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Biphotonic ionic dissociation of weak charge-transfer complexes in glassy solutions at 77 K
Volume 36, number 1
RIPHOTONIC IN GLASSY
CHEMICAL PHYSICS LETTERS
IONIC DISSOCIATION SOLUTIONS
OF WEAK CHARGE-TRANSFER
i975
COMPLEX=
AT 77 R
Yohji ACHIBA and Katsumi Fhysical
15 Octobci
Clzetnisrr_v Loboraror~.
KIMUR4 brstirrtre
of Applied
Electriciry,
Hokkoido
Unilwsity,
Sopporo
060. Japafl
Received 14 July 1975
The dependence of photoinduced radical anion formation upon exciting light intensity has been studied for glassy solutions of tetracyanobenzene (TCNB) in ether-isopentane and 2-methyl-tetrahydrofuuran (MTHF) nt 77 Ii. It has been found thatinetller-isopentnnc _Jnss theTCNB anion is formed biphotonically via the lowest excited triplet state of the 1:l TCNB-ether CT complex, whereas in MTHF glass the ionic pho:odissociation of the TCNB-XITHF CT complex OCCUIS monophotonially in its lowest excited singlet state.
i. Introduction Tetracyanoethylene, tetracyanobenzene (TCNB) and pyromellitic dianhydride (PMDA) are known to work as electron acceptors in charge-transfer (CT) complexes. Photoirradiation of these acceptors in solvents such as ether, clioxane, dimethoxyethane, tetrahydrofuran (THF), 2-methyltetrahydrofuran (MTHF) and acetonitrile gives rise to the corresponding radical anions [l-l 01. Such photoinduced anion formations have been explained in terms of photoexcitation of weak ground-state CT complexes and of subsequent ionic dissociation in their excited states [4-IO]. Recent spectroscopic studies [I l-1 j] have indicated that several CT complexes of TCNB and PMDA with n- and n-type donors in solution dissociate into ions in the lowest excited singlet and/or triplet states of the complexes. For CT complexes with the lowest excited triplet states of localexcitation character, it is very unlikely that ionic dissociation takes place in these states. ’ Therefore, we considered it interesting to study the lighht-intensity dependence of ionic photodissociation. We report here an example of biphotonic ionic dissociation of a CT complex for the first time.
2. Experimen!al For studying complex
formation
between
TCNB
and ether, electronic absorption measurements were carried out on solutions containing TCNB (1 X 10m3 M) and ether (up to 1 M) in 1,2-dichloroethane at room temperature. For low-temperature photolysis experiments two kinds of transparent rigid solvent were used, one of which is a 1:l ether-isopentane (EP) mixture and the other is MTHF. EP and MTHF glasses containing 1 X 10m3 M TCNB were photoirradiated with ultraviolet light (2.50-380 nm) which
isolated from a 250-W high-pressure mercury arc with a filter system consisting of an aqueous solution was
of NiSOq and a Toshiba UV-D25 glass filter. Neutral filters (12: SO,45 and 60% in transmission) were employed for changing exciting light intensity. A squaretype quartz cell of 1 .O-cm path length was used, immersed in the liquid nitrogen contained in a quartz dewar which has four flat window5 capable of cross illumination. Absorption measurements for the TCNB radical anion were carried out photoelectrically with a Hitachi G-3 monochromator. For preventing the samples from excitation, the monitoring light was isolated from a 150-W xenon arc with water and a Toshiba W-42 cut-off glass filter (the transmission is ,less than 1% at 390 pm). The exciting and the monitoring Light were set perpendicular to each other. TCNB was purified by vacuum sublimation after recrystallization from ethanol. Ether and MTHF were distilbd after drying over CaH2. Isopentane was purified by passing through activated alumina. 65
Volume
36, num’ber 1
CHEMICAL
PHYSICS
LETTERS
15 October
1975
a
B
1
I
t
3Oci
/
t
350 &DO Gel Wavelength (nm)
MO
Fis i_ Absorption spectra of TCNB in EP glxs at 77 K .before and after UV irradiation, represented by cuvcs z and b, respective&. In the insert, the relative yield fr) is plotted against the irradiating time fr), tile percenteges being trunsmissions of the neutral filters used.
