On the Computational Design of Azobenzene-Based Multi-State Photoswitches

Abstract

In order to theoretically design multi-state photoswitches with specific properties, an exhaustive computational study is first carried out for an azobenzene dimer that has been recently synthesized and experimentally studied. This study allows for a full comprehension of the factors that govern the photoactivated isomerization processes of these molecules so to provide a conceptual/computational protocol that can be applied to generic multi-state photoswitches. From this knowledge a new dimer with a similar chemical design is designed and also fully characterized. Our theoretical calculations predict that the new dimer proposed is one step further in the quest for a double photoswitch, where the four metastable isomers could be selectively interconverted through the use of different irradiation sequences.

Keywords: absorption spectra; azobenzene dimers; molecular photoswitches; theoretical photochemistry.

Scheme 1

Molecular structure of dimer 1 in its most stable (EE) form.

Figure 1

Computed absorption spectra of dimer 1 in acetonitrile at the optimized ground-state geometries of the four stable isomers of dimer 1. Vertical red lines indicate the position of the vertical transitions while the black contour corresponds to the convolution with a Lorentzian function depicted in order to theoretically reproduce the experimental absorption spectrum.

Figure 2

Shape of the molecular orbitals involved in the relevant excited electronic states of the EE isomer of dimer 1. The drawings correspond to an iso-level of 0.02 a.u. of electronic density. The left top corner of the figure depicts the molecular structure of the EE isomer.

Figure 3

Full energy scheme of the ground and lowest excited electronic states of dimer 1. See text for details. Relative energies of the different structures (horizontal lines) are given in kcal/mol. Green arrows indicate the wavelengths of the vertical excitations from each conformer. Black lines indicate the ground electronic state whereas excited electronic states are in blue. Red lines correspond to crossing sections between S₀ and S₁.

Figure 4

Relaxed scans of the potential energy surfaces of the ground electronic states along all the isomerization pathways (the CNNC dihedral angle of each azobenzene unit). At each point of the scan the energies of the lowest six excited electronic states are computed. The lines are drawn with different colors and adiabatically follow the energies of each electronic state so that they never cross.

Scheme 2

Molecular structure of dimer 2 in its most stable (EE) form.

Figure 5

Computed absorption spectra of dimer 2 in acetonitrile at the optimized ground-state geometries of the four stable isomers of dimer 2. Vertical red lines indicate the position of the vertical transitions while the black contour corresponds to the convolution with a Lorentzian function depicted in order to theoretically reproduce the experimental absorption spectrum.

Figure 5

Computed absorption spectra of dimer 2 in acetonitrile at the optimized ground-state geometries of the four stable isomers of dimer 2. Vertical red lines indicate the position of the vertical transitions while the black contour corresponds to the convolution with a Lorentzian function depicted in order to theoretically reproduce the experimental absorption spectrum.

Figure 6

Full energy scheme of the ground and lowest excited electronic states of dimer 2. See text for details. Relative energies of the different structures (horizontal lines) are given in kcal/mol. Green arrows indicate the wavelengths of the vertical excitations from each conformer. Black lines indicate the ground electronic state whereas excited electronic states are in blue. Red lines correspond to crossing sections between S₀ and S₁.

Figure 7

Shape of the molecular orbitals involved in the relevant excited electronic states of the EE isomer of dimer 2. The drawings correspond to an iso-level of 0.02 a.u. of electronic density. The left top corner of the Figure depicts the molecular structure of the EE isomer.

Figure 7

Shape of the molecular orbitals involved in the relevant excited electronic states of the EE isomer of dimer 2. The drawings correspond to an iso-level of 0.02 a.u. of electronic density. The left top corner of the Figure depicts the molecular structure of the EE isomer.

Figure 8

Allowed photo-isomerizations for dimer 2. (a): Activated through low-energy photoexcitation (λ > 425 nm). (b): Activated through high-energy photo-excitation (λ < 350 nm).