The reaction of biphenyls in the upper triplet states in micellar systems

J. Photochem.

Photobiol.

A:

Chem.,

55

(1991)

377-386

377

in the upper

The reaction of biphenyls micellar systems

triplet

states

in

Shuichi Hashimoto’ Chemistry

Department,

Gunma

College

of

Technology,

and Biochemistry,

University

580

Toriba-machi,

Maebashi,

Gunma

371

(Japan) J. Kerry Thomas Department (U.S.A.)

of Chemistry

of Notre

Dame,

Notre

Dame,

IN

46556

(Received April 4, 1990)

Abstract Reactions of the upper triplet states of biphenyl and its halogenated derivatives were investigated in aqueous micellar systems at room temperature with a two-step excitation method. Irradiation of the triplet states of biphenyl and its chloro and bromo derivatives produced hydrated electrons (e,,-) and a decrease in the triplet states. No evidence of carbon-halogen bond homolysis, with a rate competing with e,formation (~
in an anionic micelle, i.e. sodium dodecylsulfate. This is because of the enhanced geminate recombination of electron-cation pairs caused by the positive micellar surface charge which prevents the escape of electrons from the cationic micelle.

1. Introduction Photoexcitation of halogen-substituted organics, in particular haloaromatics, is known to cause carbon-halogen (C-X) bond homolysis and to generate the corresponding aryl radical and halogen atom. On the basis of gas phase molecular beam studies, Bersohn and coworkers [l] have proposed a general scheme for the dissociation pathway for haloaromatics: photodissociation proceeds from upper singlet states via internal conversion of SF followed by intersystem crossing to a triplet state localized in the carbon-halogen bond S2(ai--7jF) -

S,(W#)

-

T(a,a*)

-

fragments

Spin-orbit coupling is postulated to be a dominant factor which accounts for the dissociation rate for the different at-y1 halides: the lifetime of the dissociating aryl iodides is approximately 1 ps, whereas aryl bromides dissociate on a time scale approximately two orders of magnitude longer than the corresponding aryl iodides. Grieser and Thomas [2] have observed the temperature-dependent triplet state lifetimes of several aryl iodides and bromides in toluene. They propose that the temperature ‘Author

to whom correspondence

lOlO-6030/91/$3.50

should be addressed.

0 EIsevier Sequoia/Printed

in The

Netherlands

378 dependence may result from a crossing of the phosphorescent triplet state surface to a dissociative T(cr,u*) level. Picosecond spectroscopic studies of haloaromatics at room temperature in hydrocarbon solvents have been carried out by Rentzepis and coworkers [3]. The intersystem crossing rate increases with a charge in substituent from chlorine to bromine to iodine. In addition, the fluorescence lifetime has been shown to be the same as the growth rate of the triplet state population. Furthermore, direct observation of radical formation within a laser pulse (approximately 15 ps) has been demonstrated for some aryl-alkyl halides (i.e I-(chloromethyl), 1-(bromomethyl), Z-(chloromethyl) and 2-(bromomethyl) naphthalenes), whereas formation of radical species has not been directly detected for aryl halides. Since the fluorescence lifetime is much longer, e.g. 2400 ps for 1-(chloromethyl) naphthalene, than that of the dissociating state, it has been suggested that the deactivation process from the excited singlet state IS,*) proceeds via at least two competing channels: (1) direct deactivation from an &(w,?r*) to a T(u,#) dissociative state and (2) internal conversion to the Sr state with subsequent intersystem crossing. If this dissociation mechanism is applicable to aryl halides, then the rate of C-X bond scission could be much faster than that of intersystem crossing even for chloro and bromo derivatives as has been observed for chloromethyl naphthalenes [3]. The lack of direct observation of radical formation hampers the estimation of the rate of dissociation of aryl halides in room temperature solution. In this study, we have employed a different technique to attempt to observe directly the dissociation process of aryl halides, namely halogen-substituted biphenyls. We have assumed that the photoexcitation of the lowest triplet state T1 may induce cross-over to T(cr,b*) in the upper triplet state T,, followed by the formation of a biphenyl radical and halogen atom. Aqueous micellar solution is used in which twophoton ionization of aromatics can be facilitated [4] because of the large polarity of the medium and the surface electric potential. The time scale of photo-ionization and solvation of the photoejected electron is considered to be approximately l-10 ps [5]. Hence, in micellar systems, it is expected that competition between bond cleavage and electron ejection will take place for aryl halides, especially for iodo and bromo compounds in the higher triplet states, thus enabling us to estimate the rate of C-X bond scission relative to that of photo-ionization.

