Chapter 1
A Historical Overview of Photomechanical Effects in Materials, Composites, and Systems

Toru Ube and Tomiki Ikeda

Research and Development Initiative, Chuo University, Tokyo, Japan

1.1 Introduction

Photomechanical effects in materials are a topic of considerable recent research. Many papers are continually appearing in top-ranked journals reporting novel materials, demonstrations of distinctive mechanical outputs, and initial demonstrations of device utility. This book is a comprehensive review of the material development, fundamental science (photochemistry, optics, and mechanics), and application of photomechanical effects in materials. This chapter provides an overview of the historical development of the simple yet captivating idea of photomechanical energy conversion in materials. In this way, the reader will have a general awareness of the interrelated nature of the topics and themes discussed throughout the subsequent chapters.

1.1.1 Initial Studies of Photomechanical Effects in Materials

Historians might argue that the first implementation of photomechanical effects in materials was the invention of the sundial by the ancients. It is inarguable, however, that humankind has sought to harvest this plentiful resource. Many of these pursuits have found their inspiration in nature in which countless species have adapted to use and leverage light-induced motility (photomechanical effects) to harvest more energy (sunflower), protect sensitive leaves (circadian rhythm plants), or even camouflage (chameleon, cephalopods).

The emergence of the potential utility of photomechanical effects in the modern era can largely be attributed to the famous American inventor Alexander Graham Bell and his work in the late 1800s [1]. After Bell invented the practical telephone, he shifted his focus on the development of a photophone to enable communication without the necessity of a conducting wire between a transmitter and a receiver (Figure 1.1). To accomplish this, Bell used a crystalline material (selenium) as a component of a receiver, which was connected in a local circuit with a battery and an electroacoustic transducer. The sound emission changes depending on the state of light through a variation in resistance of selenium. The photophone Bell envisioned is the basis of optical communication and realized in recent times in practical applications enabled by the development of optical fibers and lasers [2]. Bell subsequently investigated nonelectronic photoresponsive receivers to make light audible without the aid of electricity. He found that diaphragms of various substances (metals, rubbers, paper, etc.) produce sounds when irradiated with light. This phenomenon is explained in terms of a vibration of the diaphragm, which is caused by a local, photoinduced temperature rise and a corresponding change in thermal expansion of the material. Recent examinations of photoacoustic tomography extend upon this fundamental tenet pursued by Bell [3]. Accordingly, Alexander Graham Bell can be considered as the originator and "father" of photomechanical effects in materials in the modern era.

Figure 1.1 Schematic illustration of a photophone proposed by A. G. Bell. LS, light source; M, mirror; L, lens; H, heat absorber; S, sound; FR, flexible reflector; C, crystal; PR, parabolic reflector; B, battery; T, electroacoustic transducer.

1.1.2 Research of Photomechanical Effects in Materials - 1950-1980

Stimuli-induced deformation of materials has attracted much attention since the 1950s. The most responsive form of these materials is a polymer gel, which consists of a cross-linked polymer network and solvent. Kuhn, Katchalsky, and coworkers demonstrated expansion and contraction of hydrogels containing carboxyl groups by successive addition of alkali and acid [4]. The carboxyl groups ionize and deionize depending on the pH, leading to the change in intramolecular electronic repulsion and subsequent expansion and contraction of polymer chains. This conformational change at a molecular scale is translated to macroscopic deformation. Subsequently, various types of the so-called smart materials have been developed, which deform when subjected to stimuli such as heat, electricity, light, magnetic field, and humidity [5].

Photoresponsive materials have potential advantages compared to these other stimuli. Light is a comparably "smart" stimulus allowing for remote and wireless controllability with spatial selectivity and also direct control of response magnitude via variation of intensity, wavelength, or even polarization. Initial research activities of photomechanical effects in polymeric materials were undertaken in the 1960s. The general approach of these initial studies remains largely unchanged today, focused on incorporating photoresponsive moieties into polymeric or crystalline materials.

