3814
J. Phys. Chem. 1989, 93, 3814-3818
Role of Water in the Multiple-Component Phosphorescences of Anthraquinone and &Halogenoanthraquinones in 2,2,2-Trifluoroethanol at 77 K Kumao Hamanoue,* Toshihiro Nakayama, Tetsuji Yamaguchi, and Kiminori Ushida Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo- ku. Kyoto 606, Japan (Received: September 7, 1988)
From the measurements of phosphorescence spectra of anthraquinone and P-halogenoanthraquinones(the 2-chloro and 2-bromo compounds) at 77 K, their lowest triplet states have been assigned to be of an na* character even in such a strong hydrogen-bonding solvent as 2,2,2-trifluoroethanol (TFE). Addition of water to anthraquinone/TFE solutions exhibits the multiple-component phosphorescences: The spectra due to the short-lived components are identical with those in the absence of water, while the spectra due to the long-lived components are very broad and remarkably different from those in the absence of water. The lifetimes of the long-lived components increase linearly with the increase of water content; the maximum lifetimes are reached with the water volume fractions of 15% for anthraquinone, 1.2% for 2-chloroanthraquinone, and 0.9% for 2-bromoanthraquinone. These results have been interpreted in terms of complex formation among triplet anthraquinones, TFE, and water molecule(s).
Introduction
It has been reported that phosphorescence spectra of aromatic carbonyl compounds, whose nx* and AT* triplet states are very close to each other, are strikingly dependent on the nature of environment' and the dual phosphorescences are sometimes observed. For example, Suter et al.* observed the dual phosphorescences of 1-indanones in ethanol; the short-lived nn*-type phosphorescences were attributed to the free triplet molecules and the long-lived na*-type phosphorescences to triplet indanones forming a hydrogen bond with ethanol. Since 2,2,2-trifluoroethanol (TFE) is well-known to have a strong hydrogen-bonding a b i l i t ~it, ~is expected that the lowest triplet state of anthraquinone might be of a nn* character due to the formation of a hydrogen bond between TFE and the carbonyl group of anthraquinone. In contrast, we have observed an na*-type phosphorescence spectrum with a short lifetime of 1.2 ms in distilled and dried T F E similar to that in EPA (ether/ isopentane/ethanol = 5:5:2 in volume r a t i ~ ) :The ~ absorption band due to the transition from the ground (So) state to the lowest excited singlet (SI;nr*) state can be seen in EPA, while no such an absorption is observed in TFE. Thus, we have concluded that the energy levels of the excited singlet nn* and an* states of anthraquinone in TFE are very close to each other; however, the lowest triplet state is still of an na* character. Recently, Yamauchi and Hirotas reported that ( I ) the time resolved EPR (TREPR) spectrum of anthraquinone in TFE was remarkably different from that of triplet anthraquinone of an nn* character. (2) In accordance with this, the phosphorescence spectrum of anthraquinone in TFE was essentially different from that in EPA, and the decay of the phosphorescence in TFE consisted of three components with lifetimes of 2, 50, and 150 ms. These results are greatly different from our results mentioned above. Thus we have reexamined the phosphorescence spectra of anthraquinone and 6-halogenoanthraquinones(the 2-chloro and 2-bromo compounds), because Yamauchi and Hirota used TFE as the solvent without further purification, while we purified and dried the solvent very carefully. ( 1 ) Li, Y. H.; Lim, E. C. Chem. Phys. Lett. 1970, 7, 15. Case, W. A,; Kearns, D. R. J . Chem. Phys. 1970, 52,2175. Goodman, L.; Koyanagi, M. Mol. Photochem. 1972, 4 , 369. Nishimura, A. M.; Tinti, D. S . Chem. Phys. Lett. 1972, 13, 278. (2) Suter, G . W.; Wild, U. P.; Schaffner, K. J . Phys. Chem. 1986, 90, 2358. (3) Mukherjee, L. M.; Grunwald, E. J . Phys. Chem. 1958, 62, 1311. Eckstrom, H. C.; Berger, J. E.; Dawson, L. R. J . Phys. Chem. 1960,64, 1458. Figueras, J. J. Am. Chem. Sot. 1971, 93, 3255. Spears, K. G.; Steinmets, K. M. J . Phys. Chem. 1983, 89, 3623. (4) Hamanoue, K.; Nakayama, T.; Kajiwara, Y.; Yamaguchi, T.; Teranishi, H. J . Chem. Phys. 1987, 86, 6654. (5) Yamauchi, S.;Hirota, N. J. Chem. Phys. 1987, 86, 5963.
