SPECTROSCOPY LETTERS, 11(7), 493-511 (1978) G.P.Gurinovi'Gch, A.E.LoSev, LI .8enkevi tch i n s t i t u t e of Physics , Academy of Sciences o f the By- e l o r u s s i a bad, Adinsk, €iSb.R/IJS$h hiTitODUCTLOi1 The i n v e s t i g a t i o n of concent ra t ion e f f e c t s i n t h e s o l u t i o n s of luminescent compounds allows us t o ob ta in t h e d e t a i l e d information concerning i n t e r a c t i o n s bet- ween molecules o f d i sso lved substance. The inc reas ing concent ra t ion of dye and pigment s o l u t i o n s is known t o l e a d t o the concent ra t ion depo la r i za t ion phenomena ( C D E ' ) and the concent ra t ion auenching o f f luorescence (CdF) . A t p resent the o v e r a l l of these phenomena i s ra- t h e r widely inves t iga t ed both i n t h e o r e t i c a l and expe- r iment a1 aspects . Nevertheless the problem of the mu- t u a l r e l a t i o n of t he concent ra t ion depo la r i za t ion and the concent ra t ion f luorescence wenching remains y e t unsolved. The improvnent o f t h e CDF theory , underta- ken i n las t y e a r s , as we l l t he u s i n g new methods of in- v e s t i g a t i o n , g ive the r e a l p o s s i b i l i t y t o c a r r y out t he comparison o f theory and experiment more s t r i c t l y . 49 3 494 GURINOVITCH, LOSEV, AND ZENKEVITCH Such comparison permits t o determine the mechanism and the r e g u l a r i t i e s of energe t ic i n t e r a c t i o n between mole- c u l e s d e f i n i t e l y and t o i n v e s t i g a t e the reasons o f C4.F more profoundly, and a s c e r t a i n i t s inf luence on CUP. In present paper the r e s u l t s o f expe.rimenta1 in - v e s t i g a t i o n of concentrat ion inf luence on spec t ra l - lu- minescent c h a r a c t e r i s t i c s of pigment so lu t ions w i l l be presented and the ana lys i s of obtained data w i t h use of c a l c u l a t i o n s according t o modern theo r i e s of CUP w i l l be c a r r i e d out. The second range o f ques t ions , i nves t iga - t e d i n the paper , is connected w i t h t he e luc ida t ion of poss ib l e reasons o f CqY and the r o l e o f in te rmolecular i n t e r a c t i o n i n t h e e l e c t r o n i c e x c i t a t i o n energy deac t i - v a t i o n processes i n concentrated so lu t ions and the aaeo- c i a t e s of pigment J D O l a O U l a 8 . Prom t h i s po in t of view the r e g u l a r i t i e s of the aeeoc ia t ion and the mixed a s soc ia t ion of d i f f e r e n t pigments and the energe t ics of such systems a r e inves t iga t ed as we l l t he inves t iga t ion o f e x c i t a t i o n t rapping e f f i c i ency e i t h e r by a s soc ia t e s o r d i r e c t l y i n mixed aggregated complexes under v a r i a t i o n of t h e i r com- p o s i t i s n is c a r r i e d out. cuiicr;NTHA'l'IUii DEPULxHLZxTIuii OF J?LUO~J$SC~~&CE The modern theo r i e s o f CDF Ci-51 a r e mainly based on the ca l cu la t ions of f luorescence emission p robab i l i t y p f r o m the i n i t i a l l y exc i ted molecules s i n c e every ener- CONCENTRATION EFFECTS IN PIGMENT SOLUTIONS 495 gy t r a n s f e r a c t r e s u l t s p r a c t i c a l l y i n f u l l emission depo- l a r i z a t i o n as shown by Galanin [6) and the re fo re t h e emis- s i o n o f i n i t i a l l y exc i ted molecules b r i n g s the ca rd ina l con t r ibu t ion t o polar ized f luorescence of so lu t ion . The- s e t h e o r i e s a r e based on the fol lowing common assump- t i ons : 4 ) the i n t e r a c t i n g molecules have not the nonge- n e r a t e d exc i t ed s t a t e 8 and have been exc i ted t o t h e i r lowest