Photophysics and photochemistry of NIR absorbers derived from cyanines: key to new technologies based on chemistry 4.0

Abstract

Cyanines derived from heptamethines were mainly discussed regarding their functionalization to broaden the solubility in different surroundings exhibiting either hydrophilic or hydrophobic properties and to tailor made the ΔG et photopysical properties with respect to absorption and fluorescence. Electrochemical properties were additionally considered for some selected examples. The cyanines chosen comprised as end groups either indolenine, benzo[e]- or benzo[cd]indolium pattern, which facilitated to shift the absorption between 750–1000 nm. This enabled their use in applications with light sources emitting in the near-infrared (NIR) region selected from high power LEDs or lasers with line-shaped focus. The absorbers considered were discussed regarding their function as sensitizer for applications related to Chemistry 4.0 standards. These were mainly photopolymer coatings, which can be found for applications in the graphic industry or to protect selected substrates. The huge release of heat on demand upon turning ON or OFF the NIR light source enables them for photothermal treatment in processes requesting heat to initiate either chemical (activated reactions) or physical (melting, evaporation) events.

NIR‐Sensitized Activated Photoreaction between Cyanines and Oxime Esters: Free‐Radical Photopolymerization

“…This potential is based mainly on UV and visible‐light excitation, but NIR excitation did was not addressed. Recently, an approach based on a photocatalytic system using Cu II in combination with a cyanine comprising a barbiturate group also demonstrated applications in NIR‐sensitized photopolymerization . Excitation based on NIR radiation has the advantage of i) deeper penetration into matter because of a significantly lower scattering coefficient, ii) embedding additives with UV absorption, and iii) the release of heat applicable for photothermal treatment of cancer cells .…”

“…Cyanines with absorption in the NIR convert most of the absorbed light energy, as shown by the fluorescence quantum yield residing between 5 and 20 % for systems requiring 800 nm excitation . This aspect has also received interest for industrial applications enabling thermally driven processes such as curing of blocked isocyanates or melting and crosslinking of powder coatings in just one step .…”

“…The question as to whether decomposition follows an entire photonic mechanism or thermal decomposition might play a certain function as well. Different structural motifs of the cyanines were chosen to determine PET leading either to chain breaking or changes of the polymethine structure with hypsochromically shifted absorption …”

NIR‐sensibilisierte kationische und hybride radikalische/kationische Polymerisation und Vernetzung

“…Die oben genannten Probleme hinsichtlich der Substitution älterer Anregungstechniken auf der Basis von Hg‐Lampen haben zu verstärkten Aktivitäten bei der Entwicklung von UV‐LEDs als alternative Strahlungsquellen geführt [11] . Diese Entwicklung war erfolgreich, obwohl noch Probleme bei Systemen auftraten, welche Additive einsetzen, die im UV‐ und sichtbaren Bereich absorbieren.…”

Near‐infrared‐induced photothermal decomposition of charge transfer complexes: A new way to initiate thermal polymerization

“…Notably, their capacities to reach elevated temperature upon photoexcitation and thus to induce a thermal polymerization have been demonstrated in previous works. 5,[7][8][9] .…”