Chalcogen-Guided Control of Azoarene Photoswitching: Tuning Excited-State Energies Through Electronic Property Modulation

Abstract

In recent years, chalcogen bonding has emerged as a promising alternative to classical supramolecular interactions such as hydrogen or halogen bonds. While its behavior in the electronic ground state has been extensively studied, its role in the excited state is gaining increasing attention. We recently demonstrated that the lack of photoswitchability of ortho-tellurated azobenzenes is due to an excitation-induced conversion of the classical chalcogen bond into a more pronounced, electron-rich three-electron σ bond. This transformation significantly strengthens the interaction between the chalcogen and the Lewis base center, effectively preventing isomerization. Based on these findings, we have now investigated the photoswitching behavior of ortho-tellurium-substituted azoarenes by modulation of the electronic properties of the aryl substituent and the oxidation state of the tellurium center. Our results show that electron-donating groups destabilize the excited-state geometry associated with the formation of a three-electron σ bond, thereby restoring photoisomerizability. Furthermore, oxidation to the Te(IV) species disrupts this bonding interaction, leading to significantly enhanced photoswitching properties. Together, these findings provide valuable design principles for the development of multiresponsive molecular switches based on chalcogen bonding and excited-state control.

Keywords: azo compounds; chalcogen bonding; excited state; molecular switches; three‐electron σ bond.

Figure 1

a) Potential energy profile of azobenzene ( A ) and chalcogen‐substituted azobenzenes ( B and C ) in the singlet ground (S₀, solid) and excited states (S₁, dashed). Upon excitation, the non‐covalent chalcogen bond present in the ground state of the trans isomers converts into a covalent three‐electron σ bond in the excited state, resulting in stabilization of B and C relative to their non‐bonded counterparts B′ and C′ . b) Change in orbital energies for tellurium‐substituted azobenzenes upon light excitation. The splitting of the (n_N + n_Te) and (n_N – n_Te) orbitals (red arrows), caused by the formation of a covalent three‐electron σ bond, is hardly affected by the substituent on the second ring. In contrast, the energy difference between the (n_N – n_Te) and π* orbitals (blue arrows) is strongly influenced by the nature of the substituent.

Figure 2

Investigated tellurium‐containing azoarenes 1–3 and selenium‐containing azobenzenes 4 and 5.

Scheme 1

Synthesis of the ortho‐tellurium substituted azoarenes 1–3 and the selenium‐containing compound 5 (for the definition of Ar see Figure 2). Reaction conditions: i) n‐BuLi, Ph₂Te₂, diethyl ether (1a: 48%; 1b: 6%, 1c: 52%, 1d: 16%, 1e: 21%, 1f: 15%). ii) SO₂Cl₂, dichloromethane (>99%). iii) Br₂, diethyl ether (3a: 78%; 3b: 71%, 3c: 77%, 3d: 84%, 3e: 82%, 3f: 70%). iv) KOt‐Bu, Ph₂Se₂, dimethyl sulfoxide, 47%.

Figure 3

a) Stereoisomeric structures trans‐I and trans‐II of 1a exhibiting two different chalcogen bonds. b) Solid state structure of 1c. The Te···N interaction is visualized as a red dashed line. The distance is given in angstroms.

Figure 4

Solid state structure of 3e. a) Inter‐ and intramolecular interactions of the tellurium center. b) Intermolecular interactions of bromine atoms. Interactions with distances shorter than the sum of the van der Waals radii are depicted as red dashed lines. Distances are given in angstroms.

Figure 5

Sections from the ¹H NMR spectra of the azoarene 1d: a) after synthesis (initial state, trans/cis: 100/0), b) after UV irradiation with λ = 365 nm (trans/cis: 58/42), c) after irradiation with λ = 405 nm (trans/cis: 72/28) d) and after irradiation with λ = 530 nm (trans/cis: 98/2) (CD₂Cl₂, 400 MHz, c = 5 mm).

Figure 6

Overview of the cis ratio [%] for the investigated azoarenes 1 after different photochemical treatments. After synthesis (IS, black), after irradiation with λ = 365 nm (red), after irradiation with λ = 405 nm (blue) and after irradiation with λ = 530 nm (green).

Figure 7

Section from the ¹H NMR spectra of the azoarene 1d: after synthesis (trans/cis: 99/1), after UV irradiation with λ = 365 nm at room temperature (trans/cis: 58/42), 0 °C (trans/cis: 60/40), −40 °C (trans/cis: 67/33), −70 °C (trans/cis: 72/28) (CD₂Cl₂, 600 MHz, c = 5 mm).

Figure 8

Change of the cis ratio of the investigated azoarenes 1d, 4, and 5 at different temperatures (CD₂Cl₂, c = 5 mm). The cis ratio at −40 °C is normalized to 100%.

Figure 9

Overview of the cis ratios [%] for the investigated azoarenes 2 and 3 after different photochemical treatments. After synthesis (IS, black), after irradiation with λ = 365 nm (red), after irradiation with λ = 405 nm (blue) and after irradiation with λ = 530 nm (green).