Polymeric Photosensitizers - ACS Symposium Series (ACS Publications)

6 Polymeric Photosensitizers D. C. Neckers

Downloaded by NANYANG TECH UNIV LIB on November 5, 2014 | http://pubs.acs.org Publication Date: May 5, 1986 | doi: 10.1021/bk-1986-0308.ch006

Center for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403

This paper reviews polymeric photosensitizers from the first case indicating that the concept was possible to polymer-rose bengal. The review focuses on the chemistry of the appended small molecule, and the role of the polymer. The most important polymeric photosensitizer to date is polymer rose bengal - a heterogeneous source of singlet oxygen; and the most extensive review is of this system. Immobilized photosensitizers derive ultimately from the work of Kautsky and deBruijn (1). In determining the mechanism of "Photodynamic Action" (2), they carried out the following experiment: Two dyes were separately absorbed at the expanded inner surfaces of a gel. One surface contained the photosensitizer adsorbate, and the other surface the acceptor sorbate. When mixed, the particles are in contact with one another, but the sensitizer and the acceptor are not. The c r i t i c a l experiment for the detection of diffusable, activated oxygen involved irradiating the sensitizer dye, and observing photochemical bleaching of the acceptor dye. The sensitizer was trypaflavine and the acceptor was leuco-malachite green. Kautsky concluded that an excited state of oxygen was involved in the bleaching of the particle immobilized methylene blue and he also reasoned that the metastable oxygen was singlet oxygen (3) • Though 30 years of erroneous conclusions and false premises led to a number of other suggested mechanisms for dye sensitized photooxidation, in 1964 singlet oxygen was confirmed as the 0097-6156/ 86/ 0308-0107507.25/0 © 1986 American Chemical Society

In Polymeric Reagents and Catalysts; Ford, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


108

POLYMERIC REAGENTS A N D CATALYSTS

reactive intermediate produced when low energy t r i p l e t dyes are irradiated i n a i r or pure oxygen. The f i r s t uv s e n s i t i z e r immobilized to a synthetic polymer for the purpose of actually carrying out a photosensitized process was reported by Moser and Cassidy (U)• These workers reported poly(aerylophenone) photoreduced to the corresponding a-phenethy1 alcohol. They concluded that the photoreduction chemistry of poly(arylophenone) was like that of the photoreduct ion chemistry of acetophenone • Peter Leermakers (5^) reported that poly(phenylvinyIketone)—a " b r i t t l e , p l a s t i c mass"—was used for heterogeneous energy transfer to three dienes: norbornadiene, cis-piperylene and myrcene• This work suffered from incompleteness, however, and when we repeated the work according to Leermakers' experimental protocol (5^), we observed that the polymer was s i g n i f i c a n t l y soluble and also contained oligomeric sens i t i z e r s . Our conelusion was that Leermakers and James (_5) did not consider an authentic heterogeneous energy transfer process, but were instead looking simply at energy trans fer react ions of dissolved small acetophenone ana logs in solution. Our own interest in polymeric energy transfer donors derived from Merrifield's work (6). Our f i r s t three papers (7) b e l i e the fact that we were cons idering polymeric uv sens i t izers at i t s incept ion, too. Thus we synthes ized acetyl co(s tyrenedivinylbenzene) only to discover ( ^ - A l C l j (7) • And we made "(?) -benzoyl" only to discover what we (8) and several others (9) already knew—that acetophenone and benzophenone t r i p l e t s we re v i r t u a l l y i d e n t i c a l in chemistry to alkoxy r a d i c a l s . Hydrogen abstraction by alkoxy radicals on benzylic centers is a well known chemical process. @-Benzoyl (10) presented to the world not an effective heterogeneous energy transfer donor, but a procedure for enhancing the photocrosslinking of polystyrene• Asai and Neckers (11) resolved this problem by fluorinating the polymeric backbone, but the results were unspectacular. Enter Rose Bengal Our love a f f a i r with rose bengal i s made clear by our recent publicat ions (12). © - R o s e bengal(13) came about for three rather obvious reasons: 1 • The rose bengal t r i p l e t energy is about 40 kcal/mol. This excited state hasn't the energy to abstract hydrogen from a M e r r i f i e l d poly(styrene-co-divinylbenzene) backbone• Rose bengal t r i p l e t s are completely quenched by oxygen at concentrations over lO'^m. 2• Heterogeneous energy transfer from immobilized dyes to oxygen had precedent in Kautsky's experiments. Long wavelength v i s i b l e l i g h t — p a r t i c u l a r l y in the r e d — i s not absorbed as extensively by polymer beads as is uv radiation. In addition, dye chromophores have huge extinction c o e f f i c i e n t s . A l i t t l e incident radiat ion goes a long way in act ivat ing the dyes. 3• Rose bengal has two nucleophilic centers—the C-2' carboxylate and the C-6 phenoxide. One or both might displace chloride from chloromethy1 funct ions in Merri f i e l d s "Bio-Beads". 1



