J. Phys. Chem. 1982,86,22-26
22
ARTICLES Mechanism of External Heavy Atom Effect on Intersystem Crossing in Fluid Solutions. Analysis Based on Fluorescence Decay Data Yuko Shlmlzu and Tohru Azuml" Department of Chemistry, Faculty of Science, Tohoku Universliy, Sendai980, Japan (Received: March 6, 198 1; I n Flnal Form: August 4, 1981)
-
The external heavy atom effect by ethyl iodide on S1 T1intersystem crossing is analyzed for fluid solutions of pyrene in various alkane solvents of different viscosity. At any concentration of ethyl iodide, the fluorescence decay is expressed as a single exponential function. Conventional Stern-Volmer plots with respect to the fluorescence decay (Le., the plots of AkF, the increment of the fluorescence decay rate constant induced by the heavy atom effect vs. C, the concentration of the heavy atom perturber) deviate from linearity and bend downward in the range of C larger than 1 mol dm-3. The approximate quenching rate constants, k,, obtained from the Stern-Volmer plots in the concentration range below 1mol dm-3are independent of solvent viscosity and are 2 orders of magnitude smaller than the rate constant for diffusion. This experimental evidence is satisfactorily interpreted in terms of reaction kinetics in which the formation and the dissociation processes of the exciplex composed of an excited pyrene molecule and an ethyl iodide molecule are quite fast. Further, the kinetic analysis reveals that the conventional Stern-Volmer plots are inadequate. We propose that plots of l / A k ~vs. 1/C are more appropriate since they should yield a straight line in a wide concentration range. Further, from the intercept and the intercept/slope ratio of the straight line, the decay rate constant of the exciplex, kE, and the equilibrium constant for the exciplex formation, K , are obtained. For the pyrene-ethyl iodide system in hexane, we obtain the following results: k E = 1.79 X lo9 s-' and K = 0.047 mol-' dm3.
1. Introduction
The addition of a heavy atom perturber such as ethyl iodide to a solution of aromatic molecules usually leads to a decrease of both fluorescence intensity and fluorescence lifetime.'* This is the manifestation of the external heavy atom effect on the intersystem crossing (isc) from the first singlet state (S,) to the lowest triplet state (Tl).This effect is attributed to the increase of the spin-orbit coupling as a result of the interaction between the aromatic molecule and the heavy atom perturber. However, the exact mechanism by which the intermolecular interaction takes place has not been well clarified. Most of the previous studies on the external heavy atom effect in fluid solutions are analyzed in terms of a mechanism which is outlined as follows: (1)By diffusion-controlled encounters of the excited aromatic molecule (lM*) and the heavy atom perturber (P), an excited complex or an exciplex is produced. Here, and in the following, the term exciplex is extended to include all 'M* P entities which interact without implying that the exciplex has a
+
(1)S.P. McGlynn, T. Azumi, and M. Kinoshita, 'Molecular Spectroscopy of the Triplet State",Prentice-Hall,Englewocd Cliffts, NJ, 1969, Chapter 8. (2)T. Medinger and F. Wilkinson, Trans. Faraday Soc., 61, 620 (1965). (3)J. Bendig, M. Siegmund, and S. Helm, Adu. Mol. Relaxation Interact. Processes, 14, 121 (1979);J. Bendig, s.Helm, and D. Kreysig, Chem. Phys. Lett., 54, 466 (1978). (4) H. Dreeskamp and J. Pabst, Chem. Phys. Lett., 61,262 (1979). (5)R. P.DeToma and D. 0. Cowan, J. Am. Chem. SOC.,97, 3283 (1975). - -,(6)D. Schulte-Frohlinde and H. Hermann, Ber. Bunsenges. Phys. Chem., 81,562 (1977). (7) T. C. Werner, Fluoresc. News, 9, 1 (1976). (8)I. B. Berlman, J . Phys. Chem., 77, 562 (1973). I--
0022-3654/82/2086-0022$01.25/0
finite binding energy. (2) The exciplex undergoes efficient isc, and in view of this fast decay process neither the dissociation of the exciplex nor the fluorescence of the exciplex is considered. In the framework of this mechanism, the Stern-Volmer plots with respect to either the quantum yield or the lifetime of the fluorescence should yield a straight line, and the quenching rate constant, k,, obtained from the slope of the straight line should represent the rate constant for diffusion, kdiff. While there appears to exist a somewhat vague notion that the external heavy atom effect is reasonably well understood in terms of the above mechanism, careful examination of the literature reveals that such a simple mechanism as that outlined above is inadequate. This recognition is based on the observation of the following anomalies: (1)As the concentration of the heavy atom perturber increases, the Stern-Volmer plots markedly deviate from linearity and bend downward.2 (2) The quenching rate constant, k,, obtained from the low concentration range of the Stern-Volmer plots is nearly independent of viscosity,M and further these rate constants are 2 orders of magnitude smalleP5than the rate constant for diffusion, kdiff. It appears that only a little attention has been paid toward these anomalies. The first anomaly, that is, the nonlinearity of the Stern-Volmer plots, was interpreted by Medinger and Wilkinson2 in terms of the decrease of the activity coefficients at higher concentrations. Kokubun? on the other hand, suggested that the nonlinearity is an indication of the inadequacy of the diffusion-controlled quenching mechanism. The second anomaly has (9)H. Kokubun, Bull. Chem. SOC. Jpn., 42,919 (1969).
