Photoluminescence color

Abstract We describe mechanochromic and thermochromic photoluminescent liquid crystals. In particular, mechanochromic photoluminescent liquid crystals found recently, which are new stimuli-responsive materials are reported. For example, photoluminescent liquid crystals having bulky dendritic moieties with long alkyl chains change their photoluminescent colors by mechanical stimuli associated with isothermal phase transitions. The photoluminescent properties of molecular assemblies depend on their assembled structures. Therefore, controlling the structures of molecular assemblies with external stimuli leads to the development of stimuli-responsive luminescent materials. Mechanochromic photoluminescent properties are also observed for a photoluminescent metallomesogen and a liquid-crystalline polymer. We also show thermochromic photoluminescent liquid crystals based on origo-(/ -phenylenevinylene) and anthracene moieties and a thermochromic photoluminescent metallocomplex. 

Kozhevnikov, Bruce, and coworkers described an LC N,C,N-coordinated plati-num(II) derivative 4 showing mechanochromic luminescence (Fig. 4) [31]. Compound 4 shows a hexagonal columnar phase up to 145 °C and another hexagonal columnar phase up to 220 °C. A pure film of compound 4 shows a red excimer-like emission at room temperature. Interestingly, the photoluminescent color of compound 4 changes from red to yellow by annealing at 110 °C and the yellow photoluminescence is preserved after cooling to room temperature. The red... 

Fig. 6 Molecular structure of compound 6 and a change of its observed photoluminescent color on the nematic-smectic liquid-crystalline phase transition...

We have recently reported that compound 7a shows a smectic-smectic phase transition associated with the change of the photoluminescent color (Fig. 7) [33]. Compound 7a has a 2,6-diethynylanthracene group as a photoluminescent core. This molecule has fork-like mesogens which consist of tetra(ethylene oxide) and p-(4-/ra .v-pentylcyclohexyl)phenyl moieties. A similar molecular design was previously applied to induce smectic liquid crystallinity for rotaxanes and catenanes [39—41 ]. 

Although the property of interest in all of the above-mentioned cyano-OPV-containing materials was a mechanically induced change of the photoluminescence color (i.e., a mechanically induced transformation from an excimer-rich to a monomer-dominated emission), certain cyano-OPVs also display a changed of their absorption color upon (dis)assembly. Thus, Kunzelman et al. reported mechanochromic blend films based on C18-RG and either poly(ethylene... 

Beyond cyano-OPVs, several other dyes that form excimers in their aggregated state have also been used for mechanosensing purposes. Pucci and coworkers have reported that poly(propylene) (PP) films containing bis(benzoxazolyl)stilbene (BBS) (Fig. 9a) change photoluminescence color upon tensile deformation [50]. In the as-prepared state, the emission spectrum (Fig. 9b) shows a broad excimer emission band around 500 nm, along with sharper monomer emission bands at 410, 430, and 455 nm. Mechanical deformation lowers the intensity of the excimer band, resulting in a photoluminescence color change (Fig. 9c). 

Fig. 11 Schematic illustration of polylactic acid (PLA) beads with mechanoresponsive luminescent micelles on the surface and change of their photoluminescent color upon application of a mechanical force. The pictures were taken under excitation with ultraviolet light. Adapted with permission from [56]. Copyright 2014 American Chemical Society...

Fluorescent small molecules are used as dopants in either electron- or hole-transporting binders. These emitters are selected for their high photoluminescent quantum efficiency and for the color of their emission. Typical examples include perylene and its derivatives 44], quinacridones [45, penlaphenylcyclopenlcne [46], dicyanomethylene pyrans [47, 48], and rubrene [3(3, 49]. The emissive dopant is chosen to have a lower excited state energy than the host, such that if an exciton forms on a host molecule it will spontaneously transfer to the dopant. Relatively small concentrations of dopant are used, typically in the order of 1%, in order to avoid concentration quenching of their luminescence. 

Figure 17.1 (A) S ize-dependent photoluminescence colorofZnS-shelled CdSe quantum dots. (B) Schematic presentation of size in A, color, and photoluminescence spectral maxima of CdSe quantum dots. (C) Size-dependent absorption (solid lines) and photoluminescence (broken lines) spectra of CdSe quantum dots. Reprinted with permission from references [4] (A) and [5] (C) copyright [1997, 2001], American Chemical Society.

Figure 17.2 (A) Absorption and photoluminescence spectra of CdSe quantum dots prepared from CdO, CdC03, and Cd(AcO)2 in the presence of different ligands. (B) Increase in the optical density (at 400 nm) of a CdSe quantum dot reaction mixture with time under reaction at 75 "C. Color pictures in the inset of B represent CdSe (a,b) andCdSe-ZnS (c) quantum...

Figure 11.2. Nanowire electronic and optical properties, (a) Schematic of an NW-FET used to characterize electrical transport properties of individual NWs. (inset) SEM image of an NW-FET two metal electrodes, which correspond to source and drain, are visible at the left and right sides of the image, (b) Current versus voltage for an n-type InP NW-FET. The numbers inside the plot indicate the corresponding gate voltages (Vg). The inset shows current versus Vg for Fsd of 0.1 V. (c) Real-color photoluminescence image of various NWs shows different color emissions, (d) Spectra of individual NW photoluminescence. All NW materials show a clean band-edge emission spectrum with narrow FWHM around 20nm. (See color insert.)...

K.Y. Musick, Q.-S. Hu, and L. Pu, Synthesis of binaphthyl-oligothiophene copolymers with emissions of different colors systematically tuning the photoluminescence of conjugated polymers, Macromolecules, 31 2933-2942, 1998. 

Sun YP et al (2006) Quantum-sized carbon dots for bright and colorful photoluminescence. J Am Chem Soc 128 7756-7757... 

Fig. 10.5 Photoluminescence spectra of dissolved C6Q. The peak at 750nm is the photoluminescence signature of the material. Photoluminescent C60 aggregates can be visualized within cells and exhibit a vivid red signature which is easily detectable (Levi et al., 2006) (See Color Plates)...

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