Nov. 20, 1963
ELECTRON EJECTION IN AQUEOUSP-NAPHTHOL
[CONTRIBUTION FROM
THE
DEPARTMESTO F
PHYSICAL
3557
CHEMISTRY, HEBREWUNIVERSITY, JERUSALEM, ISRAEL]
Electron Ejection and Fluorescence in Aqueous P-Naphthol Solutions BY MICHAELOTTOLENGHI RECEIVEDJUNE6, 1963 The ultraviolet photochemistry of aqueous (3-naphthol solutions was investigated using specific scavengers for solvated electrons ( i Y 2 0 , H30+,and acetone) and for hydrogen atoms (C2H60H). It was found that the excited (3-naphtholate ion undergoes an electron ejection process while the neutral p-naphthol molecules undergo both electron ejection and dissociation into hydrogen atoms and (3-naphthoxyl radicals. The pH dependence of the quantum yields of these processes is similar to that found by Forster and Weller for the relative fluorescence of the p-naphthol molecule and the p-naphtholate ion. I t is thus concluded that electron ejection occurs mainly from the first singlet, fluorescence-emitting, excited state, in thermal equilibrium with the medium.
Introduction In a previous paper' the photochemistry of the phenolate ion in aqueous solution was investigated in the presence of specific scavengers for solvated electrons (NzO and acetone) using monochromatic light of 2288 and 2537 A. The kinetic analysis of the dependence of the quantum yield on the scavenger concentration indicated that the excited phenolate ion undergoes a thermal ionization into a solvated electron and a phenoxy1 radical, both in the same solvent cage. The limiting constant quantum yield, I', obtained a t high [NZO], represents total scavenging of solvated electrons from the photochemical cage. The magnitude of I' is determined by the relative efficiency of the electron ejection process and the various ways of deactivation of the excited ion. A brief spectroscopic study' indicated that the absorption bands of the phenolate ion in aqueous solutions are due to internal T-T* excitations of the aromatic system and do not involve any considerable contribution of a C.T.T.S. (charge transfer to the solvent) transition. Such a transition leads to the formation of solvated electrons in the case of the C1-, Br-, I-, and OH- ions in s ~ l u t i o n . ~ However, -~ the exact nature of the excited state from which the electron ejection takes place, in the case of the phenolate io?, remained unclear. Irradiation a t 2288 or 2537 A. excites the ion to the second singlet excited state (the lBlUstate) which may, however, undergo radiationless transitions to lower excited states before ejecting the electron. Electron ejection may then take place from a lower state such as the lowest singlet (the 'Bz, state), the lowest triplet (the 3B2, state), or even a C.T.T.S. state. The purpose of the present work was to determine which of the various possible states is responsible for electron ejection from excited aromatic molecules and ions in solution. The choice of @-naphtholis due to the relatively high fluorescence efficiency of both ionized and neutral forms of this molecule. As it will become clear in the following sections, the main conclusions of this work are drawn from the analogies between fluorescence eficiencies and quantum yields for electron ejection in aqueous @-naphtholsolutions. Experimental Radiation Source and Actinometry.-The radiation source was an Osram C d / l cadmium arc operated a t 12 v . and 1.5 a m p . The cadmium lines absorbed by the 8-naphthol solutions were those in the range between 214 and 360 mp. Actinometry was carried out, employing the uranyl oxalate actinometer, with a suitable correction for the inner filter effect of the oxalic acid for the 2288 A . cadmium line. The light intensity was found to be in the range 2.0 X to 3.4 X einstein 1.-' set.-'. (1) J Jortner, M Ottolenghi. and G. Stein, J . A m . Chem. S O L 86, , 2712 (1963) (2) J Jortner, M . Ottolenghi, and G . Stein, J P h y s . C h e m . , 6 6 , 2029 (1962). (3) J . Jortner, M . Ottolenghi, and G Stein, i b i d . , 86, 2037 (1962) (4) J. Jortner, M . Ottolenghi, and G Stein, ibid., submitted for puh lication.
