Journal of Photochemistry and Photobiology, A: Chemistry, 53 (1990) 141 - 167 141

ABSORPTION AND EMISSION SPECTROSCOPY AND PHOTOCHEMISTRY OF 1 ,I O-ANTHRAQUINONE DERIVATIVES: A REVIEW

AURORA NAVAS DIAZ

Department of Analytical Chemistry, Faculty of Sciences, University of Malaga, 29071- Malaga (Spain)

(Received December 6, 1989)

Summary

The spectral characteristics, photophysical parameters and photochem- ical reactivities of l,lO-anthraquinone derivatives are reported. The effects of various parameters (substituents, intramolecular and intermolecular hydrogen bonds, solvents, concentration and chemical reaction) on the absorption and emission properties are described. The photochemical re- activities, photo-oxidations and photoreductions displayed by the various derivatives are given and compared.

1. Introduction

The first commercially important anthraquinone (AQ) vat dyes were discovered at the beginning of this century. Today, AQ derivatives form a most important group in the list of synthetic and natural colourants for several reasons. Anthraquinones are photosensitizers; they phototenderize cellulose materials, photosensitize the oxidation of a large number of sub- strates and have been suggested as photocatalysts for solar energy storage processes such as the photo-oxidation of chloride ion to chlorine and water splitting. The optical characteristics of the simpler derivatives change on chemical transformation and therefore they are widely used in analytical chemistry.

Several reviews have been published in recent years [ 1 - 61, including monographs, dealing with the photochemistry of these compounds. There are also isolated items of information to be found from many sources which might be useful in analytical applications. This work aims to provide a general review of the spectral, photophysical and photochemical charac- teristics of l,lO-anthraquinone derivatives which are most useful in analytical chemistry.

Figure 1 shows the chemical structure of the AQs and the positions of the substituents.

lOlO-6030/90/$3.50 @ Elsevier Sequoia/Printed in The Netherlands

142

Fig. 1. Chemical structure of anthraquinones and positions of the substituents.

The tradenames used by anthraquinone dyestuff manufacturers are given in the Colour Index [ 7 ] ; their chemical structures and properties are listed in Venkataraman [ 81.

Laboratory-synthesized dyes can contain extraneous substances, byproducts of the formation process, and residual amounts of electrolytes from the neutralization reaction. Commercial dyes may contain several different types of impurities [9]. The separation of anthraquinone dyes is discussed in ref. 9. There are several different chromatographic processes which are suitable for small-scale separation and purification of the dye products to analytical grade [9 - 161 and others which are more applicable to larger-scale preparation [ 17, 181.

2. Spectroscopic characteristics

Many theoretical and practical studies have been made of the parent compound [19 - 231. The AQ molecule is characterized by the electronic states So, T,,,,, T,,,,, S,,,* and S,,,, [ 241. The weak absorption at long wavelengths is caused by the S,__* transition [22]. Phosphorescence emis- sion occurs from the T,,,, level. Both transitions are due mainly to the C=O chromophore group which ensures the least energy difference between the lowest excited electronic state and the So fundamental electronic state [251. The %,,a transitions determine the intense shorter wavelength absorp- tions [Zl, 251.

The spectral characteristics of the anthraquinone compounds are related to the nature and relative positions of the electronic states and to the molecular structure of the compound. There are many studies on the spectral absorption [26 - 353 and emission [24, 35 - 391 characteristics of the l,lO-AQs.

There are four T,~T* absorption bands in the wavelength range 220 - 350 nm and one n,r* band at longer wavelengths, close to 400 nm [26, 28, 29,33, 38,40 - 421. Luminescent emission is seen between 450 and 850 nm [41 - 441. The optical characteristics of the derivatives are modified from those of AQ by the following factors: (1) the nature and position of the substituents, (2) the formation of hydrogen bonds, and (3) other inter- molecular interactions,

143

2.1. Influence of substituents on absorption 2.1.1. Nature and position of substituents The effect of the substituents on the AQ absorption spectrum (Table 1)

depends on whether they are electron donors or attractors. The electron-attracting substituents (-HS03, -NOZ, halogens) do not

appreciably alter the AQ absorption spectrum [ZS]. However, electron- donor substituents that affect the electronic states of AQ cause spectral dis- placements [ 28,291.

Donor groups have little effect on the energy of the S,,,, and T,,,* states, but due to conjugation of the aromatic system, they cause a large decrease in the energy of the S,,,, and T,,, * states. Substitution with donor groups can even invert the relative order of these states. When the sub- stituent groups are powerful electron donors (-NH2, -OH), the lowest excited states frequently present a strong charge-transfer (CT) character [28,34,39].

The principal effect of the electron-donor groups is to develop an absorption band in the visible region [28]. When powerful electron-donor groups are present in the anthraquinone ring (hydroxyl or amino groups, for example), an additional T--?T* absorption band appears in the visible region of the anthraquinone UV absorption bands and is assigned to a CT transition. This CT transition is associated with the transfer of an electron from the electron-releasing substituent to the carbonyl group [29, 32 - 34, 391. The relative position of this band is the result of the conjugation of the 2p, orbital of the nitrogen or oxygen with the anthraquinone ?r electronic system.

If the donor group is modified by a group with an electron-attracting tendency, such $LS acetyl or benzoyl [ 281, the absorption band is displaced towards shorter wavelengths. In a similar way, the band in the visible region disappears when the free electron pair is immobilized on the nitrogen atom (as in the hydrochloride) because the CT transition due to the interaction of the pair of free electrons of the nitrogen with the AQ ring is prohibited. The visible band reappears when a small amount of ethanol or water is added to the solution of the hydrochloride derivative 129, 341.

The S, electronic state energies of substituted AQs follow a predictable pattern which is determined by the type of substituent and its position. Because of the stronger donor character of the NH, group of the aromatic ring, it affects the S1 energy more than the OH group. The displacements of S1 are additive for a 1,4-substitution pattern, but not for 1,5- or 1,8-substitu- tion. For the mixed I,$-isomer only a single absorption system is found. The spectrum of f-amino-5-hydroxyanthraquinone (l-A-5-HAQ) is well ex- plained as the sum of the individual spectra of 1-aminoanthraquinone (l-AAQ) and l-hydroxyanthraquinone (l-HAQ) [ 461.

The wavelength of maximum visible absorption increases with the electron donor character of the substituents [28, 291 in the order: OH < NH* < O- < NHMe for substitution in position 1 and OMe < OH < NH2 < NHMe < O- < NMe, for substitution in position 2.

