Photophysics, photochemistry, and optical properties of polyimides

Abstract

This review discusses charge transfer (CT) interactions in wholly aromatic polyimides (PI). Discussion was extended to the influence of CT interactions on PI properties, i.e. photophysics, photochemistry, optical properties, and other properties. Section 2 deals with the electronic states and photophysics in PIs. Most of the experimental and theoretical data evidenced the presence of both intra- and interchain CT interactions in wholly aromatic PIs. A model compound approach made it possible to understand the detailed intramolecular photophysics including the intramolecular CT process. However, it is still a long way from obtaining quantitative information such as the concentration (population) and thermodynamic parameters of intermolecular CT complex (CTC) present in the PI solids. Wholly aromatic PIs show a broad/structureless fluorescence in a long-wavelength range, originating from intermolecular CTC. A few papers revealed that the CT fluorescence intensity increased corresponding to the degree of chain packing/staking. Thus, the CT fluorescence was applied to studies on local ordering and binary PI blend miscibility. In Section 3, the influence of CTC on photoconductivity, mechanical and other properties is discussed. Low molecular weight donor-loaded PI exhibited prominent photoconductivity enhancement. Transient absorption measurements revealed that the CTC formed between the added donor and the acceptor moiety in the PI chains is responsible for the photoinduced charge generation process. The donor-free PI film also showed a similar transient absorption spectrum to the donor-loaded PI film, suggesting that even wholly aromatic PI itself can form CTC. The CTC sites in PIs may act as physical crosslinks. But it is not yet clear how the thermal and mechanical properties (glass transition temperature, thermoplasticity or melt viscosity, modulus, etc.) of PI films are affected by CTC formation. A paper reported that CT-inhibited PIs, such as semi-aromatic PIs, tend to show a much higher voltage holding ratio, which is desirable for liquid crystal displays, than the corresponding wholly aromatic PIs. Section 4 reviews photosensitive PIs and their precursor systems. In this section, photochemistry in an intrinsically photosensitive polyimide containing a benzophenone unit and photodegradation in fluorine-containing PIs are also described. For benzophenone-containing PIs, the model compound approach was carried out to depict a detailed energy diagram including some photochemical kinetic parameters. The use of a series of model compounds elucidated a prominent conformation effect on the photoinduced hydrogen abstraction efficiency. The control of transparency and refractive indices and birefringence of PI films and molecular-design were focused in Section 5. This section also describes why the control of such optical properties has been demanding for practical applications. Finally, non-linear optical (NLO) properties of several PIs and their related systems, modified by attaching NLO chromophores, were reviewed.