Machwe, M K

Pandya, M L

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Indian J. Phya. 62B, 186-188 (1978)Effect of solvent on polarization of fluorescence o f eosinM. L. P a n d y a * an d  M. K . M ach w eDepartment of Physion & Astrophysics, Delhi Ufdversity, Delhi 11(KM)7 (Received 2 December 1970, revised 21 M ay 1977)T'Jie polarization o f fliiorescouce of oosin is studied, in different solvents o f varyin j^j dipolo-inoments. The results are interprotod in terms o f dipole-dipole interaction as the extrinsic cause o f  depolarization.1 . IntboduotionPerrin's formula (Perrin 1920) giving a correlation between polarization and viscosity is not able to (‘.x^dain completely tlie effect t)f solvent on polarization, parti(iiilarly wlien tlu^  viscosity is low. The dipolc^-dipoh  ^ interaction between t,b.e solvent and tlu^  solut(  ^ (Macliwe ei al 1970) (^oiild be one sucli contributing factor to explain the phcmonuma. Tlie prx^ scmt study was thereft)re, undertaken to investigate the effect of the dipoK  ^ moment of the solvent on the polarization o f fluoresceuct** o f eosin.2. Expkuim kntal  P rooedurkTl\e ^polarization o f eosin in different solvents (cone. ~ 1 0 '^* g/cc.)has been measured at room tiMiiperature ( ~  20"C) with an Aminco Bowman 8 i)ectro- photofluorometer. For solution in each solvcuit, the (excitation and emission peaks wore located and then polarization was calculated using the method adopted by Azuini ef al (1902). The (dfoct o f scattered light and background for each solvent was measured by itsing pure solvent in the cell. The contribu­tion of this effect to the intensity of fluorescence was found to be less than 0*5 pin cent. The observed fluonvsccnce. intensity was accordingly corrected. The results obtained for polarization along nith peak excitation and emission wave- Iengthsarc given in Table 1. The Table also includes the values of dipole moment and viscosity which are taken from standard tal)Ies (Kayi» <S:. Laby 1966; Hand Book of Physics and (Chemistry).3 . RBSXTIiTS AND DISCUSSION1It is obvious from the observations in Table 1 that a plot between —  andp(Perrin— plot) will not bo linear and lionce tJm observed variation o f polariza-rition can not be aiicoiuitod for as viscosity efhxjt alone. Also as the concentration is low ( ~  g/ic.), the olootromagnotic coupling o f neighbouring molecules* Pormanont addi'ess : Department of Physics, M. M. College, Modinagar (U.P.)*186Fluorescence of eoain 187and it« effect on decrea.se in polarization due to energy mi.stration is .small, Tlu' .solvents in Table 1 are so arranged that aswe go dcmi the Table, the value of the dipole moment decreases. The corresponding column giving the values of also shows a decrease in P as we move down the table, with ilu* only <*xoeption o f acetone. Howwer, it may be noted that the viscosity of acetone is very low and hence low value o f polarization is expectcHi. But comparing the values o f P for acetone and methanol, it is seen that tihe observed polarization is liigher in acetone though its viscosity is lower than jntdhanol Tins being contrary to viscosity effect, establisluss the efhvct of dipoleimmient TJu^  oh.servatioiis.therefore, cleai'ly indicate that tin* ])ercentage polarization is liigher in a solvent o f higher dipole moment.Table 1Solvent Dipol(‘momentViflOOHi ty(nm) (nm)O/ pFormamido :V2.5 .331 .750 570 7-0Aooiont* 2-8S 0-.32 .550 570 2-4Water 2-3 hOO .535 55H 5-0Ethanol 1-70 1-20 .544 .504 4-6-Butanol 1-67 290 .554 570 4-:tMethanol 1-65 0-59 542 564 2 0This effect can be explained as follows ; when a molecule, is excibni, its dipole moment changes. "But diiring the short interval of time in which the absorption <)1 excitation energy takes plaee, the orientation o f the. emission-dipole remairm unchanged. But just before a radiative transition takes place, the solute mole­cules gets itself reoriented in thc^  .surrounding solvent iTUslium due to dipole- dipole interaction to get a new equilihrium position thus resulting in depolarr/a- tion o f fluorescence. The Brownian rotation tends to destroy this equilibrium orientation and causes further depolarization. But the Brownian motion is opposed by the dipole-dipole interaction between solvent and solute mokicii es which for a given solute depends upon the dipole moment o f the solvent. Thus this interaction tries to reduce the depolarization caused by the Brownian rota- which for a given solute depends upon the dipole moment o f the solvent. Thus this interaction tries to reduce the depolarization caused by the Brownian rota- tion. The polarization therefore, is expected to be higher m a solvent with higher dipole moment.188 M. L. Pandya and M. K. Maohwefn addition to a purely mechanical environmental effect o f the above type, tiv>re may be a super imposed structural (effect. The dipole-dipole interaction can cause a distortion in the planer symmetry of the molecule. As the polariza­tion o f fluorescence depends on the orientation o f the emission dipole moment in the molecule, any change in the symmetry o f the molecule will cause a corres­ponding change in polarization. I f  an increase in dipole-dipole interaction tries to align the emission dipole with the plane o f the molecule, the polarization increases. This seems to be so in tlie present case o f eosin. Of course, it is possible to have some structurally diff(*T(mt fluorescent molecule where a reverse effect will be obtaiiu^d.The above study, thert^fore, clearly indicates t)i,e effect o f dipole moment o f the solvent on the polarization of fluorescence.A oknow lbbgmbntsThe authors are thankful to Prof. S. N. Biswas, Head of the Phvsics Depart­ment, Delhi ITniversity, for providing facilities to carry on tJie experimental Avork. One of us (MLP) gratefully acknowledgr^s the Hnamdal assistance of II.G.C,R b f e b b n c b sAzumi T. &. Me Glynn S. P. 1902 J. CJmn. Fhys. 37, 2412.Kftyo G. W. Laby T. H. 1960 Tahlen of Physical and Chemical constanis (Longmans & GreenCo. Ltd., London).Machwe M. K.. Vadhera T.. Pandya M. L., Kishore ,7. & Gopala Krishnan K. 1976 Curr. Sci. Porrin F. J. 1920 Physics Padium 7, 290.

Effect of Solvent on Polarization of Fluorescence of Eosin

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Effect of Solvent on Polarization of Fluorescence of Eosin

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