Photophysics of Fluorescent Contact Sensors Based on the Dicyanodihydrofuran Motif

Abstract

Fluorescent molecular rotors have been used for measurements of local mobility on molecular length scales, for example to determine viscosity, and for the visualization of contact between two surfaces. In the present work, we deepen our insight into the excited‐state deactivation kinetics and mechanics of dicyanodihydrofuran‐based molecular rotors. We extend the scope of the use of this class of rotors for contact sensing with a red‐shifted member of the family. This allows for contact detection with a range of excitation wavelengths up to ∼600 nm. Steady‐state fluorescence shows that the fluorescence quantum yield of these rotors depends not only on the rigidity of their environment, but – under certain conditions – also on its polarity. While excited state decay via rotation about the exocyclic double bond is rapid in nonpolar solvents and twisting of a single bond allows for fast decay in polar solvents, the barriers for both processes are significant in solvents of intermediate polarity. This effect may also occur in other molecular rotors, and it should be considered when applying such molecules as local mobility probes.

Molecular Probing of the Microscopic Pressure at Contact Interfaces

“…In previous studies, we used fluorescent molecular rotors to determine the real contact area A r . 18,51 For the rotors, the suppression of nonradiative decay pathways due to restrained molecular mobility in contact areas makes the fluorescence intensity higher there, allowing us to see whether the surfaces touch. Although the pressure should influence the local mobility gradually, the rotor probe responds in an almost binary on/off fashion to contact.…”

Molecular rotors to probe the local viscosity of a polymer glass

“…We briefly detail the protocol to embed the molecular rotor in the polymer glasses. DCDHF fluorescent probes (available from previous work 42 ) are added to a solution of PVAc in tetrahydrofuran (THF from Sigma-Aldrich) of concentration 20 wt%. The PVAc glass film is prepared by drop-casting this polymer solution on a plasmacleaned glass slide.…”

“…The dicyanodihydrofuran chromophore (DCDHF, see Fig. 1) that we use here is one example of such molecular rotors: it has been used as a probe in many studies such as fluorescence sensors for contact mechanics measurements 42,43 or single-molecule imaging in cells 44 . The molecular rotors' sensitivity to its local environment has already been used to probe the local viscosity of viscoelastic media 45,46 .…”

Suppression and utilization of Kasha's rule: realizing the transformation from blue to near-infrared emission

“…5 However, regarding these categories of compounds, some great challenges need to be addressed: (1) inapparent variation of emitting colors that bedims the discrimination, especially, with the naked eye; 5,[18][19][20][21] (2) the deficiency of near-infrared (NIR) emission that limits the general applicability in bioluminescence fields; 9,[20][21][22][23][24][25] and (3) an unclear mechanism that restricts the molecular design and applications, especially, from the aspect of the connection between the molecular configuration and the matrix. 18,19,22,[26][27][28][29][30][31][32][33] Additionally, almost all studies focus on the transition from low to high excited states rather than the other way around, which cannot dialectically expose the nature of Kasha's rule. [5][6][7][8][9]20,21,[23][24][25]34 In a previous work, we reported a series of phenazine-based compounds 14 to explore the connection between the electronic configuration of the excited state and the molecular configuration.…”