Photoinduced Luminescence of 9,lO-Anthraquinone Primary Photolysis of 9,IO-Dihydroxyanthracene Steven A. Carlson and David M. Hercules Department of Chemistry, University of Georgia, Athens, Ga. 30602
Extended photolysis of 9,lO-dihydroxyanthracene (9,lODHA) produces two primary products which fluoresce blue. One has been identified as 9-anthranol, and the other remains unidentified-called the "41 0 intermediate." 9,lO-Dimethoxyanthracene shows behavior similar to 9,lO-DHA. Both compounds show low (0.005) quantum yields. Several speculations are given about the structure of the "410 intermediate." 9,lO-DHA was found to produce some 9,lO-anthraquinone on photolysis. Phototautornerization of 9,lO-DHA to oxanthrone has been observed. The rate constant for the tautomerization of 9,lO-DHA in the ground state is 4 X l o - ' set.-' 9-Anthranol is produced by photolysis of oxanthrone. The fluorescence of 9,lO-dimethylanthracene ($f = 0.77) has been reported along with phosphorescence ( T ~= 2.2 X sec.) of oxanthrone.
Although the photochemical conversion of 9,lO-anthraquinone (AQ) to 9,lO-dihydroxyanthracene(9,lO-DHA) has been extensively studied (1-3), it has been observed that under certain conditions photolysis proceeds beyond 9,lO-DHA. For example, Hercules and Surash ( 4 ) found that the green fluorescence (Amax 480 nm) of 9,lO-DHA disappeared on prolonged photolysis, being replaced by less intense blue fluorescence (A, 412 nm). A fluorescing photoproduct (Ama, 538 nm) was also observed for AQ in alkaline ethanol. This was attributed to the anion form of one of the two fluorescing photoproducts found in the neutral solution. In a later study Gorsuch et al. ( 5 ) reexamined the blue fluorescence resulting from AQ photoreduction in ethanol. They found the blue fluorescence to be due to several compounds, one being identified as 9-anthrol from spectral measurements. Upon photolysis this blue fluorescence disappeared and was replaced by a structured emission in the 360 to 420 nm region which appeared to be a combination of anthracene and 9,9'-bianthryl. This tentative sequence of photoproducts was proposed on the basis of products formed in the chemical reduction of anthraquinone (6). Gorsuch et al. also found that the yield of bluefluorescing compounds was higher for irradiation at 253.7 than at 365 nm. Recently, Eremenko and Dain (7) also observed that the green fluorescence of 9,lO-DHA disappeared upon prolonged irradiation of 365 nm. A negligible amount of blue fluorescence, attributed to 9-anthrol, was detected. The (1) F. Wilkinson, J Phys. Chem., 66, 2569 (1962). (2) K. Tickle and F. Wilkinson, Trans. Faraday Soc., 61, 1981 (1965). (3) S. A. Carlson and D. M. Hercules, Photochem. Photobiol., 17, 123 (1973). (4) D. M . Hercules and J. J. Surash. Spectrochim. Acta, 19, 788 (1963). (5) J. D. Gorsuch, J. P. Paris, and D. M. Hercules, paper presented at the 144th National Meeting of t h e American Chemical Society, Los Angeles. Calif. April 1963. (6) E . H. Rodd, "Chemistry of Carbon Compounds,'' Vol. 1118, Elsevier, NewYork, N . Y . , 1951. (7) S. M . Eremenko and B. Y . Dain. Doki. Akad. Nauk. SSSR, 167, 380 (1966).
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small yield of blue-fluorescing products was ascribed to photolysis of 9,lO-DHA to nonfluorescent products, such as oxanthrone and anthrone. The intent of the present study was to characterize the blue-fluorescing products of prolonged AQ photolysis and to determine a sequence for their formation. Of particular interest were correlations or differences between chemical and photochemical reduction of AQ. A secondary question was the effect of irradiation wavelength on the photoproducts formed. To obtain more information on AQ photochemistry, the photochemical nature of compounds related to AQ by chemical reduction and/or oxidation was also examined.