6.

NECKERS

Polymeric

Photosensitizers

These are c h l o r o m e t h y l a t e d p o l y ( s t y r e n e - c o - d i v i n y l b e n z e n e ) beads. In f a c t , as the © - r o s e bengal p a t e n t d i s c l o s e s ( 1 3 ) , a p p r o x i m a t e l y 40 d i f f e r e n t dyes were i m m o b i l i z e d ( o r at l e a s t we t r i e d t o i m m o b i l i z e 40 d i f f e r e n t dyes) t o M e r r i f i e l d r e s i n . We a l s o r e p o r t e d i m m o b i l i z a t i o n of rose bengal t o c e l l u l o s e , DMAE c e l l u l o s e , paper, n y l o n , and p o l y e t h y l e n e i n t h a t patent a p p l i c a t i o n . However, o f the i m m o b i l i z e d dyes t e s t e d i n 1971, rose bengal i m m o b i l i z e d t o M e r r i f i e l d r e s i n was the most e f f e c tive. I n f a i r n e s s , though, the t e s t p r o t o c o l employed i n these e a r l y experiments (14) was n a i v e , and many f a c t o r s d i s c o v e r e d by my r e c e n t coworkers (15) make i t c l e a r t h a t we s h o u l d r e i n v e s t i g a t e o t h e r i m m o b i l i z e d dyes. About the o n l y t h i n g the e a r l y experiments e s t a b l i s h e d was that ( ? ) - r o s e bengal was a good source of s i n g l e t oxygen i n non-polar s o l v e n t s . The quantum y i e l d s r e p o r t e d i n the e a r l y s t u d i e s were i n c o r r e c t , s i n c e the a c t u a l quantum e f f i c i e n c y o f energy t r a n s f e r depends on oxygen pressure. Rose bengal p r e s e n t s an I n t e r e s t i n g h i s t o r i c a l case i n i t s own r i g h t . R e l a t e d t o Baeyer's f l u o r e s c e i n (16) and Emi1 F i s c h e r ' s e o s i n ( 1 7 ) , i t was s y n t h e s i z e d o r i g i n a l l y by Gnehm (18), the f i r s t p r e s i d e n t of the ETH i n Z u r i c h . Gnehm was employed e a r l y i n h i s c a r e e r by the Swiss c h e m i c a l f i r m Offenbach and Geschwanden, the predecessor i n the 19th c e n t u r y to CJhemical i n d u s t r y B a s e l — C I B A (19) . Gnehm's s y n t h e s i s of rose bengal was made p o s s i b l e because CIBA h e l d the patent on the process f o r forming p e r c h l o r o p h t h a l i e anhydride (20) from p h t h a l i c anhydride by gas phase c h l o r i n a t i o n u s i n g antimony p e n t a c h l o r i d e . The name "Rose B e n g a l " i s a commercial name and r e s u l t s because the dye i s v e r y s i m i l a r i n c o l o r t o the c i n n a b a r paste B e n g a l i women used a t that time as a c e n t e r forehead spot t o s y m b o l i z e m a r r i a g e — I n d i a n happiness wart ( 2 1 ) . Rose bengal shows up f i r s t as a s i n g l e t oxygen s e n s i t i z e r i n von Tappeiner's 1904 paper (2) d e s c r i b i n g the e f f e c t o f f l u o r e s c e n t m a t e r i a l s on enzymes and paramecia; t h i s was the f i r s t r e p o r t of something von Tappeiner r e f e r r e d t o as photodynamic a c t i o n , i . e . l i g h t , dyes and oxygen k i l l e d paramecia. Rose bengal's r o l e as a s e n s i t i z e r i n contemporary m e c h a n i s t i c s t u d i e s i n s i n g l e t oxygen c h e m i s t r y i s l a r g e l y the r e s u l t of Schenck and h i s coworkers ( 2 2 ) . Rose bengal i s a xanthene and r e l a t e d t o both the t r i p h e n y l m e t h a n e s as w e l l as t o f l u o r e s c e i n . Rose bengal d i f f e r s from f l u o r e s c e i n , which i s s t i l l among the most f l u o r e s c e n t compounds known, i n a number o f important ways. I n p r i n c i p l e o n l y one i s i m p o r t a n t : I t i n t e r s y s t e m c r o s s e s to the t r i p l e t w i t h an e f f i c i e n c y o f 78% i n p o l a r s o l v e n t s and f l u o r e s c e s w i t h a quantum y i e l d o f l e s s than 10% i n these s o l v e n t s . F l u o r e s c e i n e f f e c t i v e l y does not i n t e r s y s t e m c r o s s to the t r i p l e t . I t emits from the s i n g l e t s t a t e w i t h very h i g h e f f i c i e n c y — a n a l t o g e t h e r u s e f u l p r o p e r t y i n i t s own r i g h t . The fundamental s t r u c t u r e s of the important xanthenes are shown i n Table I . The numbering system i s o u t l i n e d i n the f i g u r e i n the t a b l e .