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 1, 1982 23
External Heavy Atom Effect on Intersystem Crosslng
been interpreted by Melhuish and Metcalflo and by Bendig? These authors suggested a mechanism in which the exciplex dissociates with a rate much faster than its decay rate. In this mechanism, the quenching rate constant obtained from the low concentration range of the StemVolmer plots is expressed as the product of the decay rate constant of the exciplex and the equilibrium constant for the exciplex formation and hence should be independent of solvent viscosity. This mechanism not only accounts for the second anomaly quite satisfactorily but also seems reasonable provided the interaction between the aromatic molecule and the heavy atom perturber is weak. In this case, the binding energy of the exciplex produced by the encounters of these weakly interacting moieties is expected to be small as compared with thermal energy. Hence, the dissociation of the exciplex to ita parent compounds should occur quite readily. As is discussed above, a variety of interpretations have been given for each of the anomalies. However, there appears to exist no mechanism which simultaneouslyaccounts for both anomalies. In view of this status, we believe that detailed examination of the external heavy atom effect is necessary in order to establish the mechanism. Such an effort is devoted in this paper to the pyrene-ethyl iodide system. While most of the previous studies are based on measurements of fluorescence intensity (or quantum yield) at steady excitation conditions, we find it more advantageous to rely on the decay data rather than on the quantum yield data for the following reasons: (1) Whether or not the decay is expressed as a single exponential function is important information that we need to obtain before anything else. (2) As far as decay data are concerned, the possibility of complexation in the ground electronic state does not have to be considered. (3) Because of the absorption of the exciting light by the heavy atom perturber, accurate determination of fluorescence quantum yield is difficult. For example, the absorption spectra of pyrene and ethyl iodide do not seem to overlap. However, we note that the concentration of ethyl iodide is as much as 10s-108 times that of pyrene. In the case of high concentrations of ethyl iodide, therefore, considerable portion of the exciting light a t 337.1 nm is absorbed by ethyl iodide. The calibration of this effect is not necessarily easy. In the case of lifetime measurements, on the other hand, no such calibration is necmsary. (4) Instability of the exciting light output harms the accuracy of the quantum yield data but does not harm the accuracy of the decay data. 2. Experimental Section Pyrene was purified by recrystallization from ethanol followed by zone refining. Ethyl iodide was distilled with a spinning band distiller just before use. In order to investigate the viscosity effect, we chose hexane, octane, and tetradecane as solvents.11 Spectroscopic-gradehexane was used without further purification. Octane and tetradecane were chromatographically purified through a Wcm column of alumina previously treated with silver nitrate.12 All of the solvents were transparent down to 220 nm and, further, did not exhibit any emission. The concentration of pyrene was kept to lo6 mol dm-3. At this concentration the excimer formation process is negligible. The concentration of ethyl iodide was varied from 0.02 to 2.5 mol dm-3. Degassing of solutions was (10) H. W. Melhuieh and W. S. Metcalf, J . Chem. SOC.,976 (1964). (11) We use the nomenclaturerecommendedby IUPAC, that is, all of the alkanes indicated are normal. (12) E. C. Murray and R. N. Keller, J . Org. Chem., 34, 2234 (1969).
4 0 . 5 I n s /CHRNNEL
-
I
:I
:
E 1.0 M
I
nL ., 0
..
+
200
400 600 CHRNNEL NUMBER
800
1000
Figure 1. Some examples of the decay of the fluorescence of pyrene in hexane solvent at 25 "C. The three decays correspond to those observed under the presence of 0, 0.5, and 1.0 mol dm3 of ethyl iodide. The ordinate refers to the logarithm of the photon count and the abscissa to the channel number. One channel corresponds to 0.51 ns.
achieved by repeated cycles of freezing-pumping-thawing-distillation in vacuo. The last step was found to be indispensable to remove the last trace of oxygen. In the case of tetradecane solution, distillation was not possible at room temperature; therefore, the sample was heated at, but not higher than, 70 "C. The fluorescence emission and excitation spectra were observed with a Hitachi MPF2A spectrofluorometer. The fluorescence decay functions were determined with a single photon counting method. The excitation light was a nitrogen discharge from an Ortec 9352 nanosecond light pulser. The decay curve8 were determined and analyzed until the intensity decreased to 1/1,,,, of the initial value. In view of the temperature dependence of the fluorescence lifetime,l3 all of the measurements were carried out at a constant temperature of 25 O C .
