Mehata, M S

Tripathi, H B

IACS Institutional Repository

Indian J. P/m.75A(2), 189-192 (2001)U P  Aan international journalPhotophysics of 6-methoxyquinoline in Nafion® polymer matrixM  S M e h a ta *  a n d  H 1Phoiophystcs Laboratory, Depai DSB Campus. Kurnaon University.£>mail . kumaonunivcrsit|T r ip a lh iacnt ot Physics. |inital-26.^ ()()2, India6? vsni.comR e c e iv e d  ^  M a y  2 0 0 0 ,  (k  c c p t e j l  4  A u f i u s i  2 0 0 0Abstract Fluorescence characteristics of 6-methoxyquinohne (6MQ) in Nafion*" polymer have been repoiiccl I he spcctial Icaluics as well as iiansu ni studies show a similarity for 6MQ in fluid media at low temperatures and in Nafion*" matrix at ambient temperature Fxcitation wavelength Icpcmlcncc of the fluorescence alongwith the decay measurements across the emission piofile show the piesencc of vaiioiis conlormcrs in the lorm I monocations that assume different geometries in the polymer matrix and these conformers undergo excited stale charge translei processKeywords ' 6-Methoxyquinoline, Nafion**, EERSP\(\S Nos. . 87 64.Ni, .^1.70 Hq, 3^ 5().DqThe llu o re scc n cc  o f  6 -m c lh o x y q u in o lin e  (6 M Q ) in v a r io u s  p o la r  Milvcni has b een  fo u n d  to  e x h ib it  e d g e  e x c ita tio n  red  sh ift (E E R S ) I ( ii sh ift o f  th e  e m is s io n  p e a k  w h e n  e x c ita t io n  is d o n e  to w a rd s  ihc ted ed g e  o f  th e  f ir s t  a b s o r p tio n  b a n d , w h ic h  is in te rp re te d  as  k in g  due to  .so lven t r e o r ie n ta t io n  r e la x a tio n  in  th e  e x c ite d  s la te  M 2|  V a r io u s  e f f e c t s  ( v i s c o s i t y ,  t h e  s o l v e n t  a n d  i t s  concen tra tion ) h a v e  b e e n  r e p o r te d  e a r l ie r  b y  I lo h  an d  A z u m i I M Ik ey am a  el al. [4 ]  h a v e  s h o w n  th a t in  th e  f lu o re s c e n c e  spectrum o f  6 M Q  in e th a n o l  a t 2 9 4 K , a  t im e - d e p e n d e n t  sp e c tra l ^hili IS ob .scrvcd  w h e n  th e  s o lv e n t  re la x a tio n  t im e  ( r ^ )  is o f  th e  Older o f  f lu o re s c e n c e  li fe t im e .S ch u lm an  etal. [5 ] h a v e  s tu d ie d  th e  p h o to c h e m is try  o f  6 M Q  ii'' a ju n c tio n  o f  p H  a n d  h a v e  s h o w n  th a t  a t a  p H < 4 , o n ly  th e  em ission o f  th e  m o n o c a t io n  w ith  e m is s io n  m a x im u maround 4 4 5  n m  is o b s e rv e d , w h e re a s  a b s o rp tio n  m a x im u m  o f  L   ^b an d s  a re  a t 3 13 n m  a n d  3 3 7  n m  re s p e c tiv e ly . F ro m  a p H  ol 4 to 6 , th e  f lu o re s c e n c e  o f  th e  c a tio n  fa lls  a n d  th e  f lu o re sc e n c e  >^1 the n e u tra l s p e c ie s  a p p e a r s  to  f in a lly  r is e  to  its  m a x im u m  '"iilue at a  p H  o f  1 2. In  a q u e o u s  so lu tio n  (p H  =  7), it w as  su g g e s te d  that 6 M Q  u n d e rg o e s  p ro to n  tr a n s fe r  re a c t io n  in  e x c ite d  s ta le  [2] to  an  in c r e a s e d  b a s ic i ty  o f  6 M Q  o n  e x c i t a t io n .  