2536
J. Phys. Chem. 1983, 87, 2536-2544
Photochemistry of 9,1O-Anthraquinone-2-sulfonate in Solution. 1. Intermediates and Mechanism I. Loeff, A. Trelnln, Department of Physical Chemistry, Hebrew University, Jerusalem 9 1904, Israel
and Henry Llnschltz’ Department of Chemistry, Brandeis lJnlversi@, Waltham, Massachusetts 02 154 (Received September 14, 1982; I n Final Form: December 28, 1982)
The photochemistry of aqueous 9,10-anthraquinone-2-sulfonate (AQS) wm investigated by using four different approaches: (1)laser photolysis of AQS in water as a function of concentration and pH; (2) laser photolysis of AQS in CH3CNand CH3CN/H20mixtures, combined with emission spectroscopy;(3) steady-state photolytic study of the effect of AQS concentration and pH on the yield of photohydroxylation in water; (4) quenching of intermediates and inhibition of photohydroxylation by inorganic anions. Our results lead to identification of triplet AQS (7 100 ns in water) and two other intermediates (Band C) which are formed by two parallel reactions of triplet AQS with H20. The nature of these intermediates is still uncertain, but evidence is presented to rule out H abstraction or net electron transfer (even in the case of OH-) leading to formation of free OH radicals. The possibility that B and C are two different water adducts is discussed. The role of preferential solvation of AQS in CH3CN/H20mixtures in determining its photochemistry is also examined. Species C (Ama 600 nm) is the only transient observed which appears to react with ground-state AQS, and this reaction is considered to be responsible for photohydroxylation. Our results provide direct evidence for the validity of the “3AQS/H20”mechanism proposed by Clark and Stonehill (CS), in which the primary step is reaction of 3AQSwith water (and not with ground-state AQS) to produce the hydroxylating agent. However, this mechanism is modified for pH 511 by proposing another hydroxylating agent which may be AQS-aOH exciplex (or radical pair) produced by charge-transfer (CT) quenching of 3AQS by OH-. Evidence is presented to establish the charge-transfer nature of quenching of triplet AQS by various anions including OH-.
-
-
Introduction The photochemistry of 9,lO-anthraquinonesulfonatesin aqueous solution, in particular that of the “strong sensitizers” 9,10-anthraquinone-2-sulfonate (AQS) and 9,1O-anthraquinone-2,6-disulfonate (AQDS), has received much attention1 because of its relevance to some important photosensitizing effects induced by anthraquinones, phototendering of cellulosic materials, and photosensitized oxidation of various substrates. Recently, interest in this area has been heightened by suggestions for utilizing anthraquinones as photocatalysts in solar energy storage as in photooxidation of chloride ion to chlorine2and splitting of water.3 However, despite very extensive research that has been conducted on the photolysis of AQS and AQDS in water, these systems display some features which have eluded all previous attempts to construct a reasonable mechanism that accounts for the overall photolytic effects in terms of the intermediates involved. With excess of efficient H donors such as alcohols, a simple C=O C(OH) mechanism appears to explain the results.’ The possibility of the abstraction of the hydroxylic hydrogen was also ~uggested.~ However, when the alcohol concentration in aqueous solutions is lowered, and in particular
-
(1) For reviews, see: (a) J . F. McKellar, Radiat. Res. Reo., 3, 141 (1971); (b) A. V. El’t.gov, 0. P. Studzinskii, and V . M. Grebenkina, Russ. Chem. Reu. (Engl. Traml.), 46, 93 (1977); (c) I. H. Leaver in
‘Photochemistry of Dyed and Pigmented Materials”, N. S. Allen and J. F. McKellar, Eds., Applied Science Publishers, Barking, h e x , England, 1980; (d) J. M. Bruce in “The Chemistry of the Quinonoid Compounds”, S.Patai, Ed., Wiley, New York, 1974. (2) H. D. Scharf and R. Weitz, Tetrahedron, 35, 2255 (1979); in “Catalysis in Chemistry and Biochemistry, Theory and Experiment”, B. Pullman, Ed., Reidel, Dordrecht, The Netherlands, 1979. (3) I. Okura and N. Kim-Thuan, Chem. Lett., 1569 (1980). (4) K. A. McLauchlan and R. C. Sealy, J. Chem. Soc., Chem. Commun. 115 (1976). 0022-36541 83/2O87-2536$O1.5OlO
in the absence of alcohol, the photolysis becomes more complex: reduced and hydroxylated products (mainly CYand P-hydroxyanthraquinonesulfonates)are formed with appreciable quantum yields depending on quinone concentration and pH.596 Since intersystem crossing is very efficient (@kc2 0.9),7 all proposed mechanisms start with some reaction of the triplet with water (Q*/H,O mechanism), ground state (Q*/Q mechanism), or with another triplet (Q*/Q* mechanism). The simplest Q*/H20 mechanism, which is favored by the Russian school,lb involves free OH radicals produced by the reaction 3Q* H 2 0 QH + OH pK = 3.9 (ref 8)) (QH F! Q- H+
+ +
-
-
or in alkaline solutions 3Q* + OH- Q- + OH However, a comparative study of the effects of pH and various OH scavengers on the photolysis and y radiolysis of aqueous AQS seems to be at variance with this OH mechani~m.~ Still, it is much in use (e.g., ref lo), probably because evidence against it is indirect (overall yields) and no other satisfactory mechanism has been established. A similar OH mechanism has been proposed for the photo(5) (a) K. P. Clark and H. I. Stonehill, J. Chem. SOC.,Faraday Trans. 1 , 68, 577 (1972); (b) Zbid., 68, 1676 (1972). (6)(a) A. D. Broadbent and R. P. Newton, Can. J. Chem., 50, 381 (1972); (b) A. D. Broadbent, H. B. Matheson, and R. P. Newton, Ibid., 53, 826 (1975). (7) K. Tickle and F. Wilkinson, T r a m . Faraday SOC.,61,1981 (1965); A. A. Lamola and G. S. Hammond, J . Chem. Phys., 43, 2129 (1965). (8) E. Hayon, T. Ibata, N. N. Lichtin, and M. Simic, J.Phys. Chem., 76, 2072 (1972). (9) K. P. Clark and H. I. Stonehill, J . Chem. Soc., Faraday Trans. 1 , 73, 722 (1977). (10) H. Inoue and M. Hida, Chem. Lett., 107 (1979).