TABL

E1

F

Ma&

mum

w

avel

engt

hs

(nm

) a&

l m

olar

ex

tinct

ion

coef

fici

ents

(1

mol

-’

cm-l

, . m

par

enth

eses

) of

lon

gest

w

avel

engt

h ba

nd

of

anth

ra-

g

quin

one

deri

vativ

es

in v

ario

us s

olve

nts

Com

poun

d So

lven

t

MeO

H

o-C

6H4C

10H

Et

OH

~-

cc14

D

ioxa

n H

epta

ne

Oth

er

R$= H

R1=N

H2

R'=N

HCHJ

R'= N(

CH3)

?

R'='

NHAc

R'=N

HBz

R’=

N(C

H?)A

c

R'=N

(CHS

)Bz

R'=N

HCOC

HJ

410a

- (60)

.475a. 48P

(6300) (6400)

503a

- (7100)

503a

- (4900)

400a

4148

(5600) (5000)

405a

424a

(6400) (6200)

- 337a

(5000)

-

335a

Rl=N

HC&

R'=O

H 402a

- (5500)

R'=(

-j-

493a

- (5000)

R1=O

CHS

378a

- (5200)

R'=O

C&

R1=C

H3

R' = C

H#O

jNa

4w

- (150)

390b

40

0e

396b

(1

000)

(1

15)

475d

45

7c

46Be

45

3b

480(

iso-Pr

OH)d

, 44

5(he

xane

)f

(630

0)

(148

0)

505.

P 49

7C

(347

0)

(720

0)

504b

50

0e

(537

0)

(510

0)

420c

(5

930)

50

5C

(780

0)

406c

33

0e

(590

0)

364c

(3

800)

336(

HzO)g

(479

0)

-.

- .-

_

_ _

- -

__

- -

- .

- _

. .

- .

_~ .

_ -

_ -

R’

= N

Oz

R’=

CN

R’=

Cl

R’

= S

OjN

a

- 4178

-

(70)

41

5a

-

(100

)

400e

400e

R’=

R3=

R4=

RS

=R

6=R

7=R

8,H

+

R2

= N

H2

4408

44

5a

449b

40

5C

422e

(4

500)

(6

400)

(3

550)

(4

400)

R

2 =

NH

CH

3 46

2’

- 47

1b

431C

(5

700)

(5

630)

(5

050)

R

2 =

N(C

H3)

? 47

2a

- 45

2=

(590

0)

(600

0)

R2

= N

HA

c 36

7a

- (4

200)

R

2 =

NH

Bz

3758

(6

900)

R

2 =

N(C

H3)

Ac

R’

= N

(CH

3)B

z -

-

R2=

OH

36

8a

- 35

4C

330e

(3

900)

(I

nso

l.)

R’=

()

- 46

5a

- (5

400)

R

” =

0C

H3

363a

-

36ac

(3

950)

(3

200)

R

2 =

CH

;, -

-

l-t2 =

CH

2SO

$a

4oog

(2

00)

R2

= N

O2

420a

-

(50)

R

2=C

N

413a

-

(100

) R

2 =

Cl

R2

= C

&

R2

= S

OjN

a

400e

35

9C

(360

0)

3%!(

&o)

e (4

680)

439(

hex

ane)

b (7

800)

370(

hex

aney

(200

) 33

3(H

# (5

130)

332(

H20

)9

(490

0)

TA

BL

E

1 (c

ontin

ued)

Com

poun

d So

lven

t

MeO

H

o-C

6H4C

10H

E

tOH

2 m

cc14

D

ioxa

n H

epta

ne

Oth

er

R1

= C

l, R

Z =

CH

J

R’

= C

l, R

Z =

CH

$OjN

a

R’

= B

r, R

2 =

CH

3

R’

= B

r, R

2 =

CH

$Ofl

a

R’

= I,

R2

= C

HS

R’

= SO

aNa,

R2

= C

H#O

aNa

R’=

R4=

OH

R’=

R4=

NH

z 59

0’

R’

= R

4 =

NH

Ac

R’

= R

4 =

NH

Bz

R2=

R3=

R4=

R6=

R7=

R8=

H+

R’

= R

5 =

OH

R’

= R

S

= C

l

R’

= R

5 =

NO

* R

’ =

RS

= N

H2

R’

= R

s =

NH

Ac

R’

= R

5 =

NH

Bz

487f

596a

(1

3800

) 47

4a

(670

0)

48B

a (7

100)

500a

(1

1900

) 43

5a

(970

0)

445a

(1

1500

)

555h

43

0h

430(

acet

one)

, 43

0(et

her)

, 41

0,

420(

CH

&l)

h 41

5(M

/E),

35

1(M

CH

/iP)g

(100

) (3

160)

34

5(H

20)g

(5

130)

41

6(M

/E),

35

1(M

CH

/iP)g

(-)

(310

0)

345(

HzO

)g

(500

0)

360(

M/E

),

360(

MC

H/iP

)g

(415

0)

(330

0)

3!25

(H20

)9

(355

0)

475h

59

2b

(155

00)

425h

484b

(7

950)

475h

42!j

h 36

0e

400e

40

0e

482e

475(

acet

one)

, 47

5(et

her)

, 47

5(C

HC

l,)h

595(

iso-

PrO

H),

57

5(he

xane

)d

420(

acet

one)

, 42

5(et

her)

, 42

5(C

HC

l#

492(

iso

-PrO

H)d

.-

R~

=R

~=

R‘+

,R~

=R

~zR

~=

H+

R’

= R

8 =

Cl

R’=

R*=

NH

*

R’=

R~=

RS

=R

~=R

~ZR

~=H

+

R2

= C

H$O

$a,

R3

= C

H3

R*

= C

H#O

fia,

R

3 =

CH

zS03

Na

.

R’=

R3=

R‘+

,R5=

R7=

R8,

H+

R

z=R

6=()

H

R2

= R

6 =

NH

1 47

4f

R’=

R3=

.R4=

Rs,

Rs=

H+

R2=

R7=

NH

2

RZ

= R

3= R

6= R

7= R

s=H

+

R’=

R4=

R5=

NH

2 60

1f

580b

(1

00)

400e

58

0e

338(

H20

)g

(525

0)

336(

H*o

)g

(575

0)

341

477,

426

b 38

4,38

2,

420b

(1

000)

(6

450)

(16

00)

398,

46

8b

(460

0) (

4300

)

610d

525h

48

0*

560h

48

!ih

383.

