Highly transparent and birefringent chromophores for organic photorefractive materials

Organic Semiconductors based on Dyes and Color Pigments

Organic dyes and pigments constitute a large class of industrial products. The utilization of these compounds in the field of organic electronics is reviewed with particular emphasis on organic field-effect transistors. It is shown that for most major classes of industrial dyes and pigments, i.e., phthalocyanines, perylene and naphthalene diimides, diketopyrrolopyrroles, indigos and isoindigos, squaraines, and merocyanines, charge-carrier mobilities exceeding 1 cm(2) V(-1) s(-1) have been achieved. The most widely investigated molecules due to their n-channel operation are perylene and naphthalene diimides, for which even values close to 10 cm(2) V(-1) s(-1) have been demonstrated. The fact that all of these π-conjugated colorants contain polar substituents leading to strongly quadrupolar or even dipolar molecules suggests that indeed a much larger structural space shows promise for the design of organic semiconductor molecules than was considered in this field traditionally. In particular, because many of these dye and pigment chromophores demonstrate excellent thermal and (photo-)chemical stability in their original applications in dyeing and printing, and are accessible by straightforward synthetic protocols, they bear a particularly high potential for commercial applications in the area of organic electronics.

Effect of polarity and structural design on molecular photorefractive properties of heteroaromatic-based push-pull dyes

A combined experimental (optical and electro-optical absorption measurements) and computational (ab initio RHF and DFT) approach has been used to investigate the molecular low- and high-T(g) photorefractive (PR) performances of neutral and zwitterionic heteroaromatic dipolar chromophores in terms of structural and solvent-polarity effects. We have found that the nature of the building units (donor, acceptor, and spacer) and the polarity of the surrounding medium strongly affect all the relevant ground-state and nonlinear optical properties involved in the PR activity, that is, the dipole moment, the polarizability anisotropy, and first hyperpolarizability of the electronic ground-state. The variation of these properties is in turn transferred to molecular low- and high-T(g) PR figures of merit. It is shown that PR molecular performance not only relies on a proper choice of structural components but varies by orders of magnitude as a function of the medium polarity, and this suggests that a combination of molecular design and host-matrix engineering is required for optimized performances of PR materials.

Chromophore design for photorefractive organic materials

During the last years, significant progress has been achieved in understanding the mechanism of the photorefractive effect in amorphous organic materials. New chromophores could be devised which provided a substantial increase in the electrooptical response and lead to photorefractive materials with unprecedented refractive index modulation (delta n = 10(-2) at E = 28 V micron-1) and two-beam coupling gain. These improvements could only be accomplished by optimizing the electronic structure of highly conjugated merocyanine dyes to perfectly balanced dyes in the charge resonance limit (such as aminothienyl oxopyridone (ATOP) and indoline dimethine oxopyridone (IDOP) derivatives), considering effects of supramolecular ordering (dipolar aggregation), and adjusting the compatibility of the dyes to photoconducting polymers (like poly-N-vinylcarbazole). In particular, optimized glass-forming dyes (such as 2BNCM and ATOP-4) combine the dual functionalities of charge transport and electrooptical response and exhibit photorefractivity even in absence of any additional photoconductor.

Dual-use chromophores for photorefractive and irreversible photochromic applications

Holographic experiments are performed on a series of dual-use chromophore molecules wherein both irreversible photochromic and erasable photorefractive holographic gratings can be written in the same storage volume. At 675 nm, the chromophore undergoes a photochemical reaction leading to the creation of irreversible holographic gratings. Alternatively, at longer wavelengths, application of an electric field during grating formation allows the storage of erasable photorefractive holograms in the same location as previously stored permanent photochemical holograms. Photochemical gratings (eta > 60%) can be written in less than 1 min, whereas photorefractive gratings (eta > 50%) can be written in less than 1 s. The photochemical gratings have a diffusion-limited dark half-life of as long as two weeks depending on the glass transition temperature of the composite.

ATOP dyes. optimization of a multifunctional merocyanine chromophore for high refractive index modulation in photorefractive materials

This paper reports synthesis, characterization and structural optimization of amino-thienyl-dioxocyano-pyridine (ATOP) chromophores toward a multifunctional amorphous material with unprecedented photorefractive performance. The structural (dynamic NMR, XRD) and electronic (UV/vis, electrooptical absorption, Kerr effect measurements) characterization of the ATOP chromophore revealed a cyanine-type pi-conjugated system with an intense and narrow absorption band (epsilon(max) = 140 000 L mol(-)(1) cm(-)(1)), high polarizability anisotropy (deltaalpha(0) = 55 x 10(-)(40) C V(-)(1) m(2)), and a large dipole moment (13 D). This combination of molecular electronic properties is a prerequisite for strong electrooptical response in photorefractive materials with low glass-transition temperature (T(g)). Other important materials-related properties such as compatibility with the photoconducting poly(N-vinylcarbazole) (PVK) host matrix, low melting point, low T(g), and film-forming capabilities were optimized by variation of four different alkyl substituents attached to the ATOP core. A morphologically stable PVK-based composite containing 40 wt % of ATOP-3 showed an excellent photorefractive response characterized by a refractive index modulation of Deltan approximately 0.007 and a gain coefficient of Gamma approximately 180 cm(-)(1) at a moderate electrical field strength of E = 35 V microm(-)(1). Even larger effects were observed with thin amorphous films consisting of the pure glass-forming dye ATOP-4 (T(g) = 16 degrees C) and 1 wt % of the photosensitizer 2,4,7-trinitro-9-fluorenylidene-malononitrile (TNFM). This material showed complete internal diffraction at a field strength of only E = 10 V microm(-)(1) and Deltan reached 0.01 at only E = 22 V microm(-)(1) without addition of any specific photoconductor.