Porous organic crystals are formed by hydrogen bond-directed columnar assembly of triphenylamine (TPA) bis-urea macrocycles 1. Upon activation, the crystals have accessible 1-dimensional channels that can be readily loaded with guests affording different crystalline complexes. Encapsulation of guest molecules within these activated crystals can dramatically alter their properties. Herein, we study the influence of electron-accepting guest molecules on the electrical conductivity of crystalline triphenylamine bis-urea macrocycles. The three guests, 2,1,3-benzothiadiazole (BTD), 2,5-dichloro-1,4-benzoquinone (ClBQ), and malononitrile (MN), differ in their reduction potentials and ability to form charge transfer interactions with the triphenylamine electron donor. The structures of the complexes are compared, and their extent of charge transfer interactions is investigated using diffuse reflectance spectroscopy, photoluminescence, and transient absorption spectroscopy. Complex 1.ClBQ displays a large ground state charge transfer band at ambient conditions, while photoinduced charge transfer transitions are observed in 1.BTD and 1.MN. While pristine TPA bis-urea macrocycle crystals exhibit non-conductive behavior, a dramatic increase in electrical conductivity is observed for the host-guest complexes, with 1.ClBQ exhibiting the highest conductivity (1.98 x 10^-5 Scm^-1). Computational methods provide insight into this behavior by determining electronic coupling values, frontier molecular orbitals, and electron reorganization energy. UV irradiation decreases electrical conductivity, and transient absorption spectroscopy elucidates the excited-state dynamics of the host-guest complexes. This study highlights the influence of charge transfer on the electrical conductivity of metal-free host-guest complexes and demonstrates their potential as organic semiconductors.

Gamage Isuri P. Wijesekera

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