TABLE I: Phosphorescence Lifetimes Various Solvents at 77 K
of Anthraquinones in
( T ~ )
r,/ms
solvents MC MTHF EPA pure TFE
AQ
2-CAQ
2-BAQ
2.8 2.8
2.7
3.4
3.0 0.9
1.7 2.7 2.4 0.8
1.2
2.6
Experimental Section
C P grade anthraquinone (AQ) and EP grade 2-chloroanthraquinone (2-CAQ) were purchased from Wako Pure Chemical Industries, Ltd. 2-Bromoanthraquinone (2-BAQ) was synthesized from 2-CAQ by the substitution of chlorine atom by bromine atom. The details of the methods of preparation of these compounds have been given elsewhere.6 The solvents used for the spectral measurements were spectral grade methylcyclohexane (Dojin) and ethanol (Nakarai), 2-methyltetrahydrofuran (Aldrich), diethyl ether (Merck), and GR grade isopentane and TFE (Wako); isopentane was further purified by passing through an alumina column, and TFE was dried over molecular sieves 3A (Wako) and distilled under nitrogen atmosphere. The distilled TFE was again stored over molecular sieves 3A. 2-Methyltetrahydrofuran (MTHF) was distilled under nitrogen atmosphere and ethanol was dried over molecular sieves 3A. Methylcyclohexane (MC) and diethyl ether were used without further purification. The phosphorescence spectra were recorded at 77 K with a Hitachi MPF-4 fluorescence spectrometer equipped with a phosphorescence photometry accessory. The photomultiplier was an HTV R446 and the phosphorescence spectra were recorded in the energy mode; that is, the spectra were uncorrected. For the measurements of phosphorescence decays, nanosecond laser photolysis was performed using the second harmonic (347.2 nm) of a Q-switched ruby laser with a half-peak duration of 20 ns.' The decays of phosphorescences were observed by using an HTV R666 or an RCA 8575 photomultiplier combined with an Iwatsu TS-8 123 storage oscilloscope and a microcomputer. Results
In Figure 1, we show the phosphorescence spectra of AQ in TFE without further purification (called crude TFE hereafter), where spectra A (full line) and B (broken line) were recorded with (6) Hamanoue, K.; Kajiwara, Y.; Miyake, T.; Nakayama, T.; Hirase, S . ; Teranishi, H. Chem. Phys. Lett. 1983, 94, 276. (7) Nakayama, T.; Miyake, T.; Okamoto, M.; Hamanoue, K.; Teranishi, H. Mem. Far. I n d . Arts, Kyoto Tech. Uniu., Sci. Technol. 1980, 29, 35. Hamanoue, K.; Yokoyama, K.; Kajiwara, Y.; Nakajima, K.; Nakayama, T.; Teranishi, H. Chem. Phys. Lett. 1984, 110, 25.
0022-3654/89/2093-38 14$01.50/0 0 1989 American Chemical Society
Phosphorescences of Anthraquinones I
I
A
AQIEPA
AQITFE
1_ _ _ , I , , ,
400
5com603660
450
Wovelength/ nrn
Figure 1. Phosphorescence spectra of AQ in crude TFE, where spectra A and B were recorded by 330-nm excitation with and without the sector of the phosphorimeter, respectively.