5(he

xane

),

430(

Bz)

, 44

6(C

H~c

N)~

(5

750)

607

( iso

-PrO

H)d

480(

acet

one)

, 48

0(et

her)

, 48

0(C

HC

l~)~

485(

acet

one)

, 48

5(et

her)

, 48

5(C

HC

l~)~

63

5(is

o-Pr

OH

)d

Bz,

ben

zene

; M

/E,

met

hano

l/eth

anol

(5

:2);

MC

H/iP

, m

ethy

lcyc

lohe

xane

/isop

enta

ne

(1:4

).

aDat

a fr

om

ref.

28.

d D

ata

from

re

f. 30

. gD

ata

from

re

f. 44

. bD

ata

from

re

f. 24

. eD

ata

from

re

f. 31

. hD

ata

from

re

f. 32

. C

Dat

a fr

om

ref.

29.

fDat

a fr

om

ref.

45.

148

In all the compounds substituted in position 1, the maximum wave- length is greater than that of the corresponding derivatives substituted in position 2, independent of whether or not an intramolecular hydrogen bond is present; this is due 129, 471 to the fact that the CT stabilization in posi- tion 1 is greater than that in position 2.

The bands of the absorption and emission spectra undergo a batho- chromic displacement under the influence of the hyperconjugation of substituted alkyl groups in position 2 in comparison with the unsubstituted l-hydroxyanthraquinone and l-aminoanthraquinone [ 36, 37 1.

The probability of the electronic transition in the visible band (E,,,) increases (like h,,, ) with the electronic conjugation [27, 303 in the follow- ing order: a-substitution < @-substitution < a&-substitution < 1,4,5substi- tution < 1,4,5,8-substitution.

2.1.2. Steric hindrance The steric hindrance between the substituents and the rest of the

molecule may decrease or increase the electron-donor efficiency accord- ing to the spatial configuration adopted by the system. Thus for I-dimethyl- aminoanthraquinone, the steric effect between the substituent in position 1 and the ketonic oxygen atom causes the group to twist towards the outside of the plane of the AQ nucleus in which the maximum mesomeric possi- bilities can occur [ 281. As a result, the wavelength of maximum absorption is similar to that of the other methylated derivatives, but the coefficient of extinction is considerably less [ 28, 291.

In contrast, in 2-alkyl-l-amino anthraquinones the alkyl group exercises a steric effect on the NH2 group which probably leads to its rotation; this promotes the formation of a more planar ring with a hydrogen bond X=0- - -HN( and a decrease in the energy of the system [37] ; therefore h max increases.

2.2. Influence of substituents on emission 2.2.1. Fluorescence and phosphorescence Table 2 summarizes the luminescence characteristics of the AQ deriv-

atives. The luminescence characteristics of the HAQs are conditioned by the formation of intramolecular hydrogen bonds. The HAQs fluoresce, but phosphorescence is only seen in the 2-hydroxy derivatives. The fluorescence quantum yields of the 1-hydroxy derivatives are much greater than those of the derivatives substituted in position 2 [49]. It has been shown [59] that for a 1,4-substitution pattern (1,4-dihydroxyanthraquinone (1,4-DHAQ), f,2,4-trihydroxyanthraquinone (W&THAQ), 1,2,5,Stetrahydroxy- anthraquinone (1,2,5,&TTHAQ) and 1,4,5,&TTHAQ), the quantum yield of fluorescence is moderate to high and.the absorption and fluorescence spectra overlap; there is no indication of dual fluorescence character. In contrast, cw-HAQs which lack the 1,4-substitution pattern (l-HAQ, 1,5- DHAQ, 1,2-DHAQ) are weak emitters with indications of dual fluorescence.

149

The luminescent characteristics of AAQs have been related to the excited states with CT character [2, 501. The I- and 2-AAQs show fluor- escence emissions in the wavelength range 520 - 630 nm [24, 41, 561, but the yield is negligible and no phosphorescent emissions are seen in any derivative 141, 49, 51, 551. The quantum yield of l-AAQ decreases as the alkylation of the amino group increases. The presence of electron-withdraw- ing substituents in the l-amino group causes a decrease in the fluorescence quantum yield [ 431. The N-substituted l-AAQs (except l-anilineanthra- quinone) show fluorescence [ 511.

The 2-alkyl-l-hydroxyanthraquinones (2-alkyl-l-HAQ) and 2-alkyl-l- aminoanthraquinones (2-alkyl-1-AAQ) show fluorescence. The fluorescence quantum yields of the 2-alkyl-1-AAQs are about twice as large as the corre- sponding I-alkyl-1-HAQs; branching of the substituent leads to a decrease in the quantum yield [36, 371. In the series of P-halogenanthraquinones, an increase in the quantum yield of phosphorescence is seen following the sequence: P-F < &Cl < P-Br < p-1.

This is explained by the increase in orbital-spin coupling which in- creases the nuclear charge of the halogen atoms which have free orbitals [ 25 1. The o-halogen AQs show lower quantum yields of fluorescence than the P-halogen derivatives due to the modification of the geometrical molec- ular structure by the interaction of the carbonyl group with the halogen atom at the o-position [60].

The aminochloroanthraquinones show fluorescence, but only the 2-amino-3-chloro derivative shows phosphorescence [ 551. Similarly, the aminocarboxyanthraquinones show fluorescence, but only the 2-amino-3- carboxy derivative shows phosphorescence 1161. The latter exhibits normal phosphorescence at longer wavelengths than the fluorescence and also exhibits anomalous phosphorescence at shorter wavelengths (produced by excitation at wavelengths shorter than 350 nm [ 541) which might result from an upper T,,, * state. However, the 2-amino-3-hydroxyanthraquinone (2-A-3-HAQ) derivative is non-luminescent [ 521.

In the aminohydroxy derivatives, l-A-2-HAQ exhibits a greater quan- tum yield than l-A-4-HAQ and l-A-5-HAQ; l-H-2-AAQ is non-fluorescent [ 531. None of the aminohydroxy derivatives shows phosphorescence [ 53].

The differences between the derivatives can be explained in terms of the potential energy diagram [ 43, 491.

The n--a* transition has a low probability and the AQs that have a 10~ energy n,R* state are not normally fluorescent, although they may emit strong short duration phosphorescence (10m2 - 10e3 s) at low temperature. The K--B* transition is very intense and the colourants which have a low energy r,r* state often show fluorescence and phosphorescence; the latter being characterized by a fairly long lifetime (0.1 - 10 s) [2].

