Abstract
Hydrogen-bonded organic frameworks (HOFs) are the focus of intense scientific research due their potential applications in science and technology. Here, we report on the synthesis, characterization, and photobehavior of a new HOF (T12F-1(124TCB)) based on a dehydrobenzoannulene derivative containing fluorine atoms (T12F-COOH). This HOF exhibits a 2D porous sheet, which is hexagonally networked via H-bonds between the carboxylic groups, and has an interlayers distance (4.3 Å) that is longer than that of a typical π–π interaction. The presence of the fluorine atoms in the DBA molecular units largely increases the emission quantum yield in DMF (0.33, T12F-COOH) when compared to the parent compound (0.02, T12-COOH). The time-resolved dynamics of T12F-COOH in DMF is governed by the emission from a locally excited state (S1, ~0.4 ns), a charge-transfer state (S1(CT), ~2 ns), and a room temperature emissive triplet state (T1, ~20 ns), in addition to a non-emissive triplet structure with a charge-transfer character (T1(CT), τ = 0.75 µs). We also report on the results using T12F-ester. Interestingly, FLIM experiments on single crystals unravel that the emission lifetimes of the crystalline HOF are almost twice those of the amorphous ones or the solid T12F-ester sample. This shows the relevance of the H-bonds in the photodynamics of the HOF and provides a strong basis for further development and study of HOFs based on DBAs for potential applications in photonics.
Highlights
Published: 30 April 2021Hydrogen-bonded organic frameworks (HOFs) have emerged as an exciting class of materials constructed entirely from organic molecules connected mainly by hydrogenbonds (H-bonds), in addition to π–π interactions [1,2,3]
HOF materials based on dehydrobenzoannulene (DBA) derivatives are under intense study because of their interesting chemical and spectroscopic properties, which can be used in lighting, sensing or imaging [18,19,20,21,22]
The spectral behavior of the short-time component indicates that it arises from the locally excited state of both molecules, while the ~2 ns one reflects the emission of species with a CT character after undergoing an intramolecular charge transfer (ICT) reaction, which is shorter than the resolution of the used setup (~15 ps)
Summary
Hydrogen-bonded organic frameworks (HOFs) have emerged as an exciting class of materials constructed entirely from organic molecules connected mainly by hydrogenbonds (H-bonds), in addition to π–π interactions [1,2,3]. We report on the synthesis and spectroscopic properties of two new DBA derivatives (T12F-COOMe and T12F-COOH) in N-N-dimethylformamide (DMF) solution (Figure 1) The crystallization of the latter makes the production of a new HOF possible; this new HOF, T12F-1(124TCB), exhibits a 2D hexagonally networked, porous sheet via. Starting with the ligands in DMF, the steadystate experiments reveal that the first allowed transition is the S0 →S2 , due to the S0 →S1 transition is forbidden Upon electronic excitation, both molecular units undergo an ultrafast intramolecular charge transfer (ICT) process in less than 15 ps to yield a charge-transfer structure fluorescing in the ns regime and phosphorescing in short (ns) and long (μs) regimes. The emission quantum yield of T12F-COOH (0.33) is remarkably higher than that of T12-COOH (0.02), in agreement with our expectations
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