Guest Inclusion Modulates Concentration and Persistence of Photogenerated Radicals in Assembled Triphenylamine Macrocycles

Abstract

Substituted triphenylamine (TPA) radical cations show great potential as oxidants and as spin-containing units in polymer magnets. Their properties can be further tuned by supramolecular assembly. Here, we examine how the properties of photogenerated radical cations, intrinsic to TPA macrocycles, are altered upon their self-assembly into one-dimensional columns. These macrocycles consist of two TPAs and two methylene ureas, which drive the assembly into porous organic materials. Advantageously, upon activation the crystals can undergo guest exchange in a single-crystal-to-single-crystal transformation generating a series of isoskeletal host–guest complexes whose properties can be directly compared. Photoinduced electron transfer, initiated using 365 nm light-emitting diodes, affords radicals at room temperature as observed by electron paramagnetic resonance (EPR) spectroscopy. The line shape of the EPR spectra and the quantity of radicals can be modulated by both polarity and heavy atom inclusion of the encapsulated guest. These photogenerated radicals are persistent, with half-lives between 1 and 7 d and display no degradation upon radical decay. Re-irradiation of the samples can restore the radical concentration back to a similar maximum concentration, a feature that is reproducible over several cycles. EPR simulations of a representative spectrum indicate two species, one containing two N hyperfine interactions and an additional broad signal with no resolvable hyperfine interaction. Intriguingly, TPA analogues without bromine substitution also exhibit similar quantities of photogenerated radicals, suggesting that supramolecular strategies can enable more flexibility in stable TPA radical structures. These studies will help guide the development of new photoactive materials.

Effects of Self-Assembly on the Photogeneration of Radical Cations in Halogenated Triphenylamines

“…In the solid state, no degradation of crystalline 1Br was detected by SC-XRD, PXRD, and solution-state NMR even after 10 h of UV-irradiation using a Hanovia 450 W medium pressure Hg arc lamp . Prior work showed lower intensity 365 nm LED’s gives identical EPR spectra for 1Br vs the mercury lamp, although longer irradiation time is required . Some broadening of the aryl and carbonyl resonances was observed in the solid-state MAS 13 C NMR of 1Br , which further indicated the delocalization of the electron density .…”

“…The halide free 1H crystallized in the orthorhombic system with centrosymmetric space group Pbcn . Both TPA units extend opposite site of the disordered methylene ureas.…”

Direct Crystallization of Diamine Radical Cations: Carbon‐Nitrogen Bond Formation from the Reaction of Triphenylamine with TiCl4, TiBr4, or SnCl4 versus Carbon‐Carbon Bond Formation with SbCl5**

“…Oxidation of triarylamines produces radical cations that are important in hole transport, and the creation of color centers and EPR active sites in materials [10,11] . With suitable protection (usually substituents in the para positions) triarylamines can be oxidized to produce stable radical cations that can be crystallized and used as powerful oxidants [12–20] .…”

Near quantitative synthesis of urea macrocycles enabled by bulky N-substituent

“…Synthesis and applications of urea macrocycles have also been explored. For instance, Shimizu reported bisurea macrocycles as nanocontainer to facilitate photodimerization, selective cycloaddition and to stabilize radicals [28][29][30] . Gong pioneered an efficient approach for one-pot formation of aromatic tetraurea macrocycles via specific oligomeric structures preorganized by their intrinsic intramolecular hydrogen bonds 31 .…”