J. Phys. Chem. 1903, 87, 4467-4467
where aLKM,lm are constants determined by the initial conditions. After a 90° pulse, CiKM,lo(O+) N S&Ky,$o/kT E 8&Koe (Bo is die magnitude of the applied field) while Cim,m(O+) = Cim,m(O+) = 0, implying that aLKM,lO = b o S K o e , ~ L K M , o o= ULKM,OO = 0. After the 7 evolution time C’LKM,~O = (8~ ~) 0 8 exp[-(W, ~ 0 ~ + 2 W h ] (A221 leading to CLKM,10(7)
= 4 8 ~ ~ exp[-(WI 8 ~ 0
+ 2Wx)71- 1)
(A231
In the text, where Tl effects are omitted from the calculation, CLKMlo(7) is set equal to zero. If, instead, it is kept, its effect on the calculation comes in through the action of the second (i.e., 9O0J pulse which transforms CLm,lA1(~) to
C L K M , ~ + ~=( ~ + ) 7 7 2 [cLKM,1-1(7)
- cLKM,11(7) 1 - (i/ 2‘”)
CLKMJO(7) (A241
4461
Cl(7)homogeneoua a
COlj exp[-(Re Aj
im
+ i Im Aj)T]OmjUm (B1)
= Cali exp[-(Re Aj + i Im A,)T] I
with alj = ~,OVOmjUm. The corresponding CW spectrum is thus a superposition of complex Lorentzian line shapes having width = Re A, and frequency position = Im Ah In the presence of inhomogeneous broadening, a range of Im Aj. values are present, call them Im Ai + x k , and are distributed among the system molecules according to some weighting function h(Xk). Equation B2 becomes Cl(7) 0: Ch(Xk)&j exp[-(Re Aj + i(Im Aj 4- xk))7] k
I
033) Following the second pulse Cl(7+)0: Ch(Xk)Calj* exp[-(Re Aj - i(Im Aj + xk])7] k
I
034)
so that c-,,1&1(7+ is)no longer the complex conjugate of CLKM,l*l(d.
The TI correction in eq A24 for the two-pulse sequence is thus s&ple even for slow motions. In more compiicated sequences, e.g., ones with three or more pulses, we must also consider the effect of the second pulse on CLKM,10(7). The second pulse can mix terms with nonzero L,Kvalues into CLKM,lo(~), which will then evolve during the subsequent evolution time (cf. eq AlSA21). The third pulse can then transfer these evolved nonzero L,Kterms back into the corresponding off-diagonal elements where their effect will show up in measurements.
Appendix B Effect of Inhomogeneous Broadening on the Quadrupole Echo. A purely homogeneous free induction decay (eq 26) written in terms of C(7), 0, A, and U is
(B2)
and during the second evolution time CP(7+7) a
[Cap&* exp[-(Art + Aj*7)](Ch(Xk) exp[-iXk(t - 7)) ri
k
(B5) where apr
= COprOlr 1
The sum over r j in eq B5 is eq 29 written in terms of matrix elements of 0 and A. The single sum over K can be taken over to an integral in the limit that the Xk differ only infinitesimally. It then becomes the Fourier transform of the distribution function h(X), i.e., h(t-7). Registry No. TMSI-d9,23726-00-3; HMB-dI8,4342-40-9.
Excited and Ionic States of Polymers with Pendant Phenanthryl Groups in Solution. Model Systems for Photophyslcs In Phenanthrene Aggregates Naoto Tamal, Hlroohl Mawhara;
and Noboru Mataga
Depamnent of Chemistry, Faculty of Engineering Science, Osaka Unlvmlty, Toyonaka, Osaka 560, Japan (Received: Mer& 3, 1983)
Using N2gas laser and picosecond NdS+:YAGlaser photolysis systems, we measured the transient absorption spectra of 9-ethylphenanthrene (EPh), 1,3-di-(9-phenanthryl)propane(DPhP), poly[2-(9-phenanthryl)ethyl vinyl ether] (PPhEXE),and poly(9-vinylphenanthrene)(PWh) in solution. The Sn S1absorption spectrum is almost common to the present compounds,indicating a very weak interchromophore interaction in the excited singlet state. The absorption spectrum of the triplet and cationic states changes from compound to compound, while the spectra of their anion radicals are identical. The triplet excitation and the positive hole can be stabilized easily by forming excimer and dimer cation, respectively, whose conformations are discussed in detail. In the case of polymer systems, plural dimer sites with different geometries exist, which is due to the stacking effect of chromophores. On the other hand, the negative charge is always trapped as a monomer anion. On the basis of the present results, the photophysics of phenanthrene aggregates is discussed.