2.2.2. In tersystem crossing and internal conversion If the fluorescence quantum yields are small, it is because the singlet

excited state is mainly deactivated by radiationless processes either to So

150

TABLE 2

Emission properties of anthraquinone derivatives

Compound Solvent

n-Hexane @LX 102 )hm

Z-Propanoi E thy I ace tote @pL x 102 &,, @LX 102 &,,

R==Rj=R4=Rs=R6,R7=Rs=H+

R’=H Fl(77 K) Ph(77 K)

R’=OH Fl( RT) Fl(77 K) Ph(77 K)

R’ = NH1 Fl(RT) 7

R’ = NHCOCH3 Fl( RT) 10-J 500 0.4 518 R’ = NHCH3 Fl( RT) 0.3 560 10-Z 612

R’ = N(CHJ), Fl(RT) R’ = pipecidino Fl( RT) <10-J R’ = benzoylamino Fl( RT) 10-S R’ = monochloro- Fl( RT) < 10-3

acetylamino R’ = anilino Fl(RT) < 10-s R’ = 1 -N-methylanilino FI(RT) c. 10-S

Rl=RS=R4=Rs=R6=R7=Rs=H+

R==OH Fl( RT) Ph(77 K)

R= = NH2 Fl( RT) Insol.

R* = NHCH, R= = N(CHJ), R= = N(C4H9)= R2 = piperidino

R2=F

R= = Cl

R2 = Br

R==I

R2 = SOsNa

R2 = CH2S03Na

Fl(RT) Fl(RT) 20 Fl(RT) Fl(RT) 27

Fl(77 K) Ph(77 K) Fl(77 K) Ph(77 K) Fl(77 K) Ph(77 K) Fl(77 K) Ph(77 K) Ph(77 K)

Ph(77 K)

R3=R4=R5=R6=R7=R8=H+ R’=R==‘-JH Fl(RT)

Ph(77 K) R’ = OH, R= = NH2 R’ = OH, R= = CH3

Fl( RT) Fl(77 K)

R’ = OH, R2 = isopropyl Fl(77 K) R’ = OH, R= = t-butyl Fl(77 K) R’ = OH, R2 = cyclohexyl Fl(77 K)

0.4 570,597(s)

530 0.3 590 0.8 567

0.69 515

< 10-3 500 10-X 528 0.13

0.11 515 0.13

<lo-J < 10-S <10-x

10-X 472,500 Insol.

< 10-3 2.4

1 495,535(s) 10-S 610

510,545(s) 10-S 630

0.4 57O(s),590

n.0. n.0.

520 509

563

151

Furan Ethnnol Benzene Other *‘Lx 102 Lrn @‘Lx 102 h,, *‘Lx 102 &m @‘Lx 102 Ln

Reference

3 558 0.82

2 505 0.41 0.2 588 0.14

< 10-J 0.5 510

< 10-a < 10-S

Insol.

5 532 0.059

4 590 0.021

4 615 0.012

0 0.39

2.2

1.7 0.76

548

7.7(MCH/iP) n.o.(EPA glass) l(cyclohexane) l.O(CHJCN) 1.2(MeOH) 6.5(toluene) 24(MCH/iP) 0.82(CH3CN) 10p2(cyclohexane) 0.16(CHBCN) 0.2(MeOH) O.S(toluenef 10-3(cyclohexane)

6 515(EPA glass) 8 529 O.S(CH&N)

0.2(MeOH) 25(toluene)

5 6 15 8.5

20(cyclohexane) 0.33(CH&N) 19(c&ohexane) O.lB(CH&N) 0 0.38 0 0.40 0 0.47 0 0.62

492(MeOH/ EtOH)

49l(MeOH/ EtOH)

n.o.(EPA glass) n.o.(toluene) S.O(MCH/iP) 7.0(MCH/iP) 6.9(MCH/iP) 8.4(MCH/iP)

48 48 49 36 49 41,43,50 41 42 42 37 41,43, 51 41.43 41 42 42 43 43 43,51 bl

43,51 43

49,52 49,52 41, 43, 50, 52 42 42 43 43 41 41,43 41 48 48 48 48 48 48 48 48 44

44

49

49

53 36 36 36 36

(continued)

152

TABLE 2 (continued)

Compound Solvent

n-Hexone @LX 102 &,,

Z-Propanol @LX 102 Xem

Ethyl acetuie @LX 10’ )bm

R’ = NH2, R2 = CHs Fl( RT) Fl(77 K)

R’ = NH2, R2 = isopropyl Fl(RT)

R’ = NH2, R’ = t-butyl Fl(77 K) Fl( RT)

R’ = NH2, R2 = bexyl Fl(77 K) Fl( RT) Fl(77 K)

R' = NH2, R2= cyclohexyl Fl(RT)

R’ = NH2, R2 = OH Fl(77 K)

R’ = NH2, R2 = carboxy Fl( RT) FI(RT)

R’ = NH*, R2 = Cl Pb(77 K) Fl( RT)

R’ =Cl, R2 = CH2SOaNa Ph(77 K) Ph(77 K)

R1 = SOINa, R2 = CH2SOjNa

Ph(77 K)

R~=R~=R~=R~=R’~zR~=H+

R’ = NH2, R’= carboxy Fl(RT)

R’ = NH2, R3 = Cl Ph(77 K) Fl( RT) Ph(77 K)

R2=R3=Rs=R6=R7=R8=H+ R’=R4=OH Fl( RT)

Fl(77 K)

R’=R4=NH2 Ph(77 K) Fl( RT)

R* = NH2, R4 = OH R’ = R4 = methylamino

Fl( RT)

R’ = NHCH3, R4 = Br Fl(RT) Fl( RT)

R~=R~zR~=R~=R~=R~=H+ Rl=Rs=OH Fl( RT)

Fl(77 K)

R1=RS=NH2 Ph(77 K) Fl( RT)

R’ = NH2, Rs = OH R’ = Rs = piperidino

Fl(RT) Fl(RT)

R2=R3=R4=R5=R6=R7=H+ R’=Ra=OH Fl( RT)

Fl(77 K) Ph(77 K)

R~=R~=R~=R~=R~zH+ R*=R4=RS,OH Fl(RT)

Ph(77 K)

21 18

19 17 17 15 19 18 21 20

2.1 540 0.84 583 1.6 563

1.7

14.3

10-J

10-S

1.1

<10-J

0.8 600 0.4 590

0.35 585 1 565

528 0.77 585 1.4 563

5 536,565

1.3 592 1.5 572

610,630(e) < 1O-3 <10-z 614,640

0.54 600 0.56 597 665,670(s) c 1W3

566 0.6 575,605(s)