EXPERIMENTAL Chemicals. A-I0,lO'-Bianthrone (Aldrich Chemical Co.) was recrystallized twice from ethanol before use. 10,lO'-Dihydro-IO,IO'-dihydrony-A-lO, lO'-bianthrone (I) was prepared by mild LiAlH4 reduction of A-lO,lO'-bianthrone. To a solution of 0.625 g of A-lO,lO'-bianthrone in 25 ml of CHC13, 0.1975 g of LiAlH4 (Alfa Inorganics) in 40 ml of ether was added slowly with stirring over a 10-min period. The reaction mixture was kept at 5 "C and under a Nz atmosphere. After 5 more minutes of stirring, 15 ml of ethanol, 20 ml of water, and 20 ml of 0.5N HzS04 were added in succession. Pale yellow solid (0.6 g) was collected by filtration. After washing with CHC13 t o remove starting material, the product was heated in boiling ethanol and filtered, giving a colorless solid (0.287 g, 46%); mp 198-203 "C dec; UV (ethanol) log c 317 nm (3.96), 248 nm (4.03); IR (KBr) (values in cm-l) (intensities are vs = very strong, s = strong, m = medium, and w = weak) 3350 (s), 3060 (m), 3020 (w), 2860 (w), 1602 (w), 1467 (s), 1450 (m), 1190 (s), 1125 (s), 1045 (vs), 948 (m), 768 (s), 724 (s). The product was too insoluble, even in boiling solvents, for NMR measurement. 9,lO-Phenanthrenequinone (PAQ) (Eastman Organic) was recrystallized from benzene. 9, IO-Phenanthrenediol was prepared by photolyzing a lO-4M solution of PAQ in deoxygenated ethanol (8). Benzil (J. T. Baker, suitable for photosensitizer use) was used without further purification. a,P-Dihydronystilbene was prepared by photolyzing a lO-4M solution of benzil in deoxygenated ethanol (9). Anthrapznacol was prepared using Newman's method of pinacolization (10). Anthrone (Eastman Organic) (50 g) was mixed with 300 ml of ethanol and 200 ml of benzene. Freshly scratched aluminum (12 g) (Alcoa foil, cut in 1-in. squares) and 0.5 g of mercuric chloride (Mallinckrodt) were added to the anthrone solution. After waiting for l min, the solution was shaken for 3 min and left to sit for 3.5 hr. Then 100 ml of ethanol was added, and the solution was refluxed for 6 hr. After cooling, 225 ml of 1N HC1 was added. Extraction with benzene and evaporation gave 36 g of colorless crystals (72%), mp 167-173 "C dec. Recrystallization from benzene raised the mp to 175-176 "C dec, (lit. (11) 180 "C): W (ethanol) 266 (2.21), 273 (2.19); IR (KBr) 3550 (m), 3355 (vs), 3060 (m), 2915 (m), 2870 (w), 2815 (w), 1555 (w), 1455 (s), 1425 (s), 1395 (s), 1160 (s), 1020 (vs), 720 (vs); NMR (dimethyl sulfox-ide) (6 values are in ppm downfield from T M S = 0) multiplet (16 H ) 6 6.84-7.50, singlet ( 2 H) 6 5.97, singlet ( 4 H) 6 3.71. The above procedure is given in detail since the method of Gomberg and Bachmann (11) gave highly variable yields, mostly poor. (8) P. A. Carapellucci, H. P. Wolf, and K . Weiss, J. Amer. Chem. Soc.,
91,4635 (1969), (9) H. Berg, Z.Chem., 2, 237 (1962). (10) M . S. Newman,J. Amer. Chem. Soc., 62, 1683 (1940). (11) M. Gomberg and W. E. Bachmann, J. Amer. Chem. Soc., 49, 236 (1927).