3. Results
and discussion
Complex formation between TCNB and ether has not been reported so far. From the present anaIysis of the concenrracion dependence of absorption inter.sity (at 330 nm) in terms of the Benesi-Midebrand method [16], it was fdund that a I : I CT complex is formed between TCNB and ether with an equilibrium constant of,kCT = 0.13 Q/M at 25”. Shimada et a!. ilO] reported that a 1: 1 CT complex. is formed between TCNB and MTHF with k, = 0.45 Q/M at room temperature. From these km vales, the amount of CT complex may be estimated to be about 39 and 82% in EP and MTHF solutions of TCNB, respectively, at room temperature. The percentage of each compjex however may be considerably g-eater at 77 K than that 3t room temperature. The 250-330 nm bands of solutions of TCNB in EP and MTHF somewhat differ in spectral shape from that in ai inert solvent such as I ,2-dichloroethane. For an MTHF solulion of TCNB, Shimada et al. [lo] pointed out thai a CT band is heavily overlapped with the TCNB 1ocaIexcitation band. The same situation may a!so occur for an EP sclution 0fTCNB. .. @j
.-. _
Fig. 2. Exiting li$t intensity dependenccs of the relative rate of anion formation for (a) the TCNB-ether CT complex and (b) the TCNB-%iTI-IF CT complex.
In each photoirradiation experiment, not only the CT complex but also the free TCNB moIecule were excited. However, since it is generally recognized that no exciplex is formed in rigid matrices at 77 K, the photoinduced anion formation described below may be interpreted in terms of a CT complex rather than an exciplex. In fact, it has been reported that a solution of pyrene’or anthracene in r-bu~lam~ne emits an exciplex fluorescence at room temperature but not at 77 K [18,19]. The photoirradiation of an EP glass of TCNB (1 X 10m3 M) at 77 K gave rise to an absorption spectrum which is shown by curve b in fig. 1. Since this spectrum resembles an available spectrum of the TCNB anion (TCNB-) [l?] , the photoinduced species stably trapped in the 77-K glass may safely be attributed io TCNB-. Such a TCNB- spectrtim was also obtained in the photolysis of the TCNB-MTHF system at 77 K. The yield of TCNB- was estimated from peak absorbzncas at 460 nm. &. is shown in the upper insert of fig. 1, the yield (Y) of TCNB- at the initial stage was confirmed to increase Enearly with the irradiation period (t) for each neutral filter. The slope of each Y-r line indicates the rate of tion formation,R = dC/dt, where Cdenotes the concentration of TCNE-. The plot of log R versus log I is : .’
Volume 36, number
CHEMICAL
1
PHYSICS LEmERS
15 October
1975
The energy levels 3f the TCNB-cther complex are schematicalIy illustrated in fig. 3a, in which the hi&er SJ (CT)-
triplet
\q*;
state
associated
with
the ionic
dissociation
is
by T,, . Since the fluorescence and phosphorescence spectra of a solution of TCNB in EP glass at 77 K resemble those in inert 1,2-dichloroethane solvent, we may consider that both the lowest excited singlet and triplet states of the TCNB-ether complex are of local-excitation character. From the relation indicated
hv i
G-
C-
(b)
(al Fig_ 3. Schenmtic ionic dissociation the monophotonic CT complex.
enera diagrams showing (a) the biphotonic of the TCNB-ether CT complex and (b) ionic dissociation of the TCNB-MTHF
shown in tig. 2, giving an appro,ximately linear line with a slope of 1.8 for the TCNB-EP system, where Iis the exciting light intensity. This means that R = .kl'-* holds for this system, X-being a constant. Similarly, TCNB- was produced by the photoirradiation of an MTHF glass of TCNB at 77 K. Plotting log R against log I, we obtained a straight line with a slope of 1.2 as is shown by line b in fig. 2. This slope suggests that in the TCNB-MTHF system the anion formation mainly occurs monophotonicnlly. Adding piperylene as a triplet quencher to the CT systems, we found that the TCNB- yield decreases with increasing piperylene concentration in the TCNB-EP system but is independent of piperylene concentration in the TCNB-MTHF system. These quenching experiments suggest that the photoinduced anion formations of the TCNB-ether and -MTHF complexes take place in the excited triplet and singlet states, respectively. For the latter complex, such a singlet-state anion formation was suggested by Shimada et al. [lo] flash experiment. From
the above
from a room-temperature experimental
results
singlet
state
of the TCNB--IMTHF
absorption
spectrum
of this complex
absorption tail on the longer-wavelength LE band ofTCNB.
