Phororhemrsrry and Photobioloyy, 1977, Vol. 25, pp. 593-598. Pergdmon Press. Prmted in Great Britain

RESEARCH NOTE

FLUORESCENCE SPECTRA OF CONDENSED AROMATIC HYDROCARBONS IN WATER AND IN

AQUEOUS SURFACTANT SOLUTION

AKWA NAKAJIMA Department of Electronic Engineering, Saitama institute of Technology, Fusaiji 1690,

Okabemachi, Saitama* and Division of Chemistry, Research Institute of Applied Electricity, Hokkaido University, Sapporo, Japan

(Received 20 October 1976;

INTRODUCTION

Rather few studies seem to have been made of the electronic spectra of condensed aromatic hydro- carbons in aqueous solutions. The main reason may be that condensed aromatic hydrocarbons have generally poor solubilities in water so that their spec- tra are difficult to study in full detail. Spectroscopic investigations of aromatic compounds in aqueous solutions, however, are of considerable significance in connection with the studies of solvent effects (e.g. Bay- liss and Hulme, 1953; Bayliss and Wills-Johnson, 1968; Kosower, 19681, applications in biophysics, bio- chemistry (e.g. Udenfriend, 1962; Stryer, 1968; Brand and Gohlke, 1972), and water pollution (e.g. Ishio et al., 1970; St. John, 1973; Kulkarni, 1975).

Further interest has been growing in the spectro- photometric and fluorometric studies of the micellar systems involving aromatic compounds as probes (Riegelman et al., 1958; Nakagawa and Tori, 1964; Hautala and Turro, 1972; Wallace and Thomas, 1973), which are of biological relevance (Cordes and Gitler, 1971). However, the electronic spectra of rela- tively large polycyclic hydrocarbons have rarely been studied in aqueous surfactant solutions.

Our aim is to carry out a spectroscopic study of polycyclic hydrocarbons in water and in micellar solution. In this work, the emission and excitation spectra have been used as more sensitive tools than the absorption spectra. The spectral behaviors of the aromatic hydrocarbons have been compared with particular regard to the effects of water as a solvent and the dissolved states of the hydrocarbons.

As has been demonstrated previously (Nakajima. 1971, 1974). the intensities of forbidden vibronic bands of weak electronic transitions are generally much enhanced in polar media. Thus, the relative in- tensities of the vibronic bands are expected to be markedly increased in water, while the band enhance- ment in the molecules solubilized in the micelles is

~ ~

*To which all correspondence should be directed. ? A similar study was also performed with anthracene

and pyrene, which is to be published elsewhere.

accepted 30 December 1976)

expected to be small as in solvents with lower polar- ity.

MATERIALS AND METHODS

The aromatic hydrocarbons studied? were naphthalene, phenanthrene, triphenylene. 1.2-benzanthracene. 1.1 2-ben- xoperylene, coronene, benzo[u]pyrene and benz&]pyr- ene, which were purified by recrystallization and vacuum sublimation. The surfactant used was sodium dodecyl sul- fate (SDS) of G.R. grade from Nakarai Chemicals, Ltd., which was twice recrystallized from methanol. Water was carefully distilled three times. Ethanol of super-special grade was used as purchased.

The sample solutions were prepared by the following three methods: a small amount (- 1 mg) of the aromatic hydrocarbon was (1) directly dissolved in 10ml pure water, (2) first dissolved in 0.5 me ethanol and then diluted with 10 me water and (3) dissolved in 10 mC aqueous solu- tion containing 0.5 g SDS.

These solutions were treated for more than 2 h in an ultrasonic cleaner to speed the dissolution of the hydro- carbon. The emission and excitation spectra indicated that the compounds were not decomposed after the ultrasonic treatments. If the solutions were turbid with the dispersed microcrystals of the hydrocarbons, they were carefully fil- tered using finest Whatman glass fibre papers. The concen- tration of SDS was 0.17M, sufficiently higher than the critical micelle concentration (0.008 M ) .

The emission and excitation spectra were measured with a Hitachi MPF-2A spectrofluorometer, whose sensitivity was corrected in these spectra. The emission and, if necess- ary, excitation spectra in ethanol and for microcrystals were also observed for comparison.

RESULTS AND DISCUSSION

For illustration, the observations with naphthalene are described in some detail. Figure 1 shows the fluor- escence spectra of naphthalene in water, in aqueous SDS solution, in ethanol and for microcrystals. In these solutions, the spectral shifts are observed to be quite small. The emission spectrum for microcrystals which appears at almost the same wavelength region as those in solutions shows a relatively small broad- ening with considerable reabsorption of shorter wavelength bands. There is no difference between the emission spectra for the solutions prepared by the methods of direct dissolution and dilution of the eth-

593

AKIRA NAKAJIMA 594

+ c 3

._

f e e t ._

x c C 0 t

._

(u

F - c?

