Photochemtstry and Photobiology. 1976, Vol. 23. pp. 3 7 4 3 .

Pergamon Press

Printed in Great Britain

DIRECT AND SENSITIZED PHOTOOXIDATION OF CYANINE DYES G. W. BYERS,S. GROSS and P. M. HENRICHS Research Laboratories, Eastman Kodak Company, Rochester, N.Y. 14650, U S A (Received 30 M a y 1975; accepted 12 September 1975)

Abstract-Direct photooxidation of cyanine dyes was found to be markedly enhanced by formation of hypsochromically shifted (H) aggregates. Singlet oxygen was found to attack many cyanines possessing low oxidation potentials with rate constants in excess of lo8 / mol- I s- I . Singlet oxygen attack yields carbonyl products consistent with 1,2 addition to on 3,3’-diethyl-8,1O-dimethylthiacarbocyanine the 2,8 bond of the methine chain.

INTRODUCTION

Fluorenone were Baker chemicals “suitable for photosensitizer use” and were used without further purification. 2,SDimethylfuran (Aldrich) was distilled (94°C) just prior to use. N-Ethylbenzothiazolone was prepared by alkaline hydrolysis of 1-ethyl-2-ethylthiobenzothiazoliumethylsulfate and was distilled just prior to use (bp 117°C at 0.7 mm). Ethyl-2-quinolone (Eastman organic chemicals) was used without further purification. Methanol-d, was Stohler 99’%D. Methylene blue (Eastman, cert.) was recrystallized once from water and three times from ethanol. Rose bengal (Eastman, cert.) was recrystallized twice from water. 3,3,4,4-Tetramethyl-1,2-diazetine-l,2-dioxide (mp 190°C) was prepared according to Singh and Ullman (1972), and was given one extra recrystallization from ethanol. The solvents were Eastman spectro quality and were used without further purification, except when concentration of the solvent necessitated its distillation in a glass apparatus to remove trace nonvolatiles. Quantitative analyses for dye degradation were performed spectrophotometrically with Cary model 14, Perkin-Elmer models 350 and 450, and Unicam model SP1800 spectrophotometers. Solutions of aqueous H-aggregates were quantitatively diluted with methanol and analyses were based upon the absorption of the monomeric dyes. NMR spectra at 100 MHz were obtained with a Varian Associates HA-100 spectrometer equipped for operation in the pulsed Fourier transform mode with a Varian 620/i computer. Spectra (4K data points) were obtained from 4x M dye solutions in 45 min. Gas chromatographic (GC) analyses were performed on a Hewlett-Packard Model 5750 flame ionization chromatograph. N-Ethyl-2-quinolone and N-ethylbenzothiazolone were analyzed on a Carbowax 20M column with hexaeicosane as an internal standard. 2,5-Dimethylfuran was analyzed on a Chromosorb 101 column employing the method of quantitative injections. Quantitative photolyses were performed on an optical bench employing the following appropriately connected components; (1) a Bausch & Lomb high-intensity monochromator (1350 lines/mm, 20W00 nm) with a 200 W Osram super pressure mercury lamp and a 1-1/2 in. quartz collimating lens, (2) a mount for 2 x 2 in glass filters, (3) an Ilex Optical Co. 3 universal manual shutter, (4) a masked sample holder for 4 em rectangular optical cells, and (5) a calibrated photomultiplier. Actinometry was accomplished with an EG & G radiometer system (+ 580) with an S-20 photomultiplier calibrated for 3W800 nm and a quartz narrow-beam adapter similarly calibrated. Chemical actinometry (3W400 nm) was effected with ferrioxalate (Calvert and Pitts, 1966), and this technique was

Cyanine dyes have found extensive commercial application as spectral sensitizers in silver halide photography. However, the vast majority of these dyes possess insufficient light-fastness for textile or similar commercial applications (Baer, 1971). Timbers and Lingafelter (1949) have noted the photooxidation of pinacyanol (I) by oxygen. More recently Krasnovskii and Drozdova (1962) and Bourdon and Durante (1962) described the photoreduction of cyanines in the presence of ascorbic acid. Many cyanine dyes are also

excellent peroxy radical scavengers (Fukuzumi and Ikeda, 1971; G. Byers, unpublished). Meier (1971) and Foote (1968) have outlined a number of paths by which dyes in general may become photodegraded. Several instances of cyanine-dye photodegradation have been observed in these laboratories during handling of concentrated aqueous solutions. Under these conditions cyanines often exhibit hypsochromically shifted absorption bands due to formation of dimers, trimers, etc. (H-aggregates) (West and Pearce, 1965). We have investigated the direct photooxidation of cyanines in both the H-aggregated and monomeric states and noted the kinetic changes associated with aggregate formation. The photosensitized oxidation of monomeric dyes was examined in detail and oxidation products for a few dyes are reported. MATERIALS AND METHODS

The dyes employed were prepared by standard synthetic procedures (Hamer, 1964). All dye samples appeared pure (single component) by paper electrophoresis and thin-layer chromatography (TLC, cellulose, silica gel, and acidic, neutral, or basic alumina). 2-Acetonaphthone, Pyrene, and

