Indian Journal of ChemistryVol. 33A, August 1994, pp. 722-726

Photooxidation of diphenylamine in micellar media

P K Gupta & N R Dahrniwal"Depart ment of Chemistry, Punjabi University, Patiala 147002

Received 4 October 1993; revised and accepted 24 March 1994

Photosensitized oxidation of diphenylamine (OPA) with methylene blue as sensitizer in sodiumdodecyl sulphate, cetyltrimethyl ammonium bromide and Brij-35 micellar solutions in aqueous alco-holic media gives N-phenyl-p-benzoquinoneimine as the product. The initial decrease in rate in thepresence of surfactants has been explained through the deactivation of singlet dye resulting in lowerconcentration of the reactive triplet dye. A mechanism suggesting the involvement of 102 and elec-tron transfer pathways is suggested in the presence of anionic, cationic and neutral surfactants. Theeffect of singlet oxygen quenchers has been studied at low substrate concentration.

The competition between singlet oxygen (102) andelectron transfer reactions in photosensitized ox-idations has been reported to be affected not onlyby the nature and concentration of the reactants 1-3

but also by the nature of solvent'> in homogene-ous media. The sensitized photooxidations involv-ing porphyrine, methylene blue, flourescein, chlor-ophyll and rose bengal as sensitizers have beenshown to proceed with the involvement of 102 inmicellar media'"!", Micelles can change the singletoxygen production and reactivityll-15 and it hasbeen reported that photooxidation proceeds bothby electron transfer and 102 mechanisms in thepresence of sodium dodecyl sulphate (SDS) andcetylpyridinium chloride where both substrate andsolvent play an important role in differentiatingthe two mechanisms":". The fluorescence intensityof the dyes is affected because of the effect ofsurfactants on their absorption/fluorescencespectra 17. The fluorescence intensity of rose ben-gal and eosin has been observed to first decreaseand then increase when more surfactant isadded 10. The effect of absorption characteristicsin the premicellar region has been reported to bedue to dye rich premicellar aggregates". Bothtriplet absorptionl'' and reactivity'? are affected bysuch an interaction.

In the present investigation we have examinedthe effect of sodium dodecyl sulphate (SDS), ce-tyltrimethyl ammonium bromide (CTAB) andBrij-35 (Bj-35) (which are anionic, cationic andneutral in nature respectively) on methylene blue(MB) sensitized photooxidation of diphenylamine(DPA) in order to establish the role of 102 insuch systems.

Materials and MethodsDiphenylamine (DPA) and 1,4-diazabicyc-

10[2:2:2]octane (DABCO) were repeatedly crystal-lized from ethanol and benzene/petroleum etherrespectively. Triethylamine (Glaxo, Qualigen),SDS (Glaxo, GR), CTAB (Loba Chemie) andBrij-35 (Loba Chemie) were used as- received.Ethanol (Bengal Chemicals) was of absolutegrade. Other solvents were of high purity gradeand were distilled before use.

Photolysis was done in a rectangular pyrexglass cell (1 em x 1 em x 4 cm) with a high pres-sure mercury lamp (250 watt, Philips) with a stab-ilized power supply. The incident light was colli-mated and filtered through a 470 om optically flatglass cut off filter placed between the collimatorand the photolysis cell.

Shimadzu (model UV-190) spectrophotometerwas used for monitoring purposes where ..thephotolysis cell was directly placed in the spectro-photometer housing at known times and absor-bance measured at 450 om where the productshowed maximum absorbance. The cell was againplaced in front of light source after the absorb-ance measurements.

Results and DiscussionMethylene blue sensitized photooxidation of di-

phenylamine (DPA) in SDS micellar medium inwater-methanol gives N-phenyl-p-benzoquinone-imine (PBQ) as the only primary product as report-ed elsewhere'', The same product has been identi=fled in the presence of CTAB and Bj-35 micellesboth in water-methanol and water-ethanol medium.