Photochemical Procedure .-Reaction vessels were adapted for vacuum photochemical experiments from 1 X 1 X 4 cm. or 1 X 3 X 4 cm. spectrophotometric cells. The procedure in experiments involving NzO was previously described . 3 The pressure of the gas evolved on irradiation was determined by means of a McLeod gage and a Pirani L.K.B. gage. The chemical composition of the gas was ascertained by combustion in the presence of hydrogen or oxygen, or by comparing the readings on the McLeod gage with those on the Pirani gage, after a suitable calibration. The p H below pH 3.1 was adjusted by means of H z S O ~ ; CH3COOH-CH3COONa buffer was employed in the range 4 < pH < 5 , K2HPOa-NaHzPOa buffer in the range 6 < pH < 8, and H3B03-KCI buffer in the range 7.9 < pH < 10.5. The buffer M. Above pH 12 concentration in all solutions was 5 X the pH was adjusted by means of NaOH. In all cases total absorption of light by the (3-naphthol molecule or the (3-naphtholate ion could be assumed. Determination of Fluorescence Intensities.-The apparatus for the fluorescence measurements consisted of a xenon arc as the light source and two Bausch and Lomb monochromators, for both exciting and emitted light. An E M 1 6256B photomultiplier was used as the detector for the emitted radiation, the intensity of which was measured a t 414 mp.
Results and Discussion Photochemistry of the @-Naphtholate Ion.-In previous inve~tigationsl-~ i t has been shown that electron ejection from excited molecules and ions in solution may be detected by means of specific scavengers for solvated electrons, such as proton donors (HsO+, H2P04-,NH4+, etc.), acetone, or NzO. Because of the restriction of the high alkalinity, N 2 0 was employed in the P-naphtholate system. Solutions of 2 X M P-naphthol, a t pH values above 12, were equilibrated with N20 a t various pressures and then irradiated. The gas evolved on irradiation was found to be pure N2,its quantum yield - y ( N P ) depending on the NzO concentration but not on the pH (when pH > 12). The experimental results are reported in Table I. As in other systems previously i n ~ e s t i g a t e d , ' - we ~ attribute the nitrogen evolution to the formation of solvated electrons from the excited @-naphtholateion followed by scavenging by NzO, according to the kinetic scheme hv
RO- +RO-* RO-* --+ RORO-* +( R O e-aq)~ (RO e-s,)I +RONzO RO O-
+
+ +
+ (RO +
(RO
--+
(1) (2) (3)
+
RO
+ NZ
+ e-nq(in bulk)
(1) (5) (6)
where the parentheses denote the photochemical cage in which the solvated electron e-aq is formed with the P-naphthoxyl radical, RO. The dependence of y(NJ on [NzO] is due to competition between the cage scavenging (eq. 5 ) and the geminate cage recombination, eq. 4. This dependence may be quantitatively represented by Noyes' equation5 y(N2) =
yr
+ r,'2a~/rrkh-,o+e-aq[Nz01
(7)
( 5 ) R . M. Noyes, J . A m Chem. SOL.,7 1 , 2042 (195.5); 78, 5486 (1956)
3558
VOl. 85
MICHAEL OTTOLENGHI
“.I”
I
u‘ 0.08
0.8
d
0.6
0.4
W
8
0.04
H
1.00
a 0.2
t 0
ao2
t/‘ 0
2
io
O\
4
d
6
n
m
8 mob’’5
10
12
16
14
1-l.
