time-resolved epr study of the photoreduction of phthalic anhydride and chlorinated phthalic...

Appl. Magia. Reson. 23, 333-347 (2003) Applied Magnet|c Resonance �9 Springer-Verlag 2003 Printed in Austria

Time-Resolved Phthalic Anhydride

EPR Study of the Photoreduction of and Chlorinated Phthalic Anhydrides in

2-Propanol

K. Hansongnern l, T. Fukuju 2, H. Yashiro 2, K. Maeda 2", T. A z u m i 2, and H. M u r a P

Department of Chemistry, Faculty of Science, Prince of Songkla University, Hat-Yai, Songkla, Thailand

2 Department of Chemistry, Graduate School of Science, Tohoku University, Sendai, Japan 3 Department of Chemistry, Faculty of Science, Shizuoka University, Shizuoka, Japan

Received September 7; revised November 25, 2002

Abstract. The photoreduction of phthalic anhydride (PA), 3,6-dichlorophthalic anhydride (3,6-DCPA), 4,5-dichlorophthalic anhydride (4,5-DCPA) and tetrachlorophthalic anhydride (TCPA) in 2-propanol has been studied with time-resolved electron paramagnetic resonance. The chemically induced dynamic electron polarization spectra show that the reaction takes place through the excited triplet states. From PA, cyclohexadienyl-type hydrogen adduct and ketyl radicals were observed, whereas 3,6-DCPA produced the 3,6-DCPA anion and hydrogen adduct radicals. With 4,5-DCPA only the anion radical appeared, whilst the TCPA system showed no apparent anion and adduct radical formation. These data show that the hydrogen adduct formation occurs at the 4 position in the benzene ring, but that 4,5-DCPA and TCPA do not undergo this reaction. The anion radicals of PAs are formed in subse- quent deprotonation reactions of the ketyl radicals. We propose that the hydrogen adduct radical formation in PA and 3,6-DCPA takes place through direct hydrogen abstraction by the excited triplet molecules, in competition with similar abstraction by the carbonyl group to form the ketyls.

1 Introduction

There are several different react ion pa thways in the photolys is o f ca rbony l compounds , inc lud ing Norr ish type I react ion, hydrogen abstract ion reac t ion and elec- t ron transfer. The ca rbonyl group o f the exci ted tr iplet molecu le u sua l ly abstracts a hydrogen a tom from reductants such as alcohols [1, 2] to form radicals . Some- t imes, however , hyd rogen adduct radica ls are formed, where a h y d r o g e n a tom

" Present address: Department of Chemistry, University of Tsukuba, Tsukuba, Japan

334 K. Hansongnern et al.

from reductants adds, for example, to unsaturated molecules to forro cyclohexadienyl radicals [3-5]. The nn* character of the excited triplet state of aromatic carbonyl compounds such as xanthone lies behind the hydrogen adduct formation, whilst the nn* character of the carbonyl group favors the ketyl formation. Adduct formation occurs especially when the reductant molecules have hydrogen atoms directly bonded to germanium, boron or tin, making them compara- tively more electronegative. Benzyl radicals are formed when flavones ate pho- tolyzed in the presence of this type of reductant [6]. With maleic anhydride, the hydrogen adduct formation is observed even in the absence of these particular compounds [7, 8]. In a recent investigation of the photoreduction of flavone, the formation of benzyl radicals by way of keto-enol conversion was reported even in the absence of tri-butyltin hydride, with which it had been observed previously [9, 10]. These particular reductants were thought only to be involved in intramolecular conversion but this remains controversial anda more general ra- tionalization of the reaction mechanism is awaited.

In this papera time-resolved electron paramagnetic resonance (EPR) study of the photochemistry of phthalic anhydride and its chlorinated derivatives in 2- propanol solution is reported, and the formation mechanisms of adduct and anion radicals in this solvent are discussed.