= E {3(@A-)*)
proposed by Nagakura et al. [20], the energy level of the higher triplet state with CT character may be estimated to be about 1.5eV higher than the lowest excited triplet state, where D and A denote the,electron
donor
and acceptor,
respectively.
.. D+) should initially be formed in the ionic photodissociation of a CT complex, followed by either geminate recombination or dissociation into radical ions. Diffusional separation of the ion pair should be very suppressed in rigid glass. In order that a photoinduced radical anion is stably trapped in rigid glass, it seems Generally
speaking,
a pair of radical
ions (A-.
to be necessary to satisfy a c,ondition that the counter ion (II’) should react with ;I neighbouring solvent molecule forming another more stable cation which prevents TCNB- from geminate recombination. men ether or MTHF is used as an electron donor as well as a solvent, the above condition may be satisfied. In fact, in radiation chemistry, it has been reported that MTHFi reacts with MTHF formir,g protona ted species [21]. It is especially interesting to note that ionic dissociation occurs even in a ri@d matrix at 77 K.
References
it may be
complex
shows
III C. L;1gercrantz and hl. Yhland, Acta Chem. Stand. 16 (1962) 1043,1799,1807. R.L. Ward, J. Chcm. Phvs. 39 (1963) 852. hf. Scfue and S. Nagakura, Bull. Chem. Sot. Japan
;:I [41
is
[51 [61
of CT character rather than LE (local-excitation) character. Pt should be mentioned that the groundstate
(D+A-)‘}
laser
concluded that the TCNB-ether complex in EP glass dissociates into ions biphotonically via the lowest excited triplet state, whereas the ionic photodissociation of the TCNB-MTHF complex in MTHF glass mainly occurs monophotonically in the lowest excited singlet states. It is also su=ested that the lowest excited
E{ ’
an
side of the
38 (1965) 1048. D.F. Illen and hf. Calvin, J. Chem. Phys. 42 (196.5) 3760. F.E. Stewart and E. Eisner, Mol. Phys. I2 (1967) 173. Y. Achiba and K. Kimura, Bull. Chem. Sot. Japan 45
(1972) 1272. 171 Y. Achiba, S. Kaisumata Letters 13 (1972) 213.
I81 Y. Achiba
and K. Kimura.
and K. Kimura, Chem. Phys. Chem
Phys.
Letters
19
(1973) 45.
67,
.Y&ume 36, number 1
15 October
CHEMICAL PHYSICS LETTERS
[9] I$ Kim&, Y. Achl’oa and S. Katsumata, J. Phys. Chem. ‘.- 77 (1973) 2520. “. [lOI M; Shimada, H. hf~suhva zncl N. Matap, BuIi. Chem. >oc. Japan 46 (1973) 1903. [ I1 1 R. Potashnik, C.R. CoZdschmidt and M. Ottolenghi, J. Phys. Chem.. 73 (1969) 3170. { 121 H. hiazuhara, M. Shlrnzda, N. Tsujino and N. &fata$a, BuiL Chem. Sot. Japnn 44 (1971) 3310. [I31 %f.Irie, S. Tomimoto 2nd K. Hayashi, J. Phys. Chem. 76 (i972) 1419. 1141 M. Irie, H. ?&suhara, 1;. Hayarhi and N. Mataga, J. Phys. Chem. 78 (1974) 341.
1975
[ 1.51 Y. Achiba and K. Mmura, J. Phys. Chem.. to bc published. f16] H.A. Bencsi and J.H. Hildebrand, J. Am. Chem. Sot. 71 (1949) 2703. 517 1 A. Ishitani and S. Na,qkura, Thcoret. Chim. Acta 4
(1966) 236. [ 181 N.. Nakashima and N. Mataga, 2. Physik. Chem. NF 79 (1972)
150.
[ 191 A. Nakajima, 13~11.Chem. Sot. Japan 42 (1969) 3409. [3-O] S. Iv&a, 5. Tanaka and S. Nagakura, J. Chem. Phys. 47 (1967) 2203. [21] W.H. Hamill, in: Radid ions, rds. E.T. Kaiser and L. Kevan (Interscience, New York, 1968) p. 332.
