The Directed Heavy Atom Effect: A Design Principle for Metal-Free Organic Phosphors.

Abstract

Phosphorescent materials are changing the way organic compounds are used in optical devices. By being more efficient, slower emitting, and uniquely sensitive they are expanding the applicability of organic materials. In order, though, for this trend to include all organic materials the critical limitation of an organometallic structure must be overcome. At the time of this publishing, organometallics alone preform well enough for applicational use. Here is presented the Directed Heavy Atom Effect (DHAE), a design principle to realize bright phosphorescence from metal-free organic compounds. By synergistically combining the phosphorescence-enhancing property of the heavy atom effect with the heavy atom directing property of halogen bonding, DHAE achieves organic phosphors with efficiencies competitive to organometallic and inorganic materials. Here the DHAE is attained by cocrystallizing an aromatic aldehyde with an optically inert, halogenated analog. These cocrystals exhibit halogen bonding, which directs the heavy atom to enhance spin-orbit coupling at the carbonyl and activate phosphorescence. The optically inert host isolates the chromophore from self-quenching, resulting in unprecedentedly bright metal-free organic phosphorescence. From this design principle a variety of materials with varying properties are synthesized. Emission color can be tuned in either fine steps of 5 nm or broad chromic steps from blue to green, yellow, and orange. Material modifications, DHAE phosphors with controllable, vapor deposited microstructures or ester functionalization, are also achievable with careful material design presented here. The novel phenomenon of delayed phosphorescence is demonstrated from crystals of pure DHAE chromophore, holding promise for enhancing emissive device efficiencies. To escape design complications, polymeric hosts are presented as an alternative to crystal systems. If well designed, polymer hosts can produce bright phosphorescence even from liquid DHAE-style chromophores by suppressing vibrational dissipation pathways. Finally a broad summation of the work is offered along with ideas about the future direction of DHAE phosphors.