-
Introduction The study of primary photoprocesses such as energy migration, excimer formation, and charge separation is now one of the most important subjects in chemistry. Micellar solutions, microemulsions,vesicles, and membrane systems involving aromatic chromophores have been investigated by means of single photon counting and laser photolysis 0022-36541a312087-4481801 .SOIO
measurements, and information on their excited-state dynamics is increasing quite rapidly. In these systems, the local concentration of aromatic chromophores is high, while their distribution is nonuniform. This will result in new kinds of dynamic behavior characteristics of molecular aggregates which may be observed by using time-resolved spectroscopy. In accordance with these developments, 0 1983 American Chemical Society
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The Journal of Physical Chemlstty, Vol. 87, No. 22, 1983
photophysical processes of polymers having aromatic groups as pendant or backbone chromophores have been investigated with considerable attention. Although carbazole’ and phenanthrene2 are exceptional molecules which do not form excimers in solution, the dimer model ~ o m p o u n d and s ~ ~polymersH having these aromatic groups show excimer emission. Therefore, these compounds are considered to be a model system suitable for elucidating the aggregate photophysics of molecules which do not associate with each other even at high concentrations. In addition, the following characteristic dynamics were observed when excited with an intense laser pulse. Namely, several fluorescent states are produced in one polymer chain under the condition ah L 1where u and h are the absorption cross section in cm2 and the light intensity in photons cm-2, respe~tively.~This was clearly demonstrated by investigating the polymers having carbazolyl groups with Q-switched ruby laser and N2 gas laser photolysis systems. An intrapolymer interaction between the excited singlet states and a transient polyelectrolyte formation were reported as new phenomena characteristic of polymers excited with an intense laser pulse.”’2 Concerning the phenanthryl groups, information on excited-state dynamics in aggregated states is scarce. Zachariasse et al. has recently reported the intramolecular excimers of some diphenanthrylpr~panes,~ while the photophysics of the related polymers were studied by Itaya et a1.8 Here we describe the results on polymer having phenanthryl groups where the interactions between excited states induced by high-intensity excitation are hardly observed by transient absorption spectroscopy because of their small a values at the excitation wavelength. This phenanthryl system seems to be one of the best polymers for elucidating excited-state dynamics by the laser photolysis method. Here, their absorption spectra of the lowest excited singlet and triplet states and the ion radicals produced by electron-transfer quenching with electron donor or acceptor are presented. These spectra are compared with those of monomer and dimer model compounds and discussed from the viewpoint of aggregate photophysics.
Experimental Section Materials. Poly(9-vinylphenanthrene)(PVPh), poly[2-(9-phenanthryl)ethyl vinyl ether] (PPhEvE), and their monomer model compound, 9-ethylphenanthrene (EPh), were the same as used in a previous work.8 The mean degrees of polymerization of PVPh and PPhEVE are 18 and 87, respectively, which were determined by gel-per(1) M. Yokoyama, T. Tamamura, T. Nakano,and N. Mikawa, Chem. Lett., 499 (1972);M. Yokoyama, T. Tamamura, M. Aauni, M. Yoehimura, Y. Shirota, and H. Mikawa, Macromolecules, 8, 101 (1975). (2)E.A. Chandrose, J. W. Longworth, and R. E. Vieco, J.Am. Chem. SOC., 87,3259 (1972). (3) G.E.Johnson, J. Chem. Phys., 61,3002 (1974);63,4047 (1975). (4)F. C. De Schryver, J. Vandendriessche, S. Toppet, K. De Meyer, and N. Boens, Macromolecules, 15, 406 (1982). (5)K. A. Zachariasse, R. Busse, U. Schrader, and W. Kiihnle, Chem. Phys. Lett., 89,303 (1982). (6)G.E.Johnson, J. Chem. Phys., 62, 4079 (1975). (7)A. Itaya, K. Okamoto,and S.Kusabayashi, Bull. Chem. SOC. Jpn., 49,2082 (1976). (8)A. Itaya, K. Okamoto, and S. Kusabayashi, Bull. Chem. SOC. Jpn., 50,52 (1977). (9)H.Masuhara, S.Ohwada, N. Mataga, A. Itaya, K. Okamoto, and