Wavelength, nm

Figure 1. Fluorescence spectra of naphthalene in water and in the aqueous solution prepared by the dilution method (-), in aqueous SDS solution (-----), in eth- anol (-) and for microcrystals , (. . . . .). The micro-

crystalline spectrum is shown, for clarity, with the intensity reduced to half relative to those of the solution spectra.

Wovelength, nm

Figure 2. Fluorescence spectra of phenanthrene in water and in the aqueous solution prepared by the dilution method (-), in aqueous SDS solution (-----), in eth- anol (-) and for microcrystals (.....) . The micro-

crystalline spectrum is shown, for clarity, with the intensity reduced to half relative to those of the solution spectra.

anolic solution. By comparing these emission spectra it can be concluded that no molecular aggregation occurs in the present concentration range, in accord with the relatively high solubility of naphthalene in water (Seidell, 1941) and the considerable absorbance observed in a saturated solution. Consequently, the emission observed in aqueous solutions is attributed to the molecular fluorescence of naphthalene.

As studied previously (Nakajima, 1971, 1974), the forbidden components in the vibrational structures of electronic transitions are remarkably intensified under the influence of the solvent polarity. Since water is one of the most polar solvents, the weak vibronic bands in the electronic spectra should increase in in- tensity in aqueous solutions. In fact, the spectral fea- ture in water is very close to those in most polar solvents such as formamide and dimethylsulfoxide (Nakajima, 1974). As shown in Fig. 1, the 0-0 band at 316 nm and the vibronic band at 331 nm are appre- ciably enhanced in water, and this enhancement is of a reasonable magnitude compared to that in eth- anol and other solvents of various polarities.

It is of considerable interest from Fig. 1 that the emission spectrum in aqueous SDS solution is some- what more broadened and shifted to the red than in ethanol and in water, but the relative intensities of the vibronic bands are intermediate between those in ethanol and in water, implying that the naphtha- lene molecules exist in a considerably polar environ- ment, probably near the micellar surface (Riegelman et al., 1958).

In Figs. 2 and 3, the fluorescence spectra of phenanthrene and triphenylene, respectively, in water and in aqueous SDS solution are shown together with those in ethanol and for microcrystals. The emission spectra in aqueous solutions are slightly red-shifted and broadened relative to those in ethanol, while the

microcrystalline spectra exhibit large broadening and red-shift with considerable reabsorption, as usually observed.

As shown in Fig. 2, the emission spectrum of phenanthrene in the solution prepared by diluting its ethanolic solution with water is much the same as that in water, though it is somewhat broader than in ethanol; this indicates that phenanthrene is rela- tively soluble in water (Seidell, 1941) and the molecu- lar aggregates are not extensively formed by this method. In aqueous SDS solution the emission spec- trum shows further red-shift with larger broadening and approaches in some degree the microcrystalline spectrum, implying that there may be some intermole- cular interaction within the micelles. The details of

Wavelength, nm

Figure 3. Fluorescence spectra of triphenylene in water (-), in the aqueous SDS solution prepared by the dilu- tion method (-.-.-), in aqueous SDS solution (-----), in ethanol (-) and for microcrystals (. . . . .). The micro- crystalline spectrum is shown, for clarity, with the intensity reduced to half relative to those of the solution spectra.

Research Note 595

Wavelength, nrn

Figure 4. Fluorescence spectra of 1,2-benzanthracene in water (-), in the aqueous solution prepared by the dilu- tion method (-.-.-), in aqueous SDS solution on exci- tation at 340 nm (------), in ethanol (-) and for micro-

crystals (. . ' . .).

the excitation spectra for the solutions prepared by the three different methods have been found fo be somewhat different from one another, especially in the first electronic transition. The band intensification in phenanthrene is comparatively small in water as well as in other solvents.

As shown in Fig. 3, the emission spectra of tri- phenylene in aqueous SDS solution and in the solu- tion prepared by diluting the ethanolic solution are more or less similar to those in water and in the crystalline state. The weak emission spectra observed on excitation at wavelengths longer than 290nm are in good accord with the microcrystalline spectrum, which indicates that considerable aggregation occurs because of the poor solubility of triphenylene in water. As clearly seen from Fig. 3. the vibronic bands at 346 (O-O), 362 and 382nm are appreciably

enhanced by the change of solvent from ethanol to water (Nakajima, 1974).

Figure 4 shows the fluorescence spectra of 1,2-ben- zanthracene. The emission spectrum in water 'is some- what broadened and red-shifted relative to that in ethanol. No band enhancement is observed in water, as the first transition is allowed in this hydrocarbon. As shown in Fig. 4, the emission spectrum in the solu- tion prepared by the dilution method almost corre- sponds to that for microcrystals, which is consider- ably red-shifted and affected by reabsorption. In aqueous SDS solution the emission spectrum consists of both the microcrystalline and molecular spectra, the ratio of which varies with the excitation wave- length. Thus, part of the solubilized molecules seem to exist in some aggregate state.

