T. FELDMANN AND A. TREININ
3768 strong transient absorptions are observed in the range from 390 to 440 nm. The evaluation of this type of spectra is done by densitometer measurement,s along the X and the t axis as well. Some registration tracks of the hydroquinone photolysis spectrum are assembled in Figure 5 . I n part a, absorption maxima at 428 and 404 nm are clearly recognizable which are due to the p-hydroxyphenoxyl r a d i ~ a l . ~ The absorption curve E,(X) of the transient products can be calculated easily by forming the difference between the film density readings before and at time intervals i after irradiation (Xo(X) - &(A)) and dividing this by the gradation curve y(X) of the emulsion
Registrations over time at constant wavelengths are presented in Figure 5b. As is t o be expected, the tracks for 402 and 426 nm yield identical radical half-life
values of about 75 psec. The absorptions of the p-hydroxyphenoxyl radical are superimposed, however, by a long-lived absorption which is seen most clearly in the 381-nm curve. The origin of this absorption could not yet be clarified. For parallel investigations by flash photolytic and pulse radiolytic methods, the system described will be complemented by the addition of a flash X-ray tube. This device, which is still in the test stage, is operated with a 160-kV condenser bank and produces X-ray pulses of O.l-psec duration. The equipment is constructed to give 1 krad per pulse inside the irradiation cuvette. Together with the z-pinch light source and the method of kinetic absorption spectroscopy, it will form a simple and compact arrangement for the flash photolytic and pulse radiolytic investigation of biophysical problems. (7) H. I. Joschok and L. I. Grossweiner, J.Amer. Chem. Soc., 88, 3261 (1966).
I- Photosensitized Reactions in Metaphosphate Glass by T. Feldmann and A. Treininl Department of Physical Chemistry, Hebrew University, Jerusalem, Israel
(Received May 7, 1968)
The photooxidation of C1- and Br- in metaphosphate (MP) glass could be sensitized by I-. The reaction was investigated by optical and esr methods. The mechanism proposed involves energy transfer from excited I- to a phosphate polymer, which subsequently dissociates to form the MP color centers. The oxidation of X- is a dark reaction whereby MP holes are scavenged by halide ions, and X atoms are converted to XZradicals. The esr of Clz- in glass is analyzed. The primary processes involving excited I- are discussed and an exciton-type mechanism is proposed for the energy transfer.
The halide ions X- have no excited states in the gas phasej2 but as shown by spectroscopic and photochemical experiments, such states (so called CTTS states) do exist in solution.3 Their natural lifetimes, estimated from their integrated absorption bands,4aare about 10-6 sec, but the actual lifetimes are much shorter.4b Dissociation to X esol- and deactivation to ground state are the competing radiationless processes in liquid solutions. The activation energy of dissociation is higher (by ea. 5 kcal for I- and Br- in aqueous solution) and this by itself can explain their low dissociation yield in ices at 77OK.6 (In alkaline ices, trapped electrons could not be detected.6) The mechanisms of these processes are not clear. Dissociation of X-* may involve diffusion of X from excitation ~ i t e . ~ bI n this case dissociation should be hampered in
+
The Journal of Physical Chemistry
rigid matrices even at room temperature. Still, efficient photolysis of I- and Br- does occur in boric acid7 and metaphosphates glasses. In the first case this may (1) On leave of absence a t the Department of Chemistry, Brandeis University, Waltham, Mass. 02154. (2) R. S. Berry, C. W. Reimann, and G. N. Spokes, J . Chem. Phys., 37, 2278 (1962). (3) For a recent review, see C. K. Jdrgensen, “Halogen Chemistry,” Vol. 1, Academic Press, New York, N. Y., 1967, p 280. (4) (a) J. Jortner and A. Treinin, Trans. Faraday Soc., 58, 1503 (1962); (b) J. Jortner, M. Ottolenghi, and G. Stein, J. Phys. Chem., 6 8 , 247 (1964). (5) P. N. Moorthy and J. J. Weiss, J. Chem. Phys., 42, 3121 (1965). (6) P. B. Ayscough, R. G. Collins, and F. S. Dainton, Nature, 205, 965 (1965). (7) A. Zaliouk-Gitter and A. Treinin, J. Chem. Phys., 42, 2019 (1965). (8) T. Feldmann and A. Treinin, ibid., 47, 2754 (1967).