I
-
---AQlEPA - iTFE
-AQiMC IMTHF
-
I
Figure 3. Phosphorescence spectra of anthraquinones in EPA and pure T F E recorded by 330-nm excitation with (-) and without ( - - - ) the sector of the phosphorimeter. I
AQlTFE!4°%H20~
, 2-CAQ!TFE104°bH20;
Wavelength Inrn
Figure 2. Phosphorescence spectra of anthraquinones in MC, MTHF, EPA, and wet TFE. All spectra were recorded by 330-nm excitation with the sector of the phosphorimeter.
and without the sector of the phosphorimeter, respectively. Spectrum A is very similar to the phosphorescence spectrum of AQ in crude T F E with lifetimes of 2, 50, and 150 ms, reported by Yamauchi and H i r ~ t athough ,~ our spectrum decayed with two lifetimes of 1.2 and 113 ms. The value of the longer lifetime changed with the sweeping time of the oscilloscope as will be discussed later. Figure 2 shows the phosphorescence spectra of AQ, 2-CAQ, and 2-BAQ in MC, MTHF, EPA, and TFE, where TFE was distilled and stored in a refrigerator for a few days (called wet TFE hereafter). Except for the phosphorescence spectra in wet TFE, all the other spectra can safely be assigned to the well-known phosphorescences from the lowest triplet anthraquinones of an nx* character,E because the clear progression of the carbonyl vibration can be seen and the lifetimes are on the order of milliseconds as listed in Table I.9,10 In comparison with the phosphorescence spectra in MC, MTHF, and EPA, those in wet TFE are very broad and structureless; they are rather similar to the phosphorescence spectra of a-halogenoanthraquinones such as 1-chloro, 1-bromo, 1,5-dichloro, 1,5-dibromo, 1,8-dichloro, and 1,8-dibromo compounds in EPA and TFE:4,6 We have concluded that the lowest triplet states of 1,8-dihalogenoanthraquinonesare of a x r * character, and those of 1-halogeno and 1,Sdihalogeno compounds are of a mixed nx*-xx* character. Since the broad and structureless phosphorescence spectra in TFE (cf. Figure 2) were taken in insufficiently dried solvent, and Yamauchi and Hirotas used TFE as the solvent without purification (crude TFE), the origin of the unusually broad phosphorescence spectra may be due to water which is contained as (8) In EPA, the relative band intensities around 45C-470 nm are very strong compared with those in our previous paper.' This is due to the fact that the spectra in Figure 2 were uncorrected, while those in our previous paper were corrected. (9) Kasha, M. Discuss. Faraday SOC.1950, 9, 14, Radiat. Res. Suppl. 1960,2,243. Drabe, K. E.; Veenvliet, H.; Wiersma, D. A. Chem. Phys. Lett. 1975, 35,469: Narisawa, T.; Sano, M.: I'Haya, Y . J. Chem. Lett. 1975, 1289. (10) Khalil, 0. S.;Goodman, L. J . Phys. Chem. 1976, 80, 2170.
Wavelength i n m
Figure 4. Effects of water on the phosphorescence spectra of A Q and 2-CAQ in pure TFE, where the spectra were recorded with (-) and without (- - -) the sector of the phosphorimeter.
Figure 5. Effects of water on the phosphorescence spectra of 2-BAQ in pure TFE, where the spectra were recorded with (-) and without (---) the sector of the phosphorimeter.
an impurity. In fact, as shown in Figure 3, the phosphorescence spectra of A Q taken with and without the sector of the phosphorimeter in distilled and dried TFE (called pure TFE hereafter) are almost identical with those in EPA and decayed following a single exponential function with a lifetime of 1.2 ms (cf. Table I).11 Thus, we have concluded that the lowest triplet state of AQ in pure TFE is still of an nx* character, in spite of the strong ~~
(1 1) The phosphorescence lifetimes of anthraquinones in the present paper are shorter than those in ref 4. This discrepancy is due to the different methods for the determination of the lifetimes: The lifetimes in ref 4 were determined by a hand-made digitization of the traces of the electrical waveforms (decay curves) on the screen of a oscilloscope, while, in the present paper, the lifetimes were determined by a computer simulation.
3816
The Journal of Physical Chemistry, Vol. 93, No, 9, 1989
Hamanoue et al.