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 11, SEPTEMBER 1973
9,9'-Epoxy-9,9', lO,lO'-tetrahydro-9,9'- bianthryl (III) was prepared (12) by gently refluxing an anthrapinacol (2.00 g) in thionyl chloride (Eastman Organic, 30 ml) solution for 3.5 hr. Evaporation to dryness followed by mixing with 200 ml of ethanol left a yellow precipitate, 0.69 g (35%): mp 235-238 "C; UV(ethano1) 341 (3.44), 359 (3.72), 379 (4.04), 401 (4.18); fluorescence spectrum (FS) (ethanol) A, 440 nm broad. Placing this yellow compound in boiling xylene and cooling gave another yellow product, mp 167 "C: UV (ethanol) peaks a t 341, 359, 379(max), and 401, c not measured; FS(ethano1) A,, 436 nm sharp. The nature of these two anthracene derivatives was not examined further. The ethanol filtrate was evaporated nearly to dryness and cooled, giving 0.92 g (46%) of a colorless product, mp 208-210 "C dec. Two recrystallizations from ethanol and one from benzene gave 111, mp 214-215 "C dec (lit. (13) 219 "C): UV (heptane) log c 315 nm (2.81); IR (KBr) 3055 (m), 3010 ( m ) , 2950 (w), 2895 (w), 1680 (s), 1600 ( m ) , 1485 (s), 1445 (s), 1237 (s), 916 ( m ) , 823 (m); NMR(CDC13) multiplet (12 H) 6 6.65-7.45, doublet (4 H ) J = 7 cps 6 6.52, AB quartet (2 H) centered a t 6 4.25, singlet ( 2 H) 6 4.14. 1,4-Naphthalenediol (Eastman Organic) was recrystallized twice from an ethanol-benzene mixture. 2-Methyl-1,4-naphthalenediolwas prepared by photolyzing a 10-4M solution of 2-methyl-1,4-naphthoquinone (Matheson Coleman and Bell) in deoxygenated ethanol (14). 9,9'-Bianthryl was prepared according to the procedure of Winkler and Winkler ( 1 5 ) , mp 308-310 "C (lit. (16) 311-314 "C). The UV and IR agreed with that reported by Weinshenker (16). Chemical reduction of 10-hydroxyanthrone (1.0 g) with zinc dust (1.0 g) and 10 ml of 2:1 glacial acetic acid:concentrated HC1 also gave 9,9'-bianthryl (60%), mp 305-308 "C: UV and FS identical to that of previously prepared compound. 9, IO-Dimethoxyanthracene (9-IO-DMA) was prepared using the procedure of Meek and coworkers ( 1 7 ) , except that it was carried out in a Nz atmosphere. The yield of 9,lO-DMA was 7270, mp 202 "C (lit. (76) 198-199 "C): UV (ethanol) 345 (3.13), 361 (3.45), 3.81 (3.63), 403 (3.56); IR (KBr) 3060 (w), 2960 (w), 1610 ( m ) , 1450 (s), 1360 (s), 1265 ( m ) , 1155 ( m ) , 1065 (s), 960 (s), 780 (s), 755 (s), 680 (s); NMR (CSz) split doublet (4 H) 6 8.08-8.24, split doublet ( 4 H) 6 7.32-7.48, singlet (6 H) 6 4.04. 10-Bromoanthrone was prepared by the method of Goldmann ( 1 8 ) . In a typical procedure, equimolar amounts (0.0925 mol) of anthrone (recrystallized from benzene) and Brz, both in CS2, were mixed over a 10-min period. Evaporation to dryness followed by recrystallization from benzene yielded 21.1 g (84%) of pale yellow crystals, mp 140-143 "C dec (lit. (19) 148 "C dec): IR and NMR were identical to that found by Koch and Zollinger (20). IO-Hydroxyanthrone (oxanthrone) was prepared according to Meyer ( 1 9 ) . The procedure is described here because deviation from the exact directions was found to result in the formation of anthraquinone (AQ). Freshly prepared 10-bromanthrone (26 g) was added to 500 ml of 60% aqueous acetone. The mixture was heated quickly to boiling and stirred until nearly all the solid had dissolved. The solution was then filtered, and 150 ml of water was added. A white precipitate formed immediately. After cooling in an ice bath, the solution was filtered. Crystallization from benzene gave pale yellow crystals (14.7.g, 74%). Two more recrystallizations from benzene resulted in colorless crystals, mp 156-157 "C dec, (lit. (19) 167 "C: UV (ethanol) 272 (4.23), 303 shoulder (3.57), 350 (2.04), 363 (1.95), 378 (1.48); IR (KBr) 3420 (s), 3060 (w), 2880 (w), 1654 (s), 1003 (s); NMR (ethanol) split doublet (2 H) 6 8.118.33, multiplet (6 H ) 6 7.31-8.03, doublet (1 H) J = 8.3 cps) 6 6.92, doublet (1 H) ( J = 8.3 cps) 6 5.68; NMR (ethanol: small amount of D20) same spectrum as in ethanol except that the doublet a t 6 6.92 disappeared and the doublet a t 6 5.68 became a singlet. Anthraquinone ( AQ ) (Eastman Organic) was recrystallized four times from ethanol, giving pale yellow needles, mp 283-285 "C (lit. ( 2 1 ) 286 "C: IR (KBr) 3065 (w), 1685 (s), 1595 (s), 1568 (m), (12) F. Bell and D. H. Waring. J. Chem. SOC.,1949, 1579. (13) E. Bergmann and W. Schuchardt, Justus Liebigs Ann. Chem., 487, 225 (1931). (14) J . Rennert, S. Japar, and M . Guttman, Photochem. Photobiol., 6, 485 (1967). (15) H. J. S. Winklerand H. Winkler,J. Org. Chem., 32, 1695 (1967). (16) N . M . Weinshenker, Ph.D. Thesis, M.I.T., Aug. 1968. (17) J. S. Meek, P. A. Monroe, and C. J. Bouboulis. J. Org. Chem., 28, 2573 (1963). (18) F. Goldmann, Ber., 20, 2437 (1887). (19) K . H. Meyer, Justus Liebigs Ann. Chem., 379, 37 (1911). (20) W. Koch and H. Zollinger, Helv. Chim. Acta, 48, 554 (1965). (21) R. Kempf, J. Prakt. C h e m . , 78, 201 (1908).