I- PHOTOSENSITIZED REACTIONS IN METAPHOSPHATE GLASS
3769
result from direct oxidation of the excited ion by the highly acidic mediuma9 I n the second case, some preliminary results suggest that the excited ion might transfer its energy to phosphate polymersa8 I n the present work, we wish to verify this view. Experimental Section Preparation of Glasses. Pure metaphosphate (MP) glasses were prepared as described elsewhere.lo Halide-containing glasses were produced by melting thoroughly ground mixtures of MP and halide salts a t 750” under an atmosphere of Nz. KI, KBr, and KC1 of Analar grade were used. Irradiation and Actinometry. Glass disks were irradiated with an Osram Cd/l cadmium arc lamp (operated at 12 V and 1.5 A) using a 1-cm layer of water as a filter. The only light absorbed by our glasses was the 2144 (weak), 2265 (weak), and 2288-A lines. (The last two will be considered together.) To cut off the 2144-A line, a solution of, KBr was used, which transmitted 80% of the 2288-A line. The intensity ratio I2144/12288 = 3.3 X 10-2 was determined from emission spectrum of the lamp. Intensity of light incident at the front of the glass was measured by differential actinometry with uranyl oxalate,” and from known intensity ratio the small contribution from 2144 A could be found (-2 x 10-8 einstein/min). From this and the absorption spectrum of the glass the amount of light absorbed by X- at each wavelength was determined. Yields of Xz- were determined from their absorption peaks, assuming emax to be the same as in aqueous solutions; emax X 10-4 M-l cm-’ = 1.2, 0.8, and 1.4 for Clz-, Brz-, and Iz-, respectively.’Z From the yield with a KBr filter and without, the yield at 2144 A was obtained. Altogether, the quantum yield of Xz- was calculated by means of the formula (px2- = (l/Ia) (ADx,-/At)(nr2/1000ex2-)
where l a is the number of einsteins absorbed by X- per unit time, Dx,- and ex2- are the absorbance and molar extinction coefficient of Xz-, respectively, and r is the radius of the disk in centimeters. Optical absorption spectra were measured with a Cary 14 recording spectrophotometer. Electron spin resonance spectra were obtained at 9.4 kMc/sec with a Varian 4500 spectrometer. Results Under conditions employed, the pure base glass displayed very weak esr signals after only a few hours of irradiation. However, a marked photoactivity was exhibited by I--doped glass (Figure 1). The two characteristic esr doublets of MP8 were produced, but the hole doublet (h1,hz) was relatively reduced in intensity. A dark reaction involving I- appeared to be responsible for annihilation of the hole center; it continued to decay after irradiation and rate of decay increased with I- concentration (Figure 1). The
I
I
I
2950
3150
3350
H 3550
GAUSS 375C
Figure 1. Esr of I--doped glasses irradiated with Cd lamp for 20 min. Curves a and b: 2.8 x 10-3 M KI, 8 and 50 min after irradiat,ion, respectively. Curve c: 0.4 M KI, 10 min after irradiation. (All concentrations refer to mixture from which the glasses were prepared.)
“electron” signal (el,ez) was rather stable. Increase in its rate of production with (I-) was due, at least partly, to increase in light absorption. Thus for the same time of treatment the ratio between intensities of “electron” and hole centers increased with (I-). I n parallel, the spectrum of Iz- grew up, peaking at 375 mc1.