110

P O L Y M E R I C REAGENTS A N D C A T A L Y S T S

Polymer based rose bengals were f i r s t r e p o r t e d by Neckers and h i s coworkers i n 1973 (13). 0-r ose bengal was the f i r s t — and i t remains the o n l y — i m p o r t a n t polymer based p h o t o s e n s i t i z e r • Three p r o p e r t i e s make i t u s e f u l : 1• Quantum y i e l d s of s i n g l e t oxygen f o r m a t i o n from polymer rose bengals are e s s e n t i a l l y the same as are quantum y i e l d s o f s i n g l e t oxygen f o r m a t i o n i n s o l u t i o n ( 1 5 ) . 2. Rose bengal i m m o b i l i z e d t o p o l y s t y r e n e beads i s not bleached. Rose bengal i n s o l u t i o n bleaches w i t h c o n t i n u e d radiation. 3. Energy t r a n s f e r from rose bengal i m m o b i l i z e d w i t h i n a p o l y s t y r e n e bead i s a slower process than i s energy t r a n s f e r from a rose bengal i m m o b i l i z e d on the s u r f a c e o f a p o l y s t y r e n e bead. There i s an i n f l u e n c e of oxygen p r e s s u r e on the observed e f f i c i e n c y o f s i n g l e t oxygen f o r m a t i o n from i m m o b i l i z e d rose bengals. 4. S e l f quenching o f e x c i t e d s t a t e dye by ground s t a t e dye w i t h rose bengals i m m o b i l i z e d t o s o l u b l e p o l y s t y r e n e s o c c u r s , b u t i t i s not observed w i t h the beads. Photochemistry

