Journal of Chemournal of Chemical Sciences Volume 104 ical Sciences Volume 104 Issue 2 1992 [Doi 10.1007%2Fbf02863363] M S a Abdel-Mottaleb; M S Antonious; M M Abo Ali; L F M Ismail;

8/10/2019 Journal of Chemournal of Chemical Sciences Volume 104 ical Sciences Volume 104 Issue 2 1992 [Doi 10.1007%2F

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Proc. Ind ian Acad. Sc i. (Chem. Sci.) , Vol. 104, No. 2, April 1992, pp. 185-196.9 Pr inted in India .

hotophysics and dynam ics of coum arin laser dyes and their analyticalimplications

M S A A B D E L - M O I TA L E B 1 ., M S A N TO N I O U S 1, M M A B O A L l 1,L F M I S M A I L 2 B A E L - S AY E D 2 a n dA M K S H E R I E F 31Dep ar tmen t of Chemistry, Facul ty o f Science, Ain Sham s U nivers i ty, Abbassia , Cairo , Egypt2Dep ar tment of Chemist ry, AI-Azhar Univers i ty, NA SR City, Cairo , E gypt3 Dep ar tmen t of Chemist ry, Helwan U nivers i ty of Technology, Helwan, E gypt

Abstract. A com parativ e study of the fluorescence of selected fex ible an d rigidized7-am inocou marin der ivatives (coum arin 6, 138 and i ts r ig idized co um arin 106) has b eenma de in a series of solvents of varied properties. Th e studies were further au gm ented b y thequ ant um chemical SC F- CI M O calculation s which provide a theoretical basis for interpretingspecif ic in teract ions and complex format ion wi th the polar solvents in the in t ramolecu larcharge transfer singh:t state of these dyes. Moreover, rotational relaxation behaviour, asreflected in the fluorescence pola rization o f these dyes, is correlated w ith solv ent viscosity/freevolume. Th roug h measurem ents of the temperature/viscosi ty dependence o f the f luorescencedepolar izat ion ( I /P) in glycerol, we are able to es tabl ish an Arrhenius- l ike e la t ionship linking(I /P) to the f ree-volume fraction of the medium . Fur thermo re, depolar izat ion data f it ted thePerr in equat ion wi th a s lope proport ional to the sensi t iv i ty of the molecular s t ructuretoward s m ediu m fluidity. Th e results im ply the p rom ising util i ty of these hig hly fluorescent

dyes as f luorescent probes for local f lu idi ty and polar i ty of the surro und ing medium ofinteres t. We dem onstra te poss ible analyt ical appl icat ions of commercia l ly avai lable coum arin6 as a pro m ising fluorescent p robe for m ed ium p roperties (e.g. fluidity, polarity an d forfollowing m icellization in th e so lution s of som e io nic surfactants).

KeywordL Co um arin dyes; fluorescence; qu an tu m chemical calculations; viscosity; probes,

1 . In t roduc t ion

I t appears tha t f luorescent dyes , in par t icu lar those of an e le c t ro n-d on or-a cce pto r

(EDA) type wi th so lvent -dependent in t ramolecular charge- t ransfer ( ICT) processinvolving exciplex formation with polar solvents and structure twist by l ightabsorp t ion , have foun d v ery broad technQlogiea l appl ica t ions . M an y of theseapplicat ions, e .g . in laser d yes, solar energy co nce ntrato rs in ph otov oltaic cel ls ,nonl inear opt ica l mater ia l s and ana ly t ica l probes , depend on the photophys ica lproper t ies of these dyes and have a l ready been used com mer iea l ly in pas t years butoth er potent ial ones are s ti ll un der dev elopm ent . I t is thus o f interest to predict an dto co nt ro l f luorescent dyes proper t ies for a b e t te r und ers tanding o f the mechan ismsinvolved in these applicat ions and thus ex tend the f ield of applicat ion. O ver the pastfew years , man y inves t iga t ions have been focussed on the ro le of the so lvent (medium)and the mechanism of so lva tion in cont ro l l ing the photo ind uced ICT and e lec t ron

* For co r respondence

185


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186 M S A Abdel Mottale b et al

t ransfer for EDA dye molecules in solut ions, in r igid polymeric matr ices as wel l asin the gas -phase em ploying a supersonic j e t expan s ion technique (R abek 1982; Dem as1983; Lak ow icz 1983; Visseret al 1983; O Co nn or and Phi l l ips 1984; W il liams 1984;

W olfbei s and Baus te r 1985; Re t t ig 1986 , 1988; M eech and Phi l lips 1987; Penget al1987; Ro ha tg i -M ukhe r jee 1987; Jones and Phi l lips 1988; Posch and W olfbe is 1988;C a z e a u - D u b r o c aet al 1989). Al though cons iderab le p rogress has been m ade , m oreinformat ion ab ou t the pho tophys ics and spec t ra l charac te ri s ti cs o f the E DA molecu lesis needed and i t con t inues to b e the sub jec t o f a m ajor cha llenge for chemica l research .Coumar in der iva t ives wi th a ro ta tab le o r r ig id ized amino group in the 7-pos i t ionhave rece ived grea t a t t en t ion f rom the theore t ica l and exper imenta l po in t s o f v iew(Schaefer 1973; Re ynold s and Drexhag e 1975; Joneset al 1980; K ub in and F le tcher1983; Ab del -M ot ta lebet al 1984; H up per tet al 1989; Van G om pel a nd Schus te r 1989) .The fund amen ta l in format ion g leaned abou t the photophys ica l p roper t ies o f th isgroup of dyes revea led a nu mb er o f in te res ting fea tures. The we ak f luorescenceintensi ty and shorte ning of the fluorescence l ife time (accelerat ion of f luorescence decay)observed for mo s t f l ex ib le am ino coum ar ins , par t i cu la r ly in p ro t ic so lven ts , have beenexpla ined in t e rms o f a nonrad ia t ive re laxa t ion p rocess o f the p lanar IC T s ta teproduc ing a twis ted IC T s ta te (ca lled (TICT)) (L ipper tet al 1987) via internal rotat io nof the amino g roup to an or thogon a l geo met ry. This twis ting process occurs in a fewpicosecond s and cr i tically depends on the kind of interact ions exer ted b y environmentalfac tors . TICT s ta tes , access ib le in f lex ib le m ul t ichrom opho r ic molecu les , have beencons idered as nonrad ia t ive tunne ls to the gro und s ta te (Vogelet al 1988) . Ho we ver,based on f luorescence l i fe t ime da ta , i t has been conc luded tha t hydrogen bondingbe tween so lven t and a coum ar in m olecu le seems no t to be a de te rmin ing fac tor fo r

