J. Phys. Chem. 1994, 98, 10771-10778
10771
Photophysical Processes in Aromatic Polyimides. 2. Photoreduction of Benzophenone-Containing Polyimide Model Compounds Masatoshi Hasegawa,' Yoichi Sonobe, Yoichi Shindo, and Tokuko Sugimura Department of Chemistry, Faculty of Science, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274, Japan
Takashi Karatsu and Akihide Kitamura College of Arts and Science, Chiba University, 1-33 Yayoicho, Inage-ku, Chiba 263, Japan Received: April 26, 1993; In Final Form: August 6, 1994@
The photochemical kinetic parameters for the model compounds of benzophenone-containing polyimide were determined on the basis of the Stem-Volmer analysis. The quantum yield for photoreduction, CPM, in dichloromethane decreased with an increase in the intramolecular charge-transfer (CT) character which depends upon the chemical structure of the amine components and the conformation around N-aryl group linkage. This could be explained well by a schematic photophysical mechanism in which the intersystem crossing followed by the hydrogen abstraction competes with the intramolecular CT process followed by effective T, absorption and deactivation. The phosphorescence spectra and lifetime at 77 K and the transient T1 emission spectra of the model compounds at 20 "C suggested that TI is of pure (n,n*) state independent of the intramolecular CT character. It was found that benzophenonebisimides have the hydrogen abstraction rate constants no less than that of benzophenone (BP).
-
Introduction It was widely accepted that both of the intra- and intermolecular CT interaction exist in solid-state aromatic polyimides (PI). The intramolecular CT interaction is attributed to an alternative sequence composed of the electron donor (aromatic diamine component) and the electron acceptor (aromatic dianhydride component). The intermolecularCT complexes formed owing to molecular aggregation between PI chains in solid state are responsible for the color of PI films' and the photoconducti~ity.~-~ The intermolecular CT complexes present in solid-state PIS were also evidenced by an experimental result in which the visible absorption tail (CT band) shifts reversibly with the change in hydrostatic pre~sure.~ The result provided a direct evidence of the intermolecular CT complex formation in the aromatic PI film. We have previously observed an increase in the intermolecular CT fluorescence intensity of various aromatic PI films by thermal annealing. This can be interpreted as result of an increase in the intermolecular CT complex site population.6-8 Recently, the intermolecularCT fluorescence was also observed in a liquid-crystalline polyester composed of 1,Cdialkyl ester of pyromellitic acid and 4,4'-biphenol in the layered liquidcrystalline state.g It is generally difficult to separate the contribution of intraand intermolecular CT in solid-state PIS to the photophysical and photochemical properties. Then, we have previously studied the photophysical processes for biphenyl-type PI with its model compounds in dilute solution to eliminate the effect of intermolecular interaction and estimated the rate constant for the intramolecular CT process, kCT, to be more than 5 x 10" sW1.lo Since the value is comparable with the rate constant for an extremely fast process of the intersystem crossing, kIsc, in aromatic carbonyl compounds (e.g., kIsc G= 2 x 10" s-' for BP)," if BP moiety was introduced into PI main chain, it is expected that the efficiency of the hydrogen abstraction via the intersystem crossing is strongly affected by the rate of the @
Abstract published in Advance ACS Abstracts, September 1, 1994.