and P h o t o p h y s i c s

o f Immobilized

Rose Bengal

The a b s o r p t i o n spectrum o f rose bengal i n MeOH i s shown i n F i g u r e 1,a. The f l u o r e s c e n c e e x c i t a t i o n spectrum o f a ^ ) - r o s e bengal i s v i r t u a l l y i d e n t i c a l . The t o t a l e m i s s i o n spectrum o f rose bengal taken a t 77°K i n MeTHF shows the expected f l u o r e s c e n c e and phosphorescence, F i g u r e 2,a. The f l u o r e s c e n c e r e l a t e s to the a b s o r p t i o n spectrum as the expected m i r r o r image r e l a t i o n s h i p and the t r i p l e t energy c o r r e s p o n d i n g t o the 0-0 band i s at 740 nm, c o r r e s p o n d i n g t o a t r i p l e t energy of 40 k c a i / m o l e , F i g u r e 2 a . The a b s o r p t i o n spectrum o f the rose bengal chromophore i s almost e n t i r e l y due t o the xanthene p o r t i o n of the m o l e c u l e . Changes i n the s o - c a l l e d A r i n g have l i t t l e or no e f f e c t on any of the s p e c t r o s c o p i c p r o p e r t i e s . Changes i n the e x t e n t of i o n i z a t i o n at C-6 do a f f e c t the spectrum, however, and t h i s i s shown n i c e l y by the s p e c t r a o f rose bengal i n MeOH and the b i s p i p e r i d i n i u m s a l t i n methylene c h l o r i d e , F i g u r e l b . The r e l a t i v e peak h e i g h t s of the s h o r t e r wavelength Xmax (-520 nm) t o the l o n g e r wave l e n g t h Xmax (558 nm) a r e 1:3 i n the f u l l y d i s s o c i a t e d form, b u t almost 1:1 i n the p i p e r i d i n i u m s a l t which i s not as d i s s o c i a t e d i n the s o l v e n t i n q u e s t i o n . Hydrogen bonding a l s o affects the r e l a t i v e p o s i t i o n o f the a b s o r p t i o n maxima c a u s i n g a red s h i f t i n the l o n g e r o f the two maxima. Aggregates o f these dyes a r e a l s o i m p o r t a n t . Rose bengal s o l u t i o n s do not f o l l o w Beer's law even at moderate c o n c e n t r a t i o n s , d e v i a t i n g n e g a t i v e l y because of a g g r e g a t i o n e f f e c t s . The dimers absorb u n i q u e l y from the monomers ( 2 3 ) , which e x i s t only i n d i l u t e s o l u t i o n . The e q u i l i b r i u m c o n s t a n t f o r t h e i r f o r m a t i o n a t 25°C i s 4 x 10"*^ M ~ * . One can assume the dimers t o be s t a c k e d ( F i g u r e 3) though no d i r e c t evidence e x i s t s to support t h i s s t r u c t u r e . The f l u o r e s c e n c e s p e c t r a of rose bengal monomers and dye dimers (24) (a) b e l i e the e x t e n t of d i s s o c i a t i o n a t C-6 ( F i g u r e 2b,c) and a r e (b) s e n s i t i v e t o p r o x i m i t y e f f e c t s . The l a t t e r i s


NECKERS

Polymeric

Photosensitizers


Table I. The Commercial Xanthene Dyes

l

R2

H Br I I

H H H CI

R

1 2 3 4

uranine* eos i n erythrosin rose bengal

X

n m

max( )

491 514 525 548

0.93 0.63 0.08 0.08

0ST

n0l

0.03 0.3 0.6 0.76

0.1 0.4 0.6 0.76

* Uranine i s the d i s o d i u m s a l t of f l u o r e s c e i n .


P O L Y M E R I C REAGENTS A N D CATALYSTS


112

Figure 1. Electronic absorption spectra of rose bengal derivatives 1a. rose bengal absorption spectrum i n MeOH 1b. rose bengal dipiperidine s a l t absorption spectrum i n CH2CI2



NECKERS

Polymeric

Photosensitizers

Figure 2. Emission spectra of rose bengal derivatives. 2a. Total emission spectra of rose bengal at 77 K. 2b. Fluorescence emission of rose bengal, d i trimethyl ammonium s a l t i n EtOH at ambient temperature. 2c. Fluorescence emission of rose bengal, d i trimethyl ammonium s a l t i n EtOH recorded at 77 K. ( X x=520 nm ). e

Figure 3.

Stacked rose bengals in r a t i o n a l l y synthesized dimers.


114

POLYMERIC REAGENTS A N D CATALYSTS

Table

II.

Do t h e D y e s I n t e r a c t ? R e g i o n f o r Rose B e n g a l


Rose B e n g a l

v-J

0

Dimer

/

x

0

Absorption Spectra i n V i s i b l e Dimer Forms i n E t h a n o l

Form

(RBj

X

l

x ^ A

x

C[mol/2]

n

2

5

0.48xl0"

5

559.0

3.24

1.02xl0

565.5

2.55

0.858xl0

5

0.539xl0"

5

565.0

2.51

0.893xl0

5

0.704xl0"

5

560

2.85

0.896xl0

5

1.35xl0"

5

553

2.07

0.77xl0

1.95xl0"

5

561

2.58

0.682xl0

1.85xl0"

5

x

ONa' —' la

( RB)

II Note:

Molar absorption c o e f f i c i e n t s molecular weight.

were c a l c u l a t e d

f o r 0.5


6.

