Triplet state properties of trichlorobenzenes: a laser flash photolysis study

JournalofPhotochemistry

and Photobiology,

A: Chemistry,

41 (1988)

147 - 155

147

TRIPLET STATE PROPERTIES OF TRICHLOROBENZENES: A LASER FLASH PHOTOLYSIS STUDY C. M. PREVITALIt Radiation

Laboratory,

University

of Notre Dame,

Notre Dame,

IN 46556

(U.S.A.)

(Received April 14, 1987; in revised form May 29, 1987)

Summary The T-T absorption spectra for 1,2,3-, 1,2,4- and 1,3,5-trichlorobenzene have been determined by laser flash photolysis in cyclohexane and methanol. The wavelength of maximum absorption is around 300 nm in both solvents for the three isomers, A second, less intense band appears at about 420 - 450 nm. The extinction coefficients for the T-T absorption and the intersystem crossing quantum yields were determined by energy transfer to anthracene. The triplet decay was found to be a function of the energy of the laser pulse and of the concentration of the trichlorobenzenes. The triplet lifetimes extrapolated to zero laser intensity and concentration were in the range 2 - 4 ps in all cases. T-T annihilation rate constants were determined and the values corresponded to the diffusional limit. Rate constants for triplet deactivation by ground state molecules were measured for 1,3,5trichlorobenzene in cyclohexane and methanol and for the 1,2,4- isomer in cyclohexane, with values in the range (4 X 106) - (3 X 107) M-l s-r. For the other isomers an upper limit of 3 X lo6 M-i s-r was estimated for this process. The role of the triplet state in the photochemistry was discussed using literature data for the photodecomposition quantum yields.

1. Introduction The photochemistry of polychlorinated benzenes has received considerable attention, particularly because of the importance of these compounds as environmental pollutants. However, from a mechanistic point of view there are many aspects which are still far from clear. In many cases there is a lack of agreement among different workers about the intermediates involved in the photoprocesses. Photodechlorination is the main reaction observed in all cases. Relatively high decomposition quantum yields are commonly found [l - 81. tpermanent address: Departamento de Qufmica y Fisica, Universidad Nat. de Rio Cuarto, 5800 Rio Cuarto, Argentina. lOlO-6030/88/$3.50

@ Elsevier Sequoia/Printed in The Netherlands

148

Quantum yields are dependent on the substitution pattern and also on the solvent. However, even in very similar solvents wide discrepancies in the reported values can be observed. Thus, for example, for the dechlorination of p-dichlorobenzene a quantum yield of 0.17 was determined in cyclohexane [7] while other workers found a value of 0.004 in n-hexane [3]. This lack of agreement could be due to several factors. In some cases the photoreactions are carried out to a high decomposition percentage, and secondary reactions become important. Another probable cause of discrepancies is that different workers employ different concentrations of the chloro compounds, and the quantum yields have been shown to be dependent on the initial concentration. There are also different views in the literature concerning the excited state involved. It is recognized that the excited singlet possesses sufficient energy to undergo homolytic C-Cl bond cleavage, but the thermally equilibrated triplet state is generally too low in energy to react in this manner. However, based on sensitization and quenching experiments the triplet state has been proposed to be photochemically active in some cases [7], and mixed photochemistry involving both the singlet state and the triplet state was claimed to take place in the case of tetrachlorobenzenes [ 61. The solvent also plays an important role in determining the yields and distribution of the photoproducts [2] and in previous studies 18 - lo] we have also observed that the triplet state properties of monochlorobenzene and of the dichlorobenzenes depend strongly on the solvent polarity. Nevertheless the change in triplet properties does not parallel the corresponding photochemical behavior, suggesting that in these cases the excited singlet may be photochemically active. To understand completely the mechanism of the photochemical decomposition of polychlorobenzenes we believe that it is necessary to know the photophysical properties of these molecules. As a continuation of previous studies on chlorobenzene [8, 101 and dichlorobenzenes [9] we now present a laser flash photolysis determination of the triplet state properties of the trichlorobenzenes (TCBs) in cyclohexane and in methanol.

