5524
J. Phys. Chem. 1991,95, 5524-5528
Sendtlzatlon of Benzene Fluorescence by ck- and trans-Decalin David B. Johnston, Yi-Ming Wang, and Sanford Lipsky* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received: December 28, 1990)
The effect of benzene to quench the fluorescenceof cis- and trum-decalin and to itself be sensitized to fluorescence has been studied for excitation of the system at 161 nm. The dependence of the quenching of the decalin fluorescenceon the benzene concentration (from 0.002to 0.1 M) is shown to be fitted well for both isomers by the standard diffusion model. Corrections for static contributions are considered. The quenching encounter between excited decalin and benzene results in the production of an emitting state of benzene with an efficiency of only 0.44 for cis-decalin and 0.33 for trans-decalin. The origin of these low efficiencies appears to be attributable to inefficiency in the conversion to the emitting state of benzene from those states that are populated via the quenching encounter.
I. Introduction The quenching of cyclohexane fluorescence by benzene has been demonstrated' to be well predicted by the standard Collins and Kimball diffusion model." However, observations on the concomitant appearance of benzene fluorescence indicate that, in the quenching encounter, the annihilation of the cyclohexane emitting state (hereafter referred to as C*) is not completely compensated for by the creation of an emitting state of benzene (i.e., B*). Conservation in the number of emitting states appears to occur with a probability, ( B e ) , of only 0.26 per encounter.' The factor of 0.26 is reasonably well predicted by assuming that there is, in fact, a conservation of total electronic energy in the C* B encounters but that the upper electronic states of benzene that are thus populated do not convert with high efficiency to B*.' The calculation utilizes Fiirster theory5 to deduce the population of upper excited states of benzene generated in the encounter and utilizes previously determined internal conversion efficiencies from these states to B*.6 In the present investigation we obtain encounter efficiencies for sensitization of benzene fluorescence by trans- and cisdecalin whose emitting states lie ca. 0.6-0.8 eV below that of cyclohexane. We find these efficiencies, as expected, to be larger than 0.26 but to remain significantly below unity and to be reasonably well understood using the aforementioned procedure. Also we will demonstrate that the standard diffusion model continues to predict well the efficiency with which benzene quenches the saturated hydrocarbon fluorescence in these much more viscous liquids.
+
11. Experimental Seetion The apparatus for these experiments has been previously described in some detail' and consists essentially of a Spex F212 spectrophotometer suitably modified to permit excitation and emission analysis in the vacuum ultraviolet. Samples are illuminated at normal incidence with a 30-W Hamamatsu L879 D2 lamp (fitted with MgF2 window) through a Spex 1680 double monochromator and the emission collected at 22S0 from the normal and spectrally dispersed through a second Spex 1680 double monochromator. Sodium salicylate screens were used to monitor the intensity of exciting light at the position of the sample. The only modification in what has previously been described was to employ somewhat smaller exciting and analyzer band passes (Le., 3 nm) and to note that under these conditions our maximal lamp intensity was found to be at 161 nm, rather than the previously reported 160 nm. Also, to CoNltlYc the sample a somewhat smaller cell was used than previously described. The new sample cell consisted of a Pyrex cylinder, 0.61 cm in length to which was (1) Johnston, D. B.; Li ky, S . J. Phys. Chem. 1991, 95, 1896. (2) Collins, F. C.; Kim& G. E. J. Collold. Scl. 1949, I , 425. (3) Weller, A. 2. Phys. Chem. (Fru urf um M u h ) 1957, 13, 335. (4) Choi, H. T.; Lipky, S. 1.Phys. C em. 1981,85,4089. (5) Fbnter, Th.Ann. Phys. 1948, 2, 55. (6) Braun, C. L.; Kato, S.; Lipky, S. J . Chem. Phys. 1963, 39, 1645.
