J. PhyS. Chem. 1983, 87, 1493-1498
pathway via the zinc triplet. The ZnMPDE fluorescence intensity of MCH solutions of the mixed monomers at 77 K, when excited at 543 mm, shows ca. 40 f 15% decrease compared to that of only ZnME'DE after correction for the CuMPDE absorption. The ESR intensity of the Am8 = 2 transition due to the ZnMPDE triplet state of solutions of the mixed solutes under white-light irradiation also shows ca. 30 f 10% decrease compared to that of only ZnMPDE. These results are interpreted by assuming either energy transfer from the singlet state of ZnMPDE to CuMPDE or enhancement of the intersystem crossing rate in the excited singlet state of ZnMPDE due to the adjacent CuMPDE followed by efficient triplet energy
1493
transfer from the triplet ZnMPDE to CuMPDE. For the case of the covalently linked zinc and copper porphyrins, no decrease in the zinc fluorescence quantum yield was observed and only a slight shortening of the zinc triplet lifetime was detected in comparison with the zinc monomer.3 Therefore, in order to elucidate the details of the energy transfer in the present case, quantitative studies are needed on the quantum yields and lifetimes of the excited states of ZnMPDE and CuMPDE for solutions of both the separate and mixed solutes. Registry No. ZnMPDE, 15376-02-0; CuMPDE, 14710-65-7; methylcyclohexane, 108-87-2; 2-methyltetrahydrofuran, 96-47-9.
Photoionization of Bis(dimethy1amino)tetrahydropyrene. Importance of Solvent-Solute Exciplex Interactions Yoshlnorl Hlrala, Noboru Mataga, Department of Ctmmisby, Faculty of Englnmring Sclence, Osaka Unlverslty, Toyonaka, Osaka 560, Japan
Yoshlteru Sakaia, and Solchl Mlsuml The InstiMe of Scientific and Industrlel Research, Osaka Unlverslty, Sulta, Osaka 565, Japan (Received: September 2, 1982; I n Final Form: November 16, 1982)
The photoionization of 2,7-bis(dimethylamino)-4,5,9,1O-tetrahydropyrene(BDATP) has been studied by means of transient absorption and transient photoconductivity measurements with the picosecond laser photolysis method. The precursor of the photoionization, which consisted of the BDATP cation and the acetonitrile dimer anion, has been directly observed in acetonitrile solution. It has been demonstrated clearly that solvent-solute exciplex interactions are quite important for the photoionization processes of BDATP in benzonitrile, pyridine, as well as acetonitrile solutions. In contrast to these solutions, no free radical ion has been detected in a,a,a-trifluorotoluene (TFT) solution, where, however, the solute-solvent ion-pair state has been confirmed to be in equilibrium with the first excited singlet (SI)state of BDATP.
Introduction There has been a great deal of work, both experimental and theoretical, concerning the electron photoejection of aromatic molecules in organic liquids and solids. However, most of the work has been performed in nonpolar solvents as well as alcoholic glasses, and studies pertaining to electron photoejection in polar liquids have been rather scarce in spite of their importance in photochemical and photobiological primary processes. The process of electron photoejection of an aromatic compound in a condensed medium was first demonstrated by Lewis and collaborators.' They irradiated N,N,N',N'-tetramethyl-p-phenylenediamine(TME'D) in EPA glass at low temperature and obtained its cation radical, which is known as Wurster's blue. Since then, the electron photoejection of TMPD has been studied extensively in order to elucidate the mechanistic details of electron photodetachment, electron-cation recombination, and and other related phenomena.2 Another type of photoinduced charge separation of aromatic compounds is ionic photodissociation, which has
been studied for the typical exciplex and excited donoracceptor complex systems such as pyrene-N,N-dipyrene-p-dicyanobenzene,6 and tetrameth~laniline,~-~ cyanobenzene-toluene' in polar solvents. Ionic photodissociation is an important decay channel of exciplexes, which quenches their fluorescence in polar solvents leading to the formation of anion and cation radicals in the case of the above typical systems. The electron photoejection of some aromatic amines like TMPD occurs monophotonically with near-UV irradiation even in nonpolar solvents. We can expect that the lower energy photon causes the ionization of such compounds because the energy of the cation-electron pair state can be lowered by the interaction with the surrounding polar solvent. Several aromatic amines such as 2,7-bis(dimethylamino)-4,5,9,1O-tetrahydropyrene (BDATP) benzidine derivative^,^ and N,N,N',N'-tetramethylpyrenedi-
(1) Lewis, G. N.; Lipkin, D. J. Am. Chem. SOC.1942,64,2801. Lewis, G. N.; Bigeleisen, J. Zbid. 1943, 65, 520.
d-_, 9 2QA
(2) Lesclaux, R.; Joussot-Dubien, J. 'Organic Molecular Photophysics";Birks, J. B., Ed.; Wiley-Interscience: London, 1973; Vol. 1, pp 455-587.
