11051
Quantum Yield and Lifetime of the Lowest Triplet State of Benzene
Concentration and Temperature Dependence of the Quantum Yield and Lifetime of the Lowest Triplet State of Benzene in the Liquid Phase Robert R. Hentz* and Ronald M. Thibault Department of Chemistry and the Radiation Laboratory,' University of Notre Dame, Notre Dame, lndiana 46556 (Received October 27, 7972) Publication costs assisted by the U.S. Atomic h e r g y Commission
The 253.7-nm photosensitized isomerization of trans-2-octene has been studied in liquid benzene and itri mixtures with methylcyclohexane at 295 and 195°K. Quantum yield of the benzene triplet state de.. creases from 0.71 for neat benzene to 0.24 for 0.05 M benzene a t 295°K and from 0.94 for 0.56 M benzene to 0.85 for 0.056 M benzene at 195°K. Such results, with complementary fluorescence results in the liter. ature, give temperature-independent specific rates of 8.7 X lo6 and 35 X lo6 sec-l for intersystem crossing from the monomer excited singlet and excimer singlet of benzene, respectively. The specific rate of internal conversion from the monomer excited singlet of benzene decreases from 25 X 106 sec-1 at 295°K. to zero a t 195°K and that from the excimer singlet is zero at both temperatures. The benzene triplet lifetime a t 295°K is found to decrease from 0.5 psec in 0.05 M benzene solution to 6 nsec in neat benzene, corresponding to a specific rate of 1.7 x lo8 sec-l for quenching of triplet benzene by ground-state benzene (with concentration expressed as mole fraction); extrapolation to infinite dilution gives 1 psec as the lower limit for the intrinsic lifetime of triplet benzene in the liquid phase a t 295°K. Isomerization of trans-2-octene photosensitized by toluene and p-xylene also was studied. From the results a t 295"K, spe.. cific rates are obtained for the decay modes of the monomer excited singlets of toluene and p-xylene. For the monomer excited singlet a t 295"K, the specific rate of intersystem crossing increases and that of the activated internal conversion decreases in the sequence benzene, toluene, p-xylene.
lar13-15 studies were reported. Results of those studies are Introduction discussed in relation to the present results. When the work reported in this paper was initiated, certain published results suggested a concentration deExperimental Section pendence of the quantum yield and lifetime of the lowest Sources, quality, and purification procedures for ben. triplet state of benzene in the liquid phase. Triplet quanzene and methylcyclohexane were the same as those in tum yields, X, of2 0.57 and3 0.58 had been obtained for the fluorescence lifetime study of Gregory and He1man.l'' neat benzene excited at 253.7 nm. However, Sandros4 had The trans-2-octene (Aldrich Reagent Grade) was deaerat. reported x = 0.25 for 0.029 M benzene in cyclohexane. ed, transferred on to LiAlH4, and stirred overnight to re. From results of a study of the fluorescence lifetime of benmove water and peroxides. The olefin then was vacuum zene in cyclohexane solutions, x = 0.21 was predicted for distilled four times by bulb-to-bulb transfers in which the the infinitely dilute s ~ l u t i o n A . ~ triplet lifetime, 37, of 7 nsec had been estimated for benzene in the neat l i q ~ i d . ~ middle fraction from each distillation was retained for the next distillation. The deaerated middle fraction of the However, Sandros4 had reported 37 = 2 psec for 0.029 M final distillation was stored in an evacuated bulb at -25' benzene in cyclohexane, a value in good agreement with and contained about 0.08% cis-2-octene and 0.05 wt % of an earlier estimate of Lipsky6 for dilute solution but inunknown impurity. No increase in the amount of cis isocompatible with results of Dubois and coworkers.' In a mer was observed after storage or preparation of solutions. study of the pulse radiolysis of benzene-cyclohexane mixtures, Thomas and Manis associated a growth in optiThe Radiation Laboratory of the