Multiphotonic photolysis of perylene and pyrene in liquid cyclohexane

225

J. Photochem. Photobiol. A: Chern., 58 (1991) 22.5-237

Multiphotonic photolysis of perylene liquid cyclohexane

and pyrene in

M. Lamotte+, J. Pereyre, R. Lapouyade and J. Joussot-Dubien Laboratoire de Photophysique et Photochimie Molt!culaire, URA CNRS 348, Universite de Bordeaux I, F3340.5 Talence (France)

(Received July 9, 1990)

Abstract The products resulting from the photolysis of perylene (Ar) in cyclohexane (Cy) using pulsed irradiation at 248 nm from an excimer laser are compared with those obtained using a low pressure mercury lamp at 185 nm. In the laser experiment, the data indicate a two-photon mechanism. The different final photoproducts obtained using the two types of irradiation are interpreted as resulting from the secondary excitation of intermediates and/or solvent excitation in the case of continuous irradiation. Several reaction schemes are proposed to account for the formation of the identified photoproducts. They are based on the initial formation of an aromatic anion+yclohexane cation pair (Ar-...Cy’), as first proposed by Warman, in addition to the usual ion pair (Ar’ ...e-).

1. Introduction The photochemistry of polycyclic aromatic hydrocarbons (PAHs) from highly excited states is poorly documented. Yet, excitation to high energy levels, either directly or by multiphotonic processes, is of growing interest. The technique of photoablation, which is used to excite molecules to high energy levels, has received considerable attention [l]. The method takes advantage of the high power of excimer lasers and, although polycyclic aromatics are often used as sensitizers, the mechanisms are far from understood. In previous experiments performed in frozen and liquid alkanes [2, 31, we have demonstrated that different mechanisms are involved in the photodegradation of PAHs in non-polar solvents depending on whether the excitation leads to highly excited triplet or singlet states. Photolysis from highly excited triplets proceeds by a radical mechanism following the sensitized decomposition of the alkane which produces hydrogen atoms and R radicals [2, 41. In contrast, the outcome of the electronic excitation from highly excited singlet states is not yet fully understood. PAHs are relatively stable on exposure to UV radiation provided that the excitation takes place exclusively in the singlet manifold and at an energy lower than the ionization threshold. When the excitation energy exceeds the ionization threshold, the’most probable event is ionization and charge separation leading to the formation of the aromatic ‘Author to whom correspondence should be addressed.

lOlO-6030/91/$3.50

8 Elsevier Sequoia/Printed in The Netherlands

226 cation and a solvated electron [5]. In non-polar liquid solvents, the separation probability of the charge pair is very low and the main reaction following ionization is fast recombination [6-81 of the cation and the ejected electron. The recombination time has been estimated to be less than 9 ps [7] and to be dependent on the shape of the solvent molecule [lo], varying from 2.2 ps in normal octane to 0.4 ps in more spherical iso-octane. With solutes of low ionization potential such as tetramethyl-paruphenylenediamine (TMPD), recombination leads essentially to a fluorescence state of the aromatic [g-12]. In some cases, however, irreversible photolysis of the aromatic molecule has been reported [7, 131. As yet, it has not been determined whether this happened following recombination or from a totally different pathway. Some time ago, Warman [14] suggested that the multiphotonic photolysis of anthracene (An) in liquid n-alkane may result in the formation of a solvent cation (S’) and a solute anion (Ar-) by a hole “injection mechanism”. A similar mechanism was invoked by Konuk et al. [15] to interpret the photoionization of perylene in dimethylformamide. Unfortunately, a two-photon laser experiment performed on a picosecond time scale for perylene in cyclohexane [3] did not reveal any transient absorption that could be assigned to one of the postulated ions. Nevertheless, the analysis of the photoproductswas interpreted within the framework of Warman’s hypothesis. In particular, the identification of cyclohexenyl addition photoproducts and evidence for the involvement of charged species were in agreement with the formation of the cyclohexane radical cation CsHr2’+, which was supposed to give rise to a reactive cyclohexene cation C6H1{’ on decomposition. Sauer et al. [13] have provided further arguments in favour of the formation of CaH1i+ during the laser photoionization of anthracene in cyclohexane, but they have concluded that GH,, + or C6Hr3+, which are formed subsequent to C6HrZ’+, must be responsible for the high ion mobility observed in cyclohexane [16]. In our previously reported photolysis experiments [3], irradiation of perylene and pyrene in liquid cyclohexane was performed only with the 185 nm radiation from a low pressure mercury lamp. At this wavelength, the absorption coefficient of neat cyclohexane cannot be neglected and we could not exclude the possibility that some of the identified photoproducts were formed from the direct decomposition of cyclohexane as a primary step and not from reactions following perylene excitation. In order to obtain exclusive excitation of the solute, we carried out further experiments which involved a resonant sequential two-photon excitation of the solute using the 248 nm radiation from an excimer laser. Under these conditions, the sequential two-photon excitation of the solute is expected to arise with a much larger probability than the direct two-photon excitation of the solvent. Another advantage of this excitation lies in its pulsed nature (At=30 ns) which limits the subsequent irradiation of the photoproducts. The mechanism of PAH photodegradation following excitation under high intensity conditions needs to be clarified. In particular, we need to ascertain whether, like photoionization, photodegradation of PAHs proceeds by a two-photon mechanism with the Sr state as intermediate. Analysis of the photoproducts by high performance liquid chromatography (HPLC) and mass spectroscopy was also performed and the results were compared with previous reports [3]. The role played by perfluorohexane (pFH), which accelerates the photodegradation, was also investigated by analysing the products obtained from the degradation of pyrene at 185 nm using the low pressure mercury lamp. For this experiment, pyrene was chosen because it gives rise to simpler chromatograms which are easier to compare