0022-385418312087-1493$01.50/0
(3) Taniguchi, Y.; Nishina, Y.; Mataga, N. Bull. Chem. SOC.Jpn. 1972, 45, 764.
(4) Taniguchi, Y.; Mataga, N. Chem. Phys. Lett. 1972,13, 596. (5) Masuhara, H.; Hino, T.; Mataga, N. J . Phys. Chem. 1975, 79,994. (6) Hino, T.; Masuhara, H.; Mataga, N. Bull. Chem. SOC.Jpn. 1976,
-.
I
(7) Masuhara, H.; Shimada, M.; Tsujino, N., Mutugu, N. Bull. Chem. SOC.Jpn. 1971, 44, 3310. ( 8 ) Hirata, Y.; Mataga, N.; Skata, Y.; Misumi, S. J . Phys. Chem. 1982, 86. 1508.
0 1983 American Chemical Society
1494
The Journal of Physical Chemistry, Vol. 87, No. 9, 1983
amine10 show pulse laser-induced photocurrent in acetonitrile solution with excitation at 347 or 337 nm. In these systems, the biphotonic ionization as well as the monophotonic one occur by nanosecond or picosecond laser pulse excitation. In the case of the laser photolysis and transient absorption measurements, the biphotonic ionization occurs rather easily because of the high excitation density. On the other hand, the laser-induced photoconductivity measurement has the advantage of high sensitivity to detect charged species and does not require such a high excitation density as in the transient absorption measurement. In the present paper, we report detailed studies by means of picosecond laser photolysis and transient absorption as well as transient photoconductivity measurements on the monophotonic ionization mechanisms of BDATP in such polar solvents as acetonitrile, benzonitrile, pyridine and a,a,a-trifluorotoluene (TFT).
Experimental Section Picosecond transient absorption spectra were measured by using a mode-locked ruby laser photolysis systems as well as a mode-lock Nd3+:YAGlaser photolysis system, the details of which are given elsewhere."J2 A N2 laser photolysis system13was used for the microsecond transient absorption spectral measurements. Fluorescence lifetimes were determined by exciting the sample with the second harmonic of the picosecond ruby laser and by observing the decay curve with a high-speed microchannel plate photomultiplier (HTV R-l194UX)-fast storage oscilloscope (Tektronix 7834-7A19-7B80) combination. The experimental setup for transient photoconductivity measurements was very similar to the one reported before.8 Some measurements were carried out by using a fast preamplifier (HP 8447D) connected to a fast oscilloscope amplifier plug-in (7A19). In this case, the rise time of the detection system was less than 2 ns. The synthesis of BDATP was described e1~ewhere.l~ N,N,N',N'-Tetramethylbenzidine (TMB) and biphenyl (BPh) were purified by repeated recrystallization from ethanol followed by sublimination in vacuo. TMPD was obtained from its dihydrochloride (GR grade) by dissolving in water and precipitating out free amine with addition of ammonia. The precipitate was purified by sublimation in vacuo. GR-grade acetonitrile, pyridine, and TFT were refluxed over calcium hydride and distilled. GR-grade benzonitrile was passed through a column of alumina twice and distilled in vacuo. Spectrograde isooctane, CC14, 2propanol, methanol, Nfl-dimethylformamide, and acetone were used without purification. For picosecond and microsecond spectroscopic studies, samples were prepared in quartz cells with 1-cm optical path and the absorbances of samples at the excitation wavelength were adjusted to 0.8-1.5. Quartz cells of 1-cm optical path with a pair of platinum electrodes separated by 5 mm were used for photoconductivity measurements, where the absorbances of the solutions at the excitation wavelength were adjusted to 0.6-0.7. All samples were (9) Hirata, Y.; Takimoto, M.; Mataga, N. Chem. Phys. Lett., in press. (10) Nogami, T.; Mizuhara, T.; Kobayashi, N.; Aoki, M.; Akashi, T.; Shirota, Y.; Mikawa, H.; Sumitani, M. Bull. Chem. SOC. Jpn. 1981, 54, 15.59.
