Taming Macromolecules with Light: Lessons Learned from Vibrational Spectroscopy

Abstract

application of vibrational spectroscopy to rotaxane-based molecular machines, [11] molecular motors, [12] and molecule-based electronics, such as molecular electronic switches, [13] are highly recommended.In this feature article, we will focus on one of the most commonly used photoswitches, both for materials engineering and biological applications, namely, azobenzene. [14] This motif is known to change its geometry upon illumination with ultraviolet or visible light, in a socalled photoisomerization reaction from the thermodynamically stable trans conformation to the bent cis conformation (Figure 1a). [15,16] This reaction can be used to both decrease and destroy existing order in materials, as in the cases of photoinduced unfolding of azobenzenecrosslinked alpha-helical peptides, [17] photo induced nematic-isotropic phase transitions [18] and isothermal solid-toliquid transitions, [19] or, on the contrary, to induce order and anisotropy in initially isotropic or less ordered materials, such as in photoinduced birefringence, photoinduced chirality, alloptical poling, and surface relief grating (SRG) formation. [2,20,21] The photoisomerization of individual azobenzene moieties is known to occur in the picosecond timescale and results in changes in its molecular dimensions and properties, such as its dipole moment, solubility and UV-vis absorption. [15] Apolar azobenzene molecules typically have well isolated cis and trans absorption bands along with long thermal cis lifetimes. They are therefore suitable for optical switching applications where two "stable" long-lived states are required. By contrast, functionalizing azobenzene molecules with electron donating and electron withdrawing substituents (or push-pull substitution) alters their photochemistry, typically leading to an overlap of cis and trans absorption bands and thus to quasi-continuous photoswitching between the trans and the cis geometrical isomers. [15,16] Examples of drastic changes in material systems resulting from bistable photoswitching between trans and long-lived cis isomers are illustrated in Figure 1b-d and include photoswitching of the tilt of liquid crystals by an azopolymer command layer, [22] the light-powered contraction of a helical ribbon of a liquid crystalline azopolymer, mimicking plant tendrils, [23] and the photoinduced decrease in helix content of a peptide cross-linked by an azobenzene. [17] Demonstrations of the use of continuous azobenzene photoswitching between the two geometrical isomers, shown in Figure 1e

Molecular-Level Study of Photoorientation in Hydrogen-Bonded Azopolymer Complexes

“…The orientation parameter (⟨ P 2 ⟩) was calculated using eq where Δ A is the dichroic difference, A 0 = ( A p + 2 A s )/3 is the structural absorbance, and α is the angle between the transition dipole moment of the vibration and the molecular axis of interest. Equation assumes that the orientation is uniaxial with respect to the laser polarization direction, which is expected to be the case considering the low ⟨ P 2 ⟩ values reached in this work …”

“…Equation 1 assumes that the orientation is uniaxial with respect to the laser polarization direction, which is expected to be the case considering the low ⟨P 2 ⟩ values reached in this work. 45 The UV−vis measurements were performed with a fiber-coupled spectrometer (Ocean Optics, USB 2000+) and a DH-mini light source, combined with the linearly polarized 488 nm laser to pump the azobenzenes for 5 min. A clean glass substrate was used as the reference.…”

Supramolecular design principles for efficient photoresponsive polymer–azobenzene complexes

“…The most common experimental method for characterizing hydrogen-bonding, halogen-bonding and ionic bonding resulting from proton transfer is Fourier transform infrared spectroscopy (FTIR), 75 the use of which in the context of azobenzene-materials has been reviewed recently. 76 It provides information on which chemical groups are involved in the supramolecular bonding and the IR wavenumber shifts upon complexation can be qualitatively correlated with the supramolecular interaction strength. 75 Additionally, quantification of the extent of supramolecular bonding can be reached by using appropriate model compounds.…”

Driving Cells with Light‐Controlled Topographies

“…In this framework, light represents a very attractive external trigger, as it can be noninvasively provided according to arbitrary spatio‐temporal patterns at different wavelengths and polarization states . Light responsivity is typically obtained by dispersing or chemically binding photoactive units within the polymer network . Azobenzenes are among the most popular molecular photoswitches because of their ability to undergo a conformational change between two isomeric states when irradiated with UV or visible light .…”