exc i ted s t a t e s ; 2 ) the quantum y i e l d of f luo res - cence doesn ' t depend on molecule concentrat ion; 3 ) t he d i s t r i b u t i o n of mOleCUleS i n s o l u t i o n i s chao t i c and i s o t r o p i c ; 4) t he r o t a t i o n a l depo la r i za t ion doesn ' t oc- cu r ; 5 ) t he energy migra t ion between molecules is des- c r ibed by the Forster-Galanin induct ive resonance theo- r y [7,8] . The theory of CDF developed by Bodunov [5] i s also based on these common assumptions b u t , i n cont- rast w i t h the o the r t h e o r e t i c a l models, s t a t e d i n works [4-4] and connected w i t h s p e c i f i c assumptions about t he c h a r a c t e r of e x c i t a t i o n movement i n system. I n h i e theo- r y the moat common case is considered with the usage of t h e Monte Carlo method of ca l cu la t ion . I n the case o f dipole-dipole i n t e r a c t i o n between molecules i n accordan- ce wi th t h e data of work [5] the t h e o r e t i c a l valuee of emission anisotropy wi th in the accuracy up t o 2% a r e packing up the curve, i n the range of O,! < $ < 40, descr ibed by t h e func t ion: 49 6 GURINOVITCH, LOSEV, AND ZENKEVITCH where El=C/co ;r i s the anisotropy emission of solu- t i o n w i t h concent ra t ion c C - 0 ; C o i s the c r i t i c a l concent ra t ion corresponding t o t h e condi t ione a t which the pa i red transfer proba- b i l i t y i s equal t o the t o t a l p robab i l i t y of a l l i n t r a - molecular deac t iva t ion processes ,co=($f I?:). I n accor- dance with induct ive resonance theory, t h e value o f the c r i t i c a l t r a n s f e r d i s t ance R, is ca l cu la t ed w i t h the formulae [7,8] : ; ro i s the an iso t ropy , when -i 00 J fn ( d ) * & A (41% ) ' 2 ) 9000. b?IO*K'* 80 i 2 8 . l w n 4 aN R6a = where 9 i s the wavenumber;EA i s the molar decimal ex- t i n c t i o n c o e f f i c i e n t of acceptor ; fa i s the s p e c t r a l d i s t r i b u t i o n of the donor molecule f luorescence (mea- s u r e d in auanta and normalized t o i i n wavenumber sca- l e ) ; N is the Avagadro number;n is t he r e f r a c t i o n index of so lvent ; 80 i s t h e donor quantum y i e l d i n ab- 2-2 -z sence of t r a n s f e r ; K' i s the o r i e n t a t i o n factor (K f o r the i s o t r o p i c s o l u t i o n s of middle v i s c o s i t y ) . The CDP phenomenon f o r chlorop4yl l and i t s analogs i n s o l u t i o n s of d i f f e r e n t na ture and p rope r t i e s has been i n v e s t i g a t e d by u s [9-11]. For the comparison of theory and experiment the viscous i s o t r o p i c pigment so- l u t i o n i n c a s t o r o i l were chosen [ 9 ] , which a r e cor- responding t o a b e s t ex ten t common assumption i n CDP CONCENTRATION EFFECTS IN PIGMENT SOLUTIONS 49 7 t h e o r i e s . I n add i t ion the high s o l u b i l i t y of chloro- p h y l l and i t s d e r i v a t i v e s i n t h i s so lvent (up t o 10- m / 1 1 permits t o carry out the comparison of t he theory and the experiment i n the wide range of concen- t r a t i o n . It i s worthy t o note that i n c o n t r a s t w i t h t h e o r e t i c a l cons idera t ion supposing the constancy of f lus rescence quantum y i e l d , i n experimental condi t ions , LqF i s observed for a l l i nves t iga t ed pigments. kesu l t s of determing R, according to the Forster-Galanin theo- r y and by f luorescence depo la r i za t ion w i t h usage of the method s t a t e d i n [42] a r e l i s t e d i n Table 1 , where the bas ic spectral- luminescent parameters of pigments a r e involved. AS shown i n Table 1 x, $ f,(4) €,(I)$ by order doesn ' t l e a d t o e s s e n t i a l devi- a t i o n of 13ih values f rom RExp ones. The analogous ac- cordance was observed f o r t hese pigments i n the de te r - gen t mice l l s [to3 and i n t h e r i g i d pol.yvinvlb.vthiral f i l m s [{{I i n v e s t i g a t e d by us e a r l i e r . These f a c t s can se rve as the evidence of t he inductive-resonance bonds between pigment molecules i n the examined systems. 