J . Phys. Chem. 1985,89, 1171-1 177
1171
Photochemistry of Naphthalene in Alcohol or Alkane Solutions at High Pressures Gerald Z. Yin and Malcolm F. Nicol* Department of Chemistry and Biochemistry, University of California. Los Angeles, Los Angeles, California 90024 (Received: November 19, 1984)
The ultraviolet photophysics and photochemistry of naphthalene dissolved in fluid alcohols or alkanes have been studied at pressures up to 6 GPa at room temperature. A series of photoreduction, photoaddition, and subsequent polymerization reactions can be excited at 3 15 nm and shorter wavelengths at high pressures; the same reactions occur with much lower efficiencies at atmospheric pressure and room temperature. The dependence of naphthalene reaction rate on the irradiation intensity indicates that the primary step of the photochemistry is a two-photon process. Microanalyses of the photoproducts recovered from the high-pressure diamond cell by GC-MS spectrometry show that they are primarily tetrahydronaphthyl derivatives. A triplet pathway and subsequent radical mechanism are suggested and discussed.
The photophysics and photochemistry of naphthalene and other simple aromatic hydrocarbons attract attention as prototypes for understanding the photoactivities of more complex systems without, for the most part, photochemical complications.',2 With a few exceptions (some photodimeri~ations,~"photoadditions of olefins,7-" and photoreactions with amine^'^-'^), naphthalene is relatively stable under ultraviolet (W)irradiation. Photoadditions and photoreductions of aroomatic hydrocarbons in "inert" solvents have been reported by Lamotte and Jous~ot-Dubien,'~J~ who detected permanent photoproducts during UV irradiation of polynuclear aromatics in rigid matrices of alkanes, alcohols, or polymers a t 4.2 and 77 K. For anthracene in cyclohexane, new absorption and fluorescence spectra were attributed to 9,lO-diThe hydroanthracene and 9-cyclohexyl-9,l0-dihydroanthracene. substitution product, 9-cyclohexylanthracene, and oxidized derivatives, bianthryl-9-anthrone and 9, l @anthraquinone, also were found.I7 Phenanthrene16 and triphenylene'* were believed to react similarly in rigid matrices at low temperature. The yields of these reactions are relatively low, and Lamotte and Joussot-Dubien claim that these reactions do not occur in degassed liquid solution^.'^*'^
(1) John B. Birks, "Photophysics of Aromatic Molecules", Wiley, New York, 1970. (2) D. Bryce-Smith, Ed., 'Photochemistry", Vol. 1-13, Part 111, John Wright and Sons Ltd., London, 1968-1981, Chapter 4. (3) Th. Forster and K. Kasper, Z . Phys. Chem. (Wiesbaden), 1, 175 (1954). (4) D. 0. Cowan and R. Drisko, "Elements of Organic Photochemistry", Plenum Press, New York, 1975, Chapter 2. (5) J. S. Bradshaw and G. S. Hammond J. Am. Chem. SOC.,85, 3953 (1963). (6) P. J. Collin, D. B.Roberts, G. Sugoqdz, D. Wells, and W. H. F. Sasse, Tetrahedron Lett., 321 (1972). (7) J. J. McCullough and C. W. Huang, Can. J. Chem., 47,757 (1969). (8) D. R. Arnold, L. B. Gillis, and E. B. Whipple, Chem. Commun.,918 (1969). (9) E. Grorenstein, T. C. Cambell, and T. Shibata, J. Org. Chem., 34,2418 (1969). (10) T. Sugiok, C. Pac, and H. Sakurai, Chem. Left., 791 (1972). (11) J. J. McCullough and W. S. Wu, J. Chem. SOC.,Chem. Commun., 1136 (1975). (12) J. A. Barltrop and R. J. Owers, Chem. Commun., 1462 (1970). (13) M. Bellas, D. Bryce-Smith, and A. Gilbert, Chem. Commun., 862 (1967). (14) D. Bryce-Smith, A. Gilbert, and C. Manning, Angew. Chem., 86,350 (1974). (15) M. Bellas, D. Bryce-Smith, M. T. Clarke, A. Gilbert, G. Klunklin, S. Krestonosich, C. Manning, and S. Wilson, J . Chem. Soc., Perkin Trans. 1 , 2571 (1977). (16) M. Lamotte, R. Lapouyade, J. Pereyre, and J. Joussot-Dubien, C. R. Hebd. Seances Acad. Sci., Ser. C, 290, 21 1 (1980). (17) M. Lamotte, et al., J . Chem. SOC.,Chem. Commun.,725 (1980). (18) M. Lamotte, J . Phys. Chem., 85, 2632 (1981). (19) M. Lamotte, et al., in "Polynuclear Aromatic Hydrocarbons", P. W. Jones and P. Lever, Eds., Ann Arbor Science Publishers, Ann Arbor, MI, 1979, p 159.
