Fluorescence of Benzoic Acid in Aqueous Acidic Media
The Journal of Physical Chemistry, Vol. 82, No. 1, 1978 81
Fluorescence of Benzoic Acid in Aqueous Acidic Media Roberto Martin and George A. Clarke" Department of Chemistty, University of Miami, Coral Gables, Florida 33 124 (Received August 16, 1976; Revised Manuscript Received July 13, 1977) Publication costs assisted by the Universlty of Miami
The room temperature fluorescence of the benzoic acid monomer is reported for the first time in aqueous and ethanolic solutions. The appearance of fluorescence, which is dependent upon the lowest l(a,a*), lying energetically below the 3(n,a*),is found to be affected by pH, buffer, solute concentration, and solvents. For the nonfluorescent monomer, in low polar organic solvents and for the benzoate anion in aqueous solvents, the l(a,a*)configuration is thought to lie above the 3(n,7r*)state. In aqueous acidic media, where the l(a,a*) lies below the 3(n,a*)and the l(n,r*),the fluorescence is assigned to the undissociated monomer of benzoic acid, and this represents the first reported fluorescence from the monomeric species of benzoic acid. In addition, the two bands in the benzoic acid fluorescence in aqueous acidic solutions and that from benzoic acid in ethanol are shown to be related, and thought to originate from planar and nonplanar excited state configurations.
Introduction Until recently, aromatic carbonyl compounds, such as benzoic acid, have been found not to flu~resce.l-~The nonfluorescent nature of these compounds has been ascribed to efficient intersystem crossing (Sl T1),ll2even though in an often overlooked study, in 1910, Ley and Engelhardt did observe fluorescence for benzoic acid in ethanoL6 In 1961, Ellman, Burkhalter, and Ladou7 found benzoic acid to fluoresce under the extreme condition of a 70% sulfuric acid solution, but no emission properties were given. Saltiel et a1.,8in 1970, observed fluorescence for a wide range of aromatic ketones, including benzophenone, in carbon tetrachloride solutions a t 23 "C. In 1971, Rusakowicz, Byers, and Leermakersg obtained fluorescence for benzoic acid in 85% phosphoric acid. Tournon and El-BayoumilOJl studied the room temperature emission of several phenyl carboxylic acids in ethanol solutions, where only the monomer form of the acid is present, and found several of these acids to fluoresce, while benzoic acid did not. This lack of emission further sustained the concept of a nonfluorescent monomer in benzoic acid. Baba and Kitamura,12in 1972, reported fluorescence for dimeric benzoic acid at 77 K in a mixture of isopentane and methylcyclohexane (6:l by volume, respectively). They also noted that the benzoic acid dimer fluoresces at room temperature. This present study shows that fluorescence can be obtained from the monomer form of benzoic acid in hydroxylic solvents.
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Experimental Section Benzoic acid, ethylene glycol, potassium hydroxide pellets, 1 N NaOH, 0.1 and 1 N HC1 solutions, and pH buffer salts of 4.0 and 6.9 were obtained from Fisher Scientific Co. Methanol was purchased from Mallinckrodt. p H 2 and 3 pHhydrions were procured from Arthur Thomas Co. The ACS certified benzoic acid was twice recrystallized from a water-methanol mixture, and vacuum dried. Phenylacetic acid, from Eastman Kodak, was twice recrystallized from petroleum ether, and vacuum dried. Spectroquality grade toluene, methanol, USP (reagent quality) absolute pure ethyl alcohol, and distilled water were used without further purification. The absorption and emission cells were cleaned by rinsing three times with acetone and then distilled water, followed by overnight soaking in nitric acid, a rinsing with distilled water, and oven drying. All volumetric glassware was detergent cleaned, washed three times with distilled 0022-3654/78/2082-0081$01 .OO/O