Wavelength / nm
Figure 6. Phosphorescence spectra of anthraquinones in TFE/water
solvents, recorded at various sector speeds of the phosphorimeter, where A, and A, are open and closed times of the sector, respectively. hydrogen-bonding ability of the solvent. For 2-CAQ and 2-BAQ, however, the phosphorescence spectra taken in pure TFE with the sector of the phosphorimeter are somewhat broad and the spectra taken without the sector of the phosphorimeter are different from those in EPA. Moreover, the phosphorescence spectra in pure TFE decayed with more than two components, though the lifetimes of the long-lived components could not be determined on account of the weak emission intensities. Figure 4 shows the effect of water on the phosphorescence spectra of AQ and 2-CAQ in pure TFE. Upon addition of water, the phosphorescence spectra became broad and structureless (cf. full lines): The positions of emission peaks were not so much affected by the addition of water, while the spectral profiles changed. As shown in Figure 5, the phosphorescence spectra of 2-BAQ, taken without the sector of the phosphorimeter, are strongly affected by the addition of water. Thus, we propose that the phosphorescence spectra of anthraquinones in TFE/water solvents consist of na*- and m*-type emissions. This is based on the following facts: (1) Generally, if a aa*-type triplet state is mixed more than 10% with an na*-type triplet state, the general spectral feature is expected to resemble those of the pure na* triplet state.I2 (2) As shown in Figure 6 , the phosphorescence spectral shapes became broad and structureless with the decrease of the sector speed of the phosphorimeter. This indicates that the phosphorescence intensities of the short-lived components decreases with the decrease of the sector speed of the phosphorimeter. (3) On the other hand, when the phosphorescence spectra were taken without the sector of the phosphorimeter, the phosphorescence intensities of the short-lived components were enhanced and the spectra were rather similar to those in pure TFE (cf. broken lines in Figures 3, 4, and 5). Although the decays of the phosphorescences in TFE/water solvents could be nearly fitted as a sum of two exponential terms, the values of lifetimes determined changed with the sweeping times of the oscilloscope; typical examples for AQ are shown in Figure 7. These results indicate that there are more than two transient species. The shortest lifetimes were identical with the phosphorescence lifetimes in pure TFE, Le., 1.2 ms for AQ, 0.9 ms for 2-CAQ, and 0.8 ms for 2-BAQ (cf. Table I)," while the longest lifetimes increased with the increase in the water content as shown in Figure 8. At higher content of water, no change in the lifetimes of the long-lived components was observed. Since the addition of water to MC, MTHF, and EPA solvents did not affect the phosphorescence spectra of anthraquinones and their decays, and since the phosphorescence spectra of anthraquinones in pure TFE were rather similar to those in MC, MTHF, and EPA (cf. Figures 2 and 3), we propose that the unusually broad phosphorescence spectra observed in TFE/water solvents (12) Sano, M.; Narisawa, T.; Izawa, K.; I'Haya, Y . J. Bull. Chem. SOC. Jpn. 1979, 52, 2685. Sano, M.; Narisawa, T.; I'Haya, Y . J; Proc. Mol. Ctyst. Symp., Kleinwalsertal, Austria 1980, 2 14.