2. Experimental details 1,2,3-Trichlorobenzene (123TCB) (Aldrich, 99%), 1,2,4+ichlorobenzene (124TCB) {Aldrich Gold Label, better than 99%) and 1,3,5-trichlorobenzene (135TCB) (Fluka, better than 99%) were used without further purification. Cyclohexane and methanol were spectrophotometric grade and anthracene was Aldrich Gold Label (better than 99.9%). The TCB concentrations were in the range 0.004 - 0.02 M. All solutions were deoxygenated by bubbling with oxygen-free argon. The laser flash photolysis facility has been described elsewhere [ 111. The excitation source was a Quanta Ray Nd:YAG laser with frequency quadruplication (6 ns full width at half-maximum, at 266 nm). The excitation

149

was at 90” to the analyzing beam and the laser beam was defocused over the ground face of 2 mm absorption cells in order to eliminate hot spots. Fresh solutions were used for each laser pulse. Actinometry was performed with standard solutions of naphthalene in cyclohexane, and the absorbance was matched with that of the sample solutions at 266 nm. The absorption of naphthalene triplet was monitored at 414 nm with narrow slits in the monochromator. All experiments were carried out at room temperature. 3. Results and discussion When TCBs are excited at 266 nm in cyclohexane or methanol a transient with a broad absorption around 300 nm is observed immediately after the laser pulse. The spectra in cyclohexane are shown in Fig. 1 and those in

Cl Cl

tT

Cl

0

Fig. 1. T-T

absorption

spectra of TCBs in cyclohexane.

150

methanol in Fig, 2. They are very similar to those of monochlorobenzene [8, lo] and dichlorobenzenes [9] except for the presence of a second, less intense band around 420 - 440 nm that was not apparent in the case of the less chlorinated compounds. For 124TCB this ‘second band appears as a shoulder to the main absorption, suggesting that in the case of monochlorobenzene and dichlorobenzenes it could be shifted to the blue and submerged under the higher energy transition. Both bands show similar decay kinetics and it was checked that both transitions are diffusionally quenched by oxygen and anthracene. In the latter case the growth of anthracene triplet could be observed. It can be concluded that both transitions correspond to the triplet of the TCBs. In the case of monochlorobenzene in methanol the typical absorption of solvated electrons was also observed [ lo]; this was not the case for dichlorobenzenes and nor was it observed in the present,experiments even at high laser energy. This suggests that photoionization does not take place when TCBs are irradiated in polar solvents. ) -

Cl Cl ti

Cl

0

I-

, -

300

400

500

A(nm) T ‘-T absorption spectra of TCBs in methanol.

151

At low laser intensity the initial absorbance of the transient was directly proportional to the pulse energy and decayed by apparently firstorder kinetics over a period of a few microseconds. Actinometry was carried out by measuring the absorbance at the maximum of each absorption band using absorption-matched solutions of naphthalene in cyclohexane. By comparison of the initial slopes of plots of initial absorbance us. the laser pulse energy for both sample and standard, the products #TUT, where #T 1s * the triplet state quantum yield and cT is the molar extinction coefficient of the triplet, were determined. A quantum yield and an extinction coefficient of 0.75 and 24 500 M-l cm-’ respectively were used for the naphthalene standard in cyclohexane [ 121. The T-T extinction coefficients were estimated using energy transfer to anthracene and following the established procedure for these determinations [13]. With anthracene concentrations in the range lop5 - 1O-4 M and 0.01 0.02 M TCB nearly all the light at 266 nm was absorbed by TCB. The small fraction of direct excitation was determined in blank experiments and was subtracted when necessary. Anthracene triplet growth was monitored at 422 nm in cyclohexane and 420 nm in methanol. The growth matched the decay of the TCB triplets at their absorption maxima. From plots of the observed first-order growth or decay rate constant as a function of anthracene concentration the bimolecular rate constants k,, for energy transfer were obtained. The values were in good agreement with those expected for diffusional processes in the given solvents. Since under our experimental conditions anthracene triplets decay by second-order T-T annihilation, the experiments were carried out at such a low excitation dose that there was no decay during at least two lifetimes of the donor. The extinction coefficients for the T-T absorption of anthracene at the maximum were taken as 64 700 M-i cm-’ in cyclohexane [ 121 and 5.5 X lo4 M-’ cm-l in methanol [lo]. By using this method the intersystem crossing quantum yields and T-T extinction coefficients for the TCBs in both solvents were obtained (Tables 1 and 2). Because of uncertainties in the extinction coefficients of anthracene triplet, the absolute values may possess considerable errors, beyond the standard deviations quoted. However, the relative values of @T for the three TCBs in a given solvent are independent of these uncertainties since they depend only on the relative absorbances of the acceptor triplet. The values quoted in the tables are consistent with this requirement. In both solvents, cyclohexane and methanol, the triplet decay follows nearly first-order kinetics. The apparent rate constant was found to be a function of laser pulse energy: the higher is the initial concentration of triplets the higher is the initially observed rate constant. This suggests that a T-T annihilation process is taking place. For 135TCB in both solvents and for 123TCB in methanol the observed decay was also found to be a function of the TCB concentration. Accordingly, the following processes were considered for the kinetic analysis: TCBT -

kl

TCB or products

(1)