2 Material and Methods

All experiments were performed on a continuous-wave (cw) X-band EPR (Bruker ESP 380 E) spectrometer modified for time-resolved observation without field modulation. Photolysis was performed with a XeC1 excimer laser (2 = 308 nm). Phthalic anhydride (PA) was obtained from Wako Pure Chemicals, 3,6-dichlorophthalic anhydride (3,6-DCPA) and 4,5-dichlorophthalic anhydride (4,5-DCPA) ffom Aldrich, tetra-chlorophthalic anhydride (TCPA) from trade TCI Mark, triethyl- amine (TEA) from Nacalai Tesque, and 2-propanol from Kanto Chemicals. The structures of the compounds are shown in Fig. 1 and all were used as received. The concentrations of the anhydride molecules were in the range of 10 -2 M in 2-propanol. Nitrogen gas bubbling was used to deoxygenate the solution which flowed through a quartz flat cell installed in the EPR cavity. Al1 experiments were performed at room temperature immediately after the preparation of the solution. Full experimental details have been provided previously [3, 8, 11].

Introductions to chemically induced dynamic electron polarization (CIDEP) have also been given elsewhere [12-14]. Here the spectral patterns did not change during the observation period, which implies that the initial ratio of spin polarization caused by geminate pair RPM (radical pair mechanism) and TM (triplet mechanism) was conserved. No contribution from F-pair polarization was observed and no contribution from the radical-triplet pair mechanism (RTPM) [15] was needed to rationalize the results. The patterns due to the different spin polarization mechanisms are superimposed in the spectra but their analysis allows us to elucidate the photolysis mechanisms.

3pAs* -'1-

,so l 1 PAs*

h.t

X3 O

PAs

O ~o O

PA

EPR study of Photochemistry of Phthalic Anhydrides 335

Hydrogen abstraction/ H-adduct radical formaSon

x6 o CH3 ~ X l ~

q OH + 0 CH3 " ~ HX4 X3

/ PAsH" / CH3 ~CH-O# Hydrogen abstraction/ CH 3 / ~ . Ketyl-type radical formation

l l k Deprotonation reaction XB H'O X 6 0

CH3"~/C-_OH + X ~ ~ O ~ X S ~ O +H+ q CH"~>~~ x.~~o ~~__~ x.~~o

.. A~o, rad,c I~e;Dc~IPA"kety'" I(TCPA?)] PAs'- I r eac i~U ; t "~~

- l' ~ - = - - c . � 9 1

CI O O Cl O cCo cC:C~ o c,~~'~o CI O O CI O

3,6-DCPA 4,5-OCPA TCPA

Fig. 1. Photochr processes of phthalic anhydrides in 2-propanol.

3 Results and Discussion

3.1 Phthalic Anhydride in 2-Propanol

Figure 2a shows the CIDEP spectrum observed 1.25 ps after laser excitation of PA in 2-propanol. It shows the presence of at least two different radicals. One has a spectrum with hyperfine coupling (hfc) constants of 4.33 mT (2 equivalent protons), 0.808 mT (1 proton), 0.752 mT (1 proton) and 0.193 mT (1 proton) and a g-value of 2.0032, which identify it as the cyclohexadienyl hydrogen adduct radical (PA'H) (see Fig. 1) [16]. The other obvious spectrum is from the alcohol-derived 2-hydroxy-2-propyl radical ((CH3)2C'OH). These spectra exhibit emission at low field and absorption at high field (E/A pattern), with the low- field lines of the adduct radical slightly stronger than those at high field. This pattern is consistent with RPM from ah excited t¡ state together with a weak emissive TM. Simulated spectra with RPM are given in Fig. 2b and d. This spectrum implies that the excited triplet state of PA reacts with the 2-propanol mole-

336 K. Hansongnem et al.

l i

Abs.

§ Em.