0022-3654/85/2089-1171$01.50/0
The photophysics of aromatic molecules at high pressures has
been studied by Robertson,ZOJ'Drickamer,22Offen,23-25 N i ~ o l ? ~ ~ ~ and their associates. Most of these studies involved crystals or rigid (polymer) solutions. Several typical effects of compression on these compounds have been established; for instance, most spectra shift toward longer wavelength^^',^^,^^^^^ and fluorescence and phosphorescence lifetimes decrease with increasing pres~ u r e . ~ *Chemistry -~~ and photochemistry at very high pressures are essentially undeveloped since it is difficult to analyze the submicrogram samples. During 1983, we reported that, in alcohols or alkanes, naphthalene photoreacts irreversibly under relatively modest excition intensities at high pressures. The reaction was indicated by changes of the fluorescence spectra and lifetimes35similar to the changes Lamotte and Joussot-Dubien observed for aromatics in rigid matrices at low temperatures. We indicated that chemical analyses were needed to describe the reactions in detail. Thus, we undertook to develop methods to recover the naphthalene and photoproducts from diamond anvil high-pressure cells for analysis by GC-MS spectrometry that are described in this report. These analyses confirm that "inert" liquid solvents photoreduce and photoadd to naphthalene at high pressures and provide direct evidence of the several photon-initiated reactions: (1) photoreduction of naphthalene to hydronaphthyl radicals and to tetrahydronaphthalenes, (2) polymerizations of the hydronaphthyl and alkyl radicals to yield dimers, trimers, and higher polymers which emitted a white fluorescence, (3) photoaddition of alkyl radicals from the solvents (methanol, ethanol, pentane, or isopentane) to naphthalene, and (4) H-D exchange between deuterium atoms on naphthalene molecules and hydrogens from the solvents when deuterated naphthalene participated in the photoreactions. We also found that the same photoreactions occur in fluids at room temperature and atmospheric pressure, although at relatively M solution low rates. The naphthalene in 0.5 mL of a 5 X (20) W. W. Robertson, et al., J . Mol. Specfrosc., 1, 1 (1957). (21) W. W. Robertson and A. D. King, Jr., J . Chem. Phys., 34, 151 1 (1961). (22) H. G. Drickamer, Annu. Reu. Phys. Chem., 33, 25 (1982). (23) H. W. Offen and R. R. Eliason, J. Chem. Phys., 43, 1096 (1965). (24) H. W. Offen, J . Chem. Phys., 42, 430 (1965). (25) H. W. Offen and R. A. Beardslee, J. Chem. Phys., 48,3584 (1968). (26) M. F. Nicol, Appl. Spectrosc. Reu. 8, 183 (1974). (27) M. F. Nicol, J. Opt. SOC.Am. 55, 1176 (1965). (28) M. F. Nicol, J . Chem. Phys., 45, 4573 (1966). (29) M. F. Nicol and J. Somekh, J . Opt. SOC.Am., 58, 233 (1968). (30) M. E. Baur and M. F. Nicol, J . Chem. Phys., 44, 3337 (1966). (3 1) M. F. Nicol, W. D. Ellenson, and R. Geffner in 'Organic Scintillators and Liquid Scintillation Counting", D. L. Horrocks and C. T. Peng, Eds., Academic Press, New York, 1971, p 21. (32) H. W. Offen and D. T. Phillips, J. Chem. Phys., 49, 3995 (1968). (33) J. J. Kim, R.A. Beardslee, D. T. Phillips, and H. W. Offen, J . Chem. Phys., 51, 2761 (1969). (34) P. C. Johnson and H. W. Offen, Chem. Phys. Lett., 6, 505 (1970). (35) G. Z. Yin and M. F. Nicol, Luser Chem., in press.
0 1985 American Chemical Society
1172 The Journal of Physical Chemistry, Vol. 89, No. 7, 1985
Figure 1. Cross section of diamond anvil cell that illustrates the location of the microcontainers (cross-hatched)used to confine the liquid solvent that entraps the sample when the cell is opened.
in mixed alcohols is consumed within 24 h by irradiation with a 100-W mercury short arc, whereas in a mixed alkane solvent, only a small portion of naphthalene reacts under these conditions. However, the larger volumes of solutions that could be reacted at atmospheric pressure yielded larger amounts of products, which was useful for identifying the many minor products of these reactions. In both solvents, the photochemistry is strongly accelerated by pressure. At pressures greater than 3 GPa, the reactions are faster in alkanes than in alcohols.35 The relative yields of polymers also increase at high pressures. These observations are consistent with changes of the enthalpy and volume during the reactions.