TIC T s ta te fo rmat ion in the case of 7 -am inocou ma r in der ivat ives s tud ied (Chu andYan gbo 1987). Us ing supersonic j e t expa ns io n technique , i t has been show n tha t aspecif ic interact ion with polar prot ic or aprot ic solvent molecules is a prerequis i tefo r T IC T s t a te f o r m a t i on i n c ase o f N ,N-d ime thy l am inobenzon i t r il e (DM AB N) (Penget al 1987) . I t seem s that invest igat ions are s ti ll req uired to ex plain the effect ofso lu te -so lv en t in te rac t ions on the f luorescence quenching o f am ino coum ar ins dueto exc i ted s ta te com plex format ion .

As a par t o f a con t inu ing s tudy on the f luorescence behav iour o f a range of coum ar indyes o r m u l t i c h ro m o p hor i c E D A t ype (Abde l -M ot t a lebet al 1989) , the pre sent w ork

H5C2)2N/V -O~ ~O6

( H 3 C ) 2 N ~ O138


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hotophysics and dynamics of co umarin laser dyes 187

was taken up to p rov ide a mo re genera l in format ion on the flex ib le coum ar ins 6 and138 and i ts r ig id ized der iva t ive coum ar in 106 . Fo r sake o f c la r ity and comple teness ,the photophys ica l and dy nam ica l behaviou r and the in te rp lay be tween dye s t ruc ture

and solven t in hom og en eou s me dia will be invest igated f irst . These s tudies are essent ia lprerequis i tes in ord er to exp lain the effect of sol ute -so lve nt interact ions, p ar t icular lyspec i f ic hydrogen bonding in te rac t ions , on the photophys ica l p roper t ies and f luore-scence quenching o f these 7-subs t itu ted amino -coum ar ins due to exc i ted s ta te complexforma tion. Th e resul ts will be discussed on the basis of the electronic s t ructure of thesed y e s o b t a in e d b y ap p l ic a t io n o f t he w e ll -k n o w n q u a n tu m c he m ic al P P P - S C F - C I M Omethod. This gives an insight into the s inglet exci ted s ta te propert ies and hydrogenbonding complexa t ion of these dyes and the dynamics and mechanisms of so lu te -so lven tre laxa t ions . S tud ies will be ex tended to and focussed on m easurements o f f luorescencedepolar izat ion in glycerol a t different temperatures . These s tudies are indispensableprel iminaries in a t tem pting a m ore ref ined analysis of the viscosi ty/free volu m econt ro l l ing fac tors a ffec t ing the m ot ion of f luoroph ores incorpo ra ted in d i ffe ren tsystems of industr ia l and biological interest, e .g . polym ers , proteins , m icelles , e tc .Guil le t 1985; W innik 1986) .

F ina lly, we repor t l inear cor re la t ion b e tween f luorescence depola r iza t ion I /P) andmed ium v iscosi ty n T ) func t ion) and d em ons t ra te i ts u t il ity in p red ic ting the averagemicroviscosi ty of some ionic micel les . As an i l lustra t ive example, we explore theana ly t ica l ap p l ica t ion of coum ar in 6 as a useful f luorescence probe to de te rm ine c r it ica lmicel le concentrat ion, cmc, in ionic micel lar solut ions, which have been consideredas mo del sys tems mim ick ing b iomem branes in b io log ica l p rocesses Tur roet al 1980;Gu il le t 1 985; W innik 1986) .

2 . Experimental

Cou ma r ins 6 , 138 and 106 laser g rade) were suppl ied by Eas tm an K od ak and w ereused as rece ived . CTAB and SDS were purchased f rom Janssen Chimica and usedas rece ived . Spec t ra l measurements , f luorescence po la r iza t ion and qua ntum y ie ldde te rmina t ions , and the s tat i st i ca l t rea tment o f da ta were descr ibed before Abdel -M o t t a l e b et al 1989) . A wide range of spectral grad e so lvents 18 solvents) belon gingto different c lasses were used. Numerical values represent ing solvent propert ies are

co l lected f rom the l i t e ra ture Re ichard t and G oern er t 1983) . Q ua ntu m chemica lc a lc u la ti on s w it hi n th e fr a m e w o r k o f t he P P P - S C F - C I - M O m e t h o d w e r e pe r fo r m e dby apply ing a p rogram suppl ied by QC P E Gr i ff iths 1976 , 1981; Gr i ffi thset al 1988).