1
50
100
150
Time / ms Figure 7. Double-exponential analyses of the phosphorescence decay curves of AQ in TFE/H,O (15%) at various sweeping times of the oscilloscope. Broken lines are the stimulated curves with two exponential time constants indicated in inserted figures. 0
r
5
10
15
20
5 I
a5
I
ID
I
I
15
20
H20 /vol% Figure 8. Plots of the longest phosphorescence lifetimes ( T ~ of ) AQ (O), 2-CAQ (A), and 2-BAQ ( 0 ) against the volume percent of water in
TFE/water solvents. are due to the complex formation among triplet anthraquinones, TFE, and water molecule(s); that is, the existence of water in TFE is essential for the complex formation. Since the lifetimes of the long-lived components increased linearly up to the maximum values with increase in water content, the "average" number of water molecules involved in a complex may change with water content and the maximum number of water molecules may be achieved at the water content of about 15% for AQ, 1.2% for 2-CAQ, and 0.9% for 2-BAQ. Discussion We have proposed that the phosphorescence spectrum of AQ in pure TFE is still of an na* character with a short lifetime of 1.2 ms (cf. Figure 3). However, the phosphorescence spectra of 2-CAQ and 2-BAQ in pure TFE are somewhat different from those in EPA, indicating that our pure TFE is not still dried absolutely. This conjecture is based on the following facts: (1) A comparison of phosphorescence spectra of 2-CAQ and 2-BAQ in Figure 3 with those in Figures 4 and 5 clearly indicates that the spectra taken in pure TFE without addition of water are interpreted in terms of the superposition of at least two kinds of
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3817
Phosphorescences of Anthraquinones phosphorescence spectra. Thus, our pure TFE may still contain a small amount of water. In fact, the phosphorescence spectra of 2-CAQ and 2-BAQ in pure TFE decayed with more than two decay components, though the lifetimes of the long-lived components could not be determined on account of the weak emission intensities. (2) The effects of water on the change of the phosphorescence spectra and their decay times are more sensitive for 2-CAQ and 2-BAQ than that for AQ (cf. Figures 4, 5, and 8), indicating the existence of an interaction between TFE/water and a halogen atom at the P-position of AQ. Since T F E is well-known to have a strong hydrogen-bonding So transitions ability and no absorptions due to the SI ( m * ) can be seen for AQ and P-halogenoanthraquinones,their lowest singlet and triplet states in TFE might be of a aa* character. However, the behavior of phosphorescences clearly indicates that the lowest triplet states in absolutely dried TFE are still of an na* character. Probably, TFE may not affect the nature of the triplet states of anthraquinones in accordance with our previous results that the ratios of the solvent stabilization energies of the second excited triplet states to those of the lowest excited singlet states are 0.43 f 0.01 and 0.39 f 0.06 for 9-bromo- and 9,lO-dibromoanthracenes, respectively,13indicating that the stabilization energy of the triplet an* state by solvents is about 40% to that of the singlet Sl(aa*)stabi1i~ation.l~ In the measurements of absorption and fluorescence spectra of diazines (pyridazine, pyrimidine, and pyrazine) at room temperature in hydrogen-bonding solvents such as methanol and water, Baba et al.15observed that a hydrogen bond formed between solute and solvent molecules gave rise to a large blue shift in the S,(na*) So absorption spectra, while the fluorescence spectra in hydrogen-bonding solvents were identical with those in aprotic solvents such as isooctane, ether, and acetonitrile. Thus, they concluded that the hydrogen bond was broken in the singlet na* excited state: Baba16 also proposed that the cleavage of the hydrogen bond may occur on a picosecond time scale. If the above conclusions are correct and the same situation holds for anthraquinones, the normal na*-type phosphorescences may be observed in the case where the time constant for the intersystem ~ ) than that ( 7 ” ) for the cleavage of the crossing ( T ~ is~ longer hydrogen bond in the Sl(na*) state. This is consistent with our present observation that the absorption bands due to Sl(na*) So transition remarkably shifted to the blue in pure TFE, while the phosphorescence spectra in pure TFE are almost identical with those in MC, MTHF, and EPA: Even if T~~~is shorter than T~ and the hydrogen bond is cleaved in the triplet manifold on a pice or nanosecond time scale, one can also expect the observation of normal na*-type phosphorescences in pure TFE. The very fast cleavage of hydrogen bond observed for diazines by Baba et al. at room temperature is not a general conclusion, because Yamazaki et al.” reported that the rate constant for dissociation of the hydrogen bond in the na* singlet state of 9,lO-diazaphenanthrene (DAP) in methanol/ethanol (1: 1 in volume ratio) was 4.3 X lo8 s-l at 113 K and the activation energy of the dissociation was 10 kJ/mol. At room temperature, the hydrogen-bonded complex between solute and solvent molecules dissociated in the singlet na* state of DAP, but the breakage of the hydrogen bond at 77 K was inhibited owing to the high viscosity of the solvent. Thus, the fluorescence spectra in methanol/ethanol solvent at 77 and 300 K were assigned to the emissions from the lowest excited singlet states of the hydrogen-bonded complex and the free DAP molecule, having the lifetimes of 1.2 and 3.5 ns, respectively. Moreover, the emission maximum (Amm = 470 nm) due to the excited singlet state of the hydrogen-bonded
