Solution-processed thin films based on sandwich-type mixed (phthalocyaninato)(porphyrinato) europium triple-deckers: Structures and comparative performances in ammonia sensing

The solution-processed thin films of a series of three sandwich-type mixed (phthalocyaninato)(porphyrinato) europium complexes with different numbers of hydroxyl groups at the meso-substituted phenyl groups of porphyrin ligand 1–3 were prepared by a quasi-Langmuir–Shäfer (QLS) method. Examination by spectroscopic methods revealed that J-type aggregates have been formed with the increasing degree of order of molecular stacking 1 < 2 < 3 films. The IR and XRD results reveal that, a dimeric supramolecular structure was formed depending mainly on the π–π interaction between the tetrapyrrole cores of neighboring triple-decker molecules, which, as the building blocks, self-assembled into the target nanostructures through intermolecular hydrogen bonds. Competition and cooperation between the intermolecular π–π interaction and hydrogen bonding for different compounds were revealed to result in thin-film microstructure with a different morphology from nano-grains for 1 and 2 and nano-sheets for 3. An increasing sensitivity to NH3 at varied concentrations in the range of 15–800 ppm, follows the order 3 < 1 < 2, revealing the effect of film structure/morphology on sensing performance. In particular, excellent sensitivity, nearly complete reversibility and reproducibility to NH3 even at room temperature are obtained for the ultra-thin solution processed films from 2.