1286 (s), 1175 ( m ) , 934 (s), 809 (m), 696 (s); NMR (trifluoroacetic acid) split doublet (4 H) 6 8.33-8.54, split doublet (4 H ) 6 7.888.10. 9,10-Dimethylanthracene (Aldrich Chemical Co.) was recrystallized three times from ethanol before use. Instrumentation. Melting points were taken on a Mel-Temp apparatus and are corrected. Infrared (IR) spectra were recorded with a Perkin-Elmer Model 237 grating spectrometer; in general only bands characteristic of the functional groups present are listed. Nuclear magnetic resonance (NMR) spectra were obtained on either a Varian A-60 or T-60 spectrometer; peak positions (6) are given in ppm downfield from an internal tetramethylsilane (TMS) standard. Photolyses were carried out in a Model RPR-100 Rayonet photochemical reaction, using one of three interchangeable light sources (Rayonet RPR-2537 A, RPR-3000 A, and RPR-3500 A reactor lamps). Solutions for large scale photolysis a t 253.7 or 300 nm were placed in Rayonet RQV-218 or RQV-323 quartz reactor vessels. Use of a fan in the photochemical reactor kept the operating temperature a t ca. 35 "C. Unless otherwise stated, all photolyses were done in the Rayonet reactor under nitrogen. Fluorescence and excitation spectra were obtained using a Turner Model 210 spectrofluorometer. This instrument produces spectra corrected for variations of source intensity with wavelength and variation in the wavelength response characteristics of the grating-photomultiplier combination. The instrument was checked against perylene as a standard. Absorption spectra were taken on a Cary Model 14 recording spectrophotometer. Phosphorescence spectra were observed with an Aminco-Bowman Instrument and are uncorrected. Procedures. The monitoring of absorption and fluorescence changes was carried out using solutions in a 1-cm quartz cell. These solutions were deoxygenated by the freeze-pump-thaw technique, and the cell was sealed under vacuum before photolysis. Large scale photolyses were carried out on solutions deoxygenated by 1 to 0.5 hr of bubbling with prepurified nitrogen in a nitrogen-filled glove bag. The reaction flasks were then stoppered tightly and sealed with paraffin wax. Thin layer chromatography (TLC) was done on Eastman Chromagram Sheet 6060, silica gel with fluorescent indicator. Spots to be measured spectrally were cut out of the TLC sheet and eluted with ethanol through a microcolumn into a 1-cm quartz cell. Alumina (Woelm neutral, activity I) was used for all column chromatograms. The progress of the chromatography was followed by absorption and fluorescence measurements on each fraction.
RESULTS E x h a u s t i v e photolysis of 10-4M AQ in ethanol was carried out until fluorescence was no longer observed (16 hr., 253.7-nm irradiation). Absorption a n d fluorescence changes followed a s t e a d y progression, becoming less intense and being shifted to shorter wavelengths as photolysis progressed. I n the later stages of photolysis, t h e r e was no absorption at wavelengths longer than 300 nm and n o fluorescence at wavelengths longer than 350 n m . The p r i m a r y p r o d u c t s in the photolysis of 9,lO-DHA fluoresce blue (A > 385 nm). For a typical photolysis of 10-4M AQ in ethanol u n d e r 253.7-nm irradiation, the blue-fluorescing species were formed within 6 m i n and disappeared completely after 60 min. Figure 1 shows the fluorescence change a f t e r 6 m i n of photolysis. It c a n be seen that the fluorescence of 9,lO-DHA is largely replaced b y a b r o a d blue fluorescence. The s m a l l peak at 362 n m is significant. As irradiation continued, this peak increased and fluorescence above 400 nm decreased. B y proper selection Qf sample irradiation time and fluorescence excitation wavelength, three blue-fluorescing products could be distinguished. These a r e shown i n Figure 2. T h e fluorescence s p e c t r u m having a p e a k at 454 nm is d u e to 9-anthrol; the other fluorescing c o m p o u n d s will be called the "362 intermediate" and the "410 intermediate" in reference to t h e i r fluorescence m a x i m a . Figure 3 shows the fluorescence intensity of 9,lO-DHA and the 362 and 410 intermediates as a function of irrad i a t i o n t i m e . 9-Anthrol fluorescence was not monitored since it was weaker t h a n t h e other fluorescence and over-
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 11, SEPTEMBER 1973
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'" I
