Oct., 1963
DELdYED
FLUORESCEXCE OF PYRENE
ethanol. The frequency is quoted as C.P.S.with reference to a high field P 3 1 signal in a saturated solution of TPP in CCI, as external reference. A plot of 1/ ( v i - Y ) vs. l/(concentration of zinc nitrate) is linear, with intercept = 0.004 and slope = 0.030. From eq. 9, the equilibrium constaiit K1 and D, are calculated to be 0.13 l./mole and 250 c.P.s., respectively. PSI
TABLE IV FREQUENCY IN 0.5 M TBEP AS A FUXCTION OF ZINCNITRATE is 95% ETHANOL COXCENTI~ATION Zinc nitrate, M
0 0.b 1.0
1.25 1.5
P31
frequency, c.p.8.
389.7 374.1 358.7 354.6 348.8
2199
SOLUTIOriS
magnetic ions, on P3l, has been used by Cohn and Hughesg to demonstrate that the complexation with adenosine triphosphate involves only the p- and yphosphate group, and not the @-phosphate. For TBP and TBEP, the addition of a small amount of C U + ~ or lliI~i+~ broadens the P31 signal so much that it becomes unobservable. This demonstrates clearly that the phosphoryl oxygen is involved in complexation. General Conclusions.-From the very small equilibrium constants obtained for the reactioiis between HA and TBP or TBEP, and between TBEP and zinc nitrate, and from the very large equilibrium constant obtained for eq. 3, we conclude that the synergistic effect arises from the interaction of L on the ZnA2 complex. A possible structure of the ZnAzL complex is CF3
CFs
Although TBEP has been used extensively as solvent extractant of metal ions from acid solutions, this is the first tirne that its complex formation constaiit has been determined. This is because in the complexation, no hydrogen ion is set free and hence no pH measurement can be carried out. Furthermore, the measurement of proton chemical shift in TBEP as a function of zinc nitrate coiicentration is useless, because the protoiis are too far away from the site of binding, namely, the phosphoryl oxygen. The site is demonstrated subsequently. Li, et G Z ~ have .,~ shown that paramagnetic metal ions broaden the p.m.r. lines of the nuclei which are adjacent to the sites of binding because the magnetic field of the ion decreases Lhe relaxation time of the nuclei and because the complexed ligand is exchanging rapidly with the free ligand in solution. A similar effect of para(8) h'. C. Li, R. L. Bcruggs, and E. D. Becker, J . Am. Chem. Soc., 84, 4650 (1962).
ll
I/
fl
C CH
\/\ S
O-C=CH(C4H3S)C=0
C-0
HC--CH HC
I
I
c=o
\ / z11 / \
0-P
(OR)3
The equation for the formation of this complex may be represented as ZnX2.H20
+ L = ZnA2.L + H20
The aquo metal-thenoyltrifluoroacetone complex, ZnAyH20, is thus rendered more hydrophobic by substitution of T B P or TBEP for the HzO. Acknowledgment.-The authors are greatly indebted to Dr. E. D. Becker for valuable discussion and advice and to Mr. R. B. Bradley for maintaining the 1i.m.r. spectrometers in top operating condition. (9) M. Cohn and T. R. Hughes, Jr., J . Bzol. Chem., 237, 176 (1962).
ON THE DELAYED FLUORESCENCE OF PYRENE SOLUTIOKS BY J. B. BIRKS The Physical Laboratories, University of Manchester, Manchester, England Received April 9, 1963 Observations of the radiative lifetimes of the pyrene singlet excited monomer and excimer are used, in conjunction aTith the data of Parker and Hatchard on the normal and delayed fluorescence of solutions of pyrene in ethanol, to evaluate the rate parameters describing the system. It is shown that triplet-triplet quenching results in the initial formation of excimers and singlet excited monomers in the ratio of 1.5: 1.