0 1983 American Chemical Society
Photochemistry of 9,lO-Anthraquinone-2-sulfonate
hydroxylation of other carbonyl compounds in water, e.g., benzophenone," and therefore it is important to examine thoroughly its possible role. A different Q*/H20 mechanism not involving free OH radicals was also presented5but the available experimental results have been insufficient to establish it firmly. In particular, little is known regarding the nature of the intermediates involved and their role in photolysis. Several flash photolytic studies have been published12-15but only recently has the short-lived triplet absorption of AQDS in water been re~0rded.l~ Another longer lived intermediate (T 50 ps) was fist observed by Phillips et al.13and falsely identified as the triplet.'* In the present work, we provide more detailed information on the intermediates involved in the photolysis of aqueous AQS and show that a modified version of the Q*/H20 mechanism as proposed by Clark and Stonehil15 (CS) can account for the experimental results on the time scale from ca. 40 ns to final products. Four different sets of experiments were conducted (1)laser photolysis of AQS in aqueous solutions at various concentrations and pH values; (2) laser photolysis of AQS in CH3CN and CH3CN/H20mixtures; (3) steady-state photolysis study of the effects of AQS concentration and pH on the yield of photohydroxylation; (4) detailed study of the effects of inorganic anions on the intermediates observed (laser photolysis) and on the final yield of photohydroxylation. Some early work on the effect of halide ions on the photochemical reduction of AQS and AQDS by alcohols had been reported16 and the effect was attributed to triplet quenching. Addition of N3-, C032-,Br-, I-, and NCS- was found later5ato diminish the rate of hydroxylation in the absence of alcohols, and the question was raised as to whether this effect might involve OH radicals or other intermediates. Here we present only those results pertaining to identification of the intermediates. Ionic effects are the subject of part 2 of this series,17in which we investigate the possible use of anthraquinones as photocatalysts in the aerobic oxidation of halide ions in solution.
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983 2537 I
l
-
Experimental Section Materials. Sodium 9,10-anthraquinone-2-sulfonate (Fluka, puriss, p.a.) and the inorganic materials (all of AnalaR, puriss., or analytical grades) and acetonitrile (Fluka "Garantie" for UV spectroscopy and Burdick and Jackson UV grade) were used without further purification. Water was triply distilled. Apparatus and Procedure. Flash Photolysis. Most of the laser photolysis experiments were conducted with an Avco-Everett N2 laser (337.1 nm, 10 ns, 0.5 mJ) as previously described,lsa with 1-cm optical cells as reaction vessels. Kinetic traces of transients with T > 200 ns were digitized and averaged (at least 16 pulses) by using a combined Biomation 8100 (digitizer)-Nicolet (computer) system and displayed on an X-Y recorder. Part of the work was repeated with a Q-switched frequency-doubled (11)R. V. Bensasson and J. Gramain, J . Chem. SOC., Faraday Trans. 1 , 7 6 , 1801 (1980).
(12)N.K. Bridge and G. Porter, Proc. R. SOC.London, Ser. A, 244, 259 (19.581. _
_
_
\
(13)G. 0.Phillips, N. W. Worthington, J. F. McKellar, and R. R. Sharpe, J. Chem. SOC.A , 767 (1969). (14)(a) V. A. Kuzmin and A. K. Chibisov, Chem. Commun., 1559 (1971);(b) V. A. Kuzmin, A. K. Chibisov, and A. V. Karyakin, Int. J . Chem. Kinet. 4,639 (1972). (15)A. Harriman and A. Mills, Photochem. Photobiol., 33,619(1981). (16)H.R.Cooper, Trans. Faraday SOC.,62,2865 (1966). (17)Part 2 of this work, to be submitted for publication. (18)(a) C. R.Goldschmidt, M. Ottolenghi, and G. Stein,Isr. J. Chem., 8,29 (1970);(b) G. Dolan and C. R. Goldschmidt, Chem. Phys. Lett., 39, 320 (1976).
a- . I
400
I
1
I
500
600
h,nm Flgure 1. Transient spectra produced in unbuffered aqueous AQS solution at various time delays after beginning of flash (Nd laser, 353 nm). [AQS] = 3 X lo-' M, pH -6, aerated solutions. Each point is the average of three measurements using fresh samples for each flash. Insert: Oscilloscope traces at 450 and 510 nm.
ruby laser (Holobeam, Series 600) and associated equipment.Ig The results (spectra and kinetics) were essentially the same as those obtained with the N2 laser. Several experiments were also conducted by irradiation at 353 nm with a frequency-tripled neodymium laser (for details see ref 18b), which was found to give better resolution below 400 nm. In the analysis of transient absorption, no correction was introduced for depletion of ground-state AQS since its extinction coefficients at all X (down to 380 nm) are much lower than that of the transient (essentially triplet AQS). For relatively long-lived transients ( T 2 50 ps) a conventionalflash photolysis apparatus (2-kJ energy, -10-ps r l I 2of flash) was also employed with a reaction vessel of 18-cm optical path and an outer jacket filled with 0.01 M potassium biphthalate solution to block light below 300 nm. The flash photolysis apparatus was similar to that described elsewherea20 Solutions were deoxygenated by bubbling with NP This simple procedure was found to give practically the same results as repeated freeze-thaw degassing on the vacuum line, even for acetonitrile solutions where the triplet lifetime is much longer than in water. To minimize thermal reaction in alkaline solutions, the appropriate amount of NaOH was added to the degassed solution (which was then further degassed) just before flashing. Neutral solutions (pH -6) were unbuffered and HC104 was used to lower the pH. Steady-State Photolysis. Solutions in 1-cm optical cells were irradiated at 365 nm by using a medium-pressure Hg lamp (Thorn Lighting, 125 W) through a Corning 7-60 filter with continuous O2 flushing during irradiation. The yield of hydroxyanthraquinone was determined spectroph~tometrically.~ No distinction between CY- and P-hydroxyanthraquinone was made as long as their molar ratio remained unchanged; otherwise, the necessary correction was introduced for the change in their molar ratio with pH5bwhen attempting to study the effect of pH on the yield of photohydroxylation. Photolysis was carried out to less than 5% conversion, and there was no significant inner filter effect at 365 nm. Emission spectra were obtained with a spectrofluorimeter (SLM, Urbana, IL, Model 4800) and emission lifetimes were measured by using the N2 laser apparatus and asso(19)L. J. Andrew, A. Deroulede, and H. Linachitz,J. Am. Chem. SOC., 82, 2304 (1978). (20)L. Dogliotti and E. Hayon, J. Phys. Chem., 71, 2511 (1967);M. Langmuir and E.Hayon, Ibid., 71,3808 (1967).
2538
The Journal of Physical Chemlstty, Vol. 87, No. 14, 7983
Loeff et al.
0,451
I
m-
I
I
I
400
I
I
500 X ,nm
600
Figure 2. Absorption spectrum of triplet AQS (transient A) in water
I
0.01 M H C I O 4
A , nm Figure 3. Spectrum of transient B in unbuffered (pH -6) and lo-* M HCIO, solutions. [AQS] = 2 X lo-, M, air free, 2.5 ps after pulse (N2 laser). The two spectra were produced at different light intensities and therefore were normalized to almost equal peak intensity.
(0) and acetonitrile (0).The spectrum in water was derived by extrapolation of the data represented in Figure 1 to t = 15 ns. The
spectrum in W,CN was taken 40 ns after flashing a 8 X lo-' M AQS solution with a frequencydoubled ruby laser (Amx = 341 nm).
ciated photometer and oscilloscope, with the measuring beam cut off.