complex was observed at shorter wavelength than that (A, = 496 nm) due to the excited singlet state of free DAP. For the short-lived phosphorescence spectra of anthraquinones in pure TFE, however, the spectral shift by solvent may be shall, because the stabilization energy of the triplet state by solvent is expected to be smaller than that of the excited singlet stafe as stated before. In fact, the positions of the emissiop peaks and phosphorescence lifetimes were independent of the hydrqgenbonding ability of solvents as shown in Figures 2 and 3 and ‘fable I. Thus, it is not clear whether na*-type triplet anthraquinones in pure TFE are hydrogen-bonded or not. Also one cannot ascribe the broad phosphorescence spectra in TFE/water solvent to the emissions from the triplet na* states of the complexes of anthraquinones/TFE/water molecule(s), based on the fact that the emission peaks shifted to the red and the lifetimes are longer than those in MC, MTHF, EPA, and pure TFE. The discussions mentioned so far are based on the assumption that the lowest excited singlet states of hydrogen-bonded anthraquinones are still of an na* character. For acridineIs and acridoneI9 in benzene containing a small amount of acetic acid, monochloroacetic acid, or trichloroacetic acid, Kokubun observed aa*-type fluorescences of the hydrogen-bdnded solute molecules. Mataga et aLZ0also observed a aa*-type fluorescence of the hydrogen-bonded complex between 1-pyrenol qnd triethylamine in hexane or benzene. Thus, if the lowest excited singlet states of hydrogen-bonded anthraquinones are also of a ar* character, the lowest triplet state may be of a mr* character, giving rise to the broad phosphorescence spectra with long lifetimes. However, this is still ruled out for the short-lived na*-type phosphorescence spectra of anthraquinones in pure TFE and TFE/water solvents. Therefore, we propose the following. (1) In pure TFE solvent at 77 K, free or hydrogen-bonded anthraquinones exist, giving rise to the lowest excited singlet states of an na* character. This may again give rise to the na*-type phosphorescence spectra. (2) In TFE/water solvent at 77 K, free (or hydrogen-bonded) anthraquinones and complexes of anthraquinones/TFE/water molecule(s) may exist; that is, the microscopic environments surrounding a solute molecule are not homogeneous. The former may give rise to the appearance of na*-type phosphorescence spectra with short lifetimes, while the latter may give rise to the population of the lowest excited triplet states of a AT* character, giving broad phosphorescence spectra with long lifetimes. For some derivatives of aromatic carbonyls, it is well-known that the lowest triplet states are of a aa* character with relatively long lifetimes.21 Based on this, Yamauchi and Hirotas interpreted the phosphorescence spectrum of AQ in crude T F E as follows. The intensities of the vibronic bands are remarkably weak compared with those of triplet na* AQ, and the vibrational frequency of the most intense bands, being 1505 cm-I, is much smaller than that (1670 cm-I) of triplet na* AQ. They ascribed this to be due to the contribution of the vibronic bands involving C = C stretching modes (1454 and 1535 cm-1),10-22 which are characteristic of the phosphorescence spectra of triplet aa* aromatic carbonyls. In the TREPR experiment of AQ in crude TFE, Yamauchi and Hirotas observed remarkably different spectrum from that of triplet na* AQ reported by Murai et al.,23and Yamauchi and Hirota determined the zero-field splitting parameters as D = -3.1 GHz and E = -0.87 GHz, which are very different from those ( D = -9.3 GHz and E = -0.21 GHz) of triplet na* AQ reported by Murao and Azumi.z2 However, we believe that the broad structureless phosphorescence spectrum of AQ observed by Yamauchi and Hirota is due to a superposition of the phosphorescences of free (or hydrogen-
(13)Hamanoue, K.; Hidaka, T.; Nakayama, T.; Teranishi, H.; Sumitani, M.; Yoshihara, K. Bull. Chem. SOC.Jpn. 1983,56, 1851. (14)Strictly speaking, however, we are not sure whether the same conclusion can be given to the n s * states or not. (15)Baba, H.; Goodman, L.; Valenti, P. C. J . Am. Chem. SOC.1966,88, 5410. (16)Baba, H. Photochemistry (published by the Japanease Photochemistry Association) 1987,11, 1. (17)Yamazaki, I.; Takeda, H.; Baba, H. Bull. Chem. SOC.Jpn. 1980,53, 541.
