Synthesis and Unexpected Optical Properties of Ionic Phosphorus Heterocycles with P-Regulated Noncovalent Interactions

Abstract

Optoelectronic properties of organic chromophores (OCPs) are to a large extent dictated by the chemical structures. Herein, we synthesized a new series of ionic phosphorus(P)-heteropines via the methylation of the P(III) center. Our studies revealed that methylation is highly dependent on the P(III) environments (NPN, NPC, and CPC), in which adjacent nitrogen atoms greatly withdraw electron density of the P(III) center. The observation of noncovalent interactions between solvent molecules and the molecular backbones of the related P-heterocycle in the single crystal structure implied tunable molecular conformations. Different from the red-shifted absorption and emission spectra of ionic P-OCPs induced by either decreased lowest unoccupied molecular orbital (LUMO) or intramolecular charge transfer (ICT) state in previous studies, current ionic P-heterocycles exhibit blue-shifted absorption and emission spectra compared to the nonionic counterparts. Our experimental and theoretical studies suggest that the unexpected photophysical characters are probably due to the counter-anion induced structure twisting via intermolecular noncovalent interactions between NH-indole and O(OTf), and/or strong intermolecular O•••F bonding between O(MI) and F(OTf). Our studies also revealed that the P-environments (NPN, NPC, and CPC) conjunctly impact the photophysical properties of the ionic P-heteropines. Overall, the fact that the P-environment-regulated noncovalent interactions induce the rich structure dynamics and photophysics offers us with a new and effective strategy to fine-tune the optical properties of OCPs.

Synthesis and Photophysical Properties of Carbazole-Functionalized Diazaphosphepines via Sequent P–N Chemistry

“…Table S1). Similar to the structures of previous heteropines, 34,42,47 ). These observations implied flexible structure characteristics for 1PO, 3PO, and 4PO.…”

“…Furthermore, the crystal structures of 3PO (42.1°and 41.6°, Figure S2) and 4PO (33.4°and 33.7°, Figure S3) showed the larger torsion angles between [d]-aryl moieties and the indole moieties compared with the reference compounds with the phenyl group (M3O: 26.6°and 31.2°, M4O: 2.7°and 2.0°) reported previously. 34,42,47 It is likely that the donor−acceptor interactions induced a steric effect that pushed indole away from the central phosphepine ring. We also observed a longrange intermolecular π−π interactions in the single crystal of 3PO (Figure S3b).…”

“…Except for 1P (11%), 2−4P series (2P: 2%, 3P: 7%, and 4P: too low to be detectable) exhibited lower photoluminescence quantum yields (PLQYs, Table 1) than those of the M2−4 series (M2: 14%, M3: 19%, and M4: 67%). 34,42,43 The lower PLQYs of 2−4P are likely due to the intramolecular charge transfer character (vide infra). Similar effects were observed in previous studies where molecules with strong ICT showed low PLQYs.…”

Synthesis, post-functionalization, and photoluminescence of contorted diazaphosphepine-based polycyclic aromatic heterocycles

“…14,23,24 Among the main group elements, phosphorus is a unique member where diverse and better-controlled P-chemistry, namely oxidation, borylation, alkylation, and metal coordination, was demonstrated to fine-tune the optoelectronic properties of OCMs. [25][26][27][28][29][30] For example, Baumgartner and coworkers conducted a seminal work on dithienophosphole systems (Scheme 1, I) where the P-chemistry had strong impacts on the photophysical properties, redox characteristics, and self-assembling properties. 25,28 Romero-Nieto and coworkers developed a new synthetic protocol to access the sixmembered phosphaphenalene system (Scheme 1, II).…”

“…[25][26][27][28][29][30] For example, Baumgartner and coworkers conducted a seminal work on dithienophosphole systems (Scheme 1, I) where the P-chemistry had strong impacts on the photophysical properties, redox characteristics, and self-assembling properties. 25,28 Romero-Nieto and coworkers developed a new synthetic protocol to access the sixmembered phosphaphenalene system (Scheme 1, II). 31 Their recent studies uncovered that the P-center of the six-membered system exhibits very distinct electronic communication with π-conjugated backbones compared with a five-membered P-system.…”