(11)Okada, T.; Migita, M.; Mataga, N.; Sakata, Y.; Misumi, S. J.Am. Chem. SOC.1981, 103, 4715. (12) Masuhara, H.; Ikeda, N.; Miyasaka, H.; Mataga, N. J. Spectrosc. SOC.Jpn. 1982, 31,19. (13) Yasoshima, S.;Masuhara, H.; Mataga, N.; Suzaki, H.; Uchida, T.; Minami, S.J. Spectrosc. SOC.Jpn. 1981, 30, 93. (14) Natsume, B.;Nishikawa, N.; Kaneda, T.; Sakata, Y.; Misumi, S.; Enoke, T.; Inokuchi, H. Chem. Lett. 1981, 601.
Hirata et al.
/
A
BDA T P / A CN 3ns
\
Flgure 1. Transient absorptlon spectra of BDATP in acetonitrile solution. Delay times after excling laser pulse are indicated in the figure.
deaerated by repeated freeze-pump-thaw cycles.
Results and Discussion BDATP in Acetonitrile. As we have reported already! for the BDATP-acetonitrile system the rise time of the photocurrent is different from that of the cation absorption. The rise times of the photocurrent and the BDATP cation absorption are 9 and 2.1 ns, respectively, and the latter is in good agreement with the fluorescence lifetime of BDATP in acetonitrile. Thus, the ionization occurs from the relaxed fluorescence state via the intermediate state which shows an absorption spectrum similar to that of BDATP cation radical but does not give a photocurrent. Transient absorption spectra of BDATP in acetonitrile solution taken at 3 ns and 10 ps after excitation are shown in Figure 1. The absorption band below 500 nm with a maximum at 470 nm was assigned to the BDATP cation radicalas The weak absorption in the region of 500-600 nm can be recognized only at short delay times and disappears at microsecond delay times as can be seen from Figure 1. Moreover, since this absorption can be observed before the rise of the photocurrent, it is not due to the free anion radical but the anion part of the ion pair or the nonfluorescent exciplex. As discussed below, this weak absorption beyond 500 nm at short delay times can be assigned to the acetonitrile dimer anion which is produced when an electron is injected into acetonitrile. In order to investigate the behavior of this species, we have tried to produce it by utilizing the biphotonic ionization of a solute with low ionization potential and to trap electrons from it with appropriate scavengers. BDATP is not suitable for this purpose, since it hardly shows biphotonic ionization by excitation around 350 nm. Therefore, we employed TMB, which undergoes the biphotonic ionization rather easily. Figure 2 shows the transient absorption spectra of the TMB/CCl,/acetonitrile system measured by means of the picosecond laser photolysis method, the sample solution being excited with the third harmonic of the Nd3+:YAG laser. It is possible to observe the monophotonic ionization of TMB in acetonitrile solution through a long-lived ionpair state by using a picosecond exciting pulse at 347 nm with relatively low i n t e n ~ i t y .In ~ the measurements indicated in Figure 2, however, the TMB cation radicals were formed immediately after excitation via two-photon absorption owing to the high intensity of the exciting pulse. The absorption bands below 500 nm and above 700 nm are
The Journal of Physical Chemistty, Vol. 87, No. 9, 1983 1495
Photoionization of BDATP
+
12-
CCIL
in
acetonitrile
.",
f (,Psi
a
LOO
500
A
InmI
-
Flgure 2. Transient absorption spectra of the TMB/CCl,/acetonitrile system. Delay times after exciting laser pulse are indicated in the figure. Concentration of CCI,: 1.0 X lo-' M. 0
MeN I-
Flgure 4. Decay curves of the photocurrent of the BDATP/biphenyVacetonitrlle system. Biphenyl concentration: (a) 0, (b) 7.0 X M. Open circles show the (l/photocurrent) lo-', and (c) 6.2 X VS. t plot.
0
NMe2
+
in
BDATP .,......
c
C
?
u5 0
c
0
c
4
ocetonitrile
r 0
200 LOO f /nsI
600
Flgure 5. Decay curves of photocurrent in acetonitrile solutions of: (a) BDATP, (b) BDATP and 7 X lo4 M of biphenyl, and (c) TMPD. See the text for the interpretation of the plots by open circles.