4 t h e changing of production 00 x aD 0 For t he comparison of t h e o r e t i c a l c a l c u l a t i o n o f t h e CDF curve by the formulae ( 1 ) with experimental data R o be taken as fa r as th i s value is determined by dipole- value calculated by the formulae ( 2 ) m u s t 49 8 GURINOVITCH, LOSEV, AND ZENKEVITCH TALlLE Spectral-Luminescent Charac t e r i s t i c s and T r a n o f e r Para- meters of Chlorophyll and Its Analogs ' Pigment p a i r :hl L-Chl Pheo 5- I-c Eheo g 4VPChl- - 4VI'Chl PChl - Chl g Chl k-Chl 2 Chl g-Chl Chl g-Chl 5 Chl E+Chl g Chl g-Chl g Chl g-Chl g , solvent c a s t o r o i l c a s t o r o i l c a s t o r o i l c a s t o r o i l c a s t o r o i l c a s t o r o i l c a s t o r o i l pyr idi . ne ace t on1 e t h e r &Tow 47000 50300 32800 32800 47000 77500 '77500 77200 83500 9 086 43 46 34 42 47 40 54 54 55 53 d ipo le i n t e r a c t i o n OnlV and is not comected with the o t h e r add i t iona l assumptions. Heal ly , i t should be no- t e d f r o m Table 1 , f o r s e r i e s of so lvents d i f f e r ing through r e f r a c t i v e index and inf luence on the pigment e l e c t r o n i c s p e c t r a , Rkheor values f o r chlorophvl l g d i f f e r unessen t i a l lv . In con t r a s t w i t h t h i s R:xp va- l u e s found f o r t he same pigment d i f f e r e s s e n t i a l l y , because the determinat ion Of R:xp was c a r r i e d out by CONCENTRATION EFFECTS IN PIGMENT SOLUTIONS 499 d i f f e r e n t methods, which suppose beforehand e i t h e r mo- d e l of t r a n s f e r i n the l i m i t s o f induct ive bond bet- ween molecules. r ' igure 4 shows the C U P t h e o r e t i c a l curve calcu- l a t e d i n work 1 5 1 and the experimental po in t s ob ta i - ned by us f o r ch lorophyl l s a and - b , pheophvtin and 4-vinyl-protochloroph~ll~ (4VPChl). As i t followed r/ro . 1.0 * =- 2 0.8 - 0.6 - 0.4 - 0.2 - 0 4.0 0.8 - 0.6 - 0.4 - 0.2 0 - FIG. i Comparison of theory and experiment on CuF. So l id cur- ve is t h e o r e t i c a l one (equat ion ( I ) ) ; A: 0 -chlorophyl l a; o -chlorophyl l b. B: 0 -pheophytin I; 0 -4VPChl; A -qVPChl+chlorophyll 2 ( 4 : I ). Solvent - c a s t o r o i l . 500 GURINOVITCH, LOSEV, AND ZENKEVITCH from t h i s f i g u r e f o r a l l pigments the good agreement i s observed up t o values r=0,330,4. A t the g r e a t valu- e s of the experimental po in t s l i e h igher than the t h e o r e t i c a l curve. For ch lorophyl l 2 and 4VPChl a t h igh concent ra t ion r w e the e s s e n t i a l discrepancy o f t h e theory and the experiment i s observed. The compa- r i s o n of these r e s u l t s with the CJF experimental curve f o r i nves t iga t ed pigments, shown i n Figure 2 , permits t o discover the r e a l r e l a t i o n between CDF and CUP. In- \ \ O / / 0.2 , , ".