(18)Kokubun, H. Z . Phys. Chem. N.F. 1957,13,386. (19)Kokubun, H. 2. Phys. Chem. N.F. 1958,17, 281; 1976,101, 137. (20)Ojima, S.;Miyasaka, H.; Mataga, N. Symp. Photochem., Sendai, Jpn. 1987,299. (21)Becker, R.S. Theory and Interpretation of Fluorescence and Phosphorescence; Wiley-Interscience: New York, 1969;p 157. Sebti, M.; Dupuy, F.; Mtgel, J.; Nouchi, G. Compt. Rend. Acad. Sci. (Paris) 1971,272B, 123. (22)Murao,’T.; Azumi, T. J. Chem. Phys. 1979,70,4460;1980,72,4410. (23)Murai, H.;Hayashi, T.; I’Haya, Y. J. Chem. Phys. Lett. 1984,106, 139.
-
-
-
3818
J . Phys. Chem. 1989, 93, 3818-3825
bonded) AQ and the complexes among triplet AQ, TFE, and water molecule(s). Thus, we are somewhat puzzled about why Yamauchi and Hirota observed the TREPR spectrum due to only triplet AQ of a nn* character, because our present study clearly reveals the existence of more than two kinds of triplet species in TFE/water solvent. Moreover, our preliminary results revealed that TREPR spectrum of AQ in pure TFE is similar to that in TFE/water. This indicates that the TREPR spectra in pure TFE and TFE/water are not due to nn*-type triplet AQ, nor is the complex of triplet AQ/TFE/water molecule(s) of a nn* character. Probably, the TREPR spectrum may be due to a triplet state of a photoproduct produced upon the repeated photolysis based on the following facts. (1) In our TREPR experiment, a sample in an EPR cavity was irradiated with a nitrogen laser repetitively at 10-20 Hz for about 15 min; Yamauchi and Hirota used a Lumonics TE861M excimer laser (XeCI, X = 308 nm). This means that a number of laser shots, say several thousand laser shots, is necessary in order to obtain one TREPR spectrum. (2) During the measurement of a TREPR spectrum, the color of the emission from the sample changed from green-yellow to yellow and the color of the sample after the measurement was red. We have presented that the addition of a small amount of water to anthraquinones/TFE solutions gives rise to the appearance of
multiple-component phosphorescence spectra. Such water effects have been also observed for the phosphorescence spectra of flavone, flavanone, 1-indanone, and benzophenone in hydrogen-bonding solvents, though the spectral changes of phosphorescences and their decay profiles are dependent on the solute molecules as well as solvents.24 Therefore, careful experiments must be carried out, especially when the multiple-component phosphorescence spectra in hydrogen-bonding solvents are weakly observable, in which case one has sometimes to avoid even humidity.
Acknowledgment. We express our sincere thanks to Professor Kinichi Obi and Mr. Kouki Yano of Tokyo Institute of Technology for the help in the measurements of the TREPR spectra. This work has been financed by a Grant in Aid for Scientific Research from the Ministry of Education of Japan (No. 62470008 and 626065 16). Registry NO. TFE, 75-89-8; AQ, 84-65-1; 2-CAQ, 131-09-9;2-BAQ, 572-83-8; H20,7732-18-5. (24) Sakurai, K.; Ushida, K.; Nakayama, T.; Hamanoue, K. Abstracts of The 56th Spring Meeting of the Chemical Society of Japan; Chemical Society of Japan, Tokyo, 1988; Vol. I, p 187. Hamanoue, K.;Nakayama, T.; Sakurai, K.; Ushida, K. XI1 IUPAC Symp. Photochem., Bologna, Italy, 1988, 310.