Results I . Intermediates. A. Aqueous Solutions. Figure 1 shows transient absorption produced by laser photolysis in neutral AQS solution at various time delays. Three transients could be readily separated by their different decay rates and spectra. One of them, designated C, is suppressed in acidic solutions and therefore the other two could be better analyzed in ca. M HC104. Transient A. Transient A is the only species observed which is totally formed within the flash duration and it has the fastest decay rate. Its rate constant kdAcould be readily determined in the wavelength range 420-470 mm, since A is followed by transient B with lifetime which is -500 times longer (Figure 1, insert). From linear plots of In (D - D,) against time, kdA= (1.0 f 0.2) X lo7 s-l was derived for neutral solutions (D-was taken as the relatively constant absorbance around 0.5 ps). No distinct effect on k d A was detected by (a) bubbling the solution with O2 M), (b) raising the concentration of AQS from to M, (c) replacing HzO by DzO, (d) changing pH in the range 1-10 by adding HC104or NaOH. In acidic solutions, the kinetics were better defined with kdA(average) closer to 1.1 X lo7 s-l, because interfering absorptions were suppressed (transient C and in basic region also transient B with lifetime shortened by alkali). Therefore, 1.1 X lo7 s-l is taken as the preferred value of kdA.Above pH -11.5, kdA was found to increase linearly with [OH-] (section ILA). To obtain the spectrum of A, free as possible from that of subsequent intermediates, In (D - D,) plots were extrapolated to within the flash duration (back to flash midpoint). The spectrum (Figure 2, curve a) displays a strong band peaking at 380 mm and a weaker band centered around 460 nm. Transient B (Figure 3). Transient B grows in parallel with decay of A as is evident from the oscilloscope traces around 500 nm (Figure 1, insert); its apparent growth constant measured in 10-3-10-2 M HC104is (1.0 f 0.1) x lo' s-l. Two isosbestic points at 480 and 550 nm are clearly exhibited in acidic solutions (Figure 4). Transient B has a broad absorption peaking around 480-490 nm. Its decay L first order with kdB = (2.0 f 0.2) X lo4 in neutral or acid solutions (up to M HC104) but it appears to react rapidly with OH- and with the Lewis bases borate and phosphate ions which were tried
A
-,: 0
r:*.o..'0. . . o
I
I
1
I
500
400
*-L I
-
-&
--*
600
X ,nm Figure 4. Transient spectra produced in lo-' M HCIO, solution at various time delays after beginning of pulse (N2 laser). [AQS] = 3 X lo-' M, aerated.
as buffers, even at lo4 M base. Its decay rate is somewhat enhanced also in more strongly acidic solutions, e.g., kdB = 3.5 X lo4s-l in 0.1 M HC104solution. No distinct effect on kdBwas observed on (a) bubbling the solution with O2 M) or (b) raising AQS concentration from 2 X to 7 X M. However, at [AQS] k M, addition of M HC104)was necessary in order to free acid (e.g., 5 X the absorption of B from that of other longer lived species. Transient C (Figure 5, Insert I). Its absorption around 600 nm overlaps that of A (Figure 4) from which it is produced. This is indicated by a buildup around 600 nm prior to decay (Figure 5). Such traces could be analyzed in terms of two consecutive reactions kc
kdC
A-Cwith any other reactions, particularly production of B, competing on A so that kdAis its overall decay constant. Assuming that the observed absorption is a superposition only of A, B, and C and that time t is short enough for no considerable decay of B to occur, we obtain D = Do eXp(-kdAt) + D,[1 - eXp(-kdAt)] +
where a, = tCkC/tA,and tc and t A are the extinction coefficients of C and A, respectively, at given wavelength. The three terms on the right are the absorbances DA, DB,
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983 2539
Photochemistry of 9,lO-Anthraqulnone-2-sulfonate
I
I
I CH CN
PSPH,~
( s c a l e for c u r v e c )
0.25
0.50
1 I
I
,
0.75
Hz0
I
I
IC
0 W
I' I
v)
! I
0
2
-
d
+ n
am Y C
I
200
t , nsec Figure 5. Kinetics at 600 nm as resolved by computer analysis in terms of the three intermediates involved, A. B, and C. [AQS] = 2 X lo4 M, unbuffered (pH -6), air free. Curve a: experimental results (0)and computed curve (continuous curve): curves A, B, and C: computed contributions of the corresponding intermediates. Insert I : longwavelength translent spectra produced in unbuffered AQS solution at various time delays after beginning of pulse (N2 laser). [AQS] = 3 X lo4 M, aerated. Insert 11: Stem-Vdmer plot for the suppression of transient C by OH-.
and Dc, respectively. A good fit with kinetic traces could be obtained by using eq 1 and a computer programz1with four adjustable parameters: Do, kdA, k d c , and CY^. The results are shown in Figure 5, together with the various contributions to the overall kinetics, from which the values kdA = 9.3 x 106 s-1, kdC = 3.0 x 106 s-1, CY, = 2 x 107 s-l were derived. More simply, k d C was obtained from first-order analysis of the declining part of the kinetic traces at 600 nm after -400 ns, when the decay of A is practically complete. As with A and B, no effect of Oz on kdCwas observed. On the other hand, C is the only intermediate with lifetime depending on the concentration of AQS: k d C = (2.8 f 0.3) X lo6 (6 f 1) X 108[AQS] s-l In alkaline solutions, the properties of C could be determined only in a rather limited pH range (-11-12). Below this range, the decay of B, accelerated by alkali, could not be well separated from that of C; above pH 12, reduction in the yield of C and interference by absorption of longer lived species limited the analysis. Nevertheless, from studies within this restricted pH range, the following conclusionswere drawn: (I) the rate constant kdC appears to be unaltered in alkaline solution; (2) the yield of C definitely decreases with increasing pH; the relative initial yield of C, as obtained by extrapolation of firsborder decay back to the flash midpoint, follows a Stern-Volmer relation (Figure 5, insert 11): D o / D = 1 + (37 f 7)[OH-] where Do and D are the extrapolated absorbance values at 600 nm in neutral and alkaline solutions, respectively. As noted previously, C is strongly suppressed in acidic solutions, a decrease being noted even at pH -4. Both its initial yield and lifetime appear to decrease with decreasing pH, and after its full decay the growth of a new long-lived species is observed at 600 nm. The behavior of the system in acidic solutions is still under investigation. B. Acetonitrile Solutions. Figure 2 (curve b) shows the transient absorption produced by laser photolysis in aer-
+
(21) A standard library program of the Hebrew University Computer Center, based on minimizing the s u m of squared deviations of the thecretical experimental values.