NECKERS

Polymeric

Photosensitizers

shown f r o m t h e a b s o r p t i o n s p e c t r a ( T a b l e I I ) o f r a t i o n a l l y s y n t h e s i z e d r o s e b e n g a l d i m e r s i n w h i c h the dye c h r o m o p h o r e s are s e p a r a t e d by m e t h y l e n e u n i t s 2 t o 6 c a r b o n s l o n g ( 1 2 ) . In the e m i s s i o n s p e c t r a of t h e s e d i m e r s , the r e l a t i v e r a t i o o f I \ / I is a s e n s i t i v e f u n c t i o n o f the l e n g t h o f the m e t h y l e n e c h a i n s e p a r a t i n g t h e two c h r o m o p h o r e s ( F i g u r e 4 ) • T h i s i s a l s o the r e s u l t o f t h e s t a c k i n g o f t h e d y e s as a f u n c t i o n o f t h e s t e r e o c h e m i c a l m o b i l i t y o f the c h a i n o f c a r b o n atoms separating t h e two c h r o m o p h o r e s . The l o n g e r m e t h y l e n e c h a i n s a l l o w f o r more rose bengal/rose bengal i n t e r a c t i o n s .


2

T h e r e a r e s e v e r a l e f f e c t s o f d y e / d y e i n t e r a c t i o n s on photochemical energy transfer. 1. Dye t r i p l e t s c a n be q u e n c h e d by a p p r o p r i a t e l y p o s i t i o n e d , p r o x i m a t e g r o u n d s t a t e dye m o l e c u l e s o f the appropriate excited state energies. This is s o - c a l l e d s e l f quenching. 2. Dye a g g r e g a t e s a b s o r b a t d i f f e r e n t w a v e l e n g t h s , e m i t a t l o w e r w a v e l e n g t h s and h a v e l o w e r t r i p l e t e n e r g i e s . This t r a n s l a t e s i n t o a b s o r p t i o n from d i m e r s , o r e m i s s i o n from exc imers• 3. Dye a g g r e g a t e s may be q u e n c h e d by 0 d i f f e r e n t l y t h a n are s i t e i s o l a t e d dyes. 2

Dye/Dye

Interactions

and

Their Effect

on S i n g l e t

Oxygen F o r m a t i o n .

The o v e r a l l e f f e c t o f one d y e i n t e r a c t i n g w i t h a n o t h e r d y e on a p o l y m e r i c s u p p o r t w i l l l i k e l y be t o d e c r e a s e t h e e f f i c i e n c y o f e n e r g y t r a n s f e r t o o x y g e n , i . e . t h e q u a n t u m e f f i c i e n c y of s i n g l e t oxygen f o r m a t i o n . We h a v e s t u d i e d t h e e f f e c t o f d y e / d y e i n t e r a c t i o n s on s i n g l e t o x y g e n f o r m a t i o n b o t h i n s o l u b l e p o l y m e r s and w i t h p o l y m e r b e a d s . Rose b e n g a l i s our p r o b e . The i m m o b i l i z a t i o n o f r o s e b e n g a l t o n o n - c r o s s l i n k e d p o l y ( s t y r e n e - c o - c h l o r o m e t h y I s t y r e n e ) i s s h o w n f o r a 3:1 c o - p o l y m e r w i t h d i f f e r i n g c o n c e n t r a t i o n of rose bengal i n Table I I I . Table

III. The Y i e l d s , % - C H C l w h i c h C o r r e s p o n d t o t h e Rose B e n g a l s Added t o the R e a c t i o n M i x t u r e o f 0 - C H C l , a n d t h e c m " o f C=0 i n © - R B 2

Number o f

1

2

Polymert

(D-RB-51* (?>-RB-102 QVRB-152 m-RB-305 @-RB-450 m-RB-610 m-RB-1520

t

mg. r o s e

Yield [g]

0.85 0.95 0.77 1.03 1.15 0.90 1.05

bengal

per

% I in Polymer

% o f CH C1 group

76.2 70.3 76.2

1.65 3.30 5.00 10.0 15.0 20.0 50.0

gram o f

2

C=0 cm""