"i
0022-3654/91/2095-5524$02.50/0
attached (via Varian Torr Seal epoxy) two MgFz windows of active diameter, 1.1 cm. Both cis- and trans-decalins (Aldrich 99%) were additionally purified by shaking with H2S04/HN03(1 :3), washing with alkaline water and then drying Over NaSO,, The resulting sample (air-equilibrated) had molar absorptivities at 190 nm of 2.1 and 1.4 M-' cm-I for the cis and trans isomers, respectively. Excitation of both samples at 210 nm showed a weak impurity emission. For the trans isomer, the impurity emission peaked at ca. 319 nm and extended from ca. 270 to 350 nm. The emission was not observed when the rrans-decalin was excited at 161 nm or when a solution containing benzene was excited at 210 nm. The implication is that the impurity concentration was sufficiently low as to be unable to compete with either transdecalin absorption at 161 nm or with benzene absorption (even at our lowest concentration of 0.002 M) at 210 nm. Accordingly, background corrections for all solutions excited at 1 6 1 and 2 IO nm were taken exclusively as contributions from dark counts and stray light. For cis-decalin, the impurity emission observed from the neat solvent at 210-nm excitation was found to peak at 290 nm and again to span generally the region of benzene emission. Here, too, the decision was made not to subtract its intensity as a background contribution to the benzene emission intensity. In this case the decision was based on the determination that such a subtraction would have noticeably (i.e., by ca. 10%)decreased the ratio of the benzene 287-272-11111peaks from their proper ratio. Thus,for cisdecalin too, only dark count and stray light contributions were considered for background subtractions. AU solutions were studied at 23-24 O C and left air-equilibrated unless otherwise noted. 111. Results
A. ciS-Deeplin. Luminescence measurements excited at 161 nm were made on 21 different concentrations of benzene in cisdecalin ranging from 0.00560 to 0.112 M. For subsequent convenience we will refer to these solutions as S, with a concentration equal to 0.01 120 M. Thus So refers to neat cis-decalin and Sloto our highest concentration solution of 0.1 12 M. Typical S,, S,, Sb, Sloare shown spectra from a set of solutions SO,SI, in Figure 1. The spectra are uncorrected for the spectral mponsc function of the analyzer system. A corrected spectrum for cisdecalin is shown in Figure 2. The ratio of the fluorescence intensity at 230 nm from So to that from S, was usually more reliably obtained by maintaining both exciting and analyzing monochromators at 161 and 230 nm, respectively, and, for each example, S, monitoring the fluorescenceintensity over a SUEciently long period of time as to make negligible (Le., 514%) statistical counting errors. For most samples at least 2 or 3 such measurements were made (for SIand SIo,6-10 measurements) usually on separately prepared solutions and over a period of several weeks. The averages, over all of these measurements arc presented in column 3 of Table I as the ratio I@(S,,>/I:g(S,) where I t is the intensity of emission at XI per incident photon of Q 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 5525
Sensitization of Benzene Fluorescence by Dccalin
1 . 0 1 .
. 1 7
I
I
,
,
,
9
I
0
150
250
0.0 250
350
350
300
WAVELENGTH, ( n m )
WAVELENGTH, (nm) Filpn 1. Emission spectra (uncorrected for spectral response of the analyzer) of cisdecalin benzene solutions excited at 161 nm. S, (neat cis-decalin), SI(0.01 12 M),s2 (0.0224 M), s4 (0.0448 M),s 6 (0.0672 M),and Slo(0.1 12 M).
+
EIgm 3. Emission spactrum of benzene from an Slo(0.1 12 M)solution in cis-decalin for excitation at 210 nm (dash line) and for excitation at 161 nm with cis-decalin contribution subtracted (solid line).
t
t
t cn Z
w
IZ -
I-
Z -
m
1.0
\ I
" t
[11
Q
0.01 150
180
300
240
WAVELENGTH, ( n m ) Figwe 2. Emission spectra (corrected for spectral response of the analyzer) of neat cis-dccalin (A) and truw-decalin (B)excited at 161 nm.
"
"
I
175
'
'
" I 1
200
WAVELENGTH, (nm) Figure 4. Intensity of cis-decalin fluorescence per unit incident photon of exciting light as a function of excitation wavelength. exciting light at X,. The absolute quenching efficiency of the cis-dccalin by benzene, %e, is related to this ratio as
. .
solution
(i#(So)/ii$( S ,)'
mM exDtl theorete theoret'
label
c.