0022-365419412098-10771$04.5010
intramolecular CT process followed by effective deactivation. The purpose of this paper is to elucidate the photophysical properties controlling the hydrogen abstraction efficiency of benzophenone-containing PI with its model compounds. Benzophenone-containing PI is known to be useful as a thermally stable photoresist materials. Lin et a1.12 proposed an acceptable photo-cross-linking mechanism in which the triplet benzophenone bisimide abstracts intermolecularly hydrogen atom from the adjacent alkyl groups and then subsequent coupling between the radicals formed so occurs to form a crosslinking point. It has been confirmed that the benzophenone bisimide group bound to PI backbone behaves as like BP. There are a few studies in which photoreactivity of the benzophenonebisimide model compounds was compared with that of BP.13-15 Reference 13 showed that the hydrogen abstraction rate constant, k,, of BP from 2-propanol is about 1 order smaller than that for the model compound. On the other hand, ref 14 reported that kr for BP is about 1 order larger than that for the model compound. Although the structure of the N-aryl group in ref 13 is different from that in ref 14, this inconsistency cannot be readily explained by only the difference of the amine moiety. The present study also describes comparison of the photoreactivity for various model compounds and BP. Experimental Section Materials. The model compounds of benzophenone-containing polyimide were prepared from benzophenonetetracarboxylic dianhydride (BTDA, Tokyo Chemical Ind.) and the stoichiometric amount of monoamines; BTDA recrystallized from acetic anhydride was added to the dried N,K-dimethylacetamide solution of the monoamine with continuous stimng at room temperature for 2 h, and then the solution was refluxed for 1 h. The acylation of aromatic and aliphatic amines with BTDA occurs quantitatively as well as polycondensation of poly(amic acid). The precipitate was recrystallized twice from a suitable solvent. 0 1994 American Chemical Society
Hasegawa et al.
10772 J. Phys. Chem., Vol. 98, No. 42, 1994
-
Benzophenone-containing Polyimide
0
M(BTDA/MCHA)
mol dm-3) and the cyclohexane solution of BP (ca. mol dm-3) were degassed by the multiple freeze-pump-thaw cycle and sealed off into a Pyrex tube (1.5 cm in diameter). The solutions were irradiated for 0.5-2 h with continuous stirring through a bandpass filter (Toshiba, UV-D36A) by using a highpressure mercury lamp (Ushio, UM-452) equipped with a voltage regulator at 25 "C on a merry-go-round together with the cyclohexane solution of BP as a actinometer. @M was detennined from the relation @M - NM/zab ----@act
M(BTDA/Z,Q-DEA)
M(BTDA/Z,CDMA)
M(BTDM3-ISA)
a
Figure 1. Chemical formulas of the model compounds.
The chemical formulas and symbols of four kinds of model compounds are listed in Figure 1. The symbols are represented by M(X/Y) where M, X, and Y mean model compound, dianhydride, and monoamine components, respectively. The abbreviations are as follows: 2-methylcyclohexylamine(MCHA), 2,6-diethylaniline (2,6-DEA), 2,4-dimethylaniline (2,4-DMA), 3-ethylaniline (3-EA). The structures were confirmed by the infrared absorption spectra. Benzhydrolbisimides were prepared by reduction of the benzophenone carbonyl group in the model compounds; the ethanol solution of the stoichiometric amount of N a B b was added gradually into the tetrahydrofuran (THF) solution of the model compound. After the residual solvent was removed under vacuum, the reduced product was then recrystallized twice from benzene. The infrared absorption band characteristic of the benzophenone carbonyl group (1670 and 1290 cm-') disappeared completely after reduction, and the 'H NMR spectrum showed that the reaction occurred quantitatively. 'HNMR for the reduced M(BTDA/2,6-DEA) (270 MHz, CDC13) 6 1.13 (12H, t, J = 7.5 Hz), 2.44 (8H,q, J = 7.5 Hz), 3.0 (lH, br s),
NBdzab
XM[M] XBPIBPl
where Qact is the quantum yield for the disappearance of BP in cyclohexane as actinometer, N the number of the photoreduced molecules, I,+, the number of absorbed photon, and X the conversion. [MI and [BPI are the initial concentration of the model compounds and BP (in mol dm-3), respectively. For all the sample solutions, the absorbances (Abs) were always high enough to absorb all of the 365 nm incident light during photoirradiation (Abs > 1.5). Therefore, l a b in the model compound systems is in principle equal to that in the BP system. Strictly speaking, the integral light intensity absorbed by the model compounds was corrected as function of conversion. Since the photoreduced products in the model compound systems have the molar extinction coefficients, E , at 365 nm no less than the original compounds unlike the photoreduced products in the BP system have little absorption at 365 nm, the integral light intensity absorbed by the model compounds decreases as the photoreduction proceeds. In eq 1, if @BP is known, @M can be readily determined from the experimental values of XM and XBP. The conversion of BP in cyclohexane was determined by the absorbance change of the (n,n*) band around 350 nm. @BP was determined on the basis of the StemVolmer relation
