Chapter 1
Photophysics, Photochemistry, and Photo-optical Effects in Polymer Solids Kazuyuki Horie
Downloaded by 80.82.77.83 on June 14, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch001
Faculty of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
Photophysics, photochemistry, and optical phenomena of polymers for microelectronics are closely related to one another. By using polyimides as typical examples, it is discussed how photophysics such as charge transfer process affects photochemistry such as photosensitivity of polyimides. Photochemistry such as photoisomerization induces the change in refractive indices providing the phase modulation in optical devices. Microstructure of a polymer liquid crystal is also elucidated by fluorescence spectroscopy.
The interaction of light with materials is one of the fundamental subjects in natural science and technology. Phenomena related to light are called in Japanese "hikari" by using a single word, but there are one word and two prefixes concerned with light in English; "light", "photo-" and "opto-". Photochemistry and photophysics deal with the change in materials by light which usually chemists are interested in, and optics deal with the change in light by materials which has been developed mainly by physicists (i). Both aspects of the interaction between light and materials as well as their hybrid phenomena will become more and more important in the field of polymers for microelectronics and photonics. Typical examples of the materials with hybrid phenomena are photorefractive polymers where non-linear optical effects are induced by the space charge generated by the photoirradiation of photoconductive polymers (2) and a command surface where the alignment of nematic liquid crystals are controlled by photoisomerization of azobenzene layer (3). Several examples of photo- and optofunctional materials are summarized in Table I. Polyimides (PI) are well-known because of their excellent thermal stability, and they have become important as high temperature insulation materials in the microelectronics field. They are also expected to be potential materials for various thermostable microelectronics and photonics devices. In the present paper, we use polyimides as typical materials for studying photophysics, photochemistry, optical phenomena and their relations in polymer solids, and report several recent results of the charge-transfer (CT) fluorescence study of aromatic polyimides, the influence of CT state on the photochemistry of photosensitive polyimides, third-order non-linear optical properties of aromatic polyisoimides, and photochemically-induced optical effects in dye-doped polymer films. Charge-transfer and dimer fluorescence studies have also been applied to polymer liquid crystals.
0097-6156/94/0579-0002$08.00/0 © 1994 American Chemical Society
Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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Photophysics in Polymer Solids
Table I. Typical examples of photo- and opto- functional materials Functions
Example
Downloaded by 80.82.77.83 on June 14, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch001
Photophysical Functions
τ— Organic Photo-Conductors — Electro-Luminescence — Excited Energy Transfer — Scintillators — Fluorescent Probes Photochemical Functions - τ — Photoresists — Photochromism — Photochemical Hole Burning (PHB) — Photoresponsive Polymers — Photocatalysts — Photo-Energy Conversion Optical Functions Optical Fibers, Wave guides Non-Linear Optical Materials Phase & Frequency Modulation Devices liquid Crystals
t
Hikari phenomena · · · The Interaction of light with Materials r-Light -Photochemistry Hikari-f PhotoL
Opto-
L
Photophysics optics
Changes of Materials by Materials • Changes of Light by Materials
Charge Transfer Fluorescence of Aromatic Polyimides Aromatic polyimides (PI) have outstanding thermal stability. Their excellent physical properties are correlated with both the rigidity of the main chains and also strong intermolecular interaction caused by the stacking of aromatic rings. Another important factor affecting the physical properties of aromatic polyimides is the state of aggregation of polymer chains. Typical examples are the high-modulus and highstrength polyimidefilmsdeveloped using thermal imidization of cold drawn polyamic acid. It is especially important therefore to control the molecular aggregation (4). Although the microscopic structures for Pis have been observed by using wideangle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS), these methods are limited to elucidation of crystalline structure and large-scale ordered structure, respectively. Fluorescence spectroscopy is known to be very useful in the
Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
Downloaded by 80.82.77.83 on June 14, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch001
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POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS
investigation of dynamics and structure of solid polymers (5) and is complementary to the X-ray methods. Several aromatic polyimide films are known to show broad and structureless fluorescence spectra with peak wavelengths in 450-600 nm region, considerably longer than those of usual aromatic molecules. Typical fluorescence spectra are shown in Figure 1 (6). As the emission spectra become red-shifted with increasing electronwithdrawing ability of aromatic imide moiety and electron-donating ability of aromatic moiety from diamine, the emission is assigned to be due to an excited-state charge transfer (CT) interaction between imide moiety and aromatic moiety from diamine. When a polyimide, PI(BPDA/PDA), from biphenyltetracarboxylic dianhydride (BPDA) and p-phenylenediamine (PDA) (No. 4 in Figure 1) prepared by stepwise imidization up to 250°C (2hr) was annealed at 330°C for 2hr, as is shown in Figure 2, fluorescence intensity at 525 nm excited by 350 nm irradiation increased 4 times without shift of peak wavelength, showing the intermolecular nature of this CT fluorescence, and a strong newfluorescenceat 540 nm appeared with 465 nm excitation due to the formation of also a ground state intermolecular CT complex (7). A linear increase in the intensity of CT fluorescence with the increase in density of polyimide films due to increase in imidization temperature observed for both PI(BPDA/PDA) and PI(BPDA/ODA), where ODA is oxydianiline, also supports the intermolecular mechanism of the CT complex (7). The study of model bisimides in solution (Figure 3) (8) shows also the intramolecular CT formation in the excited states. Strong fluorescence at 400 nm is observed for the model bisimide with cyclohexylamine, M(BPDA/CHA), (No.4 in Figure 3) which is emitted from the direct excited state of monomer because the sample has no possibility to make either intra- or inter- molecular CT interaction. The model bisimide of BPDA with aniline, M(BPDA/A), is non-fluorescent, suggesting that effective deactivation occurs via strong intermolecular CT formation of the excited state, but M(BPDA/mEA) (No. 1 in Figure 3) shows weak CT fluorescence. The fluorescence spectra of M(BPDA/o-Tol) (No. 2) and M(BPDA/i-PrA) (No. 3) show CT character in excited states although they cannot form coplaner structure. Spectroscopic studies of pyromellitic polyimides (No.l in Figure 1) and their model compounds discussing the intramolecular CT absorption or fluorescence have also been done by a few groups (9-11).
300
400 500 600 700 Wavelength (nm)
800
Figure 1. Fluorescence Spectra (300 nm excitation) for Several Polyimide Films (6).
Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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HORIE
300 Downloaded by 80.82.77.83 on June 14, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch001
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Photophysics in Polymer Solids
400
500 600 Wavelength (nm)
Figure 2. Effects of annealing (330°, 2hr) on the emission and excitation spectra of PI(BPDA/PDA). — (a) excitation (monitored at 530 nm) and (b) emission (excited at 350 nm) spectra of PI without annealing; — (c) excitation spectrum (monitored at 530 nm) and emission spectra of PI excited at 350 nm (d) and 465 nm (e) with annealing (Reproduced with permissionfromRef. 6). El
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300
600 400 500 W a v e l e n g t h (nm)
4
700 )
Figure 3. Excitation and fluorescence spectra of several model compounds in solution (8).
CH2G2
Photochemistry of Benzophenone-type Photosensitive Polyimides Benzophenone-containing polyimide prepared from benzophenonetetracarboxylic dianhydride (BTDA) and bis(4-amino-3-ethylphenyl)methane (DEDPM) is known to have excellent negative photoresist properties as well as thermal stability. Photophysical processes for the photocrosslinking reaction of PI(BTDA/DEDPM) are shown in Figure 4 (12). An excited triplet state benzophenone moiety is formed via singlet state by the absorption of a photon. A part of the excited state is deactivated back to the ground state. The excited triplet state benzophenone moiety abstracts a hydrogen atomfroman ethyl group of a neighboring polymer chain, resulting in two radicals; a ketyl radical and a benzyl radial. Some of them disproportionate and others
Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS
combine with each other to form a crosslink. The quantum yields for the photocrosslinking of PI(BTDA/DEDPM), Φ , are obtained by measuring the change in their molecular weights during photoirradiation by using GPC, and Φ under air at room temperature was obtained to be 0.001. Table II shows the quantum yields for photoreaction of model compound, M(BTDA/DMA), and benzophenone, BP, in ethylbenzene solution, and the corresponding kinetic data. The quantum yields for photoreaction for M(BTDA/DMA) under air, ir, and in vacuum,