-
-
+' /
I I
I
-
+
I
I
I
I
\ \
i
-
\
+\
y*/< 350
\ \\
400
450 500 Wavelength ( m p )
Figure 1. Fluorescence changes
550
+
4 00
450
500
X(nm)
Figure 2. Fluorescence of the three blue-fluorescing products in AQ photoreduction 9-Anthrol (--),
410 intermediate (- X -), 362 intermediate (0- 0 )
lapped significantly with 9,lO-DHA. It is apparent that the 410 intermediate is formed as 9,lO-DHA is photolyzed. The 362 intermediate continues to be formed after 9,lODHA no longer remains. The relative photostability of the 362 intermediate makes its fluorescence a characteristic of AQ photoreduction in the later stages. The 362 intermediate was found to be a secondary photolysis product and will be the subject of a subsequent paper. Production of the blue-fluorescing compounds was found to be wavelength dependent. Photolysis of 9,lODHA was 30 times faster at 253.7 than 350 nm. The quantum yield (&) of disappearance of 9,lO-DHA using 235.7nm radiation was found to be 0.005 f 0.002, using 9,lOdimethylanthracene as a standard (& = 0.12) in a rotating photochemical reactor. The absorption band of 9,lO-DHA a t 380 nm was monitored simultaneously with absorption at 325 nm as photolysis progressed. When the 380- nm band had fallen to ca. 50% of its original intensity, absorption at 328 nm had risen to about 50% of its maximum intensity. Fluorescence excitation spectra of the 410 intermediate showed maxima a t 328 nm. Therefore it can be concluded that the 410 intermediate is derived directly from the photolysis of 9.10-DHA. 1796
IO
20
30
PHOTOLYSIS T I M E ( M I N . ) Figure 3. Photoproduct fluorescence intensity as a function of irradiation time
in AQ photoreduction
1 X 10-4M AQ in ethanol, 253.7-nm irradiation, 320 nm fluorescence excitation: After 1 min (--), after 6 rnin ( -)
350
0
1 X 10-4M AQ in ethanol, 253.7-nm irradiation: 9,lO-DHA (--), intermediate (- X -), 362 intermediate (- 0 - )
410
To attempt identification of the 410 intermediate, fluorescence spectra were sought for six possible intermediates having absorption bands in the 300 to 335 nm region. The results are shown in Figure 4. With one exception, the fluorescence peak was at longer wavelengths than 410 nm. In addition a blue-fluorescing product reported (22) in the chemical reduction of oxanthrone was found to be an anthracene derivative, 9,9'-bianthryl. None of these compounds matched the absorption and fluorescence characteristic of the 410 intermediate. Column chromatographic separation of large scale photolyses of 9,lO-DHA in ethanol (0.8 g in 1800 ml) was un-, successful in isolating the 410 intermediate. Fractions having their fluorescence could be separated by elution with ether or ethyl acetate, after eluting the starting material with benzene. However, the quantity of the intermediate obtained was only a few milligrams. In addition, the fractions were contaminated with yellow products having retention times similar to those of the fluorescing products. The yellow compounds probably are photoproducts which have air oxidized. No further purification could be achieved by recrystallization, sublimation, or additional chromatography. Photolysis of 9,lO-Dimethoxyanthracene.Photolysis of 9,lO-dimethoxyanthracene(9,lO-DMA) in deoxygenated ethanol gave fluorescence and absorption changes similar to those with 9,lO-DHA. Figure 5 shows the absorption and fluorescence of 9,lO-DMA a t various irradiation times. The main photoproduct had absorption peaks at 247 and 305 nm and a fluorescence maximum at 381 nm (henceforth it will be referred to as the 381 intermediate). Upon prolonged photolysis, the 381 intermediate disappeared, and no new fluorescence was detected. If oxygen were present during the photolysis, the 381 intermediate was not observed. The quantum yield for disappearance of 9,lO-DMA was determined to be 0.003 f 0.001 using the same procedure as for 9,lO-DHA. 9,lO-DMA in deoxygenated ethanol is highly fluorescent (A, 435 nm). Using quinine sulfate as a standard, the @ e for 9,lO-DMA was found to be 0.77 A 0.05 a t 25 "C. Photochemistry of Oxanthrone. Because 9,lO-DHA gave more blue-fluorescing compounds than 9,10-DMA, the possibility that some photolysis of 9,lO-DHA might be occurring through its tautomer, oxanthrone, was investigated. In solution at room temperature, 9,lO-DHA exists (22) M. A. Matthews, J . Chem. Soc., 1926,236
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
ri
420
Xlnm)
Figure
5.