where kd and k, are the rate constants for the triplet deactivation (in s-') and the hydrogen abstraction (in M-' s-0 from hydrogen donor (RH= cyclohexane), respectively. @BP is the quantum yield for disappearance of BP as function of [RH] (in M). Measurement of Oact(intersection in eq 2) enables us to obtain @M from eq 1 without the absolute measurement of the absorbed light quanta. 6.15(1H,brs),7.24(4H,d,J=7.5Hz),7.40(2H,dd,J=7.5 Since in the model compounds the change in absorbance in and 8.5 Hz), 8.00-7.93 (6H, m). the ultraviolet region after photoreduction is not so large, the Benzophenone (Tokyo Chemical Ind.) was recrystallized conversion for the photoreduction of the model compounds was twice from benzene. Solvents used for photoreaction and determined from the change in the integral absorption intensity phosphorescence spectrum measurement, dichloromethane, THF, ratio of 1290 cm-' band characteristic of benzophenone carbonyl 2-methyltetrahydrofuran (MTHF), cyclohexane, and ethanol group to the benzene ring stretching band at 1462 cm-' as an were distilled before use. Dehydrated acetonitrile (Kanto intemal standard (the C-H stretching band near 2950 cm-' for Chemicals) was used as a non-hydrogen donor solvent without M(BTDA/MCHA)) in KBr using an IR spectrometer (Jasco, further purification. Spectroscopic grade dichloromethane (DotFTIR-5300). The IR absorption spectra of M(BTDA/2,6-DEA) ite, Luminasol) was used for the ultraviolet-visible (UV-vis) and the corresponding compound reduced with NaB& are absorption spectrum measurement without further purification. shown in Figure 2. The 1290 cm-' band was used to determine Measurements. Various photochemical kinetic parameters the conversion because the baseline is so flat that the reproducwere determined on the basis of the Stem-Volmer plots with ibility is much better than for the 1670 cm-' stretching band. respect to the quantum yield for disappearance of the model The precision of this method was confirmed from the result for compounds, (PM, vs the concentration of hydrogen donor or BP that the conversion determined from the absorption change triplet quencher (Q = naphthalene).16 For the limited solubility in the 1290 cm-' band is consistent well with that determined of the model compounds, CHZClP was chosen mainly as a from the change in the (n,n*) absorption band around 350 nm. hydrogen donor. The model compounds in the established The UV-vis absorption spectra of the model compounds composition of the mixed solvents (CHZClz/CH3CN, ca. were recorded on Jasco Ubest-30 spectrophotometer. The
Photophysical Processes in Aromatic Polyimides 0.5
J. Phys. Chem., Vol. 98, No. 42, 1994 10773
,
m
ls00
1600
1400
1200
lo00
150
Wavenumber I em"
b
Figure 2. Infrared absorption spectra of M(BTDA/2,6-DEA) (broken line) and the compound reduced with NaBh (solid line).
corrected phosphorescencespectra and lifetimes t p of the model compounds and BP in a transparent rigid glass (MTHF/ethanol = 9/1) were measured at 77 K using Hitachi 850 fluorescence spectrometer equipped with a lifetime measurement apparatus based on the sampling method. The phosphorescence quantum yields, (Pp, for the model compounds were determined by comparing with the integral fluorescence intensity of quinine sulfate in 1 N H2S04 ((Pf 0.55).17 Transient triplet--triplet (T-T) absorption spectra of the model compounds in CH3CN were measured by excitation at 355 nm (third harmonics of Nd:YAG laser, Continuum SL 1-10: 6 ns fwhm, 20 dlpulse) with a detection system (Tokyo Instrument) composed of a multichannel diode array (Princeton IRY-512G: 18 ns gate) with a SPEX 270M monochromator (resolution: 0.3 nm/channel).'* Decay profiles measured with a photomultiplier were analyzed by the Marquardt nonlinear least-squares fitting method. The rate constant for the triplet quenching by naphthalene, k,, was determined from the relation tdt 1 f k,to[Q] where to and t are the triplet lifetimes in the absence and the presence of naphthalene, respectively, measured with the T-T absorption decay at 380 nm for M(BTDA/2,6-DEA) (monitored at 525 nm for BP). The value of kr for M(BTDA/2,6-DEA)/THF/CH3CN system was also directly determined at 20 "C from the relation (Ppd(Pp = 1 zokr[THF] where (Ppo and (Pp are the phosphorescence yields in the absence and the presence of THF, respectively.