F o r  an  ‘n icrincd ia te  p H  o f  6  to  10, th e  f lu o re s c e n c e  o f  th e  c a tio n  a n d  tl^  neu tra l m o le c u le  r e m a in  c o n s ta n t . ^orresponding Author, b-mail : msmehata^yahoo.coinF ro m  th e  t e m p e r a tu r e  d e p e n d e n c e  o f  th e  f lu o re s c e n c e  c h a ra c te r is t ic s ,  it w a s  su g g e s te d  th a t at 160K . a  ra p id  c h a rg e  tra n s fe r  (C T ) fro m  th e  m e th o x y  g ro u p  to  th e  q u in o lin e  rin g  tak es  p la c e  fo l lo w e d  b y  a  so lv e n t r e o r ie n ta t io n  re la x a tio n  al a m b ie n t te m p e ra tu re  161. It h a s  b e e n  re p o r te d  re c e n tly  |7 ,  8 ] th a t the  p ro b e  m o le c u le  ( 6 M Q ) m a y  b e  u se d  as  a  c h lo r id e  ion  se n s o r  as w e ll a s  a  p ro b e  m o le c u le  fo r  th e  so lv a tio n  d y n a m ic s  s tu d ie s .In  r ig id  m e d ia , h o w e v e r , th e  d y n a m ic  e q u il ib r iu m  b e tw e e n  th e  d if f e re n t  s o lv a tio n  s ite s  is lo s t, an d  an  e m is s io n  w ill o c c u r  f ro m  a F r a n c k - C o n d o n  e x c i t e d  s la te  c o r r e s p o n d in g  to  th e  so lv a tio n  s ite  sp e c if ic a lly  e x c ite d  |9 J . l l i e  re lax a tio n  ra le  c o n s tan t o f  d if f e re n t  s o lv a tio n  s i te s  a re  d if fe re n t a n d  s m a lle r  th a n  th e  f lu o re s c e n c e  ra te  c o n s ta n t .  T h is  le a d s  to  e x c iia lio n  w a v e le n g th  d e p e n d e n c e  o f  th e  f lu o re s c e n c e  a n d  h e n c e  th e  o b s e rv a tio n  o f  red  e d g e  e f fe c t (R E E ) in  th e  e m is s io n  sp e c tra  in  r ig id  m e d ia  ( lO f T h e  s e c o n d  k in d  o f  re d  e d g e  e x c ita t io n  e f f e c t is o b se rv e d  fo r f le x ib le  m o le c u le s  lik e  th e  te r -b u iy l e s te r  oJ 9 -a n ih ro ic  a c id  (9 T B A ) a n d  te l ra  p h e n y l b u ta d ie n e  (T P B ) in r ig id  m a tr ic e s  [ 1 11, a n d  is  d u e  to  th e  d if fe re n t  c o n fo n r ie r s  o f  v a ry in g  g e o m e tr ie s  w h ic h  a b so rb  d if fe re n t e n e rg ie s . In  flu id  m e d ia , th e se  c o n fo rm e rs  in te rc h a n g e  r a p i d ly ; h o w e v e r , in  r ig id  m e d ia , th e  m o le c u le s  a re  tr a p p e d  in  c e r ta in  g e o m e tr ic  c o n f ig u ra t io n s .  T h is  d e p e n d s  u p o n  th e  n a tu re  (v isco sity , f re e  v o lu m e  a n d  th e  p o la r ity ) o f  th e  p o ly m e r m atrix  [ 1 2 ].02001 lACS190 M S Mehata and H B TripathiIn  t h e  p r e s e n t  p a p e r ,  w e  r e p o r t  t h e  f l u o r e s c e n c e  c h a ra c te r is tic s  o f  6 M Q  in a  N afio n ®  film . T h e  p o ly m e r  re a c ts  a s  a  c a tio n  ( io n )  e x c h a n g e r  113] h a v in g  a  p o la r  h e a d  g ro u p  S O ,H *  a n d  is h ig h ly  a c id ic  in  n a tu re . T h e  s tru c tu re  o f  6 M Q *  m o n o c a tio n  a n d  N aFion®  a re  g iv e n  in  F ig u r e s  1 a n d  2 re s p e c tiv e ly .e m is s io n  m a x im u m  w h e n  th e  e x c ita tio n  w a v e le n g th  is increased f ro m  3 5 0  to  4 0 0  n m  ( re d  e d g e  o f  th e  f i r s t  a b s o rp tio n  b a n d ) sh a p e  a n d  th e  fu ll w id th  a t h a lf  m a x im u m  (F W H M ) o f  the  emissionC H,0Figure 1. Chemical structure of 6-methoxyquinoline monocation(6M0*).. .  . o c  C F , - C F , - S 0 3 H*m  =  5  to  1 3 .5  • * * ®n = 1000k =  1 ,2 ,3 ...