Photochromic dopant

R.P. Lemieux, Photoswitching of ferroelectric liquid crystals using photochromic dopants. Soft Matter 1, 348-354 (2005)... 

T. Yamaguchi, T. Inagawa, H. Nakazumi, S. Irie, M. Irie, Phase transition of a liquid crystal induced by chiral photochromic dopants. Mol. Cryst. Liq. Cryst. 345, 287-292 (2000)... 

Barmatov and coworkers took an approach of forming photo-optically active polymers by using hydrogen bonding between functionalized liquid crystalline copolymers and low-molecular-mass dopants. The photochromic dopants used by them contain azobenzene components ... 

Scheme 1. The photochromic dopant spiropyran (SP) and the stable forms of its isomer merocyanine (MC).

The general concept of preparing such materials involves the synthesis of combined chiral-photochromic LC copolymers or obtaining cholesteric mixtures based on nematic polymers and low-molecular-mass chiral and photochromic dopants. 

Figure 22 schematically represents (a) light control over the helical stmcture of (b) a cholesteric photochromic polymer through the use of a copolymer containing the combined chiral-photochromic group, and (c) a mixture of a nematic homopolymer with low-molecular-mass chiral-photochromic dopant. 

Trancong, Q., Chikaki, S., and Kanato, H. (1994) Relationship between reori-entational motions of a photochromic dopant and local relaxation processes of a glassy polymer matrix. Polymer, 35, 4465-4469. 

Precise control of orientation may be difficult in PPT systems, because the system is rather based on destruction of organized structure induced by a large topological change of photochromic dopant and self-organizing character of the LC phase. 

Photochemical polarization flip was immediately extended to antiferroelectric LCs (AFLCs), and it was found that in azobenzene guest/AFLC host systems, polarization flip could be induced photochemically and effectively, as shown in Figure 3.1331 Novel photochromic molecules to induce polarization flip effectively have been explored, and molecular design on the basis of large Ps resulting from chiral cyclic carbonates has been found to be quite effective. An azobenzene dopant containing a... 

Photochromic processes are often observed both in solution and in the solid state, thus making for facile incorporation of photochromies in films, in membranes, and as dopants in host matrices—prerequisites for the construction of molecular optoelectronic devices. Section 2.3.1 focuses on the materials and supramolecular systems prepared from photochromic systems. For more comprehensive descriptions of the basic photochemical processes the reader is referred to any of the numerous reviews on the subject [47, 51, 89, 159-162]. 

The most basic photochromic systems are those that undergo a light-induced structural rearrangement. Isomerizations often involve large nuclear rearrangements which, for example, can change the symmetry or convert from a linear to bent structure. This property is especially useful for doping of liquid crystals and thin films, in which the microscopic structure of individual dopant molecules can be used to modulate the macroscopic properties of a host system. 

The photochromic properties of naphthopyrans <05PHC33> and spiropyrans <05CHE281> have been reviewed. Electronic materials which incorporate six-membered O-and iS-heterocycles include derivatives of (2,6-dimethyl-47/-pyran-4-ylidene)propanedinitrile used as dopants for red organic LEDs <05JMAC2470>. Reviews of the organosulfur donor... 

Ruslim C, Ichimura K. 2000. Conformational effect on macroscopic chirality modification of cholesteric mesophases by photochromic azobenzene dopants. J Phys Chem B 104(28) 6529 6535. 

Azobenzenes are a well-known family of photochromic compounds that can isomerize from its tmns form to cis form upon UV irradiation (Fig. 5.4a). The cis isomer can be switched back to trans form either by visible light or thermal relaxation. The rod shape of the trans form can stabilize calamitic LCs, while the cis form is bent and normally decreases the order parameters of LC phases. Owing to the dramatic molecular shape and property change between the trans and cis isomers, azobenzenes are the most investigated photochromic molecules to function as either mesogens or dopants in chiral LCs. 

There are two ways to generate photoresponsive chiral SmC LCs. The simple one is to add a photochromic molecule to SmC LCs and the other is based on chirality amplification, i.e. using a photoresponsive chiral dopant in SmC LCs to induce a phototunable SmC phase. Some examples of mesogens that showing SmC or SmC are presented in Fig. 5.22. 

A photochromic dianthryl molecule can act as a triplet energy transfer quencher when combining with a ruthenium diimine complex [123]. UV excitation of the systems (X = CH2O, C(O)O) results in a photoinduced cycloaddition of the dianthryl unit, leading to an increase of the luminescence intensity. Near-field optical addressing of this luminescent photoswitcheable system (X = CH2O) as a dopant in PMMA films has been demonstrated [124] (Fig. 14). 

The formation of hydrogen bonds between the carboxylic acid groups of the functionalized liquid crystal copolymers and the pyridine portion of the dopants leads to stable, none separating mixtures. In mixtures containing up to 30 % of the dopants no separation was observed. Induction of a nematic mesophase is observed in the case of a smectic polymer matrix doped with low molecular weight photochromic dyes. 

With the polymer chemical industry being the largest of its kind and the enormous impact of polymers on our daily life, the incorporation of photochromic dyes as dopants in polymeric matrices or as integral components in the polymeric structure is highly desirable. In this section, we discuss how photoresponsive dithienylethenes can be integrated with polymer science by briefly describing both polymer structure and materials processing. We will include in our discussions homopolymers, random copolymers and molecular dopants and will start with the last case after a brief mention of some requirements. 

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