1

1725.0 1725.0 1725.0 1741.4 1728.1 1738.3 1739.8

polymer


116

P O L Y M E R I C R E A G E N T S A N D CATALYSTS

Three f e a t u r e s of the data are i m p o r t a n t . 1. The r e a c t i o n occurs w i t h 75% + 2 y i e l d . 2. The i n f r a r e d s t r e t c h i n g frequency o f the carboxy1 o f the i m m o b i l i z e d dye i n c r e a s e s as the dyes come i n t o c l o s e r p r o x i m i t y (higher loading). 3. Dye l o a d i n g i n f l u e n c e s the ^\/^2 * ° *- both the a b s o r p t i o n spectrum and t h e e m i s s i o n s p e c t r a . P h o t o o x i d a t i o n u s i n g t h e polymers of Table I I I as sens i t i z e r s i n CH2CI2 s o l u t i o n i s g i v e n as a f u n c t i o n of polymer l o a d i n g by rose bengal i n F i g u r e 5. The r e s u l t s d i v i d e i n t o two s e p a r a t e regimes. The l i g h t l y loaded regime (a) i s not p a r t i c u l a r l y i n t e r e s t i n g s i n c e e f f e c t s i n t h i s regime a r e s o l e l y the r e s u l t of O2 m o b i l i t y e f f e c t s — a n d these depend on s o l u t i o n v i s c o s i t y . I n the more h i g h l y loaded regime (b) the e f f e c t s are those of s e l f quenching. Beyond a l o a d i n g o f 340 mg/g, s i t e / s i t e i n t e r a c t i o n s quench the dye e x c i t e d s t a t e i n s o l u t i o n and t h e quantum y i e l d o f s i n g l e t oxygen decreases beyond t h e l o a d i n g p o i n t . Based on simple s t a t i s t i c a l c a l c u l a t i o n s , t h i s suggests t h a t i f the polymer i m m o b i l i z e d dyes are c l o s e r than 50 s t y r e n e u n i t s , " s i t e / s i t e " quenching (11) o c c u r s i n s o l u t i o n , and the e f f i c i e n c y o f s i n g l e t oxygen f o r mation d e c r e a s e s . S i t e / s i t e i n t e r a c t i o n s i n s o l u b l e polymers show up i n the a b s o r p t i o n s p e c t r a of the i m m o b i l i z e d dyes ( F i g u r e 6 ) , and t h i s i n f l u e n c e i s s i g n i f i c a n t l y observed i n the c o l o r of the polymers. Thus, l i g h t l y loaded polymers are p u r p l e — m o r e h e a v i l y loaded polymers are deep r e d . The e f f i c i e n c y o f photochemical energy t r a n s f e r from rose b e n g a l i m m o b i l i z e d t o M e r r i f i e l d r e s i n beads has now been s t u d i e d i n g r e a t d e t a i l by Paczkowski, Paczkowska and Neckers ( 2 6 ) . The r a t e of p h o t o o x i d a t i o n o f 2,3-dipheny1-p-dioxene, a c h e m i c a l quencher o f s i n g l e t oxygen, can be compared f o r the v a r i o u s \ Poly-RBs ( i . e . s o l u b l e polymer rose bengals as the source of O2) or (p)-RBs w i t h the r a t e o f p h o t o o x i d a t i o n o f 2,3-dipheny1-pdioxene f o r rose bengal i n MeOH (0O2 0.76). The important e x p e r i m e n t a l parameter which must be c o n t r o l l e d when u s i n g t h i s method t o study the energy t r a n s f e r process from the t r i p l e t s t a t e of the s e n s i t i z e r t o oxygen i s to assure t h a t i t be a z e r o o r d e r r e a c t i o n . The s i n g l e t oxygen quenching p r o c e s s by 2,3d i p h e n y l - p - d i o x e n e must a l s o be a z e r o o r d e r r e a c t i o n . As G o t t s c h a l k and Neckers have shown ( 2 7 ) , t h i s depends on the l i f e time of s i n g l e t oxygen i n the s o l v e n t i n q u e s t i o n . Below the d e f i n e d c r u c i a l quencher c o n c e n t r a t i o n , the Schaap/Thayer r e l a t i v e a c t i n o r a e t r i c method i s not a v a l i d measure of the quantum 1 y i e l d of 0 formation. F i g u r e 7 shows observed e f f i c i e n c y o f s i n g l e t oxygen f o r m a t i o n ( u s i n g (F)-RB-2.0 as a s e n s i t i z e r ) as a f u n c t i o n o f the f l o w o f oxygen i n the c e l l . The r a t e of 2,3-dipheny1-p-dioxene p h o t o o x i d a t i o n u s i n g RB d i s s o l v e d i n MeOH as the s e n s i t i z e r e s s e n t i a l l y does not change as a f u n c t i o n of the r a t e of f l o w o f oxygen. The r a t e of