S. Kusabayaehi, J. Phys. Chem., 84,2363 (1980). (10)H.Masuhara, 5.Ohwada, K. Yamamoto,N.Mataga, A. Itaya, K. Okamoto,and S. Kusabayaehi, Chem. Phys. Lett., 70,276 (1980). (11) H.Masuhara, S. Ohwada, N. Mataga, I PPhEVE > DPhP > PVPh. Fluorescence decay curves of these compounds were measured by using filters whose transmittance is given in Figure 1. The decay curves of PPhEVE and DPhP were reproduced approximately with a single exponential decay function, while fast and slow decay processes with time constants of 6 and 22 ns were observed for PVPh. The emission rise times of these compounds were shorter than the time resolution of the present apparatuses. The initial intensity ratio of the fast to slow components is larger in the long-wavelength region (a Y44 filter) than in the short-wavelength region (U360 and L38 filters). The obtained decay times are listed in Table I. Time-Resolved Absorption Spectra of Quencher-Free Systems. Figure 2 shows the transient absorption spectra of THF solutions of EPh, PPhEVE, and PVPh in the picosecond time range. The spectra of the EPh and PPhEVE systems have absorption peaks at 490 nm and weak bands at 630 nm. These are assigned to the S, SItransitions of the phenanthryl group, in view of the facta that the independence of their spectral shapes with regard to the delay time in the 0 ps-a few nanoseconds region corresponds to their fluorescence lifetime and that the absorption maximum is in agreement with that of the S, S1transition of phenanthrene." A similar absorption spectrum was observed also for the PVPh system. A broadening of the band observed at 630 nm is a unique
L Ik,
Figure 2. Time-resolved absorption spectra of O-ethylphenanthrene (a), poly[ 2-(9-phenanthryl)ethyi vinyl ether] (b), and poly(9-vlnylphenanthrene) (c, d) In THF. Delay times after excltatbn are glven in ihe figure.
4
.
0 0.2
s 0 n
b n U
500
400
Wavelength( nm)
Figure 3. (A) lime-resolved absorption spectra of 1,3di(9phenanthry1)propane in THF. Delay times are given in the figure. (6) "allzed absorptbn spectra of 1,3-dl(9-phenanthryl)propane at 1 ps (1) and S-ethylphenanthrene at 500 ns (2). A difference spectrum (3) was obtained by Subtracting spectrum 2 from spectrum 1.
difference between the monomer and polymer spectra. This result is also independent of the delay time. The transient spectrum of EPh in the submicrosecond time range is considered to be due to its triplet state became of its similarity to the T, TI spectrum of phenanthrene.l8 This assignment is confirmed also by the triplet-state energy transfer from EPh to naphthalene. In the case of PPhEVE, the spectrum was identical with that of EPh. A similar absorption spectrum was also observed for DPhP, but its bandwidth becomes a little large as the delay time increases, as shown in Figure 3a. On the other hand, the PVPh system gives a new broad absorption spectrum having a maximum around 390 nm instead of the T, TI band of EPh at 480 nm (Figure 4a). The M naphthalene decreases the intensity addition of 1X at 450-500 nm and increases that around 400 nm. The difference spectra between systems with and without naphthalene are similar to the T, T1spectra of naphthalene and phenanthrene. This means that the monomer triplet component of the PVPh system is quenched by naphthalene while the component responsible for the new absorption characteristic of PVPh is not. These transient absorption spectra of DPhP and PVPh were quenched by
-
-
-
(18) E.J. Land, h o c . R . SOC.London, Ser. A , 305,457 (1968); M.W. Windeor and J. R. Novak in 'The Triplet State", A. B. Zahlan, Ed., Cambridge University Press, London, 1967, p 228.
4464
Tamai et ai.
The Journal of Physlcal Chemistry, Vol. 87, No. 22, 1983
L
I
400ps
I
L
% 400
500
Wavelength ( n m )
Flgure 4. (A) Time-resobed absorption spectra of poly(9-vinylphenanthrene) wlth (2) and without (1) 1 X lo-* M naphthalene in THF at 300 ns after excitation. The difference spectra 3 and 4 were obtained by subtracting spectrum 1 from spectrum 2 and spectrum 2 form spectnrm 1, respectively (seetext). (B)The difference spectnm obtained by subtracting the T, TI absorption spectrum of phenanthrene (4) from the spectrum of poly(9-vinyiphenanthrene) at 300 ns (see text).
