Radiation Chemistry

---

21 The Application of Pulse Radiolysis to the

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Radiation Chemistry of Organic Dyes LEONARD

I. G R O S S W E I N E R

Physics Department, Illinois Institute of Technology and Department of Radiation Therapy, M i c h a e l Reese Hospital and M e d i c a l Center, Chicago, Ill.

Pulse radiolysis studies have shown that many organic dyes are highly reactive toward the products of water radiolysis. -

Dyes with quinonoid structures are reduced by e aq to the semiquinone in encounter-limited reactions. Hydroxyl radi­ cals react by oxidation and addition processes. In general, reductive bleaching takes place via semiquinone dismutation, while oxidative decoloration is a complex process involving several O H radicals. The complexing of dyes to high molecular weight substrates leads to a marked change of

e-aq

reactivity.

Some oxidized dye intermediates and

triplet states react with e-aq in chemiluminescent processes.

>Tphe r a d i a t i o n c h e m i s t r y of o r g a n i c dyes has b e e n s t u d i e d f o r almost A

40 years.

I n early w o r k t h e emphasis w a s o n t h e c o l o r changes i n ­

d u c e d b y i r r a d i a t i o n , m o t i v a t e d i n p a r t b y the p o s s i b i l i t y that d y e s o l u ­ tions m i g h t serve as c o n v e n i e n t dosimeters

( 2 5 ) . A l t h o u g h dyes are

g e n e r a l l y m o r e c o m p l e x t h a n the other o r g a n i c m o l e c u l e s w h o s e r a d i a t i o n c h e m i s t r y h a s b e e n s t u d i e d i n d e t a i l , specific reasons c a n b e c i t e d f o r interest i n this subject.

F r o m t h e v i e w p o i n t of r a d i a t i o n b i o l o g y , dyes

c a n a c t as w e l l - d e f i n e d m o d e l s o f b i o l o g i c a l r e d o x systems.

T h i s aspect

is p a r t i c u l a r l y p e r t i n e n t i n c o n n e c t i o n w i t h recent w o r k o n the r a d i o l y s i s of dyes c o m p l e x e d t o h i g h m o l e c u l a r w e i g h t substrates.

I n addition, the

strong v i s i b l e c o l o r a t i o n of d y e d e r i v a t i v e s facilitates t h e i d e n t i f i c a t i o n of the transient a n d p e r m a n e n t r e a c t i o n p r o d u c t s , w h i c h offsets t o some extent t h e m u l t i p l i c i t y of possible reactions.

S o m e of t h e intermediates

f o r m e d i n r a d i o l y s i s are p h o t o c h e m i c a l p r o d u c t s also, w h i c h has assisted c o n s i d e r a b l y i n e l u c i d a t i n g r e a c t i o n m e c h a n i s m s i n b o t h cases.

Despite

the p o t e n t i a l a p p l i c a t i o n s of p u l s e r a d i o l y s i s t o this subject, t h e a c t u a l a m o u n t of w o r k r e p o r t e d since 1962 has n o t b e e n large.

T h e current

309 Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

310

RADIATION CHEMISTRY

1

status of the field appears to p a r a l l e l d y e p h o t o c h e m i s t r y at a p p r o x i m a t e l y a d e c a d e ago, w h e n flash p h o t o l y t i c methods w e r e i n t r o d u c e d to c o m ­ p l e m e n t the u s u a l p r o c e d u r e s of c o n t i n u o u s i r r a d i a t i o n a n d product characterization.

permanent

T h u s , the emphasis i n the pulse r a d i o l y s i s ap­

p r o a c h has b e e n o n i d e n t i f y i n g the transient species f o r m e d w h e n the d y e reacts w i t h the p r o d u c t s of w a t e r r a d i o l y s i s a n d m e a s u r i n g t h e i r f o r m a t i o n a n d d e c a y rate constants.

T y p i c a l l y , these intermediates

can

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result f r o m b o t h o x i d a t i o n a n d r e d u c t i o n processes, a n d they m a y exist i n m o r e t h a n one i o n i c state w i t h different spectra a n d reactivities. t i v e l y c o m p l e t e investigations h a v e b e e n a t t e m p t e d o n l y for tives of the thiazines a n d xanthenes.

Rela­

representa­

I n a d d i t i o n to o p t i c a l a b s o r p t i o n

spectra, n e w i n f o r m a t i o n o n c h e m i l u m i n e s c e n t reactions of dyes has b e e n o b t a i n e d b y electron p u l s e - i n d u c e d emission measurements.

This paper

comprises a s u m m a r y of recent w o r k , i n c l u d i n g a b r i e f d i s c u s s i o n of p e r t i n e n t steady i r r a d i a t i o n G values. Research Results Pulse Radiolysis of Aqueous Methylene Blue. T h e first p u l s e r a d i o l y ­ sis s t u d y of a n o r g a n i c d y e w a s c a r r i e d out o n the t h i a z i n e d y e , m e t h y l e n e b l u e (3,

C o n s i d e r a b l e emphasis was g i v e n to the different i o n i c

12).

states of the d y e , r e f e r r e d to as M B H

2

(pK

3 +

a

=

-5.1), M B H

(pK.