%I
5.60 7.20 8.96 11.2 16.8 22.4 28.0 33.6 39.2 44.8 50.4 56.0 61.6 67.2 72.8 78.4 84.0 89.6 95.2 101 112
s0.6
fh.8
Sl.0 %I s2.0
S2.J S3.0 S3.I S4.0 S4.I SS.0 SI3 S6.o s6.S %O
S7.J S8,O
s8.I
G.0 Sl0.0
1.10 1.14 1.18 1.22 1.35 1.45 1.59 1.72 1.91 2.00 2.13 2.27 2.42 2.59 2.75 2.95 3.09 3.24 3.49 3.65 3.97
1.11 1.14 1.18 1.23 1.35 1.47 1.60 1.73 1.87 2.01 2.15 2.30 2.45 2.61 2.76 2.92 3.09 3.26 3.43 3.65 3.96
1.12 1.14 1.19 1.23 1.36 1.48 1.62 1.75 1.89 2.03 2.17 2.31 2.47 2.63 2.78 2.94 3.11 3.27 3.45 3.62 3.97
Jk6' (S,,)/l#( SJ6
exutl
DrbdiCr
2.08 2.89 3.78 4.66 7.06 9.51 12.6 14.7 18.2 20.7 23.8 26.1 29.3 31.4 35.7 37.6 43.0 43.8 50.4 51.2 57.1
2.04 2.85 3.67 4.48 7.13 9.17 12.0 14.7 18.5 20.4 23.0 25.9 28.9 32.4 35.7 39.7 42.6 45.6 50.7 54.0 60.5
'This ratio is the reciprocal of that utilized in Table I of ref 1. 'It should k noted that the denominator of this ratio refers to solution S,, whereas in Table I of ref 1, the corresponding denominator referred to solution h. #See cq 4 with a = 11.79 M'I and @ = 4.433 M-I, 'Corrected for static quenching with a = 13.39 M-'and @ = 3.778 M-I. 'Predicted assuming the validity of cqr 9 and 10.
The sensitization of the benzene fluorescence is presented in column 5 of Table I as the ratio of the benzene emission intensity integrated over its spectral distribution (from X I = 264 nm to XI = 350 nm) to the intensity of the cis-decalin emission at 230 nm (Le., li$(S,,)). An integrated intensity was used for benzene since its emission spectrum is rather structured and, therefore, the intensity at a particular wavelength more sensitive to change in analyzer slit width. The choice of 264 and 350 nm for the starting and ending points of the integration was arbitrarily chosen to encompass most of the spectrum. To avoid too cumbersome a notation, we refer to this integrated intensity as JL6'(S.). As will be noted from Figure 1, the benzene spectrum must be recovered from its overlap with that from cisdecalin. This was accomplished by normalizing the neat cis-decalin spectrum to the solution spectrum and then subtracting. The benzene spectrum thus ob tained, when normalized to its peak at 279 nm, was found neither to show any significant effect of benzene concentration nor to be significantly altered from a benzene spoctrum obtained by excitation of the benzene directly at 210 nm. Figure 3 shows the comparison (for the SIsolution in Figure 1) of the X, = 161 nm and X, = 210 nm spectra. The fluorescence from neat cis-decalin (at X, = 230 nm) was also monitored as a function of X, from X, = 150 to 200 nm. The intensity of the exciting beam at X, was obtained by monitoring the fluorescence (at XI = 400 nm) from a deposit of sodium salicylate on the back side of a MgFz window identical with that used in the cells and placed in the same geometry. Comparisons of this with other monitors of the exciting light has been com-
5526 The Journal of Physical Chemistry, Vol. 95, No. 14, 1991
Johnston et al.
m Z
W
label
c, mM
exptl theoret'
s0.2
2.24 5.60 7.20 8.96 11.2 16.8 22.4 33.6 44.8 56.0 67.2 78.4 89.6 101 112
1.08 1.21 1.26 1.40 1.45 1.71 2.01 2.50 2.89 3.56 4.09 4.62 5.50 6.24 7.06
SO., S0.S s1.0
SI., s2.0
n -.-
n
150
l
%.O
250
350
WAVELENGTH, (nm) Fipn 5. Emission spectra (uncorrected for spectral response to the analyzer) of trans-decalin + benzene solutions excited at 161 nm. So (neat trans-decalin), SI(0.01 12 M), S2(0.0224 M),S4 (0.0448 M), s 6 (0.0672 M),and Slo(0.112 M).