GUPTA et al.: PHOTOOXIDATION OF DIPHENYLAMINE 723

The change of medium from methanol to ethanoldoes not have any effect on the nature of the pro-duct in the SDS micellar system as well. The ex-tent of the reaction was monitored spectrophoto-metrically at 450 nm, the A.max of PBQ, in the visi-ble region where DPA is transparent. The linearplots of absorbance versus irradiation time in thepresence of varying amounts of surfactants, 102

quenchers, varying concentrations of DPA anddifferent solvents etc. reveal PBQ to be a primaryproduct. The primary nature of PBQ was furtherconfirmed by the similarity in the general shapeof the visible spectra of PBQ and reaction mix-tures photolysed. for different times in all thecases mentioned above.

Addition of surfactants strongly influences theextent of the reaction. The effect of varying con-centrations of SDS and CTAB was examined andthe results are given in Figs 1 and 2 where therelative reaction rate, (R), is plotted against sur-factant concentration. The rate first decreases andthen increases after showing a minimum with in-crease in SDS and crAB concentrations. It be-comes practically constant at high concentrationsof the surfactants. Profound changes in the ab-sorption spectra of dyes have been observed inthe presence of surfactants 17 and the fluorescenceintensity of dye has been reported to decreaseand then increase with the addition of surfactantwhich finally reaches a plateau. In order to ex-plain the present results a reaction mechanismshowing the participation of 102 and electrontransfer needs consideration and such a tentativemechanism, IS given in Scheme 1.

The surfactants decrease the concentration ofexcited singlet MB when added in small amountsresulting in decrease in triplet MB concentrationwhich is a reactive intermediate for the formationof the product. The singlet lifetime decreases withthe increase in surfactant concentration in the mi-cellar region and aggregation may account forsuch a decrease. Hydrophobic ion pairs formed in

Scheme-l

12

8

°O~--'---~2-~I-- ••..i..-< ~6-~I--l.L2 --,...i.s.J[SDSJX'O~M

Fig. 1 - Effect of varying [SOS] on extent of reaction. R is therelative reation rate. (OPAj = 2.4 X 10-4 M; [MB] = 7.5 x 10- 5

M; Solvent =water-methanol (4: l).

10

....oxCl'

6

~~---L ~ L-__~ ~o 5 10

[CTAB]XlO~M

Fig. 2 - Effect of varying iCTABJ on extent of reaction.[OPAj=2.4 x 10 - 4 M; (MB]- 7.5 x 10 - 5 M, Solvent =water-

methanol (4: 1).

the premicellar region is surrounded by the hy-drocarbon chains of the surfactants when theirconcentration is increased further and micellesare formed. The micellization effect increases theconcentration of triplet MB and the quantumyield of PBQ formation increases when surfactantconcentration is increased further and plateau isobserved both in SDS and CTAB micellar media.

724 INDIAN J CHEM. SEe. A, AUGUST 1994

Although the decay rate (kd) of triplet MB decre-ases and energy transfer (k1) from the sensitizerincreases in micellar solution?", the initial dec-rease in the rate could be solely attributed to thedecrease in the singlet life time of MB in premi-cellar region. The absorption characteristics ofMB in micellar media may not significantly con-tribute towards the rate enhancement/decrease asall the incident radiations are essentially absorbedby MB. The [MB] effect shows that the quantumyield increases with increase in [MB] and itreaches a plateau. The [MB] used here corre-sponds to the plateau region. The decreased de-cay rate of triplet MB (~) increases the rate ofPBQ formation through step kJ followed by Is inthe presence of surfactant concentration at/be-yond their critical micelle concentrations. Thereaction is also expected to be enhanced if it goesthrough step k4• Dye molecules are present bothwithin and outside the micelles. Singlet oxygengenerated through step k, in the bulk phase reactsimmediately with DPA to give the product. How-ever, '02 generated in the micelle may diffuse outto react with DPA or enter into another micellecontaining DPA and react with it to give the pro-duct. When MB, DPA and oxygen are enclosedtogether inside the same micelle, '02 generated inthe micelle at once reacts with DPA and shouldshow a manifold increase in the rate because both102 and DPA are present in close proximity. Thepresent results show about three-fold increase inrate for thirty-fold increase in [SOSj which showsthat the presence of MB, OPA and oxygen in thesame micelle is least probable. It appears, there-fore, that the reaction occurs through formerwhere 102 moves from one micelle to the otheror bulk phase before reacting with DPA whichhas an additional support from the results ob-tained in the chlorophyll sensitized photooxida-tion of 1,3-diphenyl isobenzofuran 7