CH N ,
0.25
0.50
0.75
X H ~ O( s c a l e for c u r v e s a 8 b 1
-
HO ,
Figure 6, Decay rate of triplet AQS in H,O/CH,CN mixtures as functions of mole fraction (curve a, lower scale) and "preferential solvation parameter" (see text, c, upper scale) of the solvent. The rate constants kdAwere obtained from ekher absorption (0) or emission (0)measurements. (k,* values for curve c correspond to those taken from curve a.) Curve b: the long-lived absorption D, relative to that of the triplet DT, both measured at 470 nm, as a function of xHzo(air free). D, was taken as the nearly constant absorbance following the triplet decay: DT was measured -40 ns after flash. Insert: emission spectrum of AQS in CH,CN. [AQS] = 8 X lo-' M, air free.
ated acetonitrile solutions of AQS (ca. lo4 M). Ita shape resembles that of transient A, with the bands at X > 400 nm somewhat red shifted and less well-defined. Its decay leaves a weak, long-lived absorption, which becomes more apparent in air-free solutions. This intermediate absorbs strongly below 400 nm but bears no resemblance to intermediate B. Similarly, no transient analogues to C could be detected. The decay of A in CH3CN is first order with the same decay constant, kdA = (7 f 2) x io4s-l in air-free solution, throughout the wavelength range 400-600 nm. The apparent decay constant increases linearly with Oz concentration and becomes (1.9 f 0.1) X lo6 s-l at 1.35 X M 02.(This is the solubility of O2 in CH3CN at 1 atm, 25 OC.zz In parallel with this quenching effect, the long-lived absorption decreases and, in 02-saturated solution, transient A is practically the only intermediate observed at X 5 400 nm. The flashed acetonitrile solutions were found to emit light; the emission spectrum is shown in Figure 6 (insert). Its decay is first order with k A = (8 f 2) X lo4 s-l, irrespective of wavelength. It is also quenched by O2at the same rate as A, and therefore A is clearly responsible for this emission, directly or indirectly (e.g., delayed fluorescence). Hence, it was found more convenient to study various kinetic effects on A by following its emission; e.g., no concentration quenching occurs on raising AQS concentration from 2 X lod to 4 X lo-' M (saturated solution). The effect of water on A was studied in CH3CH/H20 mixtures by following both absorption and emission. Its lifetime decreases on adding H20and is reduced to 100 ns in pure water, but the main change occurs only at high (22) M. Brandeis, unpublished results, obtained by gas chromatography.
2540
Loeff et ai.
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983
TABLE I: Comparison of Flash and Steady-State Stern-Volmer Kinetics for Quenching of A (Triplet AQS) and Inhibition of Photohydroxylation Reaction by Anions EcT- 10-~k anion ET: eV M-'
NCSe-
-1.2
NO,' IN3BrNCSOH-
-1.0 -0.8 -0.7 0.0
c1-
0.0 0.2 0.6
c10,-
1.3
so,,-
3.7 3.2 4.2 3.1 3.8 3.9 0.3 0.5
46
102
76 93 95
420 400 307 350
95
74 58 87
4
[ B r - l , 10-3M 6 8
IO
I
6.0
ha+ KSVd p H 6 pH13 p H 6 p H 1 3
382 282 345 355
2
I
n
upper scale
\
n
2.o
48
510-3 510-3
a See text. The decay of A was measured at 4 4 0 - 4 5 0 nm, and the concentration of X- did not exceed 5 X lo'* M. In the case of OH', in addition to zero concentration (in effect pH -6), [OH-] was varied in the range 4 X 10-3-5 X M because at lower concentrations there is spectral interference with transient B, the lifetime of which is shortened by alkali. Since B is quenched more effectively by OH-, this interference becomes relatively M. The error in determining small at [OH-] >, 4 X k , is estimated as *lo%, except for OH' where a +30% error may be involved. 7 = l / k d A with k d A = 1.1 X lo' + ( 3 x 108)[OH-]. Determined from SternVolmer plots for photohydroxylation (see text). With the exception of Br- where [AQS] was varied (Figure 7 ) , [AQS] = 2 x M.
Figure 7. Stern-Volmer plots for inhibition of photohydroxylation of 0,-saturated AQS solutions by inorganic anions. Absorbance D was measured after 5 min of irradiation at 365 nm, Curve 1 (upper scale): KBr, unbuffered (pH -6), [AQS] = 1.6 X l o 3 (A), 4 X lo3 (O),and 8X (A)M. Curve 2: NaCi, unbuffered [AQS] = 2 X M (V).Curve 3: KBr, 0.1 M NaOH, -[AQSl- = 4 X lo3 (0) and 8 X lo3 (VIM.
l / d = L I M I T l N G YIELD ( [ A P S I - @ l
mole fraction, xH2o2 0.9 (Figure 6, curve a). I n parallel with this effect, transients B and C are produced at rates which equal the decay rate of A. The long-lived absorption (mainly B) relative to that of triplet, both measured at 470 nm as a function of xH (Figure 6, curve b), follows a pattern similar to that of K dt.2. Owing to low absorptions it was difficult to carry out a similar study at 600 nm. II. Quenching Effects by Inorganic Anions. A . Quenching of Intermediates. Quenching of A in water was studied by measuring its decay in the presence of various anions. The results are summarized in Table I. In some cases (e.g., NO2-),quenching is accompanied by net redox reactions leading to production of semianthraquinone; in other cases (e.g., halide ions) the redox reactions producing AQS- and X, occur only at quencher concentration which is higher than that required for almost total quenching of A.17 Special attention was paid to the quenching of A by OH- ( K = (3 f 1) X los M-' 8-9 because of its relevance for understanding the photolysis in alkaline solutions. The effect of C1-, Br-, I-, and NCS- (up to 3 X M) on transients B and C was also investigated. No distinct influence on their decay kinetics was observed, but their formation yield was found to be suppressed in parallel with quenching of A,17 as previously reported for OH- (section LA). B. Inhibition of Photohydroxylation. With the exception of OH-, the anions which quench intermediate A also inhibit the photohydroxylation of AQS. In the case of Br- which was examined in detail (up to lo-' M anion) the final spectrum showed no evidence for new stable products which result from the quenching. The yield of hydroxylated anthraquinone (a and p together) in 02saturated solutions was found to obey a Stern-Volmer relation D(O)/D(X' = 1 + KsvP-I where Do)and DCX-)are the product absorbance values in the absence and presence of quencher, respectively, all other conditions kept the same. Such Stern-Volmer plots are shown in Figure 7 for Br- (at various concentrations
P 0
I
1
I
I
I
I
2
4
6
0
IO
I2
PH
Flgue 8. Dependence of photohydroxylatknyield on pH. Absorbance D was measured after 5-min irradiation at 365 nm. 0,-saturated. Curve 3: [AQS] = 2 X lo-' M. Curves 2 and 1: d and c are the parameters of the Stern-Volmer relation, 1/D = d c/[AQS] (see text).
