Organic solids and polymers that absorb in the near-infrared (NIR) region (1000-2000 nm) represent a class of emerging materials and show a great potential for use in photonics and telecommunications. The radical anions of stacked aromatic imides, fused phorphyrin arrays, polythiophenes, sandwich-type lanthanide bisphthalocyanines, semiquinones, and mixed-valence dinuclear metal complexes are a few known examples of NIR-absorbing organic materials. Most of these NIR-absorbing materials are also electrochemically active or electrochromic (EC). This brief review covers several types of NIR-absorbing organic materials and discusses their potentials for applications in EC variable optical attenuators (VOAs).

Soluble organoiron polyethers, thioethers, and amines were synthesized via nucleophilic aromatic substitution reactions. The synthesis of these classes of organometallic polymers involved either the reaction of cyclopentadienyliron complexes of dichloroarenes with various oxygen and sulfur dinucleophiles or the reaction of etheror amine-containing diiron complexes with dithiols. Polymerization reactions with the diiron complexes gave rise to organoiron polymers with alternating ether/thioether or amine/thioether bridges. Removal of the iron moieties from the backbone of these polymers allowed for the production of the corresponding organic materials. Furthermore, the organometallic polymers had much higher solubilities than their organic analogues. Thermogravimetric analysis of the organoiron polymers indicated that the polymers lost their metallic moieties at approximately 200°C, whereas degradation of the polymer backbones occurred around 500°C.

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Nucleophilic aromatic substitution reactions of chloroarene cyclopentadienyliron complexes were utilized to prepare new classes of oligomers and polymers containing both neutral and cationic organoiron complexes in their structures. Photolysis of these polymers resulted in the removal of the cationic cyclopentadienyliron moieties, while the neutral organoiron complexes remained intact within the polymer structures. The weight-average molecular weights of these polymers after photolysis ranged from 8700 to 56 200 with polydispersities from 1.1 to 3.1. Thermal analysis established that the cationic polymers possess higher glass transition temperatures, but lower thermal stability than the neutral ferrocene-based polymers. The glass transition temperatures of the cationic polymers ranged from 65 to 161 °C, while the T gs of the neutral polymers ranged from 10 to 92 °C. Electrochemical studies showed that the iron centers in the neutral complexes were oxidized, while the cationic complexes were reduced. Viscosity studies showed that the cationic polymers exhibited a polyelectrolyte effect.