Oscillator strength and polarization of the forbidden n→π* band of trans-azobenzene: A computational study

Abstract

The trans-azobenzene molecule is thought to prefer a planar C2h geometry, in gas phase as well as in solution, according to the most recent computational studies. As a consequence, the weak n-->pi* absorption band is forbidden by symmetry at the equilibrium geometry, and its intensity depends on the effect of the vibrational motions on the electronic structure. In this computational study, we determine the contribution of the vibrational modes to the oscillator strength, taking into account the anharmonicity, the thermal distributions, and the solvent effects. The good agreement of our results with the measured absorption spectrum confirms the C2h equilibrium structure of trans-azobenzene, with a relatively easy torsion of the phenyl groups around the N--C bonds. We also address the question of the polarization of this transition, which is a preliminary step to interpret the time-resolved fluorescence anisotropy measurements [C.-W. Chang et al., J. Am. Chem. Soc., 126, 10109 (2004)], a very sensitive probe of solvent effects on the excited state dynamics.

ortho‐Fluoroazobenzenes: Visible Light Switches with Very Long‐Lived Z Isomers

“…Most noticeably, although in the calculated spectra of the E isomers of AB , F4 , and planar F4‐diester the n→π* transitions are dipole‐forbidden (oscillator strength f i =0), the n→π* transition is visible for the E isomer of nonplanar F4‐diester , and is in fact as strong as the n→π* signal of the corresponding Z form. Note that experimentally, however, the optical spectra in solution show a slight n→π* signal for all E isomers as a result of coupling of electronic transitions to molecular vibrations 24. Furthermore, the n→π* splittings between the E and Z isomers are in good qualitative agreement with those determined experimentally.…”

“…Note that experimentally, however, the optical spectra in solution show a slight n!p* signal for all E isomers as a result of coupling of electronic transitions to molecular vibrations. [24] Furthermore, the n!p* splittings between the E and Z isomers are in good qualitative agreement with those determined experimentally. Quantitatively, for the fluorinated species the Dl nÀp* values (up to about 100 nm, see Table 3) are too high, which is not surprising since the absolute excitation wavelengths are also overestimated (the computed n!p* absorptions are typically redshifted by several tens of nm with respect to the experimental bands).…”

Stochastic model for photoinduced anisotropy

“…In trans ‐azobenzene, the $ n \to \pi^* $ transition is symmetry forbidden at the equilibrium $C_{2h}$ geometry, but it borrows intensity from the lowest $ \pi \to \pi^* $ state. The orientation of the transition dipole is then well defined: it lies in the xy plane, almost parallel to the long axis of the molecule, and it makes an angle of about $50^\circ$ with the ◯ axis 23 , 24 . At $\beta=0^\circ$ and $180^\circ$ , the xy plane is orthogonal to the light polarization axis Ẑ, therefore at these angles we find the maximum of the ρ density.…”

Visible‐Light Photoswitching by Azobenzazoles

“…An obvious strategy to shift the absorption of an azobenzene‐type molecule into the visible is to extend the conjugation of the system. Although the use of symmetric systems prevents the effective use of the symmetry forbidden n–π* transition, it does avoid the potential problems of push–pull systems or relying on protonation to deliver visible‐light absorption. In this work, we report the photoswitching properties of symmetric azobenzazole photoswitches 1 – 3 .…”