Higher concentrations of Br- were require! to obtain appreciable light absorption at 2144 A. Br2(A, 350 mp) and the “electron” center were produced, but no hole center. The holes were probably scavenged by Br-. C1- hardly absorbed at 2144 A. However, when C1- or Br- was added to I- a striking nonadditive effect was observed. Figures 2 and 3 show the behavior of a glass doped with I- and large excess of C1-. Irradiation led to fast production of C12-. It was identified by comparing its optical spectrum with that induced by X-rays in C1- doped glass (Figure 2). Amax 330 mp is close to that of Clz- in aqueous solution. l 2 (The peak at 230 mp is probably due to CI3-.l3) The same glass with no I- present was not affected by identical treatment (Figure 2); after 2 hr of irradiation Clz- could just be detected. The esr spectrum of the irradiated glass (Figure 3) revealed that the hole doublet was suppressed by C1- and a new esr pattern was produced. It showed two strong signals with 50-G separa(9) H. M. Buck, W. Th. A. M. Van der Lugt, and L. J. Oosterhoff, Tetrahedron, 19, (Suppl. 2), 173 (1963). (10) T. Feldmann, A. Treinin, and V . Volterra, J. Chem. Phys., 42, 3366 (1966). (11) J. Jortner, M. Ottolenghi, and G. Stein, J . Amer. Chem. Soc., 85, 2712 (1963). Reflections a t the front of the actinometer cell and the glass disk were assumed to be the same. (12) M. E. Langmuir and E. Hayon, J. Phys. Chem., 71, 3808 (1967). (13) G. Zimmerman and F. C. Strong, J . Amer. Chem. SOC.,79, 2063 (1967). Volume 79,Number 11 October 1968
T. FELDMANN AND A. TREININ
3770
*
O
" 0.8C
-I
I
0.01 200
I
2 50
I
300
350 A, my
Figure 2. Change in absorption spectra of glasses doped with K I (10: 1) after 10 min exposure to Cd light (X), KCl and with KCl alone after 7 min X irradiation (0). Dashed curve: absorption spectrum of the KCl glass before and after 50 min exposure to Cd light.
+
expected to be broadened beyond detection. The other two should suffer the same broadening, but taking into account the intensity ratio, the mI = +2 component may not be readily detected. The average separation between the mI = 0 and +1 signals is -$a/' A(8) sin 6 do. Using available datal4 and carrying out numerical integration, we obtained A , = 52 G. Altogether this description is in complete agreement with the new esr pattern in Figure 3. The mI = +2 component can still be discerned on the low-field side of the sharp signal; the latter at 3200 G evidently corresponds to mI = + l . Similar experiments with I--Br- mixtures were more difficult to interpret since Br- also absorbed at 2144 A when in large excess. Still, under proper conditions, irradiation led to the fast production of Brz- (Figure 4; compare curves 1 and 4. The very large hyperfine anisotropy of Brz- l4 smeared its esr spectrum beyond detection.) Finally, a glass containing a mixture of Br- and C1- with little I- was irradiated with Cd light and then with X-rays. By adjusting times of irradiation, the effects produced by the two kinds of radiation were nearly the same (Figure 4), The quantum yields of X2-, produced in the I--Xglasses, were measured under conditions where no holes could be detected. They were nearly the same for Br0.2, ( ~ 2 2 8 8 5 X Similar inand C1-: (p2144 crease of cp with hv was displayed by I- alone, but the quantum yield of 1 2 - was smaller owing to trapping of I atoms in YIP.s Their release by heating resulted in growth of both 1 2 - and 1,- spectra and by other annihilation processes.
-
-
Discussion I
2950
t
I
3150
I
3350
l
I
3550
I
~
L
375
Figure 3. Esr of C1- doped glasses. Curve a: 2.1 M KCl after 10 min X irradiation. Curves b and c: 2.8 and 0.28 M KCl, respectively, with 4.6 X 10-3 M KI after 20 rnin exposure to Cd light. (See remark to Figure 1.)
From the effect of X-rays on halide doped glassess it is known that MP holes are efficiently scavenged by C1-, Br-, and I-.