(O -C F 3 -C F -C F 3 ),I[ ( C F 3 - C F 3 ) ^ - C F - C F 3]„Figure 2. Chemical structure of Nafion*®6 -M e ih o x y q u in o lin e  (A ld r ic h  C h e m ic a l, U S A ) is  u se d  a n d  is c h e c k e d  fo r  its  p u rity . Nafion®* ~  117 film  (A ld r ic h )  w a s  c le a n e d  by f irs t b o il in g  in  n i t r ic  a c id  a n d  th e n  in  d is ti l le d  w a te r  fo r  a b o u t h a l f  a n  h o u r. 6 M Q  w a s  s e p a r a te ly  lo a d e d  in  th e  N a fio n ®  film  fro m  a q u e o u s  s o lu t io n . T h e  e s t im a te d  c o n c e n t r a t io n  fo r  th e s e  m e a s u re m e n ts  w a s  a b o u t  \0r^ M .T h e  a b s o r p tio n , f lu o r e s c e n c e  a n d  e x c ita t io n  s p e c tra  w e re  re c o r d e d  w ith  J a s c o  V -5 5 0  s p e c t r o p h o to m e te r  a n d  J a s c o  F P - 7 7 7  s p e c t ro f lu o ro m e te r  re s p e c tiv e ly .  F lu o r e s c e n c e  d e c a y  w a s  m e a s u r e d  w i th  E d i n b u r g h  I n s t r u m e n t - 1 9 9  t i m e  d o m a i n  s p e c t ro m e te r  u s in g  th e  t im e - c o r re la te d  s in g le -p h o to n  c o u n tin g  te c h n iq u e . T h e  d e c a y  a n a ly s is  is  d o n e  b y  a v a ila b le  F L A - 9 0 0  s o f tw a re .  T h e  g o o d n e s s  o f  f i t  is  j u d g e d  b y  th e  r e s id u a ls  an d  th e ir  v a lu e .F ig u re  3 sh o w s  a n  a b s o rp tio n  a n d  p e a k  n o rm a liz e d  e x c ita tio n  s p e c tra  o f  6 -m e lh o x y q u in o ! in e  in  N afio n ®  f ilm . T h e  lo w  e n e rg y  a b s o r p tio n  b a n d  (3 4 1  n m )  in  th is  m a tr ix  is  s l ig h tly  re d  sh if te d  a s  c o m p a r e d  to  th e  b a n d  in  s o lu t io n  (3 3 7  n m ) . H o w e v e r , th e  b a n d  a t  3 1 3  n m  {LJ  r e m a in s  s a m e . T h e  e x c i ta t io n  s p e c tru m  is re d  s h if te d  w ith  r e s p e c t  to  th e  a b s o r p t io n  sp e c tru m  a n d  a ls o  e x h i b i t  s u c c e s s i v e  r e d  s h i f t s  c o r r e s p o n d i n g  to  l o n g e r  w a v e le n g th s  o f  th e  e m is s io n  p ro f i le .F ig u re  4  s h o w s  s o m e  p e a k -n o r m a l iz e d  f lu o re s c e n c e  s p e c t r a  f o r  d i f f e r e n t  e x c ita t io n  w a v e le n g th s  o f  6 M Q  in  N afio n ®  f ilm . A  re d  s h if t  f ro m  4 3 1  to  4 4 8  n m  ( -  8 8 0  c m “ *) is  o b s e r v e d  in  th eFigure 3. Absorption spectrum (1) and excitation spectra ot 6M(,) m Nafion® film for A,.„= 400 nm (2), = 430 nm (3) and A,„,(4).Figure 4. Excitation wavelength dependence of some peak-normal emission spectra of 6MQ in Nation® film at 296K for A ,^ = x „  = 360 nm (2), A„ = 370 nm (3). 380 nm (4), A„ = 390 nm (“iiand = 400 nm (6).,pctira rem ain  th e  s a m e  th r o u g h o u t th e  ra n g e  o f  e x c ita tio n  wavelength (/.e. 3 5 0 -4 0 0  n m ).The o b se rv ed  s p e c tra  ( a b s o rp tio n  a n d  e m is s io n )  w ith  th e  polymer m atrix  r e s e m b le  th e  s o lu tio n  s p e c tra  fo r  a  p H  o f  2 , an d  Ihe slight sh ift in b a n d  p o s i tio n  m a y  b e  d u e  to  th e  h ig h  v is c o s ity  „i the N afion '’^  m a tr ix  re la t iv e  to  so lu tio n  p h a se . T h e  sp e c tra l leaiuies in d ica te  th e  p re s e n c e  o f  m o n o c a tio n  s p e c ie s  ( 6 M Q +)11 oj in Nafion® m atrix .Tlie o b se rv ed  b e h a v io u r  can  b e  e x p la in e d  in  th e  fo l lo w in g  nianner. In N afion®  p o ly m e r  m a trix , th e  6 M Q  m o n o c a tio n  fo rm s iliKerent g e o m e tr ic  c o n fo rm e n s . E a c h  c o n fo rm e r  h a s  its  o w n  iiiiission sp