Figure 1.2 Typical photochromic molecules used to induce photomechanical effects: (a) azobenzene, (b) spiropyran, (c) fulgide, and (d) diarylethene.

By far, the most common approach to sensitizing polymeric materials to light is to functionalize these materials with azobenzene. Azobenzene is a common dye molecule and widely known to photoisomerize between a thermally stable trans isomer and a metastable cis isomer (Figure 1.2) [6]. Generally, trans-azobenzene isomerizes to the cis isomer upon irradiation with UV light, whereas cis-azobenzene reverts to the trans isomer upon irradiation with visible light or heating. The isomerization of azobenzene produces a variety of changes in properties such as molecular shapes and polarity. Photochromic behavior and applications of azobenzene derivatives have been actively studied since the isolation of the cis isomer in 1937 [7]. The photochemistry of azobenzene and other chromophores employed to generate photomechanical effects is exhaustively detailed in Chapter 2.

In 1967, Lovrien predicted that light energy could influence the conformation of polymer chains if photochromic molecules such as azobenzene were parts of polymers or bound to them [8]. In this seminal work, Lovrien proposed four strategies to achieve a conversion of light energy into mechanical energy. (i) Use of a polymer electrolyte solution containing azobenzenes in side chains (Figure 1.3a). trans-Azobenzenes in the side chains tend to contract polymers by hydrophobic interaction. When irradiated with light, the hydrophobic interaction within the side chains decreases with trans-cis isomerization and results in a local expansion of the spacing of the polymer chains driven by Coulombic interaction. (ii) Use of solutions composed of polymer and azobenzene electrolytes (Figure 1.3b). In this approach, the polymer chains are spaced by electronic repulsion between trans isomers, which Lovrien suggested would assemble on the chains. Upon trans-cis isomerization with light irradiation, the polymer chains could organize into neutral coil conformation upon liberation of azobenzenes from chains. (iii) Incorporation of photoisomerizable groups in the backbone of polymer chains. (iv) Introduction of photoisomerizable cross-links so that light can govern the distance between chains. Experimentally, Lovrein investigated the first two approaches: a polymer electrolyte solution containing azobenzene chromophores in the side chains and a polymer solution blended with azobenzene electrolytes. In both systems, photoinduced changes in viscosity were observed. This effect is ascribed to the conformational change of the material system, which was correspondingly amplified to macroscopic deformation or force. Thereafter, van der Veen and Prins prepared a water-swollen polymer gel containing a sulfonated azostilbene dye (chrysophenine) [9]. The presence of cross-links enables the translation of microscopic changes in conformation into macroscopic deformation of gels. These authors observed shrinkage as much as 1.2% upon irradiation with UV light.

Figure 1.3 Systems for photoinduced deformation of polymer chains proposed by Lovrien. (a) Polymer electrolyte functionalized with azobenzene moieties. (b) Blend solution composed of polymer and azobenzene electrolytes.

Photomechanical effects of dye-doped polymers were also observed in bulk polymeric systems. Merian first reported the photoinduced deformation of polymer fibers containing photochromic molecules [10]. Azobenzene is a common dye molecule, and in the course of using an azobenzene derivative to dye hydrophobic fibers, Merian found that the dyed nylon fiber shrank about 0.1% upon irradiation with light. He attributed this macroscopic dimensional change to the conformational change of the azobenzene moieties. Agolini and Gay observed macroscopic deformation of about 0.5% and measured photogenerated stresses when azobenzene-functionalized polyimide films were exposed to light [11]. Smets and de Blauwe reported deformation of polymer networks containing spirobenzopyran as photochromic cross-linkers, confirming that photomechanical effects in polymeric materials are not limited to azobenzene chromophores [12]. The photomechanical response of polymeric materials and gels prepared from conventional morphologies (amorphous, semicrystalline) is detailed in Chapter 4.

In these early examinations of photomechanical effects in polymeric systems, the corresponding mechanism was solely ascribed to photochemical processes. However, heat generated by nonradiative deactivation process could also cause macroscopic deformations of these...