152 TABLE

1

Triplet state properties

Amax (-1 QTeT (Mwi #T

eT (M-l

cm-’

of trichlorobenzenes 123TC33

124TCB

135TCB

290

310 4500 f 400 0.7 + 0.1 6400 f 600 (3.5 f 0.5) x 105 (1.1 f 0.2) x 107 <3x106 (6.5 +_ 0.2) x log

310 6100

3900

-c 400 0.6 + 0.1

)

cm-l)

6500 + 600 (4.3 + 0.2) x 105

kl (s-l) kzleTI

in cyclohexane

(1.1

(s-l)

k3 (M-’ s-l) keta (M-’ s-l )

f 0.1)

<3 x 106 (9.0 + 0.5)

x 10’

x 109

f 500

1.0 + 0.1

6100 + 600 (3.0 + 0.6) x IO5 (1.1 f 0.2) (3.3 f 0.5) (6.6 f 0.2)

x 10-J x 107 x 109

aEnergy transfer to anthracene. TABLE

2

Triplet state properties

9T eT (M-l

of trichlorobenzenes

cm-‘)

kl (s-l)

k2/eTl k3

keb

(s-l) (M-l

s-l )

in methanol

123TCB

124TCB

135TCB

290 1100 0.18 6100 (3.8 (1.9 (4 * (1.6

300 1400 f 200 0.24 k 0.04 5800 f 600 (2.7 + 0.2) x lo5 (2.3 + 0.2) x 10’ < 2 x 106 (1.9 f 0.1) x 10’0

300 3200 0.61 5200 (3.0 (1.5 (2.0 (1.7

f 200a + 0.04 * 600a + 0.4) x 105 + 0.2) x 10’ 2) x 106 + 0.1) x 10”

f 300 f 0.06 * 600 + 0.2) * 0.2) + 0.5) f 0.1)

x x x x

105 10-J 10’ 10’0

aAt 300 nm. b Energy transfer to anthracene.

2TCBT -

kz

2TCB

TCBT + TCB k

(2) 2TCB

(3)

where TCBT and TCB stand for the excited triplet and the ground state respectively of the TCBs. It follows from the mechanism that the initial apparent first-order rate constant should be given by

4WTCBTl haitial dt

= k1 + k3[TCBT10 + k,[TCB]

(4)

where [TCBTIo is the triplet concentration at the end of the laser pulse. Equation (4) can be rewritten in terms of the absorbance OD as dCln(OD)Jtiitial =k dt

4

+ (OD)Ok2 = k obs eTI

(5)

153

where 1 is the optical path and k,= k1 + k,[TCB]

(6)

A plot of kobs us. (OD), in cyclohexane is shown in Fig. 3 for 135TCB. It can be seen that there is a clear dependence of kq on 135TCB concentration. In Fig. 4 k4 is ploted us. [135TCB]; k, and kl were obtained from the slope and the intercept.

c1

IO

‘in m

‘0 x

x

5

0.01

0.02 A

0.03

0.04

(OD)

Fig. 3. Initial rate constants kobs for the triplet decay of 135TCB in cyclohexane us. the absorbance at the end of the laser pulse at 310 nm. 135TCB concentrations: 0, 0.0048 M; 0, 0.010 M; 0, 0.016 M.

IO

5

0.02

0.01 CONCENTRATION Fig. 4, First-order rate constants 135TCB concentration.

(M 1 k4 for the triplet

decay of 135TCB

in cyclohexane

us.

A similar treatment of the data was carried out for all three TCB in both solvents. Self-quenching of the triplet (reaction (3)) was found to be important only in the case of 135TCB in both solvents and for 124TCB in methanol_ For the other cases an upper limit for k, was estimated. All the

