Quenching of the singlet and triplet state of benzene ...

Quenching of the singlet and triplet state of benzene in condensed phase H. A. KHWAJA, G. P. SEMELUK, AND I. UNGER

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Department of Chemistry, Universiry of New Brunswick, Fredericton, N.B., Canada E3B 6E2 Received October 5, 1983 G. P. SEMELUK,and 1. UNGER.Can. J. Chem. 62, 1487 (1984). H. A. KHWAJA, Quenching of the fluorescence of benzene by halocarbons in methanol, ethanol, acetonitrile, and cyclohexane has been investigated. It proceeds via a reversible exciplex intermediate formed between excited singlet benzene and ground state halocarbon. Quenching is controlled by the polarity of the medium. Excited singlet benzene interacts with acetonitrile solvent and a longer wavelength emission due to this complex is observed. The effect of CCI,, CH,CN, CHCI,, and CH,CC13 on the triplet and singlet excited states of benzene in a methylcyclohexane-isopentane mixture at 77 K has been examined. Carbon tetrachloride and acetonitrile are far more effective quenchers than either CHC13 or CH,CCl,. In the case of the latter pair enhanced intersystem crossover competes with complex formation. G. P. SEMELUKet I. UNGER. Can. J. Chem. 62, 1487 (1984). H. A. KHWAJA, On a CtudiC le piCgeage de la fluorescence du benzkne par les halocarbones dans le methanol, I'Cthanol, I'acCtronitrile et le cyclohexane. I1 se produit via un exciplex intermkdiaire reversible qui se forme entre le benzene singulet excite et 1'Ctat fondamental de l'halocarbone. Le piCgeage est control6 par la polarit6 du milieu. Le benzene a 1'Ctat singulet excite interagit avec I'acCtonitrile et on observe que la longueur d'onde de I'Cmission est plus ClevCe a cause de ce complexe. On a CtudiC l'effet du CC14, du CH,CN, du CHC13 et du CH3CC13sur les Ctats triplet et singulet excitCs du benzene dans un mClange de mCthylcyclohexane/isopentane a 77 K. Le titrachlorure de carbone et I'acCtonitrile sont des pikges beaucoup plus efficaces que le CHCI, ou le CH,CCl,. Dans le cas de cette derniere paire, une augmentation dans les 6changes entre systkmes est en compCtition avec la formation de complexe. [Traduit par le journal]

Introduction Exciplex formation between excited singlet or triplet state species and ground state partners of a different chemical nature has been demonstrated tibe a key pathway in fluorescence and phosphorescence quenching (1-3). Some exciplexes exhibit fluorescence which is characterized by a red-shift with respect to the uncomplexed excited monomeric state, a lack of vibronic structure, and different decay characteristics. Nonemissive exciplexes have also been shown to exist (2-6). In such cases, indirect evidence is obtained for the intermediacy of exciplexes. Typical examples are the quenching of aromatic hydrocarbons and carbonyl excited states by aromatic compounds (7), olefins (€9, and amines (9). In most of these systems the interposition of exciplexes is supported by a dependence of the quenching rate constant on both the ionization potential of the donor and the electron affinity or the reduction potential of the acceptor. Since exciplexes possess total or partial charge-transfer nature, the rate of these processes should increase when the dielectric constant and the polarizability of the solvent increase (10- 12), and this dependence should reflect the degree of charge transfer. A number of experimental (13, 14) and theoretical studies (15, 16) have confirmed this view. The decrease in exciplex emission in polar solvents appears to be due to ionic dissociation of the exciplex to free solvated radical ions (17). Halocarbons, and CCl, in particular, have been shown to be efficient fluorescence quenchers for a number of molecules (18-21). It has also been shown that polar solvents increase this quenching (22, 23). In a previous publication we reported on the fluorescence quenching of benzene vapour by a number of halocarbons (6). We report here on the effect of solvent polarity on this reaction in liquid phase and on the rates of quenching by CCl,, CH,CN, CHCl,, and CH3CCl, in solid phase. Experimental Materials Benzene (spectrograde 99.9%) was obtained from Fischer Scien-