mented on in a previous paper.' The ratio of the cis-decalin to the sodium salicylate intensity is a measure of the relative efficiency for internal conversion in cisdecalin from states generated at A, to the emitting state, C*. This ratio is shown plotted in Figure 4. The falloff from A, 2 185 nm to A, zz 200 nm is a consequence of the absorption cutoff of the cis-decalin. However, from 150 to 180 nm, the conversion efficiency seems to be essentially constant. The slight dip at 165 nm is considered to be an artifact of the monitoring screen. As will be discussed below, the sensitization efficiencies per encounter, (Be)requires an absolute comparison of the benzene emission obtained at A, = 161 nm with that obtained by direct excitation of the benzene into its emitting state. To make valid this comparison, collection efficiencies must, of course, remain the same for the two excitation wavelengths. For our system (especially with its short path length cell), this requires a decadic absorption coefficient of at least 20 cm-I. Accordingly, even were we to excite at the peak of the benzene first absorption system at 255 nm, only our Slosolution would qualify. Accordingly, we have, instead, excited at A, = 210 nm which is found to give an integrated benzene emission intensit Le., P''(S,) which is constant for all n 1 1.5. The ratio JP'(S,)/~'(S,,) (which it should be recalled is already corrected by the ratio of the exciting light intensities at 210 nm to that at 161 nm), when divided by qcBis found to be independent of n. Thus, for example, in one series of experiments, the ratio JLm(S,)/JZ,"(S,) was measured to be 0.301,0.493,0.649,0.742,0.819,0.849 for n = 1.5, 3, 5 , 7, 9, and 10, respectively. Dividing these by cpcB (see eq 1 and Table I) gives, respectively, the values 1-16, 1.18, 1.16, 1.12, 1.13, and 1.14. Averaging over 24 such experiments gave finally the ratio
The effect of air on the intensity of neat cisdecalin fluorescence was obtained by comparing air-saturated samples with samples that had been extensively purged with dry nitrogen and transferred to the sample cell within a nitrogen-flushed drybox. The quenching ratio was found to be 1.13 f 0.02. Also, an air-quenching ratio of 1.47 f 0.03 was determined for benzene when directly excited at 210 nm in the cis-decalin solvent. B. bmo-Decah Fourteen solutions of benzene in trumdecalin were studied in the same manner as were the cisdecalin solutions. Figure 5 shows the emission spectra (uncorrected for spectral response of the analyzer) for six of these solutions all excited at 161 nm. A cortccted spectrum for t r a d d n is shown in Figure ( S ,the , ) sensitization 2. The uenchin ratios, f ~ ~ ~ ( S o ) / I l ~and ratios JB 1% ( S , ) / f ~f# , ) art presented in columns 3 and 5 of Table 11. To obtain J B '(S,,),the neat trans-decalin spectrum was subtracted from the S, qpectrum exactly as was done for the cis-decalin system but, as is obvious from a comparison of Figures
S4.0
SS.0 s6.0 s1.0 SS.0 Sg.0 Slo.0
theoretd
exptl
predict'
1.08 1.21 1.25 1.34 1.43 1.66 1.90 2.40 2.95 3.52 4.14 4.79 5.48 6.20 6.95
3.48 4.30 5.68 6.76 10.3 14.4 21.1 29.8 38.0 47.8 55.7 67.9 71.2 88.0
3.15 3.90 6.00 6.75 10.6 15.1 22.5 28.3 38.4 46.3 54.3 67.5 78.6 90.8
1.08 1.21 1.25 1.34 1.42 1.65 1.89 2.40 2.95 3.54 4.16 4.83 5.52 6.26 7.03
'See footnote u Table I. bSee footnote b Table I. eq 4 with a = 23.44 M-l and @ = 6.93 M-I. dCorrected for static quenching with a = 25.45 M-l and @ = 5.79 M-I. ePredicted assuming the validity of eqs 9 and 10.
2'ol- 0.0 150
175
200
WAVELENGTH, (nm) Figure 6. Intensity of tronrdecalin fluorescence per unit incident photon of exciting light as a function of excitation wavelengths.