o
100
-
O
-
0
50
7s e
Benzene
w ,
CH3CN a I
-
I
+
Results and Discussion Photoreduction of Benzophenonebisimides in Solution. A large number of the experimental results on the photoreduction of BP provides much information to understand fully the photoreaction mechanism of benzophenone bisimide compounds. So far, the photochemical and photophysical properties of BP have been examined in detail by using the steady-state photochemical analysisI6 and the transient T-T absorption measurement technique.19-23 As known widely, phthalimides are less reactive for photoinduced intermolecular hydrogen abstraction from hydrogen donor solvents compared with BP.% This is explained by the nature of the lowest triplet state TI. Coyle et aLZ5 showed that TI for N-alkylphthalimides are assigned as the (n,n*) state on the basis of its long phosphorescence lifetime in contrast to that T1 for BP is of pure (n,n*) state. For benzophenonebisimides possessing both of benzophenone and imide carbonyl groups in CH2Cl2, we observed no appreciable decrease in the absorbance of the imide carbonyl stretching band (1774 cm-') and complete disappearance of the benzophenone carbonyl bands (1670, 1290 cm-l) after pro-
where (PISCand (PTare the quantum yields for the intersystem crossing and the chemical transformation from ketyl radical to some chemically stable products, respectively. kd, k,, k,, and kq represent the rate constants for the triplet deactivation, phosphorescence emission, hydrogen abstraction, and triplet quenching by naphthalene, respectively. [RH] and [Q] (in mol dm-3) are the concentration of hydrogen donor and triplet quencher, respectively. Naphthalene has little absorption at 365 nm. For all model compounds in CH3CN at room temperature, very weak phosphorescence were observed with an usual fluorimeter, indicating that the phosphorescence quantum yield (Pp (= @IS&& k,)) 10 kJ mol-'. In the previous paper, kq was determined to be 6 x lo9 M-' s-l for BPhaphthalene system?' 0.9 x lo9 M-l s-' for BP/ Fe(DPM)3 (ferric dipivaloylmethide) system.22 We measured kq to be 2.0 x lo9 M-' s-l for the M(BTDA/2,6-DEA)/ naphthalene system and 5.7 x lo9 M-' s-l for the BP/ naphthalene system by the transient T-T absorption decay measured in CH3CN at 20 "C on the basis of the relation zdz = 1 kqto[Q] (Figure 5). Various photochemical kinetic parameters are summarized in Table 1. The value of k, determined on the basis of eq 4 for M(BTDN2,6-DEA)iTHF system (kr = 4 x lo6 M-' s-l) is roughly consistent with that
+
0.3
0.2
0.1
[RH]" t M" 30
20
10
n 0:OOO
0.002 0.004 0.006 0.008 0.010
[Q1/ M Figure 4. Stem-Volmer plots for a series of the model compounds in CHZCI&H~CN in (a) the absence and (b) the presence of triplet quencher. (0)M(BTDN3-EA), (0)M(BTDA/2,4-DMA), (0)M(BTDA/2,6-DEA), and (W) M(BTDA/MCHA).
0
0
1
2
5
4
3
[Q] / loe4mol dm" Figure 5. Stem-Volmer plot for the triplet lifetime of M(BTDA/2,6DEA) in CH3CN as a function of the naphthalene concentration [Q].