2 +

=

0.0), a n d M B . T h e transient spectra o b t a i n e d b y i r r a d i a t i n g n e u t r a l a n d +

a l k a l i n e solutions w i t h 2-fxsec. pulses of 4 M e v . electrons i n the presence of f o r m a t e s h o w e d that the d y e is b l e a c h e d via a two-step process, w i t h a c o n c u r r e n t a b s o r p t i o n increase p e a k i n g at 420 m/x. T h e

intermediate,

d e s i g n a t e d as M B - corresponds w i t h the m e t h y l e n e b l u e

semiquinone

first i d e n t i f i e d b y flash p h o t o l y s i s i n the presence of r e d u c i n g agents T h e faster step w a s a t t r i b u t e d to the r e d u c t i o n of M B

15, 17).

b y c o m p a r i n g its rate w i t h the d e c a y of e\

q

+

(11,

by

e'

m

at 720 m/x, a n d the slower

step was a t t r i b u t e d to r e d u c t i o n b y C 0 ~ r e s u l t i n g f r o m the s c a v e n g i n g 2

of O H b y formate.

T h e s e m i q u i n o n e disappears b y a second-order p r o c ­

ess a c c o m p a n i e d b y a p a r t i a l r e t u r n of the d y e , w h i c h was e x p l a i n e d b y d i s p r o p o r t i o n a t i o n l e a d i n g to e q u a l parts of M B MBH.

The

methylene

K e e n e et at

(12)

blue semiquinone

2 +

( i n 0.1N

p r o t o n a t i o n states assigned

differ f r o m the results of M a t s u m o t o ( 1 5 ) ,

p o s e d the f o l l o w i n g f o u r states: M B MBH,

a n d the l e u c o base,

+

T h e rate constants are s u m m a r i z e d i n T a b l e I.

H S0 ) 2

4

and M B H ,

3 +

• (pH >

9), M B H

( i n cone. H S 0 ) . 2

4

+

by

who pro­ (pH

3-8),

R e c e n t meas­

urements b y J. F a u r e , R. B o n n e a u , a n d J. J o u s s o t - D u b i e n [/. Chim.

Phys.

6 5, 369 (1968) ] o n the effect of the i o n i c strength o n the d i s m u t a t i o n rate constant

s u p p o r t the c o n c l u s i o n of M a t s u m o t o that the n e u t r a l s e m i ­

q u i n o n e M B - is stable o n l y above p H 9.

Keene, L a n d , and Swallow

Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

21.

GROSSWEINER

[J.

Chim.

Organic 65, 371

Phys.

311

Dyes

( 1 9 6 8 ) ] n o w agree w i t h the assignments

gested b y M a t s u m o t a a n d F a u r e et al.

a n d i n d i c a t e that the

sug­

reaction

m e c h a n i s m s g i v e n i n Ref. 12 ( w h i c h are s u m m a r i z e d i n T a b l e I )

must

be m o d i f i e d a c c o r d i n g l y . Table I.

Pulse Radiolysis Rate Constants for Methylene Blue

Reaction ' 0

+ MB

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m

pH

6

MB-

+

7.8

C0 " + MB -» MB• + C 0 +

2

COOH- + M B -» MBH +

COOH- + M B H COOH- + M B H H

+ C0

9

3 +

-» MBH

2

2

+ H

+

0

— -0.8

9

—2 X 1 0 -TO

d

+

2 +

—7 X 10

2 MB- + H 0 -» M B H + MB + OH"

7.8

3.0 X 10

2 MBH -» MBH + MBH Data from Keene, Land, and Swallow (12). Ionic states of methylene blue M B , M B H , M B H M B - , M B H , M B H ; leuco base: M B H . M B H , M B H ; leuco base: M B H . Units of liters/mole-sec. *H .

0.8"

2 +

2

+

9

9

— -5.7*

+

(10%)

10

5.6 X 1 0 ' ( 1 0 % )

1.8

2

+ C0

+

Constant

2.5 X 1 0

7.8

2

+ C0

+

-> M B H

2 +

Rate

2 +

8

(10%)

9

—1.6 X 10

9

a

b

+

+

+

2

2

2 +

2

3 +

, ionic states of the semiquinone:

2 +

2 +

c

0

T h e p u l s e r a d i o l y s i s of M B w i t h o u t f o r m a t e l e d to t h e M B • absorp­ +

t i o n p l u s another transient p e a k i n g at 520 m^t.

T h e latter

corresponds

w i t h the s p e c t r u m of s e m i o x i d i z e d d y e o b t a i n e d b y flash p h o t o l y s i s 15)

(II,

a n d w a s a t t r i b u t e d to the attack of O H . H o w e v e r , t h e a d d i t i o n of

O H to the d y e w a s not r u l e d out.

N o e v i d e n c e w a s f o u n d for t r i p l e t

f o r m a t i o n . T h e spectral changes o b t a i n e d i n m o r e a c i d i c solutions w i t h formate

s h o w that

d e s i g n a t e d as M B H

the 2

2 +

semiquinone

(pK

ffl

—

-3)

occurs

i n two protonated

and M B H

(pK

+

ffi

— 2).

states

It w a s sug­

gested that the first p r o t o n is a t t a c h e d to the c e n t r a l r i n g n i t r o g e n a t o m a n d the second p r o t o n adds to the — N ( C H ) 3

group. T h e approximate

2

f o r m a t i o n a n d d e c a y rate constants are g i v e n i n T a b l e I. Pulse Radiolysis of Aqueous Fluorescein Dyes. T h e xanthene of the

fluorescein

electrons fluorescein

(4,

dyes

t y p e w e r e i n v e s t i g a t e d u s i n g 1-jusec. pulses of 30 M e v .