X-
+ hole-MP +X + MP
(1) This explains annihilation of the hole center and formation of X2-. However, the question arises as to the mechanism by which holes are produced by Cd light. I n our experiments only little light was absorbed by the tion and few weak components. The same pattern was base glass and this led to little chemical effect even in produced by X-rays in MP containing C1- alone (Figure the presence of high concentrations of C1-, which might 3). Evidently it belongs to a C1 species, most likely compete with geminate recombination. Therefore, Clz-. The esr of Clz- in crystals is well 1~n0wn.l~ the primary excitation of I- is responsible for this effect. It consists of a septet with intensities in the ratio of However, it does not seem to involve direct photoioniza1 :2 :3 :4 :3 :2 : 1. It displays large hyperfine anisotropy; tion of I-. Hole transfer from I to MP is unlikely, since the first-order splitting A ranges from 101 G at e = 0" the reverse reaction (eq 1) is efficient zven for C1-. to 9 G a t 0 = 90" (e is the angle between molecular The most effective Cd line at 2144 A falls within the axis and magnetic field). The line mI = +1 is least onset of the CTTS band of MP. lo It is overlapped from sensitive to angular spreading, with H = 3190 f 10 G its long wavelength side by a weak band, which is at 9.3 kil'Ic/sec. Starting with this line, the anisotropy probably due to some n -+ T * transition in the phossteeply increases as mI either increases or decreases. phate polymers (similar spectra are exhibited by simple For random orientation of Clz- in glass this should lead to gradual smearing of the components on both sides of (14) T. G. Csstner and W. Kanzig, Phys. Chem. Solids, 3 , 178 mI = +1. ThosewithmI = -3, -2, -1, and +3 are (1967). The Journal of Physical Chemistry
1-
PHOTOSENSITIZED
REACTIONS IN
n/IETAPHOSPHATE
3771
GLASS
*5t
I 300
P
-0-p-0
I
Amp
400
Figure 4. Radiation-induced changes in absorption spectra of glasses doped with halides. Curve 1: 1 iM KBr 3 X lo-’ M K I after 30 min exposure to Cd light. Curves 2 and 3: 4 iM KC1 0.15 M KBr 5 X M KI, after exposure for 10 min to X-rays and 15 min to Cd light, respectively. Curve 4: 1 1%’ KBr after 45 min exposure to Cd light. (See remark to Figure 1.)
+
+
II
0-... 0-I!-0-
1
7-
I. .p-0-
II 0
Figure 5 . Exciton-type mechanism for I - photosensitized degradation of metaphosphate glass.
+
phosphate ions’6). Thus direct excitation of M P to its CTTS state a t 2144 1 should be of little probability. Efficient CTTS-CTTS energy transfer from X- to M P may still occur if other competing reactions are relatively slow. Now, from experiments at 1849 As it is known that CTTS excitation of NIP leads to formation of the “electron” and hole center. The nature of the CTTS state is not yet clear even in the case of X-. Its assignment rests on some environmental effects, which do not provide a sensitive tool for studying the detailed charge distribution. There is little reason t o believe that the same model will apply to the CTTS state of I- and a polyphosphate ion. Moreover, the question arises as to the nature of this state in glassy matrices, which possess neither the ordered structure of crystals nor the dipolar polarization properties of liquids. Still, the spectrum of I- in boric acid glass’ is rather similar to that in aqueous solution or K I crystal. For this discussion, which is concerned with primary processes involving CTTS states, a crude picture of a hole-electron pair will be used. This pair is mobile in crystals (exciton), confined to the excitation site in the case of randomly dispersed simple ions, and probably shows some freedom of migration along polymer units. Thus in XIP it is likely to move freely along the polyphosphate chain, thus more closely resembling an exciton. When such “exciton” reaches a relatively weak chemical bond, some kind of internal conversion to dissociative state can occur. Such conversion was shown to be very efficient in 1\CS-.16 Figure 5 describes a possible mechanism for formation and destruction of “excitons” in I--MP glass. A highly stretched P-0 bond, momentarily formed, should be favored as a site of degradation. Subsequent hole migration away from the ruptured bond should stabilize the hole and “electron” centers, trapped separately on two radicals. Altogether, the net reaction is Iphotosensitized production of MP color centers. According t o the proposed picture, the I- photosen-
sitized oxidation of C1- and Br- is a dark reaction represented by eq l . I 7 Halide ions compete with matrix on the holes, which are produced with a quantum yield -0.2. This view is further verified by the experiment with halide mixtures (Figure 4). Owing t o its low concentration, I- could not play any significant role in the X-ray-induced changes and still they were the same as induced by Cd light. Evidently, C1- and Br- compete on the same precursor, which can only be the MP hole center. The mechanism of energy transfer from I- to phosphate polymer is not clear. Preliminary study of emission revealed that excitation of I--MP glass led to some broad emission above 250 mp, which was not displayed by the base glass. Its nature is under investigation, but it seems to originate from MP. The emission spectrum of I- in glass is expected to be resonance fluorescence. The breadth of its absorption band presumably reflects distribution of environments and since the rigid matrix is not likely to relax fast, emission is expected t o be at the same wavelength as absorption. Thus overlap between an emission spectrum of I- and CTTS absorption of M P should increase with hv. This explains the effect of wavelength on quantum yield. The light-absorbing halide may also undergo direct photoionization, in parallel t o energy transfer. However, the absence of any esr signals that could be assigned to physically trapped electrons suggests that this is a minor process. Thus energy transfer should be much faster than the dissociation process. Owing to low absorption of M P in the region involved, the former is unlikely to be very fast; its pseudo-first-order rate constant is probably not much larger than the radiative rate constant, -los sec-’. This suggests that dissociation is hampered in glass. However, the quantum yield of I- emission should be determined in order t o verify this view. (15) M. Halmann and I. Platzner, J. Chem. Soc., 1440 (1965). (16) M. Luria and A. Treinin, J. Phys. Chem., 72, 305 (1968). (17) The possibility that some oxidation product of I- (or reduction product of MP) is the photosensitizer should also be considered. However, photosensitization was found to occur under conditions where light was in effect totally absorbed by I-.