3 Results and discussion

3.1 Solvent effect

Coumar ins 6 , 138 and 106 exhib i t an in tense so lven t -dependent v i s ib le absorp t ionband: i t s wave length i s cons i s ten t wi th a t rans i t ion which has a cons iderab le CT

cha ract e r p r op o r t i on a l t o t he do n o r- ac cep to r pow er w i th in t he mo lecu l a r subun i ts .The absorp t ion and f luorescence spec t ra show pos i t ive so lva tochromic behavior, asshown in table 1 and f igure 1. General ly, i t i s noted that , the absorpt ion spectrum isless sensi tive tow ard s the solvent polar i ty Re ichardt and G oer ner t 1983) than the


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88 M S A A bdel Mottale b et al

Table I . Spect roscop ic and fluorescence qu an tum yie ld da ta for flex ib le coum ar ins 6 an d 138 andthe r ig id ized der iva t ive coum ar in 106 toge ther w i th so lvent polar i ty pa ram eter (E~-)* .

Solvent E~

,l, ,is O t , i . , l f O I ,t , ; .y O i

Cou ma r in 6 Coum ar in 138 Coum ar in 106

1 W ate r 1.000 490 565 0-03 367 472 0-39 380 490 0-702 M eth an ol 0-765 456 508 0.78 366 454 0.65 384 471 0.903 Eth ano l 0-654 458 505 0-87 365 448 0.69 386 467 0.904/ so -B uta no l 0.506 453 502 0-87 361 443 0-90 380 462 1-005 Ac etonitr i le 0-472 455 503 0-56 358 433 0.90 . 378 448 0-746 D M SO 0-441 466 515 0.56 363 439 0.80 384 455 0-807 D M F 0.404 461 505 0-84 361 433 0-90 380 447 0 '818 Ace tone 0-355 460 500 0-93 356 427 1~0 374 442 1 029 Dic hlor om etha ne 0-32 1 455 496 0-94 359 424 0-60 380 440 1@0

10 Ch lor ofo rm 0-259 455 492 0.93 361 422 0-90 377 440 0-70I1 Eth ylac etate 0.228 450 490 0-86 353 418 0-90 368 432 0.6212 Dio xan e 0.164 440 485 1 00 356 424 0-90 376 426 0-9713 Benz ene 0-127 434 487 0.87 355 411 0-80 372 425 0.9014 p-Xy lene 0-123 430 480 0.90 348 408 0-90 372 423 0.8115 To luen e 0-096 433 485 0-86 350 410 0-90 376 425 0-8516 Cy cloh exa ne 0-077 425 465 0-77 3 46 395 0-80 369 415 0-8817 n-H exan e 0-075 424 483 0.79 343 394 0.75 368 412 0-8018 n-H epta ne 0-052 422 485 1~)1 343 395 0-7 1 360 410 1-00

* The Eg va lues hav e been rep or ted for ov er 240 so lvents and represent b oth the genera l as wellas specific solvent propert ies (Reichardtet al 1983).

7 O

9

3 4 ;~)

t

1o ~ o

Figu re 1 . Va r i a t i on o f abso rp t i on (o ) and f l uo re scence ( . ) wave l eng ths fo r coum ar in 138wi th the so lvent polar i ty pa ram eter E r va lues . (Notewo r thy i s tha t exc luding a few solventsresul t s in excel lent l inear corre la t ions be tween , l , o r ) ' s and Er va lues) . S imi lar bchaviourwas not iced for o ther coumar ins s tudied (see tab le 1) .

f luorescence spec t rum which unde rgoes a m ore rem arkable red sh if t as the p olar i tyof the solve nt increases figure 1), an d the efficiency of the fluores cenc e 0~ ) is slightlyaffected by solvent chang e table 1). Co nsidera ble decrease in Of value can be seenin water. This effect is m ore p ron ou nc ed in the case of f lexible cou m arin s 6 and 138,

re flec t ing the ro le p layed by ro ta tory f reedo m and s t ruc ture twis t and could be m ostprobably a t t r ibu ted to local spec if ic in te rac tions inc luding hydrog en bon ding complexformat ion . These resu lt s emphas ize the obvious ICT na ture of the m ajor e lec t ronic


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Photophysics and dynamics of coumarin lase r dyes 189

t rans i t ions under cons idera t ion . Same observa t ions and conc lus ions have beenrepor ted f rom c om para t ive s tud ies o f fluorescence decay k ine tics fo r s imi la r dy es(Jones et al 1980, 1985; Ch u and Y angb o 1987; Van G om pel an d Schus te r 1989).

M oreo ver, a s teady qu enching of the f luorescence in tensi ty concom i .an t wi th agradua l ba tho chrom ic sh if t o f the f luorescence wave length i s observ ed for al l dyesunder cons idera t ion in d ioxan e so lu t ions ( figure 2) on addi t ion o f increas ing am oun tsof wate r. U po n add i t ion of t race quant i t ies o f wa te r (up to abo ut 3 o f the to ta lso lu t ion vo lum e) the in tens i ty o f the ini ti a l f luorescence spec t rum of the dye in thenea t d ioxane so lu t ion i s decreased , and a new red-sh if ted spec t rum appears . N o suchchange in the abs orp t io n spec t rum o f a dye so lu t ion was no t iced for thi s lowconcent ra t ion l imi t ( < 1 .66 M) o f wate r. The appearan ce of th i s new f luorescenceco m po ne nt an d th e isoemissive poin t (f igure 3) are character is t ic of exci ted s ta tehydrog en bond ing com plex format ion of s imple s to ich iome t ry wi th wate r molecu lesas p rev ious ly repor ted for some dye molecu les o f ED A type (Lako wicz 1983;C a z e a u - D u b r o c aet al 1989) accord ing to the eq ui l ibr ium :

m D y e + n H 2 0= m Dye~:nH 2 O).