Xlnm)
Absorption and fluorescence changes in the photolysis
of 9 , l O - D M A 325
375
425
475
253.7-nm irradiation, irradiation time (rnin): 0 ( A ) , 24 (B), 44 ( C ) , 84
525
X (nm)
Figure 4. Fluorescence spectra of six compounds with absorption maxima near 325 n m I ( A ) , I l l ( B ) , 9,lO-phenanthrenediol (C), a,@-dihydroxystilbene (D),
1,4-naphthaIenediol(E), 2-rnethyl-l,4-naphthalenediol (F) in equilibrium with oxanthrone although the equilibrium is far (89%) on the side of 9,10-DHA (19, 23). In addition the equilibrium is attained slowly. of oxanthrone tautomerization The rate constant ( k to 9,lO-DHA in ethanol was measured spectrophotometrically at 25 "C. Formation of 9,lO-DHA was monitored by measuring the increase of the AQ absorbance at 323 nm after aeration. The linear plot of log (oxanthrone concentration) us. time gave a value of k - 1 of 3 x sec-l. A similar plot for oxanthrone in ethanol containing 0.1M HCl gave a value for k - 1 of 1 x 10-4 sec-l. When 1 x lO-4M solutions of oxanthrone in deoxygenated ethanol were photolyzed at 253.7 nm, 9,lO-DHA was formed in ca. 30% yields. Continued irradiation gave the\ photoproducts previously found in 9,lO-DHA photolysis. It is important to note that a 9,lO-DHA solution, photolyzed until almost no 9,lO-DHA was left, showed an increased 9,lO-DHA absorption and fluorescence after sitting in the dark for a week. In an effort to increase the concentration of 9,lO-DHA in the large scale photolysis runs, advantage was taken of the high solubility of oxanthrone in ethanol, A 10-2M oxanthrone solution in ethanol solution was deoxygenated and allowed to sit for several weeks to tautomerize. Photolysis of this mixture for 8 hr with 253.7-nm irradiation produced a large yield of precipitate (0.25 g) which was identified as AQ by its melting point, UV, and IR. The photolysis of oxanthrone in ethanol (10-4M, 253.7nm irradiation) also produced ca. 30% yields of the anthrone: 9-anthrol tautomeric pair. This was demonstrated by allowing the 9,lO-DHA produced to air oxidize and measuring the remaining 9-anthrol. Yields were also estimated by making the photolyzed solution lO-3M in KOH and measuring the 9-anthrol anion formed. It should be noted that the measured yields of tautomeric product (9,lO-DHA) and reduced products (9-anthrol and anthrone) varied with the length of photolysis time but were approximately equal in each photolysis examined. Figure 6 shows the phosphoresence spectrum of oxanthrone in EPA a t 77 OK. It has a 0-0 band at 408 nm (24500 c m - l ) with ca. 1 6 0 0 - ~ m -separation ~ in the vibra(23) 'K. Bredereck and E. F. Sornrnerrnann, Tetrahedron Lett., 41, 5009 (1966).
L
Figure 6. Phosphorescence spectrum of oxanthrone in EPA at 77 "K (310 nrn Excitation)
tional structure. Phosphorescence decay measurements gave a T~ of 2.2 msec.
DISCUSSION The extended photolysis of AQ produces, first a green fluorescent-product (9,lO-DHA) and second, several bluefluorescing species. Two of these blue-fluorescing species are derived directly from 9,lO-DHA and are called primary products. One has been identified in the present work as 9-anthranol, on the basis of its fluorescence and absorption characteristics. It is also known to be a product in the chemical reduction of AQ (6). The second product (410 intermediate) could not be identified, but it is not a compound produced in the chemical reduction of AQ. A similar intermediate is produced in the photolysis of 9,10-DMA, (381 intermediate), but no 9-methoxy anthracene could be detected. Anthracene and its monosubstituted and unsymmetrically disubstituted derivatives are well known to form
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
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pbotodimers. However, symmetrically 9,lO-disubstituted anthracenes are usually considered photochemically unreactive (24) in the absence of oxygen. The photolyses of 9,lO-DHA and 9,lO-DMA provide the first examples of photoreaction in the symmetrically 9,lO-disubstituted anthracene family. The low photochemical quantum yield of the photolyses (ca. 0.005) is compatible with the high fluorescence yield in these compounds (ca. 0.8). The similarity in photochemical behavior of 9,lO-DHA and 9,lO-DMA implies a similarity of mechanism, so far as the primary photochemical process is concerned. By analogy to the photochemical behavior of aryl esters and ethers (25), the primary event is probably cleavage of the 0-X bond, viz. ArO
-
X +hu
+
explanation can be seen by looking at the earlier method of preparing III. Barnett and Matthews (27) isolated a purple-fluorescing compound, mp 219 “C, by stopping the reduction of anthrone to 9,9’-bianthryl a t an early stage. From the elemental analysis and the acid-catalyzed conversion of this compound to 9,9’-bianthryl, it was identified as III. However, in view of the fluorescence observed, IV is more likely the compound isolated by Barnett and Matthews.