5, 8).

The

transient

spectra

obtained

with

deaerated

solutions s h o w three characteristic sets of b a n d s . A p r o m i n e n t

p e a k that shifts f r o m 355 m/* i n n e u t r a l solutions to 395 m/x i n a l k a l i n e solutions corresponds w i t h the s e m i q u i n o n e m o n o a n i o n (13).

T h i s b a n d is q u e n c h e d b y e~

m

(pK

a

=

9-5)

scavengers, s u c h as o x y g e n or H 0 , 2

a n d w a s a t t r i b u t e d to r e d u c t i o n of the d y e b y e\ . q

2

A b a n d at 4 1 5 - 4 2 0 m/x

w h i c h does not c h a n g e w i t h p H w a s i d e n t i f i e d w i t h the s e m i o x i d i z e d r a d i c a l m o n o a n i o n , a p h e n o x y l d e r i v a t i v e first o b s e r v e d i n flash p h o t o l y s i s also ( 1 3 ) .

T h i s b a n d is q u e n c h e d b y f o r m a t e a n d w a s a t t r i b u t e d to the

o x i d a t i v e attack of O H . T h e r e m a i n i n g transient consists of a diffuse

Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

312

RADIATION CHEMISTRY

a b s o r p t i o n to l o n g w a v e l e n g t h s of the

fluorescein

1

d i a n i o n b a n d (491 m/x)

a n d was d e s i g n a t e d as the " r e d p r o d u c t . " It was d i s t i n g u i s h e d f r o m the other t w o species b y its c o n s i d e r a b l y l o n g e r l i f e t i m e a n d

first-order

decay.

T h e " r e d p r o d u c t " has at least t w o constituents w i t h different b u i l d u p a n d d e c a y rates. It is q u e n c h e d e n t i r e l y b y H a n d O H scavengers

such

as f o r m a t e or e t h y l a l c o h o l , w h i l e o n l y one c o m p o n e n t w a s o b s e r v e d i n air-saturated solutions. T h e l o n g e r - l i v e d constituent w a s i d e n t i f i e d w i t h Downloaded by NANYANG TECHNOLOGICAL UNIV on June 21, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch021

O H a d d u c t s of the xanthene r i n g system, a n d the secondary w e r e assumed to be H - a t o m adducts. quinone ( R )

and semioxidized dye ( X )

10.7 are G ( R ) =

3.3 ±

components

T h e i n i t i a l y i e l d s of the

0.3 a n d G ( X ) =

i n deaerated 1.4 ±

0.2

semi­

solutions at p H

(5).

T h e results o b t a i n e d w i t h the eosin d i a n i o n are s i m i l a r to

fluorescein.

I n this case, the R absorptions at 369 m/x ( m o n o a n i o n ) a n d 405 m/x ( d i ­ a n i o n ) agree exactly w i t h the flash photolysis assignments

10,

16),

w h i l e the X m o n o a n i o n b a n d at 450 m/x is reasonably close to the

flash

photolysis spectra r e p o r t e d at 462

(10)

(7,

a n d 456 m/x (16).

The

eosin

O H a d d u c t m a x i m u m is l o c a t e d near 600 m/x c o m p a r e d w i t h 570 ± 20 m/x for

fluorescein

and G ( X ) =

T h e i n i t i a l y i e l d s at p H 9.0 are G ( R ) =

(4,5). 2.0 ±

0.3 (22).

3.3 =t 0.3

T h e smaller c o n t r i b u t i o n of the O H a d d i ­

t i o n p a t h i n eosin c o m p a r e d w i t h

fluorescein

is consistent w i t h the o c c u ­

p a t i o n of f o u r of the six xanthene r i n g sites b y b r o m i n e atoms. T h e o n l y i n ­ f o r m a t i o n a v a i l a b l e o n e r y t h r o s i n f r o m p u l s e r a d i o l y s i s locates the d i a n i o n at 450 m/x a n d the X m o n o a n i o n at 470 m/x T h e rate constants f o r d y e r e d u c t i o n b y e~

m

e~

m

as c a l c u l a t e d f r o m the

pseudo-first-order l i f e t i m e i n the presence of formate

(to

suppress

the o v e r l a p p i n g " r e d p r o d u c t " a b s o r p t i o n ) are g i v e n i n T a b l e II. b o t h f o r m a t e a n d a n e~

scavenger s u c h as N 0 or H 0 2

m

R

(6).

2

2

When

are present, the

o n l y significant r e a c t i o n is the r e d u c t i o n of the d y e b y C 0 ~ . T h e rate 2

constants d e t e r m i n e d b y a n a l o g c o m p u t e r fit to the g r o w t h rate of are g i v e n i n T a b l e II.

R

A l t h o u g h the b r o a d " r e d p r o d u c t " a b s o r p t i o n

l i m i t s the use of c o m p e t i t i o n m e t h o d s f o r d e t e r m i n i n g the O H r e a c t i o n rates, estimates w e r e m a d e b y a n a l o g c o m p u t e r s o l u t i o n to the kinetics of the

[dye/e" /OH] a q

system b a s e d o n the d e p e n d e n c e of G ( R )

G ( X ) on initial dye concentration

(Table

and

II).