0.47 575 0.7 555

0.37 598 0.85 585 < 10-a

1.5 530,570

7.3 543,572

153

Reference

Furan Ethanol Benzene 0 ther @LX 102 bm @LX 102 &,, @LX 102 &,, @LX 102

10-S 612,645 < fO-2

10-S 670,710 0.087 0.32

0.67 568

0.78 563 1.3 2.2

< 10-a

17(MCH/iP)

17(MCH/iP)

lS(MCH/iP)

lS(MCH/iP)

lS(MCH/iP)

2.5 0.24 n.o.(EPA glass) 1.8 n.o.(EPA glass)

0.76 n.o.(EPA glass) 1.5 n.o.(EPA glass)

14.9(MCH) 19.9(MCH)

590(toluene) 562(ether)

550(toluene)

562(MeOH/ EtOH)

495(MeOH/ EtOH)

558(ether)

540( toluene)

n.o.(EPA glass)

49,56 56 49

612,635 lo-’ 614,64B(M 3Cl) 43 2.2(MeOH) 42 I.O(toluene) 42 0.65 583(toluene) 53 10-s 675,72O(CH&l) 43

37 37

37 37 37 37 37 37 37 37 53 16,54 16,54 55 55 44

44

16,54 16,54 56 55

O.ll(CH&N) 41

550 540

1.4 568(CH&N) 0.92 568(2MTHF) 2.9(MCH) n.o.(EPA glass) 2.4(MeOH) 2.5( toluene) 2.3 556(CH&N) 1.3 548( 2MTHF ) 0.8 572(toluene)

P.O(MCH) n.o.(EPA glass)

n.o.(EPA glass)

49,57 57 57 56 49 42, 57 42 57 57 53 43

49 56 46

49 49

(continued)

154

TABLE 2 (continued)

Compound Solvent

n-Hexane @LX 102 A,,

2-Propanoi @.L x 102 A,,

Ethyl acetate @LX 102 x,,

R~=R~=R~=R~xH+ R’=R*=RS=R8=OH FI(RT) 0.4 550,561,591

Ph(77 K)

RZ=R3=R6=R7=H+

R1 = R4 = R5 = R8 = NH1 Fl(RT) R’=R~ER~,R~=OH Fl(77 K)

R’=R~zR~=R~=R~=R~=H+

R* = NH2, Et3 = OH Fl( RT) F’h(77 K)

R* = NH2, R3 = carboxy FI(RT) Ph(77 K)

0.2 684 0.24 672

n.0. n.0.

0.14 573 5 535

RZ = NH?, R3 = Cl

R2 = CHzSO,Na, R3 = CHS

RZ = CH2S03Na, R3 = CH2S03Na

Fl( RT) Insol. Ph(77 K) Ph(77 K)

Ph(77 K)

R’=R~=R~=R~=R~,R~EH+ R2=R6=OH Fl( RT)

Ph(77 K)

RZ=R6=NH* Fl( RT)

R2 = R, = S03H Fl( RT) FI(77 K) Ph(77 K)

0.22 558 1.8 545

10-3 463,505

(km) or to the triplet (k,sc). The radiationless deactivation by internal conversion occurs mainly by intramolecular and intermolecular hydrogen bonding. The formation of the triplet by the intercrossing system is only appreciable if the difference in energy between the excited singlet state and the nearest triplet is small.

The principal processes that affect the AQ derivatives in their excited states are summarized below.

‘AQ dye -

Scheme 1. AQ dye AQ dye

155

FlUan Ethanol Benzene Other @LX 102 L,, @LX 102 x,, @LX 102 &,, @LX 102 Lem

Reference

n.o.(EPA glass)

0.4 672 50

53(MCH) 56

n-0. n.o.(EPA glass) 6 1

0.05 8.5 0.08

0.23

0.075 592 0.23 524 0.032 3.5 0.23 < 10-Z

2(H20/glycerol

(4:l))

49 49

52 52

520(ether) 16, 54 455, 492, 534.5, 584 16, 54

(EPA glass) GlO(EPA glass) 16,54 512(toluene) 55 620(EPA glass) 55

493(MeOH/ 44 EtOH)

491(MeOH/ 44 EtOH)

532(s),560(EPA glass)

GOO(MeOH) 552(CH&N) 538(2MTHF)

470,500(H20)

49 49

57 57 57 58

58

h em3 emission wavelength maximum (nm); a~, luminescence (fluorescence (Fl) or phos- phorescence (Ph)) quantum yields; Insol., insoluble; RT, room temperature; EPA, diethyl ether: 2-methylbutane:ethanol (5 : 2 :2, by volume); MCH, methylcyclohexane; MCH/iP, methylcyclohexane/isopentane( 1:4); SMTHF, 2-methyltetrahydrofuran; n.o., not ob- served; s, shoulder.

The AQ molecule only shows phosphorescence from a T,,,, state. The lack of fluorescence is caused by strong intersystem crossing, probably from an intermediate T,,,, state 1611.

In l-HAQs, the T,,,+ state is situated above the lowest S,,,, state; thus intersystem crossing between the two states is not possible and no appreciable population of triplet states occurs. The low fluorescence yields are attributed to quenching due to a process of rapid, non-radiative deactiva- tion with the participation of the intramolecular hydrogen bond [46].

In contrast, the S,,, state of the 2-HAQs is situated, in terms of the energy diagram, only slightly higher than the T,,, state, a condition which

156

greatly favours intersystem crossing to the T,,,* state. Consequently, a decrease in the fluorescence quantum yield occurs and phosphorescence appears [49]. The respective substitution of an amino and hydroxyl group in positions 2 and 3 produces a rapid deactivation within the singlet and triplet manifolds [ 52 ] .

It has been shown that the main contributors to internal conversion of monoaminoanthraquinones are deactivation processes through the inter- molecular hydrogen bond between the solvent molecules and the oxygen atom of the carbonyl group [41,42] or the amino hydrogen [ 421. A mech- anism of thermally-assisted S1-T, intersystem crossing is proposed for di- aminoanthraquinone deactivation.

AQs with electron-donor substituents exhibit faster internal conversion when one of the quinoid oxygens participates in a hydrogen bond; this may be either intramolecular cr intermolecular. This behaviour is explained [ 57 J by invoking the stretching vibrations of the hydrogen in the hydrogen bond as accepting models for the Si-So process.

2.3. Influence of hydrogen bonding 2.3.1. Intrumoleculur hydrogen bonding The substitution of AQ with groups that form intramolecular hydrogen

bonds (IHBs) on the quinonic oxygens has a profound effect on its elec- tronic structure and photophysical characteristics.