ArO’ + X .
where X = H, CH3. Obviously, secondary reactions differ for the ArO- radicals of 9,lO-DHA and 9,lO-DMA because the intermediates produced fluorescence a t 410 and 381 nm, respectively, although the parent compounds differ only by having two hydrogens replaced by methyl groups. The dependence of blue-fluorescing compound yield on irradiation wavelength can be understood on the basis of the nature of the photoproducts. For both 9,lO-DHA and 9,10-DMA, the photoproduct absorbs around 320 nm with a second peak of similar intensity around 250 ( 6 ca. 20006000). On the other hand, the starting material has weak absorption around 320 nm and a very intense peak at 250 nm ( 6 ca. 100,000). Since the quantum yield reaction (& > 0.05) of the photoproduct is much greater than its quantum yield of formation, photolyses a t 300 and 350 nm do not build up high concentrations of photoproduct. With 253.7-nm irradiation, the high 6 of the starting material somewhat shields the photoproduct from photolysis especially during the early stages. For this reason, all large scale photolyses aimed a t producing either the 410 or 381 intermediate were carried out with 253.7-nm irradiation. An attempt was made to identify the 410 intermediate by comparison with the absorption and fluorescence spectra of known compounds. These are either demonstrated or possible products in the chemical reduction of AQ. The structures of some possible intermediates are shown below
Several qualitative observations supported this. Melting or photolyzing I11 produced a compound (tentatively assigned as IV) with a UV and FS like that of 9-methylanthracene (28). A possible pathway for these reactions is shown above. These spectra were identical to those for a compound (mp 211-213 “C, IR showed -OH group) isolated in small yield during a chromatographic purification of 9,9’-bianthryl. It appears that I11 and IV have similar melting points. This could explain a discrepancy in the literature. Several other ring systems were eliminated as possibilities for the 410 intermediate on the basis of their UV and FS (and also by TLC comparison).
yp %e HO
OH
VI
V
OH VI1
Dihydroanthracene derivatives, I1 and VII, are assumed to absorb and fluoresce a t wavelengths equal to or longer in 2-methyl-l,4-naphthalenediol. However, one can deduce some information about the nature of the 410 intermediate from the data a t hand. It is likely that an OH group is still interacting with the T electron system because of the lack of structure in the fluorescence band. Also, it is likely a less conjugated system than 1,4-naphthalenediol, i. e., probably one which has lost some aromatic character. Also, it is clearly not a product of chemical reduction of AQ. These lead to the possibility of another tautomeric species, similar to oxanthrone, such as VI11 and IX.