Volume 78, Number 11 October 1968
DAVIDC. WALKER
3772 I n conclusion, our results provide further evidence to the existence of excited halide ions, which in addition to deactivation and dissociation can undergo chemical quenching. They also indicate that apparently inert matrix material may act as a chemical quencher, when
its absorption spectrum tails into the region of exci8tation. A low intrinsic probability for energy transfer can be compensated by a large concentration of quencher and a decrease in rates of other competing processes.
An Efficient and Highly Selective Radiation-Induced
Isomerization in the Crystalline State by David C. Walker Chemistry Department, University of British Columbia, Vancouver 8, British Columbia, Canada (Received May 7, 1968)
This paper presents an evaluation and discussion of the G value for a remarkable radiation-induced isomeric transformationof a crystalline organic ester during X-ray crystallographicanalysis. The high yield [G(isom) > 571 cannot result from a chain reaction but appears to be a real case of extremely efficient energy transfer or collective excitation peculiar to the pure crystalline state.
Introduction I n a determination of the structure and absolute configuration of hirsutic acid by X-ray crystallographic analysis of its p-bromophenacyl ester, Comer and Trotterl observed an unusual molecular rearrangement induced by the analyzing X-ray beam. They found an approximately equimolar mixture of the original (I) and rearranged (11) compounds (see Figure 1) after about 10 hr exposure to the soft X-rays and that further exposure to the X-rays caused no further chemical or crystallographic change. Comer and Trotter confirmed their crystallographic results by separating an irradiated crystal, by preparative thin-layer chromatography, into two chemically distinct compounds, shown by chemical analysis and ir, uv, and nmr spectroscopy to be consistent with structures I and I1.lr2 These authors noted the following additional features: (i) the crystal structure was not disrupted as a result of the rearrangement and there were only very minor changes in lattice parameters and intensities of reflections; (ii) the two different molecules were apparently randomly distributed at lattice sites; (iii) the irradiated (but not the unirradiated) crystal contained O--Hhydrogen bonds; (iv) irradiation of hirsutic acid itself did not lead to any rearrangement, whereas the methyl ester showed a rearrangement similar to the p-bromophenacyl ester; and (v) the rearrangement could not be induced thermally or by prolonged exposure to sunlight. From a radiation chemistry standpoint it is clear that, e
The Journal of Physical Chemistry
0
for every other molecule to be altered in exactly the same manner, the rearrangement cannot be a primary radiation effect but must result from a highly selective, rather efficient, secondary utilization of the energy. The purpose of this short paper is to report some radiation dosimetry for the conditions used by Comer and Trotter and to evaluate a radiation yield for this rearrangement.
Experimental Section and Results (a) Dosimetry. The X-rays from a 40-kV X-ray tube having a Cu target and Ni filter will correspond predominantly to the Cu K a line at 8.0 kV. X-Rays of this energy will interact mainly by photoelectric processes, and the absorption coefficient will have a strong dependence on X-ray energy and atomic number of absorber. Consequently, it was decided to measure the total energy incident upon a chemical dosimeter rather than rely on the calibration of a photographic film or ionization chamber. A hole 0.8 cm in diameter was cut through the lower part of one of the ground-glass walls of a 1-cm square cross section silica spectrophotometric cell and this hole was covered and sealed with polyethylene sheet
