Application of a Rotating Photoelectrode to a Photochemical Study of Fluorenol Paul R. Gaines,‘ Val E. Peacock,2 and Dennis C. Johnson Department of Chemistry, Iowa State University, Ames, IA 500 10
The ultraviolet photolysis of fluorenol in alkaline methanol and isopropanol solutions was investigated with a rotating photoelectrode and wavelengths were selected for the maximum rate of productlon of electrochemically active photoproducts. Final products of extended photolyses in neutral solutions were identified by mass spectrometry following partial separaticns by liquid chromatography. Mechanisms for the photolytic reactions are given on the basis of the fluorenyl and oxyfluorenyl radlcais as primary photoproducts. The photocurrent spectra are explained on the basis of the proposed mechanism.
Johnson and Resnick ( 1 ) described a rotating photoelectrode (RPE) for electroanalytical detection of the intermediates and products of photochemical reactions in liquid media. The design of the RPE was inspired by the rotating ring-disk electrode which has been proved useful for kinetic studies of electrogenerated intermediates. Typical values of half-lives of reactions for which rate constants can be determined by a RRDE are approximately l msec t o 20 sec (2). This range is also expected to apply to the RPE. Lubbers, Resnick, Gaines, and Johnson ( 3 ) described a computer program for the digital simulation of the processes occurring a t the RPE which affect production, consumption, and mass transport of photoproducts. The computer program and the RPE were applied to a kinetic study of the photodimerization of benzophenone in alkaline alcoholwater solution. The rate constant determined is in fairly good agreement with literature values obtained by other methods. The qualitative application of a RPE in a study of the photochemistry of fluorenol is described here. A single report of research on the photochemistry of fluorenol was found in the chemical literature. DeGroot ( 4 ) investigated several aromatic alcohols and their derivatives by flash photolysis and ultraviolet-visable spectrophotometry. T h e absorbance spectra of the primary photoproducts for such systems result from triplet-triplet and singlet-radical transitions ( 5 ) . In attempting t o resolve the spectra, DeGroot dissolved the compounds in monomethylmethacrylate and polymerized the samples. The lifetime of the radical in solid polymethylmethacrylate was expected t o be greatly increased over that in the liquid state and the radical spectra easily obtained after decay of the triplet state. DeGroot observed, however, that fluorenol is quantitatively oxidized to fluorenone during the polymerization and could not be studied in this media. H e photolyzed fluorenol in isooctane and detected a relatively stable photoproduct with a lifetime of approximately 15 min. The photoproduct gave absorbance maxima a t 384 nm, 365 nm, and 350 nm and was proposed t o be the oxyradical of fluorenol. DeGroot also studied the compound in triisopropylborate (TIPB). The flashed solutions produced absorbance specPresent address, EXXON Research and Engineering Co., Linden, NJ 07036. Present address, Department of Chemistry, University of Wisconsin at Madison, WI 53706.
t r a with maxima a t 350 nm. No electron spin resonance signal was detected for the photoproduct and DeGroot suggested it was a stable dimer of the oxyradical. Aromatic alcohols were found by Orloff (6) to react with TIPB to produce the corresponding diisopropylborate ether derivatives of the alcohols. Hence, the photochemical behavior observed by DeGroot was probably that of the ether and not pure fluorenol. We have concluded from results reported here that both .the fluorenyl and oxyfluorenyl radicals are produced by photolysis in the presence of methanol and isopropanol. Production of all final products identified can be explained on the basis of reactions of these primary photoproducts.