Experimental Section Fluorescence spectra and lifetimes were measured with a synchronously pumped, cavity-dumped dye laser excitation source and single-photon detection techniques which are described in detail elsewhere.36 New 100-W short-arc mercury lamps (Illumination Industries Inc.) were used for excitation in some photochemical and fluorescence experiments. For the photochemical work, the UV output from the mercury arc was isolated by a Corning 7-54 filter and was tightly focused by anf = 1.2 quartz lens to irradiate naphthalene solutions contained in Pyrex tubes. For fluorescence measurements, a combination of 5 cm of 0.178 M aqueous NiSO,, 5 cm of 5 X M aqueous K2Cr04,and a Corning 7-54 filter was used to isolate the 315-nm excitation. Fluorescence spectra were collected in the transmitted mode for the high-pressure diamond cell or in the 90° scattered mode at atmospheric pressure. The spectra were scanned with a Jarrell-Ash 0.5-m Ebert monochromator with 10-nm resolution and detected with an RCA 1P28A photomultiplier and Victoreen dc amplifier. The diamond high-pressure cell used for this work was modified from Bassett's design3' by R. Brewer and A. Karim of this department. As an aid for recovering samples for analysis, the cell was further modified by attaching with epoxy two stainless steel rings (3.0-mm diameter and 0.6 mm high) around the diamonds to form microcontainers with volumes of about 5 pL (Figure 1). Diamond anvils, selected for low fluorescence and good UV transparency, were purchased from Dubbledee Corp. Naphthalene solutions, which had been deaerated by 5-10 freeze-thaw cycles under vacuum, were loaded into the cell by placing the cell inside a small stainless steel box, flooding the lower part of the cell
-
(36) M. F. Nicol, J. M. Wiget, and M. Anton, J . Mol. Srrucr., 47, 371 ( 1 97x1. \ - -
-I
(37) W. A. Bassett, T. Takahashi, and P. W. Stock, Reu. Sci. Instrum., 38, 39 (1967).
Yin and Nicol including the lower diamond and gasket with the solution, and sealing the sample with the upper diamond. These operations were performed inside of a nitrogen-filled glovebox to minimize the exposure of the solution to oxygen. The amount of naphthalene in the diamond cell was very small, often between 70 and 90 ng. For the atmospheric photochemistry, 0.1-mL naphthalene solutions were contained in either a 5-mm-diameter Pyrex tube with a Teflon stopper or an evacuable 5-mm-diameter Pyrex tube. After irradiation, the naphthalene solutions were diluted with the same solvents up to 0.5 mL for GC-MS analysis. The solutions of naphthalene or naphthalene-d8 in either 4: 1 methanol-ethanol or 1:l pentane-isopentane used for this study were 5 X M unless otherwise stated. The naphthalene (Fisher N128) was refluxed with sodium and then sublimed twice under vacuum. Naphthalene-d8 (Aldrich) was used without further purification. Other reagents were obtained from the following sources: absolute ethanol, Publicker Industries, Inc.; methanol, Mallinckrodt AR; pentane, Mallinckrodt spectAR; isopentane, Aldrich spectrophotometric grade; UV-grade hexane, Burdick & Jackson Lab., Inc.; chloroform d (99.8%), Cambridge Isotope Lab.; 1,l'-binaphthyl, Eastman Kodak Co.; ethylene glycol, J. T. Baker Chem. co. We believe that this is the first study in which samples have been collected from diamond cells for chemical analysis. Since a typical sample contained only about 80 ng of naphthalene and both naphthalene and the solvents are very volatile at room temperature, special procedures had to be developed for opening the cell and collecting the sample. These procedures were performed in a cold room (at -15 "C) where the cell, solvent, and tools had been stored for at least 2 h to equilibrate. The collection began by rinsing the area surrounding the diamond anvils with solvent before releasing the pressure. Fivemicroliter aliquots of solvent, either UV-grade hexane or methanol, were injected with a syringe into the microcontainers on both sides of the gasket, then sucked back by the same syringe, and transferred to a 5-mm-diameter Pyrex tube. This step was repeated until about 0.1 mL was collected. The combined rinsings were analyzed by GC-MS in order to detect impurities and residual naphthalene surrounding the diamonds. Sample collection continued by injecting 5 pL of solvent into the microcontainers on both sides of the gasket to surround the diamonds. Then, the pressure was slowly released, the upper piston was lifted, removed, and turned over, and a few more microliters of the solvent was quickly delivered to both microcontainers from a disposable pipet in order to cover the diamond tips. The gasket was then rinsed with about 10 pL of solvent by squeezesuck cycles of a pipet, and the rinse solutions were transferred to a second Pyrex tube. Both diamonds and the microcontainers were then washed by squeeze-suck cycles with the same pipet, and these washes were added to the second Pyrex tube. The volumes of the solutions in both tubes were then reduced to about 2 pL by gently flowing a stream of helium from a pipet over the solution to evaporate some of the solvent. The resulting concentrated solutions were immediately drawn into a 1O-kL syringe and injected into a KRATOS MS-25 gas chromatograph-mass spectrometer (GC-MS) with a 0.256 mm X 30 m DB5 fused silica capillary column (J & W Scientific, Inc.). Unless otherwise indicated, the oven temperature program for the G C column was as follows: initial temperature, 32 O C for 10 min; ramp rate, 16 OC/min; final temperature, 280 OC for 10 min. A Data General Nova computer system was interfaced with the GC-MS for data analysis. The detection limit of the KRATOS MS-25 GC-MS, about 2 ng, is adequate for the photoproduct analyses. GC-MS elution patterns of a sample of the hexane solvent used to rinse the diamond cell and a naphthalene sample recovered from the diamond cell without exposure to UV radiation are shown in Figure 2 (temperature program: initial, 32 OC/lO min; ramp rate, 16 OC/min; final, 280 OC/lO min). Since most of the heavy components in the UV-grade hexane are retained when the collected solutions are concentrated by evaporation, the C7, C8, C9, and C,, impurity peaks are much more intense in the concentrated
The Journal of Physical Chemistry, Vol. 89, No. 7, 1985 1173
Photochemistry of Naphthalene ELUTION TIME / MIN:SEC(?6:03)
ELUTION TIME 1 MINSEC “
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Gas chromatographs were obtained with a Hewlett-Packard 5710A gas chromatograph (GC) with a hydrogen flame ionization detector and a 0.256 mm X 30 m SE-52 fused silica capillary column (J & W Scientific, Inc.). Mass spectra (MS) were recorded on an AEI MS902 mass spectrometer. ‘H N M R spectra were recorded on a Bruker WP-200 FTNMR-200 spectrometer. Absorption spectra were carried out on a Cary 219 spectrophotometer.