-
I
LI U
C
0
n L
0 u)
n
a
1,
____,
, 600
I
-
\T
400
500
700
Wavelength ( nm )
Figure 6.
Time-resolved absorption spectra of poiy(9-vinylphenanthrene)containing 0.4 M “Bin DMF and reference spectra phemnthrene dimer catbn (2), and &NB of phenanthrene catkn (I), anion (3). References are gben in Figure 7C. Delay times after excitation are shown in the figure.
than 460 nm and ita relative contribution to the anion band increases as the delay time does. Similar spectral change was also demonstrated in the case of the DPhP system. The new band is due to the triplet state of phenanthrene, whose increased contribution is led by the recombination of the anion and the DABCO cation. In the m e of P W h different spectra were observed at long delay times. Time-Resolved Absorption Spectra of Systems Containing 0-DCNB. The quenching of the excited phenanthrene chromophore by o-DCNB in polar solvents leads to the production of phenanthrene cation and o-DCNB anion. This is clearly demonstrated in Figure 6 for the SIabsorption band of PVPh PVPh system. The S, observed at early delay times is replaced by the broad band having a maximum at 440 nm. Since the o-DCNB anion has no appreciable absorption band beyond 410 nm and it is difficult for the PVPh cation and o-DCNB anion to recombine with each other within a few nanoseconds, this new band is undoubtedly ascribed to the PVPh cation. The submicrosecond absorption spectra of the EPh and DPhP systems are shown in Figure 7, where the corresponding reference spectra are also given. The spectra of the EPh-o-DCNB system at 500 ns can be reproduced by superposition of the bands of the cation and the triplet of phenanthrene. The spectral change along the delay time can be explained with the triplet formation through the recombination of both ion radicals and the formation of the intermolecular dimer cation of EPh. The absorption spectrum of the DPhP-o-DCNB system seems to be different from those of the EPh system, and a subtraction of the band of the triplet DPhP from the observed one gives a spectrum having a peak at 430 nm.
-
I
LI
400
500 Wavelength ( nm
Flgure 5. Timaresobed absorption spectra of g-ethylphenanthrene ( 4 , 13-49-phenanthrylkropane(b), and poM9-vinylphenanthrene) ( 4 containing 1.0 M DABCO in DMF. Delay times are given in the figure.
air saturation and result in no permanent chemical change. Time-Resolved Absorption Spectra of Systems Containing DABCO. The quenching of excited-singlet-state aromatic hydrocarbons by electron donors in polar solvents results in the formation of aromatic anion and quencher cation.lg This ionic photodissociation is a very useful method for producing ion radicals to be measured, and here DABCO was used as a quencher. The molar extinction coefficient of this quencher cation in the visible region is small, which is a suitable condition for elucidating the absorption spectral shape of aromatic anion radicals. The absorption spectra of EPh and PVPh anions in DMF, measured at 1ns after excitation, have a peak at about 460 nm and are similar to that of the reference phenanthrene anion. At longer delay times of the EPh system (Figure 5 ) , a new band is observed in the longer wavelength region (19) Y.Taniguchi,Y.Nishina, and N. Mataga, Bull. C h m . SOC.Jpn., 46,764(1972); H.Masuhara, T. Hino, and N. Mataga, J. Phys. Chem., 79,994 (1975);T. Hino, H. h w a , H.Masuhara, and N. Mataga, ibid., 80, 33 (1976); H.Masuhara and N. Mataga, Acc. Chem. Res., 14, 312 (1981).
Discussion Interchromophore Interactions in the Excited Singlet State. As indicated in Table I, the fluorescence yield decreases in the order of EPh > PPhEvE > DPhP, which is, roughly speaking, in parallel with the change of lifetime. The quenching degree of the monomer fluorescence seems to be related to the difficulty of conformational change leading to an approach of two phenanthryl groups. Namely, the intramolecular excimer interaction is con-
The Journal of Physical Chemlstty, Vol. 87, No. 22, 1983 4465
Excited and Ionlc States of Polymers r
I
llP3P
22P3P
33P3P
99P3P
29P3P
39P3P
A
500ns
__I
...I
Flgure 8. Possible geometrical structures of some diphenanthrylpropanes. Shaded parts represent the overlap of two phenanthryl groups.
