G. DASGUPTAAND D. PHILLIPS
3668 Aclcnozv1edg;rzent. This work was supported by the Office of Waler Resources Research, Department of Interior, and by the Office of Naval Research. The
Raman instrumentation was purchased with matching funds from the National Science Foundation and the University of Rhode Island.
uenchi.ng off the Excited States of Benzene and Benzenes by Olefins
by 6. Das Gupta and D. Phillips” Uopcxrtment of Chemistry, The University, Southampton SO9 BNH, England
(Received March 16, 297.2)
The quenching of the excited singlet state of benzene and substituted benzenes in the vapor phase by various olefins has been investigated, and rate constant data have been tabulated. The possible role of ti chargetransfer quenching mechanism is discussed. Quenching of 3Blubenzene by the olefins, probably through an energy transfer mechanism, was also investigated, and rate parameters were determined.
Introduction The quenching of the excited singlet state of benzene (‘Bz,) b) various mono- and conjugated and nonconjugated oI&nsl and other a-bonded molecules2 has been studieid recently by monitoring fluorescence yields. The interaction between the excited state of the aromatic and the olefin can lead to adduct f ~ r m a t i o nand ,~ selection ruleo14based on orbital symmetry considerations, have bcen proposed for such interactions. The quenching of the excited triplet state of the aromatic (3Bl,) by but-2-ene has been studied directly5 and results in exccllent agreement with those from competitive quenching exaerirnents in which either biacety16 or but-2-ene7bs was wed as a reference compound were obtained. The triplet quenching almost certainly arises in all cases from triplet energy transfer from the aromatic to the additive. The mechanism of singlet quenching can be, however, different in the case of different additives. In the case of ketones,Z CS2,2etc., the singlet state of the additive lies lower than that of the aromatic and thus an electronic energy transfer mechanism j s, probably responsible for quenching. For monoolefins ktowevex, electronic energy transfer is an endotherinic process, and thus would have to occur to a twisted ‘ nonvertical” staLe of the olefin. I n the case of conju:gated diolefins singlet energy levels are more nearly ~egenerate,and thus the energy transfer process would bc clxpected to be more efficient than in the case of monoolefins and nonconjugated diolefins. Although measured quenching rate constants for singlet quenching by the various molecules studied to date fit the above pattern, the role of chemical interactions The Journal
Q ;
Piiysical Chemistry, VoE. 76, hro. 24, 1972
must also be considered. Thus it has been proposed that the “allowedness” of the cycloaddition reaction of ground state buta-lj3-diene and ethylene to lBzu benzene can also explain the relative quenching efficiencies for these two molecule^.^ It is of interest to note that if charge-transfer interaction between the benzeneolefin collision complex can occur, the cycloaddition reactions become much more a l l o ~ e d . ~ It seemed of interest therefore to study the quenching interactions between the excited singlet state of benzenes and olefins containing substituent groups which might enhance such charge-transfer interactions.
Experimental Section The experimental system m-as similar to that described in earlier r e p o r t ~ , ~and ! l ~ details will not be given here. (1) A. Morikawa and R. J. Cvetanovic, S. Chern. Phys., 49, 1214 (1968).
(2) E. K. C. Lee, M. W. Schmidt, R. G. Shortridge, Jr., and G. A. Haninger, Jr., J. Phys. Chem., 73, 1805 (1969). (3) A. Morikawa, S. Brownstein, and T. J. Cvetanovic, J . Amer. Chem. SOL.,92, 1471 (1970). (4) D. Bryce-Smith, Chem. Commun., 806 (1969). (5) H . E. Hunsiker and H. It. Wendt, Chem. Ph3js. Letl., 12, 180 (1971).
(6) G. A. Haninger, Jr., and E. K. C. Lee, J . Phys. Chem., 73, 1815 (1969). (7) A. Morikawa and R. J. Cretanovic. Can. b. Chem., 46, 1813 (1967). (8) M. W. Schmidt and E. K. C. Lee, J . Amer. Chenz. Sac., 92, 3579 (1970). (9) K . Al-Ani and D. Phillips, J . Phys. Chern., 74, 4046 (1970). (10) D. Gray, K. hl-Ani, and D. Phillips, J . Chem. SOC.A, 905 (1971).