RIPHOTONIC IN GLASSY
CHEMICAL PHYSICS LETTERS
IONIC DISSOCIATION SOLUTIONS
OF WEAK CHARGE-TRANSFER
i975
COMPLEX=
AT 77 R
Yohji ACHIBA and Katsumi Fhysical
15 Octobci
Clzetnisrr_v Loboraror~.
KIMUR4 brstirrtre
of Applied
Electriciry,
Hokkoido
Unilwsity,
Sopporo
060. Japafl
Received 14 July 1975
The dependence of photoinduced radical anion formation upon exciting light intensity has been studied for glassy solutions of tetracyanobenzene (TCNB) in ether-isopentane and 2-methyl-tetrahydrofuuran (MTHF) nt 77 Ii. It has been found thatinetller-isopentnnc _Jnss theTCNB anion is formed biphotonically via the lowest excited triplet state of the 1:l TCNB-ether CT complex, whereas in MTHF glass the ionic pho:odissociation of the TCNB-XITHF CT complex OCCUIS monophotonially in its lowest excited singlet state.
i. Introduction Tetracyanoethylene, tetracyanobenzene (TCNB) and pyromellitic dianhydride (PMDA) are known to work as electron acceptors in charge-transfer (CT) complexes. Photoirradiation of these acceptors in solvents such as ether, clioxane, dimethoxyethane, tetrahydrofuran (THF), 2-methyltetrahydrofuran (MTHF) and acetonitrile gives rise to the corresponding radical anions [l-l 01. Such photoinduced anion formations have been explained in terms of photoexcitation of weak ground-state CT complexes and of subsequent ionic dissociation in their excited states [4-IO]. Recent spectroscopic studies [I l-1 j] have indicated that several CT complexes of TCNB and PMDA with n- and n-type donors in solution dissociate into ions in the lowest excited singlet and/or triplet states of the complexes. For CT complexes with the lowest excited triplet states of localexcitation character, it is very unlikely that ionic dissociation takes place in these states. ’ Therefore, we considered it interesting to study the lighht-intensity dependence of ionic photodissociation. We report here an example of biphotonic ionic dissociation of a CT complex for the first time.
2. Experimen!al For studying complex
formation
between
TCNB
and ether, electronic absorption measurements were carried out on solutions containing TCNB (1 X 10m3 M) and ether (up to 1 M) in 1,2-dichloroethane at room temperature. For low-temperature photolysis experiments two kinds of transparent rigid solvent were used, one of which is a 1:l ether-isopentane (EP) mixture and the other is MTHF. EP and MTHF glasses containing 1 X 10m3 M TCNB were photoirradiated with ultraviolet light (2.50-380 nm) which
isolated from a 250-W high-pressure mercury arc with a filter system consisting of an aqueous solution was
of NiSOq and a Toshiba UV-D25 glass filter. Neutral filters (12: SO,45 and 60% in transmission) were employed for changing exciting light intensity. A squaretype quartz cell of 1 .O-cm path length was used, immersed in the liquid nitrogen contained in a quartz dewar which has four flat window5 capable of cross illumination. Absorption measurements for the TCNB radical anion were carried out photoelectrically with a Hitachi G-3 monochromator. For preventing the samples from excitation, the monitoring light was isolated from a 150-W xenon arc with water and a Toshiba W-42 cut-off glass filter (the transmission is ,less than 1% at 390 pm). The exciting and the monitoring Light were set perpendicular to each other. TCNB was purified by vacuum sublimation after recrystallization from ethanol. Ether and MTHF were distilbd after drying over CaH2. Isopentane was purified by passing through activated alumina. 65
Volume
36, num’ber 1
CHEMICAL
PHYSICS
LETTERS
15 October
1975
a
B
1
I
t
3Oci
/
t
350 &DO Gel Wavelength (nm)
MO
Fis i_ Absorption spectra of TCNB in EP glxs at 77 K .before and after UV irradiation, represented by cuvcs z and b, respective&. In the insert, the relative yield fr) is plotted against the irradiating time fr), tile percenteges being trunsmissions of the neutral filters used.
