Unusual solvent-dependent photophysical and self-assembly properties of NO2 substituted T-shaped phenazines

Abstract

This paper investigates the importance of substituent placement when designing low-molecular mass π-organogelators. The electron-deficient NO₂ substituent was systematically added to novel T-shaped phenazines to examine electronic as well as assembly properties. This T-shaped molecular platform promotes selective electronic tuning, which can be theoretically analyzed by examining the system's frontier molecular orbitals. Electronic properties were characterized by UV-vis spectroscopy and cyclic voltammetry, and comparisons were made based on number and placement of the NO₂ group. Computational chemistry (B3LYP/6-31G*) was employed for geometry optimizations, and to generate molecular orbital diagrams for all systems. The most noticeable influence of NO₂ position was found for two molecules with four NO₂ groups placed at different locations about the molecule (T-34dNT and T-35dNT). A 0.13 eV difference in E_LUMO was observed while E_HOMO was not significantly impacted by this change only in NO₂ placement. Interestingly and unexpectedly, the photophysical properties and solvent-dependent gelation properties were considerably different for T-34dNT and T-35dNT. T-34dNT exhibited a unique fluorescence (FL) solvatochromism, with FL intensity and maxima dependent on solvent polarity. This result is indicative of intramolecular charge transfer. In addition, long tailing at the solid-state absorption of T-34dNT suggests the presence of intermolecular charge transfer. The gelation of T-34dNT produced chromism ranging from red to orange to yellow when the solvents changed from acetonitrile to ethyl acetate to cyclohexane, respectively. T-35dNT gels in these solvents did not exhibit any of the same properties. Xerogel morphology characterizations were carried out using three different solvents for both T-34dNT and T-35dNT. In the case of T-34dNT, striking differences in the morphology were detected by field-emission scanning electron microscopy (FE-SEM). We conclude that numbers of substituents are not the only consideration in effective molecular design for organogelators, but that substituent position plays a critical role in certain fundamental properties of these systems.