I I

342 344

I C I ~ I ~ 1

7 Em.

I I I I

A I i q I I I I I

/ 346 348 350 352 354 356

B (mT)

PAH"

I I

345 345.5 346 343 343.5 344 344.5

B (mT)

Fig. 2. a CIDEP spectrum observed 1.25 ~ts atter laser excitation of PA in 2-propanol. The hyperfine lines marked by vertical arrows originate in the alcohol-dedved radical ((CH~)2C'OH). la Simulated speetrum of a assuming the reaction of Eq. (1) with RPM. e Expanded part of the spectrum over the field indicated by a horizontal arrow in a. The stick spectrum shows the corresponding calculated spectrum due to the hydrogen adduct radical (PA'H). The intensities have not been corrected for polariza-

tion. d Simulated spectrum of e with RPM.


cule to forro radicals, and that the reaction is not fast enough to induce strong TM polarization. Previous studies have shown the TM in radicals produced from phthalic anhydrides to be emissive [17].

Since photoreduction of aromatic carbonyl compounds in alcohol normally forms ketyl or related anion radicals, the observation of PA'H in alcohol is un- usual. The weak E/A spectrum observed at the center field may be due to the ketyl-type radical (PA-ketyl'). The failure to observe a strong signal from the ketyl-type radical as the primary intermediate may be due to the weak signal intensity of the RPM at around g = 2.00. The overall reaction is:

PA + h v--+ IpA* ---(ISC)--+ 3PA*,

3pA* + (CH3)2CHOH --+ PA'H + (CH3)2C'OH , (1)

3pA* + (CH3)2CHOH ~PA-ketyl" + (CH3)2C'OH.

Figure 3 shows time profiles of the signal intensities from PA'H and (CH3)2C'OH obtained by monitoring lines on the low-field side. It seems that the signals from (CH3)2C'OH and PA'H rise immediately after laser excitation. However, the signal of PA'H grows even after the signal of (CH3)2C'OH starts to decay, whilst the decay of the alcohol radical is much faster than that observed in the system of TCPA presented in this paper and in different system [18] where no PA is present. The time profile suggests that the initial reaction is the hydrogen abstraction reaction from 2-propanol. The fast decay of the signal of (CH3)2C'OH implies reaction with the ground state PA in the solution. It is well known that the alcohol radical reacts with carbonyl compounds efficiently to forro ketyl radicals. I f the alcohol radical reacts with PA to forro PA-ketyl" in a similar way, this explains the quick decay of the alcohol radical. The detailed reaction process is expressed as:

PA + (CH3)2C'OH ~ PA-ketyl" + CH2C(OH)CH 3. (2)

400 , ~ , , ; 11 (CHa)2C'OH

= -400 - , ,

~800 v I

I I I I I

0 1 2 3 4 5 Time (ps)

Fig. 3. Time profiles of the CIDEP signals from the hydrogen adduct radical (PA'H) (broken line) and from (CH3)2C'OH (solid line) obtained by monitoring the low-field lines with asterisks in Fig. 2c.


b

Em.

I I I I I I I

342 344 346 348 350 352 354 356 B (mT)

Em.

PA"

i i i i i

347.6 348.2 348.8 B (toT)

PA"

I I I / I I i

347.6 348.2 348.8 B (mT)

Fig. 4. a CIDEP spectrum observed 1.25 ~s after laser excitation of PA and TEA (2.10 -2 M) in 2-propanol. The three-line spectrum is assigned to the anion radical of PA (PA'-). Extra lines of low intensity due to overlap of spectra of the hydrogen adduct radical (PA'H) and of (CH3)2C'OH are observed, b Expanded spectrum between the fields indicated by a horizontal arrow in a. e Ex-

panded spectrum 3 ~ts after laser excitation when only the anion apparently remains.

Here, we should carefully analyze the signal intensi ty of the radical species, because the spin-polarized signal is not a direct measure of its concentration. When a successive reaction such as the hydrogen transfer reaction like Eq. (2) occurs, the secondary formed radical does not show the quantitative concentration of the radical species [8, 19]. Furthermore, the rise and the decay of the


CIDEP signals occasionally correspond to the growth of the magnetization and the spin-lattice relaxation, respectively. Consequently, it is difficult to determine exact kinetic parameters from time-domain cw time-resolved EPR. The time reso- lution of the spectrometer (about 200 ns) may be another problem. However, a qualitative analysis is as follows: The fast decay of the signal of (CH3)2C'OH may be rationalized by the transfer of the RPM polarization of the methyl proton of the alcohol radical to PA-ketyl" keeping its signal phase; namely, the spin polarization of the secondary-formed ketyl-type radical may not clearly appear by the cancellation of the emissive and absorptive electron spins in (CH3)2C'OH.