EXPERIMENTAL Instrumentation. The photoelectrode was constructed by Pine
Instrument Co. of Grove City, PA, and was described in Reference 1. The pertainent radii were R1, Rz = 0.5000 cm and RB= 0.6245 cm. The platinum ring electrode was coated with Hg for all experimental measurements by dipping the electrode in a pool of Hg in the manner described in Reference 7. The photoelectrode was rotated in a PIR photorotator from Pine Instrument Co. which was equipped with a variable-speed motor and a ASR speed controller. The light source was a 500-watt,high pressure, xenon arc lamp, model 959C-98, from Hanovia, Inc. of Newark, NJ. A GM 250 grating monochromator from Schoeffel Instrument Corp. of Westwood, NJ, was used for wavelength isolation. Further description and a schematic diagram of the optical system were given in Reference 3. Thc estimated spectral bandwidth of the photolysis beam in the RPE was 40 nm. Actinometric measurements were made with ferrioxalate according to the procedure described in References 1 and 8 with an uninterrupted 2-hr photolysis. The intensity of the light beam at various wavelengths is given in Table I. Transmission spectra were recorded on a Bausch and Lomb Spectronic 600. The molar absorptivity of fluorenol was determined with a 1.00-cm quartz cell and a spectrophotometer fashioned from the light source, grating monochromator, and a “blue-sensitive”photodetector from Beckman Instrument Co. A consequence of this instrumental arrangement was that the spectral bandwidth for determinations of molar absorptivity was identical to that for photolytic studies and actinometric measurements. The profile of light intensity across the surface of the optical disk of the RPE was shown in Figure 8 of Reference 3. The potentiostat and associated electronics are described in Reference 1. All measured electrode potentials are reported in V vs. SCE. Measurement of the photocurrent in the mercury-coated ring electrode was made by subtracting the dark current from the total current observed during photolysis. Photocurrent-potential ’ curves for the photoproducts were obtained according to the procedure described in Reference 3. Chromatographic separations were made on a Chromatronix model 500 liquid chromatograph with a LC-6M column. The col-
Table I. Beam Intensity at RPE 1 , nm
I o , quantalsec
240
3.6 x
270
6.0 x l o i 4 1.0 x 1015 1.3 x 1015
290 310
ANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
1373
0
,
8
0
0
I
t
h (nm)
243
Figure 1. Photocurrent and transmission spectra ( 0 )A,, vs. X for 0.10M fluorenol in 90% methanol-10% water containing 0.10 M NaOH, E, = -0.80 V vs. SCE and w = 95 rad/sec; (A)I,,, vs. ifor 0.1OMfluorenoiin 90% isopropanol-10% water containing O.iOMNaOH, E, = -0.80 V vs. SCE and w = 95 radlsec; (-) % T v s . for 5 X 10-5Mfluorenol in 90% ethanol-10% water
umn was packed with 80-100 mesh Amberlite XAD-2 macroreticular resin from Rohm and Haas Co. with bed dimensions of 6.3 mm X 14 cm. The eluent was 90% methanol-10% chloroform a t a flow rate of 24 ml/hr. The column temperature was 25 & 3 "C. The photometric detector was model 200-UV from Chromatronix and the chromatograms were recorded on a Sargent SRG stripchart recorder. Mass spectra were obtained on an Atlas CH-4 mass spectrometer. Qualitative identification of compounds in the chromatographic residues was made based on the presence of parent peaks in the mass spectra and the relative intensities of isotope peaks. High resolution mass spectra were obtained on an Associated Electrical Industries Ltd. MS902 mass spectrometer. Reagents. Unless designated to the contrary, all chemicals were Baker Analyzed Reagents. Methanol was made 2 M in NaOH and distilled from NaBH4. Isopropanol was also distilled from NaBH4. Chloroform and absolute ethanol (Commercial Solvents of Chicago, IL) were distilled before use. Purified NaClOCH20 was obtained from Fisher Scientific Co. of Fairlawn, NJ. All water was triply distilled with demineralization after the first distillation and the second was from alkaline permanganate solution. Solutions were deaerated with dispersed prepurified Nz from Air Products. A Nz atmosphere was maintained over solutions during photolytic studies with a RPE. Dissolved 0 2 was reduced electrochemically at the electrode potentials used in this study and the presence of traces of 0 2 was signaled by the observation of a large dark current. Analyzed grade fluorene, bifluorene, and fluorenone were obtained from Aldrich Chemical Co. The fluorenone was recrystallized three times from ethanol. Fluorene and bifluorene were each recrystallized three times from 