2. Experimental

details

Biphenyl (BP, Aldrich), 4-chlorobiphenyl (CBP, Alfra), 4-bromobiphenyl (BBP, Kodak) and 4,4’-dibromobiphenyl (DBBP, Kodak) were recrystallized twice from ethanol. Sodium dodecylsulfate (SDS, BHD specially purified for biochemical work) was used as received, whereas cetyltrimethylammonium bromide (CTAB, Sigma) was recrystallized three times from a methanol-acetone mixture. Two methods were employed for the present two-step excitation experiments. One was carried out at Notre Dame using a nanosecond laser photolysis system [6] (apparatus I; rise time of the total system; 8 ns) in which a steady state concentration of the triplet state was initially produced by illumination with a pulsed 500 W xenon lamp (pulse width; 3 ms) with a Corning 9-54 filter (50% cut-off wavelength, 230 nm). This light source also served as a monitoring lamp for the transient absorption. A pulse of laser light from a Lambda-Physik EMG-100 (h,= 337 nm; pulse width, 6 ns; energy, 5 mJ pulse-‘) was used to excite the lowest triplet to the upper triplet manifold. Apparatus I has a better time resolution than the second technique. Apparatus II

379

(described later) produces a balanced population of triplet compared with other transient species, which is appropriate for observation of the transient absorption spectrum. However, this method was only applicable to BP and CBP where the triplet state is relatively long lived, and in the millisecond range. The other experiment was performed at Tohoku University, with a conventional flash photolysis apparatus [7] (apparatus II), which delivered two stepwise flashes of light (pulse width, 10 /.Ls;input energy, 24-320 J) separated by a variable delay. The second flash can be replaced by a pulsed laser (A_= 347.1 nm; pulse width, 20 ns; energy, 100 m3 pulse-‘). The time resolution of this system improved down to 20 ns when the laser was used as the second-step excitation source. A fresh sample was used for each experiment, thus avoiding side effects of any photoproducts. Samples were deoxygenated by bubbling with nitrogen for at least 30 min. All experiments were carried out at ambient temperatures (23-25 “C). Doubly distilled water was used for all the experiments.

3. Results 3.1. Excitation of triplet biphenyl (BP) and I-chlorobiphenyl (CBP) in miceilar systems Figure 1 shows the time course of T-T absorption of BP (A = 370 nm) in nitrogensaturated SDS micellar systems observed, at slower time response, with apparatus II (conventional). Figures l(a) and l(b) show the experiments performed in the absence nm), which followed 80 fl after the and presence of the second flash (A,, =31MOO first flash. On irradiation with the second flash an immediate bleaching of the T-T absorption was observed. Using apparatus II, we failed to obtain a transient absorption spectrum other than that of the triplet partly because of the large absorbance of the T-T absorption over a wide wavelength range and partly to interference by scattered light immediately after the flash. The A,,, value and first-order decay constant (k,) of the T1-T, absorbance for all the compounds studied were measured with apparatus II and are listed in Table 1. The T-T absorption spectrum was obtained from the transmittance change observed between samples irradiated with a Corning 9-54 filter present, i.e. no excitation of the sample, and samples irradiated with a Corning of ground state BP was in good agreement with employed apparatus I to obtain a transient spectrum triplet state. Irradiation of the sample with a pulsed xenon 4.50 W; pulsed current, 300 A; pulsed voltage, 90 V)

0.

time / ps

500

0

Fig. 1. Decay of T-T absorption of 8.0 x 10-j (a) and presence (b) of the second flash.