SCAN NUMBER
Figure 2. GC-MS elution patterns of (A) concentrated hexane used to
rinse the surroundings of the diamonds and (B) naphthalene recovered in hexane from diamond cell without photochemistry. TABLE I: Quantitative GC and GC-MS Analyses of 10-ng Naphthalene Standards and of the Naphthalene Recovered from Diamond Cells Which Were Loaded with about 80 f 10 ng of Naphthalene but Were Not Irradiated
sample no. 5-18-83 5-20-83 5-23-83 5-20-83 5-23-83 6-2-83
sample no. 5-27-83 5-21-83 6-13-83 6-13-83 1-29-83 1-29-83
GC Analysis samvle re1 intens 1.86 standard 1.61 standard 1.68 standard recovered 3.48 recovered 4.62 4.58 recovered GC-MS Analysis sample total counts standard 42 660 recovered 105518 standard 32 492 82 162 recovered standard 224 141 recovered 140 619
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10f 1 10f 1 20 f 1 26 f 1 26 f 1
quantity,)ng 10 f 2 21 f 2 1oi2 26 2 10 f 2 33 2
*
solutions than in the original hexane. Quantitative analysis indicated that, without UV exposure, loaded naphthalene can be recovered reproducibly from the diamond cell (Table I). Out of 80 f 10 ng of naphthalene loaded into the diamond cell, between 26 and 33 ng of naphthalene can be collected after releasing the pressure.
Results Photochemistry at High Pressure. When a 5 X M solution of naphthalene in 1:l pentane-isopentane a t 4.7 GPa was irradiated for 30 min a t 310 nm with a mercury arc, the same photoproduct fluorescence spectra and decay kinetics were observed that had previously been produced with a picosecond laser pulse train.33 Two new bands appear in the emission spectra: a broad band (300-550 nm) with a short (5 ns) lifetime and a structured band as shorter wavelengths (280-310 nm) with an intermediate lifetime (12-15 ns). After limited (30 min) exposures, the growth of the intensity of the broad band (relative to the naphthalene emission) could be partially reversed by heating the sample to 180 OC for 1 h. However, after irradiation for 2 h, the naphthalene emission could not be restored by heating the sample. These observations suggest that, at an early stage of this photochemistry, some naphthalene forms weakly bound dimers which can be decomposed by heating or by releasing the pressure. At the same time, naphthalene reacts with solvents to give reduction and addition products. Although dimers form easily and rapidly at the beginning of the process, all of the naphthalene eventually reacts irreversibly with the solvent to form at least two different types of products. One, with the 300-nm emission band, was described previ~usly.~~ A second, whose fluorescence is similar to that of a dimer (a broad emission band and short lifetime), is stable to moderate heating at low pressures and has not been discovered before. These products were identified by a combination of M S and GC-MS methods. The lighter photoproducts were determined by GC-MS analysis of the type illustrated in Figures 3-5 for the products collected by methanol from a 5 X M solution of naphthalene in 1:l pentane-isopentane after 3-h irradiation by a mercury arc. These spectra were taken a t higher sensitivity than the spectra of the standards and clearly show that no detectable naphthalene was recovered. The mass spectra for three photoproducts, G C peaks 487, 528, and 532, have the same abundance maxima (at m l z
1174 The Journal of Physical Chemistry, Vol. 89, No. 7, 1985
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Yin and Nicol '
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400
500
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Figure 7. Fluorescence spectra of a 5 X lo-* M degassed solution of naphthalene in 4:l methanol-ethanolbefore and after a 24-h irradiation with an Hg arc: 1, original solution before irradiation; 2, solution and precipitate after irradiation; 3, solution after irradiation diluted by half; 4, diluted solution after heating to 120 OC for 1 h.