TABLE 1: Photochemical Kinetic Parameters of the Model Compounds and B P 20 OC model
@M
(PIX
k, (M-ls-')
-196°C kd
(SKI)
t ~ ( m s ) (PP ~
0.38 M(BTDA/MCHA) M(BTDA/2,6-DEA) 0.41 M(BTDA/2,4-DMA) 0.22 M(BTDN3-EA) 0.096 BP 0.40 Dichloromethane was
1.0 0.87 0.35 0.19
1.3 x 10' 3.3 x 3.4 x lo5 7.6 x 4.6 x IO5 9.2 x 2.3 x 10' 2.9 x 1.0 3.0 x lo5 6.6 x used as hydrogen-donor
15.6 lo6 7.8 IO6 9.9 lo6 6.6 lo6 2.6 solvent. lo6
0.71 0.50 0.12 0.051
0.82
determined by the change in transient phosphorescence intensity as function of [THFI (k, e lo7 M-' s-' 1. Relation between the Photochemical Kinetic Parameters and Photoinduced Intramolecular Charge-Transfer Character of the Model Compounds. We reported previously1° that the fluorescence spectra of biphenylbisimides in CH2Cl2 depend strongly on the chemical structure of the amine component. The fluorescence and excitation spectra are shown in Figure 6. Compound 4, possessing an alicyclic group on the nitrogen atom, emits a normal fluorescence peaking at 385 nm (@f = 0.05), but in compounds 1-3 possessing alkyl-
J. Phys. Chem., Vol. 98, No. 42, 1994 10775
Photophysical Processes in Aromatic Polyimides
I
500
.
700
600
Wavelength I nm
0 250
Figure 6. Fluorescence and excitation spectra of a series of biphenyl bisimides in CH2Clz at room temperature.
350 Wavelength I nm
400
450
b l
h F,C
300
350
400
t N
CF,
450
Wavelength I nm
400 Wavelength I nm
450
Figure 8. UV-vis absorption spectra of a series of bisimide compounds derived from various dianhydrides and cyclohexylamine in CH&: (a) 5 x M; (b) 5 x M.
Wavelength I nm
Figure 7. W-vis absorption spectra of a series of bisimide compounds derived from various dianhydrides and 3-ethylaniline in M; (b) 5 x M. CHzCl2: (a) 5 x substituted phenyl groups on the nitrogen atom, broad fluorescences were observed at longer wavelength (2500 nm) instead of the normal fluorescence peaking at 385 nm. These are assigned as the intramolecular CT fluorescence because the peak position red-shifts with an increase in solvent polarity. The CT fluorescence for compound 1 derived from BPDA and 3-EA was located at wavelength longer than that for compounds 2 and 3 derived from biphthalic dianhydride (BPDA) and orthoalkyl-substituted anilines. It is widely accepted that for CT complex systems composed of a fixed electron acceptor and a series of donors, both of the CT fluorescence and CT absorption bands red-shift with an increase in the electron-donating character of the donors (namely, increase in the CT chara~ter).~' Hence, judging from the criteria, the intramolecular CT character
decreases in the following order depending on the position of the alkyl substituents on the N-aryl groups and the aromaticity of the amine components: meta- > ortho-alkyl-substituted phenyl group > aliphatic group. With the ab initio method (Gaussian 92, STO-3G), we calculated previously the most stable dihedral angle between the phthalimide plane and the N-aryl group for N-(2,6diethylpheny1)phthalimide (ortho-substituted compound) and N-(3-ethylphenyl)phthalimide (meta-substituted compound) and obtained 73" and 33", respectively.28 The calculated rotational banier indicated that the rotation around N-Ar linkage is nearly free for the metal compound but is practically inhibited for the ortho compound because of the steric hindrance. The effect of alkyl substitution at ortho and meta position on the intramolecular CT fluorescence peak wavelength for biphenylbisimides is rationalized by the calculated results; the meta compound of which the dihedral angle is smaller is probably favorable for the conjugation required for the CT process in comparison with the ortho-substituted compound. Parts a and b of Figure 7 show the UV-vis absorption spectra of a series of bisimides derived from various aromatic dianhydrides with 3-EA and their longer wavelength absorption tails, respectively. As shown in Figure 7b, the broad absorption bands extending up to 450 nm are observed. By contrast, non-CT compounds composed of various dianhydrides with MCHA have little absorption over 400-450 nm as shown in Figure 8. Comparing the wavelength (1)at a constant E (100 M-' cm-') because there is no absorption peak in the longer wavelength region, a linear relation between il-'and the electron affinity EA of the dianhydride components (taken from ref 29), Le., 2-l
Hasegawa et al.