Concerning the geometrical structure of the excimer, Zachariasse et al. have recently reported the formation of the intramolecular excimers of 1 (m-phenanthryl)-3-(nphenanthry1)propane where m and n are 1, 2, 3, and 9 (abbreviated as mnP3P according to Zachariasse et aL5). The monomer fluorescence of 29P3P and 39P3P was not quenched, while a broad excimer emission was observed for symmetrically substituted diphenanthrylpropanes such as l l P 3 P , 22P3P, 33P3P, and 99P3P. Using Buchi Dreiding stereomodels, we examined possible geometrical structures with different overlaps between two phenanthryl chromophores and they are schematically displayed in Figure 8. In the case of 29P3P and 39P3P, two kinds of structures with different partial overlaps are suggested, while the sandwich type of structure is possible in the case of m = n. Combining this geometrical information with the emission studies given by Zachariasse et al., one may conclude that only the sandwich type of structure is suitable for the singlet excimer stabilization of phenanthrene systems. Therefore, two phenanthryl groups forming a sandwich type of structure are considered to trap the singlet excitation in the present polymer. Interchromophore Interactions in the Triplet State. Since the submicrosecond absorption spectrum of DPhP is a little broader than that of EPh, both are normalized in Figure 3B. A new absorption spectrum was obtained by subtracting the monomer triplet band. In view of their behavior and the spectral identity with that reported by Zachariasse et al.,5 this band is ascribed to the triplet excimer of phenanthryl groups. The geometrical structure of this excimer is considered to be a sandwich one, since asymmetrically substituted diphenanthrylpropanes do not show this absorption.6 In the case of PVPh a new submicrosecond absorption band was obtained by subtracting the contribution of the phenanthrene monomer triplet from the measured spectrum, which is given in Figure 4B. The effect of air saturation and ita transient behavior suggest that the present spectrum is due to the triplet PVPh. Why this new apecies is not quenched by naphthalene is explained by assuming that the stabilization energy of the triplet state in PVPh from the triplet EPh is larger than an energy difference of the triplet state between phenanthrene and naphthalene (-500 cm-I). This assignment is supported by the fact that the present spectrum is identical with that of the PVPh-DABCO system at long delay times (Figure 5c). As described in the Results Section, the EPh anion and the DABCO cation formed by electron-transfer quenching recombine with each other, leading to the phenanthrene triplet state. This recombination in the polymer system is considered to produce the triplet state of PVPh. A broad absorption having a peak at 390 nm and descending to the long-wavelength side is characteristic of PVPh. This spectral shape might suggest that a new
-
e
7. (A) Tlmwesolved absotptkm spectra of 94hylphenanthrene containing 0.4 M oOCNB in DMF. Delay times are ghren In the figure. (6) (a) Absorptkn spec" of the 1,3dW$henanthryl)propane system contalnlng 0.4 M oOCNB In DMF, observed at 300 ns after excitation. (b) Absorption spectrum of the triplet DPhP. (c) The dlfference spectrum obtained by subtractlng spectrum b from spectrum a. (C) Reference absorption spectra of phenanthrene catlon (l),phenanthrene dimer cation (2), oDCNB anion (3),and phenanthrene triplet (4). See: T. Shkh and S. Iwata, J . Am. Chem. SOC., 95, 3473 (1973);A. Kira and M. Imamura, J . Phys. Chem., 83, 2267 (1979); N. Tamai, H. Masuhera, and N. Mataga, to be submitted for publication.
sidered to be a main cause leading to fluorescence quenching and the produced excimer is considered to have a low fluorescence quantum yield and a short lifetime. Actually, a new broad emission was observed in the case of PVPh (Figure 1A-f), whose maximum position and decay time are 385 nm and 6 ns,respectively. A possibility that this new emission is due to a copolymerized impurity is excluded by examining the preparation method.8 The stacking of phenanthryl groups in PVPh may increase the excimer formation efficiency compared to PPhEVE and DPhP. The Stokes shift of this excimer is -1800 cm-l and its stabilization energy is small, which is consistent with the theoretical estimation.20 As described in the Results Section, the excimer formation process of PVPh could not be detected by fluorescence measurement because of the large overlap of monomer and excimer emissions. However, the fact that we could measure the short decay time of 6 ns of PVPh means that the excimer formation time of this compound is shorter than this decay time. The transient absorbance of PVPh obtained immediately after excitation is almost the same for EPh and PPhEVE, and no appreciable spectral change from 33 ps to 5 ne was observed for PVPh. Since the excimer bands should decrease in this time range, it is concluded that there is no appreciable difference of S, SIabsorption spectra between the monomer fluorescent and sandwich excimer states. This is consistent with the small Stokes shift of emission due to the excimer formation. These results clearly demonstrate the weak interaction between the excited-singlet-state and ground-state phenanthrene chromophores.
