Santhanam, K S V

Vaz, Francis

English

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Indian J. Phys. 51B, 446-»51 (1977)Mixed system electrochemilumincscencc involving a nucleoside or a nucleotideF r a n c is  V a/, and K. S. V . S a n t i i a n a mChemical Physics Group, Taki Institute o f Fimdumental Research, Bom bayElccLrochcmilumincscence has been generated in a mixed system through the electron transfer reaction involving riboflavin or flavinmt^nonucleotidc anion and a cation of an aromatic hydro­carbon. In each case an emission from the excited singlet statd of the hydrocarbon is observed. Triplet states appear to be involved in the electron transfer reaction and it has been supported by the observation of magnetic field efteot on emission intensity.Previous reports of clcctrochemilumincscence (E C L ) have demonsitraled the formation of excited slates by radical ion annihilation reactions in several mixed systems! (1-6). W e report here the first example of EC L of a mixed system involving a micleotide. The EC L has been generated from reactions of riboflavin (R F )  anion or flavin mononucleotide (FM N ) anion with either rubrene cation or 9, 10-diphenylanthracene (D PA ) cation in N, N-dimethyl formamide.It is interesting to note that one of the imfledules (FM N ) studied here plays a key role in the biological electron transport chain and in bacterial bioluminesceince (7-10).The cyclic voltammetric behaviour of a solution containing R F  and rubrcnc and 0.1 M tetra-n-butylammonium perchlorate as supporting electrolyte is shown in Figure 1. The wave L  is identified as due to reduction of R F  to riboflavin anion (R F "') by comparison wil'h the cyclic voltammogram of R F  ( 1 1 ). The reversible couple observed' at wave 11, is due to rubrene-rubrene anion (12) and that at wave is due to rubrene-rubrene cation (12).  Th& peak potentials for the different waves are listed in Table 1.When the potential of the platinum electrode is cycled at 2.5 Hz (corres­ponding to the duration of forward electrolysis of tj. =  0.2 sec. and reverse electrolysis of L =  0.2 Sec.) between =  — 0.60 V and =  +  1.30 V , an orange— red E C L  emisision is observed. The emission spectrum shows amaximum at 565nm and is shown in Figure 2. For comparison, the fluores- ce n ^  spe^ra of riboflavin and rubrene are shown in the same figure. ECL emission, is also observed when the potential swing is adjusted between wave He and wave la but 'the initensity of this emission is stronger by a factor of ten. (1 3 ). We could not observe an FC L emission if the potential excursion was adjusted to wav© and a potential short ol wave I ,, or if the potential of the platinum electrode was cycled between O V and wave 1., (16).I cM ixed system electrochemiluminescence 4 4 7ing electrcxic at a sweep rate o f 13 mv/secAlthough the fluore'sccncc speotra RF" and rubrene overlap in certain regions of respective emissions, it is unlikely that FX L emission occurs from RK since the enthalpy (AH*^) of the radical ion iicaclion is very much lessthan the energy of the triplet state of R F  ( F t — 1-s — ®V)(18 ). For rubrene, the spectroscopic values arc F-r =  1-2 eV and E *  — 2.30 eV and the triplet state of rubrene is easily accessible in the electron transfer reaction.On this basis the most reasonable explanation lor the observation of EQ ,. in the above experiments is the radical ion reaction betiween riboflavin anion and rubrene cation.R  F .-  +  R  ^SR f  ■‘RJR* --------3R + R F  ^  »R *+  R R  +  hv( 1)(2)(3>4 4 8 Francis Vaz and K. S. V. Santhanamwhere R F  =  riboflavin, and R  =  Tubrene<. The free radical R F ."  has pre  ^viously been identified by esr (11) during electrochemical reduction at wave F.X ( nm )Fig. 2. Upper curve : Fluorescence Spectrum in DMF of rubrcnc {—) or riboflavin (— ),ower Curve ; E C L  Spectrum during electrolysis af a solution cemtaimng riboflavin and rubrenc obtained by cycling the potential of the working electrode between — 0.60V to +  1 44 V vs Ag.Experiments were also carried out with anather oxidant DPA+, and the results arc essentially similar. On cycling the potential of the electrode between wave and wave in a solution containing DPA and R F , a blue emission (Xmax =  440 nm) characteristic of the fluorescence of DPA is observed. Thisi negative result is explained on the basis of the non- accessabilily of triplet state of R F  : the values of the spectroscopBC states of R F  and DPA and enthalpy of the reaction (DPA+ +  R F " )  are given in Table 1.hX^L reaction involving a NucleotideThe cyclic voltammetric pattern of a solution containing FM N  and rubrene is similar to Figure 1. The peak potentials of the various waves are given in Table 1. A strong E C L  emission is observed by programming the poten­tial of the platinum electrode between wave and wave T,. The emission spectrum shows maximum at 560 nm and is: attributed to emission from *R *.M ixed system electrochemiluminescence 44^ >Energetically, the triplet state of FMN is not accessible (see Table I ) in the electron transfer reaction.