It w a s s h o w n that R a n d X react together d u r i n g the early stages of t h e i r d e c a y i n deaerated bleaching yields under

6 0

solutions, w h i c h accounts f o r the l o w d y e

C o i r r a d i a t i o n unless a n e'

m

or O H

scavenger

is a d d e d . T h e d e c a y of R i n the absence of X is second o r d e r ; h o w e v e r , the rate constant decreases m a r k e d l y w i t h i n c r e a s i n g d y e c o n c e n t r a t i o n . T h e p r o p o s e d e x p l a n a t i o n is that the s e m i q u i n o n e forms a c o m p l e x w i t h u n r e a c t e d d y e , so that the a c t u a l d e c a y rate is c o n t r o l l e d b y the e q u i ­ l i b r i u m c o n c e n t r a t i o n of free s e m i q u i n o n e . T h e analysis l e d to the s e m i ­ q u i n o n e d i s p r o p o r t i o n a t i o n rate constant

a n d a l i m i t i n g v a l u e of

Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

the

21.

GROSSWEINER

Organic

e q u i l i b r i u m constant.

313

Dyes

T h e " r e d p r o d u c t " d e c a y is first order a n d p a r a l l e l s

the r e t u r n of c o l o r a t i o n i n the r e g i o n of the d y e a b s o r p t i o n . T h e m e c h a ­ n i s m p r o p o s e d is s l o w w a t e r ( o r O H " ) e l i m i n a t i o n f r o m the O H a d d u c t l e a d i n g to the f o r m a t i o n of X a n d e v e n t u a l l y a c o l o r e d p e r m a n e n t p r o d u c t (Table

II).

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Table II.

Pulse Radiolysis Rate Constants for Fluorescein and Eosin Fluorescein

Reaction

a

e~

m

+ S + H -> R +

Rate Constant*

pH

Ref.

1.4 X 10!° ( 1 5 % ) 2.0 X 1 0 ( 1 0 % )

10.7 13

5 20

2.6 X 10

7

(35%)

10.4

2.0 X 10

7

(20%)

10.4

10

CO," + S + H R + C0 2 R + H 0 -> L + S + OHOH- + S - » X + OHO H - 4- S ^ S O H R + X —> 2 S SOH- - > X + O H R + S ^ RS

Eosin Rate Constant

pH

11

Ref.

2.2 X 1 0

10

(20%)

9.0

c

5

4.2 X 10

8

(20%)

8.8

4

5

1.6 X 10

7

(20%)

9.0

c

10.7

5

.7

5

10.7 10.7

5 5 5

1.7 X 10 ( 2 0 % ) 0.6 X 10 ( 3 0 % ) 6.5 X 10 ( 2 0 % ) 70 sec." (10%) K < 5 X 10 M "

+

2

2

1.6 X 10 ( 2 0 % ) 1.4 X 10» ( 4.7 X 10 ( 2 0 % ) 100 sec." ( 2 0 % ) K < 4 X 10 M " 9

1 5 % )

8

1

6

1

1 0

10.4

9

9

8

1

5

1

9.0 9.0 8.5 9.0 9.0

c c

4 c c

Dye dianion (S); semiquinone dianion ( R ) ; semioxidized dye monoanion ( X ) ; O H adduct (SOH •); leuco base ( L ) ; complex (RS). In units of liters/mole-sec. unless indicated otherwise. Unpublished data.

a

b c

Table III.

Pulse Radiolysis Rate Constants for Other Dyes

Reaction Dye + e~ m

Dye + O H Dye + C 0 " Dye + D-glucose 2

a

ox

Dye Acriflavine Rhodamine B Acridine orange M e t h y l green Rhodamine B Rhodamine B Methylene blue

Rate 3.3 X ^3 X 3.2 X 4.3 X ^9 X 1.8 X 2 X

Ref. 20 20 2 2 20 20 2

Constant" 10 (10%) 10 10 10 10 10 (25%) 10 10

10

10

10

9

8

9

Units of liters/mole-sec. Pulse Radiolysis of Other Dyes in Aqueous Solution. I n c o n n e c t i o n

w i t h a n i n v e s t i g a t i o n of e l e c t r o n p u l s e - i n d u c e d l u m i n e s c e n c e b e l o w ) , P r i i t z a n d L a n d (20)

(discussed

r e p o r t e d rate constants for the

of r h o d a m i n e B a n d a c r i f l a v i n e ( T a b l e I I I ) .

reactions

The rhodamine B reduction

p r o d u c t ( i n the presence of f o r m a t e ) absorbs most strongly at 410 m/x, w h i c h corresponds w i t h the s e m i q u i n o n e transient o b t a i n e d b y the p h o t o l y s i s of the d y e i n aqueous f o r m e d w i t h N 0 present 2

a l c o h o l solutions

absorbs b e t w e e n

350

(24).

a n d 500

The

species

m/x. It

Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

flash was

314

RADIATION CHEMISTRY

1

o b s e r v e d that b o t h the s e m i q u i n o n e a n d the o x i d a t i o n p r o d u c t are l o n g l i v e d i n deaerated

solutions, w h i c h suggests that the latter i n c l u d e s a

s u b s t a n t i a l c o n t r i b u t i o n f r o m the O H a d d u c t . a n i o n complexes b y B a l a z s et at.

w e r e r e p o r t e d f o r the r e a c t i o n of e~

m

m e t h y l green ( M G

)

2 +

(Table III).