-
(20) J.
B. Birks, Chem. Phys. Lett., 1,304 (1967).
4400
The Journal of physlcal Chemlstry, Vol. 87, No. 22, 1983
electronic state characteristic of PVPh where the excitation is delocalized over several chromophores is produced. An alternative and more probable explanation is as follows. The band of the sandwich type triplet excimer is overlapped with that of another excimer with a partial overlap between neighboring chromophores. The latter excimer is considered to be stabilized only in PVPh by the stacking effect of the surrounding chromophores. At any rate, the interaction in the triplet excimer state is concluded to be stronger than that in the singlet excimer sate in these phenanthrene systems. The measurement of the triplet excimer emission is an interesting topic in molecular photophysics and phenanthrene was studied by Langelaar et al.21 In spite of their detailed investigations,the observed emission was ascribed to an impurity. Later, the time-resolved photon-counting technique was applied to this phenanthrene system by Aikawa et al.22and a broad and structureless emission at 520 nm was concluded to be due to the triplet excimer. Concerning the polymer system, room-temperature studies were not given, and the phosphorescence spectrum of this polymer a t 77 K is different from that of EPh, showing an interaction between neighboring phenanthryl group^.^ Furthermore, the phosphorescence spectrum of phenanthrene dimer with a sandwich type of structure was reported to have a maximum at about 515 nm.23 All these experimental results suggested that the absorption spectrum of the triplet phenanthrene excimer could be measured by the present laser photolysis method, which has been actually demonstrated here. Interchromophore Interactions in the Ionic States. Interaction between the anion radical and the parent ground-state molecule is expected to result in the formation of the dimer anion, which was demonstrated for olefin derivative^^^ and a ~ e t o n i t r i l e . ~However, ~ no report on the dimer anion of aromatic hydrocarbons has been given except for the anthracene system where the dimer anion was produced from the anthracene photodimer.26 Therefore, whether the dimer anion is stabilized in bichromophoric molecules and polymers having aromatic groups is one of the most interesting subjects. The spectral shape of the phenanthrene anion band is almost common to the EPh, DPhP, and PVPh systems. This indicates that the dimer anion cannot be stabilized even in the polymer with significant stacking of chromophores. On the other hand, the formation of aromatic dimer cation is a quite general phenomenon.n-29 The absorption spectral shape of the dimer cation is very sensitive to the overlap of the molecular orbitals of the adjacent chromop h o r e and ~ ~ formation ~ ~ ~ of various kinds of dimer cations are observed in the carbazole systems.32 In the case of (21)J. Langelaar, R. P. H. Rettschnick, A. M. F. Lambooy, and G.J. Hoytink, Chem. Phys. Lett., 1, 609 (1968);J. Langelaar, R. P. H. Rettschnick, and G. J. Hoytink, J . Chem. Phys., 54,1 (1971). (22)M. Aikawa, T.Takemura, and H.Baba, Bull. Chem. SOC.Jpn., 49,437 (1976). (23)E. A. Chandross and H. T. Thomas, J. Am. Chem. SOC.,94,2421 (1972). (24)S.Arai, A. Kim, and M. I”ura, J. Phys. Chem., 81,110(1977). (25)Y.Hirata, N.Mataga, Y. Sakata, and S. Misumi, J. Phys. Chem., 87, 1493 (1983). (26)T. Shida and S. Iwata, J. Chem. Phys., 56,2858 (1972). (27)B. Badger and B. Brocklehurst, Trans. Faraday SOC.,65,2582 (1968);65,2588 (1968). (28)A. Kira, M. Imamura, and T. Shida, J. Phys. Chem., 80, 1445 (1976). (29)A. Ekstrom, J. Phys. Chem., 74,1705(1970);75,1178 (1971);B. Brocklehurst, ibid., 75,1177 (1971). (30)A. Kira, S. Arai, and M. Imamura, J . Chem. Phys., 54, 4890 (1971). (31)E. R. Biihler and W. Funk, J. Phys. Chem., 79, 2098 (1975).