10776 J. Phys. Chem., Vol. 98, No. 42, 1994 I
1
2.9 1
Phthalimide Phenyl group
2.3’ 1.2
1.4
1
1
1.6
1.8
I
2.0
E, J eV b
0
0
0
0
2.4 1.2
1.4
1.6
1.8
1
Phthalimide Phenyl group
Phthallmide Phenyl group
Figure 10. Schematic diagram of the electronic configuration for N(3ethylpheny1)phthalimide. 3
0
E, J eV
Figure 9. Relation between the reciprocal wavelength and the electron affinity EA of the anhydride components: (a) plotted from Figure 7; (b) plotted from Figure 8. The EA values taken from ref 29 are 1.90 for PMDA, 1.55 for BTDA, 1.38 for BPDA, and 1.30 for oxybiphthalic dianhydride.
7
3
2
i -5 \
u t
+
= -EA C (C is a constant) according to Mulliken’s CT theory2’ was observed for the former system as demonstrated in Figure 9a. On the other hand, no linear plot was obtained for the latter system (Figure 9b). Consequently, the existence of the intramolecular CT transition was evidenced for benzophenonebisimides. This transition corresponds to the HOMO LUMO transition in the model compound of PI(PMDA/ODA) (PMDA: pyromellitic dianhydride, ODA: oxydianiline) reported by LaFemina et al.30 The 365 nm incident light used in the present study gives rise to not HOMO LUMO transition corresponding to the excitation at the longer wavelength absorption region, but the photoexcitation of the benzophenonebisimide moiety (electron acceptor), and subsequently, the intramolecular CT process (electron transfer) from the N-aryl group to the benzophenonebisimide moiety is possible to occur at the singlet excited state. This process is termed the photoinduced electron transfer. For simplicity, we explain the process using N-arylphthalimide. Figure 10 shows the schematic diagram for the electronic configuration based on the n-electron density calculated with the semiempirical quantum-chemical AM1 method for N-(3ethylphenyl)phthalimide.** This indicates that the electron transfer subsequent to the excitation of the phthalimide moiety (third HOMO LUMO transition) leads to the CT state, as well as the HOMO LUMO transition. Figure 11 gives that the UV-vis absorption spectra and their absorption tails of benzophenonebisimides with various amine components. The spectral shift for the absorption bands around 300 nm was only slight with varying amine component, suggesting that the conjugation between the phthalimide plane and the N-aryl group is not so large. Therefore, the 365 nm incident light is mainly absorbed by the benzophenonebisimide moiety. We estimated the intramolecular CT character of benzophenonebisimides from the spectral shifts of the longer
0
250
300
450
350 Wavelength I nm
-
b
I!
-
--
350
400
450
Wavelength / nm
Figure 11. UV-vis absorption spectra of benzophenonebisimides derived from BTDA and various monoamines in CH3C12: (a) 5 x lo+ M, (b) 5 x M. (1) M(BTDA/3-EA), (2) M(BTDA/2,4-DMA), (3) M(BTDA/2,6-DEA), (4)M(BTDA/MCHA).