In the presence of TEA, the spectrum (Fig. 4) was totally emissive. Its hfc constants were 0.221 mT (2 equivalent protons), and 0.038 toT (2 equivalent protons) and the g-value was 2.0033, close to those of the PA anion radical with the hfc constants of 0.227 and 0.034 mT, and g-value of 2.0035 reported previously [16]. Slightly different hfc constants have been reported in dimethylforma- mide [20] and in acetonitrile in a TEA 3:1 mixture [21]. These differences may be due to different solvent polarities. The emissive pattem with slight E/A dis- tortion is explained by TM polarization RPM from a triplet precursor. TEA is a better electron donor than 2-propanol, as shown by its low ionization potential (7.50 eV) [22]. The formation of the anion radical is explained by direct electron transfer:

3pA* + Et3N ----> PA'- ---> Et3 N'+.

The failure of the observation of the TEA cation radical is not peculiar because the cation radical of amines is generally known to be very weak or difficult to detect. The weak signal of the adduct radical was observed on both sides of the spectrum of the PA anion radical, and a trace of the alcohol radical signal was recognized. The observation of the weak signals of these species may be explained by the existence of the slow competing reaction with alcohol.

3.2 3,£ Anhydride in 2-Propanol

In the photolysis of 3,6-DCPA in 2-propanol the formation of the neutral hydrogen adduct radical (3,6-DCPA'H) and the 3,6-DCPA anion radical were confirmed by interpretation of the time-resolved EPR spectmm shown in Fig. 5. The CIDEP spectmm observed at low field is emissive and that at high field is absorptive with a weaker intensity: an E*/A pattern. The spectrum of the hydrogen adduct radicals is recognized easily, even though some hyperfine lines are superimposed on those from other radicals. The 3,6-DCPA'H radical shows a pattern of a triplet of doublets with hfc constants of 4.16 mT (2 equivalent protons) and 0.809 mT (1 proton). It indicates that the hydrogen atom is attached to the 4 position to f o r m a cyclohexadienyl type of radical. The signals at the center field are superimposed on those from the 3,6-DCPA'H and alcohol-derived radicals and show strongly emissive triplet lines with a hfc constant of 0.244 mT and g-fac-


I I I [ I I 3 '6"DcPAH"

Abs. ~

§ Em.

i [ I I I f I I

342 344 346 348 350 352 354 356 B (mT)

Abs. [ ~.~ 3 , 6 - D C P A ' "

I i I I I

346 347 348 349 350 B (toT)

Fig. 5. a CIDEP spectrum observed 1.25 p.s after laser excitation of 3,6-DCPA in 2-propanol. The hyperfine lines marked by the vertical arrows are from (CH3)2C'OH. The stick spectrum shows the line positions of the hydrogen adduct radical (3,6-DCPA'H), whilst the center triplet is from the PA anion radical (3,6-DCPA'-). b Expanded spectrum of the field indicated by a horizontal

arrow in a.

tor of 2.0036. This spectrum was assigned to the anion radical of 3,6-DCPA (3,6- DCPA'-). The emissive spin polarization of the anion radical can be explained by the TM, whereas the hydrogen adduct radical spectrum shows a rather strong RPM pattem due to its larger hfc constants. No other signals were observed during the observation time of 4 ~ts.