water, soaked overnight in nitric acid, washed three times with distilled water, and oven dried. Pipets were cleaned by overnight soaking in detergent solution, washing with distilled water, overnight soaking in dichromate solution, distilled water rinsing, and oven drying. All absorption spectra were obtained using a Cary-14 spectrometer. Emission spectra were taken on an Aminco-Bowman spectrophotofluorometer equipped with an R44S phototube, and a 500-nm grating blaze. The monochromator dial settings were aligned to within k l - 2 nm using a mercury pen lamp. The xenon light source calibration was made with a rhodamine B solution, containing 3 g of this solute in 1 L of ethylene glycol, while the detector system was calibrated by placing a mirror in the cuvet holder, a t 45" to the incoming light beam.13 Absorption and emission spectra were taken a t 22 "C. Quantum yields of the benzoic acid solutions were measured by comparison with the fluorescent yield of toluene. A quantum yield of 0.13 was used for toluene in ethan01.l~
Results Water. The intramolecular absorption charge-transfer band (ITCT) of benzoic acid in water shows a small shift, 3 nm, with concentration between 5.0 X 10" (225 nm) and 1.0 X M (228 nm), and is indicative of the anion-cation equilibrium. Fluorescence is observed throughout the M benzoic acid, stated concentration range. At 5.0 X fluorescence is detected in the pH range 0-5 (Figures 1and 2). In water, we found the fluorescence intensity of the benzoic acid solutions to be of the same order of magnitude as that for phenylacetic acid, a moderately fluorescent compound, in water. No fluorescence was detected above pH 5, in benzoic acid concentrations between 1.0 X and 1.0 X M. The emissions were optimized by exciting at 250 nm, and yielded a maximum intensity between 325 and 335 nm, with a shoulder at 340-345 nm. The shoulder in the spectrum of benzoic acid in aqueous acidic solutions was found at all the concentrations and pHs studied (Figure 3), and its relative height with respect to the maxima changed both as a function of concentration (Figure 4) and temperature. No fluorescence is detected for the buffered benzoic acid solutions in the concentration range 1.0 X to 1.0 X M at pH values 2, 3 , 4 , and 6.9. Apparently, the buffers act as total quenchers of the benzoic acid fluorescence. Ethanol and Methanol. The ITCT band maxima for benzoic acid, in both ethanol and methanol, remained 0 1978 American Chemical Society
82
The Journal of Physical Chemistry, Vol. 82,No. 1, 1978
R. Martin and G. A. Clarke
0.4
0.2
I
I
I
.i'
i
i
0.1
I
0.3
I 0.2
1
0.1
1
I
i
0.0 2,
o.a 2
Figure 1. Relative fluorescence intensity of 5.0 X IO4 M benzoic acid in water, as a function of wavelength.
Figure 3. Relative fluorescence intensity of 5.0 X M aqueous benzoic acid as a function of wavelength at several pH values: (-) pH 0.3, (---) pH 2.0, and (----) pH 3.2.
0.4
0.3
I 0.2
0.1
0.0
Figure 2. Relative fluorescence intensity of 5.0 X I O w 4 M aqueous benzoic acid as a function of pH.
invariant (225 f 1 nm) for 1.0 X lov5to 1.0 X M solutions. This indicates the sole presence of the monomeric species of benzoic acid in ethanol and methan01.l~ In methanol, benzoic acid was nonfluorescent in the aforementioned concentration range, both a t 22 "C and at 77 K. On the other hand, in ethanol, benzoic acid was found to be weakly fluorescent (Figure 5). Phenylacetic Acid. The room temperature absorption and emission of phenylacetic acid at 5.0 X M in water, methanol, and ethanol was studied. The wavelength of maximum emission, at 280-285 nm, was invariant in these three solvents.
Figure 4. Relative fluorescence intensity of aqueous benzoic acid as a function of wavelength at several concentrations: (-) IO-' M I X 3, (----) IO-* M, and (---) M.
Discussion
In general, the ultraviolet absorption spectra of the undissociated benzoic acid consists of three principal bands:15 the weakest transition is the AI, Bzu(T,T*) type which lies at the longest wavelength (270 nm); the strongest transition is the Alg B,, (T,T*) type and lies at the shortest wavelength (200 nm), while a relatively strong charge-transfer band derived from the phenyl-carboxylic interaction lies at 225-230 nm. The extinction coefficient and the location of the charge-transfer transition of the undissociated benzoic acid is found to be relatively invariant in the two alcoholic solvents, 225 f 1nm, and in Furthermore, in water, the acid media (Table I).
