Pulse-radiolytic investigation of 1,1' - diphenylethylene at low temperature

Radiat. Phys, Chem. Vol. 26, No. 5, pp. 543-545, 1985 Printed in Great Britain.

0146-5724/85 $3.00 + .00 © 1985 Pergamon Press Ltd.

P U L S E - R A D I O L Y T I C I N V E S T I G A T I O N OF 1,1' D I P H E N Y L E T H Y L E N E AT LOW T E M P E R A T U R E J. SCHMIDT, J. Bos, H. MAI, U. DECKER and M. HALBIG Central Institute of Isotope and Radiation Research, Leipzig, German Democratic Republic Abstract--I,l'-Diphenylethylene (DPE) has been studied in various organic matrices and at temperatures of 90-150 K using the pulse-radiolysis technique with optical absorption spectroscopy. The dependence of the absorption spectra on the kind of matrix such as internal electron or cation scavenger, solute concentration or temperature was interpreted and the absorption bands were assigned to anionic, cationic and radical species. A simplified reaction mechanism is assumed.

INTRODUCTION AFTER INVESTIGATIONSof the aromatic olefins styrene and l-methylstyrene tl) DPE was chosen as suitable compound for further studies of the radiation-induced reactions of monomeric and dimeric species at low temperatures. Steady-state experiments on DPE at 77 K of Shida and Hamill cz'3~ showed optical absorption bands of anions in methyltetrahydrofuran at 410 and 1150 nm and of cations in sec-butyl chloride at 390, 550 and 1150 nm. Later works using pulse radiolysis usually in solution at room temperature confirmed these absorption spectra, t4-6~ However, the definite assignment of single absorption bands is partly contradictory. The aim of our pulse radiolysis and steady-state experiments on DPE in nonpolar glasses and highly viscous liquids at low temperatures is to contribute to the identification of the radiation-induced species and to derive a reaction mechanism from the kinetic behaviour. EXPERIMENTAL

2-Methyltetrahydrofuran (MTHF), /-butyl chloride (BuCI) and 3-methylheptane (3-MHp) were purified by chromatography using molecular sieves 4A and 13X (Wolfen Zeosorb) and then fractionally distilled. DPE was synthesized by a Grignard procedure from brombenzene and acetophenone, followed by a catalytic dehydratization of the tertiary alcohol. The olefin was dried and fractionated by distillation under vacuum. The pulse radiolysis experiments were carried .out with a cryostat system cooled with liquid nitrogen at temperatures from 90 to 150 K. The solution was placed in a brass cell, degassed, the cell

was sealed, and then cooled in a cryostat and irradiated with 8 to 40 ns pulses of 1 MeV electrons from an ELIT-type accelerator. The doses per pulse were 30 and 70 Gy, respectively, determined with KSCN solution. The optical detection system consisted of Schoeffel grating monochromator GM 252, photomultipliers 1 P 28 (for 300-700 nm) and 7102 (for 650-1100 nm), automatic back-off system and storage oscilloscope S 8-12A. The analyzing light was provided by a pulsed XBO lamp.

RESULTS AND DISCUSSION The aromatic olefin DPE is a cation as well as an electron scavenger. By irradiation in hydrocarbons such as 3-MHp radical cations and electrons are generated in the matrix and the charge after that is transferred to the olefin. In 3-MHp we therefore have a common absorption spectrum of cationic and anionic DPE transients. In order to investigate these transients separately it is necessary to use other scavengers in excess. We used triethylamine (TEA) for cationic and CCI4 for electron capture or as internal scavengers and suitable glassy matrices M T H F and BuCI, respectively. Anion spectra In irradiated pure M T H F ejected electrons are trapped by the matrix itself and give an absorption band at 1200 nm. The simultaneously generated cations are immobilized by proton transfer. Therefore a selective investigation of anions is possible. In the presence of DPE in M T H F the absorption band of eft is reduced or nonexistent. Figure 1 shows the absorption spectrum of transients with maxima at 340, 410, 480 and >1000 nm obtained by pulse radiolysis of a highly viscous so-

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Fig. 1. Absorption spectra of DPE anions in MTHF (1) and of cations in BuCI (2) for 0.15 mol.dm -3 DPE at 110 K, immediately after the 8 ns pulse. Insets: anion (la) and cation (2a) absorption time profiles (wavelength, in nm).

lution of M T H F containing 0.15 mol.dm -3 DPE at 110 K, using 8 ns, 50 Gy pulses. The spectrum corresponds with our steady-state experiments at 78 K and to those published in Refs. (2, 3, 6). The typical DPE anion spectrum is observed at small concentrations (0.005 mol.dm -3 DPE) in M T H F glass at 90 K after the end of the pulse. Upon raising the concentration the band at 410 nm increased and the band of e~ at 1000 nm decreased. An electron transfer reaction is assumed according to (1): (1)

err + DPE--* D P E ' - .