1 and 5, the overlap was now not as severe. The relative efficiency with which neat trumdecalin converts to its emitting state is shown plotted in Figure 6. A slightly greater decline in this efficiency is observed for the trans than the cis isomer, but, still, at 161 nm there does not appear to be much departure from unity. The absolute comparison of JL6'with 4" was performed as described for cis-decalin with the result that the ratio divided by enwas again found to be independent of n (for n > 2, Le., large enough to avoid concentration dependence of the collection efficiency) and to take the value
(3) Finally, by comparison of air-equilibrated and nitrogen purged samples, we obtain an air quenching factor for neat tramdecalin of 1.27 and for directly excited benzene in tram-decalin a value of 1.58. IV. Discussion From the Collins-Kimballz solution to the twcbparticle diffusion equation, it is simply d e r i ~ e dthat ~ * ~the reciprocal of the survival probability for quenching is related to the quencher concentration, c, by the relation (1
- wB)-' = (1 + crc)[l - d/zJ'ep erfc (531-1
where J' = @c/( 1
+ ~ r c ) ' and /~
CY
(4)
and @ are related to the C+and
The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 5527
Sensitization of Benzene Fluorescence by Decalin
In aerated solutions these are reduced by the quenching factors of 1.13 and 1.27 to 2.1 and 2.3 ns, respectively. (That 7 0 / 7 is indeed the steady-state quenching factor, has been confirmed by Lyke'O for a variety of saturated hydrocarbons). Accordingly, we would predict values of the mutual diffusion coefficient, D, of 0.84 X lC5 and 1.4 X l C 5m2/sfor the cis- and transdecalin systems, respectively. For the self-diffusion coefficients of the decalins, we have utilized Dullien's14 semiempirical equation that predicts these coefficients for 32 liquids with a standard deviation of 4%, i.e.
H
\ 0
H
D = 0.124 X 10-16V~/3RT/v~t) 1 .o
0
.100
.05
Figure 7. Quenching of cis-decalin fluorescence expressed as the ratio of the intensity of fluorescence at 230 nm from neat cis-decalin to the intensity of fluorescence at 230 nm from a cisdecalin solution containing benzene, as a function of benzene concentration. The filled circles are experimental points. The solid line is a fit to eq 4. I " ,
'
I
" " I " " . l
'
'
2
'
I
"
I
\ 0 M
8. Quenching of transdecalin fluorescence expressed as the ratio
of the intensity of fluorescence at 220 nm from neat rruns-decalin to the intensity of fluorescence at 220 nm from a rruns-decalin solution containing benzene, as a function of benzene concentration. The filled circles are experimental points. The solid line is a fit to eq 4.
B mutual diffusion coefficient, D (in cm2/s) and the effective encounter radius, R - g (in cm) via the equations
(7)
where V, and VM are the critical and molar volumes, t) is the viscasity, and all units are cgs. Using for cisdecalin the parameters (at 23.5 "C) of t) = 0.0314 P,IS VM= 154 cm3,15and V, = 512 cm3 gives D& = 0.41 X lC5 cm2/s and for the trans isomer, and V, 505 ~ mgives ~ with t) = 0.0205 P,15 VM= 159 m3,15 = 0.60 X lC5 cm2/s. The trans value is in good agreement k t h a t recently reported by Luthjens et al. (0.6 X lV5 cmz/s) but their reported cis value of 0.3 X l C 5cm2/s seems too low." For benzene in the two decalins, we compute DB = 0.49 X and 0.78 X lC5 cm2/s for the cis and trans isomers respectively, by scaling to the decalin self-diffusion constant using either the molar or van der Waals volumes18or by using the Lusis-Ratcliff equation.193 Thus finally we obtain mutual diffusion coefficients cm2/s for the cis and trans and 1.4 X of D = 0.89 X isomers respectively, both of which are in good agreement with those determined via eq 4. From eq 6, the effective encounter radii, R - g, for the cis and trans isomers are predicted to be 8.8 and 9.5 A, respectively. Since these exceed the van der Waal's contact distance of ca. 7.3 A,I8 the quenching reaction probability must be unity at 8.8 and 9.5 A, respectively, and, therefore, g can be taken qual to m.' Since R exceeds the contact distance, &, a correction should properly be made in eq 4 to accommodate those quenchers that lie within the annular volume, u, between R and R, and which generate a nondynamic, Le., 'static" quenching of the fluorescence. This effect has been treated theoretically by Andrt et al.zl but at the expense of introducing additional parameters whose magnitudes the quality of our data are inadequate to assess. Accordingly, we have simply assumed that, at the moment of light absorption, a fraction, e* of quenchers instantaneously reduce the fluorescence and, therefore, modify eq 4 to (1
- e-)-' = (1 + ac)eDf(l- d/2{ep
erfc
(c)]-*
(8)
where a and {are as previously defined and u = 4 4 R 3- R,3)/3. A similar procedure has been recently utilized by Luthjens et a1.I' It is not difficult to deduce from eq 8 that the introduction of a u # 0 will tend to decrease /3 and increase a (so long as u