+
and pH values) and for C1-. All the Ksv values thus obtained are given in Table I. III. Effect of p H and AQS Concentration o n Photowe hydroxylation. In agreement with previous found that the rate of photohydroxylation,at constant pH and light intensity, follows a Stern-Volmer relation
1/D = d
+ c/[AQS]
where D is the absorbance of hydroxy derivatives after a fixed time of irradiation and c and d are constants. Figure 8 shows the dependence on pH of (1) c / d (2) l / d , and (3) rate of hydroxylation (absorption of product per minute of irradiation at constant [AQS]). Again, in agreement with previous work, c/d appears to be constant in the pH range 1-13, although we find that its average value, c l d = (3.4f 0.5)X M, is -50% higher than that previously reported.68 But contrary to previous reports, the rate of hydroxylation is not constant all through the pH range 2-11; our experiments show a gradual decline below pH -4. The same decline is displayed by l / d , which represents the rate at infinite concentration of AQS. Similar results were obtained with two different acids: HCIOl and H2S04. Discussion A. Triplet AQS. The initial transient absorption observed in acetonitrile resembles previously reported ab-
Photochemistry of 9,1O-Anthraquinone-2-sulfonate
sorptions of triplet 9,lO-anthraquinone in hydrocarbon glass2%and in benzene,%and of triplet AQDS in water.15 Moreover, this absorption transient is accompanied by an emission having the same lifetime and sensitivity to oxygen, and whose spectrum is similar to the phosphorescence of 9,lO-anthraquinone in various The transient species A may thus be safely identified as the AQS triplet. The apparent 0,O emission band at 464 nm corresponds to E T = 61.6 kcal/mol in acetonitrile. The oxygen quenching measurements give kq(oz)= 1.4 X 108M-' s-l, which is much lower than the value 1.5 X lo9 M-ls-l reported for oxygen quenching of triplet 9,lO-anthraquinone in benzene. The reason for this low k, of AQS in CH3CN is not clear. Replacement of acetonitrile by water has a small effect on the triplet AQS spectrum but very sharply reduces its lifetime. These observations and the parallel production of transients B and C at the same rate as triplet decay (Figure 1,insert, and Figure 5) indicate that 3AQS*reacts rapidly with water. The reaction-limited triplet lifetime in water is so short that kq(Oz)could not be measured at convenient oxygen pressures. At 1atm, and assuming k lo9 M-' 8 , the value of k,[o2]is only about 10% of kdA: Thus, no effect of aeration on the triplet lifetime in water can be seen. The long-lived residual absorption seen in acetonitrile following triplet decay, and not yet identified, suggests that a relatively slow 3AQS*-solventreaction occurs here also, and may account for the reduction of triplet lifetime from 425 ps in Freon23bto 20 ks in acetonitrile. The dependence of triplet lifetime and the yield of the long-lived transient (essentially B) on the composition of the acetonitrile/water medium, shown in Figure 6, together with the interpretation given above, suggests that the acetonitrile component is strongly retained around the AQS center, thereby blocking the water reaction until the mole fraction of water exceeds 0.9. A quantitative measure of such preferential fractionation of acetonitrile into the AQS solvation shell may be derived from NMR studies of the rate of ligand exchange in [Cr(NCS),13-, in acetonitrile/water mixtures.% Here, one defines a "preferential solvation parameter", P S P C H s C N = (V/nO)CH3CN, where n and no are the number of acetonitrile molecules in the solvation layer in the given mixture and pure solvent, respectively. Similarly, PSPH,ois defined for water in the mixture and its value (PSPH 0 = 1- PSPcH CN) for various compositions can be derive6 from availabfe data2, by interpolation. Figure 6 shows that the triplet decay rate varies almost linearly with PSPHzO,i.e. with the number of H 2 0 molecules in the solvation layer. This linear relation found for two solutes as widely different as [Cr(NCS),I3- and AQS supports the view that the fractionation effect induced by AQS in acetonitrilelwater mixtures is mainly due to rejection of CH3CN by the water structure rather than to specific solutesolvent interactions.n Thus, the PSP scale established for [Cr(NCS),13- may have general applicability for relatively large (low electrical potential) solute molecules.
-
(23) (a) W. C. Neely and H. H. Dearman, J. Chem. Phys., 44, 1302 (1966); (b) S. A. Carbon and D. M. Hercules, J. Am. Chem. SOC.,92,5611 (1970); (c) B. Nickel and R. Roden, Chem. Phys. Lett., 74, 368 (1980). (24) The shoulder displayed in emission around 430 nm (Figure 5, insert) is probably due to thermally activated delayed fluorescence (see ref 23b). This is now under investigation. (25) B. E. Hulme, E. J. Land, and G. 0.Phillips, J. Chem. SOC., Faraday Trans. I , 68, 2003 (1972). (26) S. Behrendt, C. H. Langford, and L. S. Frankel, J. Am. Chem. SOC., 91, 2236 (1969). (27) D. W. Watts in 'Physical Chemistry of Organic Solvent Systems", A. K. Covington and T. Dickinson, Eds., Plenum Press, New York, 1973.
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983 2541
The assignment of A as 3AQS*and the value given above for its energy are consistent with a charge-transfer mechanism for the quenching of A in water by inorganic anions. It has been shown28that the rate of such CT quenching of an excited molecule, 3M*, by the anion Xis related to the quantity Em - E T , where Em is the energy of the charge-transfer state, M-X(aq), and ETis the triplet energy. ETof 3AQS* was taken to be 2.68 eV, as in acetonitrile. ECT values were calculated by using the expression28
EcT = Ex(so1) - E1I2(AQS)- 4.4 eV where Ex(sol) is the ionization potential of X- in solution, and El12(AQS)= -0.60 eV is the one-electron redox potential (vs. SCE) of AQS in water.29 Table I gives the results of such calculations for the several anions considered here. The theory, previously established for other triplets,28fits the AQS case very well; the quenching rate becomes diffusion controlled for E C T - E T 5 0. The kinetic properties of transient A in water (e.g., insensitivity to 02,response to anionic effects) are in agreement with that of triplet AQS as gathered from steady photolysis (see below). B. Chemistry of 3AQS in Water. The finding that the the decay rate of 3AQS* is not affected by AQS concentration up to M rules out the 3Q/Q mechanism. It puts an upper limit k 5 108 M - ' d for the reaction between triplet and ground-state AQS, in both water and CH3CN (10% variation in k d A was within experimental error). Triplet-triplet annihilation, even with k = 1O1O M-' s-l and [3AQS*] as high as lo4 M, can hardly compete with first-order triplet decay in water, and certainly plays no role whatever in steady-state photolysis. Still another possibility that can be discarded is that 3AQS* undergoes some unimolecular reaction, e.g., fragmentation or isomerization, leading to transients B and C. It this were the case, it is difficult to see why such reaction does not occur in CH3CN. We are thus left with the Q*/H20 mechanism. A simple OH mechanism like that initiated by the reaction 3Q* + H 2 0 (or OH-) to yield free OH radicals and then followed by
-
OH + AQS AQSOH k2 = 2.7 X lo9 M-' s-l (r ef 9)
(2)
clearly cannot explain the effect of AQS concentration on photolysis as reported here and in previous W O ~ ~ S Still, . ~ ? ~ it is possible to construct a more complex free-OH mechanism that overcomes this diffi~ulty.~ However, the following observations appear to prove that no free OH radicals exist in the AQS system, even under conditions of efficient photohydroxylation, that is, alkaline solutions and (AQS) k M. (1) Our results concerning the inhibition of photohydroxylation by inorganic anions X- rule out any possibility of reaction 2 competing with reaction 3 in the system.