50% ethanol-50% benzene solutions. Fluorenol was synthesized from fluorenone by reduction with LiAlH4 followed by three recrystallizations from petroleum ether. Fluorenyl ether was prepared and purified according to a procedure given by Klieg1 (9). All solid organic compounds were stored in a vacuum over Anhydrone from G. F. Smith Chemical co. Procedures. Operation of the RPE was described in Reference 4 . Separation and qualitative identification of photoproducts required extended photolysis. Samples of fluorenol weighing 0.910 g were placed in 500-ml volumetric flasks, dissolved, and diluted to volume with absolute isopropanol or absolute methanol. Each solution was thoroughly mixed and one half placed in each of two electrolysis cells of the type used with the RPE. The solutions were deaerated for 20 min witlh Nz and thermostated a t 25.0 0.1 "C. One solution was irradiated for 5 hr a t a specified wavelength with the beam transmitted by the RPE. The second solution was kept in the dark. A Nz atmosphere was maintained over both solutions during the photolysis period to exclude 0 2 . Following irradiation, 50-111aliquots of each solution were chromatographed. A special cell was used for photolysis periods greater than 5 hr which could be tightly sealed following deaeration. Use of this cell resulted in conservation of Nz. The cell was constructed from a 1.0-cm X 7.5-cm Pyrex tube and contained the solution to be pho1374
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
0
I
I
1
260
I
I
283
I
I
IhQ,
30C
I
320
h(nrn)
Figure 2.
Photocurrent and transmission spectra
Data taken as in Figure 1 except E, = - 1.4 V vs. SCE
tolyzed. Following deaeration of the solution, the tube was connected by a short piece of Tygon tubing to a 2-cm length of Pyrex tubing sealed a t the upper end with a quartz disk. The test tube was then immersed in a 1.75-cm X 15-cm test tube containing Hg. The tube with the quartz disk was inserted into the rotator port where the RPE is normally attached. Hence the photolysis beam was identical to that used for studies with the RPE. A water bath was placed around the cell and thermostated a t 25.0 f 0.1 "C. The Hg surrounding the inner vessel served as a mirror to reflect back into the solution transmitted radiation and as a thermal conductor between the solution and the bath. A sample of fluorenol weighing 19 mg was dissolved in 3 ml of absolute methanol and irradiated in the special cell a t 290 nm for 48 hr. At the conclusion of the photolysis period, 0.2-ml aliquots of irradiated solution were chromatographed. Each chromatographed fraction for several injections was collected in separate flasks. The liquid in each flask was evaporated by passing dry Nz through the solutions. The residue on the walls was washed down with a few drops of alcohol and evaporation completed. Mass spectra were obtained for each residue. Sodium hydroxide was not present in solutions subjected to extended photolysis because of resultant difficulties in separations and obtaining mass spectra.
RESULTS AND DISCUSSION Photocurrent Spectra. Plots of photocurrent as a function of wavelength ( I h v - A), hereafter referred to as photocurrent spectra, were obtained at E , = -0.80 V and -1.40 V and are shown in Figures 1 and 2 for 90% methanol-10% water solutions which were 0.10M in NaOH. Transmission spectra for the solutions are also given. Anodic maxima at 240 nm and 310 nm are observed in the photocurrent spectrum at E , = -0.80 V for the isopropanol solution (Figure 1). Very small photocurrents were obtained at Er = -0.80 V for the methanol solution. This comparison of solvent effects is useful in recognizing products formed by hydrogen-abstraction reactions between a primary photoproduct and the organic solvent. We conclude that the anodic photocurrent at E , = -0.80 V results from oxidation of the radicals of methanol (hydroxymethyl) and isopropanol (2-hydroxy-2-propyl) produced in hydrogen-abstraction reactions with the primary photoproducts. Isopropanol is known to be a more effective hydrogen donor than methanol which is consistent with the observed larger anodic photocurrent in isopropanol. Nekrasov and Korsun (IO) determined that the radical anions produced by the one-electron reductions of benzophenone and benzaldehyde in alcohol-water solutions with 0.5N NaOH are oxidized a t a Hg electrode at E > -1.5 V. Hence, it is probable that the 2-hydroxy-2-propyl and hydroxymethyl radicals in our studies are oxidized at all values of E , used in this study. A very abrupt increase in
2
,
0
0
0
0.401
I
0.90
3.73
1.10
-E&
1.3C
1
1.50
%E)
L
I
42
84
‘26 ”68 2;: j : (rat!