\ -4 -3 -2 - 1 eg 0 -5 FIG.2 C Rela t ive f luorescence y i e l d of pigments in c a s t o r oil as a func t ion of concentrat ion: A: 0 -chlorophyl l s; 0 -chlorophyll k; B: 0 -pheophgtin 2; 0 -4VPChl. CONCENTRATION EFFECTS IN PIGMENT SOLUTIONS 501 deed, as one can s e e f r o m Figure 2 the concent ra t ion emission quenching of s o l u t i o n s i s y e t i n s i g n i f i c a n t u p t o concent ra t ion of order 0 ~ 9 5 ) . In t h i s case the good agreement o f the theory and the experiment i s marked f o r t h e CUP phenomena. Aubsequent r i s i n g of pigment concent ra t ion l eads t o tne e s s e n t i a l quenching of luminescence quantum y i e l d and correspondingly , t o the dev ia t ion of experimental resu- l t s f r o m CDF t n e o r e t i c a l curve ca l cu la t ed as has a l r ea - dy been noted f o r the case of cons tan t auantum y i e l d o f molecule emissiou. The des i c ive importance shouldLt be probably a t tached t o the coincidence of t h e o r e t i c a l ca- l c u l a t i o n s and experimental data i n the case of 4VPChl u p t o C = f O - 2 y as f a r as i t had been assumed i n ca lcu l - a t i o n s of Ho e x t i n c t i o n c o e f f i c i e n t had been eaual t o protochloro- phyl l one which i s known from l i t e r a t u r e [ 4 51. 10-3m/l ("/So= 0 , g + s o f o r t h a t pigment the m o l a r decimal It should b e noted t h a t the discovered dependence of t he emission depo la r i za t ion on the concent ra t ion change of t h e luminescence auantum y i e l d i n t h e inves- t i g a t e d systems have not f u l l phys ica l i n t e r p r e t a t i o n a t p resent , though the inf luence of f luorescence quen- ching o n CDP was inves t iga t ed i n s e v e r a l works [46- 183. ~ r r work[t6] the inc rease of t he f luorescence polar iza- t i o n degree of pigment concent ra ted so lu t ions i s sup- posed t o be condi t ioned by shor ten ing of t he l i f e t i m e 502 GURINOVITCH, LOSEV, AND ZENKEVITCH of the molecule exc i ted s t a t e s . B u t under the same co- nd i t i ons of the in te rmolecular dipole-dipole in t c rac - t i o r i s and of t he high v i scos i ty of so lvent the shor te - ning of l i f e t i m e due t o quenching processes ahould'nt change the r a t i o between the i n i t i a l l y exc i ted molecu- l e s and the molecule-acceptor of energy. In con t r a s t w i t h t h i s the r i s i n g of f luorescence p o l a r i z a t i o n deg- r e e i r i comparison wi th the t h e o r e t i c a l curve observed a t x > o , 4 s i g n i f i e s t h a t the po r t ion of nonpolarized luminescence i n t o t a l emission becomes smaller . Such s e l e c t i v e luminescence quenching of molecules of se- cond type may be explained as follows: quencher mo- l e c u l e s which Quan t i ty increases under concent ra t ion r i s e a f f e c t the whole system of molecules taking p a r t i n energy t r a n s f e r during the e x c i t a t i o n presence i n system. A t f i r s t , by shortening the l i f e t i m e of i n i - t i a l l y exc i ted molecules, the quencher r e s u l t s i n decrease of the second-excited molecule po r t ion f o r which energy migrat ion takes place. B u t under t h i s condi t ion r a t i o between the i n i t i a l l y and the second- exc i t ed molecules r e t a i n s constant . Then when exc i ta - t i o n l o c a l i z e s a t second molecule ensemble due t o mig- r a t i o n the qunecher a f f e c t s t h e second molecules, deac- t i v a t i n g t h e i r addi t ion p a r t and t h e r e f o r e , decrea- s i n g por t ion of nonpolarized luminescence i n t o t a l emission. The experiments with 4VPChl s o l u t i o n s con- CONCENTRATION EFFECTS IN PIGMENT SOLUTIONS 503 t a i n i n g the ewirnolecular concent ra t ion of t h e chloro- p k v l l 2 as a quencher a r e the d i r e c t confirmation of supposed explanat ion [!2J . AS one can s e e from F i - gure 4 , experimental po in t s corresponding t o CDP of 4VPChl alone a r e arranged considerably lower than the experimental po in t s obtained f o r 4VYChl and eh lorophyl l mixture a t the same condi t ions . The s l o w i n g down of ex- per imental C3P i n comparison w i t h the t h e o r e t i c a l cal- c u l a t i o n s observed, i n separa ted s o l u t i o n s of 4VPChl and chlorophvl l a t C > may be a l s o explained by the s e l e c t i v e auenching of monomer luminescence due t o migrat ion processes t o forming pigment aggregated f o r m s [ ! 