Fig. 1.--Cage scavenging of solvated electrons by S i O : curve I , plot according to eq. 7; curve 11, plot according to eq. 8
+
re’is the yield of formation of the pair RO e-aq in the photochemical cage I, a is a parameter depending on the solvent and on the nature of the radicals in the cage,5 and yr is the “residual yield’12representing the quantum yield of radicals escaping cage recombination by diffusion into the bulk, when [S20] = 0. Equation 7 is an approximation valid only a t relatively low [NzO] values. A more general relation valid over the whole [ N 2 0 ]range, derived2 on the basis of Noyes’ theoretical treatment,4is
Equations 7 and 8 are not applicable a t very low NzO concentrations when the rates of scavenging and bulk recombination are comparable, ie., when ~ ( N z< )y r . In our system (see Table I) total scavenging is obtained M , when r(Nz) reaches the lima t [NZO] > 1.4 X which should thus iting value of r ( N z ) = 9.2 X be identified as the yield of formation of e-aq and RO in the photochemical cage a t 25’. We may therefore assume re’= 9.2 x 1 0 P . The plot of the experimental results of Table I according to the cage scavenging eq. 7 and 8 is given in Fig. 1. The deviation from linearity a t high [NzO],in the plot according to eq. 7, is due to the above-mentioned limitation of this equation. The slight deviation in the plot of eq. 8 has been discussed for other cage-scavenging systems2 and attributed to a time dependence of the diffusion-controlled rate constant ~ N , O+ e-84. From the slopes in Fig. 1 one gets 2 a d a k ~ , +e-., o = 8.7 l,‘/*mole-‘/2 when this parameter is calculated from the plot of eq. 7. When the calculation is according to eq. 8, the value 2 a d a k N , o + e-sq = 17.5 1.’’’ mole-’/’ is obtained. A factor of 1-3 between the values of the parameter 2 a d ? r k as calculated from the slopes of eq. 7 and 8 has been observed in all systems in which these equations are applicable and have been previously discussed.2 In both treatments we get yr < 8.0 X indicating a very efficient cage recombination of RO and e-aq, We may thus conclude that our experimental results are quantitatively consistent with the operation of the cage scavenging kinetic scheme (eq. l-R), thus excluding other charge transfer mechanisms, such as a direct interaction of NzO with the excited ion, which could also lead to electron capture and nitrogen evolution. I t should be noted that the quantum yields reported in Table I refer to the polychromatic cadmium radiation in the range 214-360 mw. An experiment in which
+ 0
i
0
0
250
u1
0.50
L
I
I
I
az
0.1
0.8
I
a4
I 0.6
cMuoNu mote C‘
I
I
a7
0.6 (%LE
OF CURVE
I ae r).
Fig. 2.-Curve I, the dependence of y ( Hs) on acetone concentration a t pH 2.0 in the presence of 0.75 M ethanol; curve 11, test of the dependence of y( H2) on the acetone concentration according to eq. 13.
a 4 N CH3COOH filter (cutting off light below 2400 A,) was placed between the light source and the cell showed that 65% of the chemical change was due to the 214 and 229 mp lines. The value of re’ for both of these lines, which excite the molecule to the second singlet state, has the value of 0.102. That for the longer wave lengths below 360 mg, where excitation is to the first singlet, has the value of 0.077. TABLEI THEDEPENDENCE OF y( N2) ON NzO CONCENTRATION IN SOLUTIONS OF 2 X l o e 3 M 6-XAPHTHOL AT pH 2 12 [NzO], mole
-ANd
PH
1.-1
1.95 X 1.95 X 1 . 9 5 X loe2 1.95 X 2 . 0 0 x 10-2 1.47 X 1.23 X 8.8 X 7 . 3 x 10-3 5 . 8 x 10-3 4 . 7 x 10-3 2 . 4 x 10-3 1 . 0 6 x 10-3 3 . 1 x 10-4 ...