Figure 3. I,,” vs. E, 0.10M fluorenol in 9 0 % methanol-10% water containing 0.10MNaOH: (-) X = 310 nm and (- - -) X = 240 nm; o = 95 rad/sec
252
jet)
Figure 5. IhV vs. w 0.10M fluorenol in 9 0 % methanol-10% water containing 0.10M NaOH; (-) X = 310 nm and (- -) A = 240 nm: (0)E, = -1.40 V and (0) E, = -0.80 V ~
h
f0.2 C H C
I
-02 0
-0.60-
t . . .
*..p - - - - ...a...
._.. ,-‘a
- --_q_ 2.90 L .YU
0.70 U ./U
1.13 -F$5, XE)
:.3c
1.5C -----a____
_ - - - _a -
-0.50
Flgure 4. lhuvs. E,
I
0.10M fluorenol in 90% isopropanol-10% water containing 0.10M NaOH: X = 310 nm and (- -) X = 240 nm; o = 95 radlsec
(-)
-
I
42
I
84
I
126
168
I
210 252
d((raC/S&C)
photocurrent is observed for an increase of wavelength from 295 nm to 305 nm. This is concluded to correspond to a change in the primary photolytic process and a change in the predominate hydrogen-abstracting species. A large cathodic maxima a t 290 nm is observed in Figure 2 a t E , = -1.40 V for both the methanol And isopropanol solutions and the magnitude of the photocurrents is nearly independent of solvent. The maxima for isopropanol is slightly less than that for methanol but it must be kept in mind that the hydroxypropyl radical produced a t 290 nm is probably simultaneously oxidized a t E , = -1.40 V. The cathodic photocurrents decrease markedly with increasing wavelength through the region 305-320 nm in which a sharp anodic maximum is obtained a t E’, = -0.80 V in isopropanol (Figure 1). Photocurrent-potential ( I h u - E,) curves for the photoproducts a t 240 nm and 310 nm are shown in Figure 3 for methanol. Very small anodic photocurrents are observed for E , > -1.15 V. and relatively large cathodic currents are observed for E , < -1.15 V. This large disparity between current magnitudes is consistent with the conclusion that the species reduced is not the same species oxidized a t the ring electrode. Ihv - E , curves for the photoproducts in isopropanol are shown in Figure 4. Large anodic currents are observed for E , > -1.10 V relative to those in Figure 3 which is consistent witch data shown in Figure 1 and 2. The ratio of cathodic photocurrent a t E , = -1.40 V to anodic current a t E , = -0.80 V is strongly dependent on wavelength and this
Flgure 6.
b,, vs. w
0.10M fluorenol in 90% isopropanol-10% water containing 0.10M NaOH; A = 310 nm and (- -) A = 240 nm: (0)E, = -1.40 V and (0).Er = -0 80 v
(-)
-
is concluded to be evidence that the primary photoproducts a t the two wavelengths are not the same. Variation of Rotational Velocity. The dependence of the photocurrent on the angular velocity of electrode rotation, w , is shown in Figure 5 for methanol solutions. The digital simulation of the RPE (3) showed that I h u - w plots exhibit maxima and the position of the maxima is a function of the rate of homogeneous reaction of electroactive intermediates produced photolytically. The maxima shift toward smaller values of w as the rate of the homogeneous reaction decreases. The positions of the maxima for the two plats a t E, = -1.40 V in Figure 5 are a t different values of w in agreement with the earlier conclusion that more than one primary photoproduct is formed in the wavelength range of Figures 1 and 2. The anodic photocurrents a t E , = -0.80 V are very small in agreement with data shown in Figure 1. The I h u - w plots for isopropanol solutions are shown in Figure 6. The data a t E , = -1.40 V are in agreement with the earlier conclusion of more than one photoproduct. The shapes of the plots a t E , = -0.80 V are virtually the same in agreement with the conclusion that a single species (alcohol radical) is responsible for the anodic current a t both wavelengths. ANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
1375
Table 111. Chromatographic P e a k Areas LVavelength, nm
Solvent
Methanol Methanol Methanol Isopropanol n.d. = no detection.
