2267
Photolytic Interactions of Aromatic Carboxylic Acids
Intramolecular Photolytic Interactions of Aromatic Carboxylic Acids in Solution Lalitha J. Mittal,lagb J. P. Mittal,la and E. Hayon* Pioneering Research Laboratory, U. S. Army Natick Laboratories, Natick, Massachusetts 01 760 (Received April 30, 1973)
The primary photolytic processes in aqueous solutions of ionized and nonionized phenylalkylcarboxylic acids and esters, Ph(CH2),COOR (where R = H, CH3, and CzH5 and n = 1, 2, and 3), at 20" were studied using the technique of flash photolysis. The main observed processes are Ph(CH2),COOR PhCH2. -(CH2),-1COOR (25) and Ph. .(CHz),COOR (26). No evidence was obtained for eithei photoionization or rupture of the Ph(CHz),COO-R bond. The nature of -R was, however, found to influence the relative importance of the two photoprocesses. The ratio (b26/(b25 was found to be 0.7, 1.5, and 3.0, respectively, when R = H, CH3, and C2H5. The major process observed in the photolysis of ionized phenylalkylcarboxylic acids was photoionization, leading to the production of eaq- and carbon dioxide: Ph(CH2),COO- -?!! Ph(CHZ),. eaqCOz ( 2 7 ) and Ph. .(CHz),COO- (29). The quantum yield of process 27 was found to increase with increase in n, with $27 ratios of 1.0:1.18:1.45 when n = 1, 2, and 3, respectively. The concentrations of the radicals in processes 25-27 and 29 were found to be directly proportional to the square of the incident light intensity, F,revealing the biphotonic nature of these processes in water at 20". The effects of low concentrations of specific quenchers, such as ethyl pyruvate and Ni2+ ions, showed that the excited state precursors of these photoprocesses must be relatively long lived, probably the triplet excited states. Since under the experimental conditions used, optical excitation energy was absorbed initially only by the aromatic ring and the photoejected electron comes mainly from the -COO- group, intramolecular photolytic interactions are clearly indicated. Such interactions are probably enhanced if spatial configuration is favorable by increased electronic overlap and intersystem crossing.
+
+
+
+
Introductioin Considerable interest exists in the photochemistry of benzyl derivatives and, in particular, of phenylalkylcarboxylic acids. With the former compounds, Ph(CH2),X, the interaction of the side chain on optical excitation of lBzu benzene absorption band has been found the lAlg to be quite strong. Such interactions have been the subject of various studies by photochemists and spectroscopists. The interest in phenylalkylcarboxylic acids is a more particular case (X = COOH) and stems from the important role of the aromatic amino acids phenylalanine and tyrosine as spectroscopic probes for the study of polypeptides and proteins, and as major participants in their photochemistry. The absorption and fluorescence spectra of a number of aromatic carboxylic acids have been examined a t room temperature as a function of pH (see ref 2-4, and references cited therein). The observed changes in the intensity and in the vibronic structure of the IA1, lBzu benzene absorption band have been attributed233 to the inductive effect of the carboxyl group. The quenching of fluorescence due to the substituent X has been interpreted334 in terms of an intramolecular charge-transfer interaction between the carboxyl group and the aromatic ring. The substituent is suggested to enhance spin-orbit coupling leading4 to a quenching of fluorescence, enhancement of the intersystem crossing (ISC) rate constant and the phosphorescence yields, and a decrease in the natural phosphorescence lifetime. Various mlechanisms have been proposed for the photochemistry of aromatic carboxylic acids in solution (see ref 5 and 6, and references cited therein). Photoionization and the formation of benzyl radicals have been observed in aqueous solutions5 at. room temperature. While no direct evidence is available with regard to the nature of the excited stat e precursors in the photochemistry of
-
-
+
Ph(CH2),COOH compounds, it was suggested5 that electron ejection occurs from the singlet excited state of these aromatic systems. This conclusion differs from that arrived a t in more recent work where the photoionization of tyrosine7 98 and the photoionization and photodissociation of phenylalaninegJ0 in water at 20" were shown to occur from the triplet excited state uia a biphotonic process. In this work, we report the results obtained on flash photolysis of phenylacetic acid, phenylpropionic acid, and methyl and ethyl phenylacetate in aqueous solutions. The effects of pH, light intensity, and specific quenchers on the transient species observed were examined in detail. Experimental Section The flash photolysis set-up11 and