Tamai et al.
polymers, Irie et al. have reported that the most probable geometry of the cation in poly(2-vinylnaphthalene)and polystyrene is a nonparallel sandwich one in which the interchromophore distance is close to that of the diarylethane.33 The absorption spectra of the present DPhP and PVPh extend from 400 to 550 nm and show peaks at 430 and 440 nm, respectively. These are different from the spectrum of the EPh cation and also from that of the dimer cation obtained by the y-irradiation method. At the present stage of investigation, we have no conclusion on the geometry of the phenanthrene aggregate cations. The geometrical structure of the PVPh cation may be different from that of the DPhP cation, or another type of structure may be produced in addition to that of DPhP because of the chromophore stacking.
Summary Until now, the singlet and triplet excimers and the dimer cation have been reported as molecular associate systems of phenanthrene in solution and in rigid solvents. On the basis of the results on some intramolecular excimer model compoun& given by Zachariasse et al.6 it is considered that the sandwich type of structure is the most probable conformation for both excimers. No other aggregate states have been reported as far as we know. Since phenanthryl groups bound in the polymer chain can interact with each other more easily compared to the monomer and dimer model compounds, more detailed information on aggregate photophysics is expected. Summarizing the transient absorption spectral measurements of the present phenanthryl systems, the spectral difference among polymer, dimer, and monomer compounds in various states increases in the following order: the anionic state < the excited singlet state < the triplet state = the cationic state. The anion radicals of PVPh, DPhP, and EPh give a common absorption spectrum, indicating no appreciable interaction even in the polymer. A very weak interaction is confirmed in the excited singlet state, since the absorption spectrum of the singlet excimer is similar to that of the monomer fluorescent state. In the triplet state, the sandwich type of structure corresponds to the DPhP excimer, while an additional structure with a partial overlap is probable for the PVPh. The absorption spectra of DPhP and PVPh cations are different, which may indicate that the positive charge is stabilized in a plural sites in PVPh. One of the most interesting properties of molecular aggregates is efficient energy and charge migration processes. On the basis of the present results electron migration is expected to be very efficient, while the positive charge may be rapidly trapped in the dimer site. Concerning the singlet energy migration, the phenanthrene aggregate is considered to be a suitable substance because of the poor ability to form excimers. The triplet excitation will be captured as the triplet excimer more easily than the singlet excitation. This information is believed to be useful for understanding the electronic processes of various kinds of molecular aggregates. Acknowledgment. We express our sincere gratitude to Profs. A. Itaya, K. Okamoto, and S. Kusabayashi and Profs. M. Furue and S. Nozakura for supplying polymer and dimer compounds, respectively. Thanks are due to Dr. K. A. Zachariasse for his discussion on the stable conformation of phenanthrene excimers. Thanks are also due to Profs. S. Minami and T. Uchida, and Mr. T. Iwata (32)H. Masuhara, N.Tamai, N. Mataga, F. C. De Schryver, and J. Vandendriessche, J. Am. Chem. SOC.,in press. (33)S.hie, H. Horii, and M. Irie, Macromolecules, 13, 1355 (1980).
J. Phys. Chem. 1983, 87, 4467-4470
for permitting the use of their photon counting system. The cost of the present investigation was partly defrayed by the Japanese Ministry of Education, Science, and Culture through a Grant-in-Aid (56540261) to H.M. and a Grant-in-Aid for Special Project Research on Photo-
Rate Constant of the OH -tH,O,
-
HO,
4467
biology to N.M. and H.M. Registry No. 9-Ethylphenanthrene, 3674-75-7; 1,3-di(9phenanthryl)propane,82901-39-1;poly[2-(9-phenanthryl)ethyl vinyl ether] (homopolymer), 86900-66-5; poIy(9-vinyIphenanthrene) (homopolymer), 86885-30-5.
+ H20 Reaction
John J. Lamb,+ Luisa 1. Moiina,* Craig A. Smith, and Mario J. Molina"' Department of ChemlStW, Universtty of California, Imine, California 92717 (Received: October 25, 1982)
-
Absolute rate constants for the reaction OH + H202 H 0 2 + H 2 0 have been measured by using the flash photolysis resonance fluorescence technique over the temperature range 241-413 K and at 1 atm of helium. H202concentrations were measured by ultraviolet and infrared absorption spectrophotometry. The rate constant cm3molecule-l s-l, in agreement with recent literature values. at 294 K was found to be kl = (1.8 i 0.3) X The results yield a curved Arrhenius plot, with a slight increase in the rate constant at the lower temperatures.