4 \ 1 2 3 4
OO
t i n s )
Flgure 3. (a) Transient absorption spectra of the TMB/biphenyl/aca tonitrile system. Concentration of biphenyl: 4.9 X lo-* M. (b) Time depence of absorbance measured at 640 nm (0)and its semilogarithmic plot (0)against the delay time. The rise time of the biphenyl anion radical is determined to be 700 ps.
due to the TMB cations.15 When the delay times are increased to 3 ns, the absorption between 500 and 700 nm disappears, while the intensity of the TMB cation band does not change much. In the case of the TMB/acetonitrile system, we have confirmed that neither the absorption in the 500-700-nm region nor the TMB cation band changes its intensity within the delay times of a few nanoseconds. Moreover, we have examined the TMB/ (15)Gratzel, M. 'Micellation, Solubilization and Microemulsions"; Mittal, K. L., Ed.; Plenum Press: New York, 1977;p 531. Shida, T.; Hamill, W. H. J . Chem. Phys. 1966,44,2369.
biphenyl/acetonitrile system, the results of which are indicated in Figure 3. In this case, we can observe the buildup of the biphenyl anion band at 640 nm and the bimolecular rate constant for biphenyl anion formation has been estimated to be 2.9 X 1O'O M-' s-l. The results shown in Figures 2 and 3 imply that CCll and biphenyl react with a species X- which is a negative charge carrier and gives a broad absorption between 500 and 700 nm. The negative charge carriers which are produced immediately after the electron injection into acetonitrile might be solvent anions or solvated electrons. Williams et al. studied the product of y irradiation of crystalline acetonitrile at 77 K by means of ESR and optical absorption measurements.16 Solvated electrons were not detected but the formation of acetonitrile dimer anions which show broad absorption in the 400-700-nm region was confirmed by ESR. The acetonitrile monomer anion shows absorption not in the visible region but in the near-IR region.17 Therefore, it is quite reasonable to assign X- to the acetonitrile dimer anion radical, which is produced (16)Holloman, L.;Sprague, E. D.; Williams, F. J. Am. Chem. SOC. 1970,92, 429. Takeda, K.;Williams, F. J . Phys. Chem. 1970,74,4007. Sprague, E. D.; Takeda, K.; Williams, F. Chem. Phys. Lett. 1971,10,299. (17) Tagawa, S., private communication.
1496
The Journal of Physical Chernlstty, Vol. 87, No. 9, 1983
I
OA
5
IO
[BPh] I 10-4M 1 Figure 6. Plot of the pseudo-first-orderdecay constant of the fast component of the photocurrent decay curve against the biphenyl concentration of the BDATP/biphenyl/acetonitrile system.
always when electrons are injected into acetonitrile. The decay curves of the photocurrent of the BDATP/ acetonitrile system in the 10-100-ps region with various amounts of added biphenyl are shown in Figure 4. Figure 5 shows the decay curves of the photocurrent in the 1100-ns region for the systems of BDATP/acetonitrile (a), BDATP/biphenyl (BPh)/acetonitrile (b), and TMPD/ acetonitrile (c). The decay curves in Figure 5 consist of two components and only the slow ones are shown in Figure 4. Substracting the slow component from the decay curves in Figure 5, we have confirmed that the fast component can be reproduced satisfactorily by a single exponential. The exponential decay cannot be ascribed to the homogeneous recombination of a pair of free positive and negative ions. Although the geminate pair recombination should show an exponential decay, we cannot expect that the geminate pair gives such a large photoconductivity signal as observed here. Impurity scavenging may result in an exponential decay but such a possibility can be excluded since we have confirmed that the decay rate of the fast component is not affected by the purification of the solvent. For elucidation of the mechanism of the exponential decay of the photocurrent, the results given in Figure 5 seem to be important. Namely, the exponential decay rate of the TMPD/acetonitrile system is the same as that of the BDATP/acetonitrile system. This result suggests strongly that the acetonitrile dimer anion produced by the dissociation of the ion pair or exciplex reacts with the solvent molecules and forms the more stable anion radical, the mobility of which is smaller than that of the dimer anion radical. The formation of the stable anion radical with higher degrees of aggregation will probably lead to the shifting and further broadening of the absorption band so that its observation in the 500-700-nm region will become very difficult. Actually, the absorption in the 500700-nm region is no longer observable at microsecond delay times, while the BDATP cation band can be observed at the same delay times. The lifetime of the fast component becomes shorter with addition of biphenyl, which can be explained as due to the electron transfer from the acetonitrile dimer anion to biphenyl. A plot of the pseudo-first-order decay constant of the fast component against the biphenyl concentration is displayed in Figure 6, which is in accordance with the simple Stern-Volmer relation. From the slope of the straight line, assuming the electron transfer from the
Hirata et at.
