Single crystal fluorescence behavior of a new HOF material: a potential candidate for a new LED

Abstract

Hydrogen-bonded Organic Frameworks (HOFs) have emerged as exciting new materials, due to several promising applications in science and technology. Using fluorescence microscopy, we report on the spectral and dynamical characterization of a HOF (T12-apo), its parent compound (T12) and its ester-methylated derivative (T12-Ester) in the solid state. In contrast to the behavior in solution, the absorption spectra of these solid materials exhibit a strong S₀ → S₁ transition. Fluorescence microscopy studies of the crystals of T12 reveal no crystal size effect on the emission spectra and lifetime (∼38 ns). However, those of the T12-Ester reveal a photodynamic behavior that depends on their size. Large crystals exhibit a monoexponential behavior (∼28 ns), while the smaller ones present an additional shorter component (∼5 ns) due to the relaxation of species having a charge-transfer character. For T12-apo, the results suggest two types of crystals: (i) those formed by π–π stacking, showing a photobehaviour similar to those of T12 and T12-Ester, and (ii) crystals in which the H-bonds between the building blocks play an important role and result in different emission spectra and shorter fluorescence lifetimes compared to the former. Moreover, the anisotropy measurements of large T12-apo crystals confirm the importance of π–π stacking of the aromatic core and of H-bonding interactions to form the HOF, and their relevance to the photobehaviour. We suggest the use of the HOF to build a white-LED, when combined with a blue-LED. Our results provide new information on the photobehaviour of this HOF at the single crystal level looking for its use in optoelectronic devices.