Introduction The gas-phase reaction of OH with H202is an important component of the atmospheric HO, cycle: OH
+ H202
+
HO2 + H2O
(1)
H202is formed in the atmosphere mainly by the recombination reaction of H 0 2 radicals (reaction 2) and its HO2 + HOP
H202 + 0
2
(2)
principal loss processes are reaction with OH radicals (reaction 1) and photodissociation (reaction 3). In adH202 + hv
+
OH
+ OH
(3)
dition, rainout and washout processes represent significant sinks for H202in the troposphere. Several previous studies of reaction 1 have been reported: early measurements of the rate constant using steady-state photolysis methods,1*2flash ph0tolysis,4~and discharge flow techniques5 gave results in the (0.62-1.2) X cm3 molecule-l s-l range at 298 K. More recently, measurements by two different methods-discharge floW69' and flash photolysis resonance fluorescenceel0-yielded rate constants that are about a factor of 2 larger at 298 K and which showed linear Arrhenius plots, with the rate constant decreasing by -10% at 250 K. In this work we essentially confirm these new rate constant values using the flash photolysis resonance fluorescence technique at 1atm of He; however, we observe a curved Arrhenius plot, with the rate constant increasing below room temperature and becoming 10% larger at 250 K.
Experimental Section Instrumentation and Chemicals. An all-Pyrex jacketed reaction cell with an internal volume of -150 cm3was used in the experiments. The cell was maintained at constant temperature by flowing a thermostated liquid through the outer jacket surrounding the cell. The temperature was monitored a t five different positions on the cell with copper-constantan thermocouples. 'Current address: Hydrocarbon Research Institute, University of Southern California, Los Angeles, CA 90089-1661. Current address: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109.
*
0022-365418312087-4467$01.50/0
OH radicals were produced by flash photolysis of H202 or HNOBusing a nitrogen flash lamp equipped with a Suprasil quartz lens and operated around 15 kV. An adjustable iris controlled the amount of photolysis light reaching the cell without affecting the beam diameter. An OH microwave-dischargelamp excited resonance fluorescence in the 0-0 band of the A2Z+-X211system (A 308 nm). This fluorescence was monitored perpendicular to both the flash lamp and the resonance lamp radiation flux by an EMI-9782QA photomultiplier fitted with an interference filter (290-320 nm, 20% T at.307 nm) and a Corning 7-54 filter. The resonance lamp and the photomultiplier were also fitted with quartz lenses which mildly focused the radiation. The signals, in the form of photon counts, were processed by a signal averager-computer system (Inotech Ultima 11,Data General-Nova 3) using a nonlinear leastsquares routine to analyze the decays; data in the first few channels ( t < 500 ps) were not considered in the analysis. For each decay rate sufficient flashes (usually 400) were averaged to obtain a well-defined temporal profile of OH concentration over at least three l / e times. The residuals and the autocorrelation coefficients were plotted routinely; in no case was there statistical evidence for departures from the single exponential decay expected for pseudofirst-order conditions. In order to avoid accumulation of products and to minimize the decomposition of H202,the reactant mixture was flowed through the reaction cell with a linear velocity of about 8 cm s-l. The flash lamp repetition rate was about 1 Hz, so that the cell was refilled with a fresh reactant mixture for every flash. H202was introduced into the system by passing He gas through a glass bubbler containing several milliliters of
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(1) R. A. Gorse and D. H. Volman, J.Photochem., 1, 1 (1972). (2)J. F.Meagher and J. Heicklen, J. Photochem., 3, 455 (1979). (3)N. R. Greiner, J. Phys. Chem., 72, 406 (1968). (4)G.Harris and J. N. Pitta, Jr., J. Chem. Phys., 70, 2581 (1979). (5)W. Hack, K. Hoyermann, and H. Gg. Wagner, Int. J. Chem. Kinet. Symp., 1, 329 (1975). (6) U. C. Sridharan, B. Reimann, and F. Kaufman, J. Chem. Phys., 73,1286 (1980). (7)L. F. Keyser, J.Phys. Chem., 84,1659 (1980). (8)P.H. Wine, D. H. Semmes, and A. R. Ravishankara, J. Phys. Chem., in press. (9)M. J. Kurylo, J. L. Murphy, G. S. Haller, and K. D. Cornett, Int. J. Chem. Kinet., 14, 1149 (1982). (lo)W. J. Marinelli and H. S. Johnston, J. Chem. Phys., 77, 1225 (1982).
0 1983 American Chemical Society