A fur ther increase in the w ate r concen t ra t ion in the mixed so lven ts causes an add i t iona lgradua l ba tho chrom ic sh i ft due to the gradu a l increase in the bu lk po la r i ty o f themixed solvent as not iced in f igure 2.

At tem pted ana lys i s o f the spec t ra l da ta in the low concen t ra t ion l imi t o f wate r (upto 1 .66M) by us ing the equi l ib r ium re la t ionI s K x [H 20 ] ' , whe re I s i s t hef luorescence in tensi ty due to com plex format ion and n is the num ber o f wa te r molecu lesinvolve d in the com plex, is successful. F ro m the s lop es and the intercepts of the l inearl e as t squa r e s p l o ts o f the l og ( I s ) v s l o g ( [H 20 ] ) we ob t a ined t he va lue s fo r n andK, respect ively (with a correlat ion coeff ic ient of 0-99) . All dyes form very w eak

400 480 500 Il l

Figure 2 . F luorescence spect ra of r 138: 0) pur e-d io xan e , f rom 1) to 8) in mixeddioxan e-wa ter so lvents in which wa ter conte nt i s gradual ly increased , and in 9) pure w ater.S imi l a r behav iou r was obse rved fo r coum ar in s 6 a nd 106 .


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190 M S A A bdel M ot ta l eb et a l

>

zu J

ttn lr r0

~

I |11I~ t

, ' y

l e e

/ x x x x \ xP ~ , x x x xi t ~ ~ x x ~ x x \

/ ~ t xx~ xxx \ ~

t~ x \

, \ , \ ~ x x ~ x

e . . : . . . . . .

5

h nm)

Figure 3 . Effec t o f add i t ion of sma l l qua n t i t i es o f wate r (up to 1 66M , s p e c t r a 1 - 6 ) o n t h ef l u o r e sc e n c e s p e c t r u m o f c o u m a r i n 1 3 8 in p u r e d i o x a n e ( 0 ). N o c h a n g e s i n t h e c o r r e s p o n d i n ga b s o r p t i o n s p e c t r u m w e r e n o t i c e d i n d i c a t i n g e x c i t e d s ta t e c o m p l e x a t i o n w i t h w a t e r m o l e c u l e s.Curv e 7 represen ts spec t rum in pure wa te r ( see w A s imi la r e ffec t wa s observ ed forc o u m a r i n s 6 a n d 1 0 6 .

(dye :H 2 O) co m plexes o f s imple s to ich iomet r i c r a t io (1 : 1 ). Sam e resu lt s fo r o the r ED A

dyes have been repor ted ear l ier (Visseret al 1983 ; Meech and Ph i l l ips 1987 ; Penget al 1987 ; Jones and Ph i l l ips 1988 ; Posch and Wol fbe i s 1988 ; Cazeau-Dubrocaet al1989).

I n c o n c l u s io n , i t i s s h o w n t h a t u p o n t h e f o r m a t i o n o f a n e w p o l a r s o l v a t i o n e n v e l o p eto accom m oda te the c rea ted e lect ron red i s t r ibu t ion in the 1 s t a t e o f these dyes areversible ph oto asso cia t io n o f s imp le s to ichiom etry wi th w ater mo lecules i s es tablished.M oreover, in the limi t o f a la rge p ro t ic so lven t concen t ra t ion , the so lv a t ion p rocessc a n b e r e g a r d e d a s c o n t i n u o u s ( D e t o m a 1 9 8 3 ) a n d t h e f l u o r e s c e n t s y s t e m i s u n d e rthe dyna mic con t ro l o f so lven t r e laxa t ion p rocess . S imi la r con c lus io n has beenprev ious ly repor ted (De tom a 1983) .

The resu lt s ab ove can be ra t iona l i zed on the bas i s o f qu an tum chemica l da tao b t a i n e d b y a p p l ic a t io n o f t h e w e l l - k n o w n S C F - C I - M O m e t h o d ( G rif fit hs 1 97 6,1981; G rifflthset al 1988) wh ich pe rmi t us to e s t ab l ish the na tu re o f the low es t exc i teds ing le t s t a t e and to examine the cha rge dens i ty d i s t r ibu t ions fo r the g round (So) andexc i ted S 1 s t a t e s o f coum ar ins 6 , 138 and 106. Th ese a re p resen ted in t ab le 2 . Thegrou nd s t a t e el ec t ron dens i ty d i s t r ibu t ions a re s imi la r and charac te r ized by a l t e rna ted i s t ribu t ion . I t can be seen tha t l igh t ab sorp t ion resu lt s ma in ly in a m ore po la r cha rgetransfer 1 ( ii ii*) s ta te a result which ra t iona l izes the spect ra l shi fts observ ed (seetable 1 and f igure 1). M oreo ver, in the 1 s ta tes of these dyes , e lec t ron d ensi tyal ternat ion is des t royed especia l ly for the lac tone r ing. This should resul t in u l t rafas tso lven t -depen den t e l ec t ron ic r e laxa t ion . I t i s thus expec ted tha t a sh or t r ange (a smal l

am pl i tude) geomet r i ca l r e l axa t ion i s ach ieved in p ro por t ion to the s t r eng th o f so lven tin teract ions . This i s ref lec ted in the re la t ively smal l Stokes shi f t observed and theh igh f luorescence quan tum e ff i c i ency in ap ro t i c so lven t s . I t s eems reasonab le to


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Photophys ics and dynamics of coumar in laser dyes 191