•

The mechanism of photooxidation is stronglydependent upon the concentration of the substr-ate both in homogeneous'< and heterogeneousmedia'v". In order to sort out the dependence ofrate on substrate concentration the effect of [DPAj

. was examined in the presence of SDS, CTAB andBj-35 surfactants in water-methanol and water-ethanol media and the results are given in Figs 3and 4. The rate of PBO formation first increasesand then decreases after reaching a maximum va-lue with the increase in concentration of DPA inSDS, CTAB and Bj-35 micellar systems both inwater-methanol and water-ethanol media. At low[DPAj the interaction of DPA with triplet MB isan unfavourable process because of the competi-

8

•

Fig. 3 - Effect of varying [DPAI in presence of SOS (0).CTAB (6) and Bj-35 (e). [SOSj=8.0X 10-3 M[CTAB) = 8.0 x 10-3 M; [Bj-35)=0.4% (v/v); [MB)=7.5 x'

10-5 M; Solvent = water-methanol (8:17).

12

....~l!Ca: 6

3

o 2 It 6(OPAl Xl0~M

8

Fig. 4 - Effect of varying [OPAl in presence of SOS (0).CTAB (6) and Bj-35 (e). [SOSj=8.0X 10-3 M;[CTAB)=8.0X 10-3 M; [Bj-35] =0.4% (v/v); [MB]= 7.5 x 10-5 M;

Solvent = water-ethanol (8: 17).

tion between energy transfer process (k1) andelectron transfer-exciplex formation (k4) in whichthe former is very fast in view of it being a diffu-sion controlled process. The steady state treat-ment of Scheme 1 at low [DPAj region where k4and ks are negligible gives Eq. (1) for the rate ofPBO formation:

GUPTA er al.: PHOTOOXIDATION OF DIPHENYLAMINE 725

Equation (1) predicts a flattening if R is plottedagainst [DPAj at constant [02] in the region where[DPAj> > Is/ Is which is not as observed. The de-crease in rate of the product formation at higher[DPAj shows that some other process starts com-peting with k.. Interaction of DPA with tripletMB followed by reaction with oxygen resulting inPBO is less efficient as compared to the otherpathway (102 participation) and more percentageof reaction will go through electron transfer-exci-plex formation resulting in the decrease in rate athigh [DPAj. A linear relationship between R-Iand [DPAj has been observed in water-methanolmedium at low [DPAj as predicted by the inverseof Eq. (1). However, in the presence of Bj-35 mi-celles the plot rs not as good as in SDS andCTAB system possibly because Bj-35 is a neutralsurfactant compared to SDS and CTAB whichare ionic in nature. The reciprocal plots are linearin the presence of all the three surfactants in wa-ter-ethanol medium as well, confirming the valid-ity of Eq. (1) in the low [DPAjregion.

In the region where [DPAj is high, process k4starts competing with k) and the rate dcecreases.Process k, is several times faster than k4 becausethe former involves the energy transfer which isdiffusion controlled and the latter being a less ef-ficient electron transfer process+. At still higher[DPAj the deactivation of singlet MB becomes anobvious additional pathway! and must be consid-ered for the derivation of rate equation. Theother possible pathways like singlet-singlet energytransfer at high [DPAj and interaction of singletMB with [DPAj are least probable because of en-dothermicity of the process (the first excited sing-let state energy of MB is 44.3 kcal and that ofDPA is more than 75 kcaI) and high decay rateconstants of singlet MB ( <= 2.5 X 109 s-)) respect-ively. With the inclusion of step k, and possibilityof deactivation of singlet MB, rate of PBO forma-tion is given by expression (2).