227

than those of perylene. for perylene irradiated

2. Experimental

The influence of pFH on the degradation with the excimer laser.

rate was also studied

details

W irradiation of deaerated cyclohexane solutions of perylene and pyrene was performed at 185 nm with a helicoidal low pressure mercury lamp and at 248 nm with a KrF excimer laser (Lambda Physik, EMG 200). In the first case, the irradiated solution was contained in a Dewar-like flask described in ref. 3. In the second case, a stainless steel cell with silica windows and a path length of 2 cm were used. Absorbance changes on irradiation were measured with a Cary 2200 spectrophotometer. Cyclohexane was of spectroscopic grade (Fluka). For irradiation at 185 nm, it was washed with sulphuric acid, distilled and kept over molecular sieves. Perfluorohexane (pFH) (Fluka purum) was used as electron scavenger without further purification. The analysis of the photoproducts was carried out after evaporation of the cyclohexane and redissolution in acetonitrile for HPLC separation. A Cl8 column (Spherisorb S-5) was used in conjunction with a UV absorbance detector (Pye Unicam) set at 254 nm and with mixed acetonitrile-water (85:lS) as eluent. The collected fractions were concentrated and injected in a gas chromatograph coupled to a mass spectrometer (GC/MS) for analysis.

3. Results

and discussion

3.1. Photodegradation of perylene in cyclohexane using excimer laser irradiation at

248 nm 3.1.1. Evidence for a two-photon mechanism Experimental evidence for a two-photon mechanism is not straightforward to obtain and generally requires suitable irradiation conditions. This is particularly true with pulsed irradiation which can easily introduce spurious effects due to absorption saturation. In a first step, we determined the optimum range of laser fluence to be used, according to the photophysical properties of perylene. The system of differential equations corresponding to the various photophysical and photochemical events following pulsed laser excitation of petylene was solved by a summation method [17, 201. The pulse shape of the laser was modelled using a gaussian profile and the changes in the concentration of the various species with time (AC/At) were computed for 0.1 ns time increments. The degradation of perylene (Ar) in cyclohexane (Cy) is expected to proceed after the formation of an ion pair (either Q’... Ar- or e-...Ar+; first-order rate constant chosen arbitrarily as 2~ 10M6 s-l). Other important parameters are the Sr lifetime (6.4 ns [21]) and the triplet yield (0.014 [22]). In the absence of experimental data, the absorption coefficient from Sr at 248 nm was taken to be equal to that from So (0.3 x 10’ M-l cm-‘). These approximations are not expected to affect the results, at least from a qualitative point of view. Figure 1 displays the calculated variation in the formation rate of the photoproducts (expressed as the concentration of photoproducts per pulse) us. the initial number of photons per molecule which, for a given concentration, is related to the laser fluence. From this simulated curve, it is concluded that, in order to obtain evidence for an