14 13.5 12 13 13 13 13 13 13 13 13 13 13 13 13
9 . 0 x 10-2 9.2 X 9.2 X 9 . 1 x 10-2 9.2X 9.2 X 8.3 X 7 . 7 x 10-2 7 . 3 x 10-2 6.8 x 6 . 2 X 10+ 5.05 X 3 . 0 x 10-3 2 . 0 x 10-3 < 4 . 0 x 10-4
Photochemistry of the Neutral p-Naphthol Molecule. -The photochemistry of un-ionized @-naphthol was investigated in acid solutions a t pH < 2 in order to ascertain whether photochemical electron ejection is a characteristic of the solvated anion or may also occur in the case of the neutral molecule. Experiments were carried out in acid solutions where HaO+ acts very efficiently as electron scavenger, according to H30+
+ e-aq +H + HzO
(9)
Ethanol was employed as a scavenger for hydrogen formed either by reaction 9 or, possibly, by a direct dissociation of the excited neutral molecule. On irradiation pure H2 was evolved. According to the experimental results of Table 11, there is no gas evolution in the absence of ethanol, indicating that Hz formation proceeds v i a H atoms, scavenged by the al-
ELECTRON EJECTION I N AQUEOUS @-NAPHTHOL
Nov. 20, 1963
3559
coho1 according to H
+ CzHbOH +Hz + CzH4OH
(10)
The quantum yields in Table I1 also indicate that total scavenging of H atoms is obtained when [C2H50H]> 0.75 M. However, no information can be obtained from the data as to the mechanism by which hydrogen atoms are formed. In order to clarify this point we investigated the effect of added acetone on the quantum yields of H2 evolution. The experimental results are presented in Fig. 2 , curve I. I t has been TABLE I1 QUANTUM YIELDSFOR Hz EVOLUTIOX FROM ACID P - ~ A P H T H O L SOLUTIONS IN THE PRESENCE OF CzHjOH pHa
Scavenger system--For solvated electrons For hydrogen atoms
0 . 6 2 . 5 X IO-' M H 3 0 + 0 . 7 5 U E t O H 0 . 9 1.26 X lo-' M H 3 0 t 0.75 M E t O H 1 . 7 5 1.8 X IO-' M H 3 0 + 0 . 7 5 M E t O H 1 . 7 5 1.8 X lO-'MHsO+ 1.2 MEtOH 1.75 1.8 X 10-zMHsO+ , . . 2.0 lo-' M H30+ 0 . 7 5 M EtOH The pH was adjusted by means of H2SOI
+ CH3COCHs
lo-' lo-' lo-' lo-' 10-4
lo-'
-+ (CH3COCH3)- -+
nongaseous prod.
EtOH
1EtOH
Rd-+ HZ ROH+
+
Hz where we assume that all radical pairs are formed in photocheiiiical cages. The independence of 7(H2) on HaO+ and ethanol concentrations (see Table 11) indicates that total scavenging of solvated electrons and hydrogen atoms may be assumed when [H30+] > lo-* A2 and [C2H60H]> 0.75 91. In this concentration range we may therefore neglect the recombinations of pairs 11, 111, and I v and set r H = 3.8 X and re = 1.18 X lo-*, where r H is the quantum yield of formation of hydrogen atoms in cage I11 (derived from 7(H2) a t high acetone concentrations) and re that of solvated electrons formation in cage I (derived frorn the value y(H2) = 1.56 X lo-* in the absenceof acetone, ;.e., re r H = re 3.S X lou3= 1.56 X lo-*). I t should be noted that the influence of the
+
0
2
4
6
0
10
12
pH.
Fig. 3.-The p H dependence of the quantum yield for gas evolution in the photochemistry of @-naphthol; p H adjusted with: A, NaOH; 0,borate buffer; e, phosphate buffer; a, acetate buffer; A, unbuffered; 0, H2S04.