rinse them. For reasons described below, these deposits were identified as high molecular weight compounds in which several 50 loo solvent and aromatic moieties are linked. The permanent, broad (300-550 nm) emission spectra are attributed to this material. MfZ Photochemistry at Atmospheric Pressure and Room TemFigure 5. Mass spectrum for GC peak 528 of Figure 3, pentyltetraperature. Two questions arose from Lamotte's results in rigid hydrobinaphthalene. matrices at low temperature and our results in liquid solution at high pressure: 29 1. Can naphthalene undergo photoreduction and photoaddition in alcohols or alkanes at atmospheric pressure and room teml o o r perature? 30 2. If so, can considerable amounts of the photoproducts, both soluble and insoluble, be obtained for more detailed analyses? 5 Experiments with naphthalene or naphthalene-ds in either 4: 1 methanol-ethanol or 1:1 pentane-isopentane answered both questions a f f i i t i v e l y . In each case,the naphthalene fluorescence was gradually supplanted by a bright white emission after the 0 I'~"I''''Ifl sample had been strongly irradiated by a mercury arc for a few 150 200 250 50 100 hours. At the same time a white precipitate formed, and most of it deposited on the tube wall. MfZ Figure 7 depicts fluorescence spectra of a 5 X M solution Figure 6. Mass spectrum at GC peak 596 from another experiment with of naphthalene in alcohols. After a 24-h irradiation a fairly strong 5 X M naphthalene in 1:l pentane-isopentane, pentyldihydrcdiand broad emission band peaking at 370 nm (curve 2) replaced naphthalene. the original naphthalene fluorescence (curve 1). When the exposed solution was diluted by a factor of 2 with methanol, the solution 129 and 130) which are the mono- and dihydronaphthyl ions. without precipitate showed a little weaker but much broader These three products also have higher mass fragments which emission band (curve 3), and this band remained after heating indicate that naphthalene reduction occurs by addition of solvent to 120 "C for an hour (curve 4). Absorption spectra of these molecules. Although these three products cannot be identified solutions also were measured, and both the 'La and 'Lb absorption unambiguously and the substituted positions cannot be determined bands irreversibly weakened and finally disappeared after 24-h from these mass spectra, these results gave the first trace analysis M solution irradiation. The fluorescence spectrum of a 5 X for chemistry in a diamond cell. A product from another exof naphthalene in the pentanes (Figure 8) retained the naphthalene periment under these conditions, recovered at GC peak 596, gave band as its main component after 36-h exposure, but broadening strong ion peaks at m / z 43,57,71, and 129 which clearly indicates was obvious at longer wavelengths (curve 2 in Figure 8). A very photoaddition of pentane to naphthalene (Figure 6 ) . Products broad emission, from 300 to 600 nm, was observed for the chloof such addition reactions to a single naphthalene have only one roform extract of the precipitates from this solution (curve 3). aromatic ring; thus, the emission band at 280-310 nm can be Broad fluorescence spectra that persist upon heating or dilution attributed to them. Although it was difficult to determine the are typical of aromatic polymers. The fluorescence spectrum of total amount of products recovered by this procedure, experience polystyrene in ethyl acetate, for example, consists of two bands, with recovering samples that had not been irradiated suggested a structured band between 270 and 300 nm that is the fluorescence that these products represented .bout 25% of the initial naphof benzene ring and weakens as the concentration increases, and thalene. a broad structureless band from 300 to 400 nm.38 The Evidence for the second type of product was obtained by fluorescence spectra of poly(2-vinylnaphthalene) (at M in carefully examining the diamond tip and gasket after the lighter 2-vinylnaphthalene) in solution also show a broad emission band molecules had been collected. These surfaces were thinly coated from 300 to 550 nm,39 The breadths of these emission bands with a white precipitate that did not dissolve in either hexane or methanol. Because epoxy was used to form the microcontainer around the diamond tips, stronger solvents could not be used to (38) T. Nishihara and M. Kaneko, Mocromol. Chem. 124, 84 (1969).
4
The Journal of Physical Chemistry, Vol. 89, No. 7, 1985 1175
Photochemistry of Naphthalene
300
400
500
600
W a v ~ l m ght /nm
Figure 8. Fluorescence spectra of a 5 X M aerated solution of naphthalene in 1: 1 pentane-isopentane before and after irradiation: 1, original solution before irradiation; 2, solution with precipitate after a 36-h irradiation; 3, precipitate dissolved in chloroform.