O-53 filter present. The excitation the literature [S]. Therefore we attributable to excitation of the analyzing produces

tim / p

M BP

light (OSRAM XBOa steady concentration

500

in 0.1 M SDS at 370 nm in the absence

380 TABLE

1

Amaxand

first-order decay constant of T-T absorption Qclohexane A,

BP CBP BBP DBBP

(nm)

0.1 M SDS k1 (s-l)

A,

(nm)

365

5.1 x 103

365

4.5 x 102

378 386

1.4x 103 1.3 x lo4

380 386 390

3.9 x 102 4.5 x 104 2.3 x lo4

k, (s-‘1

of BP triplets. Figure 2(a)-2(c) depict the changes in transmittance on laser irradiation after pulsed xenon lamp irradiation. The time delay for firing the laser was set 1 ms after pulsing the monitoring-exciting lamp. Bleaching of the triplet at 300-380 nm (band (a)), a sharp absorption centered at 395 nm (band (b)) and a broad absorption at around 720 nm (band (c)), which decayed faster than band (b), were observed. A transient difference absorption spectrum was obtained from the data shown in Fig. 2, and is shown in Fig. 2(d). When the solution was bubbled with N20 instead of nitrogen, the broad absorption centered at 720 nm was instantaneously suppressed, but the bleaching and absorption band (b) were left intact; this is shown by filled circles in Fig. 2(d). NzO is known to scavenge hydrated electrons (e,,-); therefore absorption band (c) is ascribed to eaq- [9]. The biphenyl cation (BP+) is known [lo] to possess two absorption bands at 330-400 nm, with two peaks at 365 and 387 nm (~(365 nm) =9 X 103 M-l cm-’ and ~(387 nm) = 8 X 103 M-l cm-l), and 600-800 nm (~(703 nm) =4_5 x lo3 M-’ cm-‘). Subtraction of the eaq- absorption spectrum from the main transient absorption band gives a spectrum that is reminiscent of BP+. Hence, excitation of triplet BP gives rise to photo-ionization in the SDS micellar system. Excitation of the BP triplet state was also carried out in the CTAB micellar system instead of SDS using apparatus I and, for comparison, exactly the same experimental conditions were employed as those used in the SDS system. Figure 3 shows a transient difference absorption spectrum in the CTAB system obtained in a similar fashion to that in,SDS. Although the amount of bleaching of the T-T absorption in CTAB is similar to that in SDS, the efficiency of the photo-ionization process is markedly reduced compared with the SDS system. Figure 4 shows the transient difference absorption spectra on excitation of the CBP triplet in an SDS micellar system observed with apparatus I (open circles represent nitrogen-saturated solution and filled circles represent N20-saturated solution). Similar spectra to those observed for BP were obtained, again indicating that photo-ionization also takes place for CBP via the triplet state. Indeed, the radical cation of CBP (CBP+) has been shown to posses two absorption bands, one of which has two peaks at 385 and 414 nm, while the other is located at around 800 nm [ll]. A notable reduction in the photo-ionization yield was observed for CBP in CTAB compared with that in SDS. 3.2. Excitation of triplet 4-bromobiphenyl (BRP) and 4,4’-dibromobiphenyI (DBBP) Only apparatus II (conventional flash) was used for the investigation of BBP and DBBP since the shorter lifetime of these triplet states hampered the production of signific:ant steady state concentrations of triplet with apparatus I. A laser pulse from

381

500ns/d

iv

b

Es _‘,._. C_--~_.~_~_._~_-_‘-_-,‘-.“.-‘~i-’-’-’-. $ ---‘.k-y.i ,_ 5l s : 500ns/d iv 0.04~ .-> P

SDS/

0 0.02 -

0 0

8 -

8 El_ =I ._

O

N2 N20

C

‘.L- .... -.- i----r,..,...Y,<, -.-.*.--_-1 f I u) -_-,>,.<,.A

E 5 $

Biphenyl/

,a

0

0 0 0 0 8,

0

0

o

0

.wot .

l

.

l

l

l

. .