OH + X-
2 OH- + x
(3)
(At this stage OH- is not treated as a quencher X-.) Apart from Cl- all other anions have k3 comparable with kZmand (28) A. Treinin and E. Hayon, J. Am. Chem. SOC.,98, 3884 (1976). The constant 4.7 eV is replaced here by 4.4 eV,because, contrary to what was previously stated, 4.7 eV corresponds to Ag/O.Ol M Ag+ and not to the saturated calomel electrode (SCE) to which Ell2refers. E(AgIO.01 M Ag+) vs. SCE in CH,CN is 0.29 V; see A. I. Popov and D. H. Geske, J. Am. Chem. SOC.,79, 2074 (1957). (29) Reference 5a, and also D. Meisel and P. Neta, J.Am. Chem. SOC., 97, 5198 (1975).
2542
The Journal of Physical Chemistty, Vol. 87, No. 14, 1983
Loeff et ai.
therefore, under the conditions employed, such a competition requires that the hydroxylation rate, relative to that in absence of quencher, should be a function of [AQS]/ [X-1. But this is not the case, as is evident from the behavior of systems containing the same concentration of Brand different concentrations of AQS (Figure 7). On the other hand, our results are in agreement with a simple triplet quenching mechanism whereby only the triplets are quenched. With this type of mechanism, whatever the exact chain of events leading from triplet to hydroxylated anthraquinones (ROH), the following Stern-Volmer relation should apply:
and in particular to establish the role of these intermediates in photolysis, we consider the following properties: (1) In neutral solutions both B and C undergo first-order decay, apparently to give back AQS and H20. (2) The decay of B is strongly enhanced by Lewis bases, which may be interpreted in terms of base-catalyzed elimination of water from the adduct as observed in related systems.34 (3) On the other hand, C is strongly suppressed in acidic solutions. This,and its long-wavelength absorption around 600 nm, may suggest an OH- adduct, but it is certainly not the basic form of B since it is not produced from B nor converted to B by change of pH. (4) Transient C is "quenched" by AQS. d&H/'$k%k = 1 + kqT[x-] C. Mechanism of Photohydroxylation. Let us summarize the main features of steady photolysis of AQS as where k , is the rate constant of triplet quenching, 7 is the reported here and in previous works: 5,6~13 (1) at constant triplet lifetime in the absence of X-, and &AH and pH the quantum yield c # Q ~ increases ~ with AQS concenare the quantum yields of ROH in the absence and prestration following a Stern-Volmer relation, reaching a limence of X-, respectively. This equation is in agreement iting value at [AQS] > M; (2) at constant [AQS], &OH with our results (Figure 7), and the values of the Sternis almost constant in the pH range 4-11 and then rises Volmer constants Ksv derived from steady photolysis exsteeply to a limiting value at pH 14, which is -4 times periments are in good agreement with kqT obtained directly higher than for neutral solutions. The rise is most profrom the laser experiments (Table I, columns 4 and 5). nounced around pH 12.5 (Figure 8); (3) below pH 4, 4ROH This agreement should convince us not only that there are drops with pH (Figure 8). no free OH radicals involved but also that triplet AQS is According to the Q*/H20 mechanism proposed by Clark the only intermediate involved in the photohydroxylation and Stonehill (the CS mechanism), one hydroxylating that is quenched by these inorganic anions under the agent AQSOH- operates at all pH values; the dependence conditions employed. Indeed, no such quenching effects of 4ROH on [AQS] results from a competition between were observed on transients B and C. decay of AQSOH- and its reaction with AQS finally to (2) In our laser photolysis experiments no indication was produce ROH; the dependence on pH reflects different found of products produced from free OH radicals. Thus, yields of triplet conversion to AQSOH-, by either H 2 0 or addition of 10-3-10-2 M KI to a solution of M AQS at OH-, the latter being -4 times more effective. (No special pH 13 produced no enhanced absorption at 390 nm where attention was given to the acidic solutions, because, in 12-has its absorption maximum,31although, even at contrast to our findings, the rate of hydroxylation was M KI, the photohydroxylation reaction is still efficient. supposed to be constant down to pH 2 (section III).) In Under these conditions any OH exists mostly as 0- which other words, the pH effect was supposed to reflect a change should preferably react with I-, since k(O-+I-) = 2.3 X lo9 from triplet interaction with H20 to triplet interaction with M-ls-l and k(0-+AQS) = 6.1 X lo7 M-l s-l (ref 9 and 30). OH-, both yielding the same intermediate AQSOH-. The same applies to neutral solutions where production Our new results provide direct information on the nature of X; and AQS- could be detected only at relatively high of intermediates involved and thus can be used to check concentrations of halide ions or NCS- (section 1I.A). Also, the validity of the CS mechanism. First we notice that, in 02-saturated solutions at pH 13, no absorption was with kq(OH-) = 3 X los M-l s- (Table I) and k d A = 1.1 x 10' detected around 430 nm that could be assigned to 03-, s-l, the quenching of 3AQS* by OH- and H 2 0 should produced by the reaction 0- + O2 F! 03-.31 proceed at the same rate at pH 12.6, and that the main (3) Direct abstraction of H from water by the triplet pH effect should occur in the pH range 11.5-13.5, as acshould lead to a considerable isotope effect on replacing tually observed and in agreement with the CS mechanism. H 2 0 by D20, which is contrary to our finding. The question now arises as to whether a single hydroxyAfter excluding the free-OH mechanism, it seems that lating agent is involved, as required by the CS mechanism, any reasonable Q*/H20 mechanism should start with ador whether the switch from H 2 0 interaction to OH- indition of water or OH- to 3AQS*, as already proposed.6 teraction also involves a change in the nature of this agent. Hence, we postulate that transients B and C are the enTo answer this question we shall first try to identify the suing addition products. Thermal and photochemical hydroxylating agent in the "H20-interaction" region. reactions of various quinones with oxygen nucleophiles H 2 0 Mechanism (PH -4-11). The only intermediate seem to involve such adducts,32with the nucleophile adding observed at X k 380 nm with properties suitable to function either to the ring or to the carbonyl group. For the phoas an hydroxylator is transient C: (1) It appears to react tochemistry of AQS these two possibilities were examined5 with AQS. (2) Unlike B, it is not affected by OH- and and it was argued that addition of H,O or OH- to the other bases through this pH range; B cannot be the hycarbonyl group is more probable. We have no clue as to droxylator, even in alkaline solution where its lifetime the structure of transients B and C, but one should exdecreases, because this decrease is significant already at amine the possibility that they are the carbonyl and benpH 9, whereas the hydroxylation yield remains constant zoid-ring adducts, respectively. (One supporting piece of up to pH 11. As stated, the effect of [AQS] on the lifetime evidence is that anthraquinone and tert-butyl alcohol form of B is difficult to determine, except in acid solution, bea quite stable carbonyl p h o t ~ a d d u c t . ~For ~ ) this purpose cause of transient absomtion overlaD which leads to an apparent increase in its-lifetime with increasing [AQS].