The signal from the alcohol radical, (CH3)2C'OH, disappeared faster than those of the 3,6-DCPA'H and the 3,6-DCPA anion radical, as shown in Fig. 6. The decay of the (CH3)2C'OH radical is followed by a slow rise of the signal due to the anion radical of 3,6-DCPA. The signal of the anion radical starts to decay after 4 ~ts as shown in the Fig. 6. This suggests that the anion radical could be produced as follows:


3,6-DCPA + h v--+ 13,6-DCPA" ---(ISC)---~ 33,6-DCPA*,

33,6-DCPA* + (CH3)2CHOH ~ 3,6-DCPA-ketyl" + (CH3)2C'OH, (3)

3,6-DCPA + (CH3)2C'OH --+ 3,6-DCPA-ketyl" + CH2C(OH)CH3, (4)

3,6-DCPA-ketyl" ~ 3,6-DCPA'- + H +. (5)

The reaction of Eq. (4) followed by that of Eq. (5) may be plausible since the time profile of the formation of the anion is nearly correlated to the disappear- ante of the alcohol radical. During this process of Eqs. (4) and (5), the emissive TM polarization of the anion radical is brought from the alcohol radical by way of the ketyl radical. The formation of the hydrogen adduct radical may be by a similar mechanism to that with PA, the direct hydrogen abstraction from (CH3)2CHOH to form the cyclohexadienyl radical:

33,6-DCPA* + (CH3)2CHOH--~ 3,6-DCPA'H + (CH3)2C'OH. (6)

Reactions (3) and (6) compete in this system. In the presence of TEA, the spectrum (not shown here) looked similar ex-

cept for a weaker signal intensity from 3,6-DCPA'H and (CH3)2C'OH compared with that of the anion radical. The photoreduction of 3,6-DCPA through electron transfer reaction should be the predQminant pathway:

33,6-DCPA* + Et3N-+3,6-DCPA'- + Et3 N'+.

However, the amine cation radical was not observed in this system either. The observation of weak signals from (CH3)2C'OH and the adduct radical implies that hydrogen abstraction from alcohol also occurs.

=. m

,-1000

-2000

I I I I I

~t; . . . . . 3,6-DCPAH' ~ . - ........ 3,6-DCPA-" ,.

i i ' ~ ' ' " , - - ( C H 3 ) 2 C O H ~ -

' ".. , 1 ,~. , ' ~ - / ~ "

I \ ,,, -. / ,:,, . . . . -""" ' hv ~ X ~ ' ~ " ~ ' " x ~ ' ' 4 "- ', ' ,, ' """'~"J'

" % ' " - . . . . - - . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xl/5 I I I ~ I

1 2 3 4 5 Time (ps)

Fig. 6. Time profiles of the CIDEP signals of the hydrogen adduct radical (3,6-DCPA'H) (broken line), the PA anion radical (3,6-DCPA'-) (dotted line) and (CH3)2C'OH (solid line). The signals

of the lines with asterisks in Fig. 5a are presented.


3.3 4,5-Dichlorophthalic Anhydride in 2-Propanol

The CIDEP spectrum observed from 4,5-DCPA in 2-propanol shows only one absorptive peak (Fig. 7a and b) with a g-value of 2.0036. The linewidth (fwhm)

AbS.

4 I I I I I I I I

342 344 346 348 350 352 354 356 B (mT)

b

Abs.

• I I I 1 r I

346.5 347 347.5 348 348.5 349 349.5 350 B (mT)

Em.

I I ~ I I L I I

346.5 347 347.5 348 348.5 349 349.5 350 B ( m T )

Fig. 7. a CIDEP spectrum observed 2.0 us after laser excitation of 4,5-DCPA in 2-propanoI. The single-line spectrum was assigned to the anion radical o f 4,5-DCPA (4,5-DCPA'-). The vertical arrows show weak fines due to the alcohol-derived radical, b Expanded spectrum of the center portion of a. e CIDEP spectrum observed 2.0 ~ts after laser excitation of 4,5-DCPA and high concentration of TEA (1 .10 -~ M) in 2-propanol. The weak spectrum (arrows) was assigned to the TEA neutral radical. Attention is drawn to the change in the phase o f the pola¡ signal due to the anion in

the absence and presence of TEA.