wavelength bands (intramolecularCT bands) as shown in Figure l l b . The model compounds can be arranged in order of decreasing intramolecular CT character as follows: M(BTDA/ 3-EA) M(BTDA/2,4-DMA) > M(BTDA/2,6-DEA) > M(BTDA/MCHA). This order coincides with the result expected from the effect of the amine component structure on the fluorescence peak position for biphenylbisimides in Figure 6. Table 1 suggests that the values of @M are closely related with the intramolecular CT character. With an increase in the
’
Photophysical Processes in Aromatic Polyimides
. I Phys. . Chem., Vol. 98, No. 42, 1994 10777
t
Figure 12. Schematic energy diagram for the model compounds of benzophenone-containing polyimide. CT character, @M decreases parallel to @ISC. The change in @ISC determined on the basis of eq 4 at 20 "C corresponds well to the change in @p measured spectroscopically at 77 K. On the other hand, no correlation of @M with kr and with kd is observed. Assuming that a process competing with the intersystem crossing from S1 to T1 is the intramolecular CT, the results summarized in Table 1 can be satisfactorily explained with a schematic energy diagram depicted in Figure 12. M(BTDA/MCHA) possessing no CT character provided the theoretically highest value of @ISC (=1.0) as observed in BP. Since the intramolecular CT process ( ~ C T> 5 x 10" s-l for compound 1 in Figure 6)'') competes with the intersystem crossing (for example, kcr = 2 x 10" s-l for BP)," @ISC should decrease with an increase in the CT character (increase in k a ) , resulting in a decrease in @M. In fact, M(BTDN3-EA) having the strongest CT character provides the lowest value of @ISC and OM as shown in Table 1. The photoreactivity of benzophenonebisimides is thus strongly affected by the intramolecular CT character associated closely with the conformation around the N-AI linkage. If TI of the model compounds is not of pure (n,n*) (a mixed state of (n,n*) with (n,n*) or with CT state), the photoreactivity should lower markedly compared with BP of which T I is of pure (n,n*) state. For example, 4-phenylbenzophenone of which T1 is of (n,n*), 4-aminobenzophenone and 4,4'-bis(NJJ'dimethy1amino)benzophenone of which T1 are of CT state are extremely less reactive (very low quantum yield for photoreduction) even in isopropyl alcohol.16c We observed a strong phosphorescence (@p = 0.98, z p = 200 ms) of 4,4'-bis(N,iV'dimethy1amino)benzophenone in a rigid glass at 77 K, indicating that @ISC is rather high. Hence, a very low kr is responsible for the extremely low photoreactivity for the compound even in isopropyl alcohol.16c Thus, the mixing of 3CT state and 3(n,n*) lowers k, markedly. But in fact, the value of kr for M(BTDN3-EA) showing the lowest value of @M is no less than that for more reactive M(BTDA/MCHA) possessing no CT character. This leads us to expect that T I is of pure (n,n*) state regardless of the CT character varying with the amine component. The phosphorescence spectra of the model compounds also supports this assignment. Figure 13 indicates that the phosphorescence spectrum of M(BTDA/MCHA) possessing no CT character is similar to those of other model compounds having CT character in both of the spectral shape and peak position. This means that the nature of T I is independent of the intramolecular CT character. There is a criteria'& for assignment of T1 with respect to aromatic carbonyl compounds in which tp < 20 ms at 77 K for Tl(n,n*), zp > 100 ms at 77 K for both Tl(n,n*) and Tl(CT). All the values of t p measured at 77 K for the model compounds were less than 20 ms, corresponding to the Tl(n,n*) state. Consequently, the lower photoreactivity of M(BTDN3-EA) is attributable to not the
na
Figure 13. Phosphorescence spectra of various benzophenonebisimides at 77 K. (a) M(BTDA/MCHA),(b) M(BTDN2.6-DEA).(c) M(BTDA/ 2,4-DMA), and (d) M(BTDN3-EA).
0.4
30.3
. I
P
2 0.2 U
0.1
0.0' 350
'
400
.
'
450
.