In l-HAQs and l-AAQs stable intramolecular hydrogen bonds are formed that persist to 280 - 300 “C [ 353; the IHBs are stronger in hydroxy derivatives than in amino compounds [ 30,471. In the fundamental electronic state the positions of the amino group of I-AAQ and the carbonyl oxygen do not permit the formation of strong IHBs [ 37, 471. The IHB in l-AAQ could be much stronger in the S,,, excited state because the electronic excitation changes the geometry of the pseudo-ring that contains the IHB t471.

The hydrogen bonds in 2-alkyl-1-AAQs are different from those in 2-alkyl-l-HAQs in which, even in the fundamental state, a flat quasi-aromatic ring is formed by an IHB between the C=O and OH groups; the alkyl groups do not exert any appreciable influence on this bond [ 371.

Hydrogen bonds greatly modify the electronic spectra of the AQ derivatives; they cause both spectral band displacements and quenching of fluorescence.

2.3.1.1. Spectral shifts. The spectral band displacements are caused by the additional rings formed by the IHBs which increase the length of the system conjugation chain [24,28,29,35,61]. In anthraquinonessubstituted in position 1, the effect of the IHB on the maximum absorption increases as the donor capacity of the substituents increases: methylamino > amino > acetylamino > hydroxy [29].

The formation of IHBs in the HAQs is shown by the fact that the long- wave bands in these compounds are displaced towards longer wavelengths compared with those of the methoxy derivatives 1351.

157

, H....*. '0 \ cz-J(l3 \

O\,.J? (Cl

Fig. 2. A representation of the photodynamics of 1,5-dihydroxyanthraquinone. (A) The enol tautomer in So; (B) the enol tautomer in Sl; (C) the keto tautomer in S1.

The hydrogen bond can also markedly affect the character of the excited state of the molecule; it can cause a weakening of the quinoid character [27]. In this way, the spectra of 1,4-DHAQ, 1,2,5,&TTHAQ and 1,4,5,8-TTHAQ mainly show those frequencies due to the hydrocarbonate skeleton as a whole [ 351. In the 1,4,5&TTHAQ molecule, each C=O group participates in the formation of two hydrogen bonds and, consequently, the four quasi-aromatic rings interact with the electronic system in the common seven-ring system. The spectrum of this compound is characterized by similar frequencies to those of the coronene and anthracene rings [25, 351. Thus, although short-lived phosphorescence is observed in AQ from the T x,?T* level, in 1,4,5&TTHAQ fluorescence is seen from the S,,,, leve1 [25].

2.3.1.2. Radiationless deactivation. The hydrogen bond is responsible for a significant increase in the non-radiant speeds of decay. In l-AAQs, the IHB between the carbonyl oxygen and the hydrogen atom of the amino group accelerates the radiationless deactivation of the charge-transfer excited states [41]. Consequently, in aprotic solvents such as benzene, the excited I-AAQs are deactivated faster than the 2AAQs [ 411 because the IHB accel- erates the radiationless deactivation.

2.3.1.3. Excited state proton transfer. The quenching of luminescence in hydrogen-bonding derivatives is explained by the intramolecular inter- actions, most of which cause rapid deactivation through keto-enol tau- tomerism.

It is assumed that the IHB favours the non-radiative deactivation of S, by tautomerism in the excited state [42, 57, 59, 611, i.e. by proton transfer along the IHB in a short timescale (compared with the lifetime S1); the tautomeric form undergoes a rapid internal conversion to the fundamental So state [47]. Thus for 1,5-DHAQ, there is evidence [57, 591 for the oc- currence of excited state proton transfer across the intramolecular hydrogen bond and this compound shows tautomerism in the excited state (Fig. 2).

158

1,SDHAQ exhibits both a structured fluorescence and a stronger fluorescence displaced towards the red which are attributed to the tautomeric form [ 591. This type of fluorescence is called dual intrinsic fluorescence [ 571. In the same way, there is evidence that in l,&DHAQ at least two emitting excited states exist [36,63, 641, and the lower energy transition is thought to be a process involving a proton transfer mechanism [ 631. The rather low yields of fluorescence exhibited by l-HAQs are attributed to the quenching produced by a rapid non-radiant deactivation process of intramolecular transfer of an oxygen atom between the carbonyl group and the l-hydroxy substituent [ 23. The conditions for tautomerization (proton transfer) in the excited state are not as favourable in the AAQs as in the HAQs [42].

However, the existence of an intramolecular hydrogen bond between a proton donor and a proton acceptor in a molecule does not guarantee that an intramolecular proton transfer takes place in the excited state [34]. Thus, in l-AAQ [ 47 ] and 1,4-DHAQ [59, 65 - 681 tautomeric excited state transfer and emission are not observed. In 1,5-DAAQ, the internal conver- sion is due to intramolecular hydrogen bonding without any appreciable change in the molecular structure 1571.

23.2. In terrnoleculur hydrogen bonding Protic solvents cause the formation of intermolecular hydrogen bonds

in AQ derivatives. The spectroscopic indications of an intermolecular hy- drogen bond are similar to those of the IHB. The intermolecular hydrogen bond causes small but measurable displacements of the bands in the elec- tronic spectrum and a noticeable increase in the speeds of non-radiant decay.

2.3.2.1. SpectrcaE shifts. Bathochromic displacements are produced in the derivative spectrum if the formation of an intermolecular hydrogen bond

Fig. 3. Complex formed between ethanol and methoxyanthraquinones.

159

with the solvent molecules is accompanied by the formation of an additional ring which lengthens the conjugation chain of the system.

Except for anthraquinones substituted in position 1, it has been shown that the intermolecular hydrogen bond causes more displacement than other types of hydrogen bond [ 291.

The spectra of methoxyanthraquinones in ethanol show more pro- nounced bathochromic displacements than those of hydroxyanthraquinones [35] which can be explained by the formation of complexes of the type shown in Fig. 3. This results in the formation of a six membered ring, which increases the length of the conjugation chain [35].

The displacement towards the red and the broadening of the spectra at low concentrations of 2hydroxyanthraquinone and 2,6_dihydroxyanthra- quinone can be explained by the persistence, even in neutral solvents, of association complexes due to intermolecular hydrogen bonds and other intermolecular interactions [ 353.