&$A
-
(30) Farhataziz and A. B. Ross, Natl. Stand. Ref. Data Ser. ( U S . , Natl. Bur. Stand.), No. 59 (1977). (31) G. L.Hug, Natl. Stand. Ref. Data Ser. (US., Natl. Bur. Stand.), No. 69 (1981). (32) K.T.Finley in 'The Chemistry of the Quinonoid Compounds", S.Patai, Ed., Wiley, New York, 1974.
(33) G.G.Wubbeh, W. J. Monaco, D. E. Johnson, and R. S. Meredith, J.Am. Chem. SOC.,98, 1036 (1976). The UV absorption of the adduct,
-
430 nm,was measured by us as a difference spectrum (before and after irradiation) synthesizing the adduct aa described by Wubbels et al. (34) D.Samuel and B. L. Silver, Adu. Phys. Org.Chem., 3,123 (1965).
,A
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983 2543
Photochemistry of 9,lO-Anthraquinone-2-sulfonate
TABLE 11: Rate Constants of Some Primary and Secondary Reactions Involved in the Photolysis of Aqueous AQS reaction rate constant methoda data from other sources 3AQS* total decay f k d A = 7 x 10, s-' 3AQS* + H,O B k J k , = 7.4 X lo-' M C f and sp "QS* t H,O C f and sp k , l k , = 5.2 X low3MC f 3AQS* + OH- total decay 3AQS* + OH- Z f and sp 3AQS* + OH- AQS f and sp k 7 / k , = 1.7c f B AQS + H,O kdB = 1.5 X io4 S-' -+
-+
+.
+.
-+
--t
--f
f
C t AQSC AQS t H,O (or OH-) a
5a.
k , / k , = 2.3 X M,C (3.5 f 0.5) x 10-3 ~e
f
-+
f and sp stand for "flash (laser) photolysis" and "steady-state photolysis", respectively. Reference 14b. e This work (sp).
(3) Suppression of C in acidic solutions parallels the drop in the rate of hydroxylation. With this assignment the following modified CS scheme is proposed:
+ + - ...+-
3AQS* + HzO
B
-+
AQS
HzO
3AQS* C
B HzO
+ AQS
C
ROH H20
(4) (4') (5) (8)
C AQS (9) In this scheme the reactions are numbered as before" (with B as a new intermediate by which the triplet decays) and the sequence of events leading from C (presumably AQSOH-) to ROH is omitted for simplicity. (Actually, all that is needed for our discussion is to assume that the chain of reactions leading from C to ROH in the presence of O2 does not include any more competitions involving AQS.) We have also ignored (although it is not crucial) any decay of 3AQS* not involving reaction with water, assuming that this is relatively very slow: in Freon this reaction proceeds with k 2 X lo3 s-1,23bwhereas kdA = k4 + k5 = 1.1 x IO7 s-l in water. From this mechanism we obtain k9 1 - =1- + - -1 (10) ~ R O H d~c d ~ c b[AQSI with 4~ = 4isck5/(k4 + k5) This expression is the same as found experimentally (section 111),with c / d = k g / k 8 . Table I1 summarizes the rate constants of the various reactions in this scheme. The values of k8 and kg,measured directly from the flash experiments (section LA), give the ratio k g / k s = (4.7 f 1)X M, which is in good agreement with the ratio c / d = (3.5 f 0.5) X M obtained from steady photolysis (section 111). This agreement lends very strong support to the CS mechanism, at least over the pH range 4-11. The rate constants K4 and k5 were calculated as follows: from eq 10, the limiting yield of ROH at relatively high [AQS] is &OH = 4c = 4isck5/(k4 + k5). Using the values @kc= 1,.&OH = 0.067,58 and k4 + k5 = k d A = 1.1 x lo7 S-l, we obtain k5 = 7 X lo5 s-l and k4 = 1.0 X lo7 s-l. OH-Mechanism (PH 211). With OH- as a quencher, the above scheme should be extended to include the following reactions: 3AQS* + OHZ (6) 3AQS* + OH- AQS + OH(7) Z + AQS -* ... ROH (8') Z AQS + OH(9') Transient Z is the hydroxylator in this region. According +
-
-
--
Reference 25.
Reference
to the CS mechanism, Z is identical with C and the enhanced hydroxylation on raising pH is due to k6/k7being larger than k5/k,. But if this is the case, then the yield of C should increase sharply with pH in the same manner as $ROH (Figure 8). On the other hand, if Z differs from C, then reactions 5 and 6 are two competing reactions and rjc should follow a Stern-Volmer relation
48 _ -- 1 + -kq(OH-) 4C
[OH-] = 1 + (30 f 10)[OH-]
(11)
kdA
where 4: and 4c are the respective quantum yields of C in neutral and alkaline solutions, and the SV constant should be the ratio of our directly measured values, kq(0H-) = (3 f 1)X 108M-ls-l and kdA = (1.1f 0.2) X lo7s-l. The agreement with the experimental result, Do/D = 1 + (37 f 7)[OH] (section LA), is satisfactory. This leads us to conclude that, if C is indeed the hydroxylator in neutral solutions (Le., no other intermediate which escaped our detection is responsible for this effect), then a new hydroxylator Z operates at pH 211. However, no spectral evidence for such a transient has been found. This may be due to its concealment by overlapping absorptions. The rate constants pertaining to the OH- mechanism are also included in Table 11. The constants k6 and k7 were derived as follows: at 0.1 M OH- the experimental value of &H (limiting yield at high [AQS]) is 0.26." At this pH about 73% of the triplets are quenched by OH-. The remaining 27% react with water to contribute 0.274; = 0.02 to 4iOH (putting 48 = 0.0675a)which leaves 0.26-0.02 = 0.24 to the reaction with OH-.Converting from 73% to 100% reaction with OH-, we obtain OH = 0.33 = k 6 / & k7). From this, and k6 + k7 = k q ( 0 ~ = - ) 3 X lo8 M-' s?, we obtain k6 = 1.0 X lo8 M-' s-l and k7 = 2.0 X lo6 M-' s-l. The ratios k 7 / k 6 = 2.0, k 4 / k 6 = 0.1 M, and k 5 / k 6 = 7 X M, thus obtained from our scheme, are in reasonable agreement with those derived from steady photolysis exM, and 5.2 X M, respecperimentd (1.7, 7.4 X tively), which strongly supports the overall mechanism. However, the following difficulty is encountered in the two-hydroxylator hypothesis: from steady-state measurements of the effect of [AQS] on 4ROH (section 111), it appears that, within the limits of error, k , ' / k i = k g / k 6= M over the whole pH range, 2-14. This con3.4 X stancy, perhaps fortuitous, is yet impressive evidence for a single hydroxylator. The increase in photohydroxylation yield at high pH may then be explained by an increase in the efficiency of the final hydroxylating process, thus
+
--c + I
AQS
2 complex
1
II_ OH-
...