Equation (2) predicts an inverse relation bet-ween ~ate and [DPAJ though not linear and thedecrease in rate with increase in [DPAj is expect-ed which is also observed (Figs 3 and 4).

In order to establish the participation of 102,

the effect of 1,4-diazabicyclo [2:2:2] octane (DAB-CO) and triethylamine (TEA), which are wellknown quenchers of singlet oxygen1.23,was stud-ied. The quantum yield of oxidation decreaseswith increase in quencher (0) concentration.Quenching process (ko) competes with the pro-cess giving products (k3) and radiationless deacti-vation of 102 (Is).

kQ10

2+0 ....•Q+30

2

Inclusion of step ko in Scheme 1 in the low [DPA]region gives the Stern-Volmer equation (3)

... (3)

Where ¢o and ¢ are the quantum yields in theabsence and presence of the quenchers and theirratio will be equal to the ratio of the reactionrates. Figures 5 and 6 give the Stern-Volmer plotswith DABCO and TEA as 102 quenchers in thewater-methanol and water-ethanol media respect-ively. All the plots are linear with intercepts equal

[TE A] X1(I~M8 16o

o

Fig. 5 - Stern-Volmer plots for DABCO and TEA in presenceof SDS (t.); CTAB (0) and Bj-35 (e). [DPA] = 2.4 x 10 -4 M;[MB]=7.5X 10-' M; [SDS]=8.0x 10 • M; [CTABj=8.0X

10-. M;[Bj-35] = O.4%(v Iv) Solvent = watermethanol(2:3)

726 INDIAN J CHEM. SEe. A, AUGUST 1994

o(TEAJX1~M

8

o

":$

o

3

2

$

Fig. 6 - Stern-Volmer plots for DABCO and TEA in presenceof SDS, CTAB and Bj-35. SDS (~); CTAB (0); Bj-35 (e)for DABCO. SDS (A); CTAB (<J); Bj-35 (_) for TEA.[DPA]=2.4x 10°-4 M; [MB]=7.5 X 10-5 M; [SDS]=8.0X 10-4

M; [CTAB] '= 8.0 x 10-4 M; [Bj-35j=0.4% (v/v) Solvent=wa-ter-ethanol (2:3).

to unity which is valid for reactions where 102 isan established intermediate. TEA can quench thetriplet state of the dye as well, in addition toquenching of 102, A part of TEA may be used inquenching the triple MB and remainder can act asa qucncher of 102,

The quenching of triplet MB can have an effecton the quantum yield of 102 formation but therate constant for quenching of triplet MB (!::: 107

MiS - I) by TEA is much less than the diffusioncontrolled energy transfer process k, and at theconcentrations of TEA used here, quenching oftriplet MB seems unreasonable.

The foregoing discussion shows that the MBsensitized photooxidation of OPA in the presenceof anionic, cationic and nonionic surfactants pro-ceeds with the participation of 102 at low [OPA]and electron transfer-exciplex formation at itshigh concentrations. The switch over from 102

mechanism to the other in the case of ionic surfact-ants occurs at lower [OPA] as compared to the

non-ionic Bj-35 as predicted by the appearance ofthe maximum in the rate versus [DPAl plot at re-latively higher [OPA] than in the case of SDS andCTAB. The stabilization of the ionic intermedi-ates by the ionic surfactants may be responsiblefor the reaction to proceed by electron transfermechanisms at relatively lower [DPA] in the pres-ence of ionic surfactants.

AcknowledgementFinancial assistance from the UGC, New Delhi

in the form of teacher fellowship to one of us(PKG) in ~ratefully acknowledged.

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