2.3.2.2. Radiationless deactivation. The anthraquinone derivatives undergo non-radiant deactivation by internal conversion. Certain solvents are fundamentally important to the deactivation process because they form intermolecular hydrogen bonds with the anthraquinones. The vibrational modes of the hydrogen bonds act as accepting modes for the energy from the excited anthraquinone to accelerate the S1 + SO process [ 41,421 and a large part of the excitation energy is dissipated through these bonds.

In ethano1 (a protic solvent) the fluorescence of aminoanthraquinones is almost quenched. In general, in protic solvents the non-radiant deactiva- tion from S1(‘CT) of the aminoanthraquinones occurs mainly through the vibrational modes of the intermolecular hydrogen bonds with the solvent molecules. It is thought that the deactivation of the anthraquinones occurs through the hydrogen bonds at the carbonyl groups rather than at the amino groups [ 411. The decrease in the measured lifetimes of the singlets and the decrease in the quantum yields of the aminoanthraquinones on changing the solvent from toluene to methanol have been interpreted as the result of non-radiative deactivation caused by intermolecular hydrogen bonds [ 421.

The relative degrees of participation of the intramolecular hydrogen bond and the intermolecular hydrogen bond in the Si + SO internal conver- sion depend on the characteristics of both the solvent and the molecule.

For aminoanthraquinones, the contribution of the intramolecular hydrogen bond to internal conversion is less than that of the intermolecular hydrogen bond [41,42] _ The compounds 1,5-DHAQ, 1,5-DAAQ and 2,6-DAAQ show internal conversion due to intermolecular hydrogen bond interaction [ 571. The two compounds with IHBs, 1,5-DHAQ and 1,5-DAAQ, exhibit rapid internal conversion in a wide range of solvents; however, 2,6-DAAQ, which has no intramolecular hydrogen bonds, exhibits rapid internal conversion only in alcohols [57].

It has been shown that 1,4-DHAQ undergoes a rapid intermolecular proton exchange [ 591.

160

2.4. Other intermolecular interactions In addition to intermolecular bond interactions, the AQ derivatives can

also undergo dipole-type interactions with solvent molecules and may also display intermolecular interactions between two conjugated x systems.

2.4.1. Solvent effects The effect of the solvent on the absorption spectra of AQ derivatives

has received much attention [29, 32, 333. The dispersion force and the electrostatic interaction cause red shifts

for all monosubstituted anthraquinones [29]; this is understandable, because the solute polarity increases in the excited state.

The symmetry of molecules such as 1,5-DHAQ, 1,5-DAAQ and 2,6- DAAQ dictates that the dipole moment should be zero or very small; con- sequently, the spectroscopic displacements caused by dipolar solvent effects are also small for this type of derivative [ 571.

The wavelength of maximum fluorescence and the quantum yield of 2-AAQs are rather more sensitive to solvent polarity than the corresponding parameters of l-AAQs [43]. The quantum yields of 2-AAQs decrease rapidly as the polarity of the medium is increased [41,43]. However, the polarity of the solvent hardly influences X,,, of 1,4-DAAQ and 1,4,5&TTAAQ [ 501.

The aminochloro derivatives display quantum yields of fluorescence that depend on the polarity of the medium. 2-Amino-3-chloroanthraquinone displays a larger quantum yield than 1-amino-2-chloroanthraquinone and 1-amino-3-chloroanthraquinone with a decrease in the polarity of the medium [ 551.

The wavelength of maximum fluorescence of 2-amino-3-carboxyan- thraquinone is more dependent on the solvent than that of the l-amino- carboxy derivative 1621. The fluorescence quantum yields of l-acetylamino- anthraquinone, l-monochloroacetylaminoanthraquinone and l-benzoyl- aminoanthraquinone are greater in polar than in non-polar solvents [ 511.

The displacements (or lack of displacements) of the absorbance and luminescence bands of the AQ derivatives which are observed when the solvent is changed have been used to identify and attribute these bands to the type of electronic transition that occurs [ 33, 361. This is summarized in refs. 2, 35 and 69.

The energy of the n,r* state is quite insensitive to solvent polarity and the n-r* transition generally displays only a small displacement towards the blue when the medium is changed from non-polar to polar.

However, the ~T,w* state is much more polarizable than the fundamental state. A change in the polarity of the medium therefore causes an appre- ciable displacement of the X-T* transition towards the red.

The CT transition is thought to be greatly affected by dipolar interac- tions because of the large permanent dipole moment of the CT state. Chang- ing from a non-polar to a polar medium produces a large reduction in the energy of the CT state and, consequently, the absorption spectrum is markedly displaced towards the red [ 361.

161

24.2. Aggregation effects Another type of intermolecular interaction is that which occurs be-

tween the solute molecules. Several studies of the aggregation of anthra- quinones in aqueous solutions have been performed 170 - 7 51.

The absorption spectra of the AQ aggregates display hypochromic and slight bathochromic effects when the concentration is increased [75]. The concentration of the ionic associate increases with an increase in the concen- tration of the anthraquinone derivative [ 731. When the concentration of the aggregate is increased beyond a certain point, it ceases to conform to Beer’s law; the coefficient of extinction decreases. The molecular aggregation of AQ derivatives at concentrations above 10s4 M and the presence of aggre- gates with a medium degree of association (2 - 5) are described in ref. 75.

In dilute solutions and low polarity media, the solute-solute interac- tions assume less importance than those which take place between solute and solvent. The aggregation decreases with a decrease in the polarity of the medium [74]. The ionic association constants increase as the solvent dielectric constant decreases [7 31. Temperature increases also reduce the aggregation of the AQ derivatives [ 741.

From the absorption and emission data, it is deduced that 1,4-DHAQ behaves as a monomer [61]. Similarly, emissions from aggregates (or ex- timers) of l&DHAQ are ruled out [64]. The formation of aggregates of the 1,5-DH-4,8-DAAQ sodium salt in water has been reported. The aggrega- tion decreases as the proportion of ethanol in the medium is increased [ 741. Dioxan-water mixtures behave in a similar way until the proportion of organic solvent reaches 70%; then, a further increase in the organic solvent content increases the electrostatic interaction between the dye and the counter-ion (Na’) [73].

2.4.3. Chemical reactions The spectroscopic characteristics of AQ derivatives are also modified

by chemical reactions that occur in the fundamental state. Changes in acid-base equilibria can affect the absorption and emission

spectra. Whether the AQs are acidic or basic depends on factors such as the IHBs [28], the character and position of the substituents [33,76, 771 and the solvent [76, 78, 791. The acidity of the AAQs and HAQs is greater in the singlet excited states [ 761 and triplets [ 801. In the latter, the acidity value is closest to that of the fundamental state.