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Fluorescence of Benzoic Acid in Aqueous Acidic Media
The Journal of Physical Chemistry, Vol. 82, No. 7, 1978 83
351
- '(n.n") ---__
0.0
3 0ENERGY x 10-~
1
(cm-')
25.0.0
0
oa
i
I
I
320
340
360
G
\ 380 '$1,
hlnml
Flgure 5. Relative fluorescence intensity of ethanolic benzoic acid solutions: 1.0 X M (----)and 1.0 X lo-' M (-), vs. wavelength.
TABLE I: Wavelengths of M a x i m u m Absorption for 5.0 M Benzoic Acid Solutions as a Function of pH x hax,
DH
nm
0 0.3 1 2 4.2 4.3 5.0 12.7 14.0
230 232 231 230 225 225 222 223 221
f,
M-'
&!lax,
cm-l
nm
11540 12000 11000 12100 9700 9900 9200 9300 9300
27 1 27 2 27 2 272 271 269 267 268 267
f,M-I cm-I
97 0 960 995 990 746 720 630 620 550
wavelength maximum and the extinction coefficient for the charge-transfer process change, as expected, with the fraction of the undissociated acid. In addition to the above mentioned transitions, there should be another one, albeit weak, in the region of the lowest I(r,r*)absorption, and it would correspond to a carbonyl type l(n,r*) transition. This transition should be sensitive to pH and substituent changes, and perhaps detected by such means. However, for example, in the study of Baba and Kitamura12 where the OH group of benzoic acid was replaced by a chlorine (benzoyl chloride), the l(n,r*) band could not be located. Furthermore, in this study, the l(n,r*) transition was not observed in either acid or basic media. The latter is probably due to the low extinction coefficient of the '(n,r*) which is probably buried within the l(r,r*) band.16 Recent MO calculations of intermolecular hydrogen bonding of disubstituted carbonyls have shown that the l(n,r*) transition energy of the complex can be blue shifted by an amount approximately equal to the stabilization of the resulting complex,17and CNDO/S-CI calculations by SeliskaP for benzaldehyde and its protonated species have demonstrated the dramatic blue shifting of the l(n,r*) with respect to the l(r,r*) transition. Aromatic carbonyl compounds, such as benzoic acid, have generally been regarded as nonfluorescent and
1
I F
/P
G-0
Figure 7. Energy level diagram of fluorescent monomer of benzoic acid. Dashed lines represent approximate energy levels.
strongly phosph~rescent.l-~The only reported studies of benzoic acid fluorescence are those in three solvents: a 70% sulfuric acid solution at room temperature: an 85% phosphoric acid solution? and a mixture of isopentane and methylcyclohexane (6:l by volume, respectively) at 77 K.12 This study reports, for the first time, the room temperature fluorescence of benzoic acid in aqueous acidic solvents. We envisage fluorescence in benzoic acid as originating from the monomer configurations in acid media and ethanol, and from the dimer state in low polar organic solvents.12J9 In order to clarify the nature of the emission processes for benzoic acid, we make use of energy level diagrams, for the fluorescent and nonfluorescent monomer forms (Figures 6 and 7 ) . This scheme is based on solvent and substituent effects of simple aromatic carbonyls.lJ2J8 The l ( r , ~ *state ) is generally found to be red shifted by both electron-donatingsubstituents and increasing solvent polarity. For example, addition of an OH group to benzoic acid, as in salycilic acid, shifts the fluorescence by 54 and 123 nm in n-hexane and carbon tetrachloride, respectively.lg Benzoic acid is also found to undergo a red shift in the emission maxima, in going from carbon tetrachloride
84
The Journal of Physical Chemistry, Vol. 82, No. 1, 1978 400
350
300
R. Martin and G. A. Clarke h(nln)
2 0
350
400
300
1
BOO
600 E
too
too
I I
and fluorescence of a 5.0 X M aqueous benzoic acid solution, exciting at 250 nm. Excitation spectrum (with A,, 330 nm) is shown by dashed curve (----). Flgure 8. Absorption