tific Company and was used without any further purification. CC1, and CH,Cl, were GC spectrophotometric quality obtained from the J. T . Baker Chemical Company. CH3CC13 (analytical reagent grade 99.5) was obtained from BDH Chemicals Ltd. These halogenated methanes were further purified by gas-liquid chromatography using a Hewlett-Packard Model 7620 A chromatograph attached to a 579 A collecting unit, with He as the carrier gas; detection was by flame ionization. The chromatographic procedure gave a purity of >99.9% for each of the materials. CFCI, was commercially available from Matheson Company (>99%) and was used as received. The sources of the solvents were as follows: hexane, Sigma Chemical Company; ethanol, Consolidated Alcohols Ltd.; methanol and acetonitrile, BDH Chemicals Ltd.; methyl cyclohexane and isopentane, Matheson, Coleman and Bell. The solvents were used as received. Blank runs were made with solvent alone which showed no fluorescence emission at the maximum sensitivity of the instrument, in the spectral region of interest. Apparatus and procedures Absorption spectra were obtained with a Perkin-Elmer 402 spectrophotometer. Fluorescence spectra were recorded with a modified Aminco-Bowman spectrophotofluorometer (SPF). Most details of the apparatus and data collection and processing have been given previously (6, 24). All quenching experiments were carried out in deoxygenated solution at room temperature with A,, = 254 nm. Samples were degassed prior to use by the freeze-pump-thaw method with a minimum of Torr. Sample cells (1 cm x 1 cm) three cycles at better than 5 x were made of quartz and had a side arm made of Pyrex tubing to facilitate the freeze-pump-thaw procedure. Preparation of samples involved adding an aliquot of stock solution of benzene to each sample along with the required quantity of quencher and diluting to volume with solvent. The benzene concentration used was chosen such that it would be approximately 20% absorbing in a 1 cm cell at 254 nm; a 2 X lo-, M solution was found to be satisfactory. The quencher concentrations were in the 2 x lo-' M to 8 X M range. At A,, 254 nm and the concentrations used in this study, CC1, has an extinction coefficient of -0.14 L mol-I cm-I. Thus absorption due to C6H6is -200 times greater than that due to CC14 and k,'s should not be negligibly affected by absorption of the latter. Fluorescence yields are based on that of C6Hhbeing 0.06,0.04, and

CAN. J . CHEM. VOL. 62. 1984

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0.029 in hexane, ethanol, and methanol, respectively, at A,, = 254 nm (25). The solid phase studies were carried out in a 1 : I methylcyclohexane, isopentane mixture at 77 K. The cells were 2 mm cylindrical quartz tubes; these were placed in a quartz microdewar flask whose bottom was in the shape of a finger transparent to uv light. Dry warm NZ was circulated around the Dewar to prevent condensation on the outside of the Dewar and to reduce ice formation within the liquid nitrogen in the Dewar. An Aminco-Kiers rotating shutter operating at a chopping rate of 50 Hz was used to isolate and record phosphorescence spectra. All solutions were outgassed several times and hermetically selaed in vacuo prior to usage. The benzene concentration was 2 X 1 0 - 9 while quencher concentrations ranged from 0 to 0.5 M.