QUENCHING OF
THE
EXCITED STATESOF BENZENE
Materials. Allene. We wish to thank Dr. M. C. Flowers of this department for his gift of a sample of highly purifizd allene. Details of the other materials have been given in earlier reports.’ Ix Care was taken to ensure that diolefins were no t present in any of the quenching gases which might lead t o spurious results. I n all cases the level of impurity was such that the observed quenching could be attributed only to the additive itself and not impurity levels
lA,
-+
3Ae
lA, -+ A,X lAe 4- 0 -+ quenching
3A, --+ A
+ B +A + 3B 3A, + 0 quenching 3A,
-+
where A = benzene, B
=
biacetyl, 0
=
olefin,
Results and Discussion It has been found that at a pressure of 15 Torr of benzene vibmtional relaxation is incomplete, and the addition of satura ted hydrocarbon molecules causes reduction in tze fluorescence quantum yield of this pressure of b e n m x excited at 235.7 nm.’ In the liquid phase, the Auorrseence quantum yield is only 0.06,12 and this reduction compared to the gas phase value is attributable to some nonradiative decay process which is effected by collisions with the environment of the excited molecule, possibly via vibrational effects. In the present exoeriment, therefore, it is possible that vibrational relaxation-redistribution plays a part in the singlet quenching by some of the olefins used. However, it must be stated that addition of up to 100 Torr of CF4 caused no discernible decrease in the fluorescence quantum yield of 20 Torr of benzene excited a t 254 nm. This result is ur-expected if vibrational relaxation were playing if significant role in the present case. hforikawa and Gvetanovic have shown that the rate constant for apparent quenching of 15 Torr of benzene excited a t 2E14~ i mis of the order of lo8 1. mol-’ sec-l,l and thus mechsnisms other than vibrational relaxation can only be isonsidered where quenching rate constants significantly in excess of this value are obtained. It has been d e m ~ n s t r a t e d ~that ~ - ~thermal ~ equilibration produces a wice distribution of emitting levels of the excited state. There is no apparent reason to suppose that different Xiibrational levels, which have different fluorescence decay I imes, l6 will be quenched with equal efficiency by :tdded molecules. The quenching parameters quoted here thus represent average values for the vibrationa; dktribution produces by Boltzmann relaxation at 23”, but as such are directly comparable to those obtained by other workers under similar circumstances.’ The average fluorescence decay time and fluorescrnee quantum yield of this distribution of vibrational levels h a w been given as 77 f 3 nsec” and O.lSll* respectively, leading to an average value for the radiative lifetirnc. ( T R : of 428 17 nsec. The quenching of the excited singlet (‘Bz,) and triplet ( 3RI,) can be represented by the simple kinetic scheme
*
+ hv +‘A, ‘Ae--+ A + hvf A
(1) (2)
5
5
20 OLEFIN
40
60
PRESSURE
BO TORR
100
Figure 1. Plots of the reciprocal of the quantum yield of fluorescence ( @ f - l ) against pressure of added olefin; benzene pressure 20 Torr, temp 23”, exciting wavelength 253.7 nm: 0 , CHF=C=CH~; 6,CHZ=CHF 0, ClHC=CClH; 0, CF,CF=CFCF,; a, CH~=CFZ;0 , ClHC=CClH. (11) P.A. Ilackett and D. Phillips, J . Chem. Sac., Faraday Trans. 1 , 6 8 , 335 (1972). (12) J. B. Birks, “Photophysics of Aromatic ,Molecules,” WileyInterscience, New York, N. Y., 1970. (13) H. F. Xemper and M. Stockburger, J . Chem. Phys., 5 3 , 268 (1970). (14) C. S. Parameter and M. W. Schuyler, ibid., 52, 5366 (1970). (15) J. M. Blondeau and M. Stockburger, Ber. Bunsengas. Phys. Chem., 75, 450 (1971). (16) K. G. Spear8 and S. A. Rice, J . Chem. Phys., 55, 5561 (1971). (17) M. Nishikawa and P. K. Ludwig, ibid., 52, 107 (1970). (18) W. A . Noyes, Jr., W. A. Mulac, and D. A. Rarter, i b i d . , 44, 2100 (1966). The Journal of Physical Chemistry, Vol. 7 G , N o . 8.4, 1972
G. DASGWPTA AND D. PHILLIPS
3670
-
-__D---
Table I: Quenching of lBlu Benzene Vapor by Olefins; Exciting Wavelength 253.7 nm ks
x
10-8,a 1. mol-1 8ec-l
Additive
ua
1 . 7 5 f0 . 1 2.35 (0)" 2.35 f 0.1 2 . 5 =k 0 . 2 3.45 (0.75p 4 . 3 (1.5)" 4 . 7 (1.86)' 5.2 5.5 6.1 7.7 f0 . 7 7 . 6 f0 . 7 10.4 f 0 . 9 5 5 . 8 (52.0)' 57 95 (92)' 137 140 (136)' 149
CHz=--CIIF CHp~CHCEX3 CF3CI"CPCF'g CH-. 2-.C.F2 cEIaC:E3==CH(;H3 Trime thy lethylene 1,4-Penta.diene Tetratne thylet.hylene 1,5-Hexadiene 1,2-ButatIieiia CH2=C=:=GH;! trans-(~lEC=(XCl cis-ClRC ===C€ICl C€I2=:CErcEx==CH2 CH2=:CF[CH=&!I-In Isoprene trans-l.,3..Pentsdiene cis-1,3-Pentadiene cis-l,3-Pentadiken~: "
x
Ionization potential,c
loa?
