintained at 300K throughout.

    Results and Discussion

    The absorption spectra of luminol show two bands, one at 300 nm and another at 360 nm in al l the solvents used, whereas the fluorescence spectra showed bands at 430, 4 1 5, 405 and 390 nm in water, ethanol, DMSO and DIO, respectively as reported earlier" , 1 2 . When the concentration of TEA was increased gradual ly, the emission intensity decreased significantly as a consequence of luminol fluorescence quenching without any appreciable change in posjtion and shape of the emission band (Fig, I ) . It was also observed that by the gradual addition of TEA, the intens ity of the absorption band

    increased gradually with a small red shift (- 2.3 nm ) (Fig, I ) and then tended to reach a constant value above a certain concentration of TEA (5 .5 x 1 0-5M), This indicates that intermolecular hydrogen bond formation is almost complete at such concentrations of TEA with the formation of a I : I complex between luminol and TEA molecules, Since water comprises bulk of the solution in which the relative concentration of luminol is low, the effect due to change of polarity, without hydrogen bonding

    4 6 8 10 [TEA] • 105 mol/dm3

    1 2 14

    Fig.2-Stern-Volmer (S-V) plot for luminol fluorescence quenching in aqueous medium. The broken line corresponds to the requirements for the S-V plots.


    2.6 � -e r- S ..

    -e .. 1.8

    0.8 1.6 [ TEA ) x to-l nio) I dm3

    Fig.3-Stern-Volmer(S-V) plots tor luminol fluorescence quenching in DIO (e) and DIO:water (2:8) mixture [foil (�), toft (0») . The broken line corresponds to the requirements for the S-V law.



    TEA - I cX:1


    � I I ! - - - TEA o o

    3-arninophthalate 3-aminophthalbydrazide (nonfiuorescent)

    (fluorescent in presence of TEA) I (lwninol)


    Table I -Lifetime (tr ) and bimolecular rate constants (kq, kq') in different solvents for the luminol fluorescence

    quenching by TEA

    Solvent tr. kq, (kq)* Range of (ns) (MI S· I ) [TEA], (M)

    Water 1 0.4 2.4x 1 0 1 2 1 .0-4.9x I O's

    010 1 . 8 3 .8x 1 09 0.28- 1 . 7x I 0· 1 (4.0x I 09)

    OM SO 2.4 6.5 x 1 09 0.9- 1 .2x 1 0.2

    (3 .2x 1 09) Ethanol 5 .5 3 . 5 x 1 0

    1O 0.2-5 .2x 1 0.2

    (2 .2x I 09) Ethanol: Water 7.2 9.0x 1 01 1 2.2-8.8x 1 0.5 ( 1 : 1 )

    *Values in the parentheses . are the kq' values obtained from l ifetime measurements.

    interaction wi l l not be of any great significan'ce and the observed spectral changes should presull)ably be attributed to i ntermolecular hydrogen bonding. It can also be said that i ntermolecular hydrogen bonding interaction is almost complete in the l imit of TEA concentration used here (� 1 0.5 M). From the change in absorption spectra, the ground state equil ibrium constant (Kg ) for hydrogen bond formation has been evaluated by Ketelaar's methodl6 using the fol lowing equation

    .( I )

    Table 2-Lifetime (tr), quenching rate constant (kq ) and effective TEA concentration in luminol fluorescence in DIO/water solvent

    Range of [TEA], M 0 .41 -3. l x I 0· 1 0.62-2. 1 x I 0. 1 0.95- 1 .5x 1 0.3 1 .00-2.8>< 1 0.3 3 .90-6.3x I 0-4 O.56-2.8x 1 0.4

    mixtures tr, kg,

    (ns) (M ·I S· I ) 1 .6 4.5x 1 09 2. 1 7.2x 1 09 3 .8 8 .5x 1 09 4 .6 9.2x l OI I 7 .2 9.8x 1 0 1 1 8 .5 I . I x l 01 2

    DIO:Water (v/v) 9 : 1 8 :2 6:4 5 :5 2 :8 1 :9

    where Ar and Ab are absorbances of the free donor (luminol) and the hydrogen-bonded complex, respectively and Ac is absorbance of a solution with donor where c is the concentration of TEA. On extrapolation of the straight l ine obtained from plot (Fig. I d) of I /(Ac -Ar ) vs. I /[TEA] to a point where I /(Ac -Ar ) = 0, Kg is obtained as Kg = - I I[TEA] . The value of Kg obtaine