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

We investigate the effect of assembly on charge transfer, charge recombination, and the persistence of radical cations in halogen-substituted triphenylamine (TPA) dimers. A series of urea-tethered TPA derivatives 1 (X = H, Cl, Br, and I) are compared, which have one phenyl group modified at the para position with a halogen. Ureas direct the assembly of these derivatives while halogen substituents influence the packing of the TPA units. These modifications affect the generation and persistence of TPA radical cations as monitored by electron paramagnetic resonance (EPR) spectroscopy. The formation and degradation pathways of the radical cations in solution and gas phase were probed by ion-mobility spectrometry mass spectrometry. In contrast, supramolecular assembly enhanced the stability of these materials as well as the persistence of their photogenerated radical cations, which appear to undergo charge recombination without degradation. Greater quantities of these radical cations are observed for the bromo and non-halogenated derivatives (1Br, 1H). Time-dependent density functional theory (TD-DFT) calculations on single molecules and hydrogen-bonded dimers suggest the stability of TPA radical cations largely depends on initial photoinduced charge separation and electronic coupling between assembled TPA dimers. The latter was found to be about 7 times stronger in 1I than in 1Br dimers, which may explain faster charge recombination and shorter lifetimes of 1I radicals. Transient absorption (TA) spectroscopy and TD-DFT were able to identify the charged species for 1Br along with the kinetic traces and measured lifetime of ∼80 ns. Fluorescence quenching studies are consistent with initial charge separation and subsequent charge transfer event between nearby TPAs. Future exploration will focus on the mobility and application of these TPA assemblies as hole transport materials.