The 410 nm band is ascribed to the DPE radical anion. Further support for this assignment is provided by the kinetics of the growth of the 410 nm band and the decay of the 1000 nm band obeying a pseudo-first-order rate law. The absorption time profiles indicate for the NIR band a fast and slower decay. The estimated rate constant for the first part is nearly identical with one for the formation of DPE radical anions with k = 3 × i0 s dm3.mol- J-s- ~for 0.005 mol.dm -3 DPE and at 110 K. The somewhat smaller value of the rate constant of the fast decay can be attributed to the growth of a D P E ' - band

also at > 1000 nm. Because of the superposition of the decreasing et7 band and of the increasing D P E ' - band the decay at 1000 nm appears slower. Therefore a rising of the concentration from 0.005 to 0.15 mol.dm -3 DPE did not accelerate the decay of the 1000 nm band. A formation of the D P E ' band at >1000 nm was also observable in a 3-MHp matrix with TEA. For 0.15 mol.dm -3 DPE and 110 K the 410 nm band decreased immediately after the pulse. With further increase of the temperature all processes become faster. A comparison of the decay of the 410 and 1000 nm bands at 110-130 K indicates a good coincidence in the time behaviour. As the absorption band of et7 decays within ~5 p,s after the pulse end, the further decay at 1000 nm can only be explained by DPE" decrease. A reaction product was spectroscopically not observable in the time range up to 250 ~s after the pulse. When the reciprocal absorbance against time was plotted a linear dependence was obtained. This demonstrates a second-order rate law and a neutralization reaction, e.g. with solvent impurity, can be assumed: (2)

DPE'

+ A + ~ product.

A rate constant k = 1 × i 0 6 dm3.mol ~.s- ~ at 110 K is estimated. The absorption band at 480 nm has not been clearly identified yet due to a superposition of the 410 nm band. As the relatively stable band was observed not only in MTHF but also in BuCI glass at 90 K we suppose a transient of a possible impurity in DPE. The absorption maximum at 340 nm is described later. Cation spectra

Low-temperature pulse-radiolysis experiments of BuCI indicate after the pulse only a broad absorption band at 520 nm assigned to the BuCI "+ (7) By addition of 0.05 tool.din -3 DPE in the optical spectrum immediately after the pulse small peaks at 340, 390 and >1000 nm appeared. Within 45 p,s the band of BuC1 '+ at 520 nm and the NIR band decreased, the both of the other bands increased, and a band at 550 nm appeared. Figure 1 (curve 2) shows transient absorption spectra of 0,15 mol.dm 3 DPE in BuCI at 110 K. where BuCI is a highly viscous liquid, with bands at 340, 390, 480, 550, >1000 nm and a shoulder at 440 nm. Inset 2(a) presents the time behaviour of the maxima. The fast initial decay at 1000 nm corresponds to the growth at 390 nm up to a maximum at 10 Ixs after the pulse. As the NIR band already decreased at low DPE concentrations this band was assigned to the monomer radical cation in agreement with Refs. (2,3,5) but in contrast to Ref. (4).

545

l-l'-Diphenylethyleneat low temperature Table 1.

The charge transfer is explained according to (3): Matrix (3)

Anionic A fast process within the pulse duration is demonstrated by (4): Cationic (4)

h + + DPE--* D P E ' + .

The first-order plots from the decay at 1000 nm and the growth at 390 nm show a linear dependence on the time. A rate constant was estimated to be k = i × l06 dm3.mol-J.s - j at 110 K. The reaction is assumed a dimerization according to (5): (5)

DPE" ÷ + DPE ~ "(DPE)f.

The maximum at 390 nm is assigned to the dimer radical cation of DPE. With the growth of the concentration from 0.05 to 0.5 tool. dm -3 DPE the band decreased faster. A comparison of kinetic data shows that a second-order reaction is probable and the decrease of "(DPE))f could be explained by neutralization with CI- from BuCI (6): (6)

kmax (nm)

Assignment

BuCI" + + DPE ~ DPE" + + BuCI. 410 and >1000 monomer radical anion 480 possible DPE impurity 340 free radical 390 and 520 > 1000 and 550 440 480 340

dimer radical cation monomer radical cation (radical or carbonium ion) possible DPE impurity free radical

in all optical spectra of irradiated DPE in BuCI as well as M T H F matrices. As the band is not influenced by both types of scavengers it was assigned to the DPE radical in agreement with Refs. (4-8). The radical band was considerably reduced when to the M T H F matrix containing 0.01 mol.dm -3 DPE 0.001 mol.dm -3 CC14 was added. From the time behaviour one can realize a delay in the formation of DPE radicals when a BuCI matrix was applied instead of MTHF. Both e - scavengers influence the radical formation due to the capture of radicals or of precursors of the radical formation. The results are summarized in Table 1.

"(DPE)f + CI- --~ products.

REFERENCES A possible trimerization product was not observable in the optical spectrum. A band at 550 nm was already present in the spectrum of DPE/BuCI glass at 90 K and decreased immediately after the pulse. From the similar time behaviour as the 1000 nm band this maximum can also be assigned to the DPE" +. During the decay a superimposed band at 520 nm was observed which showed the time behaviour of the 390 nm band and therefore could be ascribed to the "(DPE)~-. The superposition of both bands makes the interpretation difficult. The shoulder at 440 nm, not observable in all cases, disappeared when CCI4 as a stronger e scavenger was added. A band at 340 nm appeared

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1976, 80, 1968. 8. N. GETOFF, A. RITTERand F. SCHWt3RER,J. Chem. Soc. Faraday Trans. 1983, 79, 2389.