228

2-

10

20

1

0.5

co

L

i

I

'2

fluence(mJ/cm2) 80 160 3C'6

c N

1oq.lNphotons/molecule)

Fig. 1. Simulated rate of photoproduct formation (calculated for at 248 nm) IX. excimer laser fluence. A two-photon mechanism was assumed. The expected slope of two is obtained only for Perylene concentration, 1.6~10~’ M; excimer laser, 248 nm;

perylene in cyclohexane irradiated with the S, state as intermediate fluences lower than 30 mJ cm-*. fluence, 21 mJ pulse-‘.

eventual two-photon mechanism, it is necessary to use laser fluences lower than about 30 mJ cm-* pulse-‘. For larger fluences, the slope decreases from its theoretical value of two to reach an asymptotic value of unity. On the basis of this result, we determined the variation in the degradation rate of perylene as a function of the laser intensity for fluences lower than 27 mJ cm-* pulse-‘. The progress of the reaction was followed by measuring the relative absorption change of perylene at 410 nm during repeated laser pulse irradiation. It was supposed that the absorption of the photoproducts at this wavelength was negligible because of the low conversion attained (less than 10%). The results are shown in Fig. 2. A slope of 1.8fO.l is deduced from these experiments. It is concluded that the photodegradation of perylene in cyclohexane, for excitation above the ionization potential threshold, proceeds (as ionization [18, 191) by a twophoton mechanism with the S1 state as the probable intermediate. 3.1.2. Analysis of the photoproducts The absorption spectra of perylene in cyclohexane obtained after continuous irradiation at 18.5 nm (mercury lamp) and after pulsed irradiation at 248 nm (excimer laser) are shown in Fig. 3. For laser irradiation, the spectrum corresponds to a larger conversion of the products than for continuous irradiation; the absorption spectra exhibit obvious differences which indicate that different photoproducts are formed. The most striking difference is the absence, in the case of pulsed irradiation, of bands attributed to dihydro derivatives in the 290-360 nm range [3]. This is confirmed by the analysis of the photoproducts by liquid chromatography and mass spectroscopy. In contrast with the 185 nm irradiated solution, dihydrocyclohexenyl derivatives (I and/or II) were not detected in the 248 nm irradiated solution. Instead, a I-cyclohexenylperylene derivative (III) was identified. In both cases, however, l- and 3-cyclohexylperylenes (V and VI) were formed.

229

100 8

50 1

150 I number

200 I of

250 I

300 I

350 I

l

pulses

Fig. 2. Effect of excimer laser fluence on the rate of photolysis of perylene in cyclohexane at room temperature. The 100% curve corresponds to a fluence maximum of 21 ml pulse-r. The plot of the logarithm of the photolysis rate vs. the logarithm of the laser fluence is also shown (*). The broken line corresponds to a slope of two.

I

I

250

300

1

, 350

1

-400

1

450

nm

Fig. 3. Absorption spectra recorded after the photolysis of perylene in cyclohexane using continuous irradiation at 185 nm (mercury lamp) ( - - -) and a number of pulses at 248 nm (excimer laser) (-)a

230

I

t$p‘I &’$g VII

\’ I

\’

The identification of some of the photoproducts was confirmed by comparison of their retention times and absorption and fluorescence spectra with those of synthesized reference compounds. Figure 4 shows a chromatogram of a solution irradiated at 248 nm with the excimer laser (approximately 500 pulses, approximately 200 mJ pulse-‘). The broken line corresponds to a chromatogram of a reference solution containing synthesized l- and 3-cyclohexylperylenes whose retention times fit quite well with the F3 and F5 fractions respectively. The presence of 3cyclohexylperylene in fraction Fs is also confirmed by the similarity of its fluorescence spectrum with that of the synthesized molecule (Fig. 5(B)). The identification of cyclohexenylperylene in fraction F2 is confirmed by the mass spectrum, which indicates the presence of a cyclohexenyl fragment, and by its fluorescence spectrum (Fig. 5(A)) whose blue shift, compared with the perylene fluorescence spectrum, is indicative of a substitution at the 1 position. We could not identify with

I

I

5

10

15

20 Retcnt

ion

25 ttme

30

35

40

Fig. 4. HPLC analysis of the photoproducts formed in irradiated solutions of perylene in cyclohexane (laser pulses, 248 nm). Absorbance detection centred at 254 nm. Column: reversed phase CIs. Eluent: acetonitrile-water (85%). The broken line corresponds to a chromatogram displaying (from left to right) the reference peaks of perylene, 1-cyclohexylperylene and 3cyclohexylperylene.

