External Heavy-Atom Effect via Orbital Interactions Revealed by Single-Crystal X-ray Diffraction

Abstract

Enhanced spin-orbit coupling through external heavy-atom effect (EHE) has been routinely used to induce room-temperature phosphorescence (RTP) for purely organic molecular materials. Therefore, understanding the nature of EHE, i.e., the specific orbital interactions between the external heavy atom and the luminophore, is of essential importance in molecular design. For organic systems, halogens (e.g., Cl, Br, and I) are the most commonly seen heavy atoms serving to realize the EHE-related RTP. In this report, we conduct an investigation on how heavy-atom perturbers and aromatic luminophores interact on the basis of data obtained from crystallography. We synthesized two classes of molecular systems including N-haloalkyl-substituted carbazoles and quinolinium halides, where the luminescent molecules are considered as "base" or "acid" relative to the heavy-atom perturbers, respectively. We propose that electron donation from a π molecular orbital (MO) of the carbazole to the σ* MO of the C-X bond (π/σ*) and n electron donation to a π* MO of the quinolinium moiety (n/π*) are responsible for the EHE (RTP) in the solid state, respectively.

Shorter Exciton Lifetimes via an External Heavy‐Atom Effect: Alleviating the Effects of Bimolecular Processes in Organic Light‐Emitting Diodes

“…[24] Moreover, it has been well documented that absorption features and photoluminescence behavior of solutions can be influenced by the mere proximity of heavy atoms that are not even covalently bound to the respective molecule. [25][26][27][28][29][30][31] This phenomenon is known as the external heavy atom effect. It was already successfully utilized in white OLEDs by combining blue TADF fluorophores and yellow phosphors.…”

Intrinsic and Extrinsic Heavy‐Atom Effects on the Multifaceted Emissive Behavior of Cyclic Triimidazole

“…The possibility of switching on RTP and/or modulate phosphorescence properties (e.g.,e fficiency,l ifetimes and excitation/emission energy) of organic luminophores in the solid state throughs pecifici ntra-and intermolecular interactions represents av aluable tool in view of functional applications. [3] In particular,t he heavy-atom effect can be tuned, for example, by either introducing into the molecules (intrinsic effect) ah alogen atom (Br or I) [4] or by exploitings pecific intermolecular interactions (extrinsic effect) based on halogen bondingw ith other molecules of the same type (one component) [5] or ad ifferentt ype( two-component), such as for example in co-crystals. [6] According to the different patterns of halogen bonds (XBs),v ariouse ffects can be producedeither in fluorescenceo r phosphorescence.…”

“…In this regard, we have recentlyr eported the photophysical behavior of two triimidazo[1,2-a:1',2'c:1'',2''-e][1, 3,5]triazine (TT)d erivatives, namely3 ,7-dibromotriimidazo[1,2-a:1',2'-c:1'',2''-e][1, 3,5]triazine (2Br)a nd 3,7,11tribromotriimidazo[1,2-a:1',2'-c:1'',2''-e][1, 3,5]triazine (3Br; Scheme1), in which such an extrinsics ingle component-effect, combined with the intrinsic one, is clearly demonstrated. [7] In fact, compound 2Br,t he structure of which displays strong and rigidB r 4 XB intermolecularu nit, shows an induced longlived RTP,w hich is absent in 3Br that has al ooser non-planar Br 3 XB unit. Here, to gain further insighti nto the extrinsic/intrinsic effects, we investigated the structural and emissive properties of 3-iodiotriimidazo[1,2-a:1',2'-c:1'',2''-e][1, 3,5]triazine (TTI)a nd TTCo,t he 1:1c o-crystal self-assembled through I···N XB betweent wo acceptor nitrogena toms of TT and the two iodine atomsof1 ,4-diiodotetrafluorobenzene (DITFB)tof orm a1Di nfinite chain structure (see Scheme 1). TTI is the prototype of intrinsich alogen-atom effects andi ts comparative study with the bromine TT analogues allows for the evaluation of the role playedb yIversus Br.T his investigation was furtherd eepened by preliminary photophysicala nd structurala nalysis of TT2I (3,7-diiodiotriimidazo[1,2-a:1',2'-c:1'',2''-e][1, 3,5]triazine).…”

“…[7] In fact, compound 2Br,t he structure of which displays strong and rigidB r 4 XB intermolecularu nit, shows an induced longlived RTP,w hich is absent in 3Br that has al ooser non-planar Br 3 XB unit. Here, to gain further insighti nto the extrinsic/intrinsic effects, we investigated the structural and emissive properties of 3-iodiotriimidazo[1,2-a:1',2'-c:1'',2''-e][1, 3,5]triazine (TTI)a nd TTCo,t he 1:1c o-crystal self-assembled through I···N XB betweent wo acceptor nitrogena toms of TT and the two iodine atomsof1 ,4-diiodotetrafluorobenzene (DITFB)tof orm a1Di nfinite chain structure (see Scheme 1). TTI is the prototype of intrinsich alogen-atom effects andi ts comparative study with the bromine TT analogues allows for the evaluation of the role playedb yIversus Br.T his investigation was furtherd eepened by preliminary photophysicala nd structurala nalysis of TT2I (3,7-diiodiotriimidazo[1,2-a:1',2'-c:1'',2''-e][1, 3,5]triazine). Alternatively, TTCo,i nw hich I···N XB guarantees as trong non-covalent interaction between iodine atom and the luminophore, allowsf or the isolation of the extrinsic two-component effect by comparison with the TTI and TT photoluminescent properties.…”

Meta‐Dimethoxy‐Substituted Difluoroboron Dibenzoylmethane Poly(Lactic Acid) Nanoparticles for Luminescence Anisotropy

“…The P1 and P2 nanoparticles showed unusually strong RTP for metal‐free phosphors. Typically with BF 2 bdk materials, heavy atoms are introduced as substituents on the arene rings of the dye to increase RTP intensity relative to fluorescence . However, this ICT scaffold, with enhanced phosphorescence, shows progress toward eliminating the heavy atom altogether for oxygen‐sensing boron dyes.…”