added acetone on y(Hz) is not concerned with the radical recombination in cage I1 and its only effect is that of competing with H30 + for the solvated electrons, which a t pH < 2, are totally scavenged even in the absence of acetone. y(H2) in acetone-containing solutions when pH < 2 and [CzHbOH] = 0.75 M should thus be represented by
which may be rearranged to
(11)
However, the fact that y(Hz) does not decrease below indicates the limiting value of y(H2) = 3.8 X that some other process is responsible for part of the H atom formation, besides reaction 9. We suggest that this limiting yield of hydrogen is due to hydrogen atoms formed directly from excited 0-naphthol molecules. This may be represented by the general scheme
(RO -f H),"
0.04
?(He)
1.60 X 1.48 X 1.52 X 1.56 X a 0x 1.56 X
that acetone reacts very efficiently with solvated electrons in a process which does not lead to gas evolution, while its reactivity with H atoms is relatively low.' An examination of curve I in Fig. 2 shows that y(H2) decreases from the value y(H2) = 1.56 X loF2,in the absence of acetone, to a constant limiting value of y(Hz) = 3.8 X obtained a t high acetone concentrations. As the acetone concentration is too low to compete for H atoms with 0.75 M ethanol,?we conclude that the decrease in 7(H2) with increasing acetone concentration is due to competition between reaction 9 and the reaction e-.q
0.08
+
(fi) J . Rabani and G . Stein, J . Chem. P h y s . , 97, 1865 (1962). ( 7 ) S. S e h a r i and J Kabani (to be published) report the value kethaool+ d k a c e t o n e + H = 5-10,
The agreement of the experimental results with eq. 13 is clearly proved by curve I1 in Fig. 2. From the slope of the straight line the rate constant ratio k g / k l l = 3.3 was calculated. This value is the same as that obtained in radiation chemistry experiments6 and in our previous treatment for the solvated electrons formed from the excited phenolate ion.' We may thus conclude that the results in acid solutions agree quantitatively with the scheme suggested above. Photochemistry at Intermediate pH Values.-The quantum yields for gas evolution in the intermediate range 3.1 < pH < 12 are presented in Table 111. In all experiments the concentration of the solvated electron scavenger, [N2O] = 1.95 X loF2M o r [ H 3 0 + ] > IO-? M , was sufficient for total scavenging of solvated electrons either from cage I or from cage 11, and the same for the scavenging of H atoms by 0 75 M ethanol from cages I11 and IV (the only exception are the experiments in strongly alkaline solutions, a t pH > 12, where ethanol could not be used because of the formation of CzH50- ions). Figure 3, which summarizes the data of Tables I , IT, 111, gives the pH dependence of the sum y(H2) ~ ( N P over ) , the whole pH range. Considering the high scavenger concentrations this sum may be identified as that of Y H (the quantum yield of H atoms formed by direct dissociation of KOH*), Y~ (the quantum yield of solvated electrons due to electron ejection from ROH*), and Ye' (the quantum yield of solvated electrons due to electron ejection from RO-*). Fluorescence Intensities in the Presence of N20.The fluorescence intensity in M P-naphthol solu-
+
Vol. 85
MICHAEL OTTOLENGHI
3560
TABLE I11
QUANTUM YIELDSOF H z AND Nz ni
IHE
RANGE2
< pH < 12 IN
AQUEOUS SOLUTIONS OF 2 X
Scavengers For solvated electrons F o r hydrogen atoms
144
P-XAPHTHOL
I -
PH
Buffer system
r(Hd X 108
tion a t pH 13 in the presence of 1.9 X M N20 was measured and compared to that in the same solution, evacuated and without N20. The fluorescence was excited either a t 229 mp or a t 349 mp. In both experiments there was no significant effect of N 2 0 on the fluorescence intensity. I
I
I
I
I
1.00
0.75 0.50
/ 40.
-
x
-\
/
x
r(Nd
3.08 1 . 9 5 X lo-’ -44 NzO 0 . i 5 M EtOH 1 . 6 =t1 . 2 “ HzSO4 4.05 Acetate 1 . 9 5 X lo-’ M XzO .75 M EtOH 2 . 0 -I 1 . 2 “ Acetate 5.0 1 . 9 5 X lo-’ M K20 .75 M EtOH 1.6 f 1.2” 5.0 Unbuffered 1 . 9 5 X lo-’ M ?;zO .75 M EtOH 2 . 0 i 1.2“ Acetate 5.6 .75 M EtOH 2.8 f 1.2“ 1.95 X lo-’ M XzO Acetate 5.6 75 M EtOH 3.0 i0.3 6.24 Phosphate 1 . 9 5 x 10-2 M NZO .75 M EtOH 2 . 0 f 1.2n 6.98 Phosphate 1 . 9 5 x 10-2 M Nz0 .75 M EtOH 1 . 6 i 1.2“ 7.17 Phosphate 1.95 X lo-’ M NzO .75 M EtOH 2.4 i 1.2” 7.73 Phosphate 1 . 9 5 X lo-’ M Nz0 .75 M EtOH 1 . 6 + 1.2” Phosphate 7.73 ... .75 M EtOH 3 . 0 i0 . 3 7.73 Phosphate ... ...