The derivatives act as chromogenic [Sl - 831 or fluorogenic ligands [84 - 863 and as redox reactants 187 - 941 with many inorganic ions. The optical characteristics of these coordination compounds are due to intra- ligand transitions [95]. In the complexation process, the wavelength of maximum absorption is only slightly displaced. Non-fluorescent derivatives do not give rise to fluorescent complexes.

Oxidation-reduction reactions with inorganic ions give rise to products whose optical characteristics may be very different [92 - 941 from those of the reactants.

162

3. Photochemical reactivity

Anthraquinone dyes are characteristically light-fast. One undesirable effect of some of these compounds is that they cause the degradation of their supporting substrate. Many investigators have tried to resolve this problem by exploring the photochemistry of the simpler anthraquinone derivatives. A discusssion of the mechanisms of these photoreactions is beyond the scope of this review; they are explained and reviewed in refs. 1 - 6. The photoreactions are of interest to the analytical chemist because they are different from those which occur in the dark. In addition, in the deactivation of the excited states of some AQ derivatives, the deactivating photochemical reactions may compete with the luminescence process; both of these phenomena may be exploited in analytical methods.

3.1. Influence of substituenk on photoreactivity The photoreactivities of the AQ derivatives differ widely. AQ deriva-

tives can be classified as either “good” or “poor” sensitizers of environ- mental oxidation [ 96, 971. The “good” sensitizers give phosphorescent emission with the formation of semiquinone radicals, whereas “poor” sen- sitizers only display fluorescence [ 971.

There have been a number of systematic studies of the spectral charac- teristics and behaviour of a series of AAQs and HAQs in flash photolysis reactions which were designed to establish the factors controlling or in- fluencing photostability [ 2, 16, 43, 49 - 55, 981.

AQ derivatives with electron-acceptor substituents (-SOsH, -NOZ, halogens) have high photochemical reactivities [2,3, 99 - lOl], but the substituent positions greatly influence the photochemical behaviour [ 2, 961. This explains why AQ-2-sodium sulphonate and the 2,6- and 2,7disulpho- nates are better sensitizers than the l- and 1,5-sulphonates, and why 1,8- disulphonate is inactive [ 96, 991.

AQ derivatives with strong electron-donor groups (-OH, -NH*, NR1R2) are less photochemically reactive than the non-substituted AQs [3, 8, 921; a substituent in position 1 normally has a more marked attenuating effect than the same substituent in position 2 [ 2,8].

Differences between the AQ derivatives can be explained by potential energy diagrams [2, 41, 501 because, although there are many factors which affect photostability, it is chemical structure that appears to be fundamental; the photochemical reactivity of the AQ derivatives depends on the relative positions of the lowest excited electronic states which are determined by the molecular structure [25].

Most photochemical reactions are initiated by the lowest energy triplet state so that the T,,, * state is more active than the T,,,* state. Those AQs with a low T,,,, state have high photochemical activities [2, 3, 25, 961. In low T n,n* states, the oxygen atom of the carbonyl group is electron deficient and either reacts with any convenient hydrogen atom or shows a strong

163

affinity for electrons [2]. Photoexcited anthraquinones with low energy T n,77* levels are very effective oxidants [ 31.

On changing the solvent, or introducing a specific substituent in the AQ nucleus, the characters of the lowest energy levels can change [ 100, 1011; If these are T r,K* states, particularly of the CT triplet, the photoexcitation causes an electronic displacement towards the oxygen atom which loses its affinity for the hydrogen atom and for accepting electrons. Those AQs which have short-lived T n,n* states or CT triplets are less reactive [ 23.

In addition, the intramolecular CT state is the active state in photo- substitution reactions of aminohaloanthraquinones [ 1021, e.g. the photo- reduction of 2-piperidineanthraquinone [153 has recently been found to occur through a higher T,,,, excited state [40].

3.2. Photochemical reactions Fundamentally, the photoreactions of the AQ derivatives are of two

kinds: photoreduction and photo-oxidation. Because oxygen participates in certain stages of photohydroxylation, photoamination, photochlorina- tion, substitution and elimination, these may be considered as photo-oxida- tions [2].

3.2.1. Photochemical reduction The photochemical reduction of AQs is the simplest and most ex-

tensively studied reaction; it involves the transfer of an electron or hydrogen atom from an appropriate donor to the oxygen atom of the carbonyl group of the AQ derivative in the excited state [2, 3, 251. One of the main causes of the photoinstability and photoreactability of AQs is that they can easily be reduced to anthrahydroquinones by light 121.

The photochemical reduction of AQs can be intermolecular and intra- molecular. Water and almost all organic compounds (hydrocarbons, alcohols, aldehydes, ketones, ethers, amines and amides) that can act as hydrogen donors may serve as reducing agents in intermolecular photochemical reduc- tion [3,100,101]. Those AQs with side-chain substituents such as organic or inorganic anions that donate electrons (hydrogen) undergo intramolecular photochemical reduction [ 3, 101, 1031.

Photoexcited AQs without a hydrogen donor can be photoreduced by removing an electron. In this way, AAQs which are relatively non-reactive to the removal of hydrogen atoms because of the CT character of the lowest excited state, can be photoreduced by the transfer of electrons from hy- droxide ions in solutions with high pH [15,103].

In the presence of oxygen, photochemical oxidation of the reducer can only be achieved in those cases in which the oxygen does not quench, or only quenches slightly, the excited state of the AQ; this occurs in those derivatives with short-lived excited triplets [ 101, 1041,

164

3.2.2. Photochemical oxidation In aqueous solutions or hydroalcoholic solutions, photochemically

reduced AQ-&sulphonates can donate electrons to many compounds. In this way, fl-sulphoanthraquinones also behave as sensitizers of the reduction process of electron acceptors [ 31.

Anthraquinonoid dyes sensitize the triplet state by generating singlet molecular oxygen ‘02 [4]. This process transfers energy between the triplet state of the dye and molecular oxygen

sens* + 302(3Z) - sens + 102( lA)

Those AQs with relatively long-lived triplet states are potential sen- sitizers of singlet oxygen [2]. l-A-4-HAQ and its 2-methoxy derivative, and seven of the components of C.I. Disperse Blue 35, are extremely efficient sensitizers of singlet oxygen production; the most photoactive of the com- ponents is l&DA-4,5-DHAQ [ 1051.

Acknowledgments

This work was supported by the Direction General de Investigation Cientifica y Tecnica (PB 86 - 0247). The author is grateful to Mr. David W. Schofield for editing and translating the manuscript.

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