Fluorophores as Optical Sensors for Local Forces

Abstract

The main aim of this study is to investigate correlations between the impact of an external mechanical force on the molecular framework of fluorophores and the resultant changes in their fluorescence properties. Taking into account previous theoretical studies, we designed a suitable custom-tailored oligoparaphenylenevinylene derivative (OPV5) with a twisted molecular backbone. Thin foils made of PVC doped with 100 nM OPV were prepared. By applying uniaxial force, the foils were stretched and three major optical effects were observed simultaneously. First, the fluorescence anisotropy increased, which indicates a reorientation of the fluorophores within the matrix. Second, the fluorescence lifetime decreased by approximately 2.5% (25 ps). Finally, we observed an increase in the emission energy of about 0.2% (corresponding to a blue-shift of 1.2 nm). In addition, analogous measurements with Rhodamine 123 as an inert reference dye showed only minor effects, which can be attributed to matrix effects due to refractive index changes. To relate the observed spectroscopic changes to the underlying changes in molecular properties, quantum-chemical calculations were also performed. Semiempirical methods had to be used because of the size of the OPV5 chromophore. Two conformers of OPV5 (C(2) and C(i) symmetry) were considered and both gave very similar results. Both the observed blue-shift of fluorescence and the reduced lifetime of OPV5 under tensile stress are consistent with the results of the semiempirical calculations. Our study proves the feasibility of fluorescence-based local force probes for polymers under tension. Improved optical sensors of this type should in principle be able to monitor local mechanical stress in transparent samples down to the single-molecule level, which harbors promising applications in polymer science and nanotechnology.

Mechanical Switching of Aromaticity and Homoaromaticity in Molecular Optical Force Sensors for Polymers

“…Recent advances in the incorporation of force‐responsive units, so‐called mechanophores, into the backbones of polymers have allowed the synthesis of a prototypical molecular force sensor, in which the mechanically induced isomerization of spiropyran to merocyanin is accompanied by a color change of the material from yellow to red . While several molecular force sensors based on changes in their vibrational, UV/Vis, fluorescence,, or chemiluminescence spectra have been reported, an ongoing challenge in the design of novel molecular force sensors is the need to generate a measurable spectral change well before material failure occurs. Using computational methods, it is here demonstrated that the mechanically induced switching of aromaticity and homoaromaticity in molecular force sensors are convenient approaches to achieve substantial changes in color at low stretching forces.…”

Electrostatic Mechanophores in Tuneable Light‐Emitting Piezopolymer Nanowires

“…These variations may involve mass transfer, heating, mechanical stress generation/relaxation, or redistribution of electronic or ionic charges . In particular, the capability to control the optical properties of molecules by a mechanical stress can activate or deactivate chemical reactions, and enable new functionalities for monitoring and sensing . This is both a fundamental challenge in the broad field of nanosciences and a significant advantage for applications in medical diagnostics and for signal transduction in biointerfaces.…”

“…Recently, anisotropic compressive stresses at nN‐scale have been found to induce photophysical changes in conjugate polymer nanoparticles and in single organic dye molecules . In another report, uniaxial tensile forces have been applied to polymer foils doped with an oligo‐paraphenylenevinylene derivative, leading to a blueshift of the emission by 1.2 nm . With focus on practical applications, the capability to control electro‐optical processes through the modification of a piezoelectric potential, in turn led by an external stress, has been introduced by the Wang's group in 2010 .…”

“…[1][2][3] In particular, the capability to control the optical properties of molecules by a mechanical stress can activate or deactivate chemical reactions, and enable new functionalities for monitoring and sensing. [2,4] This is both a fundamental challenge in the broad field of nanosciences and a significant advantage for applications in medical diagnostics, and for signal transduction in bio-interfaces. Mechano-photo conversion has been reported, based on dynamic molecular recognition at the air-water interface.…”

“…[9] In another report, uniaxial tensile forces have been applied to polymer foils doped with an oligoparaphenylenevinylene derivative, leading to a blue-shift of the emission by 1.2 nm. [4] With focus on practical applications, the capability to control electro-optical processes through the modification of a piezoelectric potential, in turn led by an external stress, has been introduced by the Wang's group in 2010. [10] This piezo-phototronic effect is based on the use of a straininduced piezo-potential at a metal-semiconductor interface, which tunes the charge transport across the junction [11,12] and the quantum efficiency of nanostructured light emitting diodes.…”