3 ] . T h u s , the d i f fe rence observed between the t h e o r e t i c a l and t h e experimental r e s u l t s on CDF f o r pigment s o l u t i o n s a t 8 > 0 , 4 a r e s t i p u l a t e d by the in- f luence of concent ra t ion f luorescence quenching only. CONCU4THATIdN QUL2\CHIlJG UP FLUUHUCLNCS AiJD I N T J ~ R ~ O - LECULAH I N T W C T I O N The concent ra t ion quenching of f luorescence is c h a r a c t e r i s t i c f o r ch lorophyl l a and i t s d e r i v a t i v e s (F igure 2 ) a8 f o r manv dyes b u t i t is not always ex- p la ined by t h e presence of admixture o r a s s o c i a t e s [ 9,43,!8,20-22] The fact that t h e quenching occurs i n so lven t s of h igh and low v i s c o s i t y w l t h equal ef- f i c i e n c y r u l e s out d i f fus iona l proceeses as a r e s n l t of quenching. P o r t e r e t al. suppose tha t the meoha- 504 GURINOVITCH, LOSEV, AND ZENKFJITCH nism of concentration quenching i n chlorophyll solu- t ions i s energy t r ans fe r between chlorophvll molecules by inductive resonance followed by capture a t a non- fluorescent trap. The l a t t e r , i n polar so lvents , i s me- r e ly a p a i r o f c h l o r o p ~ ~ y l l molecules the separation of which, although a p a i r of the equilibrium s t a t i s t i c a l d i s t r i b u t i o n , is l e s s than a c r i t i c a l distance H o , and the electronic spec t ra of these quasi-dimers a re not d i f f e ren t f r o m monomer ones 148,221 . Such in t e rp re t a - t i o n of r e s u l t s is e s sen t i a l ly analogous t o Vavilov’s ideas [23] and the mechanism of exc i ta t ion deac t iva t i - on a t such auenching i n wencher molecules remains un- known as before. Pa ra l l e l inves t iga t ion of CqF and the formation of pigment t r i p l e t s t a t e s i n so lu t ion can give an a d d i t i - onal information i n this plan. F i r s t measurements of such s o r t were undertaken i n work 1241 i n which i t w a s shown tha t up t o C= l e s t e r o l the parallelism was observed i n the course of fluorescence quenching and t r i p l e t s t a t e formation. B u t i t i s impossible t o disseminate t h i s conclusion t o the most i n t e re s t ing concentration range fO-* - i-!o-’m/l, where average intermolecular distances change f rom 341 t o 45 A and the in t e rac t ion between molecules can es- e e n t i a l l g influence on the energetics of intramolecu- l a r t rans i t ions . Bperimental r e s u l t s of measurement i*10-2m/1 f o r chlorophyll 2 i n cho- 0 CONCENTRATION EFFECTS IN PIGMENT SOLUTIONS 50 5 of f luorescence and t r i p l e t s t a t e formation r e l a t i v e quantum y i e l d s depending on pigment concent ra t ion (ch- l o rophy l l 2 , pheophytin p), which we obtained i n wide concent ra t ion range [ i3,25,26] , a r e represented i n F igure 3 . A s i s seen f rom Fig.3 a t C 7 t h e */B0 curve f o r ch lorophyl l s t r e t c h e s lower than the curve of r e l a t i v e changing of t r i p l e t s t a t e formation quahtum y i e l d " d o . Analogous d i f f e rences a re observed f o r kl-C1 phta loc ian ine i n c a s t o r o i l - dimethylforma- mide mixture ( ! : I [27] . Nei ther changes o f e l ec t ro - n i c s p e c t r a , showing t o molecular a s soc ia t ion , are dis- covered f o r a l l pigments up t o C- 4 . i O m 2 E Therefore, t h e coincidence of B/J30 and x / x curves, observed up t o CO 4*40-2 t e s t i f i e s the basic channel of e x c i t a t i o n deac t iva t ion i n t h i s concent ra t ion range i s the pro- 10'2m/1 . 