Results and discussion Liquid phase The effect of CCl,, CH,CCl,, and CH2C12,and CFCl, on the fluorescence intensities of solutions of C6H6 in hexane was examined. In each case quenching obeys a Stern-Volmer relationship. Figure 1 shows plots of @:/al versus the concentration of quencher, where and (Dl refer to fluorescence yields of C6H6 in the absence and presence of quencher, respectively. The least-squares fit of the data to a straight line was obtained in all cases by excluding the Stern-Volmer intercept (0,l) from the calculation as a more stringent test of how closely the best fit line extrapolates to an intercept of 1. The effect of CC14 on the excited singlet state of C6H6 was also examined in CH,CN, C2HSOH, and CH,OH. The results in C2HSOHand CH,OH are summarized in Fig. 2. Bimolecular quenching rate constants, k,, were derived from the slope of Stern-Volmer plots and the singlet fluorescence lifetimes T of C6H6.Values for T were obtained from the literature, for conditions similar to those used in this study. The data are given in Table 1. Halocarbons efficiently quench C6H6fluorescence in hexane with k,'s ranging from 3.4 x 10' L mol-' s-' for the C6H6CFC1, system to 6.7 x 10' L mol-' s-' for the C6H6-CC14 pair. Quenching is more efficient by about two orders of magnitude in aromatic-halocarbon systems than in aliphatic ketone halocarbon systems (1 8). In the absence of a distinct exciplex emission quenching cannot automatically be ascribed to the formation of a charge transfer complex. Other quenching mechanisms need to be ruled out and typical exciplex behaviour such as a linear dependence of In k, vs. EA (electron affinity of acceptor) or IDD (ionization potential of donor) must be established (28). In the present instance, singlet-singlet energy transfer between C6H6 and halocarbons can be ruled out since the fluorescence spectrum of C6H6does not overlap with the absorption spectrum of any of the halocarbons. Quenching due to the enhancement of nonradiative processes is unlikely. No evidence was observed for photochemical reaction subsequent to excitation nor was there any evidence for ground state complex formation, the absorption spectrum of C6H6remaining unchanged on addition of each of the halocarbons. There is a reasonably good linear correlation between In k, and EA (see Fig. 3). This, coupled with gas phase studies on the same systems (6) and improbability of other quenching paths, supports a charge-transfer complex as being responsible for the quenching of the C6H6 fluorescence. Finally, k, is sensitive to solvent polarity which is typical of charge transfer exciplexes (29). A slope of 3 . 3 eV-I is obtained from Fig. 3, a value similar to that obtained by us (6, 30) for the quenching

FIG. 1. Stern-Volmer plots for the quenching of benzene fluorescence by CC14, CH3CC13, CH2C12, and CFCI, in hexane solution M , hCx= 254 nm, T = 298 K). ([C&j] = 2 X 4

I

I

I

I

0 Ethanol

FIG. 2. Stern-Volmer plots for the quenching of benzene fluorescence by CCI, in ethanol and methanol solutions ([C6H6] = 2 X M, A,, = 254 nm, T = 298 K).

E A (eV) FIG. 3. Plot of In k , vs. EA for benzene singlet quenching by CC14, CH3CCI3, CHZCIZ, and CFCI, in hexane solution.

of singlet aromatics by chloromethanes in the vapour phase. The magnitude of the gradient is much less than 1/RT = 39 eV-I, the value expected for a full electron transfer from excited aromatic to quencher.

KHWAJA ET AL.

TABLEI. Solution phase singlet state quenching of benzene by halocarbons (A,, = 254 nm, T


Quencher

Solvent

Dielectric constant

=

298 K)

Viscosity" (cp)

CC1,

Hexane

1.88

0.31

CH3CC13

Hexane

1.88

0.31

CHrClz

Hexane

1.88

0.31

CFCI,

Hexane

1.88

0.31

CCI,

Ethanol

24.6

1.08

CCl,

Methanol

32.7

0.55

Slope (L mol-I) % st. dev.

(ns)

k , x lo-' (L mol-' s-I)

33.6'

6.72

33.6'

1.57

33.6'

0.47

33.6'

0.34

28"

10.31

20'

8.60

T

225.8 5.2 52.6 8.1 15.8 5.8 11.5 4.9 288.8 5.5 171.9 9.8

"Reference 26. bReference 27. "Reference 25.

Solvent viscosity has been postulated to have an effect on the rate of formation and dissociation of exciplexes (3 1). Though not extensive, our study indicates that solvent viscosity does not play an important role in exciplex formation in the C6H6- halocarbon systems. On the other hand, solvent polarity clearly does enhance quenching and a mechanism consistent with the results is given below as reactions [ l ] to [6]

where D and Q are the ground state benzene and halocarbon molecules, respectively, an asterisk indicates the electronically excited state, and the prefixed superscript is the multiplicity of the electronic state. Employing the usual steady-state treatment we obtain eq. [71. = 1 + KsJQ1 [71 @;I@, The Stern-Volmer constant K,, is given by eq. [8].