'
"
500
550
'
600
Wavelength / nm Figure 14. Transient emission spectra of M(BTDN2.6-DEA) in CH3CN at 20 "C at (a) 150 ns and (b) 500 ns after excitation. mixing of 3(n,n*) and 3CT state but most likely to the decrease in QISC resulting from the increase in kc^. To confirm the photophysical scheme depicted in Figure 12, the transient emission spectra of the model compounds were measured in CH3CN at 20 "C. Although no emissions are observed by means of a normal fluorimeter, the excitation by Nd:YAG laser permitted to observe the transient emission spectra. Figure 14 shows two kinds of transient emission spectra. We believe that the broad emission observed at 150 ns after excitation is the sum of the intramolecular CT fluorescence and the phosphorescence. At 500 ns after excitation, the CT emission disappeared, consequently, the phosphorescence spectrum similar to that observed at 77 K remained. The results support the photophysical processes proposed in Figure 12. Figure 15 shows the TI T, transient absorption spectra of various model compounds in CH3CN at 20 "C. The spectrum of M(BTDN2,6-DEA) resembles to that of M(BTDN2-iPA) reported by Scaiano et al.13 However, the position of the longer wavelength band changes with varying amine component, indicating that the energy levels of the higher energy triplet states depend on the amine component structure, unlike the energy level of T1 is regardless of the amine component. Table 1 also gives the comparison of the photoreactivity of the model compounds with BP. Since the hydrogen abstraction of the triplet BP is an electrophilic reaction, one may expect that the substitution of an electi-on-withdrawing group onto BP enhances its photoreactivity. However, 4-cyano substituent on BP did not enhance actually the hydrogen abstraction efficiency.23b By contrast, substitution of a 4-CF3 group onto acetophenone enhanced its hydrogen abstraction ability up to about 2 times.31 Acetophenone tends to be subjected to a large substituent effect because of the very small energy gap between Tl(n,n*) and T & T , ~ * ) .On ~ ~ the contrary, little effect of an electron-
-
10778 J. Phys. Chem., Vol. 98, No. 42, 1994
Hasegawa et al. of the intramolecularCT process competing with the intersystem crossing followed by photoreduction. T1 was assigned as a pure (n,n*) state for all the model compounds. Benzophenonebisimides have the value of kr no less than that of BP.
0.6
300
0.9
1
400
.
500
600
700
M(BTDA/ZP-DMA)
Acknowledgment. We are indebted to Dr. J. Kawakami for his technical support and Dr. T. Furuta and Prof. M. Iwamura of Toho University for useful discussion. References and Notes
0.0
300
400
500
600
700
400
500
600
700
80.2 0.1
0.0 300
Wavelength / nm Figure 15. Transient T-T absorption spectra of various model compounds in CH,CN at 20 "C at 200 ns after excitation.
withdrawing substitutent onto BP molecule is explained in terms of the larger energy gap between Tl(n,n*) and T2(n,n*). In fact, all of the model compounds possessing two electronwithdrawing imide groups provided kr no less than that of BP as shown in Table 1. Our data differ from both of the results in ref 13 (kr for M(BTDN2-iPA) in isopropyl alcohol is about 10 times larger than that for BP) and ref 14 (k, for M(BTDN 2,4-DMA) in ethylbenzene is about 10 times smaller than that for BP). We believe that benzophenonebisimides provides kr no less than BP unless there is specific interaction with solvents. The low value of @M for M(BTDN:!A-DMA)/ethylbenzene system in ref 14 may be attributed to CT interaction between the triplet state of the model compound and the ground-state ethylbenzene. As mentioned above, if the marked decrease in (PM by the addition of benzene to the CH2Cl2 solution of M(BTDN3-EA) shown in Figure 3b is attributable to physical quenching due to intermolecular CT interaction between the triplet M(BTDN3-EA) and benzene, ethylbenzene having the ionization potential lower than benzene should behave as an electron donor. Such a CT process is known to make complicated the hydrogen abstraction mechanism. For example, the hydrogen-transfer reaction for BP/N,N'-dimethylaniline system was demonstrated to occur via the CT process, and the hydrogen-transfer reaction competes with the deactivation from the CT state which becomes effective with an increase in the solvent p0larity.33.3~ Conclusions The photophysical kinetic parameters of the model compounds of benzophenone-containingpolyimide were determined to discuss the photophysical processes associated closely with the hydrogen abstraction of the model compounds from a hydrogen donor solvent. The quantum yield for photoreduction, @M, depends on the structure of the amine component. The value of @M decreases with an increase in the intramolecular CT character related with the aromaticity of the amine components and the dihedral angle between phthalimide plane and N-aryl group. This could be explained well by the existence
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