Table 2. Pi-electron density distributions in the So an d Sl.a statesof coum arin 6 and coumarin 138 and i ts rigidized derivative 106.*

SO S l r

A tom N o. 6 138 106 6 138 106

1 1 ' 85 8 1 . 8 5 7 1 ' 8 5 7 1 ' 8 7 3 1 - 8 8 4 1 .8 842 0 .7 2 7 0 . 7 2 9 0 . 7 2 9 0 . 7 5 3 0 - 8 7 4 0 -8 753 1 - 06 0 1 . 0 4 2 1 - 0 4 6 1 - 0 6 6 1 - 0 7 7 1 .0 854 0 - 90 9 0 . 9 2 7 0 9 2 7 1 - 1 7 5 1 - 1 8 3 1 .1 835 0 -9 6 9 0 . 9 7 4 0 - 9 7 3 1 . 0 4 2 1 . 0 8 6 1 -0 896 1 .1 0 2 1 1 0 5 1 . 11 2 1 . 0 3 4 0 . 9 9 8 1 .0 0 47 0 ' 9 7 3 0 - 9 7 8 0 - 9 8 9 1 . 0 0 4 1 . 0 1 7 1 0 368 1 .1 4 0 1 . 1 4 3 1 . 1 5 3 1 - 0 6 3 1 - 0 2 9 1 .0 299 0 - 95 2 0 - 9 6 2 0 . 9 6 1 0 9 9 2 0 . 9 8 6 0 -9 8 8

10 1 .0 9 5 1 . 0 9 3 1 . 0 9 9 0 9 9 1 0 - 9 4 9 0 -9 5 611 1 .4 7 6 1 . 5 0 2 1 . 5 0 4 1 . 4 6 4 1 - 5 5 8 1 ' 5 6512 1 .6 7 5 1 6 9 0 1 ' 6 5 1 1 - 4 9 0 1 - 3 6 1 1 .3 0 413 0.828 0-90814(N) 1-130 1'33115(S) 1'890 1.903

* The p arameters used in the S C F- C I-M O calculat ions are thoseof G riffithset at (1988). Perturb ations due to the m ethyl grou pswere taken into consideration to get best agreement betweencalculated and observed longest wavelength electronic transition.

2 N ~ ~ ~ Nc o n c l u d e t h a t a s i g n i fi c a n t a m o u n t o f t h e s o l v e n t - s o l u t e l o c a l h y d r o g e n b o n d i n gi n t e r a c t i o n s w i t h t h e m o r e b a s i c c e n te r s in t h e l a c t o n e r i n g b e c o m e s t r o n g e r i n t h eS I s t a t e o f t h e s e d y e s l e a d i n g t o a m o r e c o n s i d e r a b l e r e l a x a t i o n t o t h e s o l v a t e dSl as t a t e a n d s t r u c t u r e t w i s t c a n b e s t a b i l iz e d in c a s e o f f le x i b le c o u m a r i n s i n p r o p o r t i o nt o t h e d e g r e e o f f l e x ib i l i ty w h i c h i s l a r g e r f o r c o u m a r i n 6 (d u e t o p o s s i b l e r o t a t i o na r o u n d t h e b o n d c o n n e c t i n g t h e b e n z o t h i a z o l y l g r o u p ) . T h is a c c o u n t s f o r t h ee x p e r i m e n t a l l y o b s e r v e d f l u o r e s c e n c e q u e n c h i n g i n w a t e r ( se e t a b l e 1), i n p a r t i c u l a rf o r c o u m a r i n 6 w h i c h i s c h a r a c t e r i z e d b y a l a r g e r n u m b e r o f b a s i c c e n t e r s a c c e s si b l ef o r l o c a l h y d r o g e n b o n d i n g i n t e r a c t i o n s w i t h w a t e r .

3 .2 Fluorescence depolarization

T h e o b t a i n e d f l u o r e s c en c e q u a n t u m y i e l d o f o u r d y e s i s i n s e n s i ti v e t o s o l v e n t v i s c o s i t yd u e t o a m i n o r c o n t r i b u t i o n o f t h e in t r a m o l e c u l a r t o r s i o n a l d y n a m i c s t o t h e o v e r a lln o n r a d i a t i v e e n e rg y r e l a x a t i o n p r o c es s . F l u o r e s c e n c e p o l a r i z a t i o n m e a s u r e m e n t s


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92 M S A b d e l - M o t t a l e b e tal

Ta b l e 3 . F l u o r e s c e n c e d e p o l a r i z a t i o n d a t a f o rcu,~m-arins 6, 138 and 106 as a f u n c t io n o f n T ) a n d o f th ef ree vo lum e f rac t ion of g lycero l a t d i ffe ren t t empera -tures see text) .

l pr r / .(K) (K/cP) l f 6 138 106

288-0 0'144 14"59288-7 0-144 14'52294'5 0"245 14"00294'8 0-245 13'98299"0 0'352 3"62303"3 0"513 13"28303"7 0'514 13"25

309.3 0"814 12"83315-3 '401 12'41318'7 1"713 12'19324"3 2.270 11"83329'2 3'359 11"54337"3 5.190 11"08346-0 7"854 10'63351"0 10. 320 10'39351"6 10 '3 40 10.36

2.13

2"162"19

2"58 2"55

2"80 2"64

2"98 2"723'07 2"83

2"20

2'24 3"37 3"233"82 3"41

2"292"31 4'72 4"022"44 5"50 4"902"65 6'76 6"292"82 9"01 8'48

10"75 10"313"05

prov ide an a l t e rna t ive me thod to p rove medium f lu id i ty va r i a t ion and a re ve ryin fo rmat ive abou t the dynam ics o f the f luorescen t sys tems . Po la r i za t ion measurem entsare a lso inform at ive abo ut the or ien ta t ion d is t r ibut ion o f f luor oph ores in d ifferentsys tem s of indu st r ia l and biological in teres t (e .g . polym ers , prote ins e tc .) (Turroe t a l1980; G ui l le t 1985; W innik 1986) .

