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Theoretical basis for the stabilization of charges by radicals on electrified polymers
Mazur, T.,Grzybowski, B. THE ROYAL SOCIETY OF CHEMISTRY 2017 Chemical Science Vol.8 No.3
<P>Quantum mechanical calculations at various levels of theory indicate that charges (both '+' and '-') on organic polymers can be stabilized by radicals on nearby polymer chains. The stabilization mechanism is based on the formation of intermolecular odd-electron, two-center bonds with possible concomitant spin density redistribution (depending on the polymer and the number and type of proximal heteroatoms). This result is in line with our previous experimental demonstrations that on various types of polymers charged by contact electrification, radicals co-localize and help stabilize proximal charges (of either polarity). The principle of intramolecular charge-radical stabilization we now confirm on a fundamental level might have ramifications for the design of other macromolecular systems in which chemical reactivity is controlled by radicals flanking the charged groups or by charged groups flanking the radicals.</P>
Tasior, Mariusz,Hassanein, Khaled,Mazur, Leszek M.,Sakellari, Ioanna,Gray, David,Farsari, Maria,Samoć,, Marek,Santoro, Fabrizio,Ventura, Barbara,Gryko, Daniel T. The Royal Society of Chemistry 2018 Physical chemistry chemical physics Vol.20 No.34
<P>A three-step synthetic route to a structurally unique π-expanded pyrrolo[3,2-<I>b</I>]pyrrole derived bis-ketone has been developed. In contrast to all previous ladder-type pyrrolopyrroles, the new dye exhibits a low-energy absorption band in the visible region which is responsible for its red-purple color. Interestingly, even though the compound is centrosymmetric, this band coincides with the lowest energy two-photon absorption (TPA) transition. This non-typical behaviour has been computationally rationalized by finding two close lying excited states, one of which (S1) is active for OPA and the other (S2) for TPA processes, which arise from the mixing of two symmetric partial charge-transfer states. The ultrafast excited-state dynamics was characterized by means of transient absorption analysis. A relaxation process involving S1 symmetry breaking occurs in a few ps, leading to the formation of the lowest energy charge-transfer state. This is weakly emitting, with a measured lifetime in the order of tens of picoseconds. Interestingly, two-photon polymerization has been achieved using this new ketone. The high yield of radical photo-initiation upon two-photon excitation was demonstrated by the fabrication of woodpile photonic crystal templates by direct laser writing using a zirconium-silicon hybrid composite.</P>