cm8
eV
1 . 9 f.0 . 1 2 . 6 (O)@ 3 . 7 f 0.15 3 . 9 f0 . 4 4 . 0 (0.9)e 5 . 2 (1.9)" 5 . 7 (2.3)" 6.9 7.3 6.9 8 . 3 f0 . 8 10.5 f 1 14 =k 1 . 5 63 (58)' 66 116 167 173 (168)" 184
10.41 9.77 11.24 10.31 9.30 8.81
Referonce
d
f
d d
f f f f
. . *
8.41
...
f
9.32 10.32 9.86 9.65 8.99 8.99 8.97
f d d d
f 9
f B
. * .
8.64 8.64
f B
Results based upon a radiative lifetime for benzene vapor of 428 nsec. 6 Cross section 8 obtained from equation rZ = k~,(87rkT/ This work. e Figure in c Averaged data from R. P. Blaunstein and L. 0. Christophorou, Radiat. Res. Rev., 3, 69 (1971). parentheses is that eEtiniated to be due to electronic quenching alone. First figure includes effects of vibrational relaxation. f Reference 1. p Reference 2. a
P)-'/~.
scripts refcr ,o multiplicity, subscript e refers to the equilibrium vibrational distribution assumed to be reached prior t o nimther process occurring. The nature of the quenching steps 5 and 8 will be discussed later. The mechanism predicts the usual Stern-Volmer relationship that, Qr-'
E
1
+
k3To
+
16470
+
k570[0]
(1)
benzene a t the pressures used in the biacetyl-sensitized emission experiments the expression for the reciprocal of % reduces to
+ +
where @.is0 = k3/(kz k~ k4). Thus from the ratio of slope to intercept of plots in
where T~ := k2-l =: 428 4 17 nsec. Thus plots of a*--'againtit olefin concentration are expected to yield straight lines of slope k570. Actual plots are shown in Figure 1, and rate constants kj and corresponding quenching cross sections for the various olefins studied here are givcn in Table I together with comparable data for other olefins obtained previously. If the fate 3f the triplet biacetyl is accounted for by reactions 9 and 10 the complete expression for the quantum yield of sensitized phosphorescence from biacetyl is given by eq 11. Under thc conditions of the
3B -+ B
-+ hv,
3B +B @
=
P
0
/
(9) (10)
ka
+ + kP1)
_____._____I_
(kz i- k3
experiments carried out here k,[B]>> IC@. For olefins which do not significantly quench the singlet state of The Journal of Physical Chemistry, Vol. 76, No. 24,1972
10 .
2.0 3P PRESSURE OF
40 OLEFIN
5.0
6.0
TORR
Figure 2. Plots of the reciprocal of the quantum yield of phosphorescence from 0.375 Torr of biacetyl sensitized by 20 Torr of benzene (%-I) a t 253.7 nm and 23" as a function of added olefin pressure. Symbols as for Figure I.
QUENCHINGOF FEE EXCITED STATESOF BENZENE
3674.
Table 11: Quenching Parameters for 3Blu Benzene by Olefins in Competition wilh Biacetyl; Benzene Pressure 20 Torr, Biacetyl Pressure 0.375 Torr, Exciting Wavelength 253.7 nni, Temp 25' ka Quencher
x lo-@,@
1. mol-1
880-1
2.0 1.3I0.1 0.07 ir0.005 65 I 5 65 ir 5 8.91 8.95f 0.33 17 & 1 1.8 2.5 187.2
I
X 10le,* ems
0.2 0.15 ir 0.01 0.01ir0.001 9 rt 0.7 9 ?lc 0.17 1.01 1.02 2.7 i 0.15 0.18 0.25 21.7
Referenoe C
d d d d C
e d d
20
io
60
80
100
I20
140
200
120
f C
a Computed from best straight lines of plots in Figures 2 and 3, and using rate constant k? = 3.43 X 10101. mol-' sec-1 [C. S. Parmenter and B. L. Ring, J . @hem. Phys., 46, 1998 (1967)]. * Derived from the equation s2 = ks(8rrkT/fi)-'/Z. Reference 6. This work. *Direct measurement of Hunziker and Wendt.6 f Reference 8.