The d ynam ics o f molecu la r ro ta t iona l r e l axa t ion o f m olecu les in so lu t ion has beenex tens ive ly inves t iga ted by va r ious spec t roscop ic t echn iques (Lou t fy and Arno ld 1982).M os t o f the s tud ies have been pe r fo rmed on ion ic dyes in a va r i e ty o f so lven t s inorde r to t e s t the va l id i ty o f the DSE hydrodynamic mode l aga ins t the app l i cab i l i tyof the fr ee vo lum e concep t . For dyes w hich u nderg o a nonrad ia t ive in t ramolecu la rto r s iona l r e l axa t ion the f luorescence quan tum y ie ld was found to be h igh mediaf ree -vo lume dependen t .

Our exper imen ta l ly de te rmined po la r i za t ion da ta in g lyce ro l over a r ange o ft empera tu re (2 94 .4 - 346 K) ( t ab le 3 ), f it t ed the we l l -know n Perr in equ a t ion (Per rin1929):

( l / P ) = ( 1 /P o ) + [ ( 1 /P o ) - ( I / 3 ) ] ( R T / V n ) r ,

w h e r e P o i s character is t ic value for the same f luorophore in v i t r i f ied solut ion, R isthe gas cons tan t , an d r is the f luorescence l if e time o f the f luorop hore o f a molecu la rvo lum e V. The es tab l ished equa t ion s o f co r re la t ion (wi th a co r re la t ion coeff ic i en t o f0-99) are

2.14 + 0.10T / n , 9 or coumar in 6 ,I / P =, 2 .69 + 0 .79T / n , fo r coumar in 138 ,

2.42 + 0.76T / n , fo r coumar in 106 .


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Photophysics and dynamics of coumarin laser dyes 193

p I ~t

1 1 f I 1 4

F i g u r e4. Grap hical representa t ion of the not iced var ia t ion of degree of f luorescencedepolar isa t ion of coumarin s 138 , ) and i ts r ig idised der ivative 106 e) wi th the f - ~ funct ionfor glycerol at different temp eratures see table 3). De pola rization decreases as free volu meD of med ium glycerol) decreases.

Thus , vary ing the ra t ioTin for g lycero l by tem pera ture change (Lan dol t -Boerns te in1969) and studying the ro tat iona l diffusion using f luorescence polar iza t ion techniq ueshould assis t in establ ishing the dominant solvent relaxat ion process and provideinsight into the role of the mo lecular s t ructure a nd i ts sensit ivity to the av erageme dium microviscos ity. Co mp ar ison of the s lopes ref lec ts a h igher sens i tiv ity of th emo re s t ruc tura l ly re la ted coum ar ins 138 and 106 towa rds average med ium viscos i ty /temp erature. Th e resul ts suggest the p romising ut i l i iy of these comm ercial ly avai labledyes as probes to exp lore microsco pic f luidity pertain ing to the inter ior of indus tr ial ly

and biological ly important systems such as polymers, proteins and micel les .Fur the rmo re , to tes t the appl icabi li ty of the f ree-volume concept in descr ib ing the

dynam ics of these f luorophores we have ca lcu la ted the ch ange in the f ree-volume ofglycerol with temp eratu re by using the equat io n f = fg + t r( T - Tg) w here fg = 0.025,tr = 4-4 x 1 0-4 de gre e - 1 an d Tg = 189K for glycerol (Loutfy and A rno ld 1982). W ehave fo und that , as the free volum e Or) fract ion o f the m ediu m decreases f luorescencedepo larisat ion (I /P) non linearly decreases (figure 4).

O n correlat ing (I /P) with the calculated 1/f values for glycerol at differenttemp era tures (303 3- 346K ) ( tab le 3), we ob ta ined the fo l lowing Arrhenius- l ikeregression equat ions:

1 8 e x p ( - 0 . 1 7 / f ,1 / P = 1033 ex p (- 0 .45/f ),

l 1166 e x p ( - 0 .47/f),

for coumar in 6 ,for cou m ar in 138,for cou m ar in 106 .

The se f indings confirm the ap plicabi li ty of the free-vo lume conc ept in describing themo lecular ro ta t iona l d i ffus ion of these coum ar ins . M oreover, i t re flects the s imi la rdepolar i sa t ion beha viour of the c lose ly re la ted co um ar in 138 and i ts r ig id izedderivat ive coumarin 106.

Since depolarizat ion (I /P) is a thermally act ivated process involving molecularrotat ion al diffusion, i t can be expressed in an A rrhen ius form as

1/P = Ae x p ( - AE/RT ,

wh ere A i s the preexpo nent ia l fac tor and AE is the ac t iva t ion energy o f depolar iza tion .