I n a s t u d y of d y e - p o l y -

( d i s c u s s e d b e l o w ) rate constants

(2)

w i t h a c r i d i n e orange For A O

+

(AO )

and

+

the d i s a p p e a r a n c e

of the

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d y e is a c c o m p a n i e d b y the g r o w t h of a transient a b s o r p t i o n at a p p r o x i ­ m a t e l y 390 m/ji, w h i c h was a t t r i b u t e d to the one-electron a d d i t i o n p r o d u c t . Pulse Radiolysis of Dye—Polymer Complexes. P u l s e r a d i o l y s i s studies of d y e - p o l y a n i o n complexes h a v e y i e l d e d i n f o r m a t i o n o n the r e a c t i o n of r e d u c i b l e dyes w i t h e~

w h e n the d y e is b o u n d to a h i g h m o l e c u l a r w e i g h t

m

substrate (2).

T h e i n v e s t i g a t i o n of the m e t h y l e n e b l u e - h e p a r i n c o m p l e x

w a s b a s e d o n the c o r r e l a t i o n of b i n d i n g w i t h the rate constant for d y e r e d u c t i o n b y e~ , m

i n the presence of D-glucose as O H - a n d H - a t o m scav­

enger. T h e o c c u r r e n c e of b i n d i n g w a s d e d u c e d f r o m t h e " m e t a c h r o m a t i c " s p e c t r a l shift, i n w h i c h the M B a b s o r p t i o n at 665 m/x is s t r o n g l y q u e n c h e d +

i n the c o m p l e x w i t h the a p p e a r a n c e s u c h c o n d i t i o n s the rate constant

of a n e w b a n d at 580 m/x. U n d e r for the r e a c t i o n

of e~

with

m

MB

d i m i n i s h e s f r o m the v a l u e i n free s o l u t i o n ( T a b l e I ) to as l o w as 1.1 10 M 9

_ 1

sec." . T h e d e c a y of the e~ 1

+

X

a b s o r p t i o n w a s a c c o m p a n i e d b y the

m

b l e a c h i n g of the c o m p l e x at the " m e t a c h r o m a t i c

wavelength" and

the

g r o w t h of a transient species at 420 m/x. T h e latter w a s a t t r i b u t e d to the s e m i q u i n o n e M B - , w h i c h w a s stable f o r a c o n s i d e r a b l y l o n g e r p e r i o d c o m p a r e d w i t h the case of free d y e .

( A slower g r o w t h of M B - f r o m the

r e a c t i o n of the d y e w i t h o x i d i z e d glucose was o b s e r v e d a l s o ) . T h e d i r e c t r e l a t i o n s h i p b e t w e e n c o m p l e x i n g a n d the l o w e r e~

m

b y a l t e r i n g c o n d i t i o n s to reverse i n w h i c h case the e~

m

reactivity was shown

the m e t a c h r o m a t i c

wavelength

shift,

rate constant increases t o w a r d s the free s o l u t i o n

v a l u e . F o r e x a m p l e , a d d i n g N a C l o r r a i s i n g the t e m p e r a t u r e leads to a n almost c o m p l e t e restoration of t h e h i g h r e a c t i v i t y . A l o w e r e~

m

w a s o b s e r v e d also w h e n M B

+

reactivity

is c o m p l e x e d to other p o l y m e r i c substrates

( s o d i u m h y a l u r o n a t e , s o d i u m p o l y e t h y l e n e sulfonate, s o d i u m p o l y s t y r e n e sulfonate,

s o d i u m c a r b o x y m e t h y l cellulose,

metachromatic

and D N A ) and with

d y e , a c r i d i n e orange c o m p l e x e d to p o l y anions.

m o r e , the l o w e r i n g of e~

m

r e a c t i v i t y w a s f o u n d f o r the

d y e , m e t h y l g r e e n a n d several other cations

the

Further­

non-metachromatic

(cetyl pyridinium chloride,

p r o t a m i n e sulfate, a n d p o l y l y s i n e h y d r o b r o m i d e )

when complexed

to

h e p a r i n or D N A . T h e last t w o cases are p a r t i c u l a r l y interesting because t h e y represent

p o l y c a t i o n s b o u n d to p o l y a n i o n s , i n w h i c h the

strong

interactions c a n b e d e m o n s t r a t e d b y p u l s e r a d i o l y s i s i n the absence of complicating

precipitation

effects

which

occur

with

turbidimetric

methods.

Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

21.

GROSSWEINER

Organic

315

Dyes

Luminescence of Dyes Induced by Electron Pulse Irradiation. P r i i t z , S o m m e r m e y e r , a n d L a n d (19)

m a d e the r e m a r k a b l e d i s c o v e r y that the

e l e c t r o n p u l s e i r r a d i a t i o n of r h o d a m i n e B ,

fluorescein,

or a c r i f l a v i n e i n

d i l u t e aqueous s o l u t i o n leads to v i s i b l e l i g h t e m i s s i o n that b u i l d s u p a n d decays over tens of m i c r o s e c o n d s .

