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Low-observable technology, often known as stealth technology, hides soldiers, aircraft, ships, submarines, missiles, satellites, and ground vehicles from radar and infrared sensors (preferably invisible). Stealth coating reduces radar cross section and makes aeroplanes harder to detect. Stealth planes employ radar-absorbing polymers. Stealth technology camouflages vehicles and buildings from radar. A radar-absorbent material may alter an items radar cross section at specific radar frequencies, but it does not make it \"invisible\" at any frequency. Stealth technology reduces radar reflections using radar-absorbing materials and geometry. Polymer composite-based stealth are graphene, carbon black, carbon nanotubes, and carbon fibres as carbonaceous material and filler. Stealth coating resin market size is based on epoxy, polyurethane, and polyimide. It also protects automobiles against ultraviolet rays, chips, scratches. Tanks and ships employ stealth coating technology to avoid hostile radar. The coating absorbs and scatters radar radiation, hiding the device. Military aircraft uses stealth coating to avoid radar. Commercial airlines are using stealth coating technologies to improve safety and security. The coating reduces aircraft drag and weight, improving fuel economy. Stealth coating absorbs radar signals, hiding the aircraft from radar detection systems. Military and private planes utilise it for security. Stealth coating reduces radar detection by absorbing or deflecting radar emissions. Military operations need stealth and secrecy, but civilian aviation may employ this technology to avoid hijackings and identify unlicensed planes radar-absorbent polymers cover stealth aircraft. These and other design features may weaken the aircraft's radar signal.

Dibenzo crown ether-substituted units of fluorene and acene-bearing copolymers have been successfully synthesized. FTIR, 1 H and 13 C NMR spectroscopy, DSC, TGA and GPC were used to characterize the resulting copolymers (P1–P5). With fascinating quantum yield, the polymers showed large band gaps. With a polydispersity index of 1.58–2.21, these polymers showed a higher molecular weight (16,080–20,480). Strong thermal stability was indicated by DSC and TGA studies. The cyclic voltammetry analysis found that these polyfluorenes were stable in the range of 0.43–0.53 eV under redox conditions with HOMO and LUMO. For light-emitting materials, thermal stability and emission studies display strong results that are desirable. Graphic abstract

The efficient photoluminescent polymers containing crown-substituted fluorene and diphenyl anthracene units were synthesized by aromatic nucleophilic substitution reaction. The light-emitting polymers contain benzo-15-crown-5 groups at the C-9 position of fluorene and diphenyl anthracene-emitting segments in the main chain. Introduction of a crown ether group in polymer backbone leads to its high thermal stability at varying temperature ranges and good solubility in common organic solvents such as CH 2 Cl 2 , CHCl 3 , THF, and toluene. The novel combination of pendant crown ether and anthracene moiety enhanced the electroluminescence of polymers. The synthesized polymers were effectively characterized by elemental analyses, GPC, TGA, FTIR, 1 H, 13 C NMR spectroscopy and UV–Visible spectroscopy. The electrochemical measurements and optical properties of the polymers were also studied. The obtained polymers showed good photoluminescence and electroluminescence properties with blue fluorescence.

In the present study, we report the synthesis of a series of anthracene based polyfluorenes containing alkyl substituted 9,10-diphenylanthracene and hydrazone substituted fluorene moieties. The polymers were synthesized via copper catalysed Ullmann coupling, which comparatively is inexpensive as compared to palladium and platinum used in Suzuki coupling. The synthesized polymers were characterized by various spectroscopic techniques. All the polymers exhibited blue emission having band gap in the range of 2.7–2.83 eV. The polymers showed good thermal stability with decomposition temperature over 330 °C and glass transition temperature in the range of 125–140 °C. All the polymers were soluble in common organic solvents with weight average molecular weight in the range of 21,000–25,000. The electrochemical study reveals that the HOMO energy levels of the polymers were in the range of − 5.16 to − 5.26 eV which had elevated compared with that of polyfluorene (5.7 eV). It matched the work function of ITO and ITO/PEDOT: PSS (4.7 and 5.0 eV respectively). These results indicated that the synthesized polymers could be promising materials for applications in light emitting diode.

5,8-Dibromo-13,13-bis(alkyl)dibenzofluorene derivatives functionalized with different alkyl molecules [M2a–M2f] and 2-tert butyl-9,10-di ( p -hydroxyphenyl) anthracene [M1b], were designed to research the impact of molecular structure on the optical properties of polymer. Polymers were synthesized by aromatic nucleophilic substitution in moderate to strong yields (60–65%). After structural characterization through several techniques (e.g. FTIR, 1 H, and 13 C-NMR), special emphasis was put on the study of their optical properties through UV–Vis and photoluminescence spectroscopy. The polymers demonstrated strong thermal stability up to 350 °C and high glass transition temperature (108–133 °C). All of the polymers showed blue emission within a range of 416–433 nm and had a band gap of 2.87–2.99 eV. The electrochemical study reveals that, HOMO levels for polymers were estimated in the range of − 5.22 to − 5.26 eV and LUMO of − 2.23 to − 2.38 eV. The polymers demonstrated strong thermal stability and blue emission within a range of 416–433 nm. These polymeric materials have demonstrated high photoluminescence which may be useful for future display applications as a good source. Graphic abstract