3. Results
and discussion
Complex formation between TCNB and ether has not been reported so far. From the present anaIysis of the concenrracion dependence of absorption inter.sity (at 330 nm) in terms of the Benesi-Midebrand method [16], it was fdund that a I : I CT complex is formed between TCNB and ether with an equilibrium constant of,kCT = 0.13 Q/M at 25”. Shimada et a!. ilO] reported that a 1: 1 CT complex. is formed between TCNB and MTHF with k, = 0.45 Q/M at room temperature. From these km vales, the amount of CT complex may be estimated to be about 39 and 82% in EP and MTHF solutions of TCNB, respectively, at room temperature. The percentage of each compjex however may be considerably g-eater at 77 K than that 3t room temperature. The 250-330 nm bands of solutions of TCNB in EP and MTHF somewhat differ in spectral shape from that in ai inert solvent such as I ,2-dichloroethane. For an MTHF solulion of TCNB, Shimada et al. [lo] pointed out thai a CT band is heavily overlapped with the TCNB 1ocaIexcitation band. The same situation may a!so occur for an EP sclution 0fTCNB. .. @j
.-. _
Fig. 2. Exiting li$t intensity dependenccs of the relative rate of anion formation for (a) the TCNB-ether CT complex and (b) the TCNB-%iTI-IF CT complex.
In each photoirradiation experiment, not only the CT complex but also the free TCNB moIecule were excited. However, since it is generally recognized that no exciplex is formed in rigid matrices at 77 K, the photoinduced anion formation described below may be interpreted in terms of a CT complex rather than an exciplex. In fact, it has been reported that a solution of pyrene’or anthracene in r-bu~lam~ne emits an exciplex fluorescence at room temperature but not at 77 K [18,19]. The photoirradiation of an EP glass of TCNB (1 X 10m3 M) at 77 K gave rise to an absorption spectrum which is shown by curve b in fig. 1. Since this spectrum resembles an available spectrum of the TCNB anion (TCNB-) [l?] , the photoinduced species stably trapped in the 77-K glass may safely be attributed io TCNB-. Such a TCNB- spectrtim was also obtained in the photolysis of the TCNB-MTHF system at 77 K. The yield of TCNB- was estimated from peak absorbzncas at 460 nm. &. is shown in the upper insert of fig. 1, the yield (Y) of TCNB- at the initial stage was confirmed to increase Enearly with the irradiation period (t) for each neutral filter. The slope of each Y-r line indicates the rate of tion formation,R = dC/dt, where Cdenotes the concentration of TCNE-. The plot of log R versus log I is : .’
Volume 36, number
CHEMICAL
1
PHYSICS LEmERS
15 October
1975
The energy levels 3f the TCNB-cther complex are schematicalIy illustrated in fig. 3a, in which the hi&er SJ (CT)-
triplet
\q*;
state
associated
with
the ionic
dissociation
is
by T,, . Since the fluorescence and phosphorescence spectra of a solution of TCNB in EP glass at 77 K resemble those in inert 1,2-dichloroethane solvent, we may consider that both the lowest excited singlet and triplet states of the TCNB-ether complex are of local-excitation character. From the relation indicated
hv i
G-
C-
(b)
(al Fig_ 3. Schenmtic ionic dissociation the monophotonic CT complex.
enera diagrams showing (a) the biphotonic of the TCNB-ether CT complex and (b) ionic dissociation of the TCNB-MTHF
shown in tig. 2, giving an appro,ximately linear line with a slope of 1.8 for the TCNB-EP system, where Iis the exciting light intensity. This means that R = .kl'-* holds for this system, X-being a constant. Similarly, TCNB- was produced by the photoirradiation of an MTHF glass of TCNB at 77 K. Plotting log R against log I, we obtained a straight line with a slope of 1.2 as is shown by line b in fig. 2. This slope suggests that in the TCNB-MTHF system the anion formation mainly occurs monophotonicnlly. Adding piperylene as a triplet quencher to the CT systems, we found that the TCNB- yield decreases with increasing piperylene concentration in the TCNB-EP system but is independent of piperylene concentration in the TCNB-MTHF system. These quenching experiments suggest that the photoinduced anion formations of the TCNB-ether and -MTHF complexes take place in the excited triplet and singlet states, respectively. For the latter complex, such a singlet-state anion formation was suggested by Shimada et al. [lo] flash experiment. From
the above
from a room-temperature experimental
results
singlet
state
of the TCNB--IMTHF
absorption
spectrum
of this complex
absorption tail on the longer-wavelength LE band ofTCNB.
