J. Phys. Chem. 1989, 93, 6836-6837
6836
Basicity of Phenazine in the First Triplet State J. 1. del Barrio, J. R. Rebato, and F. M. C.-Tablas* Departamento de Quimica, Facultad de Ciencias, Universidad Autbnoma de Madrid, Cantoblanco, 28049 Madrid, Spain (Received: March 1, 1988; In Final Form: March 28, 1989)
The pK, of the monovalent phenazonium cation in the first triplet state has been estimated from transient triplet-triplet absorption spectra obtained by flash photolysis. The value obtained, pKa = 1.9, differs considerably from the previously reported pKa = 4.
Introduction
The estimation of the acidity constants in excited states started with Forster' and was further developed by Jackson and Porter2 and Vander Donckt et aL3 In most cases the pK,, of the ground and the first excited triplet states show rather similar values, while those for the first excited singlet are considerably different. However, polynuclear diazines containing the pyrazine ring seem to show a different behavior with considerable values for ApK,(T,) = pK,(T,) - pKa(So). For example, in the cases of 1,Cdiazaphenanthrene and benzo[a]phenazine, Grabowska et aL4gShave reported the values summarized in Table I, where H2B2+and HB+ refer to the corresponding diprotonated and monoprotonated cations, respectively. These data show values of ApK,(T,) comparatively large, up to 5.5 pK units, in contrast with the generalization by Jackson and Porter concerning the similarity of PKa(S0) and P K ~ ( T I ) . The excited states of polynuclear 1,4-diazines show other "anomalous" properties as Grabowski et al. have indicated.6 So, in most diazines the following changes are typical
TABLE I: pK.'s of Some 1,4-Diazaaromatics' in the S, S,,and T, States from References 4, 5, and 10
state So SI
TI
1,4-DAP pK,' pK," -4.94 0.86 3.46 9.14 2.56 6.36
BPhe pK,' pK," -4.15 1.52 6.45 8.80 4.45
pK,' -6.14 (6)b
1,4-DAT pK." -0.36 (lO.l)c
6.7: 6.3L
1,4-DAP, 1,4-diazaphenanthrene; BPhe, benzo[a]phenazine; 1,4DAT, 1,4-diazatriphenylene. *Estimated from absorption spectrum only. Estimated from absorption and low-temperature fluorescence. From phosphorescence. From T-T absorption. TABLE 11: pK.'s of Diprotonated and Monoprotonated Phenazine in the S, S,. and T,States from Reference 7
state
PK,' -4.30 4.10 5.70
PK," 1.21
6 4; 1.9O
"Result from our experiments. ApKa"(S,) = pK,"(S,)
- pK,"(SO) > 0
while in 1,4-diazines as can be seen from Table I, both ApK,'(SJ and ApK,"(S,) are positive. Here, the superscript indexes I and I1 refer to the diprotonated and monoprotonated cations, respectively: H2B2+.- HB+ + H + HB+
.- B + H +
K,' Kalr
Grabowska et al.,' from phosphorescence measurements, have estimated the basicity of phenazine (Phe) in the first excited singlet and lowest triplet states. Their results, which appear in Table 11, show that ApK,(T,) reaches 10 and 2.79 for the diprotonated (H2Phe2+)and monoprotonated (HPhe+) phenazine, respectively. Although these values are in analogy with other members of the 1,Cdiazines family, the inequality
obtained for the lowest triplet state of phenazine is rather unusual even for this diazines family; for example, Table I shows that for 1,4-diazaphenanthrene the pK,'(T,) is about four units smaller than the pK,**(T,). This fact, together with the interest of the ( I ) Forster, T. 2. Electrochem. 1950, 54, 42. (2) Jackson, G.; Porter, G. Proc. R. SOC.London A 1961, 260, 13. (3) Vander Donckt, E.; Dramaix, R.; Nasielski, J.; Vogels, C . Trans. Faraday SOC.1969, 6S,3258. (4) Waluk, J.; Grabowska, A,; Pakula, B. J . Lumin. 1980, 21, 277. (5) Mordizinski, A.; Grabowska, A. J . Lumin. 1981, 23, 393. (6) Grabowski, Z. R.; Grabowska, A. Z . Phys. Chem. N.F. 1976,101, 197. (7) Grabowska, A,; Pakula, B. Photochem. Photobiol. 1969, 9, 339.
0022-3654/89/2093-6836$01.50/0
particular properties exhibited by 1,Cdiazaaromatics, stimulated us to restudy the basicity of the phenazine from triplet-triplet absorption transitions. Experimental Section
Phenazine (Hopkin and Williams) was purified by sublimation. Bidistilled water was used as solvent. The pH was measured with a pH meter (Beckman Expandamatic SS-2) previously calibrated and was varied by adding HCl (Merck) for the acid, and NaOH (Merck) for the basic solutions. All solutions were carefully buffered to guarantee that proper equilibria are established. Using the classical flash photolysis technique,* transient tripT,) were recorded with a let-triplet absorption spectra (TI Hilger medium spectroscope on Kodak Panchromatic Plus X film plates. Figure 1 shows a scheme of the experimental apparatus. The Ar-filled exciting photolytic lamp was 14 cm long and produced flashes with a width at half-maximum of 30 ps, discharging 500 J (10 KFat 10 kV). The spectroscopic lamp used was 7 cm long and had a flash duration of 18 ps at half-maximum (2.5 pF at 18 kV). The recorded spectra were analyzed with an Optronic Photomation P-1700 microdensitometer that registered the optical density versus the distance from a reference point on the plate. The wavelength was calibrated with sodium and low-pressure mercury lamps. To obtain the correct triplet-triplet absorption spectra, the spectrum obtained by discharging only the spectroscopic lamp is subtracted from the one discharging both the
-
(8) del Barrio, J. I.; Rebato, J. R.; G.-Tablas, F. M. Chem. Phys. Lett. 1985, 114, 397.