The fluorescence spectra of 1,12-benzoperylene are shown in Fig. 5. In water this compound appears to be only slightly soluble and its emission spectrum is observed with weak intensity and large broadening, composed of the microcrystalline and molecular spec- tra. The emission spectrum in ethanol is relatively sharp. In aqueous SDS solution, however, the mol- ecular fluorescence occurs more strongly from the solubilized molecules; the longer wavelength region of the fluorescence spectrum is somewhat blue-shifted relative to the microcrystalline spectrum, suggesting some specific interaction probably caused by aggrega- tion in the micelles. The emission spectrum in the solution prepared by diluting the ethanolic solution agrees very well with that for microcrystals. The band enhancement is poor in 1,12-benzoperylene.

Figure 6 shows the fluorescence spectra of cor- onene. In water the emission could not be observed. No molecular emission spectrum similar to that in ethanol appears in the solution prepared by the dilu- tion method and in aqueous SDS solution, where only the microcrystalline emission spectrum is

Wavelength, nm

Figure 5 . Fluorescence spectra of 1,12-benzoperylene in water (-), in the solution prepared by the dilution method (---.-), in aqueous SDS solution (-----), in ethanol (-) and for microcrystals (. . . . .). The microcrystalline spectrum is shown, for clarity, with the intensity reduced to half relative

to the solution spectra.

596 AKIRA NAKAJIMA

Wavelength, nrn

Figure 6. Fluorescence spectra of coronene in the solution prepared by the dilution method (--.--.-), in aqueous SDS solution (-----), in ethanol (--) and for microcrystals (. . I . .). The spectrum in

ethanol is shown with the relative intensity reduced to half.

k. -. -. C.C.-.-._.

'.

400 420 440 460 480 500 520 540 560 580

Wavelength, nm

Figure 7. Fluorescence spectra of benzo[a]pyrene in water (-), in the solution prepared by the dilution method (-.-.--), in aqueous SDS solution (-----I, in ethanol (--), and for microcrystals

(. . . . .). The relative intensity of the microcrystalline spectrum is reduced to half.

Wavelength, nrn

Figure 8. Fluorescence spectra of benzo[e]pyrene in water on excitation at 320 nm (-), in the solu- tion prepared by the dilution method on excitation at 320nm (----), in aqueous SDS solution (-----) , in ' ethanol (--) and for microcrystals (. . . . ). The relative intensity of the microcrystalline

spectrum is reduced to half.

Research Note 597

1: \

8" 300 320 340 360 380 400

_. 420

Wavelength, n m

Figure 9. Excitation spectra of benzo(a]pyrene in water (-), in the solution prepared by the dilution method: i,,,, - 410nm (-1 and ,Iq,,, <460nm (--.--.-) and in

aqueous SDS solution (-----).

observed, possibly effected by the sensitized emissions from impurities (Kajiwara et a!., 1969).

The fluorescence spectra of benzo[a]pyrene and benzo[e]pyrene are shown in Figs. 7 and 8, respect- ively. Their excitation spectra are also shown in Figs. 9 and 10. These hydrocarbons are sparingly soluble in aqueous solutions, so their emissions are only weakly observed.

As shown in Fig. 7, the emission spectrum of ben- zo[u]pyrene in the solution prepared by the dilution method is clearly comprised of those in water and for microcrystals, as is also demonstrated by the exci- tation spectra shown in Fig. 9. The emission spectrum in water is much more broadened than in ethanol. This broadening seems to be connected with the sol- vent polarity. Thus, the spectral feature in aqueous SDS solution is intermediate between those in ethanol and water, indicating a considerably polar environ- ment in the micelles. Since the first electronic transi- tion in benzo[u]pyrene is fairly allowed, no band en- hancement is observable in its emission spectrum.

As shown in Fig. 8, the fluorescence spectrum of benzo[e]pyrene in water roughly corresponds to that of its microcrystals with an overlap of the weak mol- ecular emission spectrum at shorter wavelengths. The emission spectrum in the solution prepared by the dilution method shows an anomalous feature, poss- ibly due to the sensitized impurity emission from the aggregates.

In aqueous SDS solution in which benzo[e]pyrene

280 300 320 340 360 380

Wavelength, nrn

Figure 10. Excitation spectra of benzo[e]pyrene in water: I,, - 420 nm (-), in the aqueous solution prepared by the dilution method: A,, -410 and 434nm(-----) and

in aqueous SDS solution (-----).

is somewhat soluble, its emission spectrum is similar to that in ethanol and it can be attributed to the solubilized molecules, as indicated by the excitation spectrum in Fig. 10. In the vibrational structure the intensities of 378 (O-O), 398 and 421 nm bands are much increased compared to those in ethanol, imply- ing that the environment surrounding the molecules is more polar than in ethanol. These molecules are probably located near the micellar surface, where the ionic environment is considered to be in effect.