acetonitrile dimer anion radical to biphenyl as in the case of Figure 3, the bimolecular rate constant has been estimated to be 2.5 X 10'O M-' s?, which is in a good agreement with the result of picosecond spectroscopy for the TMB/biphenyl/acetonitrile system (2.9 X 1O'O M-' s-l). The slow component of the photocurrent shown in Figure 4 seems to correspond to the homogeneous recombination of the positive and negative ion radicals and second-order decay is observed for the sample with high biphenyl concentration, where more than 90% of the acetonitrile dimer anion has transferred its charge to biphenyl. The rate constant of the homogeneous recombination cannot be determined since the concentrations of the ionic species are unknown. Only the relative value may be estimated according to the relation l/(photocurrent) 0: kt The sample without biphenyl shows a quite slow decay of the photocurrent and the decay curve cannot be reproduced by the simple bimolecular reaction mechanism. The sample with low concentration of biphenyl shows almost the same decay as that with high concentration of biphenyl in the longer delay time region, while an obvious deviation from the second-order decay can be observed at shorter delay times. The above results of the slow components of the photocurrent decay curve may be explained as follows. As argued already, a stable anion radical with higher degrees of aggregation seems to be formed in the course of the fast decay of the photocurrent. The recombination rate of the BDATP cation with this stable anion may be considerably smaller than that with biphenyl anion. Moreover, several types of stable anion radicals may be formed, resulting in complex decay curves of photocurrent in the case of the BDATP/acetonitrile system or the BDATP/biphenyl/ acetonitrile system with low concentrations of biphenyl. It should be noted here that, although the recombination rate of the BDATP cation with the acetonitrile stable anion radical is smaller than that with biphenyl anion, the mobilities of these anions are almost the same, since the intensity ratio of the total photocurrent to the slow component extrapolated to zero delay time is not much affected by adding biphenyl. Summarizing the above results, we may outline the behaviors of the excited BDATP in acetonitrile solution with and without added biphenyl as follows: BDATP*(S,I
t BDATPISo)
a
-
BDATP+***A2-
-
BDATP+
-
BDATF'
w-
-
-
: ;
\
I L /
i XATP+
+
eo--
where A represents acetonitrile, AT its dimer anion, and A,-. the stable anion radical with higher degrees of aggregation. The structure of A,-. is not clear, however, at the present stage of investigation. BDATP in TFT. The time-resolved transient absorption spectra of BDATP in TFT are shown in Figure 7. These spectra and also those of the BDATP/TFT/isooctane system with high TFT concentration are quite similar to that of the BDATP cation radical. Therefore, BDATP cation seems to be formed in the excited state of these systems. The decay of the BDATP cation absorption measured at 470 nm is shown in Figure 8. The lifetime has been determined to be 1.1ns in TFT and the analysis of the absorption band in the wavelength region longer than 700 nm which is also due to the BDATP cation has given the same value for the lifetime. In these experiments,
The Journal of Physical Chemistry, Vol. 87, No. 9, 1983
Photoionization of BDATP
TABLE I: Lifetimes ( T ) of the Ion-Pair State and the BDATP Fluorescence for the BDATP/TFT as well as BDATP/TFT/Isooctane Systems
I
BDATPiTFT BDATP/TFT/isooctanea
1500ps a
lOOOps
500ps
200ps
9ops I
60ps
i
20ps -lops
L 00
500 600 WAVELENGTH l n m )
Flgwe 7. Transient absorption spectra of the BDATPlTFT system. The delay times from the exciting laser pulse are indicated in the figure.
m
?0
O' 0
1497
I
2
1
t Ins1
Flgure 8. Decay of the BDATP cation absorbance in the transient spectra of the BDATPITFT system measured at 470 nm (0). Its semilogarithmic plot (0)shows a good straight line.