.

0

-0.02.; SOOns/d

400

iv (d)

500 Wavelength

600

700

800

/ nm

Fig. 2. Bleaching at 370 nm (a), absorption at 395 nm (b), absorption at 700 nm (c) and difference spectrum (d) on excitation of the BP triplet with a laser pulse (A=337 nm) in 0.1 M SDS (0, nitrogen-saturated solution; 0, N,O-saturated solution).

a Q-switched ruby laser (A= 347 nm) was employed instead of the second flash for excitation of the triplet state. Figure S(a)-S(d) show the time-dependent changes in transmittance at 390 nm for BP and DBBP in both SDS and CTAB; the signals at 700 nm are also shown in Fig. 6(a)-6(d)_ Immediate bleaching of the triplet state was observed for both BBP and DBBP regardless of the surface charge of the micelles. resulting from this triplet bleaching was observed Nevertheless, the formation of e,,system was only in SDS micelles; the transient absorption owing to eaq - in the CTAB very weak, even though the extent of bleaching of the triplet in CTAB was practically the same as that in the SDS system. In the SDS system, transient species with longer

382 Biphenyl /CTAB/

No

0.04-

0.02-

0” Q

& 0

’

0.00

0

0

0

00

0

0

0

~

4

0

000

-o.g2-

0

4

400

800

600 h/nm

Fig. 3. Transient difference absorption spectrum on excitation of the BP triplet in 0.05 M CTAB.

4-Cl-Biphenyl/

N2 , N20

SDS/

0.04* 0 0 0 O

0

0.02 -

.

0 0

0

.

0

go,

0

0

0

0

*a *a

l l

O+

.

0

0

0

0

l

l

l%

.

l

4

0 0

l -

0.02 -

0

.

0

8%0 8

400

h/nm

600

800

Fig. 4. Transient difference absorption spectrum on excitation of the CBP triples (A,=337 in 0.1 M SDS (0, nitrogen-saturated solution; 0, NPO-saturated solution).

nm)

than eaq- were observed at 825 nm, and are assigned to the cation radicals [12] of BBP and DBBP. The products of C-Br bond dissociation were also looked for. It has been suggested that the BP radical has no distinct absorption above 300 mn [13]. Therefore studies were initiated for both BBP and DBBP in SDS to find the bromine atom, by the addition of Br-, which on reaction with the bromine atom gives Br,- (~(360 nm) = 12 000 M- ’ cm-’ [14]). The transient absorption spectrum on laser excitation (second step of the excitation) at 370-850 nm, in the presence and absence of 0.1 M NaBr in 0.1 lifetimes

383 0

C i-

.._ ;--.

--..

;

-\. -----.

:

=: -

time / ps

100

200

-

0

20

time/ ps

Fig. 5. Transient absorption on excitation of triplet (&=347 nm) at 390 nm: (a) BBP (4x lop5 M) in SDS (0.1 M); (b) BBP (4 X 10-j M) in CTAB (0.05 M); (c) DBBP (4x lo-’ M) in SDS (0.1 M); (d) DBBP (4 x 10-j M) in CTAB (0.05 M).

C

-

I-8

4

.t!

E

2

:--..___ _-e-_ ~----______

“0.-.-.

.._’

0

I

time/ p

100

b

c

tS

d

5c . E s .!I! B -,o,

...-.I1- I._--I.*C__.l-._._. 0

time / ps

Fig. 6. Transient

absorption

8 100

on excitation

of triplet

M) in SDS (0.1 M); (b) BBP (4X 10-j M) in CTAB (0.1 M); DBBP (4x10-j M) in CTAB (0.05 M).

M SDS, gives no indication of Brz-, No evidence was obtained for C-Br and DBBP.