The fluorescence spectrum of pyrene in solution consists of a struotured violet band 11.11 due to excited moiiomers and a structureless blue band Dl due to excited dimers (excimers) formed by the association of excited and unexcited monomers.' Parker and Hatchard2have observed the normal aiid delayed fluorescence spectra of deoxygenated solutions of pyrene in ethanol at room temperature. The normal fluorescence results from the direct excitation of monomers, which then may emit or associate to yield M1 or D1, respectively. The delayed fluorescence, the intensity of which is proportional to the square of the incident light intensity, is attributed to (1) T. Fdrster and K. Kasper, Z . Elektrochem., 59, 976 (1955). ( 2 ) C. A. Parker s n d C. G. Hatchard, Trans. Faraday Sac., 59, 284 (1963).
the delayed excitation of the monomer-excimer system by triplet-triplet quenching. Parker and Hatchard have analyzed their results on the assumption that this process results in the initial formation of excimers only, which may then emit or dissociate to yield D1 or MI, respectively. The reaction scheme, which has been considered in detail else~vhere,~ is the same as that of Parker and Hatchardj2 but their notation differs from ours and is ~ kt), and shown in parentheses. kfni (kfj, k l (ko k ~ l f (kacj c are the monomer rate parameters of emission, internal quenching, and excimer formation, respectively; k f ~(kf'), k1= ( k ~ ' ) aiid , k ~ \ l (rk~d ) are the excimer rate parameters of emission, internal quenching, and dis-
+
(3) J. B. Bilks, D. J. Dyson, and I. I€. Munro, submitted for gublicstion.
J. B. BIRKS
2200
sociation into excited and unexcited monomers, respectively; and c is the concentration. I M and I D are the fluorescence quantum yields of 31, and D1, respectively. We consider three cases. (i) If only monomers are excited initially
where
is the fluorescence quantum efficiency of the monomer a t infinite dilution kfD
=
qD
kfD
+
(3)
kiD
is the fluorescence quantum efficiency of the excimer a t infinite concentration, and
Vol. 67
cence. Equation 6 reduces to (1) or (5) for a = 0 or a = a , respectively. Parker and Hatchard2 have measured the following quantities for solutions of pyrene in ethanol a t room temperature: q M = 0.65; K = 2.0 X IOs 1. mole-'; K1 = 1.70 X IOs1. mole-'; Kz = 0.706; K3 = 3.16 X lo31. mole-1. Birks, Dyson, and iLfunroa have obtained the following parameters from lifetime measurements of solutions of pyreliein cyclohexane: kf&~= 0.15 X 107sec.-l, k f =~ 1.1 X 107sec.-l. They have also shown that (7) where 7 is the solvent viscosity a t absoIute temperature T, so that for pyrene in ethanol at room temperature, ICDX = 6 X lo9 1. mole-' sec.-l. From the quantities and equations indicated in parentheses we obtain the following additional parameters for the room temperature pyrene-ethanol system.
is the "half-value" concentration a t which I ~ =I ' / g ~ r , I D = l/zqD. These relations apply to the normal fluorescence. (ii) If only excimers are excited initially
ID - --
kfD(kft1
Is1
+
kit1
f kDn1c)
(5)
kflIkMD
This corresponds to the assumption of Parker and Hatchard2 concerning the delayed fluorescence. (iii) If M o monomers and DO(= cuA10) excirners are excited initially
IDIM
=
kfD{kDVc
+ + + +
kfM { ( k f D
{
k f D a(76fX
kfM { (ICfD K2
+K~C
a(kfbl
kiD
k.111)
kiD)
+
kiL1
f kDnIc) ]
+ + (1 + + (1 +
f
k3ID)
1
@kXD
@)kDXIc] a)khlD]
(6)
This corresponds to the general case of delayed fluores-
We now are able to evaluate a from either KZ or R, and the rate parameters using (6). A value of a = 1.5 is obtained in each case, showing the consistency of the analysis. Thus the experimental data indicate that the triplet-triplet quenching results in the initial formation of excimers and excited monomers, in the ratio of 1.5: 1, respectively. It appears likely that the triplet-triplet combination occurs instantaneously into a higher excited state of the dimer with excess vibrational energy, and that this is either rapidly internally coiiverted into the normal excimer state or rapidly decomposed into excited and unexcited monomers, the probabilities of these two processes being in the ratio indicated.