231 77K/MCHl

3c@e

77K/MCH

Pe

440

460

460

500

Wavelength

420

(nm

440

460

480

500

1

Fig. 5. Fluorescence spectra of the chromatographic fractions F2 and FS (see Fig. 4) evaporated and redissolved in methylcyclohexane (MCH). The spectra were taken at 77 K for a better comparison with the reference spectra of perylene (Pe, - - -) and 3-cyclohexylperylene (3CyPe, -. -). confidence the other derivative contained in fraction Fz. It might be 3-cyclohexenylperylene (IV) which was not detected in any of the main fractions collected. The F4 fraction was found to obtain a disubstituted perylene which is best identified, according to its fluorescence spectrum, as a 1,6-dicyclohexenyl derivative. 3.1.3. Effect of perfuorohexane (pFH) We have previously observed [3] that, on continuous irradiation at 185 nm, the rate of degradation of PAHs is substantially increased by the presence of an electron scavenger such as perfluorohexane (pFH). This effect was interpreted as evidence for the involvement of charged species in the photodegradation process. A similar effect was also observed in the two-photon excitation of perylene and pyrene with the excimer laser as illustrated in Fig. 6 for perylene. This result indicates that the two types of irradiation most probably involve the same primary mechanism with the formation of identical charged species as a first step. It is important to understand the role played by pFH. Does it participate in the formation of new photoproducts or is its effect limited to a temporary trapping of electrons (or other negatively charged species) as is usually proposed? To answer this question, we investigated the role of pFH by analysing the photoproducts formed during irradiation of pyrene in cyclohexane at 185 nm. 3.2. Photo&is

of pyrene in cyclohexane

in the presence

of pFH

The chromatograms obtained from solutions of pyrene in cyclohexane irradiated at 185 nm in the presence (pFH, 10-r M) and absence of pFH (about 50% conversion) are shown in Fig. 7. The two chromatograms are’ similar and therefore we can conclude that pFH does not participate in the formation of new photoproducts. It prevents the fast

232

a 5

10

15 number

20

25

30

35

COP

of pulses

Fig. 6. Effect of perfluorohexane

(pFH)

on the degradation

rate of perylene

during pulsed laser excitation at 248 nm (perylene concentration: pFH; 0, with 10-l M pFH.

I

0

retention

lime

0

relentIon

t,mc

in cyclohexane

1.5X1O-s M): 0, without

*

Fig. 7. HPLC analysis of the photoproducts formed in irradiated solutions of pyrene in eyclohexane (continuous irradiation at 185 nm) in the absence (left) and presence (10-l M) (right) of pFH. Same conditions as for chromatograms in Fig. 4.

recombination of charged species by trapping temporary electrons and/or eventually the aromatic anion (ArH-). Furthermore, all the peaks appear with about the same relative intensity which indicates that pFH has no influence on the distribution of the final products. The photoproducts contained in the collected fractions were readily identified by GC/MS analysis and from their absorption spectra. In contrast with perylene, no cyclohexenylpyrene derivatives were observed for pyrene. Nevertheless substituted

233 dihydropyrenes (VIII and IX) were identified in fractions Fs and F4 (Fig. 7). The fraction F3 was found to contain both l- and 4cyclohexylpyrene (X and XI).