3. Results The spectral locations of the individual vibronic bands of the fluorescence of pyrene are unaffected by the concentration of ethyl iodide. The relative intensities of the vibronic bands are however slightly dependent on the concentration of ethyl iodide. For example, the relative band intensity of the 384-nm band with respect to the 372-nm band is 1.7 in pure hexane, whereas the ratio gradually decreases as the concentration of ethyl iodide increases and becomes 1.5 a t 1mol dm-3 of ethyl iodide. The slight intensity change is analogous to the well-studied solvent effect by polar solvents and therefore is likely to be ascribed to the change of solvent pr~perties'~J~ induced by ethyl iodide. However, at the same time, we cannot discard the possibility of complex formatiod6between the excited pyrene and ethyl iodide. The fluorescence lifetime decreases significantly as the concentration of ethyl iodide increases. It should be emphasized here that, a t any concentration of ethyl iodide which we have investigated, the decay is expressed as a (13) B. Stevene,M. F. Thomaz, and J. Jones, J. Chem. Phys., 46,405 (1967). (14) A. Nakajimn, Bull. Chem. SOC.Jpn., 44, 3272 (1971); Spectrochim. Acta, Part A , 30, 860 (1974); J . Mol. Spectrosc., 61, 467 (1976). (15) K. Kalyanaeundaramand J. K. Thomas, J . Am. Chem. SOC.,99, 2039 (1977). (16) P. Lianos and S. Georghion, Photochem. Photobiol., 29, &13 (1979); 30,356 (1979).
24
Shimizu and Azumi
The Journal of Physical Chemlstv, Vol. 86, No. 1, 1982
TABLE 11: Kinetic Scheme and the Rate Parameters'
I
200'
reaction I -+
'M*
,4
-2 100.
\
a
3 A
-+
OCTANE TETRADECANE
r
'(MP)* '(MP)* '(MP)*
c'
05
15
I O
20
25
C/m~ldm-~
the concentration of ethyl iodlde. The experimental points obtained for three different alkane solvents are nearly on one smooth curve. The deviation from linearity is evident at concentrations greater than 1 mol dm3.
TABLE I: Comparison o f the Experimentally Determined Quenching Rate Constant,% k , , with the Calculated Diffusion Rate Constant,c k d s f
10-'k,l q/cP
(s-' m o l - ' d m 3 )
hexane octane tetradecaned
0.33 0.51 2.22
8.6 8.5
8.1
~
V
+P
-+
'(MP)* '(MP) +
-+
'(MP)
-+
isc fluorescence nonradiative decay exciplex dissociation isc in exciplex Efluorescence o f exciplex nonradiative decay
M
~
V
2 } k M / X kGM
ME
3'"
~ F Ek~
kGE
I
' The reactions and the rate parameters are expressed in terms of notations given by Birks (ref 20) except that the concentration o f heavy atom perturber, [PI, is denoted by C.
Figure 2. Plot of AkF, the increment of the fluorescence decay rate constant of pyrene induced by the external heavy atom effect, vs. C ,
solvent
'M t
'M* -+ 'M '(MP)* -+ 'M*
HEXANE
d
LL Y
0
exciplex formation kEMC
'M* t P '(MP)* 1M* -+ 3M*
1501
rate parameter
comment
10-'kdB/ (s'l
mol-' dm3) 2002 1295 296
' Based o n the experimental values obtained at 25 "C. Pa s. Calculated from the formula k d s r = CP= 8RT/3q where q is viscosity. See footnote 1 7 . q and kdiff at 23 "c.
single exponential function. Some examples are shown in Figure 1, where linearity of the semilog plots is well exemplified. The Stern-Volmer plots based on the decay data are shown in Figure 2 for three alkane solvents of different viscosity. As is revealed from Figure 2, the plots deviate from linearity and bend downward. Further, all of the experimental points obtained from the three different solvents are nearly on one smooth curve. In the concentration range below 1mol dm-3, the plots are regarded to be linear, and hence k, may be obtained. The results are compared in Table I with kdiffcalculated from the formula" kdjff = 8RT/37 (1) As Table I indicates, k, is nearly independent of viscosity (from octane to tetradecane viscosity increases as much as 7 times, whereas k, differs only by 5 % ) , and further k, is 2 orders of magnitude smaller than the calculated kdiff. The validity of the expression of kdjffin the form of eq 1 is questionable in view of the supposition that the local viscosity around the aromatic molecule might be very different from the bulk viscosity. In fact, various methods to modify the formula have been proposed.18 None of the modifications, however, lead us to suggest a correlation between kdiffand k,. We therefore conclude that k, is (17) The rate constant for the diffusion is usually shown as kdilr = 8 R T / 3 m . This equation is valid only when all of the constants in the right-hand side are expressed in cga units and the left-hand side is expressed in units of s-' mol-' dm9. The equation which is independent of the unit system (either cgs or SI unit system) should be expressed as eq 1 of the text. (18) A. H. Alwattar, M. E. Lumb, and J. B. Birks in "Organic Molecular Photophysics", Vol. 1, J. B. Birks, Ed., Wiley, New York, 1973.
considerably smaller than
kdiff.