was about 0.14 mT. The rise of this signal was very slow compared with the other PA derivatives. We assign this sharp single-line spectrum to the anion radical of 4,5-DCPA by comparison with the spectrum observed in the presence of TEA shown below. The hfc constant to the equivalent protons at the 3 and 6 positions was small and unresolved. By careful inspection a very weak trace spectrum of the alcohol-derived radical was observed with an E/A* pattern. In the cases of PA and 3,6-DCPA, hydrogen addition occurred at the 4 position on the benzene ring, but in DCPA, the reactive 4 and 5 positions were replaced by chlorine atoms. The reaction processes may be as follows:

34,5-DCPA" + (CH3)2CHOH ~4,5-DCPA-ketyl" + (CH3)2C'OH ,

4,5-DCPA + (CH3)2C'OH --~ 4,5-DCPA-ketyl" + CH2C(OH)CH3,

4,5-DCPA-ketyl" --+4,5-DCPA'- + H "§ (7)

In the presence of TEA (1 �9 10 -~ M), electron transfer from TEA to 4,5-DCPA should occur as with the other PAs. A single emissive peak (due to the TM) at the center field was observed as shown in Fig. 7c. This spectrum demonstrates a g-value of 2.0035 which is close to that observed without TEA. Along with the single-peak spectrum, there exists a weak multiline spectrum that is tenta- tively assigned to the neutral radical from TEA (Et2NCH'CH3) [23, 24]. The photoreduction of 4,5-DCPA to form the anion radical should be similar to that of PA and 3,6-DCPA, and/or the direct hydrogen abstraction followed by the deprotonation (Eq. (7)) may occur:

34,5-DCPA* + Et3N--+ 4,5-DCPA'- + Et3 N'+,

34,5-DCPA* + Et3N ~ 4,5-DCPA-ketyl" + Et2NC'HCH 3.

In low-concentration conditions of TEA (2. 10 -2 M), the weak signal with emissive TM appeared immediately after photolysis and it rapidly converted to absorption. This was a result of net effect RPM polarization, the g-value of the TEA cation radical (2.0040) [25] being larger than those of the 4,5-DCPA anion radical and the PA-ketyl'.

The single line was a bit broader than that without TEA, which may be due to the complex formation between the TEA molecule and the 4,5-DCPA anion radical. The formation of complexes between amines and the benzophenone ketyl radical is known [26]. The absorptive signal observed without TEA (Fig. 7a) may be due to a slow reaction of the excited triplet state of 4,5-DCPA with alcohol. Ir the triplet relaxes before it reacts with alcohol in this system, then the radical species is observed with a weak absorptive polarization [27]. This mechanism is supported by the slow rise of the absorptive signal due to the slow reaction of the excited triplet state of 4,5-DCPA. The alcohol radical may react with excess 4,5-DCPA, which would account for its poor signal.

Comparison of the results from 3,6-DCPA and 4,5-DCPA with that from PA shows that the existence of the chlorine atoms as substituents on the benzene


ring can enhance the deprotonation of the ketyl radical, with the chlorine atoms acting as the electron-withdrawing groups.

3.4 Tetra-Chlorophthalic Anhydride in 2-Propanol

In the photolysis o f TCPA in 2-propanol, the alcohol radical showed an E*/A pattern a n d a broad emissive (a little distorted) single line was observed with a linewidth o f ca. 0.8 mT, as shown in Fig. 8a. Although ir is hard to assign the single-line spectrum, the width may imply formation of the ketyl radical with one proton through the mechanism:

3TCPA* + (CH3)2CHOH ~ TCPA-ketyl" + (CH3)2C'OH

or, alternatively, the anion radical by

TCPA-ketyl" ---~TCPA'- + H +.

The spin pola¡ of these species is explained by emissive TM plus an E/A pat- teta from the RPM. Since no hydrogen adduct radical was observed, the direct hydrogen abslraction by the unsaturated portion is unlikely, as may have been antici- pated. It is consistent with a nonreactive character for the chlorinated 4 and 5 positions in this compotmd. In this system, the decay of the alcohol radical was very slow compared with other PA systems (about one fifth of the decay rate in the PA system). This implies that the reaction of the alcohol radical with TCPA is inefficient.