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194 M S A Abdel Mottaleb et al

N o n l i n e a r l e a s t - s q u a r e s a n a l y s is o f o u r d a t a r e s u lt s in t h e f o l l o w i n g r e g r e s si o ne q u a t i o n s

f ~i 4 e x p ( - 1 0 7 0/ T ),1/P = ] 6 . 7 6 4 x 1 04 e x p ( - 2 8 8 8 / T ),

1 5 0 9 1 x 1 0 4 e x p ( - 3 0 2 5 / T ) ,

f o r c o u m a r i n 6 ( 3 1 8 . 7 - 3 5 1. 6 K ) ,f o r c o u m a r i n 1 38 ,f o r c o u m a r i n 1 06 ,

( in t h e t e m p e r a t u r e r a n g e 3 03 3 - 3 4 6 K f o r c o u m a r i n s 1 38 a n d 1 06 ), f r o m w h i c h t h ev a l u e s fo r t h e a c t i v a t i o n e n e rg y o f d e p o l a r i z a t i o n o f c o u m a r i n s 6 , 1 38 a n d 1 06 a r eca lcu la t ed , 2 .1 , 5 -78 and 5 .84 kca l /m ol , r e spe c t ive ly, re f l ec t ing the g re a te r f l ex ib il i ty fo rc o u m a r i n 6 . T h e s e v a l ue s c o m p a r e d t o t h e a c t i v a t i o n e n e r g y o f g l y c e ro l , f lo w( = 1 4-2 5 k c a l / m o l ) ( L o u t f y a n d A r n o l d ) i m p l y t h a t t h e s o l u t e m o l e c u l e i s r o t a t i n gr e l a ti v e l y u n p r o h i b i t e d w i t h i n a s o l v e n t c a g e ( f r e e -v o l u m e ) a r i s i n g , f r o m m u l t i p le -h y d r o g e n b o n d i n g s i t e s i n g l y c e r o l .

3.3 Photophysical properties in micellar solutions

O w i n g t o t h e i m p o r t a n c e o f m i c e l la r a g g r e g a te s ( f o rm e d b y i o n i c a n d n o n i o n i cs u r fa c t a n ts ) a s m o d e l s y st e m s m i m i c k i n g m o r e c o m p l e x b i o a g g r e g a t e s s u c h a sb i o m e m b r a n c e s , w e e x t e n d o u r s t u d y t o e x p l o r e t h e p o t e n t i a l o f t h e c o m m e r c i a l lya v a i l a b l e a n d r e l a t iv e l y c h e a p e r c o u m a r i n 6 l a s e r d y e a s a p r o b e f o r m i c e l l i z a t i o np r o c e s s e s i n c a s e o f t w o i o n i c s u r f a c t a n t s; C T A B a n d S D S . O f t h e v a r i o u s p h y s i c a l

f I / / ~ ~

Figure 5. Three-dimen sionalplot showing the spectra of coumarin 6 in aqueous CTAB

solutions of different concentrations.The heavy arrow points to 2~,x of coumarin 6 in purewater. The points on the left-side represent fluorescence ntensity at 510 nm as a function ofCTAB conc entration. The e valuation of data gives a cmc of 1 10 -~ M. Similar plo t wasobtained in case of SDS (see text).


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hotophysics and dynamics o fcoumar i n laser dyes 95

m e t h o d s e m p l o y e d , f l u o r e s c e n c e e m i s s i o n p r o b e s h a v e b e e n w i d e l y u s e d b e c a u s e o fthei r s impl ic i ty, wide appl icabi l i ty and extreme sensi t iv i ty a t very low probeconcen t ra t ions (Tur roet a11980 . M oreover, by com par ing the spec tra l da ta in mice l le s

wi th those in hom oge ne ou s so lven t sys tems , mor e can be lea rned abou t the s t ruc tu ralde ta il s o f the mice l la r m ic roenv i ronm ent (Tur roet a l 1980).

W e have obse rved tha t the f luorescence in tens ity and energy o f coum ar in 6 aqueo usso lu t ion exh ib i t mark ed change on add i t ion o f the su r fac tan t (CTA B or SDS ) . Thef lu o r e sc e n ce m a x i m u m ( = 5 65 n m ) i s b lu e - s h if t ed ( 1 9 0 0 c m - 1 ) a n d t h e f lu o r e sc e n cein tens ity o f coum ar in 6 i s enhanced . M oreover, a r ap id inc rease in the f luorescenceintensity is observed ab ove the cr it ica l micel le concen tra t ion of both C TA B (1 x 10 - 3 M )and S D S (8 x 1 0-a M) . A th ree -d im ens iona l r ep resen ta t ion o f these obse rva t ion s incase o f CT AB i s sho w n in f igure 5 . In add i t ion , bo th ob se rva t ions po in t to the ex is t enceof coum ar in 6 p robe mo lecu les in a l e s s po la r (o f E T = 0 55 p red ic ted f rom the l inea rdep end ence of~,y on the E T value , 22. = 480 + 45.6 E T, exclud ing w ater, table 1) apro t icinter ior of the micel lar aggregates o f these ionic surfactants . Thus , th is be hav iourprov ides a s imp le and excep t iona l ly h igh sens i t iv i ty m e tho d fo r fo l lowing mice l li za tion .

F ina l ly, i t i s wor th men t ion ing tha t in fo rmat ion on the ave rage mic rov i scos i ty o rorganized assembl ies i s access ible in a re la t ively s imple way by us ing the l ineard e p e n d e n c e o fl i p on n (T) func t ion above es tab l i shed . The de te rminedl i p values forc o u m a r i n 6 i n 2 5 x 1 0 - 3 M C TA B a n d 2 5 x 1 0 - 2 M S D S s o l u t i o n s ( a b o v e cm c ) a t29 8K were 3 .1 and 3-7, respectively, an d are readi ly conver te d to the averagemic rov i scos i t i e s in the r ange 18-30cP which a re comparab le wi th these in theliterature (Law 1981).