= E {3(@A-)*)
proposed by Nagakura et al. [20], the energy level of the higher triplet state with CT character may be estimated to be about 1.5eV higher than the lowest excited triplet state, where D and A denote the,electron
donor
and acceptor,
respectively.
.. D+) should initially be formed in the ionic photodissociation of a CT complex, followed by either geminate recombination or dissociation into radical ions. Diffusional separation of the ion pair should be very suppressed in rigid glass. In order that a photoinduced radical anion is stably trapped in rigid glass, it seems Generally
speaking,
a pair of radical
ions (A-.
to be necessary to satisfy a c,ondition that the counter ion (II’) should react with ;I neighbouring solvent molecule forming another more stable cation which prevents TCNB- from geminate recombination. men ether or MTHF is used as an electron donor as well as a solvent, the above condition may be satisfied. In fact, in radiation chemistry, it has been reported that MTHFi reacts with MTHF formir,g protona ted species [21]. It is especially interesting to note that ionic dissociation occurs even in a ri@d matrix at 77 K.
References
it may be
complex
shows
III C. L;1gercrantz and hl. Yhland, Acta Chem. Stand. 16 (1962) 1043,1799,1807. R.L. Ward, J. Chcm. Phvs. 39 (1963) 852. hf. Scfue and S. Nagakura, Bull. Chem. Sot. Japan
;:I [41
is
[51 [61
of CT character rather than LE (local-excitation) character. Pt should be mentioned that the groundstate
(D+A-)‘}
laser
concluded that the TCNB-ether complex in EP glass dissociates into ions biphotonically via the lowest excited triplet state, whereas the ionic photodissociation of the TCNB-MTHF complex in MTHF glass mainly occurs monophotonically in the lowest excited singlet states. It is also su=ested that the lowest excited
E{ ’
an
side of the
38 (1965) 1048. D.F. Illen and hf. Calvin, J. Chem. Phys. 42 (196.5) 3760. F.E. Stewart and E. Eisner, Mol. Phys. I2 (1967) 173. Y. Achiba and K. Kimura, Bull. Chem. Sot. Japan 45
(1972) 1272. 171 Y. Achiba, S. Kaisumata Letters 13 (1972) 213.
I81 Y. Achiba
and K. Kimura.
and K. Kimura, Chem. Phys. Chem
Phys.
Letters
19
(1973) 45.
67,
.Y&ume 36, number 1
15 October
CHEMICAL PHYSICS LETTERS
[9] I$ Kim&, Y. Achl’oa and S. Katsumata, J. Phys. Chem. ‘.- 77 (1973) 2520. “. [lOI M; Shimada, H. hf~suhva zncl N. Matap, BuIi. Chem. >oc. Japan 46 (1973) 1903. [ I1 1 R. Potashnik, C.R. CoZdschmidt and M. Ottolenghi, J. Phys. Chem.. 73 (1969) 3170. { 121 H. hiazuhara, M. Shlrnzda, N. Tsujino and N. &fata$a, BuiL Chem. Sot. Japnn 44 (1971) 3310. [I31 %f.Irie, S. Tomimoto 2nd K. Hayashi, J. Phys. Chem. 76 (i972) 1419. 1141 M. Irie, H. ?&suhara, 1;. Hayarhi and N. Mataga, J. Phys. Chem. 78 (1974) 341.
1975
[ 1.51 Y. Achiba and K. Mmura, J. Phys. Chem.. to bc published. f16] H.A. Bencsi and J.H. Hildebrand, J. Am. Chem. Sot. 71 (1949) 2703. 517 1 A. Ishitani and S. Na,qkura, Thcoret. Chim. Acta 4
(1966) 236. [ 181 N.. Nakashima and N. Mataga, 2. Physik. Chem. NF 79 (1972)
150.
[ 191 A. Nakajima, 13~11.Chem. Sot. Japan 42 (1969) 3409. [3-O] S. Iv&a, 5. Tanaka and S. Nagakura, J. Chem. Phys. 47 (1967) 2203. [21] W.H. Hamill, in: Radid ions, rds. E.T. Kaiser and L. Kevan (Interscience, New York, 1968) p. 332.