(A,,=347 nm) at 700 nm: (a) BBP (4X 10m5 (0.05 M); (c) DBBP (4x 10m5M) in SDS

i.e. of bromine bond

scission

atoms, for either BBP or DBBP. in the upper triplet states of BBP

384

4. Discussion Table 2 summarizes the energetics of the processes suggested in this study. Excitation of T, to upper manifolds provides more energy than is required for dissociation of the C-X bond, and dissociation of the C-X bond should occur if cross-over to dissociative states takes place in the T, manifold. The energy levels of the upper triplet states produced by excitation of T1 are less than the gas phase ionization potential (1,) of the molecules studied. However, it has been shown [4] that the ionization threshold (If,.,) is markedly reduced in condensed phases, e.g. approximately 2 eV (1.6X IO4 cm-‘) in liquid alkane and more than 3 eV (2.4 x lo4 cm-‘) in aqueous micellar systems. From an energetic point of view, photo-ionization in the upper triplet states is favored. Two-photon photo-ionization of aromatic hydrocarbons in condensed phases has been extensively studied and an ionization pathway via excited singlet states has been unequivocally established [17]. Nevertheless, some ambiguity exists for photo-ionization via excited triplet states. Steady state photolysis studies at low temperatures suggest that two-photon photo-ionization takes place via triplet states [18]. However, photoionization via the triplet states has been rejected for pyrene [19] and perylene [20] in polar organic media (ethanol, acetonitrile, etc.). Very recently, Grellmann and coworkers [21] have studied the photo-ionization of diphenylamine (DPA); they investigated the laser intensity dependence of the fluorescence intensity ([DPA(Sf)]), triplet concentration ([DPA(T:)]), cation radical concentration ([DPA+]) and solvated electron concentration ([e,-1) in methanol at ambient temperature with a nanosecond laser photolysis apparatus. They observed that the slope of the triplet yield vs. intensity decreased much faster than that of the fluorescence, and concluded that photoionization takes place from T1 via absorption of a second photon. They also showed that this conclusion was not applicable to naphthalene where photo-ionization involves the ST state as an intermediate level in the two-photon process. It should be noted that their conclusion was deduced from a kinetic model and lacks direct evidence of the involvement of triplet states for the two-photon process. In this study, we have shown directly the involvement of the triplet state in photoionization: bleaching of T: on photoexcitation gives rise to a concomitant formation of hydrated electrons and cation radicals. The use of anionic micellar systems helps

TABLE

2

Triplet energy (E(TI))a, sum of triplet energy and photon energy (E(T,) +hv), bond dissociation energy (O(C-X))a and ionization potential (IJb or ionization threshold (I,,)b

BP CBP BBP DBBP IBP’ aRef. 15. bRef. 16. ‘4-Iodobiphenyl.

E(Tl) (lo3 cm-‘)

E(T,) +h v (lo3 cm-‘)

D(C-X) (ld cm-‘)

21.0 21.0 20.8 20.6 20.8

50.7 50.7 49.6 49.4

36.8 30.6 25.5 21.4

I&h)

(lo3 cm-l) 67.4(64.9) 68.2(64.9)

385

to separate the photogenerated cations and eaq-, by preventing geminate recombination via the micellar surface charge. The micellar surface charge has a marked effect on the yield of photo-ionization. In cationic micelles (CTAB), the yield of eaq - is drastically reduced compared with that in SDS. This can be explained by the positive surface charge of the CTAIB micelle which hampers the escape of eaq- from the micelle, thus enhancing rapid recombination with the cation radical. For the two-photon ionization of pyrene via singlet excited states, Thomas and coworkers [22] have noted no difference in the yield of photoionization between SDS and CTAB micelles, although the subsequent kinetics of the ions are different. This is due to the larger excess energy of the electron in the pyrene studies 141. Direct evidence of C-X bond cleavage on excitation of T1 was not obtained in this study. However, the results of product analysis showed that enhanced dehalogenation occurred in the two-photon experiments. McGimpsey and Scaiano [23] have observed C-Br bond scission on excitation of the triplet state of 9,10-dibromoanthracene in benzene at room temperature with a two-step excitation method. They also estimated the lifetimes of T, and TI as 200 ps and 20 ps respectively. The present study indicates that electron ejection from higher triplet states is much faster than the rate of 20 ps reported for carbon-halide bond scission.