240 290 310 290
Peak 2 , cm
2
Peak 3 , cm
n.d.a 5.04 5.18
2
1.03
7.18 0.62 3.55
n.d.
Figure 7. Chromatogram 50-pl sample and 24 ml/hr flow rate
Table 11. Retention D a t a Compound
Retention t i m e , min
Peak 1 Fluorenol Peak 2 Fluorenone Peak 3 Fluorene Bifluorene
11 11 31 30 57 50 60
Chromatographic Separation. Liquid chromatography was used to separate the photoproducts from a 5-hr photolysis. The chromatogram obtained for the methanol solution irradiated a t 290 nm is shown in Figure 7. A wavelength setting of 290 nm was chosen for the photolysis because that value corresponds to the assumed transition between two photolytic processes. Considering the 40-nm bandpass of the monochromator, products should be found from both reactions. The solution kept in the dark produced a single chromatographic peak with a retention time identical to Peak 1 of Figure 7. The values of retention times for the peaks of Figure 7 are given in Table I1 together with retention times for various reference compounds suspected to be photoproducts. On the basis of data in Table 11, Peak 2 is identified as resulting predominantly from fluorenone. The residue obtained following evaporation of the fraction corresponding to Peak 2 was milky yellow in color whereas pure fluorenone is bright yellow. This fact is indicative of the presence of a contaminating species. The retention time of Peak 3 is intermediate the times for fluorene and bifluorene. This could be explained if Peak 3 corresponded to a mixture of the two compounds; certainly this mixture is highly possible since a fluorenyl radical intermediate could lead to both products. Fluorenone is reduced electrochemically in alcoholwater solutions by a two-step process described below ( 1 1 ) . Step 1 (Ell2 = -1.13 V):
rather than an aliphatic alcohol because the coupling of nonbonding electrons of oxygen with the aromatic system leads to partial delocalization of the anionic charge (11). The acidity of the pinacol is typical of alcohols. Fluorenone was identified as a photolysis product in methanol; however, the characteristic waves for fluorenone are missing on the Ihu - E , curves for irradiation a t 290 nm in methanol (Figure 3). Apparently the production of fluorenone from the primary photoproduct is slow on the time scale of mass transport a t the RPE and only the reaction intermediate is detected a t the ring electrode. The areas of chromatographic Peaks 2 and 3 are given in Table I11 for photolysis of fluorenol in methanol a t 240, 290, and 310 nm. Fluorenone (Peak 2) is only produced in the low energy region of the wavelength range in Figures 1 and 2. No fluorenone is produced in isopropanol a t 290 nm. Apparently the reaction of the primary photoproduct leading to fluorenone in methanol is precluded by the rapid hydrogen abstraction in isopropanol, The area of Peak 3 in methanol has approximately the same dependence on wavelength as the cathodic photocurrent a t E , = -1.40 V in Figure 2; note particularly the rapid fall off a t 310 nm. Fluorene and bifluorene are not electroactive a t potentials used in this study. Hence the photocurrent a t E , = -1.40 V must result from electrolysis of the photogenerated intermediate or a reaction by-product in the fluorene andlor bifluorene production. Fluorene and bifluorene were also determined present following irradiation a t 290 nm in isopropanol. Proposed Mechanism. Mechanisms for the photochemical and electrochemical reactions in alkaline alcohol-water solutions were formulated with reliance on the identifications of photoproducts in the neutral alcohol solutions. Production of fluorene and bifluorene (220-320 nm):
FLUORENOL
6 3+
+
2 FLUORENONE
Step 2 (E112 = -1.38 V):
The pK, of the radical intermediate was determined by Kalinowski e t al. ( 1 1 ) to be 9.5 f 0.4. The intermediate dimerizes to produce a pinacol with pK, = 13.7 k 0.2 ( 1 1 ) . The acidity of the radical intermediate is similar to phenol 1376
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8 , JULY 1975
0
FLUORENYL RADICAL
CH30H C,H,OH
- &b + F L U O R ENE