result from intrachain overlapping and coiliig of the aromatic rings that enhance interactions among the a orbitals. The products of these reactions were studied by irradiating samples with a mercury arc for one or more days, diluting the resulting solution fivefold with additional solvent, and injecting 500-nL aliquots of the diluted solutions into the GC-MS system. These analyses showed that, during a 24-h irradiation, almost all of the naphthalene in 4: 1 methanol-ethanol photoreacted with solvent molecules, whereas in 1:l pentaneisopentane, only a small portion of naphthalene had reacted even after 30-60 h. The analyses are discussed in detail in the doctoral dissertation of Dr. Yim4 Among the products that were identified are 1,2,3,4tetrahydronaphthalene, l-methoxy-2,3,4-trihydronaphthalene, 2-methoxy-2,3,4-trihydronaphthalene, hydroxymethylnaphthalenes, 1-( 1-methyl-hydroxymethy1)-2,3,4-trihydronaphthalene, and the dimers of hydronaphthalenes and their photoproducts. These GC-MS results clearly demonstrate that naphthalene photoabstracts protons from solvents and is reduced to hydronaphthyl radicals. These radicals may combine with each other to produce dimers or react further with solvents to form photoadducts. All of the napthalene photoproducts found in the solution are estimated to contain less than half of the original naphthalene. This suggests that more than half of the naphthalene is in the precipitate. Similar photoproducts were obtained when naphthalene-d8 was the starting material; however, all of the GC-MS fragmentation patterns were broadened. For example, when a 5 X 10-2 M solution of naphthalene-d8 in alcohols was weakly irradiated by a mercury arc, the mass spectra included, in addition to a strong feature at m / z 136, fairly intense features a t m / z 135, 134, and even 133. These features indicate that one to three deuteriums exchange with hydrogens from the solvents. The GC-MS analysis of a 5 X 10-2 M solution of naphthalene or naphthalene-d8 in 1:1 pentane-isopentane after 36-h irradiation also shows clear evidence of photoreduction and photoaddition. The most intense GC peak represents unreacted naphthalene-d8. Although no tetrahydronaphthalene was found, a group of G C peaks with their heaviest ions at m / z 208, the sum of the molecular weights of naphthalene-d8 and C5H12,and abundance maxima a t m / z 136-139 indicate that naphthalene-d8 is photoreduced. Although it is difficult to assign the isomers for the individual G C peaks, these compounds clearly are adducts of pentanes to dihydronaphthalene. Other peaks could be assigned to photo(39) R. B. Fox, T. R. Price, R. F. Cozzens, and R. McDonald, J . Chem. Phys., 57, 534 (1972). (40) G. Z. Yin, Ph.D. Dissertation, University of California, Los Angela, 1984. Available from University Microfilms, Inc., Ann Arbor, MI.
substitution or photoaddition products of pentanes to hydronaphthalene with double bonds between them and to photoadducts of reduced photosubstitution products of naphthalene dimers. The intensities of the G C peaks in the Clo alkane region increased as the naphthalene decreased and other main products increased, especially in ane solutions. Clo impurities from the solvents cannot accoun‘ such intense peaks. The fragmentation patterns of these peak gest that deuterated pentane radicals, which came from tht -D exchange reaction, reacted with protonated pentane to rm deuterated Clo. The precipitates from the naphthalene photoreactions would not elute at the highest temperature of the G C column and could not be analyzed by GC-MS methods. These materials were characterized by direct mass spectra, NMR, and luminescence spectroscopy. Direct mass spectra were measured as these solids slowly evaporated while they were heated from 200 to 400 O C . Figure 9 shows a mass spectrum of the precipitates from a degassed solution of naphthalene in 4:l methanol-ethanol which had been irradiated for 36 h. The fragmentation pattern clearly shows ions of monomers ( m / z 129), dimers ( m l z 258), trimers ( m / z 387), and tetramers ( m l z 516) of hydronaphthalenes with extensive ion series of -(CHI)- or -(CH2)0H. The dominant ions, from m / z 128 to 132, imply multiple reductions. Similar multiple peaking throughout the spectra implies that the precipitate is a mixture of polymerized hydronaphthalenes and solvent molecules. Precipitates from alkane solutions have similar features.
Mechanism and Conclusions The primary steps of the photochemistry of aromatic hydrocarbons in solutions or rigid matrices of “inert” solvents have been debated for many years. Because aromatic hydrocarbons are relatively stable under UV irradiation, the UV-induced processes are generally believed to be photophysical.18 However, the subsequent processes have not been carefully considered. The photoproducts of aromatic hydrocarbons recovered in these experiments demonstrate that the UV-induced primary steps open irreversible photochemikal channels. Thus, the question arises: What is the relationship between the photoexcitation processes and the chemistry? The formation of solvent radicals by energy transfer from excited aromatics has been established by several s t ~ d i e s . ’ ~ , ~ ~ ~ ~ Voevodskii and c o - ~ o r k e r sfirst ~ ~ observed this phenomenon in an ESR study of the solvent radicals. Bagdasar’yan, Sinitsyna, and Murontsev studied radical formation in aromatic amine solutions and suggested that this involves a two-photon process with the lowest triplet as an i ~ ~ t e r m e d i a t These e . ~ ~ early studies were reviewed by T e r e x ~ i n .Siegel ~ ~ and Eisenthal showed that free radicals were produced by sensitized solvent decomposition at a rate proportional to the rate of the disappearance of the triplet state ESR signal, which is consistent with a stepwise two-photon absorption mechanism.44 A delicate set of ESR experiments by Brocklehurst, Savadatti, and co-workers showed that excited molecules of several aromatic hydrocarbons sensitize the dissociation of 3-methylpentane and other solvents to give free radicals and hydrogen atoms;45a biphotonic mechanism of the process was inferred from their study. Migirdicyan demonstrated that, in naphthalene-d8-durene mixed crystals, a stepwise biphotonic absorption process was favored by the measured 1.65 f 0.40 power dependence of the rate of duryl radical formation on UV excitation intensity.46 In a rigid methylcyclohexane matrix at 77 K, Lamotte measured a 1.8 power dependence of the formation rate of dihydrotriphenylene on the excitation intensity and, by two-beam (41) B. N. Shelimov, N. N. Bubnov, N. V. Fok, and V. V. Voevodskii, Paramagn. Rezon. Dokl. Sooeshch., 1959,31(1960); see also B. N. Shelimov, Kinet. Catal. (Engl. Transl.) 7, 543-5 (1966). (42) K. S. Bagdasar’yan and Z. A. Sinitsyna, Dokl. Akad. Nauk SSSR, 160, 625 (1965). (43) A. N. Terenin in ‘Recent Prograss in Photobiology”,E. J. Bowen, Ed., Blackwell, Oxford, 1964, p 34. (44) S.Siegel and K. Eisenthal, J . Chem. Phys., 42, 2494 (1965). (45) B. Brockelhurst, W. A. Sibbons, F.T. Lang, G. Porter, and M. I. Savadatti, Trans. Faraday SOC.,62, 1793 (1966). (46) E. Migirdicyan, J. Chem. Phys., 55, 1861 (1970).