References

A b d e l - M o t t a l e bM S A, Antonious M S, Abo-Al i M M, Ismaie l L F, EI-Sayed B Aa n d S h e r i fA M K1989 J. Photochem. Photobiol.50 259

C a z e a u - D u b r o c aC, Lyaz idi S A , Cambou P, Pe i r igua A , Cazeau P ha n d P e s q u e r M 1989 J. Phys. Chem.93 2347

C h u G a n d Ya n g b oF 1987J. Chem. Soc. Faraday Trans. I83 2539C u n d a l l R B and D ale R E eds) 1983Ti m e r e so l v ed f l u o r e s ce n c e sp e c tr o s c o p y i n b i o c h e m i s t r y a n d b i o l o g y

New York a n d L o n d o n : P l e n u m P r e s s )D e m a s J N 1983E xci ted s ta te l i f e t ime measur ementsN e w Yo r k a n d L o n d o n : A c a d e m i c P r e ss )D e t o m a R P 1983Tim e reso lved f lu orescence spec t roscopy in b iochemis t ry an d b io log yeds) R BC u n d a l l

a n d R E Da le New Yorka n d L o n d o n : P l e n u m P r e s s) p. 393Fendler J H a n d F e n d l e r E J 1975Cata lys i s n mice l l a r an d macrom oleeu la r systemsN e w Yo r k : A c a d e m i c

Press)Griffi th s I 1976Colour and constitution oforoan ic m olecu les London: Academic Press )Griffiths J 1981Rev. Prog. Coloration37Gri ff i ths J , Lasch J Ga n d S c h e r m a i e rA G 1988 Q CPE ,Ind iana Univers i ty,USA)Gui l le t J 1985 P o l y m e r p h o t o p h y si c s a n d p h o t o c h e m i s tr yLondon: Cambr idge Univers i ty Press )H u p p e r t D , I t t a h V a n d K o s o w e rE M 1989Chem. Phys. Lett.159 267,and references thereinJ o n e s A C and Phi l l ipsD 1988 L a s er C h e m .9 317Jones G II , Jackso n W R, Choi Cand Bergmark W R 1985J. Phys. Chem. 89294Jones G II , Jackson W Ra n d H a l p e r n M A 1980Chem. Phys. Lett.72 391Kess le r M A a n d W o l f b e isO S 1989C h e m i s t r y a n d p h y s i c s o f l i p i d s

Kubin F and F le tcherA N 1983Chem. Phys. Lett. 9949Lakowicz R J 1983Principles off luor escence spec t r oscopy N e w Yo r k a n d L o n d o n : P l e n u mLandol t -Boerns te in 1969Z a h l e n w e r t e u n d u n k t i o n en Berlin: Springer-Verlag)I I Band, 5Tei lL a w K Y 1981Photochem. Photobiol.33 799


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1 9 6 M S A A b d e l - M o t t a l e b e t a l

Lipper t E , Ret t ig W, Bonacic- Koutecky V, Heise l F and Mieche J A 1987Adv. Chem. Phys. 68 1Loutfy R O and Arnold B A 1982J. Phys. Chem. 864205Meech S R and Phi l l ips D 1987J. Chem. Soc., Faraday Trans. II83 1941O 'C onn or D V and Ph i l l i p s D 1984Time correlated single photon counting(New York and London :

Academic Press)Peng L W, Dan tu s M , Zewa i l A H , Kem ni t z K , H icks J M and E i sen tha l K B 1987J. Phys. Chem.91 6162Perr in F 1962Ann. Phys. Paris)12 169Posch H E and W oifbeis O S 1988Sensors and Actuators15 77Rab ek J F 1982Experimental methods in photochem istry and photophysics Par t 2(Chichester, New York:

Wiley-Interscience)Re icha rd t C and Goe rn e r t E H 1983Liebigs Ann. Chem.721Rett ig W 1986Angew Chem.. Int. Ed. Engl.25 971Rett ig W 1988Appl. Phys. i45 145Reyno lds G A and Drexhag e K H 1975Opt. Commun.13 222Roha tg i -Muk he r j ee K K 1987Fundamentals of photochemistry(New Delhi : Wiley Eas tern)Schaefer F P 1973Laser dyes(Berlin: Springer-Vedag)

Schaffar B P H a nd W olfbeis O S 1988Analyst 113 693Thomas J K 1980Chem. Rev. 80283Tur ro N J , Grae t ze l M and Braun A M 1980Angew. Chem., Int. Ed. Engl.19 675Van Go mpe l J A and S chus t e r G B 1989J. Phys. Chem.93 1292Visser R J , Va rm a C A G O, K oni jne nberg J and Berg erf P 1983J. Chem. Soc., Faraday Trans. II79 347Vogel M, Ret t ig W, Fiedelde i U and Baugaer te l H 1988Chem. Phys. Lett.148 347Vogel M, Ret t ig W, Sens R and Drexhage K H 1988Chem. Phys. Lett.147 452, 461Von Buen au G a nd W ol f T 1988Adv. Photochem.14 273W ill iams D J 1984Agnew. Chem., Int. Ed. Enol. 23 690Winnik M A (ed.) 1986Photophysical and photochemical tools in polymer science, N A T O A SI series C 182

(Dordrecht: Reidel)Wol tbe is O S and Baus ter t J H 1985.J .Heterocyclic Chem.22 1215Zacha r i a s se K A , Van Phu N and Kozank iewicz B 1981J. phys. Chem.85 2676

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Journal of Chemournal of Chemical Sciences Volume 104 ical Sciences Volume 104 Issue 2 1992 [Doi 10.1007%2Fbf02863363] M S a Abdel-Mottaleb; M S Antonious; M M Abo Ali; L F M Ismail;