+
*CH20H
(4 )
*C3H60H
0 0 0 0
(5)
B I FLUORENE
The presence of appreciable quantities of bifluorene produced by Reaction 5 indicates that the pseudo-first order production of fluorene is fairly slow. Photocurrent by reduction of the fluorenyl radical ( E , = -1.40 V):
N
0
9'7
3
c
o
o
c
3
o
4*'?s?g N O O O O m O
N
3
* 0 3 0
s O 0
g
W 3
0
3
m w 3
w c - c o o w w w c o 3
3
3
4
Li C
ANALYTICALCHEMISTRY, VOL. 47, NO. 8 , JULY 1975
1377
--_.-
Production of fluorenone (280-330 nm):
Table V. Residue Analysis by High Resolution Mass Spectrometry m le
OXYFLUORCNYL
RADICAL
Choice of the oxyfluorenyl radical as the intermediate in Reaction 7 rather than the ketyl intermediate produced by the one-electron reduction of fluorenone (Reaction 1) was made because no pinacol was detected by mass spectrometry to be a photoproduct. This may a t first seem surprising because the 0-H bond dissociation energy of aliphatic alcohols is greater than that for the C-H bond of saturated hydrocarbons. However, as stated above, the energy of the 0-H bond in fluorenol is substantially decreased because of the adjacent x-system. Reaction 8 could possibly involve formation of a peroxide intermediate; however, no evidence was obtained indicating the peroxide as an intermediate state. No fluorenone was detected by chromatography in isopropanol probably because of rapid hydrogen abstraction by the oxyfluorenyl intermediate.
+
C3H,0H
2
330.141
i
Intensity
Identip
Cj3H9 Fluorenyl radical C,,II,, Fluorene C,,H160 9-(2-Hydroxy2 -propyl)fluorene C26H18 Bifluorene
0.003
0.4 6.5 0.4
0.4
_ _ _ I _
is proposed to be 9-hydroxymethenylfluorene which can be formed by the reaction of fluorenyl and methanol radicals,
M W 196 Chromatographic retention data are presented above to support the designation of Peak 2 as resulting from fluorenone (mol wt 180). An intense mass spectral peak was obtained a t mle 180 in the corresponding residue and is concluded to result from fluorenone. The abundance ratios for mle 181 and 182 are not those expected for a sample of pure fluorenone, however. The coloring of the residue from Peak 2 is cited above as evidence of the presence of a second compound with fluorenone. The second compound is identified as 9-hydroxymethenylfluorene (mol wt 196) on the basis of the mass spectral pattern a t rnle 196, 197, and 198. The mass spectrum for the residue from Peak 3 of the chromatogram contains peaks a t rnle 329 and 330 with relative intensities consistant for the identification of bifluorene (mol wt 330). Mass peaks a t rnle 166, 167, and 168 have relative intensities consistant for the identification of fluorene (mol w t 166). Mass peaks at rnle 196, 197, and 198 are also observed and again concluded to result from 9hydroxymethenylfluorene. There was no evidence of coupling between solvent radicals and the oxyfluorenyl radical to form the respective ethers. The compound 9-hydroxymethenylfluorene produced by coupling of methanol and fluorenyl radicals (Equation ll), was present in the residue of each chromatographic fraction with the most intense mass peaks in chromatographic Peak 2 which probably corresponds approximately to the maximum of a very broad chromatographic band. Photolysis of fluorenol in isopropanol is expected, therefore, to lead to production of 9-(2-hydroxy-2-propyl)fluorene(mol wt 224). A 48-hr photolysis was performed in isopropanol a t 290 nm and the residue of photoproducts was analyzed by mass spectrometry. Mass spectral peaks were obtained a t mle 224, 225, and 226 with intensity ratios in agreement with expected isotopic abundance ratios for 9-(2-hydroxy2-propy1)fluorene. High resolution mass spectrometry was used for analysis of the residue following photolysis a t 240 nm of an isopropanol solution of fluorenol. The results are given in Table V. Fluorene was the major product with a small quantity of bifluorene detected. This is explained because of the very effective hydrogen donating property of isopropanol. Also detected was 9-(2-hydroxy-2-propyl)fluoreneas expected for the isopropanol solution.