(305-315 nm) to water (325-335 nm). Furthermore, fluorescence from the l(a,n*) can be observed in acid media. In our experiments, it was detected in the pH range 0-5 (Figure 11,and the wavelength of maximum emission was unchanged by this pH variation. Both of these characteristics are in sharp contrast to those which would be expected for a l(n,7r*) fluorescent state.20-22 The fluorescence intensity for benzoic acid in aqueous solutions is of the same order of magnitude as that for phenylacetic acid, a moderately fluorescent compound (Figure 7 ) . The phosphorescence of benzoic acid is presumed to originate from the 3 ( 7 r , ~ * )state since it meets the following criteriaz3for 3(7r,7r*) emission: (a) a singlet-triplet split greater than 3000 cm-l (4-7000 ~121-l);~(b) a relatively long lifetime (varying from 1.0 to 3.8 s),*~and (c) no vibrational structure due to the carboxylic group in the phosphorescence spectra of benzoic acid in n-hexaneeZ5 Monomer Absorption. Tournon and El-Bayoumil' have noted that the lowest ~(H,T*) transition in phenylacetic acid and toluene, in ethanol, are similarily structured. However, this transition for benzoic acid is perturbed by the close proximity and coplanarity of the aromatic nucleus with the carboxyl group. Thus, this band for benzoic acid is red shifted, intensified, and less structured with respect to phenylacetic acid and toluene. The extinction coefficient of the anionic form of benzoic acid is about 30% lower than that of the undissociated acid (Table I). This decrease may be seen to arise from a reduction in the charge delocalization into the carbonyl group in the ground state, which diminishes the contribution of the quinoid type structure, thereby allowing for a nonplanar configuration in the ground state and a diminuition of charge transfer in the excited state. In the more acidic solutions, where the undissociated benzoic acid predominates, there is a distinct red shift and intensification of the ITCT band with respect to the anionic form (Table I). The I ( . T T , P * ) band is also more intense in the undissociated form of benzoic acid than its anionic form (Table I). Monomer Fluorescence. The room temperature, 22 OC, absorption and fluorescence of benzoic acid overlap (Figure 8), as has been found for phenylacetic acid.19 However, in the case of benzoic acid in water, there is a distinct shoulder to the red of the maximum. There is also a large blue shift in the (0,O) band (288 to 266 nm) and the
0.0
1
i 25
Figure 9. Absorption
in ethanol.
\ I
i
1
31
3 4I
tloo 37
4 0o
&-3
and fluorescence of 1.0 X
M benzoic acid
emission maxima (325-335 to 280-285 nm) in going from benzoic to phenylacetic acid, respectively. This underscores the interaction of the phenyl with the carboxyl group causing the maximum for benzoic acid to be red shifted with respect to both toluene and phenylacetic acid. Parallel results have been obtained by Maria and McGlynnZ6in phosphorescence studies of toluene, phenylacetic, and benzoic acids. They found a blue shift in the emission maxima and a lengthening of the lifetime with increasing chain length between the phenyl and carboxyl groups. Under our experimental conditions, no emission for benzoic acid has been detected in methanol. This may be attributed to the inordinately high background signal observed for this solvent. For phenylacetic acid, a stronger emitter than benzoic, fluorescence is detected in methanol. For benzoic acid in ethanol there is no overlap between its absorption and fluorescence (Figure 9) indicating a change in the fluorescent species. This lack of overlap is unique for benzoic acid since phenylacetic acid in ethanol has overlap between its absorption and fluorescence,lgand its (0,O) band and maximum (at 266 and 280-285 nm, respectively) remain unchanged in water, ethanol, and methanol. Interestingly, this broad and very weak fluorescence of benzoic acid in ethanol encompasses the region of fluorescence maxima in water. At 1.0 X M benzoic acid in ethanol, there is a broad band (Figure 5) which we believe to consist of the two bands observed in water. This is confirmed by the concentration changes which show the prominence of the second band for benzoic acid in ethanol at the higher concentration, 1.0 X M (Figure 5). The monomer emission of benzoic acid in the hydroxylic solvents thus appear to be similar; two emissions are present and their location is unchanged by solvent. The two bands observed for benzoic acid fluorescence in water suggest the occurrence of two different fluorescent species, having an energy difference of approximately 5 kcal. The second band in water could be accounted for by: (a) two ground state species, (b) emission from the undissociated acid and its anionic form, (c) excimer formation, (d) interactions between the undissociated acid and its anionic form, or (e) a structural change in benzoic acid, through the formation of a nonplanar excited state. Excitation spectra (Figure 8), recorded with the emission set at 330 and 342 nm, have identical shapes and maxima.