1' 0 m 0.5 0.4 0.3 0.2 0.1 0 P I G . 3 Concentration dependence of r e l a t i v e auantum y i e l d s of f luorescence and t r i p l e t s t a t e formation of chlorophyl l a so lu t ions i n c a s t o r o i l . - 506 CURINOVITCH, LOSEV, AND ZENKEVITCH c e s s , caused by i n t e r n a l conversion inc reas ing from sinp;- l e t exc i ted l e v e l . Probably, the e x c i t a t i o n migrat ion between molecules increases quenching ac t ion o f such cen t r e s . The ana lys i s of povsible reasons, r e s u l t i n g i n t he d i f fe rence of B/Bo and c a r r i e d out i n [25-271 , permits t o conclude that i t can be connected w i t h the increas ing of in te rsys tem cross ing p robab i l i t y which f o r chlorophyl l - a"at C= 4 !O-'m/l is 5 times g r e a t e r than analogoue value in low concent ra t ion so lu t ion . The changing of i n t r a - molecular t r a n s i t i o n p r o b a b i l i t i e s i n pigment molecu- l e s a t high concentrat ions i s probably conditioned by in te rmolecular i n t e r a c t i o n which does ' n t .ye t r e s u l t i n t he formation of t r u e a s soc ia t e s b u t a s a per turbing f a c t o r in f luences on the c h a r a c t e r i s t i c s of intramole- c u l a r t r a n s i t i o n s . Thus , data obtained point o u t again t h e formation of s p e c i f i c cen t r e s i n the concentrated pigment so lu t ions which have sharplv decreased f luo res - cence quantum y i e l d due t o increas ing o f intersystem cross ing and i n t e r n a l conversion p r o b a b i l i t i e s . It i s important t o note that the in te rmolecular i n t e r a c t i o n does be the p r i n c i p a l reason of such c e n t e r formation. For 4VPChl and pheophvtin 2 such i n t e r a c t i o n r e s u l t s i n formation of t r u e aggregates , presence o f which is r e a d i l y observed a t C > / )JOcurves a t G > t - 1 0 - 2 y , by the changing of absor- CONCENTRATION EFFECTS I N PIGMENT SOLUTIONS 50 7 p t ion and luminescence spectra[13,28] . A t the same ti- me these cen t r e s f o r ch lorophyl l don’t p r a c t i c a l l y d i s t i n g u i s h from monomers b.v i t s s p e c t r a l p rope r t i e s . A t t h i s po in t of vies i t i s i n t e r e s t i n g t o e s t i - mate e x c i t a t i o n deac t iva t ion r a t e s i n systems with sh- a rp ly changed t r a n s i t i o n p r o b a b i l i t i e s due t o the mu- t u a l in f luence of molecules aga ins t each other . dhe- t h e r i t i s probable cons iderable inc reas ing of i n t e r - system cross ing p robab i l i t y and correspondingly, de- c reas ing of luminescence auantum y i e l d i n a s soc ia t ed pigment complexes? Heal ly , r e s u l t s of t he s p e c t r a l - luminescent i n v e s t i g a t i o n o f ene rge t i c s of chloro’phvll and bacter iochloroph?rl l i n dry petroleum e t h e r showed t h a t the a s soc ia t ion of molecules r e s u l t s i n decrea- sing of in t e r s s s t em cross ing p robab i l i t y ( i n 3-10 ti- mes) a t i nva r i ab le r a d i a t i v e deac t iva t ion p r o b a b i l i t y 1 2 9 1 . The p r i n c i p a l channel of s i n g l e t e x c i t a t i o n ene- rgy deac t iva t ion i n such systems a r e nonradia t ion tra- n s i t i o n s by i n t e r n a l and in te rsvs tem cross ing conver- s ion . It i s also observed ( f o r assoc ia ted pigment co- mplexes i n water-dioxane mixtures ) the e s s e n t i a l shor- ten ing of exc i ted s t a t e l i f e t i m e and the decreasing of f luorescence quantum y i e l d up t o values to-* + (0-3, t h a t is t he e x c i t a t i o n d e a c t i v a t i o n p r o b a b i l i t y cor- responds t o i O i 0 f i O 4 4 sec’’ [ 3 0 , 3 { ] . 