Acknowledgments We wish to thank Drs. K. Kikuchi their generous heip with the experiments, for support of the research.

and H. Kokubun at Tohoku University for and the U.S. National Science Foundation

References 1 A. Freedman, S. C. Yang, M. Kawasaki and R. Bersohn, .I. Chem. Phys., 72 (1980) 1028. M. Kawasaki, S. J. Lee and R. Bersohn, J. Chem. Phys., 66 (1977) 2647. M. Dzvonik, S. Yang and R. Bersohn, J. Chem. Phys. 61 (1974) 4408. 2 F. Grieser and J. K. Thomas, J. Chem. Phys., 73 (1980) 2115. 3 E. F. Hilinski, D. Huppert, D. F. Kelley, S. V. Milton and P. M. Rentzepis, J. Am. Chem. Sot., 106 (1984) 1951 _ D. F. Kelley, S. V. Milton, D. Huppert and P. M. Rentzepis, J. Chem. Phys., 87 (1983) 1842. D. Huppert, S. D. Rand, A. H. Reynolds and P. M. Rentzepis, J. Chem. Phys., 77 (1982) 1214. 4 J. K. Thomas, The Chemistry o,fExcitutiun atInterfaces, American Chemical Society, Washington DC, 1984. K. Kalyanasundaram, Photochemistry in Microheterogeneous Systems, Academic Press, London, 1987. 5 G. A. Kenney-Wallace and C. D. Jonah, Chem. Phys. Let& 39 (1976) 596. 6 S. Hashimoto and J. K. Thomas, J. Phys. Chem., 89 (1985) 2771. 7 S. Kobayashi, K. Kikuchi and H. Kokubun, Chem. Phys. Left., 42 (1976) 494. 8 W. Heinzelmann and H. Labhart, Chem. Phys. Lett., 4 (1969) 20. 9 E. J. Hart and M. Anbar, The Hydrated Electron, Wiley, New York, 1970. 10 T. Shida, Electronic Absorption Spectra of Radical Ions, Elsevier, Oxford, 1988. 11 J. Monig, K.-D. Asmus, L. W. Robertson and F. Oesch, J. Chem. Sex., Perkin Trans. 2, (1986) 891. 12 P. N. Bajaj, H. Mohan and R. M. Dyer, Radiat Phys. Chem., 26 (198.5) 253. 13 K. Sehested and E. J. Hart, J. Phys. Chem., 79 (1975) 1639.

386 D. Zehavi and J. Rabani, J. Chem. Phys., 76 (1972) 312. N. J. Bunce, S. Safe and L. 0. Ruzo, J. Chem. Sot., Perkin Tmns. I, (1975) 1607. J. J. Dynes, F. L. Baudais and R. K. Boyd, Can. .I. Chem., 63 (1985) 1292. G. E. Hall and G. A. Kenney-Wallace, Chem. Phys. Let& 28 (1978) 205. Y. Nakato, N. Yamamoto and H. Tsubomura, Bull. Chem. Sot. Jpn., 40 (1967) 2480. B. J. Kelsall and L. Andrews, J. Chem. Phys., 76 (1982) 5005. 19 K. H. Grellman and A. R. Watkins, J. Am. Chem. Sot., 9.5 (1973) 983. 20 K. H. Grellmann and A. R. Watkins, Chem. Phys. Letc., 9 (1971) 439. 21 R. Rahn, J. S. Troe and K. H. Grellmann, .J. Chem. Phys., 93 (1989) 7841. 22 S. C. Wallace, M. Graetzel and J. K. Thomas, Chem. Phys. Left., 23 (1973) 359. 23 W. G. McGimpsey and J. C. Scaiano, J. Am. Chem. Sot., 211 (1989) 335. 14 15 16 17 18