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M/z Figure 9. Mass spectra of the precipitates obtained from a 5 X lo-* M solution of naphthalene in 4:l methanol-ethanolafter a 36-h irradiation with
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0.39
20
30
40
50
Irradiation T i m e / Min.
1.0
c
0.0
Figure 10. Raw (broken line) and corrected (solid line) fluorescence
intensities of naphthalene at 340 nm vs. irradiation time for several M solution of naphthalene was loaded intensities at 315 nm. A 5 X in the diamond cell at 1.0 GPa. The relative intensities of the mercury arc irradiation were 0.39, 0.25, 0.16, and 1.00, respectively. methods, also showed that the triplet state of triphenylene is the intermediate.'* In this study, the dependence of the naphthalene concentration in liquid solution at high pressure on the excitation intensity was determined by monitoring the fluorescence intensity at 340 nm while the intensity of the 315-nm excitation of a 5 X IO-* M degassed solution of naphthalene (in 4: 1 methanol-ethanol at 1.O GPa) was varied with a series of filters with relative transmissions of 0.39,0.25,0.16, and 1.OO. A smooth decay curve was obtained after the measured intensities were corrected for variations of the excitation intensity by multiplying the ratios of the emission intensities measured immediately before and after the filters were changed. Both the raw and corrected fluorescence intensities vs. exposure dependences are depicted in Figure 10, and the observed dependence of the reaction rate of naphthalene on the irradiation intensity is illustrated in Figure 11. The error bars for each point represent deviations of time and intensity measurements. Systematic errors from the slow decrease of the intensity of the mercury arc are avoided by the strong-weak-strong sequence of excitations. The slope of 2.0 f 0.3 conformsthe two-photon nature of the naphthalene photochemistry in solution at high pressure. The preceding discussion, two-photon experiment, and product analyses strongly suggest that the reactions follow from photoproduction of solvent radicals, although direct ionization of the excited triplet aromatic or the direct H abstraction by the excited triplet aromatic from the solvents cannot be excluded. A likely reaction scheme is illustrated in Figure 12. In the first three photophysical steps, a highly excited triplet naphthalene molwule is produced by stepwise absorption of two photons, and its excitation is transferred to a solvent molecule which is excited
3.0
5.0
4.0 In
I (Rehtive
6.0
7.0
Intonrity)
Figure 11. Plot of the logarithm of the rate, R , of decrease of the naphthalene fluorescence intensity (340 nm) vs. the logarithm of the relative UV irradiation intensity, I(315 nm). The slope of the line, calculated by a linear least-squaresfit program, is 2.0 f 0.3. The error bars for each point were calculated from the deviations of time and intensity readings.
to a dissociative state. The chemistry begins with (a) dissociation of the solvent to a hydrogen atom and solvent radical and (b) addition of at least one of these radicals to a naphthalene molecule (AH) to form a hydronaphthyl radical, .AHz, or alkylnaphthyl radical, qAHR. At high pressures, the dissociation and addition may be concerted which would help to overcome the tendency of any positive volume of activation and the positive overall volume change for the dissociation to decrease the rate and yield of this reaction. Several radical chain propagating and terminating reactions then open up which consume naphthalene. For example, -AH2 may react with an H atom or solvent radical to form adducts, AH3 or AH2R. These adducts can react further with H atoms or solvent radicals to form tetrahydronaphthlene (AH5), trihydronaphthyl adducts (AH,R), or multiadducts (AHzRR'). Two .AH2radicals may combine to give hydronaphthyl dimers (AH2-AH2). Hydronaphthyl radicals also may attack naphthalene to form dimer radicals of reduced naphthalenes which may attack yet other naphthalenes to form trimer radicals and eventually polymers, and the reductive photoadducts may represent side reactions to the main polymer chain. H atoms form the dissociation of protonated solvents can attack naphthalene-d8 and replace D atoms
The Journal of Physical Chemistry, Vol. 89, No. 7, 1985 1177
Photochemistry of Naphthalene
I
R
R
R
-
POLYMER
Figure 12. Proposed scheme of the twephoton-induced radical reactions.