r a t

n

355

2



NECKERS

Polymeric

Photosensitizers

A

(nm)

Figure 4• The fluorescence emission spectra of rose bengal dimers in EtOH recorded at 77°K ( X « 520 nm). e x

Compounds numbered i n Table I I .

Figure tion

of

5.

Quantum the

amount

yield of

of

singlet

RB a t t a c h e d

to

oxygen a

formation

as

a

polymer.


func-

P O L Y M E R I C R E A G E N T S A N D CATALYSTS


118

Figure 7. Observed e f f i c i e n c y of singlet oxygen formation ( f o r (p)-RB-2.0) as a function of the flow of oxygen.


6.

NECKERS

Polymeric

Photosensitizers

2,3-dipheny1-p-dioxene p h o t o o x i d a t i o n u s i n g (F)-RB as s e n s i t i z e r , however, i n c r e a s e s as the oxygen f l o w i n c r e a s e s . I t i s c l e a r t h a t there must be a b a r r i e r t o p e n e t r a t i o n of oxygen i n t o the ( F ) - R B beads which i s 0 f l o w dependent, and s a i d p e n e t r a t i o n by 0 i n t o the bead occurs at a r a t e which i s slower than i s the r a t e o f thermal d i f f u s i o n i n f l u i d s o l u t i o n . Higher r a t e s o f oxygen f l o w i n f l u e n c e the quantum y i e l d o f s i n g l e t oxygen f o r mation i n the heterogeneous polymer s e n s i t i z e r case, b u t have no i n f l u e n c e on the r a t e s e n s i t i z e d by RB i n s o l u t i o n , because i n s o l u t i o n the quantum y i e l d o f quenching by 0 i s d i f f u s i o n c o n t r o l l e d and not i n f l u e n c e d by s o l u t i o n v i s c o s i t y . For p u r poses of quantum y i e l d s t u d i e s a l l s o l v e n t s have the same v i s c o sity. For a (F)-rose b e n g a l , however, two d i f f e r e n t phases e x i s t ; a s o l u t i o n of the 1Q a c c e p t o r i n C H C 1 s o l u t i o n o u t s i d e the bead and another w i t h i n the s w o l l e n polymer beads. The b e a d / s o l u t i o n i n t e r f a c e and the s o l v e n t / a c c e p t o r system i n the bead have a much h i g h e r e f f e c t i v e o v e r a l l v i s c o s i t y . Since d i f f u s i o n of oxygen i n t o the beads from the s o l u t i o n i s a f u n c t i o n o f the e f f e c t i v e v i s c o s i t y i n the bead's i n t e r i o r , the r a t e o f oxygen f l o w must be much h i g h e r t o f o r c e 0 t o quench a l l o f the rose bengal t r i p l e t s formed i n the i n t e r i o r of the bead. The p e r t i n e n t o b s e r v a t i o n i s t h a t there i s a c r i t i c a l oxygen f l o w r a t e below which not a l l R B ^ i n the i n t e r i o r of the beads are not quenched by 0 . Rose bengal l o a d i n g on M e r r i f i e l d beads does not a f f e c t the quantum y i e l d o f s i n g l e t oxygen f o r m a t i o n s i g n i f i c a n t l y , Table IV. The h i g h e s t observed value i s 0 . 9 1 ( f o r (P)-RB-0.4); o t h e r v a l u e s are only 15% lower, e.g. (0 = 0.76 f o r (F)-RB-0.2). Both these v a l u e s are unexpectedly h i g h e r than those observed by Schaap, Thayer, B l o s s e y , and Neckers ( 1 4 ) and t h e i r coworkers (

Polymeric Photosensitizers - ACS Symposium Series (ACS Publications)

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