University of Notre Dame is operated under contract with the U. S. Atomic Energy Commission. This cal absorption a t 320 nm with a transient species having is AEC Document No. COO-38-869. triplet benzene as precursor and obtained half-lives of 3 R. B. Cundall and W. Tippett, Trans. Faraday Soc., 66, 350 (1970). and 20 nsec from the growth curves for neat and 10% benR. R. Hentz and L. M. Perkey, J. Phys. Chem., 74,3047 (1970). K . Sandros, Acta Chem. Scand., 23, 2815 (1969). zene, respectively. Thus, the present study was undertak(5) W. P. Helman, J. Chem. Phys., 51, 354 (1969). en to examine such apparent effects of concentration in a (6) S. Lipsky, J. Chem. Phys., 38, 2786 (1963). systematic manner. (7) J. T. Dubois and F. Wilkinson, J. Chem. Phys., 38, 2541 (1963); J. T. Dubois and F. Wilkinson, ibid., 39, 377 (1963); J. W. van Loben The 253.7-nm photosensitized isomerization of trans-2Sels and J. T. Dubois, ibid., 45, 1522 (1966). octene has been studied in liquid benzene and its (8) J. K . Thomas and I. Mani,J. Chem. Phys., 51, 1834 (1969). mixtures with methylcyclohexane a t 295 and 195°K. The (9) F. Hirayamaand S. Lipsky,J. Chem. Phys., 51, 1939 (1969). (10) T. A. Gregoryand W. P. He1man.J. Chem. Phys., 56, 377 (1972). results elucidate concentration dependence of the lifetime (11) R. V. Bensasson, J. T. Richards, and J. K. Thomas, Chem. Phys. of the lowest triplet state of benzene and, with compleLett., 9, 13 (1971). R. B. Cundall and D. A. Robinson, Chem. Phys. Lett., 13, 257 (12) mentary fluorescence quantum yields9 and lifetimes,lO (1972), make possible a quantitative description of decay pro(13) R. B. Cundall, L. C. Pereira, and D. A. Robinson, Chem. Phys. Lett., 13, 253 (1972). cesses of the lowest excited monomer and excimer singlet (14) R. B. Cundall and D. A. Robinson, Chem. Phys. Lett., 14, 438 states of benzene in the liquid phase. While the present (1972). work was in progress, some closely related1°-12 and simi(15) K. Sandros, Acta Chem. Scand., 25, 3651 (1971). The Journal of Physical Chemistry, Voi. 77, No, 9, 7973
1106
Toluene (Matheson Coleman and Bell, Spectroquality) was distilled twice with a Nester-Faust spinning-band column; the middle third from the first distillation was redistilled, and the middle third from the second distillation was retained. The p-xylene (Eastman Organic Chemicals, Red Label Grade) was recrystallized three times with rejection of about one-fifth of the starting material each time. Purity of all reagents used was checked by gas chromatography. Solution preparation has been described.1° Solutions were deaerated either by about nine freeze-pump-thaw cycles or by a modified method described by Gregory and Helman.lo The method of deaeration had no effect on isomerization yields. Procedures, cells, and equipment used in photolyses and actinometry a t 295°K were the same as those used in previous work3 except that the cells were individually calibrated in the present work. A previously describedlO dewar cell was used for photolyses a t 195°K. Deaerated solution was transferred from a storage bulb with break seal via a vacuum manifold into the dewar cell. The filtered3 emission of an Amersil low-pressure mercury lamp was monitored by a Cintra quantum radiometer with 253.7-nm probe. With electrical and thermal regulation and a 24-hr warmup period, lamp intensity varied less than 5% during photolysis as determined by averaging sets of three actinic measurements performed before and after each set of six photolyses. Solutions were stirred rapidly during photolysis, and exposure time was determined accurately with a shutter. In all photolyses more than 99.5% of the incident light was at 253.7 nm and more than 99.9% of the light entering the cell was absorbed. Incident intensities were near 1018 and 1019 quanta cm-2 hr-l for photolyses at 295 and 195"K, respectively. A Beckman GC-5 gas chromatograph with flame-ionization detection and two columns in series was used for determination of