to form naphthalene-di (i < 8), and the D atoms can react with the solvent molecules. Two solvent radicals also can combine to yield longer molecules which may explain the large amounts of Cl0alkanes found in some of the pentane runs. The reversible dimerization of napthalene may be an intermediate step for this irreversible photochemistry. Bonds between two molecules in the naphthalene dimer will certainly destabilize the a-bond networks of the naphthalene moieties; thus, under UV excitation, the dimers should react more readily with hydrogen atoms or solvent radicals than the monomers. In this circumstance, the hydronaphthyl naphthalene radical (.AH,-AH) will be the key intermediate. After photoreduction or photoaddition, the dimer may dissociate or the relation may persist to the final product. Aromatic hydrocarbons absorbed on a silica surface are known to form dimers,*' and we found that, at atmospheric pressure, photopolymerization easily occurs on the walls of Pyrex tubes. Thus, dimers absorbed on the glass wall may function as an intermediate for the subsequent irreversible reactions at low pressures. With one glaring exception, most of the products that can be derived from the scheme outlined in Figure 12 were detected by GC-MS analyses. The exception is that few derivatives or dihydronaphthalene were detected which implies that any dihydronaphthalene derivatives which formed were efficiently converted to tetrahydronaphthalene derivatives. This can be readily understood in terms of the enthalpies and volumes of activation and reaction for the relevant steps:
+ HR AHZR + HR' AH
-
AHzR
(1)
AH3RR'
(2)
+
If both H R and HR' add to the same ring of the original naphthalene, the standard enthalpies of reactions for these steps at atmospheric pressure should be about +IO0 and -1 30 kJ/mol, respectively, since the resonance energy of naphthalene (relative to benzene) must be supplied in reaction 1. Both steps have
positive enthalpies of activation, but again in this respect, step 1 should be more endoergic than step 2. Thus, since the photoexcitation supplies radicals of sufficient energy for the more endoergic step 1, the similar but less endoergic step 2 should proceed facilely under the same conditions to convert dihydro derivatives to tetrahydro derivatives. Further conversion to hexahydro derivatives involves an even more endothermic step in which the resonance energy of benzene must be supplied, which may not be possible under the conditions of these experiments. Pressure can affect these reactions in several ways which are not unambiguously determined in these experiments. However, some factors have been established. Measurements in aerated solutions of the fluorescence lifetimes of naphthalene show that, up to 2 GPa, the rates of diffusional quenching of excited states decrease rapidly with increased pressure. This effect more than compensates for the slight decrease of the fluorescence lifetime with pressure observed in very viscous solvents so the observed fluorescence lifetime increases with pressure. If similar changes of these diffusional processes were to increase the yield or lifetime of the intermediate triplet state, as is likely, the efficiency of the two-photon excitation process would increase a t high pressures. Thus, the rate of radical production would increase. The net effect of the reduction of diffusion rates at high pressures on geminate recombination rates or radical lifetimes is less predictable. However, in the presence of unsaturated hydrocarbons, it is likely that radical addition to unsaturated bonds will be the dominate reaction pathway at high pressure since the kinetics of these steps should be strongly accelerated by pressure. During each addition step, including subsequent additions to AH,RR', the molar volume of the system decreases by between -10 and -20 cm3. The volumes of activation should have similar signs and magnitudes. Thus, for each step, the enthalpies of activation and reaction decrease by between -10 and -20 kJ/GPa, and the rate of each reaction step increases by about a factor of e4 per gigapascal. Thus, by pressures of the order of 10 GPa, step 1 should be exothermic. At somewhat higher pressures, it may not be possible to neglect the conversion of the tetrahydro derivatives to nonaromatic species. Just as graphite is thermodynamically unstable with respect to diamond at pressures of the order of 5 GPa, aromatic hydrocarbons and, more generally, unsaturated C-C bonds become thermodynamically unstable with respect to addition reactions leading saturated products at pressures of the order of 5-10 GPa. However, until somewhat higher pressures, some mechanism such as the photoexcitation used here, the high stress gradients of a shock wave,"8 or the injection of mobile charges may be required to surmount activation barriers for the initial radical production step of the reaction. Acknowledgment. The GC, MS, and GC-MS analyses reported here would not have been possible without the able assistance and advice of Dr. Dilip Senharma and Mr. John Wells of the Instrumentation Facility of the Department of Chemistry and Biochemistry. We are grateful to Drs. K. D. Bayes, M. A. El-Sayed, C. S. Foote, J. Joussot-Dubien, and M. Lamotte for helpful comments and suggestions. This work was supported by N S F Grant DMR80-25620 as supplemented by a UCLA Foundation-College of Letters and Sciences Fellowship (to G.Z.Y.) and instrumentation grants from N S F (CHE79-10965, CHE77-09271, CHE76-05926, and GP32304), the University Research Committee, and Spectra-Physics, Inc. (48) See, for example, G. A. Adadurov, I. M. Barkalov, V. I. Gol'danskii,
(47) R. K. Bauer, P. DeMayo, W. R. Ware, and K. C. Wu J. Phys. Chem.,
86, 19 (1982).
A. N. Dremin, T. N. Ignatovich, A. M. Mikhailov, V. I. Tal'rose, and P. A. Yampol'skii, Polym. Sci. USSR (Eng!. Trans!.),7, 196 (1965).