0 1989 American Chemical Society
6837
J. Phys. Chem. 1989,93,6837-6843
l*O
tc
+
trations of these species, and C = C A CBis the total concentration of triplet state formed, which we shall assume constant in all cases. Defining X as the molar fraction of unprotonated triplet, CB = CX, and C A = C(l - X ) , we have
0
1
-D= DB
t ~ ( 1- 4 f X tB
t1 A and
1
3
5
7 P"
9
1
1
Figure 1. X = (D- DA)/(DB - DA)of phenazine versus pH.
spectroscopic and photolytic lamps.
Results and Discussion The optical density of phenazine corresponding to triplet-triplet transitions does not change for pH's above 4,according to our measurements. This seems to indicate that the unprotonated phenazine is already the predominant species present at pH = 4;then the phenazinium monovalent cation (HPhe') must be a rather strong acid. Since in most cases pKal < pK,", we may assume that the phenazinium divalent cation (H2Phe2+)is a very strong acid in the first triplet state and would exist only at extremely low pH's. So in the range that we have covered, from 1 to 12, the monoprotonated and unprotonated phenazine are only present according to the following equilibrium: HPhe+ e Phe
-
+ H+
Karl
Assuming that D is the optical density for a selected triplettriplet absorption (T, T,) at a given wavelength, measured when both HPhe' and Phe are present, and DB the optical density measured at a very high pH when only the unprotonated species is present, then3
D - eA(C - CB) + EBCB _ DB
4
with tA and tB the extinction coefficients of the acidic and basic forms, respectively; CA and CB are the corresponding concen-
where DA is the optical density at the PITSwhen only HPhe+ is present. Since the pK, for an acid dissociation corresponds to that value of the pH at which the concentration C Aof the protonated form equals that of the unprotonated CB,plotting the values of ( D DA)/(DB- DA), obtained experimentally from the microdensitometer data, versus the pH, we obtain the pK,"(T,) of HPhe+ in the first triplet state as the pH value where X = 0.5. Figure 1 shows the results obtained that yield a pK,(T,) = 1.9 f 0.2. The results obtained match very well the idea that only two species, HPhe+ and Phe, are predominant in our pH range. The pK,"(T,) obtained seems to indicate that the phenazine in the lowest triplet state does not follow the trend of most 1,4 polynuclear diazines and is not a much stronger base than in the ground state. Table I1 summarizes the results obtained for phenazine and compares our result with Grabowska's. The transient spontaneous Raman technique, used by Beck and Brus for studying the reaction dynamics of triplet quinoxaline in aqueous s o l ~ t i o n would ,~ be useful to clarify the behavior of phenazine.
Acknowledgment. The support of the CAYCIT is appreciated. Registry No. Phenazonium, 22559-72-4. (9) Beck, S. M.; Brus, L. E. J . Chem. Phys. 1981, 75, 4934. (10) Bulska, H.; Chodkowska, A.; Grabowska, A.; Pakula, B.;Slanina, Z. J . Lumin. 1975, 10, 39.
Nonequlllbrlum (Ca,Mg)O Solid Solutions Produced by Chemical Decompositiont Giorgio Spinolo* and Umberto Anselmi-Tamburini CSTEICNR and Department of Physical Chemistry, University of Pavia, Male Taramelli, 16, I 27100 Pavia, Italy (Received: October 6, 1988; In Final Form: February 10, 1989)
In situ X-ray diffraction experiments on thermal decomposition of dolomite (CaMg(C03)2)at low temperatures and low pressures indicate that the first reaction product is a homogeneous, equicomposition,rock-salt-structured oxide solution, which unmixes into coupled Mg-rich [(Ca,Mg,,)O, c 0.11 and Ca-rich [(Mg,Ca14)0, 6 0.21 phases. This process is analyzed along the lines of the classical (linear) theory of spinodal decompositions. In a general way, the thermal decomposition of suitable precursors, such as double carbonates, gives a new method for preparing nonequilibrium oxide solutions. This paper discusses the thermodynamic, kinetic, and structural requirements for the feasibility of the method and outlines its applications to the synthesis of complex oxides.
-
Introduction
-
This paper deals with two related subjects: mechanism of the thermal decomposition of dolomite ( c ~ M ~ ( c o ~ )and ~ ) spindal
decomposition in the (Ca,Mg)O binary system. Concerning the first subject, calcite-structured carbonates have been deeply investigated (see ref 1-9 and the references therein
Based on a thesis submitted by U. Anselmi-Tamburini in partial fulfillment of the requirements for a Ph.D at University of Pavia.
(1) Beruto, D.;Searcy, A. W. J . Chem. Soc., Faraday Trans. 1 1974, 70, 2145.
0022-3654/89/2093-6837$01.50/0 0 1989 American Chemical Society