the experimental conditions12 used have been described elsewhere. Most of the work was carried out using flash intensities of -2000 J. The light output from the flash lamps was varied by changing the charging voltage across the lamps, typically from -17 to -23 kV, at constant capacitance. The light output was checked and found to be directly proportional to the charging (voltage)2 in the far-uv region (where optical excitation occurs). Quartz optical cells of 20-cm path were used throughout. Solutions were prepared with triply distilled water and were degassed by bubbling with prepurified nitrogen gas. Appropriate solution cut-off filters were placed in the outer jacket of the optical cell. Solutions were buffered using perchloric acid, potassium hydroxide, and -1 m M phosphate and borate. The chemicals used were the best grade available commercially. Phenylacetic acid was obtained from MCB and phenylpropionic acid from K & K. The esters (Eastman and Aldrich) used were further purified by distillation. All experiments were carried out in oxygen-free solutions at room temperature (-20"). The Journal of Physical Chemistry, Vol. 77, No. 19, 1973
Lalitha J. Mittal, J. P. Mittal, and E. Hayon
2268
Results a n d Discussion
-
Unless stated otherwise, mainly the first absorption transition lAlg 1Bzu of the benzene nucleus of the phenylalkylcarboxylic acids and esters studied was optically excited. This was done by using a 240-nm (15% aqueous acetic acid solution) cut-off filter in the outer jacket of the optical cell. Small but distinct differences exist3 (-20% lower integrated intensity for the -COOH acids) between the absorption spectra of the ionized and nonionized carboxylic acids. The spectra of the esters are almost identical with those of the corresponding nonionized acids. It should be clearly stated that, due to certain limitations of the flash photolysis technique, photolytic processes other than those observed and discussed below may also be occurring. However, due to lifetime (flash duration 10 psec) and spectral detection limitations, certain photolytic processes may not be observed. Phenylacetic A c i d . The flash photolysis of oxygen-free aqueous solutions of 2 m M phenylacetic acid (PKa = 4.31) produced transient optical absorptions which are dependent upon the pH of the solution. Figures 1 and 2 show the transient spectra obtained at pH 2.2 and 6.3, respectively. The spectrum at pH 2.2 shows mainly the formation of the characteristic transient absorption of the benzyl PhCH2. radical. A closer examination of the experimental curve and a comparison with the recently redetermined13 absorption spectrum of PhCH2- in water (dotted spectrum in Figure 1) reveals the presence of other absorbing transient species. The following primary photolytic processes can be considered.
d 0
N
PhCH2COOH
+
PhCH2. COOH Ph. .CH,COOH PhCH,. C02 (ea[
5
+
+
+
(1)
+ H’)
(2) (3)
Concomitant with the formation of PhCH2. is the formation of COOH radicals. The spectrum of this radicall4 has a Xmax 235 nm and €235 3 x 103 M - I cm-1 and undergoes ionization with a PKa -1.4.l5 The spectrum of the basic form COOH
+ .COP-
i- Hf
(4)
is essentially indi~tinguishablel~ from that of the acid form. Its maximum could not be observed here due to the strong absorption by phenylacetic acid in this wavelength region. However, based on the known13 extinction coefficients of the benzyl, €318 9.0 X IO3 and €307 4.7 x lo3 cm-1 and .COOH radicals, and assuming equimolar concentrations of C O O H and PhCH2. (based on reaction 11, the absorption due t o the contribution from *COOH has been derived and is shown in Figure 1. The sum of the absorptions of these two radicals can be seen n o t to account completely for the absorption of the experimental curve, particularly above 330 nm. It is suggested (see also below) that process 2 also occurs and that the residual absorption is due to the carboxylalkyl radical .CH&OOH. The spectrum of this radical has been determined,14 Xmax 320 nm and €320 650 M-I cm-1. It undergoes an acid-base reaction .CH,COOH
.CH,COO-
+ H’
(5)
with a pKa = 4.5.14 The absorption of the aCH2COO radical is red shifted and has14 a Xmax 350 nm and €350 800 M - 1 cm-1. The absorption of the phenyl radical, reaction 2, is not observed. In the gas phase, the Ph. radical is reported16 The Journalof Physical Chemistry, Vol. 77, No. 19, 7973
0.04
h,nm
Figure 1. Transient spectrum produced in the flash photolysis of 2 mM phenylacetic acid at pH 2.2, NZ (0).Spectrum of the P h C H z - ( - - - - ) , the .COzH ( - - ) , and the sum of PhCH2-
plus .COzH used to synthesize the exuerimental curve obis shown. The spectrum of the CHzCOOH radical tained by difference is also shown. A 240-nm cut-off filter was (-.-a)
(.e*)
used.