Fluorescence of Benzoic Acid in Aqueous Acidic Media
The Journal of Physical Chemistry, Vol. 82, No. 1, 1978
280
300
350
A(nm)
400
85
450
Flgure 10. Relative fluorescence intensity of 1.0 X lo-* M benzoic acid solutions, exciting at 265 nm (---), 250 nm, I X 2 (----), and 240 nm, I X 3 (-), vs. wavelength.
Flgure 11. Relative fluorescence intensity of 1.0 X M benzoic acid solutions in ethanol, exciting at 245 nm (-), 250 nm (----) and 265 nm (---), vs. wavelength.
The maxima coincides with the absorption. The excitation spectra rules out the presence of two ground state species being the source of the structured spectrum of aqueous benzoic acid solutions. Simultaneous (n,r*) and ( T , R * ) fluorescence may be ruled out in these static experiments since (n,r*) fluorescence is not observable in acid media.z0 Figures 2 and 3, for the intensity vs. pH and the intensity vs. wavelength as a function of pH, rule out both fluorescence from the anion, and interactions between the undissociated benzoic acid and its anionic form as the source of the structured fluorescence of benzoic acid in water. This conclusion is based on the fact that the structured spectra is seen at all pHs for which emission is detected, and that no emission is observed for the anion. Because the excimer requires a collision between an excited singlet state and a ground state molecule, it is a diffusion-controlled process and would occur in a time scale which is long compared to that required for solvent stabilization, hence excimer formation must occur after the initial solvation of the singlet fluorescent state. Nevertheless, the relative intensity ratio of the emission from the uncomplexed singlet excited state to that from the excimer state should be wavelength independent, since they are both derived from the same excitation process. However, the relative heights of the two bands are found to be wavelength dependent (Figures 10 and 11)thereby ruling out the excimer pathway. This argument is further sustained by the lack of strong concentration dependence of the fluorescence of the two bands (Figure 4). Moreover, at pHs of 4.2 (Figure 3), the intensity difference between the two bands is about 1070,while a t pHs of 1 and 2, it is about 20-25% with a corresponding decrease in the relative height of the second band. This behavior would not be expected for excimer formation, since the reduction of pH (lower pH, that is) increases the concentration of the undissociated acid which leads to increased concentrations of the excited singlet and to an enhancement of the probability for excimer formation. The latter would result in the two bands for benzoic acid having at least the same height in going from pH 4.2 to 2.0, while a decrease in the second band with respect to the first is seen (Figure
3). The observed small concentration dependence of the relative heights of the fluorescence of the two bands for benzoic acid in water (Figure 4),the change in the two bands as a function of excitation wavelength (Figure lo), the observed pH effects, and the reduction of the second band at 77 K are consistent with the formation of two fluorescent conformers, that is a planar and a nonplanar benzoic acid. The appearance of an unusually red-shifted emission has been previously detected by Werner and Herculesz7 in a study of 9-anthroic acid, an aromatic carboxylic acid. They attributed it to a nonplanar excited state. In basic solutions, benzoic acid does not fluoresce and this may be associated with the aforementioned reduction of planarity in the anionic form. The lack of emission from the anion compared to the undissociated benzoic acid, due to the accrued charge delocalization in the anionic form, may be conceived in terms of a lowering of the 