&&j&&t& 1X

VIII

X

XI

3.3. Discussion Depending on the type of irradiation (continuous at 185 nm or pulsed at 248 nm) and the nature of the irradiated polycyclic aromatic (perylene or pyrene) different final photoproducts were observed. Four types of photoproduct were identified. (i) Cyclohexylaryl derivatives were observed in all cases, For 3cyclohexylperylene (VI) and 1-cyclohexylpyrene (XI), the positions of substitution are as expected from the known reactivity of the neutral aromatic in radical reactions. However, for lcyclohexylperylene (V) and 4cyclohexylpyrene (X), their formation cannot be accounted for solely by this mechanism and other processes are required. (ii) Dihydrocyclohexylaryl derivatives were observed only for pyrene irradiated at 185 nm (compound VIII). (iii) Dihydrocyclohexenylaryl derivatives (I and/or II) were observed only for perylene irradiated at 185 nm; the position of the cyclohexenyl substituent could not be determined. (iv) A cyclohexenylaryl-substituted compound was observed only for perylene irradiated at 248 nm; the compound was substituted at position 1 (compound III). The variety of the photoproducts can be rationalized if various photochemical pathways and different reactivities are proposed depending on the aromatic. Following the photoexcitation of aromatics above the ionization threshold in nonpolar solvents, two primary ionic pairs could be formed ArH-

ArH** -

(ArH’+

ArH -

ArH* * + c-C6Hlz -

. . . e-) (ArH’-

(1) . . . c-GH,;+)

(2)

As a working hypothesis, we will neglect process (1) as the main primary route for the disappearance of the aromatic molecules. The poor reactivity of aromatic cations supports this assumption. They are known to react with nucleophilic reagents [23], but alkenes cannot be considered to possess this property. Indeed, in the absence of other reactive species, it is difficult to find a chemical reaction that will give rise to the transformation of the aromatic cation. Furthermore, the extremely fast recombination rate [5-71 of the ion pair produced in process (1) may drastically limit the efficiency of any consecutive chemical reaction which may occur during the aromatic cation lifetime. Secondary reactions with electron scavengers such as SF6 or pFH have sometimes been invoked to account for the consumption of the aromatic during photoionization experiments [7, 131. However, as mentioned above for pyrene, the absence of photoproducts resulting from a combination with pFH provides clear evidence that such reactions are negligible. The most probable event ‘following process (1) is ArH’+ . . . e- geminate recombination yielding excited aromatic molecules [8-121 which may relax either radiatively or non-radiatively. It can be envisaged that, on geminate recombination, some chemical

234

processes may take place from molecules formed in highly excited singlet or triplet states. However, subsequent photochemical reactions of the aromatic solute from singlet or triplet states at energies lower than the photoionization threshold appear to be improbable since no photodegradation was observed on excitation at wavelengths of 250 nm or greater. Therefore, on the basis of Warman’s hypothesis, we propose that the disappearance of the aromatic molecules in cyclohexane during two-photon irradiation at 248 nm takes place mainly via reaction (2), i.e. from the formation of the cyclohexane cation. However, this cation is suspected to be very unstable and to decompose according to three possible processes. (i) A reaction following recombination with an electron yielding either a cyclohexyl radical or a cyclohexene molecule [16, 24, 251 c-C~H~;+ +(e-)

-

c-CclH1;+ + (e-) -

c-&H,,‘+R

(3)

C-C6HI0 + H2

(4)

(ii) A spontaneous monomolecular or light-assisted decomposition yielding a cyclohexene cation (C6H10.+), which may account for the presence of the cyclohexenyl group as substituent in some of the photoproducts c-&HI;+

*-

c-CsHlo’+ +H,

(iii) A fast proton transfer to a neutral cyclohexyl radical and a c-C~H~~ + cation C-C6HI;+ + c-C~H,~ -

c-CsH,l’ + c-C~H~~+

(5) cyclohexane

molecule

giving rise to a (6)