to absorb a t -440 nm. A recent17 indirect method indicated that the maximum is at 260 nm and t 630 M-1 cm-1, in aqueous solution. The absorption spectrum of phenyl in water was rechecked. The method utilized dissociative electron capture by chlorobenzene as a means of PhCl
+
eaQ- --+ Ph.
+
Cl-
generating phenyl radicals. A 5 m M PhCl aqueous solution was pulse radiolyzed at pH 9.2 in the presence of 1.5 M tBuOH. The transient spectra were determined in argon (1 atm) and N2O (1atm) solutions. The “difference” spectrum is taken to be due to the Ph- radical. The spectrum was similar to the one recently reported,l7 with Xmax 230 nm) to 2 m M PhCH2COO-, pH 6.3, using a 240-nm cut-off filter, produced a relatively very weak and completely different transient absorption, see Figure 2, with Xmax -315 nm. A similar spectrum was also obtained on flash photolysis in presence of N20 (1 atm) and 1.0 M t-BuOH. This spectrum is similar to that of Ni+ produced22 from the reduction of Ni2+ by eaq-. In the present experiments it could be produced by energy transfer to Ni2+ followed by reduction of *Ni2+ by H20. Ethyl pyruvate has also been used as a quencher; its triplet energy level ET is -2.4 eV (65 kcal/mo1).23 The quenching of the benzyl radical was monitored at 318 nm, and Stern-Volmer plots are given in Figure 3 for results obtained on flash photolysis of phenylacetate at pH 6.8
-
-
The Journalof Physical Chemistry, Vol. 77, No. 19, 1973
Lalitha J. Mittal, J. P. Mittal, and E. Hayon
2270
~CH~COOH 1’
i1o - j~
100
0
1’ ( 10-3)
300 0
200
100
200
300
0.16
0.32
mm
m -. m
d
d
6
0.08 0
0.16
0
,
,, 0
0
200
1r
I
I
400
600 0
2 p
:3
~
400
200
,.,,~ I ,
I
,
,
~
0.05
0
0.025
2 00
0
Figure
400 ( I and / z ) of
600
0.01
100
250
500
1000
Dependence upon the light intensity the concentration of the transient species produced in the flash photolysis of 2 mM phenylacetic acid at different pH values and different wavelengths. A 240-nm cut-off filter was used. 4.
TABLE I: Decay Kinetics of the Intermediates Produced on Flash Photolysis of Aromatic Carboxylic Acids in Oxygen-Free
Aqueous Solutions Systema
Phenylacetic acid Phenylacetate ion Phenylpropionate ion Methyl phenylacetate Ethyl phenylacetate
PH
2.2 6.7 6.6 6.5 6.7
Second-order decay, 2 k / c , c m sec-‘
I 0.3 x 3.1 0.2 x 4.7 f 0.2 x 3.0 f 0.3 X 7.7 f 0.6 x
5.2
105
105 105 lo5 105
a 2 mM solutions of aromatic carboxylic acids and esters were used, and a 240-nm cut-off filter. Decay monitored at 318 nm.
and phenylacetic acid a t pH 1.5. Using appropriate cut-off filters,
