The contribution of singlet oxygen to the photofading of triphenylmethane and related dyes

D>vsand Pipms

3 (1982) 49-58

THE CONTRIBUTION PHOTOFADING

OF SINGLET OXYGEN OF TRIPHENYLMETHANE RELATED DYES

NOBUHIRO

TO THE AND

KURAMOTO

Deporrmenr o/ Chemisrr>., Osaka Prejccrural lndusrriaf Research Z-I-53, Enokojima. Nishi-ku. Osaka 550. Japan

Insrirure.

and TEIJIRO KITAO

Drparrmcnr of Applied Clrcnrisrr_v. Coltcgr 01 Engineering. Uniwrsirj. o/- Osaka Prefcrrurc. J-804. h4ozrr-Unwnrachi. Sakai 591. Japan (Received:

19 June, 1981)

The conrriburiotl oj’singler os)*gen (‘A&I,) 10 ihe phorofading of Cr~*srnl Vioier in solccrl~s is examined. The I’CIICSof pholofuding it1 diclllo~~o~?~crhanc or acetone ure uccclct-ared in ~hcprescncc oj’singfcr o-x>*gcn sensirizers, e.g. Alerh~~lcm~ Blue. The t-arcs are relurdcd bjy adding ~#2cricc singlcr os>.gcn quenchers szrch as /$cnrorcne or nickel din~c~h~~ldirl~iocarhar,~a~e. II SCLWIS rhar Cr_~*srnl Violer is ruainl~- phoroosidixd IO give AficJlJer’s k-cro17c atld p-dil?7eiill.far,7inopherlo/ via rhe reaction ,r,irh si?iglcr os)*gcn. which can be gcrleralcd 6~; rhe dJ*e irselj’or bj- an addedsensirirer. TJte cjjkt of lTarious singfer os)gen quenchers ~JI ~hephorostabilit~~ of’coloured rnareriafs derived)-on1 coforrr jbrrncrs. such as Crixraf Violer Lacfone and 3-dierJ7_~.lnnlitlo-6mc~h_~*l-7-anilinojhrorar7. is esamimd also 017 silica gel. SOINC

1_

Several

INTRODUCTION

of the photofading behaviour of triphenylmethane dyes in solution have been reported_ ’ - ’ Henriquez3 proposed that in aqueous solutions of Crystal Violet and Malachite Green, dealkylation of dimethylamino groups on the phenyl rings occurred to form Parafuchsine or Doebner’s Violet; On the other hand, Iwamotos isolated Michler’s ketone [4,4’-bis(dimethylamino)benzophenone] from the photofading-products of Crystal Violet in the 49 or

on

studies

textile

materials

and Pigmenrs Ol43-7208/82/0003-0049/502-75 Printed in Great Britain

Dyes

0

Applied

Science Publishers Ltd. England,

1982

50

NOBUHIRO

KURAMOTO,

TEIJIRO

KITAO

presence of air. Porter and Spears6 exposed Malachite Green to a carbon arc source solution or on carboxylated cellulose film. and isolated 4in aqueous 4-methylaminobenzophenone. p-dimethylaminodimethylaminobenzophenone. phenol and an additio:lal product. the carbinol form of Malachite Green. In recent years. the contribution of singlet oxygen to the photofading of some dyes has been examined. For esample. it has been reported that arylazonaphthols.‘.” indigo.” quinophthalones” and aminoanthraquinone dyes’ 3 undergo self-sensitized or dye-sensitized photo-oxidation in solution. Furthermore. the contribution of singlet oxygen to catalytic fadins in dye mixtures has been reported recently.‘~.” In the present \vork.wc wish to report the photofading behaviour ofCrystal Violet (1) in some solvents under aerobic conditions. together with the mechanism of the photo-decompositions. We have rilso examined the effect of singlet osygen quenchers on the photostability of coloured materials derived from colour formers such as Crystal Violet Lactone ;tnd 3-diethylnmino-6-methyl-7-anilinofluoran.

3 -.

2.1.