the cis and trans isomers of 2-octene. Separation of methylcyclohexane and the aromatics from the olefins was achieved on the first column, for which ys-in. columns of various lengths (0.5-5 ft) packed with 20% squalane on 80-100 mesh firebrick were used. Complete separation of the 2-octene isomers was accomplished on the second column, a l/g-in. x 10-ft column packed with 80-100 mesh firebrick coated with 20% by weight of a solution of 33% AgN03 in glycerol. In some solutions 3methylheptane was used as an internal standard. Measurements of fluorescence quenching were performed with calibrated quartz cells and a modified Cary Model 14 spectrophotometer. Fluorescence was measured from the cell face used for excitation by 253.7-nm light from the combination of a 735A7 Hanovia lamp with a high-intensity Bausch and Lomb monochromator. Absorption spectra were determined with a Cary Model 14-R spectrophotometer. Results The 253.7-nm photosensitized isomerization of trans-2octene was studied in benzene and its mixtures with methylcyclohexane (MCH) and in dilute MCH solutions of toluene and p-xylene a t 295 and 195°K. Except for determination of the photostationary isomer ratio, the amount of isomerization was usually about 3% and never exceeded 6%. Within the precision of measurement ( - 5 % ) , no loss of total 2-octene was detectable by gas chromatography. A photostationary ratio of cis to trans isomer concentrations of unity was obtained in neat benThe Journal of Physical Chemistry, Voi. 77, N o . 9, 1973
Robert R. Hentz and Ronald M. Thibault
32r---l
Figure 1. Concentration dependence of the corrected quantum yield for 253.7-nm photosensitized isomerization of trans-2-octene in neat benzene at 295°K.
zene and 0.05 M benzene solution at 295°K and in 0.56 M benzene solution at 195°K. Quantum yields of trans-to-cis isomerization were calculated with corrections for back reaction and presence of the small initial fraction of cis isomer16 and (as described in the Discussion) for a small amount of quenching of benzene excited singlets at trans-2-octene concentrations greater than 0.02 M . Dependence of such "corrected" quantum yields, @(t c), on concentration of trans-2octene is illustrated for neat benzene and 0.05 M benzene solution a t 295°K in Figures 1 and 2, respectively, which show plots of @ ( t c ) - l us. [trans-2-octene]-l. Vertical bars represent the uncertainty (one standard deviation) in values of @(t c)-l; the computer-drawn line is a leastsquares fit to the points with the standard deviation of each point as a weighting factor. Similarly good straight lines were obtained from such plots for each solvent composition studied at each temperature. From the slope and intercept of each such line, values were calculated (as described in the Discussion) for the quantum yield of triplet benzene, X , and the product 37312 of its lifetime and specific rate of triplet transfer to trans-2-octene. The results are presented in Table I. Corresponding results obtained in the same manner for toluene or p-xylene in MCH are presented in Table 11. The intensity I of benzene fluorescence at 282.5 nm, entirely that of the monomer excited singlet, was measured at 295°K for each benzene concentration with each concentration of trans-2-octene (including zero for which I Io) that was used in the isomerization experiments at 295°K. For each solvent composition, the plot of I o / I us. concentration of trans-2-octene was linear. From the slopes of such plots, l h 1 ~= 1.5 M - l was obtained (in fair agreement with previous work3) as the product of specific rate of quenching
-
-
-
( 1 6 ) A. A . Lamola and G. S. Hammond, J. Chem. Phys., 43, 2129
(1965).
1107
Quantum Yield and Lifetime of the Lowest Triplet State of Benzene
O
12
4
20
[trons-0ctene-2]-', units of lo2M:' Figure 2. Concentration dependence of the corrected quantum yield for 253.7-nm photosensitized isomerization of trans-2-octene in 0.05 M benzene in methylcyclohexane at 295°K.
