Communication: Time-resolved fluorescence of highly single crystalline molecular wires of azobenzene

Abstract

We report the enhanced fluorescence with the remarkably long lifetime (1.17 ns) in the first excited state (S1) of highly crystalline molecular wires of azobenzene at the excitation wavelength of 467 nm for the first time. This observation suggests that trans-cis photoisomerization through the rotation or inversion mechanism may not be a favorable pathway after excitation to the S1 state in highly single crystalline molecular wires of azobenzene due to the hindered motion within densely packed crystal structure. We also measured the fluorescence lifetime image of a single crystalline molecular wire of azobenzene, indicating that the lifetime was remarkably uniform and that there was only a very minor variation within the crystal.

Photophysics of Azobenzene Constrained in a UiO Metal–Organic Framework: Effects of Pressure, Solvation and Dynamic Disorder

“…However, fluorescence has been reported for a crystalline nanowire of trans ‐azobenzene. [12] A fluorescence lifetime of 1.17 ns was measured, corresponding to a fluorescence quantum yield of about 10 −3 (based on a radiative lifetime of ∼600 ns), [8] but the crystal structure of the nanowire could not be determined, and the emission may have arisen from azobenzene aggregates rather than the individual chromophore. Bahrenburg et al.…”

“…However, this is consistent with the very low values reported for azobenzene in other highly constrained environments, as discussed above. [ 11 , 12 , 13 ] It is important to recall that efficient non‐radiative decay does not require attainment of the cis isomer equilibrium geometry, it only needs relaxation to a conformation that facilitates efficient internal conversion. Although the azobenzene is ostensibly constrained in the UiO framework, our previously reported crystal structure revealed dynamic disorder due to local twisting of the azo moiety.…”

Discovery of Functional Luminescence Properties Based on Flexible and Bendable Boron‐Fused Azomethine/Azobenzene Complexes with O,N,O‐Type Tridentate Ligands

“…In addition, azobenzene is also well-known to a non-emissive scaffold due to fast internal conversion and excited deactivation through the non-radiative deactivation processes accompanied to the photoisomerization. [74][75][76][77] Although many challenges to obtain luminescence from azobenzene derivatives have been carried out on the basis of the strategies for suppressing photoisomerization, such as constructing metal complexes, [78] hydrogen bonding, [79][80][81] highly crystalline structures, [82,83] cyclic compounds, [84] it is still difficult to observe intense emission from the azobenzene scaffold. Kawashima and coworkers successfully obtained significant emission from the azobenzene scaffold by the boron complexation at the N=N bond.…”

Photoswitchable Fluorescent Crystals Obtained by the Photoreversible Coassembly of a Nucleobase and an Azobenzene Intercalator

“…[14][15][16][17][18][19] These materials were however usually morphologically ill-defined and poorly structured. Conversely, well-defined azobenzene crystals with bright fluorescence properties were obtained but did not display any photoswitching characteristics [20][21][22] . Finally light-responsive organized assembly of azobenzenes were reported but the fluorescence properties were either not documented 23 or modestly affected by light.…”

“…The typical strategy has consisted of controlling with light the aggregation behavior of azobenzene derivatives, which, by exploiting the fluorescence enhancement effect upon aggregation, led to a variety of light-dependent aggregating systems with optically tunable emission characteristics. − These materials were, however, usually morphologically ill-defined and poorly structured. Conversely, well-defined azobenzene crystals with bright fluorescence properties were obtained but did not display any photoswitching characteristics. − Finally, light-responsive organized assembly of azobenzenes was reported, but the fluorescence properties were either not documented or modestly affected by light . Note that most of the above-mentioned examples were based on the assembly of hydrophobic or amphiphilic compounds in organic solvents.…”