-
ROH
I11
Here, step I is taken to be pH independent while the partitioning ratio kn/kIn rises with pH. This alternative
2544
J. Phys. Chem. 1983, 87, 2544-2556
would have to assume that this latter increase occurs in such a manner as,first, to overcome strongly the observed decrease in yield of C with pH and, second, to coincide more or less with the pH region in which quenching of A by OH- becomes significant. These requirements seem rather unlikely.35 The evidence that the interaction of anions with 3AQS* involves charge transfer suggests that the proposed intermediate, Z, may be writtzn as AQS-.OH, i.e., an OHsemiquinone exciplex or radical pair. In this case its spectrum should closely resemble that of AQS-, but with much shortened lifetime. This may be the reason that transient Z escaped our detection, being masked by the extensive absorption of AQS- in alkaline solution. This radical pair may then undergo various types of reactions, e.g., back-electron-transfer leading to physical quenching (35)A reviewer has suggested that the strongly base-sensitive process, 11, could be rapid autoxidation of an enolate form of intermediate C.
(reaction 9'), scavenging of OH by another AQS molecule (reaction 89, migration of electrons in the ring system and ultimate ejection of another anion, as in the photosubstitution of S032-by OH-, or C1- in the case of anthraquinone-l-sulfonate.lb Whether the lifetime of such a species may be sufficiently long to account for the observed dependence of yield on [AQS] is still uncertain. Only by identifying transient Z in alkaline solution and directly measuring kgl and k,' can the two-hydroxylator mechanism be definitely established. Acknowledgment. We thank Mrs. N. Shalitin and Prof. E. Hayon for their assistance in the emission studies. We much appreciate support of this work by grants from the US.-Israel Binational Science Foundation (Research Grant No. 2657/81) and from the US. Department of Energy, Division of Basic Sciences (Contract No. DEAC02-76ER03117). Registry No. 9,l0-Anthraquinone-2-sulfonate, 84-48-0.
Detailed Analysis of the Optical Absorption and Emission Spectra of Eu3+ in the Trigonal (C,) Eu(DBM),*H,O System Andrew F. Kirby and F. S. Richardson' Department of Chemistry, Universny of Virginia, Charbttesvllle, Virginia 2290 1 (Recelved: September 20, 1982; In Final Form: December 20, 1982)
Orthoaxial 6- vs. *-polarized absorption spectra are reported for single crystals of the trigonal EU(DBM)~-H~O ligand) system at 77 and 295 K throughout (DBM = dibenzoylmethanato(or 1,3-diphenyl-l,3-propanedionato) the 7Fo 5Do,1,2, 7F1 5D0,1,2, and 7F2 5Dotransition regions of Eu3+. Nonpolarized excitation and emission spectra are reported for microcrystalsof this same system dispersed in a KBr/silicone grease matrix. The emission spectra were obtained at sample temperatures between 30 and 295 K and were recorded over the 520-770-nm spectral region. With excitation at h 5 466 nm, all of the emission features obee~edin this region can be assigned to crystal field origin lines or vibronic lines split out of the 7Fo,1,23,4 6Do and 7F0,12,3 5D1multiplet-to-multiplet transitions of Eu3+. The relative intensities of the 'FJ 5Doand 7FJ 5D1emissions exhibit a temperature dependence, and at T < 150 K splittings in both the origin lines and vibronic lines associated with A E crystal field transitions reveal distortions away from exact trigonal (C3) site symmetry (at the Eu3+ions). Energy level calculations based on a semiempirical crystal field Hamiltonian are reported, and calculations of transition oscillator strengths are also reported. The latter were based on a model in which the electric dipole strengths of 4f 4f transitions depend explicitly on distributions of charge and dipolar polarizability within the ligand environment about the Eu3+ion. The unusually strong emissions observed in the 7Fo 6Doand 7F3 5Do transition regions are satisfactorily accounted for by the calculations based on this model, as are the relative intensities of all the 7FJ 5Doemissions.
-
-
-
+-
-- -
-
-
-
Introduction The study reported here is one in a series dealing with the optical absorption and emission properties of Eu(II1) in crystalline environments of octahedral, near-octahedral, and trigonal symmetry.'* The primary objectives of these studies are (1)the assignment of observed spectral lines to individual 4f 4f crystal field transitions (including --+
(1) J. P. Morley, T. R. Faulkner, and F. S. Richardson, J. Chem.Phys., 77, 1710 (1982). (2)J . P. Morlev. J. D. Saxe, and F. S. Richardson. Mol. Phvs.. 47.379 (1982). (3)A. F.Kirby, D. Foster, and F. S. Richardson, Chem. Phys. Lett., 96,507 (1983). (4)A. F. Kirby, M. T. Berry, and F. S. Richardson, unpublished resulta. ( 5 ) A. F. Kirby and R. A. Palmer, Inorg. Chem., 20, 1030 (1981). (6)A. F. Kirby and R. A. Palmer, Inorg. Chem., 20, 4219 (1981). (7)A. F.Kirby and R. A. Palmer, Chem. Phys. Lett., 80, 142 (1981). (8)F. S.Richardson, J. D. Saxe, S. A. Davis, and T. R. Faulkner, Mol. Phys., 42,1401 (1981). OO22-3854103/2087-2544$01SO10
-
-
phonon-assisted transitions), (2) the characterization of 4f-electron crystal field potentials in a variety of crystalline environments, and (3) correlations between ligand (or crystal) structure and observed 4f 4f electric and magnetic dipole intensities. In the present study, we report a detailed analysis of the optical absorption and emission spectra associated with the 7F0,12,3,46Do19transitions of Eu3+in the structurally well-characterized tris(l,3-diphenyl-l,3-propanedionato)europium(II1) monohydrate system (denoted by Eu(DBMI3.H20). As a class, the LII(DBM)~.H~O systems crystallize in a form which is enantiomorphic and which belongs to the rhombohedral space group R3,with one molecule per unit celL9 The Ln3+ ion is coordinated trigonally by the six oxygen atoms of the three nearly
-
-
(9)A. Zalkin, D. H. Templeton, and D. G. Karraker, Inorg. Chem., 8, 2680 (1969).
0 1983 American Chemical Society