extreme care was taken to excite the sample with a rather weak light pulse since we have confirmed that high-intensity excitation causes a two-photon ionization and forms the free BDATP cation radical which shows no decay in the nanosecond region. Therefore, the BDATP cation observed in the present experiment, which is formed monophotonically, should not be the free cation radical but a part of the ion pair or exciplex. Moreover, the BDATP/TFT as well as BDATP/ TFT/isooctane systems show fluorescence due to the locally excited excited state of BDATP and we have confirmed that the fluorescence lifetime agrees with the lifetime obtained from the decay of the BDATP cation absorption as shown in Table I. The above results may be explained by assuming that the rapid equilibrium as shown below is established, where the ion-pair state, BDATP+- .TFT-, is nonfluorescent. BDATP*(SJ + T F T F? BDATP+*..TFT(2)
-
In the range of relatively low concentration of TFT added to the BDATP/isooctane system, the BDATP fluorescence was quenched according to the Stern-Volmer equation and the bimolecular quenching rate constant was determined to be 4.2 X lo9 M-l s-l. In polar solvents, the
Tion/ns
rdns
-1.0 -2.2
1.3 f 0.1 2.1 + 0.2
[TFT] = 0.35 M.
quenching rate constant becomes larger compared to the value in isooctane. For example, it is 7.8 X lo9 M-'s-I in 2-propanol and 3.2 X 1O'O M-' s-l in methanol. In polar solutions with high TFT concentration, however, BDATP was decomposed gradually by UV irradiation and some unknown product which gave a new emission was formed. The quenching rate constant of the BDATP locally excited state fluorescence in isooctane is almost 8 times smaller than that in the BDATP/TFT/methanol system where the reaction is diffusion controlled. This result implies that the back electron transfer in eq 2 cannot be neglected in isooctane. In polar Solvents, however, the back electron transfer will become less important because the ion-pair state is stabilized by solvation. Exciplex Type of Interaction between Excited BDATP and Solvent Molecules. In the case of the BDATP/ pyridine and BDATP/benzonitrile systems, results of our transient absorption measurements show that the formation of the BDATP cation is very rapid, being completed within the exciting picosecond pulse (20-30-ps fwhm).The rise of the photocurrent is also very rapid in both systems and within the response time of the apparatus. It has been confirmed that the formation of charged species in these systems occurs monophotonically. The BDATP fluorescence is quenched completely and no new emission is observed in both systems. It has been demonstrated that the quenching of the BDATP fluorescence by benzonitrile is a diffusion-controlled process with rate constant of bimolecular reaction, 3.6 X 1O'O M-ls-l, leading to the formation of fluorescent exciplex in isooctane solution,ls although the benzonitrile solution of BDATP shows no fluorescence. At relatively low concentration of added benzonitrile, fluorescent 1:l exciplex is formed. However, at high benzonitrile concentration (-0.2 M), a new type of higher aggregate is formed in the excited state. The new type of exciplex shows weak fluorescence in the longer wavelength region and seems to be a triple complex with two acceptor molecules, (D+A2-).lsWith further increase of the benzonitrile concentration up to 1.0 M, the solution becomes almost nonfluorescent.18 These results indicate strongly that the photoionization of BDATP in BDATP/benzonitrile/isooctane with high concentration of benzonitrile and in the BDATP/benzonitrile system occurs through the ion-pair or exciplex state formed by electron transfer from excited BDATP to benzonitrile aggregates. On the other hand, the quenching rate constant has been determined to be about 2 X lo8 M-l s-l for the BDATP/pyridine/isooctane system from the Stern-Volmer plot at relatively low concentrations of pyridine. Contrary to the case of BDATP/benzonitrile/isooctane no exciplex fluorescence has been detected. A t high concentrations of pyridine, however, the Stern-Volmer plot shows a remarkable upward deviation from a straight line as shown in Figure 9, which indicates the formation of ground-state loose complexes between BDATP and pyridine at high pyridine concentration, although no specific (18) Hirata, Y.; Takimoto, M.; Mataga, N.; Sakata, Y.; Misumi, S. Chem. Phys. Lett. 1982, 92, 76.
1408
J. Phys. Chem. 1083, 87. 1498-1502
O
I
15.