OH I1
I
I11
Although these and the other compounds giving the spectra of Figure 4 are similar to the 410 intermediate, it is clear none gives a good match. I had a very weak fluorescence ( $ f < 0.01) centered at 425 nm. This probably results from deactivation in the excited singlet state by rotation about the double bond. A similar argument has been used (26) to explain the lack of fluorescence in tetraphenylethylene. 111 has been reported (27) to have a purple fluorescence, but no fluorescence was found in this study. A possible (24) J. 8. Birks, and
J. 8. Aladekomo, Phofochem. Photobioi., 2, 415
(1963). (25) C. E. Kalmus and D. M. Hercules, Tetrahedron Lett., 16, 1575, (1 972). (26) H. Stegemeyer. Ber. Bunsenges. Phys. Chem., 72, 335 (1968). 1923, 380. (27) E. Barnett and M . A . Matthews, J. Chem. SOC., 1798
OH VI11
IX
Intermediates such as VI11 or IX could account for the spectroscopic properties of the 410 intermediate, as well as its reactivity and oxygen sensitivity. It also is not likely such a species would be formed in the chemical reduction of A$. Also, because the 410 and 381 intermediates would involve cleavage of OH and OCH3 bonds, respectively, the nature of the “tautomers” found could be sufficiently different to account for the difference in their emission properties. (28) A . S.Cherkasov, I z v . Akad. Nauk SSSR, Ser. Fiz., 20,478(1956)
ANALYTICAL CHEMISTRY, VOL. 45, N O . 11, SEPTEMBER 1973
The above interpretation is consistent with the results of the large scale photolysis of 9,lO-DHA (10-2M).Under deoxygenated conditions, this produced a good yield of AQ. The solution did not produce any AQ precipitate on standing for 2 weeks, so the AQ produced by 8 hr of irradiation does not result from residual oxygen attack on 9,lO-DHA. The most likely precursor to AQ would be AQH., formed by photolytic scission of one of the 9,lODHA 0 - H groups. This radical is known to disproportionate as 2AQH
-
AQ
hrf
+ 9,lGDHA
Oxanthrone. The rate contants ( k l and DHA-oxanthrone conversion OH
k-1)
for 9,lO-
0 H. ‘H
410 Intermediate
UH K,,=ki/k-i
Ncnfluorercem Products
could not be measured by Bredereck and Sommermann (23) because the change in concentration was too small for their polarographic technique. They were able, however, to measure K,, = 0.13. Since we have determined k - 1 = 3 x 10-6 sec-1, then k l = 4 x 10-7 sec-1. This means that only 0.13% of the 9,lO-DHA would tautomerize by the ground-state route to oxanthrone during an hour of photolysis. This amount is too small to account for the quantity of photoreaction (notably formation of 9-anthrol) resulting from the presence of oxanthrone. Therefore, excited-state tautomerization must be occurring. In support of this, a 9,lO-DHA solution, photolyzed at 253.7 nm (11 min irradiation) until about 75% of the 9,lO-DHA had photolyzed, showed an increased amount of 9,lO-DHA after sitting in the dark for a week. This probably arises from tautomerization of oxanthrone back to 9,lO-DHA. The acid-catalyzed enhancement of k - 1 is in agreement with observations by Meyer (19). When he measured Keq (0.03), dilute HCl was added to the solution of oxanthrone in ethanol to hasten the reaching of equilibrium (3 days). This could explain the lower value of K,, since acid conditions favor the enol form (29). The phosphorescence spectrum and lifetime of oxanthrone are characteristic of an n, K* lowest excited triplet state. A similar result has been found for anthrone (30). It is not surprising then that oxanthrone phototautomerizes since the same process has been observed for anthrone (31-34). The mechanism is probably like that recently established for anthrone ( 3 4 ) . 0
Figure 7.
Photochemical sequence for 9 , l O - D H A in ethanol
Kanamaru and Nagakura (34) did not observe any 9,10-dihydro-9-anthrol or anthracene in the photolysis of anthrone. This lack of reduction products is in contrast to the formation of 9-anthrol in oxanthrone photolysis. To explain photoreduction in the case of oxanthrone, it is of no use to invoke the intermediate proposed for chemical reduction since X will not convert to 9-anthrol (by loss of water) under the deoxygenated conditions ( 3 5 ) .Instead a pathway involving cleavage of C-OH bond, rather than loss of water, from some intermediate such as XI appears to be more likely.
X OH
XI
SUMMARY Figure 7 summarizes the primary photochemical sequence for the formation and photolysis of 9,lO-DHA. It is quite clear that 9-anthranol formation arises from oxanthrone, and that photolysis of 9,lO-DHA can produce AQ. The best possibility of the 410 intermediate appears to be a n isomer of oxanthrone, but the exact nature of this species remains to be proven. Received for review January 19, 1973. Accepted March 28, 1973. This work was supported, in part, through funds provided by the U.S. Army Research Office, Durham.
OH l
I
OH
(29) H. Baba and K. Takemura, Bull. Chem. SOC.Jap., 37,1241 (1964). (30) R. Shimada and L. Goodman, J. Chem. Phys., 43,2027(1965). (31) J. Gorsuch. unpublished studies, Juniata College, 1963. (32) N. Kanamaru and S. Nagakura, Bull. Chem. SOC. Jap., 39, 1355 (1 966), (33)G. Lober, Z. Wiss. Photogr. Photophys. Photochem., 59,20 (1965). (34) N. Kanamaru and S. Nagakura, J. Amer. Chem. SOC., 90, 6905 (1968). (35) C. Prevost, C. R. Acad. Sci., 200,408 (1935).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
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