3(n,r*)state (Figure 7) which would enhance the intersystem crossing to the nonradiative triplet, and account for the nonfluorescent nature of the anionic species. It is also noted that the absorption coefficient of the anionic form of benzoic acid is reduced by about 30% compared to the undissociated acid (Table I). This reduces the probability for fluorescence to occur from the anionic form of benzoic acid. A t those pHs for which the undissociated benzoic acid is in significant concentrations, pHs less than 5.0, fluorescence is detected and increases with decrease in pH up to 3.2, at which about 90% of the benzoic acid is in its undissociated form. Beyond a pH of 3.2, the intensity levels off, and decreases below a pH of 2.0. In the region of increased fluorescence there is a corresponding increase in the extinction coefficient which parallels the increase in concentration of the undissociated acid and the disappearance of the anion up to a pH of 3.2. Beyond this point the fluorescence and the extinction coefficient of the l(r,r*) and the ITCT band remain constant (Table I). The leveling off in the fluorescence intensity may be associated with concentration and electrostatic quenching from the increased benzoic acid concentration, and the added
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The Journal of Physical Chemistry, Vol. 82,No. 1, 1978
hydrochloric acid. White has found cationic species to quench fluorescence.28 The benzoic acid fluorescence, both from its monomeric and dimeric forms, can be detected, but only a t concentrations which are unusually high for fluorescent studies. The very weak fluorescence for benzoic acid in ethanol indicates that, for this solvent, the energy gap between the l(n,x*) and the ‘ ( x , x * ) states is very small (close to that of the nonfluorescent monomer species), while this gap is considerably wider for benzoic acid in aqueous acidic media, where its fluorescence is of the same order of magnitude as that for phenylacetic acid. Thus, the benzoic acid in acid media is represented by the fluorescent monomer configuration (Figure 7). For aqueous solutions, in contrast to most other aromatic carboxylic acids, no emission is observed for the anionic form of benzoic acid. Furthermore, under buffered conditions, the fluorescence of the undissociated benzoic acid species is quenched. In order to obtain fluorescence from the neutral species, one must therefore work in regions of high concentrations and low unbuffered pHs. The sum of these observations accounts for the inability of previous researchers to effect fluorescence for the monomeric species of benzoic a~id.’-~Jl An understanding of these pH, buffer, solvent, and solute concentration effects has uncovered the dynamics of monomeric benzoic acid fluorescence. References and Notes (1) M. Kasha, Radiat. Res., Suppi. 2 , 243 (1960). (2) S. K. Lower and M. A. El-Sayed, Chem. Rev., 66, 199 (1966).
D. M. Rayner, P. K. Tolg, and A. G. Szabo (3) R. S. Becker, “Theory and Interpretation of Fluorescence and Phosphorescence”, Wiley-Interscience, New York, N.Y., 1969, pp 156- 157. (4) D. M. Hercules, Ed., “Fluorescence and PhosphorescenceAnalysis”, Wiley, New York, N.Y., 1966, pp 91-92. (5) American Instrument Company, “Luminescence Data Sheet-No. 2392-1 l A ” , Silver Spring, Md., p 10. (6) H. Ley and K. V. Engelhardt, Z. Phys. Chem., 74, 1 (1910). (7) G. L. Ellman, A. Burkhalter, and J. Ladou, J. Lab. Clin. Med., 57, 813 (1961). (8) J. Saltiel, H. C. Curtis, L. Metts, J. W. Mlley, J. Winterle, and M. Wrighton, J. Am. Chem. SOC.,92, 410 (1970). (9) R. Rusakowicz, G. W. Byers, and P. A. Leermakers, J. Am. Chem. Soc., 93, 3263 (1971). (10) J. Tournon and M. A. El-Bayoumi, J. Am. Chem. SOC.,93, 6396 (197 1). (11) J. Tournon