(4 )The E C L  mechanismFMN- +  R» iFMN +8R +  «R iR * 4  Rappears to reasonably explain the above resultsAdditional experiments have been conducted with a mixed system involv­ing DPA and FMN : EC L emission is observed at 440 nni corresponding to the emission from ^DPA.*T ab le 1. Peak poftenitials and enthalpies'Wave 1^ WavL n Wave 1 AH"*>Mixed System E p /1 y \ e VRiboflavin - 0 .6 6 - 1  32 1.20 1.70+  Rubrcnc (1 niM ) ( -0 .8 2 ) ( -  i 48) (1 04)Riboflavin ^ 0  66i - 1  71 1 M 201+  9, lO-diphenyl- ( - 0 8 2 ) ( - 1  87) (1anthracenc (1 m M )Flavin M ono­ - 0 .6 8 - 1 .3 2 1 20 1 72nucleotide -f- ( » 0.82) ( -  1 48) (1 04)Rubrene (Im M )Flavin mono­ - 0  68 -- 1 71 I.M 2.03nucleotide -I- ( - 0 8 2 ) ( - 1  87) (1 35)9, 10-diphenyl anthracene (1 m M )^Potentials are measured with respect to Ag electrode.Values in brackets refer to vs vSEC=  2.84 eV.: ET,(,ik) =  2.06 eV and E ET(a„b.ene) =  ' 2.84 eV: 205 eV,E,(op^,= 3.20 eV and E t(d,>a) ^(b )  - a h * =  ( E ^ ,  -  )  -  0 .16  :Table 2. Magnetic field enhancement of E C L  intieinsity*450 Francis Vaz and K. S. V. SanthanamMag. field (kgauss)Pulse height (volts)%  increase over initial value01.82.63.74.00.280.290.320.360.3;>03.6 14 3 28.628.6* Solution contained riboflavin and rubrenc (Im M ) in N» N— dimoth>l formamide.Potential programming is adjusted to— 0 66 V to -f  1.44 V vs Ag.The existence of triplet state intermediates in ECL mechanism is supported by the effect of a magnetic field on the emission inltensiities (12, 15, 17) Support for the involvement of triplets in the FvCL mechanism of a mixed system involving R F  or FMN is obUiincd from the data given in Table 2 on magnetic field enhancemcmt of emission intensity.The expcTimeints cond'ucled in this study dcmonstratie the intermolecular electron between FM N “, or R F ” (donor) and an acceptor (hydrocarbon cation,) in producing the triplet sitates. The sugar and/or phosphalte present in the donors togjother with an associated solvation do not appear to localise the odd electron which would slow down the electron transfer reaction and thus render difficult the detection of the EC L emission.R e f e r e n c e s1. J. T  Maloy and A. J. Bard J. Am. Chem. Soc. 93, 5968 (1971).2. H. Tachikawa and A. J. Bard Chem. Fhys. Lett. 19, 287 (1973) and referencescontained therein.3 D. J. Freed and L. R. Faulkner J Am. Chem. Soc. 93, 3565 (1971) c Ibid. 94, 4790 (1972).4. A. J. Bard, C. P. Keszthelyi. H. Tachikawa and N. E. Tokel, in Chemiiumine  ^scene and Bioluminescene, Eds. M J. Cormier, D. M. Hercules and J. Lee» Plenum press ; N. Y., 1973.5. N. Periasiamy and K S. V. Santhanam Proc. Ind. Acad. Set. 80A, 194 (1974).6. N. Periasamy and K. S. V. Santhanam Can. J. Chem., 53, 76 (1975).7. D. lE. Green, Compr. Bicx^cm. 14, 309 (1956).8. E. Baldwin Dynamic Aspects aj Biochemistry, 5th Ed., University Press, Cambridge, 1967.Mix€d system electrochemilwninescence 4519. Q. H. Gibson and J W. Hastings Biochem. J. 83, 368 (1%2).10. J. W. Hastings, A. Eberhard, T. C. Baldwin, H. /.. Nicoli. T. W. Cline and K H. Nealsoti, in Chemiluminescence and Biolumincscence, Eds. M. J. Connier, D. M. Hercules and J. Lee, Plenum press, N. Y., 197311. (a) S. Tatwa^^adi, K. S. V. Santhanam and A. J. Bard J hleawanal Chem. 17, 4 H(196«).(b) P. J. Elving, J. E O’Reilly, and C. O. vSchmakel, in Methods of Biochemical Analysis, Vol 21, Ed. D. Click, Intersciencc, Ne^ v York, 1973(c) The electrochemistry of riboflavin in non aqueous dimethyl sulfoxide has been reported (11) ; the reduction of riboflavin at wave gives initially its free radical, which has been shown to decompose slowly (during the time scales ot controlled potential electrolysis of 30 mts.) Since ECL experiments arc conducted at much faster time scales (milliseconds), the postulation of a mechanism involving RF“ is more probable.12. J. (Thang, D. M. Hercules and D. K. Roe E iec tro ih en u ca  A cta  13, 1197 (1%8).13. E C L  of pure rubrenc system has been reported (12, 14, 1,5) previously. The emission in these studies have been observed in thi.s potential pn^ramniing14. R. E. Visco and E. A. Chandross Electrochemica Acta 13, 1187 (l%-8).1,5. N Periasamy, S. J. Shah and K, S. V Santhanam. 7. Chem Phys., 58, 821 (1973).16. Pre-annihilation emission of rubrene pure system has been reported (17) during this potential programme This emission is not obscived in the present studies due to purity of the solvent. Bard et al. (17) have shown purity of the solvent plays a role in pre-annhilation17. H. Tachikawa, L. R. Faulkner ajid A. J Bard J Am. Chem Soc. 94, (i9| (1972).18. W. E. Kurtin, T. A. Moore and P S Song, in Molecular I.umincsccnccc, Ed. E C.Lim, W. A. Benjamin, N Y . 1%9. p 569.

Mixed System Electrochemiluminescence Involving a Nucleoside or a Nucleotide

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Mixed System Electrochemiluminescence Involving a Nucleoside or a Nucleotide

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