Finally, the observed effects for these aromatic hy- drocarbons are summarized in Table 1.

CONCLUSION

It has been shown that the molecular fluorescence spectra can be observed in aqueous solutions of small condensed aromatic hydrocarbons which are fairly soluble in water, while only the emission spectra for aggregates or microcrystals appear with somewhat larger hydrocarbons. Further, with a few large poly- cyclic hydrocarbons which are almost insoluble, the aggregate formation takes place even in aqueous micellar solutions, thus preventing their emission spectroscopic studies in aqueous solutions.

The vibronic bands in the fluorescence spectra in water are much broadened compared to those in organic solvents, suggesting appreciable specific inter- actions in water.

Table 1. Observed effects in emission spectra of condensed aromatic hydrocarbons

Compound

Naphthalene Phenanthrene Triphenylene 1,2-Benzanthracene 1,12-Benzoperylene BenzoCalpyrene Benzo[ e] pyr ene Coronene

Solubility in water'

+ + + + + + + + ++ + + + + 0

Band intensification

in water

fairly large slight

fairly large no

small perhaps no

somewhat large (very large in polar

organic solvents)

Dilution method?

mol. mol.

agg. + mol. mic.

mic. + mol. mol. + mic.

mic. agg.

In SDS solution?

near the surface mol. interaction

mol. + agg. mic. + agg. + mol.

mol. + agg. mol.

near the surface mic.

~ -

* + + + : fairly soluble, + + : somewhat soluble, + : only slightly soluble, 0: insoluble. t mol. : molecular spectrum, agg. : aggregate spectrum, mic. : microcrystalline spectrum.

598 AKIRA NAKAJIMA

The method of preparing solutions of condensed aromatic hydrocarbons by diluting their ethanolic solutions with water has proved unsuited for obtain- ing homogeneous aqueous solutions. Though it may be a convenient technique for developing molecular aggregates, the aggregate states appear to be consider- ably complex, depending on various conditions of preparation.

In several hydrocarbons, remarkable intensity en- hancement of the vibronic bands has been observed in water, the extent of which is dependent on solvent polarity. It has been demonstrated by employing

these hydrocarbons as probes that the band intensifi- cation can be utilized for investigating the polarity of the environment within the micelles. It is of interest that the environment in the micellar interior has been found to be more polar than in ethanol and, in some cases, the probe molecules are preferably located near the micellar surface.

The effect of the intensity enhancement may be applied to further studies of micellar solutions and investigations in biological systems, and part of the results may be useful for the quantitative analysis of trace amounts of the aromatic hydrocarbons in water.

REFERENCES

Bayliss, N. S., and L. Hulme (1953) Aust. J . Chem. 6, 257-277. Bayliss, N. S., and G. Wills-Johnson (1968) Spectrochim. Acta 24A, 551-561. Brand, L., and J. R. Gohlke (1972) Ann. Rev. Biochem. 41, 843-868. Cordes, E. H., and C. Gitler (1971) Prog. Biachem. 2, 1-53. Hautala, R. R., and N. J. Turro (1972) Mol. Photochem. 4, 545-550. Ishio, S., T. Yano and R. Nakagana (1970) Adu. Waf. Pollut. Res. 2, 111-18/1-8. Kajiwara, T., K. Ohno, S. Iwashima and H. Inokuchi (1969) Bull. Chem. SOC. Jpn. 42, 27342738. Kosower, E. H. (1968) An Introduction to Physical Organic Chemistry, p. 259. Wiley, New York. Kulkarni, R. N. (1975) Water, Air, Soil Pollut. 5, 231-237. Nakagawa, T., and K. Tori (1964) Kolloid 2. Z. Polymere 194, 143-146. Nakajima, A. (1971) Bull. Chem. SOC. Jpn 44, 3272-3277. Nakajima, A. (1974) f. Luminesc. 8, 266269. Riegelman. S., N. A. Allawala, M. K. Hrenoff and L. A. Strait (1958) J. Colloid Sci. 13, 208-217. Seidell, A. (1941) Solubilities of Organic Compounds. Van Nostrand, New York. St. John, P. A. (1973) In Water and Water Pollution Handbook (Edited by L. L. Ciaccio), Vol. 4,

Stryer, L. (1968) Science 162, 526533. Udenfriend, U. Wallace, S. C., and J. K. Thomas (1973) Radial. Res. 54, 49-62.

pp. 1687-1750. Marcel Dekker, New York.

(1962) Fluorescence Assay in Biology and Medicine. Academic Press, New York.