Spectroscopy and dynamics of a HOF and its molecular units: remarkable vapor acid sensing

Abstract

Hexaazatriphenylene (HAT) derivatives are being used for the fabrication of hydrogen-bonded organic frameworks (HOFs). Here, we report on studies of two HAT derivatives: CBPHAT and its butylated partner, CBPHAT-C₄H₉, in N,N-dimethylformamide (DMF) solutions and solid state. CBPHAT displays different species and no planar conformations when interacting with DMF. CBPHAT-C₄H₉ undergoes an intramolecular charge transfer (ICT) reaction in 50 ps, while for CBPHAT, the ICT reaction is faster (∼4.5 ps). Their emission lifetimes in DMF solution change from 50 ps to 2.3 ns. The slower relaxation of CBPHAT-C₄H₉ without oxygen quenching involves three long living states (∼2, ∼18 and ∼100 μs). The CBPHAT-1a HOF (crystals) exhibit both proton and charge transfer reactions (60 ps), leading to different emitters with fluorescence lifetimes spanning from 100 ps to 2.66 ns. This HOF shows a remarkably strong response to HCl vapors in a reversible way in ambient conditions. This ability is reflected in its absorption and emission spectra, making it a new smart sensor of acid gas. The provided theoretical and experimental results should trigger more research on this family and related HOFs to further advance its science.