0
510 OO
0
o o o
,
0
00
1 [pyridine I
I
1
2
IM1
Figure 9. Stern-Volmer plot of the fluorescence quenching for the BDATP/pyridine/isooctane system.
change can be recognized in the absorption spectra. The aggregates, when excited, will undergo immediately the formation of a nonfluorescent electron-transfer state. In contrast to the above results, we have confirmed that BDATP cannot be ionized in acetone and N,N-dimethylformamide by monophotonic excitation with a 347-nm light pulse but can be ionized only through the biphotonic process. This result also clearly demonstrates that the monophotonic ionization in acetonitrile, benzonitrile, and pyridine cannot be explained by the simple scheme of lowering the ionization threshold in polar solvents. A more specific interaction between excited solute and solvent molecules should be taken into consideration.
The importance of the exciplex type of interaction in the monophotonic ionization of BDATP has been demonstrated also in the BDATP/TFT/isooctane and BDATP/TFT systems where the nonfluorescent ion pair or exciplex, BDATP+..-TFT, is formed, which is in rapid equilibrium with the BDATP locally excited state but does not show dissociation. The latter fact may be ascribed to the low polarity of this solvent. Therefore, although the existence of the exciplex type of interaction is crucial for the monophotonic ionization, whether the ion-pair or exciplex state can dissociate or not depends on the solvent polarity. As described above, we have demonstrated clearly for the first time the important role of the ion-pair or exciplex state formed by electron transfer between the excited solute and solvent or solvent aggregates in the monophotonic ionization of BDATP. Although the photoionization processes of BDATP are mainly discussed in the present paper, we have obtained similar results also in the case of other amines including TMB and related molecule^.^ Therefore, the mechanism established in the present work seems to be a rather general one for photoionization in polar solutions. The results obtained for other systems together with a more general discussion on the ionization mechanisms will be published shortly elsewhere.
Acknowledgment. This work was supported in part by a Grant-in-Aid for Special Project Research on Photobiology from the Japanese Ministry of Education, Science, and Culture to N.M. Registry No. BDATP, 78687-14-6;BDATP cation radical, 85096-92-0;TFT, 98-08-8;TMB, 366-29-0;Bph, 92-52-4; acetonitrile, 75-05-8;benzonitrile, 100-47-0;pyridine, 110-86-1.
Radiolysis Reactions of Colloidal Sulfur in Water-Dioxane Solutions Francis J. Johnston Department of Chemistty, University of Georgia, Athens, Georgia 30602 (Received: September 2 1, 1982; I n Final Form: November 23, 1982)
Colloidal sulfur suspensions in aqueous 0.1-0.4 M dioxane solutions are solubilized upon absorption of ionizing radiation. Reaction rates with colloids labeled with 355 were measured and correspond to G values of 6.4 molecules per 100 eV in N20-saturated solutions and 3.5 molecules per 100 eV in Ar-saturated systems. The sulfur becomes organically bound through reaction with free radicals produced by reaction of OH with dioxane. A t a total M. colloidal sulfur acts as an efficient scavenger for these radicals. concentration of
tested, the reaction is, evidently
Introduction
When heterogeneous mixtures of sulfur and water absorb ionizing radiation, oxidation of sulfur occurs with a decrease in pH. H2 above that formed from pure water is Laser Raman spectra of the supernatent indicate the presence only of Infrared spectra of the barium salt precipitated from the solution was identical with that of BaS04. Although the stoichiometry was not (1) G. W. Donaldson, M.S. Thesis, University of Georgia, Athens, GA, 1968. (2) R. A. DellaGuardia, M.S. Thesis, University of Georgia, Athens, GA, 1977. (3) R. A. DellaGuardia and F. J. Johnston, Radiat. Res., 84, 259 (1980).
In the presence of the same solid sulfur surface, the reaction is accelerated when the liquid phase is saturated with NzO in place of He. On the basis of the total energy absorption in a typical irradiated system comprised of 1.2 g of sulfur and 10.0 mL of HzO, apparent G values of 0.06 and 0.2 molecules of HzS04per 100 eV were observed2for the same sample in He and N20 atmospheres, respectively. Considering that reaction really occurs in a relatively small volume close to the solid surface suggests a fairly high reactivity of elemental sulfur for reactive intermediates from water. On the other hand, no assessment was possible
0022-365418312087-1498$01.50/00 1983 American Chemical Society