T h e intensity is — 1 0

3

times h i g h e r

t h a n the e m i s s i o n i n d u c e d b y d i r e c t electron excitation a n d w a s a t t r i b u t e d to a c h e m i l u m i n e s c e n t process i n v o l v i n g the w a t e r d e c o m p o s i t i o n p r o d ­ ucts. T h i s l i g h t e m i s s i o n is q u e n c h e d b y either e~ Downloaded by NANYANG TECHNOLOGICAL UNIV on June 21, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch021

m

scavengers or f o r m a t e ,

w h i c h l e d to a m e c h a n i s m i n v o l v i n g the r e a c t i o n of e~

aq

dye i n t e r m e d i a t e .

w i t h the o x i d i z e d

I n a recent extension of this w o r k u t i l i z i n g s e q u e n t i a l

l i g h t flash a n d e l e c t r o n p u l s e i r r a d i a t i o n s , G r o s s w e i n e r a n d R o d d e

(9)

d e d u c e d that the f o r m of o x i d i z e d fluorescein or eosin w h i c h reacts w i t h e~

m

is the O H a d d u c t a n d not the s e m i o x i d i z e d species X .

Furthermore,

the t r i p l e t states of these dyes also react w i t h e~ , l e a d i n g to a n intense a(l

c h e m i l u m i n e s c e n c e f r o m the e x c i t e d s e m i q u i n o n e . T h e latter w o r k i n d i ­ cates that studies of metastable d y e species are feasible w i t h c o m b i n e d flash p h o t o l y t i c a n d p u l s e r a d i o l y t i c m e t h o d s .

Discussion I n a s u r v e y of d y e r a d i a t i o n c h e m i s t r y S w a l l o w (25)

n o t e d that a

n u m b e r of dyes i n aqueous s o l u t i o n react as f o l l o w s : ( a ) I r r a d i a t i o n of aerated solutions i n the absence of o r g a n i c s u b ­ strates leads to i r r e v e r s i b l e o x i d a t i o n , b u t oxygen-free solutions are b l e a c h e d via i r r e v e r s i b l e o x i d a t i o n a n d r e v e r s i b l e r e d u c t i o n . ( b ) W h e n o x i d i z a b l e substrates are present, aerated solutions are r a d i a t i o n resistant, w h i l e oxygen-free solutions are r e v e r s i b l y r e d u c e d . T h e s e effects w e r e e x p l a i n e d b y a general m e c h a n i s m i n v o l v i n g o x i ­ d a t i o n of the d y e b y O H a n d r e d u c t i o n of the d y e b y H atoms a n d the o x i d i z e d o r g a n i c substrate. and

T h e p u l s e r a d i o l y s i s results s h o w that

not H atoms m a k e the greater

e~

m

c o n t r i b u t i o n to d y e r e d u c t i o n i n

n e u t r a l a n d a l k a l i n e solutions a n d that H a n d O H a d d i t i o n m u s t c o n s i d e r e d as w e l l as electron-transfer reactions.

be

T h e f o l l o w i n g general

r a d i o l y s i s m e c h a n i s m indicates the reactions l i k e l y to be significant i n deaerated aqueous solutions of dyes possessing the q u i n o n o i d structure (e.g., azines, t h i a z i n e s , acridines,

D + e

a q

-

D

xanthenes).

r e d

Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

(1)

316

RADIATION CHEMISTRY

DOH-

1

( ) 2 a

D + OHD

o x

+ OH"

(2b)

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(3a)

(3b)

D

D

r e d

r e d

+ D

+ D

E>ox + D

o x

r e d

o

* 2D

(4)

» D + leuco base

(5)

» products

x

(6)

F o r t h e fluoresceins i t has b e e n p r o p o s e d that t h e O H a d d u c t decays b y slow water (or O H " ) elimination ( 5 ) : DOH- ->D

0 X

+ OH"

(7)

a n d that t h e O H a d d u c t is the species r e s p o n s i b l e f o r t h e c h e m i l u m i n e s cence i n d u c e d b y electron-pulse i r r a d i a t i o n (9,

20):

D O H - + e~ -> D * + O H "

(8)

m

[ A n intense l u m i n e s c e n c e has b e e n o b s e r v e d also w h e n I" w a s present (20, 21), a t t r i b u t e d to t h e r e a c t i o n of i o d i n e atoms w i t h t h e r e d u c e d d y e . ] T h e r e d u c t i v e attack is suppressed b y o x y g e n via Reactions 9 a n d 10. +0 ->0 "

e- (U')

2

aq

D

r e d

(9)

2

+ 0 -*D + 0 2

(10)

2

R e a c t i o n 10 is t h e process r e s p o n s i b l e f o r restoring t h e d y e i n p h o t o ­ c h e m i c a l a u t o x i d a t i o n s . T h e r e a c t i o n of 0 " ( o r H 0 - i n a c i d i c solu­ 2

2

t i o n s ) w i t h dyes is n o t e x p e c t e d to b e fast. T h e o x i d i z a b l e o r g a n i c sub­ strate scavenges H a n d O H . AH

2

+ O H - ( H ) —> A H - + H 0 ( H ) 2

2

(11)

I n a n u m b e r of cases i t has b e e n s h o w n that the p a r t i a l l y o x i d i z e d sub­ strate c a n r e d u c e t h e d y e (2, 4, 5,12, 20): D + A H - —» D

r e d

+ A

Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

(12)

21.

GROSSWEINER

Organic Table IV.