and M. A. Ebbyoumi, J. Chem. Phys., 56,5128 (1972). (12) H. Baba and M. Kitamura, J . Mol. Specfrosc., 41, 302 (1972). (13) R. F. Chen, Anal. Biochem., 20, 339 (1967). (14) J. Tournon and M. A. El-Bayoumi, J. Chem. Phys., 56, 5128 (1972). (15) H. Hosoya, J. Tanaka, and S. Nagakura, J. Mol. Spectrosc., 8, 257 (1962). (16) C. Seliskar, 0. Khalil, and S.P. McGlynn in “Excited States”, Vol. I, E. Lim, Ed., Academic Press, New York, N.Y., 1974. (17) J. Del Bene, J . Chem. Phys., 63, 4666 (1975). (18) C. Seliskar, J. Phys. Chem., 81, 660 (1977). (19) R. Martin, Ph.D. Thesis, Universlty of Miami, 1977. (20) M. Kasha, Discuss. Faraday SOC.,9, 14 (1950). (21) J. W. Sidman, Chem. Rev., 58, 709 (1958). (22) K. Yoshihara and D. Kearns, J . Chem. Phys., 45, 1991 (1966). (23) R. S.Becker, ref 3, p 156. (24) R. Martin and G. A. Clarke, unpublished results. (25) Y. Kanda, R. Shlmada, and Y. Takenoshita, Specfrochim. Acta, 19, 1249 (1963). (26) H. J. Maria and S.P. McGlynn, J. Chem. Phys., 52, 3399 (1970). (27) T. C. Werner and D. M. Hercules, J. Phys. Chem., 75, 2005 (1969). (28) A. White, Biochem J., 71, 217 (1959).
Phosphorescence Spectra of Benzophenone in Aqueous Acetate Solutions at 77 K. Relevance to the Determination of pK(T,) of Benzophenone’ D. M. Rayner,“
P. K. Tolg, and A.
G. Szabo
Division of Biological Sciences, National Research Council of Canada, Ottawa, Ontario, Canada K1A OR6 (Received August 2, 1977) Publication costs assisted by the National Research Council of Canada
Phosphorescence spectra of benzophenone at 77 K have been measured in aqueous sodium acetate solutions of pH 6. Spectral changes are observed which have previously only been seen in aqueous solution at 77 K on increasing the acidity. Therefore the spectral changes cannot as before be taken as evidence for the existence of a benzophenone-hydroxonium complex and for the involvement of such a complex in the triplet state acid-base chemistry of benzophenone at room temperature. The spectral changes are attributed to the effect of the additive (acid or salt) on the frozen solvent structure allowing the formation of a benzophenone-water complex in the ground state at 77 K.
Introduction The triplet state pK value, pK(T1), of protonated benzophenone has been shown to be 1.5 by a direct determination based upon measurements of the initial intensities and lifetimes of the transients observed in laser flash experiments.2 This value coupled with the widely different values found for the characteristic lifetimes of the triplet B and BH’ species and 6.2 X lo-* s, respectively) satisfactorily explains the disappearance of the phosphorescence observed in aqueous ~ o l u t i o nas~ the ~~ acidity increases through pH 5. Recently Favaro and Bufalini5 have reported an investigation of the behavior of triplet benzophenone in aqueous solution at room temperature using a luminescence sensitization technique. Their results, for lifetimes and pK value, are in very good agreement with the direct laser flash technique. However 0022-3654/78/2082-0086$0 1.OO/O
they propose that the excited state reaction which causes the drop in phosphorescence is not the straightforward acid-base equilibrium B(T,) + H,O+
+BH(T,) + H,O
but an equilibrium involving the formation of a hydrogen bonded complex between the benzophenone triplet and a hydroxonium ion as a distinct species B(T,) t H,O+ + B(T,)H,O+
+BH(T,)t H,O
with the true protonated benzophenone triplet only being formed at an undetermined higher acidity. Their proposition is based on the observation of phosphorescence emission at 77 K from bengophenone in aqueous matrices in the acidity interval pH 3 to Ho = -4 which is significantly different, not only from that observed in neutral 0 1978 American Chemical Society