e c tru m . A ll th e  c o n fo rm e rs  o f  6 M Q +  m o n o c a tio n  a t  room tem perature  sh o w s  e m iss io n  fro m  lo ca lly  ex c ited  s ta te  (L E ) jK well as c h a rg e  tr a n s fe r  s ta te  (C T ) . H o w e v e r , d e p e n d in g  on  ihc g e o m e try , th e  e m i t t in g  s ta te  is  d i f f e r e n t  fo r  d i f f e r e n t  conlonncrs. O n  e x c ita tio n  b y  re d  e d g e  o f  th e  a b so rp tio n  b a n d , only those m o lecu les  a re  ex c ited  w h ich  h av e  p ro p e r c o n fig u ra tio n  corresponding to  th e  e x c ita tio n  e n e rg y  a n d  h e n c e  th e  e m iss io n  lot those c o n fo rm e rs  w ill b e  re d  sh if te d  re la tiv e  to  th e  em i.ssion  l»i short w a v e le n g th  e x c ita tio n . S im ila r i ty  o f  th e  sh a p e  a n d  th e  W H M  o f  th e  e m is s io n  s p e c tra  fo r  sh o r t a n d  lo n g  w a v e le n g th  I’u 'ita tions, sh o w s  th a t th e  d e -e x c i ta t io n  p a th w a y s  a re  s im ila r  lor all the c o n fo rm e rs  a l th o u g h  th e  ra te  m a y  b e  d if fe re n t .ITie life tim e d a ta  o f  6 M Q  in  N afion®  film  a re  g iv en  in T abic 1 . l ioni the tab le  it m a y  b e  .seen th a t th e  d e c a y  d a ta  fit w ith  a  double e x p o n e n tia l fu n c tio n  th ro u g h o u t th e  e m is s io n  p ro f ile s .\  sing le  e x p o n e n t ia l  d e c a y  f u n c t io n  ta i l s  to  d e s c r ib e  th e  observed d e c a y  as  e v id e n c e d  b y  Ihe la rg e r  -v a lu e , la rg e  n o n - landom re s id u a ls  a n d  a  c o n s e q u e n t ly  la rg e  s ta n d a rd  d e v ia t io n  (biguic 5). T h ro u g h o u t th e  e m is s io n  w a v e le n g th s  p ro b e d , th e  deuiy lo llo w s  th e  fu n c tio nPhotophysics o f  6-methoxyquinoline in Nafion'^ polym er matrixamplitude a ,  corresponding to t, decreases while ttj increases with increasing emission wavelength, r r191Time /10*^ s100Chi Sq = 7 120/{A,/) = a ,  expj — - + «2 exp1 /t^ 2where / ( A )  is  th e  in te n s i ty  a t th e  e m is s io n  w a v e le n g th  A» and ly  a re  th e  tw o  f lu o re sc e n c e  l ife tim e s  a n d  a ,  a n d  a re  th e  decay^associated p a ra m e te r s  re sp e c tiv e ly . It m ay  b e  n o te d  th a t die H uo resccn ce  l i fe t im e s  r ,  a n d  T2 b o th  in c re a se  w h en  th e  decay c u rv e s  a rc  c o lle c te d  a t lo n g e r  e m is s io n  w a v e le n g th s . T h e*able 1. Lifetime parameters of 6 MQ in Nafion* matrix at 296K  ^360 nm).Inm) r ,  (ns) r ,  (ns) X'-190 6 .S9 (0.17) 18.37 (0.13) 43.72 56.28 1.07410 8.25 (0.13) 21.99 (0.22) 42,66 57.40 1.09430 9.58 (0.18) 24.58 (0.27) 42.10 57.90 1.08450 10.53 (0.18) 26.41 (0.26) 39.61 60.39 1.10470 10.69 (0.19) 26.94 (0.25) 38.15 61.85 1.13500 10.78 (0.21) 27.41 (0.24) 34.33 65.67 1.12Figure 5. (a) Two component <ii with two llumcsccncc lifetimes foi the fluorescence decay of 6MQ in Narion*^  film at 360 nm , = 450nm The residuals and the -values an- lor (b) the single exponential til and (c) the two exponential fitT h e  r e s u l t  ( E E R S )  c a n  b e  e x p la in e d , w ith  th e  h e lp  o f  a  b r o a d  d i s t r i b u t i o n  f u n c t i o n  m o d e l  .s im i la r  to  4 - d im e th y la m in o b c n / .o n itr i le  (D M A B N ) 114), r e p re s e n tin g  v a r io u s  c o n fo rm e rs , w h ic h  are  n o