Dye

Dye Bleaching G Values G(-D) Oxidative

Conditions

Methylene blue

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317

Dyes

Deaerated Aerated Deaerated w i t h organic substances present Deaerated w i t h formate

Fluorescein

0.6 ± 0.1° 0.68 ±= 0 . 0 8

0.59 0.66

a b c

25 25

a

25 12

a

22 18 22

b 6

2.32 0.95 0.85

22 22 22 22 22 22

b c

c

2.06 1.47° 1.71

2

c

2

Aerated

Safranine T

0.2

a

a

Deaerated Aerated Deaerated w i t h formate Deaerated w i t h H 0 Deaerated w i t h N 0 Aerated w i t h formate 2

1.6 ±

Ref.

2.9 ± 0 . 1 3.15 ± 0.15

Deaerated Aerated Deaerated w i t h formate

Eosin

G(-D) Reductive

0.00

0.65 ±

6

14

0.01

Average of literature values cited in reference. Total of oxidative and reductive decoloration yields. G values corrected for absorption of permanent oxidation product. It is interesting to c o m p a r e the p u l s e r a d i o l y s i s m e c h a n i s m w i t h d y e

b l e a c h i n g y i e l d s r e p o r t e d for l o w intensity, steady i r r a d i a t i o n s IV).

(Table

If n O H r a d i c a l s are n e e d e d to d e c o l o r one d y e m o l e c u l e , b l e a c h i n g

y i e l d s u n d e r i d e a l o x i d i z i n g c o n d i t i o n s s h o u l d be G O H / n i n air-saturated solutions, ^

G ) / n i n nitrous

(GOH +

(GOH +

G

E

e

G ) / n i n the

+

H

p e r o x i d e concentrations H 0 ). 2

2

T a k i n g 2.65,

of

solutions,

moderate

and

0.55

fluorescein,

T h e possible i n t e r m e d i a t e

for

G

O

H

and

, G

e

G

H

radical and molecular products.

T h e values of G ( — D )

T h i s p r e d i c t i o n is i n excellent

m a n y experiments

on methylene

e

+

unstable

i n the

pres­

GOH +

G )

b l u e w i t h different substrates

0.8 w i t h the

H

agreement w i t h the average

benzoate, e t h y l a l c o h o l , lactate, f o r m a t e ) . p l a i n e d d i s c r e p a n c y of ^

to 3-4

steps i n c l u d e d i s p r o p o r t i o n a t i o n

ence of o x i d i z a b l e o r g a n i c substrates s h o u l d be 1 / 2 ( G 3.0.

by

leads

a n d safranine T a n d n =

of s e m i o x i d i z e d d y e a n d successive reactions of O H w i t h the

—

and

hydrogen

(i.e., w h e n H a n d O H are not s c a v e n g e d 2.8,

n — 4 for m e t h y l e n e b l u e , for eosin.

oxide-saturated

presence

of

(e.g.,

H o w e v e r , there is a n unex­

fluoresceins

w h i c h m a y be c a u s e d

b y the o x i d a t i o n of the s e m i q u i n o n e b y p r i m a r y H 0 . 2

2

T h e extent of

b l e a c h i n g i n deaerated solutions s h o u l d b e c o n t r o l l e d b y the c o m p e t i t i o n b e t w e e n Reactions 5 a n d 6 w i t h the b a c k process, R e a c t i o n 4. T h e l o w values of G ( — D ) f o r the

fluoresceins

are consistent w i t h the m e a s u r e d

Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

318

RADIATION CHEMISTRY

rate constants of the r a d i c a l d e c a y reactions

1

T h e methylene blue

(4).

results f o r d e a e r a t e d solutions suggest that the b a c k r e a c t i o n is u n i m p o r ­ tant c o m p a r e d w i t h the r a d i c a l d e c a y processes because the

observed

values of G ( — D ) i n d i c a t e that f o u r O H r a d i c a l s are r e q u i r e d for p e r ­ m a n e n t o x i d a t i o n a n d that a l l H a n d e'

m

Reactions 1, 3, a n d 5.

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OH-

l e a d to r e v e r s i b l e r e d u c t i o n via

(It is not k n o w n w h e t h e r m e t h y l e n e b l u e forms

or H - a t o m a d d u c t s ) .

T h e low bleaching yields obtained w h e n

aerated solutions of v a r i o u s dyes are i r r a d i a t e d i n the presence o x i d i z a b l e substrate (24)

are e x p l a i n e d b y the s c a v e n g i n g of e~

m

atoms b y o x y g e n to g i v e u n r e a c t i v e 0 ~ 2

of a n and H

( R e a c t i o n 9) a n d the r e a c t i o n

of O H ( a n d p o s s i b l y H ) w i t h the o r g a n i c substance ( R e a c t i o n 1 1 ) .