ESPEHISIENTAL

,\ 1trrc~rirrls

The chloride of Crystal Violet (Cl Basic Violet 3. &,,_,, 588 nm) (1) and Malachite Green (Cl Basic Green 4. i,,, 420 and 616 nm) (2) were commercial reagents (Tokyo Kasei Co. Ltd). Compounds (1) and (2) were considered chromatographicnlly pure. Cryst:ll Violet Lactone (8) and 3-diethylamino-6-methyl-7-anilinofluoran (IO) were obtained from commercial products. ’ 3 PIYKcYlrrlY~ jor plwIq~uding 0) ( 1) _.-. Solutions (2-O x IO- s mol litre- ‘) of 1 in 400 ml of dichloromethane. acetone or merhanoi were prepared. These air-saturated solutions were placed in an insideirradiation-type flask (500 ml) equipped with a thermometer. condenser and mngnetic stirrer and were irradiated at 22-25 “C with a IOOW high-pressure mercury lamp (Ushio Electric Inc.. U M-l 02). A Pyres glass filter was used for filtered r:idialion (;. > 300 nm). Dye solutions (400 ml. 1-O x IO- ’ mol litre- ’ ) containing Methylene Blue (3 mg). &carotene (107 mg). nickel dimethyldithiocarbamate (NMC) (9.6 mg). 1.4diazabicyclo[2.2.2]octane (DABCO) (890mg) or 2.6-di-r-butyl-p-cresol (MO mg) \vere irradiated in a similar manner. The absorption spectra of the dye solutions were measured before and after irradiation using a Shimadzu recording spectrophotometer (UV-201 type), and the relative percentage fading was determined at the absorption maximum of 1. Similar experiments were carried out with 2.

THE

2.3.

CONTRIBUTION

Idenrificnrion

OF SINGLET

OXYGEN

TO THE

PHOTOFADING

OF DYES

51

of prodltcrs

A 285 mg/500 ml solution of Crystal Violet (1) in dichloromethane was irradiated for 10 h with filtered radiation (i- > 300 nm). After irradiation, most of the solvent was reduced in volume to 10 ml using a rotary evaporator under reduced pressure. The resulting residuewas put on an activated alumina column, and then eluted using benzene and dichloromethane-acetone (9: 1, v/v). Four fractions were taken. The UV spectrum and tic analysis of the first fraction coming off the column revealed pdimethylaminophenol together with smaller amounts of minor products. A compound in the second fraction coming off the column was identified as 4,4’bis(dimethylamino)benzophenone (Michler’s ketone) by comparing its melting point ( 172- 173-5 “C). R, values [methanol-acetone (1: 1. v/v) 0.80. benzene-acetone (4: 1. v/v) 0.58 and chloroform-acetone (7:3. v/v) O-641 and UV-spectrum (&,.,,, 245 and 370 nm) with those of an authentic sample. The visible spectrum of the third fraction showed absorption maxima at 538 nm (shoulder) and 586 nm. The former was nearly identical to one of Parafuchsine. and the latter was identical with one of unchanged starting dye. The analysis of the fourth fraction showed the unchanged starting dye. In addition, the formation of p-dimethylaminophenol could be also observed in the aqueous alkaline solution extracted from the reaction mixture. Therefore the irradiated solution was extracted with aqueous sodium hydroxide. The extract WAS neutralized with hydrochloric acid solution and once again was extracted with benzene_ The benzene extract was evaporated under reduced pressure, and the crude crystal-like residue was recrystallized once from ligroin. The material obtained was identified as p-dimethylaminophenol by comparison of its melting point (72-75 “C) and R, value with those of an authentic sample prepared by a method described in the literature.6

A solution of 1 (2-5 x lo-’ mol litre- ’ ) in 100 ml acetone was prepared and 4 ml of the solution was mixed with 1 ml of N MC solution at various concentrations. The solutions (5 ml) were sealed in a Pyrex ce!l (10 x 10 x 60 mm) with a stopper. and irradiated with filtered radiation (i- > 300nm) through the Pyrex glass filter from outside of the vessel. After irradiation. the relative percentage fading of dye was determined spectrophotometrically. and the relationship between the reciprocal of the rate of decomposition of the dye and the concentration of NMC was plotted (Fig. 3). 2.5.