If reactions (3) and (4) are important, the addition of an electron scavenger such as pFH, which is supposed to increase the lifetime of the c-C~H~~+ cation, should result in a decrease in the aromatic degradation rate. The reverse effect observed indicates that these reactions can be neglected. Reaction (5) has been observed by Tabata and Lund 1261 for a light-excited cyclohexane cation in a freon matrix at 142 K. The cyclohexene radical cation, instead of the expected cyclohexane radical cation, was also identified during the radiolysis of cyclohexane by Melekhovet al. [27] using an optically detected electron spin resonance (ESR) spectroscopy technique. These workers concluded that the cyclohexane cation, when engaged in an ion pair with 2,5-diphenyloxazol (PPO), decays monomolecularly to give the cyclohexene cation-anion pair c-CsHlo’+-PPO’-. However, in neat cyclohexane, in contrast with other alkanes, Werst and Trifunac [28] expressed some doubt about the spontaneous formation of the alkene radical cation from the alkane radical parent cation. Negative results obtained by these workers were interpreted by Veselov et al. [29] as the result of the temperature dependence of the electron paramagnetic resonance (EPR) spectra of the cyclohexene radical cation. In spite of this controversy, and in the absence of other pathways, we will retain reaction (5) as a source of the formation of a cyclohexenyl moiety. Reaction (6) has been found to be approximately thermoneutral in the case of n-butane [30] and may be proposed as a route for the postulated formation of ccyclohexane following aromatic solute ionization. C&13 + in liquid The work of Klassen and Teather [31] is in favour of the possible involvement of this reaction. They have suggested that this process may explain the fast decay of the initial cations reported by Louwrier and Hamill [32] in irradiated neat alkanes. Further evidence also arises from the work of Sauer and coworkers 1161 and Iwasaki [33, 341 who have provided experimental arguments in favour of C6H13+ instead of

235

as the species responsible for the high ion mobility found in irradiated c-&HI;+ cyclohexane. Moreover, it is commonly accepted that reaction (6) is implicated in the production of alkyl radicals in alkane radiolysis [25, 341. A similar proton transfer mechanism may also be envisaged with the cyclohexene cation formed from reaction (5) yielding a cyclohexenyl cation c-CsHIO-+ + C-CgH12c-C~H~ + c-CsHr9 + (7) Referring to the known reactivity of polycyclic aromatic compounds and considering the radicals which can be produced by the primary processes (5), (6) and (7) (ctowards the aromatic), CsHr3 + which is not a radical is expected to be non-reactive two types of radical reactions can be proposed to account for the formation of the final photoproducts identified in our experiments. (i) Attack of the aromatic molecules either by alkyl [35] or alkyd cation [36] radicals. This may explain the formation of the 3-substituted perylene derivatives (IV, VI) and the l-substituted pyrene derivative (X) which reflect the preferred reactivity of the 3 and 1 positions of the neutral parent molecules with radicals [37]. (ii) Attack of the aromatic anion by cyclohexyl or cyclohexenyl radicals which constitutes the most probable route to the formation of the l-substituted perylene derivatives (III, V) and the 4-substituted pyrene derivatives (VIII, XI). The presence of these photoproducts is reminiscent of the nucleophilic alkylation of the parent aromatic by alkyllithium reagent which, in the case of perylene, has been shown [38] to produce l-substituted perylene derivatives by a different mechanism from that involved in the radical attack of the neutral aromatic proposed in (i) above [39]. Dihydrocyclohexenyl derivatives (I and/or II) were only observed for perylene irradiated at 185 nm (continuous irradiation). In contrast with irradiation at 248 nm, irradiation at 185 nm may excite either cyclohexane molecules, yielding predominantly C-C6HIO+H2 [40, 411, or intermediate species which then become reactive. Due to the continuous nature of the 185 nm irradiation, photoexcitation of cyclohexene followed by reaction with perylene may be envisaged.

4. Conclusions

The results reported here show that the nature and the distribution of the photoproducts obtained from a pulsed (biphotonic) irradiation of a cyclohexane solution of perylene at 248 nm differ substantially from those obtained by a continuous (monophotonic) irradiation at 185 nm. Nevertheless, in both cases, adducts containing a cyclohexenyl group were clearly identified in the case of perylene. Therefore, although the two types of irradiation lead to different secondary chemical processes as the main routes to aromatic degradation, they probably involve a similar primary process which is responsible for the formation of a reactive derivative. containing a cyclohexenyl moiety. The involvement of this species is consistent with the initial formation of an aromatic anion-cyclohexane cation pair as formerly proposed by Warman [14], followed by a rapid decomposition of the cyclohexane cation. From reactions (6) and (7), it is proposed .that this process yields the cation cGHr3+, which is responsible for the high conductivity observed in cyclohexane [13, 161, and reactive radicals, which are responsible for the subsequent degradation of the aromatic.

236 Unfortunately, as yet none of these radicals have been detected, and the details of the reaction pathways remain unclear and need to be investigated.

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