In the presence o f TEA, the single-broad-line spectrum disappeared and a multiline spectrum was observed, as shown in Fig. 8b; this was assigned to the neutral radical o f TEA [23, 24]. The reaction is as follows:

3TCPA" + Et3N ~ TCPA-ketyl" + Et2NC'HCH 3.

The observation of the alcohol-de¡ radical is due to a competing reaction of the excited TCPA with alcohol. No ketyl or anion radicals were observed with TCPA, in contrast to the other PA systems. This may be due to the fast reaction of the ketyl or anion radical with TEA. TCPA appears to provide an exception to the general photoreactions of PA de¡

The reaction mechanisms derived from the experimental observations of four different PA de¡ in 2-propanol are presented in Fig. 1, where X~ denotes a hydrogen or chlorine atom. The substitution of hydrogen by chlorine atoms en- hances deprotonation of the intermediate ketyl radical to form the anion. Substi- tution at the 4 and 5 positions blocks hydrogen abstraction at the particular position.

At this stage there is no evidence for intramolecular hydrogen migration through the intermediate ketyl radical to forro the adduct radical, but this mechanism has not been excluded:

PA-ketyl" --~ PA'H.


Em.

r

a

342 344 346 348 350 352 354 356 B (mT)

2 ~

342 344 346 348 350 352 354 3 6 B (toT)

I I t I

345 346 347 348 B (mT)

I

349

Fig. 8. a CIDEP spectrurn observed 1.75 p.s after laser excitation of TCPA in 2-propanol. The hyperfine lines rnarked by arrows ate due to (CH~)zC'OH. b CIDEP spectrum observed 1.75 q after laser excitation of TCPA and TEA (5- 10 -z M) in 2-propanol. e Expanded spectrum of the center portion of b. Emissive many lines marked by upward arrows represent the spectrum of the TEA

neutral radical (EhNC'HCH3).

4 Conclusion

The photolysis of phthalic anhydride and its chlorinated derivatives occurs largely through a common reaction scheme. The excited triplet state of the PA first reacts with alcohol through two competing reaction routes to form the hydrogen adduct radical (in PA and 3,6-DCPA) and the ketyl radical (all PAs). In the case


o f 3,6-DCPA and o f 4,5-DCPA the ketyl radical subsequently deprotonates to forro the anion radical. The data suggest that the alcohol radical reacts with the PAs to forro the same ketyl radicals, except with TCPA. With 4,5-DCPA, the reaction o f the exci ted triplet state is s low compared with triplet relaxat ion anct the radicals are observed with absorptive polar izat ion resulting from reaction o f ther- mal ly equi l ibrated triplet. With 4,5-DCPA and TCPA, no hydrogen adduct for- mat ion takes p lace due to the nonreact ive nature o f chlor ine at the 4 and 5 posit ions. When TEA is added, the reaction with it predominates and anion radicals are formed except with TCPA, where a neutral radical formed by H abstraction from TEA is observed.

Acknowledgements

We are indebted to K. Kuwata for his encouragement and to Y. Sugie for his help with this research project that started initially at Osaka University. K.H. is grate- fui to the Japan Society for Promotion o f Science (JSPS-PS95679) for the support o f a visit to Tohoku University. This work is defrayed by Grant- in-Aid on Prior- ity Area Research on "Photoreact ion Dynamics" (07228241, 07228206, 08218205) and Grant-in-Aid for Developmental Scientific Research (07554064, 08740437) from the Ministry o f Education, Science, Sports, and Culture of Japan. This work was part ial ly supported by CASIO foundation for scientific studies.

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(1998)

Authors' address: Hisao Murai, Department of Chemistry, Faculty of Science, Shizuoka University, Shizuoka 422-8529, Japan

time-resolved epr study of the photoreduction of phthalic anhydride and chlorinated phthalic...

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