Phorojriding

oj cotour jormcrs

on silica

gel

A solution containing 238 mg of 3-diethylamino-6-methyl-7-anilinofluoran (10) in 200 ml of acetone-chloroform (1: 1, v/v) was prepared. Nickel bis(dithiobenzi1) (NBDB. 27-2 mg or 2.72 mg). nickel dibutyldithiocarbamate (NBC. .23.4 mg). DABCO (5.6 mg). 2.6-di-I-butyl-p-cresol (HP. 1 1 mg) or NBDB-HP (27-2 mg and

52

NOBUHIRO

KURAMOTO,

TEIJIRO

KITAO

5-5 mg) were added to 20 ml aliquots. The solutions (20 4) were spotted on silica gel tic plates using a microcylinder and dried for 30 minutes at room temperature_ The plates were exposed to air and light through a Pyrex glass filter from a IOOW mercury lamp cn 5cm from the plate. The apparatus was kept at 30-33°C. After irradiation for 20 h. plates were developed with methanol. The spot of separated dye was scanned using a dual-wavelength chromatoscanner (Shimadzu. CS-900). The conversion ( :a) was calculated from the integration values obtained by the scanning. Similar operations with the solutions of Crystal Violet Lactone (8) were carried out using nickel dibutyldithiocarbamate. nickel bis(dithiobenzil) and 2.6-di-f-butylp-cresol.

3. _

RESULTS

AND

DISC-USSIOK

A solution of Crystal Violet (1) in dichloromethane \vas faded in the presence of oxygen on exposure to filtered radiation (2 > 300 nm) from a high-pressure mercury lamp source. Figure I shows the photofading behaviour of 1 in the presence of various additives on exposure to filtered radiation. The rate of the photofading was accelerated by adding a singlet oxygen sensitizer such as Methylene Blue. On the other hand the rate was retarded in the presence of p-carotene or N MC. which act as the rate of the photofading was only quenchers of singlet oxygen. Furthermore.

Irradiation time t h) Fig. I : Effect of additives on the photofading of Crystal Violet (1) in dichloromethane. Dye solution, 400mi of 2.0 x 10m5 M. 1OOW internal high-pressure mercury lamp. Pyrex glass filter (I. > 300nm). Additives: 0). none; a, nickel dimethyldithiocarbamate (8.0 x lont); A. j&carotene (5.0 x 10-4~); 0. Methylene Blue (2.0 x 10m5 M): 0. 2,6-di-c-butyl-pcresol (5.0 x 1Om3 M).

THE

sli_ghtly

CONTRIBUTION

altered

by

the

OF SlNGLET

addition

OXYGEN

of

TO THE

PHOTOFADING

2,6-di-i-butyl-p-cresol.

OF DYES

a good

53

free-radical

scavenger. These results suggest the involvement of singlet oxygen (presumably ‘AJ in the photochemical oxidation of 1. The phot0fadir.g behaviour of Malachite Green 2 was similar to that of I. Figure 2 shows the relative rates of the photofading of 1 in various solvents without a sensitizer. The rate of reaction depended on the solvent, apparently as a function of singlet oxygen lifetime.16I9 To obtain additional evidence for the intermediacy of singlet oxygen, a competitive photo-oxygenation experiment with two different substrates, as described by Wilson”o and our previous papers,’ 1.12*‘5 was performed. Crystal Violet (1) and nickel dimethyldithiocarbamate (NMC) were used as acceptors (N MC acts as a quencher only. while I undergoes oxidation as well as acting as its own sensitizer). For steady-state concentrations of I* and ’ O2 the reciprocal of the rate of dye decomposition should be linearly dependent on NMC concentration. Figure 3 shows the result in thiscase. and N MC was shown to react faster than 1 with ’ 0, by a factor of C(I Il. When 1 was irradiated with filtered radiation (2 > 300nm) in dichloromethane.

1001 0

1

2 Irradiation

Fig. 2.

3

4

time(h)

Pbotofading behaviour of Crystal Violet (1) in various solvents. Dye solution. 4OOmI of 2-O x 10m5 M. 1OOW internal high-pressure mercury lamp. Pyrex glass filter (I. > 300nm).

54

NOBUHIRO

KURAMOTO,

TEIJIRO

KITAO

0

15 ./

67 &lo x I

-

E E G ln z Ti5

0

z v

of_.

0

05

1.0

[ NMCI ( mol like-’ x105) Fig. 3. Inverse of the r;LIe of decomposition of Crysral Violet (1) ( - Ar/A[D]) as a function of NMC concenrralion. Initial concenrration of (1) . 2.0 x IO- ‘~1 in acetone. Self-sensitized (filtered radiation. L > 300nm for 3 h).