50 8 GURINOVITCH, LOSEV, AND ZENKEVITCH B u t even w i t h such h igh exc i t a t ion deac t iva t ion p robab i l i t i e s intermolecular i n t e r a c t i o n i n assoc ia tes of chlorophyll and i t s der iva t ives can r e s u l t i n f l u o - rescence quenching. The d i r e c t experimental confirma- t i o n is the discovering of intracomplex energy trans- f e r i n mixed pigment assoc ia tes f rom weakfluorescent o r nonfluorescent donors (quantum y ie ld of which i s B 4 lo-') and a l s o f o r systems wi th weak overlapping spec t r a [30] . Estimates obtained with using the spec- tral-luminescent cha rac t e r i s t i c s of donor-acceptor p a i r s ( including c i r c u l a r dichroism spec t r a ) showed t h a t the exc i t a t ion t r a n s f e r probabi l i ty i n mixed pig- ment aggregates account for F-3.5 .10'2+ 1,2. fOt3sec-' and upper l i m i t of i n t e r a c t i o n energ.y corresponds t o v ,,=75 cm-' [ 3 ( ] . In that case as long as the mig- r a t i o n process probabi l i ty i n complex i s comparable with the v ib ra t ion re laxa t ion probabi l i ty i n complex organic molecules ( W i 0 ' 2 t 1 0 43 sec") i n such 8.~9- terns i t i s possible t o discover phenomena which take place before o r during the process of Bol tzmann d i s - t r i b u t i o n o f v ib ra t iona l energy i n the exci ted elec- t ron ic s t a t e s o f i n t e rac t ing molecules. The bef ore-relaxation e lec t ronic exc i t a t ion energy t r ans fe r which was discovered i n mixed asso- c i a t e s containing donor component (protochlorophyll CONCENTRATION EFFECTS IN PIGMENT SOLUTIONS 509 o r 4VYChl) and i n c r e a s i n g a c c e p t o r q u a n t i t y (ch loro- p h v l l 2; Pa-pheophytin 2; Cu-pheophytin a) [ 32,331 i s b e l e i v e d t o such processes . It was shown ( i n o u r ex- periments ) t h a t under s imi la r c o n d i t i o n s ( r a t i o of do- n o r and a c c e p t o r components, resonance c o n d i t i o n s ; 1 * y, - S A = A E > 7 K T ) t h e quenching of p r o t o c h l o r o p h y l l luminescence by Cu-pheophvtin 2 molecules i n mixed ag- g r e g a t e e x c e l s i n o r d e r t h e emis j ion quenching of t h i s pigment by c h l o r o p h y l l 2 and Pd-pheoph.vtin molecules (F ig .4) . L i s t e d e s t i m a t e s of e x c i t a t i o n m i g r a t i o n pro- b a b i l i t y v a l u e s f o r complex and a v a i l a b l e experiment- a l fac t s p e r m i t t e d t o conclude t h a t i n systems pro- t ochloroph.vl1 - c h l o r o p h y l l fi and p r o t o c h l o r o p h y l l - - 0.2 - -8 -7 -6 -5 ‘g c, (T, PIG.4 Changing of p r o t o c h l o r o p h v l l luminescence r e l a t i v e quantum y i e l d i n mixed a s s o c i a t e s under increasing of r e l a t i v e c o n c e n t r a t i o n of d i f f e r e n t a c c e p t o r s ( 4 - c h l o r o p h y l l 2; 2-Pd-pheophgtin 2; 3-Cu-pheophytin - a ) i n water-dioxane ( 4 : 4 ) mixtures . 