Optical absorption and excitonic coupling in azobenzenes forming self-assembled monolayers: a study based on density functional theory

Abstract

Based on the analysis of optical absorption spectra, it has recently been speculated that the excitonic coupling between individual azobenzene-functionalized alkanethiols arranged in a self-assembled monolayer (SAM) on a gold surface could be strong enough to hinder collective trans-cis isomerization-on top of steric hindrance [Gahl et al., J. Am. Chem. Soc., 2010, 132, 1831]. Using models of SAMs of increasing complexity (dimer, linear N-mers, and two-dimensionally arranged N-mers) and density functional theory on the (TD-) B3LYP/6-31G* level, we determine optical absorption spectra, the nature and magnitude of excitonic couplings, and the corresponding spectral shifts. It is found that at inter-monomer distances of about 20 Å and above, TD-B3LYP excitation frequencies (and signal intensities) can be well described by the frequently used point-dipole approximation. Further, calculated blue shifts in optical absorption spectra account for the experimental observations made for azobenzene/gold SAMs, and hint to the fact that they can indeed be responsible for reduced switching probability in densely packed self-assembled structures.

Optical properties of azobenzene-functionalized self-assembled monolayers: Intermolecular coupling and many-body interactions

“…This mechanism has been rationalized also for azobenzene dimers from a quantum-chemistry perspective. 23,24 In the present work we demonstrate that such spectral blue-shift is directly associated to LFE, expressed by the short-range e-h exchange interaction (Eq. 3).…”

“…23,24 In the present work, we investigate optical absorption properties of azobenzene-functionalized SAMs from a solid-state physics perspective. We treat the ordered SAMs as periodic systems, such that the electronic wave-functions are allowed to spread over the infinitely extended structure.…”

“…13,14 A number of experimental strategies have been suggested to overcome this problem, such as replacing the aliphatic linker by an aromatic one 15 or even bulkier groups, [16][17][18] or diluting the azobenzene moieties in densely-packed alkanethiolate SAMs. 14,[19][20][21][22] From a theoretical viewpoint the issue of hindered photo-isomerization in densely-packed azobenzenefunctionalized SAMs has been addressed by a few works, mainly focused on the excitonic coupling between the chromophores, 23,24 as well as on the effects of a metal substrate. 25 While these investigations have significantly contributed to describe the excited-state properties of azobenzene derivatives, a very recent publication elucidates the role of defects in the photo-isomerization of such SAMs.…”

UV/Vis Spectroscopy Studies of the Photoisomerization Kinetics in Self-Assembled Azobenzene-Containing Adlayers

“…However, for azobenzene, which is separated from the surface, these effects should not be able to suppress photoswitching according to a recent theoretical study by Benassi and Corni 26 . Alternatively, de-excitation may occur via excitonic coupling to the neighboring molecules, an effect recently studied using density functional theory (DFT) by Utecht et al 27 . In this study a noticeable reduction in the switching probability was found at typical intermolecular distances in azobenzene-containing SAMs.…”

Supramolecular design principles for efficient photoresponsive polymer–azobenzene complexes

“…2. To a good approximation, the transition dipole moment of the trans azobenzene typically coincides with an axis connecting the midpoints of the two benzene rings, 22 rendering the molecules that are perpendicular to the polarization plane of the incident light inert to the incident irradiation. Linearly polarized light thus orients the molecules perpendicular to the polarization plane, and the resulting anisotropy can be quantified by measuring at least three different interdependent figures of merit : dichroic ratio, order parameter, and photoinduced birefringence.…”