37

38

G. W. BYERS,S. GROSSand P. M. HENR~CHS

used repeatedly to check the calibration of the radiometer system in this region. Aging lamps were changed before maximum intensity variations reached 10% (generally less than 100 h). Solutions were prepared and handled under appropriate safelight conditions. Stirring was accomplished by bubbling a solvent-saturated gas (usually MeOH-sat. 0,)through the rear portion of the cell. Degradations were generally carried to ca 20% conversion to allow accurate determination of dye consumption. Quantum yield determinations for sensitized dye degradations as a function of conversions indicated that the quantum yields remain relatively constant through about 25% conversion. Qualitative photolyses of aqueous aggregated dyes were carried out in Pyrex test tubes (15 x 125mm) or 1 cm square Pyrex tubes. The samples (7 m / ) were photolyzed in a Rayonet photochemical reactor with white fluorescent lamps and were supported by a merry-go-round apparatus (Southern New England Ultraviolet Co. MGR-100). The tubes were generally deoxygenated by flushing with solvent-saturated Nz for 10 min. Prcparative photolyses were performed in a Rayonet reactor. Sample solutions M in dye) were prepared, with redistilled solvents. Solutions were irradiated in volumetric flasks (5W2000 nx") with magnetic stirring and flushed with solvent-saturated air. Photolyzed solutions were then concentrated in the dark by rotary evaporation a t or below room temperature. Photodegradations for NMR analyses were done by irradiation in 5 mm NMR tubes of methanol-d, solutions containing 4 x 10-3 M dye plus 1 mol % methylene blue sensitizer. Excitation was by a Spectra-Physics model 124A He-He laser operating at 632.8 nm with 15 mW output. A cylindrical lens was employed to spread the beam optimally over the entire sample.

i'\*A"/ oi /*\

/x\

*-

:+ @

v

I

El

VI

Varying the concentration of VI from 1.5 x 10 to 1.5 x M yielded a linear plot of Go/@ vs [VI] with an intercept of 1.0. Assuming values of 2.7 x M for the concentration of 0, (Hodgman, 1960) and 3 x lo9 t mol-' s-' for ko, (Kilsson et al., 1972) in air-saturated watcr one obtains a minimum of 3 x lo9 / mol-' s-' for kvl. It is cvidcnt that the diazetine dioxide (VI) is quenching dye (V) RESULTS AND DISCUSSION triplet near the diffusion-limited rate and that the dye A number of cyanines, including pinacyanol (I), triplet state is an intermcdiatc in thc photodegradathiacarbocyanine (11), oxacarbocyanine (111), krypto- tion. H-aggregated cyanines are recognized to have cyanine (IV), and 3,3'-diethyL4,5,4,5'-dibenzothiacyanine (V) were photolyzed in both monomeric and enhanced triplet yields (Pant ct a!., 1971 ; McRae and aggregate forms. As dilute monomers (10-5-10-6 M ) Kasha, 1958) in accord with molecular exciton theory in methanol, the dyes were quite stable to direct irra- (McRae and Kasha, 1963; Cooper and Liebert, diation into their visible absorption bands with quan- 1972). Triplet-sensitized formation of singlet oxygen tum yields less than As aggregates in 98 : 2 (v:v) ('0,)is well documented (Kearns, 1971). Thus, singlet water -methanol, the dyes degraded with quantum oxygen appears to be a likely participant in the degrahowever, flushing dation of H-aggregates. In contrast. monomeric yields ranging from lo-' to the aggregate solutions with N, to remove 0, gener- cyanines generally have short singlet lifetimes and in(O'Brien ally reduced the quantum efficiencies about two tersystem crossing yields less than a few orders of magnitude. Monochromatic irradiation into et al., 1974). These properties limit singlet oxygen prothe H-aggregate band (440 nm) of 3.5 x M duction as well as possible compcting redox reactions 3,3'-diethyl-4,5,4,5'-dibenzothiacyanine (V) in oxygen- of the excited dyes in accord with our observations saturated 95 : 5 (v :v) water-methanol degraded the of low bleaching quantum yields (< 1 Obh) for dilute dye with a quantum yield of 3.4 x The photo- monomers. degradation of V was inhibited by the triplet Quantitative kinetic analysis of thc oxidation of quencher 3,3,4,4-tetramethyl-1,2-diazetine-1,2-dioxideaggregated dyes is complicated by the distribution in VI (Singh and Ullman, 1972) according to Eq. 1, the size of the aggregates. The number of monowhere @ and Go are the quantum yields for dye degra- meric components in an aggregate affects the extent dation in the presence and absence of VI, respectively; of molecular exciton splitting and probably thc interk , , and ko, are the rate constants for quenching triplet nal kinetic processes, such as intersystcm crossing 10 respectively; and kd is the rate yield triplets. To avoid thc complexities associated dye by VI and 302, constant for natural decay of dye triplet. with aggregation and determine the sensitivity of cyanines to '0, attack, we have photosensitized the @O k V i CVIl -= 1+ oxidation of monomeric cyanines in methanol. @ ko,CO21 + k , The photosensitized oxidation of monomeric cyanines proceeds rapidly in methanol. 2-Acetonaph*Oho, = 0.75 If- 0.1 obtained from 2,5-dimethylfuran thone is an effective* singlet oxygen sensitizer and scavenging experiments.

:',

Photooxidation of cyanine dyes has no visible absorption to interfere with spectrophotometric dye analysis. If the dye concentration is kept sufficiently low (

Direct and sensitized photooxidation of cyanine dyes.

Photochemtstry and Photobiology. 1976, Vol. 23. pp. 3 7 4 3 . Pergamon Press Printed in Great Britain DIRECT AND SENSITIZED PHOTOOXIDATION OF CYANI...
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