5 10 GURINOVITCH, LOSEV, AND ZENKEVITCH -pd-pheophytin 5 back before-relaxat ion energy t r a n s f e r taok place. It occurs before o r during the process of establ.ishment of thermal equi l ibr ium among v i b r a t i o n l e - v e l s o f acceptor exc i ted s t a t e s . l ixc i ta t ion migrat ion from protochlorophvl l t o Cu-pheophytin a i n such condi- t i o n s i s e s s e n t i a l l y i r r e v e r s i b l e due t o the high exci- t a t i o n deac t iva t ion p r o b a b i l i t y i n acceptor which i s not lower t h a n i o S- according t o o u r es t imat ions. '3 1 It may be considered that a g g r e g ~ t e s of complex molecules a r e the intermediate l i n k between cr ,ys ta l sta- t e and s o l u t i o n s , s o far as t he re is d e f i n i t e order i n t h e s t r u c t u r e of such snstems [34], and the intermole- c u l a r i n t e r a c t i o n energv can correspond t o the caae of l o c a l i z e d exciton. Unfortunatelv, the genera l and str i- c t theory of such i n t e r a c t i o n s i n assoc ia ted systems i s absent and t h i s i s the se r ious obs tac le f o r f u l l i n t e r - p r e t a t i o n of the whole complex of experimental facts. It may be hoped tkiat s o l u t i o n of t h i s problem permits t o consider the r e g u l a r i t i e s of d i f f e r e n t concent ra t ion e f f e c t s more d e f i n i t e l y and d i f f e r e n t i a l l y , and a l so t o t ake i n t o account t h e i r m u t u a l inf luence. m 1 . F.d.Crawer, H,S.Knox. Mol.Phvs., 2 2 , 385, 1971. 2. C.Bojarski, J.Luminescence, 41r 1972. 3. I.M.Rozman, R.Sh.Sichinava, AN SSSH, aer . f iz . , 4. a.N.Bodunov, O p t i k a i Spektr . , 41, 990, 1976. a, 1863, 1975. CONCENTRATION EFFECTS IN PIGMENT SOLUTIONS 511 5. E.N.Bodunov. Zh.Prikl.apektr., 26, 1 1 2 3 , 1977. 6. h1.D.Galani.n. Trudy PIAN, 2, 34171950. 7. Th.Porster. Yluorezcenz organisher Verbindungen. Vandenhoeck und Auprecht in Gottingen, 1951. 8. ul.o.tialanin. Trudy FUN, I2, 3 , 1960. 9. k.P.Losev, d.I.Zenkevitch. Z-h.Prikl.Spektr., 9, 10. E.I.Xenkevitch, A.Y.Losev, G.P.Gurinovitch. Idol. 11. s.I.benkevitch, A.P.Losev, G.P.Gurinovitch. Izv. 12. C.Bojarski, A.Kawski, Ann. der Yhys., 2, 31, 1959. 13. d.I.Lenkevitch.xvtoreferat kand.diss. iUinsk,1973. 14. L.Szala.v, Acta Phys.Pol., 26, 511, L964. 15. V.iui.Koski, J.fi.C.hmith, J.Amer.Chem.Soc. ,z,3558, 16. P.P.Feofilov.PolJarizovannaja lyuminestsentsija atomov molekul i kristallov,Pizmatgiz,iii.,I959, a tr.16b. 1 ' 7 . L.Lzalay, Ec.5:arkaw ,b. I'ombacz ,Ac ta Phys. Chem., Lzeged,LI,21,1965. 18. S.S.Beddard,C.Porter, Nature, a, 366, 1976. 19. E.I.Zenkevitch, A.P.Losev.1~~. AN SSSH, ser.fiz., 20. ~~.F.Natson,n.Livingston,J.Chem.Phys. ,18,802,1950. 21. k.G.l'weet, G.L.Gains, iV.D.Bellamy, J.Eem.Phys., 22. k.k.iielly, L.K.Patterson,Yroc.Boy.Soc.,k L 117,1971 23. S.I.Vavilov,Sobr.soch ,t .2,Izd-vo A& SSS&%52. 24. G.Porter, ti.Strauss, Proc.Roy.Soc., I, 295, 1966. 25. x.P.Losev,Int .=eminar on Energy l'ranzfer in Conden- sed Phase, Yrague,June 29-July 2,P976(in print). 26. A.P.LOSeV, E.I.Zenkevitch,E.I.Sagun. Zh.Prik1. spektr., a, 244, 1977. 27. E.I.Sagun.Avtoreferat kand.diss., Minsk,I977. 28. G.P.Gurinovitch,E.I.Zenkevitch,A.P.Losev.V sborn. Whlorofilltt,Izd. "L?auka i Technika",Liinsk ,1974 ,st74. 29. G.P.Gurinovitch,A.P.Losev,~.I.~agun,Zh.Prikl.Spek- tr., 26, 1028, 1977. 30. E.1.Zenkevitch,k.P.Losev,G.P.Curinovitch,~ol.biol., 31. E.1. Zenkevi tch ,G .A .Kochubeev ,A.P.Losev. ,G . P. Guri- novitch, blol.biol., II, 1039, 1977. 32. E.I.Zenkevitch,A.P.Losev.,G.A.Kochubeev,G.P.Guri- novitch,Abstracts of XI11 European Congress on mo- lecular Spectroscopy,vVroclaw,Poland,I977,p.383. 3 3 . E.I.Zenkevitch,k.P.Losev,G.A.Kochubeev,G.P.Guri- novitch,Izv. AN SSSR, ser.fiz. ,1978 (in print). 144, 1968. biol., 824, 1972. Ah SSSFC, ser.fiz., 2, 979, 1972. 1942. 2, 1845, 1975. 41, 1008, 1964. 2, 516, 1975.