e+ *C3H60H
(9)
In contrast, negligible anodic photocurrent a t E, = -0.80 V was detected in methanol solutions which is evidence of a very slow hydrogen-abstracting reaction in that media. The zhu - w studies (Figure 5 and 6) led to the conclusion that the nature of the photoproducts undergoing electrolysis a t E , = -1.40 V differed a t X = 290 and 240 nm. At 240 nm, the electroactive species is the fluorenyl radical (Reaction 6) and, a t 290 nm, the mixture of fluorenyl and oxyfluorenyl radicals is electrolyzed, The cathodic electrolysis of the oxyfluorenyl radice: is probably that described by Reaction 10.
The life-times for the solvated hydroxyl radical (Reaction 3) and electron (Reaction 7) are undoubtedly short. Subsequent oxidation and reduction of the solvent occurs. Mass Spectrometry. Mass spectra were obtained for the residues remaining from evaporation of the collected chromatographic fractions following 48-hr photolyses a t 290 nm of fluorenol in methanol solution. The chromatogram had three peaks with retention times identical to those reported in Table 11. The mass spectra are summarized in Table IV together with spectra for reference compounds and data for isotopic abundance ratios of compounds with the specified empirical formulas (12). The observed intensities are given in arbitrary units. Also given are the intensities relative to the base peak of each rnle grouping. The intensities of peaks a t mle 182, 183, and 184 for chromatographic Peak 1 are consistant with the conclusion that fluorenol (mol wt 182) is eluted under Peak 1. Some fluorenone (mle 180) was detected in the spectra for both the reference sample of fluorenol and Peak 1. Some fluorenone is produced in the spectrometer by thermal oxidation of fluorenol. Mass peaks were also obtained a t mle 196, 197, and 198. The relative intensities of these peaks are consistant with the abundance ratios for the empirical formula C14H120 (mol wt 196). The identity of this species 1378
Formula
* *
165.070 0.001 166.078 i 0.001 224.098 0.002
SOL.VATED
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
i .
C,H,OH
-+
HH (12)
M W 224
A maximum was obtained a t 240 nm on the anodic photocurrent spectrum for isopropanol shown in Figure 1. Yet, a minimum appears on the cathodic spectrum in Figure 2. Mass spectrometry revealed that the ratio of fluorene-bifluorene produced was very large a t 240 nm relative to that at 290 nm. I t is our conclusion that the fluorenyl radical produced photolytically at 240 nm is in a higher electronic state of excitation than a t 290 nm. The higher energy is manifested by a "hotter" hydrogen-abstracting character. Hence, the alcohol radical and not the fluorenyl radical is detected a t the ring electrode at 240 nm. For slow hydrogen abstraction, the fluorenyl radical rather than the alcohol radical is detected. Quantum Yield for Photoproducts. The area under a chromatographic peak obtained by a spectrophotometric detector for injection of V, ml of a solution containing a colored species at concentration C, (moles/liter) is given by Equation 13. A , =