154

kinetic parameters obtained through this treatment are collected in Tables 1 and 2. The rate constants in both solvents are very similar. The T-T annihilation process is diffusion controlled as is also the case for energy transfer to anthracene. The pure first-order rate constants for decay, k 1, do not reflect any influence of solvent polarity and they are relatively fast processes when compared with the decay normally found for aromatic molecules in fluid solution. The most noticeable difference between methanol and cyclohexane exists between the intersystem crossing quantum yields. The values in the polar solvent are lower than those in the hydrocarbon, and the difference is clearly beyond the experimental error. Several cross-checks of the relative values were performed employing solutions with matched absorbances of the same TCB in both solvents_ The values in the tables are consistent with those determined by this checking procedure. In Table 3 we have collected the intersystem crossing quantum yields together with the values determined by Bunce et al. [7] in cyclohexane using the biacetyl-sensitized phosphorescence technique. In Table 3 the literature values for photolysis quantum yields are also included, but in this case the data from different research groups are difficult to compare since in most cases the experimental conditions are not the same, especially with regard to deaeration, initial concentration and percentage of decomposition. In any case, some conclusions can be extracted from Table 3. First, there is good agreement for the intersystem crossing quantum yields between our results and those in ref. 7 within the experimental error quoted. Second, in the cases of 123TCB in methanol and cyclohexane and 124TCB in methanol, the photolysis quantum yields are higher than those for triplet formation. This can be taken as an indication that the excited singlet is most probably involved in the photochemistry. Third, for 124TCB and 135TCB in cyclohexane the triplet quantum yields are nearly the same or are higher than TABLE

3

Photolysis

and triplet yields of trichlorobenzenes

123TCB

$T

@R

124TCB

Cyclohexanq

Methanol

0.6 (0.7)a

0.18

1.08

0.6gb

$R, photolysis

Cyclohexane

I35TCB Methanol 0.24

(X,

quantum yield. aFrom ref. 7. bFrom ref. 1. CFrom ref. 2, in isopropanol.

Cyclohexane

0.7a

0.61 (:::)a

0.59b 0.46c

Methanol

0.16a

155

those of photolysis. In this case there exists the possibility that the photochemically active intermediate is the triplet state alone. Bunce et al. [7 3 reported that the photodecomposition quantum yields in cyclohexane decrease with increasing TCB concentration. The compound for which this dependence is more noticeable is 135TCB. Their interpretation was that an unreactive excimer was formed between the excited singlet state of the chlorinated compound and a ground state molecule [ 7,14]. However, the laser flash results suggest that at least for 135TCB this effect is most likely due to self-quenching of the triplet state since it is for this case that the rate constant k, for triplet self-quenching is higher. There are still some inconsistencies between the kinetic parameters determined in this work and the photochemical results. From a plot of the inverse of the photolysis quantum yield VS. [ 135TCBJ Bunce et al. [7] found an intercept and slope of 6.2 and 120 + 10 M-l. With these data, assuming that eqns. (1) and (3) apply to the photolysis mechanism, kJ/k 1 can be estimated as 20 M-‘, which must be compared with 110 M-’ derived from the rate constants in Table 1. The difference is beyond the experimental error and the origin of the discrepancy is not clear.

Acknowledgments The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. Thanks are also given to the Consejo National de Investigaciones Cientificas y TQcnicas (Argentina) for a travel grant. This is Document NDRL 3026 from the Notre Dame Radiation Laboratory. References 1 A. Basinski and E. Latowska, Rocz. Chem., 40 (1966) 1747. E. Latowska and T. Latowski, Rocz. Chem., 40 (1966) 1977. 2 B. Akermark, P. Baeckstrom, U. E. Westlin, R. Gothe and C. A. Wachtmeister, Acto Chem. Stand., Ser. B, 30 (1976) 49. 3 H. Parlar and K. Forte, Chemosphere, 8 (1979) 797. 4 G. G. Choudhry, A. A. M. Roof and 0. Hutzinger, Tetrahedron Lett., (1979) 2059. 5 T. Nishiwaki, M. Usui, K. Anda and M. Hida, Nippon Kagaku Kaishi, (1979) 1343. 6 M. Usui, T. Nishiwaki, K. Anda and M. Hida, Nippon Kagaku K&hi, (1982) 638. 7 N. J. Bunce, P. J. Hayes and M. Lemke, Can. J. Chem., 61 (1983) 1103. 8 C. M. Previtali and T. W. Ebbesen, J. Photochem., 27 (1984) 9. 9 Z. B. Alfassi and C. M. Previtali, J. Photochem., 30 (1985) 127. 10 C. M. Previtali and T. W. Ebbesen, J. Photochem., 30 (1985) 259. 11 P. K. Das, M. V. Encinas, R. D. Small and J. C. Scaiano, J. Am. Chem. Sot.. 101 (1979) 6965. 12 R. Bensasson and E. J. Land, Photochem. Photobiol. Rev., 3 (1978) 163. 13 I. Carmichael and G. L. Hug, J. Phys. Chem. Ref. Data, 15 (1986) 1. 14 N. J. Bunce, J. P. Bergsma, M. D. Bergsma, W. DeGraaf, Y. Kumar and L. Ravanal, J. Org. Chem., 45 (1980) 3708.