4. Discussion
In order to understand the above experimental results, we try to analyze them on the basis of transient reaction kinetics. The reactions that we consider and the rate parameters are shown in Table II.19 We do not have any concrete evidence for exciplex formation. However, in an effort to retain the generality of the treatment, we analyze the experimental results without removing the exciplex formation process in the kinetics. Similarly, we do not neglect the fluorescence of the exciplex. As is discussed above, the fluorescence spectra do not show positive evidence of the existence of exciplex fluorescence. From this standpoint we must assume that the fluorescence of the aromatic molecule and that of the exciplex are very close to each other such that they are not spectroscopically separated. The observed fluorescence should be regarded as a superposition of the fluorescence spectrum of the aromatic molecule and that of the exciplex. The fluorescence response function should then be expressed as21
I(t) =
x)+ kFEkEMC exp(-X,t)
k ~ ~ ( -h 2 A2
-
- A1
~ F M ( X I- X ) + ~ F E ~ E M C exp(-M) (2) A2
- A1
where X1,2
=
((x- n2+ 4kMEkEMC]1/2]
1/2[x+ Y
(3)
and further, as indicated in Table I1
x = kE& + k~ = kEMC + ~ T M + ~ F M+ ~ Y =
ME + ~
kME
G M
(4)
+ ItE
X + E ~ F + E ~ G E
(5)
Thus, in general, the observed fluorescence response function is expressed as the sum of two exponential functions. We next examine how the general expression derived above is reduced in our pyreneethyl iodide system. Since we do not find any experimental evidence of exciplex formation, we expect that the binding energy of the exciplex is small as compared with thermal energy. Conse(19)We use the symbols for rate parameters by analogy with those adopted by Birks (ref 20). (20)J. B. Birks, "Photophysicsof Aromatic Molecules",Wiley-Interscience, New York, 1970. (21)Reference 20, p 304.
The Journal of phvsical Chemisity, Vol. 86, No. 1, 1982 25
External Heavy Atom Effect on Intersystem Crossing
quently, as is discussed above, the exciplex formation and dissociation processes are regarded to be much faster than the other decay processes. Namely, the following conditions are expected to be satisfied:
These correspond to the requirements adopted by Melhuish and Metcalflo and by Bendig et aL3 However, as is discussed in section 1, in most of the previous studies kE was assumed to be much larger than km as opposed to eq 7. Under these conditions, by expanding the square root that appears in eq 3, we obtain the following expressions: I
(9) kEMC + k M E The fluorescence response function is then reduced to the following expression: A2
I(t) =
~ F M ~ M+E~
~ E M+ C
I .o
0.5
0
1.5
2 3
C-' / mol"dm3
N
FE~EMC exp(-Xlt)
ME
+
Figure 9. Plots of the reclprocal of AkF, the increment of the rate constant Induced by the external heavy atom effect vs. the reciprocal of C, the concentratknof ethyl kdkle, obtained for the hexane solution. The excellent llnearlty of the plots is evident.
way of plotting. Equation 11 is rewritten in the following way: 11 1 1 -= (14) AkF k ~ - k k ~ ~ - KC k ~ Hence, the plots of l/AkF vs. 1/C should yield a straight line in the full concentration range provided the conditions in eq 6 and 7 are not violated. Such plots are shown in Figure 3 for the hexane solutions. As is visualized in Figure 3, all of the experimental points are exactly on a straight line. From the intercept and the intercept/slope ratio of the straight line, we obtain the following parameters for the hexane solution: kE - k~ = 1.79 x 10' 5-l (15)
+--
This may still look like a s u m of two exponentials. However, the second term of eq 10 has almost no weight compared with the first term. For, first of all, kFM - km is expected to be negligibly small since the external heavy atom effect on the radiative rate constants should be negligible. Second, X, is so large that the experimental time resolution (on the order of nanoseconds in the present case) would not see this component even if it had considerable weight. In this way, the fluorescence response function should be essentially expressed as a single exponential with a rate constant k F equal to X1. The increment of the rate constant induced by the external heavy atom is then expressed as AkF =
-W ~ E M ~ E M+ C ME
C
(11)
In the low concentration region where kEMC