t in te r-c o n v e rtib le  d u rin g  the  life tim e  o f  th e  so lu te . T h e re fo re  th e  m u lti-e x p o n e n tia l d ecay  can  b e  u n d e rs to o d  in  te rm s  o f  a c tiv a te d  p ro c e s se s  d u e  to  v a r io u s  tra p p e d  g e o m e tr ie s  o f  th e  m o le c u le s . In  a  b ro ad  d is tr ib u tio n  m o d e l, th e  m u lti-c o m p o n e n t d e c a y  c u rv e  c a n  b e  f itted  w ith  tw o  o r  th re e  c o m p o n e n t (e x p o n e n tia l)  m o d e l. It is in te re s tin g  to  n o te  th a t th e  d e c a y  p a ra m e te rs  fo r  6 M Q  in  N afion®  m a tr ix  a t ro o m  te m p e ra tu re  h a v e  a  c lo s e  re s e m b la n c e  w ith  th e  d a ta  fo r  6 M Q  in IN H 2S O 4  ( g ly c e ro l; w a te r ) a t  1 6 0 K [6 ]. S im ila r  to  g la ssy  m ed ia , th e  tw o  d e c a y  c o m p o n e n ts  fo r  6 M Q  in  N afion®  re s u lt  fro m  the  n o rm a l s p e c ie s  a n d  c h a rg e  t r a n s f e r  s p e c ie s .  H o w e v e r , th e  in c re a se  in  th e  d e c a y  tim e  a c ro s s  th e  e m is s io n  p ro f ile  m ay  b e  d u e  to  d if fe re n t g e o m e tr ie s  o f  th e  so lu te . T h e  m o le c u le s  trap p ed  in  d if fe re n t g e o m e tr ie s  h a v e  d if fe re n t f lu o re sc e n c e  life tim e s  fo r  n o rm a l a n d  c h a rg e  tr a n s fe r  e m is s io n s .192 M S Mehata and H B TripathiFrom above study, it is apparent that in the polymer Nafion® matrix, 6M Q exhibit em ission due to monocation which are in different geometric configurations and exhibit Red Edge Effect (REE) in emission. The conformers on excitation undergo a slow charge transfer from methoxy group to quinoline ring (activated processes) and thus the em ission shows a biexponential decay corresponding to the normal and the charge transferred species. However, different conform ations have different lifetimes for nomial and charge transfer emissions as exhibited by an increase o f lifetime acro.ss the emission profile.Acknowlet^mentOne o f the authors M. S. M ehata is thankful to the University Grants Com m ission (SAP), New Delhi for providing financial assi.stancc.References( i | D Pant, H B Tripathi and t) I) Pant J. Pliotorhem Plwialuol A • Chm 56 207 (tWI)12) D Pant, H B Tripathi and D D Pant J. Lumin 51 22.t (It92)(3) K Itoh and T  Azurai J. Chem. Phys 62 3431 (1975)(4) T ikeyama, T Azumi, T Murao and t Yaina/aki Chem /‘.n, /96 419 (1983)(5) S G Schuirnan, R M Thrcatlc, A C Campomacchja and W ) p, .J. P ham  Sd. 63 m  ( \ m )(61 D Pant» H B Tripathi and D D Pant 7. PlwunUem Plu,inhu,i ; Chem. 54 23  ^ (1990)[71 M S Mehata and H B Tripathi Anal, Lett (coimnunicaud)[81 A Martin and R Narayanaswaniy Senstns ami AvtiuiUn^  B38..V) 330 (1997)[9] K A Al-Hassan 7. Polym. Sa. B26 1727 (I98H)[10| K A Al-Hassan and M A EUBayoumi 7, Polym Sa 25 iij'; (1987)[IIJ K A Al-Hassan, M A El-Bayouini Chem. Phy.s Leu 123 tt, (1986)[12] H C Joshi, A Upadhyay, H Mishra, H B Tripathi, I) I) Ku,i ; Photot hem Pluttohiitl A : Chem 122 185 (1999)[13J 0  J Rolinski and H J S Birch Mea,\ Sa Techno! 10 1:: (1999)jl4) K A Al-Hassan and T A/umi Chem Phw. Lett 163 (1989)

Photophysics of 6-Methoxyquinoline in Nafion® Polymer Matrix

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Photophysics of 6-Methoxyquinoline in Nafion® Polymer Matrix

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