Polymorphism in the self-assembled nanostructures of a tris(phthalocyaninato) europium derivative: Phase-dependent semiconducting and NO2 sensing behaviour

Abstract A new trifluoroethoxy-substituented tris(phthalocyaninato) europium complex Eu 2 [Pc(OCH 2 CF 3 ) 4 ] 3 {[Pc(OCH 2 CF 3 ) 4 ] = 2(3),9(10),16(17),23(24)-tetrakis(2,2,2-trifluoroethoxy)phthalocyaninate}, is designed and synthesized. Introduction of trifluoroethoxy substituents at the phthalocyanine periphery within the triple-decker complex ensures their good solubility in conventional organic solvents. Significantly, depending on the fluorine-participated multiple hydrogen bonding and the π–π interactions in different solvent systems ( i.e. n -hexane and water), the self-assembly of the Eu 2 [Pc(OCH 2 CF 3 ) 4 ] 3 molecules results in well controlled morphologies (nano-bowling bowls, nanobelts and nano-particles) with totally different phases (χ, β and α phases). Furthermore, the sensitivity to NO 2 at varied concentrations in the range of 20–1600 ppb, follows the order of nano-bowling bowls in χ phase (SA-1) » nanobelts in β phase (SA-2) > nano-particles in α phase (SA-3), revealing the effect of both phase structure and morphology on sensing performance of the Eu 2 [Pc(OCH 2 CF 3 ) 4 ] 3 . In particular, SA-1 displays unexpected high percent current change of 6.52% to NO 2 gas as low as 20 ppb at room temperature, representing thus far the best result of the room-temperature NO 2 gas sensing performance in terms of the lowest detection limit with largest response. The present work not only represents the first observation of χ, β and α polymorphs for the sandwich-type triple-decker tetrapyrrole-based rare earth complexes, but more importantly provides an efficient strategy to obtain high performance organic semiconductor-based gas sensors through molecular design and solvent-triggered phase transformations.