WAVELENGTH (nm) Flgure 3. Absorption spectrum of the photoproduct in 253.7-nm photolysis of solutions of benzene and trans-2-octene in methylcyclohexane at 295'K. TABLE II: The 253.7-nm Triplet Quantum Yield, x,and 3 k 3for ~ Triplet Transfer to trans-2-Octene for the Solutes, S, Toluene and p-Xylene in Methylcyclohexane [SI, M
r,
0.05
295 195
3 k 3 r ,M-'
X
OK
TABLE I: The 253.7-nm Quantum Yield, x,of Triplet Benzene and
a. Toluene
3k37 for Triplet Transfer from Benzene, B, to trans-2-Octene in Methylcyclohexane
0.056
0.419 f 0.004a 0.024 0.707
*
b. p-Xylene
0.05
a. 295°K 0.05 0.10 0.25
0.50 2.00 6.00 11.2
0.236 f 0.005= 0.003 0.276 0.007 0.313 0.385 0.010 0.56 f 0.02 0.02 0.69 0.71 f 0.04
* * * *
b. 195'K 0.056 0.111 0.56
0.851 f 0.036 0.007 0.866 0.943 f 0.036
*
1600 f 120a 1170 f 60 410 f 30 340 60 98 f 9 31.6 f 1.6 20.4 f 1.6
*
* *
2300 400 820 40 250 f 20
Standard deviation
and lifetime of the excited singlets of benzene." The value of IOII obtained for each solution was used for calculation of the corrected quantum yield from the measured quantum yield of isomerization. No products other than the cis isomer were detected by gas chromatography in any of the experiments. However, in experiments a t 295°K with concentrations of trans-2octene greater than 0.02 M , the absorption spectrum of a photoproduct (shown in Figure 3) was observed in benzene solutions and with reduced yield, in toluene solutions but not in p-xylene solutions. For 0.1 M trans-2-octene in benzene at 295"K, is estimated for the photoproduct quantum yield which increased with increase in concentration of trans-2-octene and with decrease in benzene concentration. The photoproduct slowly disappeared in solutions kept a t room temperature. The photoproduct
0.056 a
0.566 f 0.009 0.008 0.608
295 195
*
*
2460 180a 2700 f 900
*
620 40 1300 f 100
Standard deviation
absorption spectrum was not detectable in any of the experiments a t 195°K.
Discussion The results are interpreted with the following kinetic scheme in which benzene is denoted by B and 2-octene is denoted by 0 with subscripts t and c for the trans and cis isomers, respectively; multiplicity of an excited state is indicated by a left superscript (its absence indicating the ground state), and the subscript 2 denotes excimer.
hv
'B
B 'B, 'B2
- + -
+
B
B
'B
B
'B
-
+
'B
'B
(1)
hv,
(2)
3B 'B,
-- ++ 'B 2B
'B,
(3)
2B
B hv,'
(4)
(5) (6) (7) (8)
(17) Absence of an observable dependence of lklT on benzene concentration IS attributable to the monomer excited singlet and excimer
singlet having approximately equal decay specific rates at 295"K1° and approximately equal quenching specific rates The Journal of Physical Chemistry, Vol. 77, No. 9 , 1973
1108
Robert R. Hentz and Ronald M. Thibault
- + + - + + - + lB,
0, 0,
B
3B
'B
B
'Bp
2B
(9)
0, 0,
(10) (11)
x f d e + f d m = fck,/he + / , k , / k , (IV) gives eq V. Thus, with values of +(t --c c) calculated from the measured values of and l / I o for each solution, the
-.
+
Q(t ~ 1 - l = ( o . ~ x ) - ~ (+I 1/3k3~[0,]) (v) straight line obtained in a plot of +(t - * c)-1 us. [ U t ] - ] (such as those in Figures 1 and 2) is identified with eq V, (13) B + 3B - 2 B and values are obtained for x and 3 k 3 7 from the slope and 0, + 3B 3O B (14) intercept. Because of the experimental difficulty of accurate fluorescence quenching measurements a t 195"K, values of 30- P O , + (1 (15) x and 3k31 were calculated by assuming negligible quenchThe following additional symbols are introduced to siming of the excited singlets of benzene by trans-2-octene in plify mathematical expressions: k , = k2 + k3 + k4; k , = all solutions studied a t 195°K; from the assumption k 1 1 [ 0 t ]