I
40 PRESSURE
80
OF ADDITIVE
--".-*--"-
2AO
280
TORR
Figure 4. Stern-Volmer plots of the ratio of the fluorescence quantum yield of substituted benzenes in the stbsence of additive (ao)to the fluorescence quantum yield a t any pressure of additive (ap,)as a function of added buta-1,3-diene and but-2-ene: e, lI2-difluorobenzene; 6, lI3-difluorobenaene; 8, 1,4-difluorobenzene; 0, l,4-bis(trifluoromethy1)benaene; Q, l-fluoro-2-(tr~uoromethyl)benzene; @, I-fluoro-3-(trifluoromethyl)benzene;-____ , buta-1,3-diene additive; - - -, cis-but-2-ene additive. Aromatic pressure = 2 Torr, exciting wavelength 266 nm.
Where the olefin also quenches the singlet state efficiently, plots of @ Q / @ ~against [Q] allow estimation of the rate constant for the triplet>quenching. Thus
These plots are shown in Figure 3, and values of k8 given as before in Table 11. Nature of Reaction 6. Excluding vibrational relaxation the possible mechanisms by which the singlet state of benzene may be quenched include (a) electronic en-----s. 20 40 Bo 80 100 ergy transfer, (b) charge-transfer complex formation, PRESSURE OF OLEFIN TORR (c) chemical reaction, and (d) enhancement of SI+T1 Figure 3. Plots of +r/% against pressure of added olefin. intersystem crossing. In the case of the olefins used Conditions as for Figure 2, symbols as in Figure 1. here, energy transfer to the olefin from lBg, benzene would be an endothermic process and can thus be ruled out. Studies with high pressures of methyl halides and Figure 2 of %--. againrt [O], the ratio k8/k,[B] can be obtained. JC7 is Imown to be 3.43 X 1Olo 1. mol-l ~ e c - ~ , ' ~more heavily fluoro- and chloro-substituted alkanes have not revealed any evidence of enhancement of [R] is known, and thus k8 can be evaluated. Table I1 gives values of IC, with correspo1idin.g qucnching cross (19) C. S. Parmenter and B. L. Ring, J . Chem. Phys., 46, 1998 sections. (1967). The Journal of Physical Chemistry, Vol. 76, No. 2.4, 197%'
G. DASGUPTAAND D. PHILLIPS
3672 Table 111: Quenching of the Excited Singlet States of Substituted Benzenes by cas-But-2-ene and Ruta-I ,&diene, Vapor Phase Quenching rate constant X 1010, 1. mol-1 sec-1
Quenching crosa seotion X 10-11,
Refer-
Quencher”
Exciting wavelength, nm
om2
ence
0
BD B
254 254
0.57 0.035 (0.008)
0.66 0.04 (0.009)
b
@-CIT,
BD
254
1.54
1.87
b
BD B
266 266
2.23 0,066
3.2 0.085
d d
BD B
266 266
5.57 0.93
7.4 1.22
e e
BD B
266 266
2.68 i0.15 0.28 4 0.02
3 . 4 f 0.17 0.35 4 0.03
f
BD B
266 266
2.97 f 0.15 0.77 =k 0 . 3
3.75 4 0.19 0.97 i 0.4
RD B
266 266
3 . 6 rt 0 . 3 0.58 f 0.03
4.6 f 0 . 5 0.73 f 0.04
BD B
266 266
10.67 2.45
14.7 3.4 f 0 . 3
BD B
266 266
10.8 2.8 f 0 . 2
14.9 3.7 f 0.3
BD
266
0.9
x .3
h
BD
266
1.16
1.15
h
BD
266
2.98
3.83
h
BD B
266 266
6.9 f 0.3 0.7 =t0 . 1
9.4 4 0.5 0.9 f 0 . 1
f
BD B
266 266
5 . 6 rt 0.3 1.25 4 0 . 1
8 . 3 =k 0 . 5 1.6 f0 . 1
Aromatic
molecule
0CI.‘ L
6
F
CF,
CF;S>--CF;
s
F’
F
b 4 H J
CH
F