the products were Michler’s ketone [4.4’-bis(dimethylamino)benzophenone]. pdimethylaminophenol and Parafuchsine. which were identified by comparison with authentic samples. On the other hand. the photo, decomposition products of Malachite Green (2) have been identified by many workers. 3- 6 From irradiation of 2 in aqueous solution or on carboxylated cellulose. Porter and Spears6 isolated oxidative products such as 4-dimethylaminobenzophenone. 4-meuhylaminobenzophenone and p-dimethylaminophenol. together with the carbinol form of 2 as the minor product. Iwamoto5 also obtained the same two ketones from the photofading mixture. 3.2. Mechanism of rhe phorojading of Cr>-sral Violcr and Malachite Green The most probable mechanism of the photofading and the formation of the products is shown in Scheme 1. Attack of singlet oxygen on Crystal Violet should lead to the unstable dioxetan intermediate. which could produce dimethylaminobenzophenones and dimethylaminophenol. The structure of the dioxetan intermediate was supported also by the calculation of the a-electron density of

(I) X = -NMe, (C.V.) (2) x =-H (M-G.)

i,,

‘I

:

:

!-4

‘, ‘,

‘St

+

Dealkylation product

‘.

‘. ‘\, ‘, hv

h#OJ

X (5) Scheme I

\ 3

/

c=o

i

(3)

X

f

3

I

(6)

0

I

SMe, He,

\ 3

/

(7)

OH

NMe,

(4)

X

56

NOBUHlRO

KURAMOTO,

TEIJIRO

KITAO

Crystal Violet using a Pariser-Parr-Pople molecular orbital calculation (PPPMO) method.*l The mechanism is appropriate also for the photofading of Malachite Green. In the case of Crystal Violet. the contribution of dealkylation was also suggested. since smail amounts of Parafuchsine could be detected in the reaction mixture. The extent of thecontribution of the dealkylation. however. was very much smaller than that of the singlet oxygen mechanism. Similarly. it is known that Malachite Green gives Doebner’s Violet by the dealkylation of the dimethylamino gro~p.~ 3.3. E/j,cf oj single1 0.x-j-gcn yircnchcrs I0 r/w phoIosInbilir_l- 01 colourjorf?lcrs Crystal Violet Lactone (8) and 3_diethylamino-6-methyl-7-anilinofluoran (IO) are e.g. business forms for register or estensively used for technical reproduction. computer. The colour materials derived from these colour formers show a bluetinged violet colour (9) and black (11). Generally. the dyes derived from the colour MQN_o_C~NM~I

M‘,NaCaNMcz

\ c=o ’ b

I N Me, (8)

5

C_02H ’ b

I NMcz (9)

THE CONTRIBUTJON

OFSINGLET

OXYGEN

TO THE PHOTOFADJNG

OF DYES

57

formers have a poor fastness to light. and an improvement of the properties is required.2’ In this work, the effect of various additives on the photostability of the dyes 9 and 11 derived from 8 or 10 was examined on silica gel plate. The parent structure of 9 resembles that of Crystal Violet (1). Therefore it is assumed that the photofading of 9 on silica gel may involve the contribution of singlet oxygen. The influence of some additives on the photostability of 9 is summarized in Table 1. When 9 on silica gel plate was exposed for 10 h to filtered radiation (2 > 300 nm) and air. it showed 47.7 “/;,conversion, and in the presence of

TABLE EFFECT

OF ADDITIVES

ON

THE

Run no.

D_W

2 3

9b

,

4

2 i b

9 10 II

I

PHOTOSTABlLlTY

Addiriced

None NBC NBDB HP None DABCO

NBC 11’

NBDB’ NBDB !BPDB-HPI

OF 9 ANC

11

ON SlLlCA

Conversion

GEL-

( y;)4

47-7 16.0 13.0 48.s 54.1 38.0 28-8 24.7 8-3 54.3 10.7

0 In all runs. tic silica gel plates bearing 20111 of the dye solutions (I : I. v/v)] were exposed to I3.5 x IO-‘hf. in 2Oml acetone -chloroform air and light (i z 300nm). ’ The dye was exposed for IO h. ’ The dye was exposed for 20h. d As additives. DABCO. nickel dibutyldithiocarbamote (NBC). nickel bis(dithiobenzil) (NBDB) and 2.6-di-r-butyl-pcresol (HP) were used. Additive/dye = I-O (mol ratio). ” NBDBjdye = 0.1 (mol ratio). I NBDB and HP were added. y From tic analysis.

equivalent molar amounts of NBC or NBDB, good singlet oxygen quenchers’3-“h showed 16 y/dand 13 y0 conversion. respectively. On the other hand. 2,6-di-I-butyl-pcresol had no influence on the rate of photofading. These results indicate that the photo-oxidation of 9 on silica gel plate also takes place predominantly via the singlet oxygen mechanism. The fading behaviour of 11 derived from 10 on silica gel plates was examined in the same manner as above. Although the photo-reactioqmechanism of 11 has not been defined yet. similar effects of the additives were observed on silica gel. Thus it was found that the transition metal compiexes have suppressing effects on the photofading of 11.