Conductive Polymer-Semiconductor Junction: Characterlzation of Poly(3-methylthiophene):Cadmium Sulfide Based Photoelectrochemicaiand Photovoltaic Cells Arthur J. Frank,* Spyridon Glenis, and Arthur J. Nelson Solar Energy Research Institute, Golden, Colorado 80401 (Received: September 15, 1988)
The solid-state and electrochemical properties of the junction between highly doped metallic-like PMeT [PMeT = poly(3-methylthiophene)] and n-CdS are described. Electronic contact between the polymer and the semiconductor produces a Schottky barrier in the solid state. The rectification number, barrier height, quality factor, and photovoltaic response of the solid-state n-CdSPMeTAu cell compare favorably with those of n-CdS:metal Schottky barrier devices. When the solid-state device contacts various redox electrolytes, the junction characteristics are not altered appreciably as long as the polymer is doped to the metallically conductive state. Auger depth profile studies show that the PMeT film is relatively impermeable to redox species in solution. An energy-band model is invoked that accounts for both the independent behavior of the barrier height on the redox electrolyte and the impermeable nature of the film. The open-circuit photovoltage of the bare (uncoated) n-CdS electrode is also independent of aqueous redox couples (Fe3+/2+,I-/&-, Fe(EDTA)'-/*-, S?-/S2-) in solution, suggesting that a large surface-state density probably limits the barrier height at the n-CdS:PMeT contact. Potential measurements of PMeT coatings show that the Fermi level in the film is controlled by the redox electrolyte and is not altered by the semiconductor. During band-gap illumination of the n-CdS:PMeT liquid-junction cell, the Fermi level in PMeT shifts to positive potentials because of the influx of valence-band holes from the semiconductor into the polymer. The steady-state density of holes in the film is found to depend on the concentration of Fe2+ions, which scavenge holes at the polymersolution interface. The measurements of the film potential give the first direct observation of Fermi level movement in a polymer and provide important insight into the role that conductive polymer coatings play in interfacial-charge-transfer processes and in protecting n-type semiconductor electrodes against photodegradation. Photocurrent-time data reveal that the Fe2+ film competes with n-CdS for photogenerated valence-band holes. In the presence of ferrous ions, however, the steady-state concentration of holes in the film declines markedly, inferring that the oxidative degradation of the polymer is suppressed. Before photolysis, the PMeT film adheres strongly to the n-CdS surface. After photolysis, the film can be detached easily from the surface with adhesive tape, suggesting bond disruption between PMeT and n-CdS.
Introduction Electronically conductive organic polymers are of considerable fundamental and technological interest because of their unusual electronic properties. This class of polymer is oxidized or reduced more easily and reversibly than traditional polymers. The change in oxidation state of the polymers affects their electrical, optical, redox, and mechanical properties, as well as their stability. In many cases, the oxidized or doped state of the polymer displays nearly metallic-like behavior, whereas the reduced or neutral undoped form exhibits insulative or semiconductor-like charac0022-3654/89/2093-3818%01.50/0
teristics. Because electronically conductive polymers can exist in either the metallic-like or the semiconductor-like state, several types of rectifying junctions are theoretically possible when electronic contact is made between either the polymer and a metal or the polymer and a semiconductor. In particular, it has been demonstrated in a few cases1 that the polymersemiconductor (1) (a) Ozaki, M.; Pebbles, D.; Weinberger, B. R.; Heeger, A. J.; MacDiarmid, A. G. J . Appl. Phys. 1980, 51, 4252. (b) Inganls, 0.; Skotheim, T.; LundstrBm, I. Phys. Scr. 1982, 25, 863.
0 1989 American Chemical Society