[81

Ksv

=

kq7

The lifetime of the excited singlet benzene, T, is ( k l + kJ1 and the measured quenching rate constant, k,, is given by eq. [9].

In nonpolar solvents k6 is zero; in polar solvents it has a nonzero value which increases with increasing dielectric constant of the solvent. As k6 increases, the numerator in eq. [9] increases more rapidly than the denominator; thus k, would be expected to increase with increasing solvent polarity which is consistent with experimental observation. Studies in CH3CN solvent showed that a complex is formed

I 250

300

350 400 Wavelength (nrn)

450

I 500

FIG.4. Fluorescence and phosphorescence emission spectra of benzene perturbed by CCI, in MCH/ISPT at 77 K ([C6H,] = 2 X M, A,, = 254 nm).

between it and excited singlet C6H6. In addition to the normal fluorescence band of C6H6centered around 276 nm, two new structureless bands were observed with maxima at -355 and 405 nm. These data do not warrant any further conclusions at this time.

Solid phase Quenching of C6H6 fluorescence and phosphorescence in solid phase by CC14, CH3CN, CHCl,, and CH,CC13 is depicted in Figs. 4 to 7. Our experimental setup for solid phase studies


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CAN. 1. CHEM. VOL. 62. 1984

250

300

350

400

450

500

Wavelength (nm)

FIG. 5. Fluorescence and phosphorescence emission spectra of benzene perturbed by CH3CN in 3 MP at 77 K ([C6H6]= 2 X M, A,, = 254 nm).

I

I

I

,

I

I

250

300

350

400

450

500

Wavelength ( n m

FIG. 6. Fluorescence and phosphorescence emission spectra of benzene perturbed by CHCI, in MCH/ISPT at 77 K ([C6H6]= 2 x M, A,, = 254 nm). did not allow for the determination of amount of light absorbed; consequently, we were not able to determine quantum yields. However, care was taken to ensure that the amount of light absorbed would be approximately the same for each of the glassy matrix solutions. Thus, Figs. 4-7 are expected to

Wavelength (nm)

FIG. 7. Fluorescence and phosphorescence emission spectra of benzene perturbed by CH3CC13in MCH/ISPT at 77 K ([C6H6]= 2 X M, A,, = 254 nm). present a reasonable comparison picture of the effect of these quenchers since the same experimental setup when used for vapour and liquid phase studies showed very little variation in absorbed light intensity from run to run. Inspection of Figs. 4 and 5 shows that CCl, and CH3CN are comparable and efficient quenchers of the emission of C6H6 excited at 254 nm. The phosphorescence emission is quenched to the same extent as the fluorescence and this indicates that CCl, and CH3CN have little effect on the triplet state of C6H6 with the decrease in its production being due to a decrease in singlet state population. Inspection of Figs. 6 and 7 shows that CHC13 and CH3CC13are also comparable quenchers but far less effective than CCl, and CH3CN. It takes 4 X lo-' M of CHCl, or CH3CC13to give approximately the same amount of fluorescence quenching as that produced by 4 x M CCl,. At these high concentrations of CHC13 and CH3CC13the enhanced intersystem crossover apparently competes with fluorescence quenching and a relative increase in phosphorescence is observed (see Figs. 6 and 7). Clearly, halocarbons can promote two reactions viz., reactions [lo] and [ l l ] .

For CCl, (and probably CH3CN) klo%= kl whereas for CHC13 and CH3CC13,klo must be of the same order of magnitude as kll.

Acknowledgement The authors are grateful to the Natural Sciences and Engineering Research Council of Canada for financial support of this work. 1. A. WELLER. The exciplexes. Edited by M. S. Gordon and W. R. Wale. Academic Press, New York, NY. 1975. p. 23.


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