The

extent of b l e a c h i n g w o u l d s t i l l r e m a i n s m a l l even if the r e a c t e d scavenger reduces the d y e ( R e a c t i o n 12) because of R e a c t i o n 10. F i g u r e 1 shows a c o r r e l a t i o n of the r e p o r t e d e~

&q

the values o b t a i n e d f r o m the D e b y e e q u a t i o n .

rate constants w i t h

(The

calculations

are

b a s e d o n Stokes' l a w f o r the d y e d i f f u s i o n constants, the e x p e r i m e n t a l v a l u e of 4.7 X

10~ sq. cm./sec. for the e~ 3

d i f f u s i o n constant (23),

m

dye

r a d i i f r o m spheres of d e n s i t y 1.3 g r a m / c c . a n d the a p p r o p r i a t e m o l e c u l a r w e i g h t , a n d a n assumed e~

m

r a d i u s of 2.7 A . ) . T h e agreement is reason­

a b l y g o o d i n v i e w of the uncertainties i n the c h o i c e of parameters a n d indicates that the n u c l e o p h i l i c attack of the electron o n these dyes is c o n t r o l l e d b y the encounter rate. T h e a v a i l a b l e d a t a s h o w that r e d u c t i o n b y C 0 ~ is m o r e selective, w i t h rate constants r a n g i n g f r o m 2.6 X 10 2

fluorescein

to 5.6 X

10

9

for

7

for m e t h y l e n e b l u e . It w a s suggested ( 5 )

that

the rate constants o b t a i n e d w i t h the xanthenes correlate w i t h the c h a r g e d i s t r i b u t i o n of the q u i n o n o i d structure, w h i l e steric effects at the

re­

d u c i b l e c e n t r a l c a r b o n a t o m a n d changes i n the b r i d g e structure m a y be i n v o l v e d i n c o m p a r i s o n s b e t w e e n the various d y e types. A l t h o u g h i t has b e e n o b s e r v e d that the O H r e a c t i o n p r o d u c t s i n the xanthenes b u i l d u p m o r e s l o w l y t h a n those of e~

(5, 20),

m

accurate d e t e r m i n a t i o n s of the

rate constants are c o m p l i c a t e d b y the o c c u r r e n c e of b o t h electron transfer a n d a d d i t i o n reactions. F u r t h e r m o r e , the strong v i s i b l e c o l o r a t i o n of these p r o d u c t s l i m i t s the use of c o m p e t i t i o n m e t h o d s w h i c h h a v e b e e n a p p l i e d to smaller a r o m a t i c molecules. rate constants of 3 ± 10

9

K i n e t i c estimates h a v e l e d to t o t a l O H

1 X 1 0 for the

for r h o d a m i n e B ( T a b l e I I I ) .

of O H w i t h h y d r o q u i n o n e (1.2 X (1)

fluoresceins

9

(Table II) and —

9 X

A c o m p a r i s o n w i t h the r e a c t i o n rate 10 ) 10

a n d b e n z o q u i n o n e (1.2 X

10 ) 9

suggests that the xanthene d y e results are the correct m a g n i t u d e . T h e c h e m i l u m i n e s c e n t reactions of e~

m

w i t h dyes represent a n e w

process w h i c h m a y h a v e b r o a d e r i m p l i c a t i o n s . T h e f o r m a t i o n of e x c i t e d d y e i n R e a c t i o n 8 s h o u l d l e a d to t r i p l e t f o r m a t i o n via intersystem crossing, a l t h o u g h the y i e l d w o u l d be l o w because of c o m p e t i t i o n f r o m R e a c t i o n 1.

Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

21.

GROSSWEINER

Organic

Dyes

319

MG

2 +

/

MB ©

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AO

2 2 , -

< / ©

'

©

/

/

^RHB / 2

0EO 0

/ FL "" k. ' ' i

2

2

»

5

6

IO- x k meas ,0

Figure 1. Rate constants for reaction of e~ with dyes (liters/mole-sec). Ordinate: calculated from Dehye equation for encounter-limited reactions; abscissa: experimental results aa

T h e r e a c t i o n of t r i p l e t d y e w i t h e~n l e a d i n g to t h e excited s e m i q u i n o n e q

has b e e n o b s e r v e d o n l y w i t h

fluorescein 3D + 6 T

a q

a n d eosin thus f a r ( 9 ) : ^D*

r e d

(13)

A l t h o u g h R e a c t i o n 13 w a s i d e n t i f i e d b y u s i n g l i g h t t o excite t h e t r i p l e t dye a n d a n electron p u l s e to generate e~ , i t is possible that b o t h species aq

c a n b e f o r m e d p h o t o c h e m i c a l l y i n c e r t a i n systems i n v o l v i n g s o l v a t e d or t r a p p e d electrons.

T h i s is e q u i v a l e n t to a t w o - q u a n t a process, i n w h i c h

the r e d u c t i o n p o t e n t i a l of t h e e l e c t r o n m a y b e t r a n s f e r r e d t o t h e d y e species d u r i n g t h e l i f e t i m e of t h e e x c i t e d state. It w o u l d b e i n t e r e s t i n g to l e a r n w h e t h e r R e a c t i o n 13 occurs w i t h other aromatics i n p o l a r solvents that c a n solvate electrons i n c l u d i n g b i o l o g i c a l photosensitizers. Acknowledgment T h e a u t h o r is p l e a s e d t o a c k n o w l e d g e t h e s u p p o r t of t h e N a t i o n a l Institutes of H e a l t h o n G r a n t s N o s . G M - 1 0 0 3 8 a n d G M - 1 2 7 1 6 d u r i n g t h e p r e p a r a t i o n of this p a p e r . I n a d d i t i o n , h e thanks G . O . P h i l l i p s of S a l f o r d U n i v e r s i t y f o r p r e p r i n t s of w o r k n o t e d i n t h e text a n d A . H u s a i n a n d A . F . R o d d e , Jr. of M i c h a e l Reese H o s p i t a l a n d M e d i c a l C e n t e r f o r m a k i n g a v a i l a b l e u n p u b l i s h e d results o n eosin p u l s e r a d i o l y s i s .

Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

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Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.