58

NOBUHIRO

KURAMOTO,

7-EiJIRO KITAO

REFERENCES I

1. 3. 4. 5. 6. 7. 6. 9. IO. II 12.

13. 14.

15. 16. 17.

IF. 19.

K. hlCLAREN. J. Sac. Dwrs and Colowisrs. 72. 86 (1956). J. W~crtaw. ~\lcllfand Tc.~rilbcr. .3?. =+08 (1958). P. C. HENRIQLIEZ. Rec. Trot. Chim.. St. 998 (1933). G. S. EGERTOS and A. G. MORGAK. J. Sot. D_wrs and Colourisrs. 86. 242 (1970). K. iWAhlOT0. Bu//. Circm. SO‘--. J/m. 10. 420 (1935). J. J. PORTER and S. B. SPEARS. Tcstilc Chew. Co/our. 2. 191 (1970). C. Xl. DE.SAI and B. K. VAIDYA. J. lndian Chenr. Sot.. 31. 261 (1954). N. S. ALLEN:. B. MOHAJERASI and J. T. RICHARDS. Dves and Pinnrenrs. 2. 31 (19811. J. GRImITIisand C. HAWKISS. J.C.S. Char. Comm.. 463 (1971): ~C/rmr.Soc. Pfrkirr ii. 747 (1977). N. KLIRANOTO and T. KITAO. ,\‘ippopr A’agakrr Kaislri . 258 (1977): J. SOC. DIIVS and Colorrrisrs. 96. 529 (1980). N. KLIRAXIOTO and T. KITAO. ibid.. 95. 157 (1979). N. ~L&AW&i,“dT. KITA0.J.C.S. Clrcm. C;m”r.:379(1979): J.CJrrr,~.Soc. Pcrkin II. 1569( 1980). N. KLlRAhmTo and T. KITAO. Dw.x and Pi.~nkvz~.~. 2. 133 (1981). M. W. REUBOLD rend H. E. A. ~RALIER. J.‘Soc. D_t,rr.v ond~Colottrisr.\. 94. 12 (1978): ibid.. 96. 122 ( I9SO). N. KLIRA~IOTO and T. KITAO. J. Chw. ~ccl~nol. Biorccirnol.. 30. 129 (1980). P. B. hlmim. and D. R. KEARSS. J. Amer. Chm. Sor.. 94. 1029. 724-I (1972): D. R. ADAW and F. WILI;ISSOX. J. Chcrrr. SW. FabraJo.l. Trms. I. 586 (1972). C. S. FOOTF. Y. C. CIIASC and R. W. DESS:Y. J. .-1nwr. Clwn. Sot-.. 92. 5116 (1970). R. W. DESSY and A. N~caos. Orp~rtrr ru.zcriom. Volume 20. p. 132. Ncu York. John Wiley &Sons ( 1970). hj. ~orzvw. S. Knro. ht. hlATAGA. 7. ~(IATSUWZA nnd T. Usal. f’horoscwsiri~c~d rcucrions. p. 226.

Kyoro. Kugakudojin (1976). ‘0. T. ~~‘ILXX J. .jttwr. C/rcwr. SM.. 811. X98 ( 1966). 21. RI. hlATSC.oK;Aand T. KITAO. unpublished work. 2’. K. \‘A>lA\lOTO. SWr>Y? ‘I& )‘crX-Zt/IZr,.19. 130 (1971). 23. A. ZNIIG and \V. A. HESDtRSOS. J. PO/~.. Sri.. PO/~. C/u-nr. E&I.. 13. 717 (1975). 71. J. P. GLILLORS and C. F. c-001;. i&l.. II. 1927 (1973). 2 5 D. J. CARL~SW. G. D. hItslxsii,\u_. T. SUPKL~NC~IUK and D. hl. WILES. J. .4tm-r. C/WWI.SW.. 9-1. S960 (197’). 26. H. H. WASSEKW~X rend R. N’. illb~~n~.

Sing/ur

n.v,vgm.

p.

160. N~lw ‘x-ork. Academic

Press (1979).