Ultrafast Dynamics of Encapsulated Molecules Reveals New Insight on the Photoisomerization Mechanism for Azobenzenes

Abstract

Spatial confinement can have a profound impact on the dynamics of chemical reactions, especially for isomerization reactions that involve large-amplitude structural rearrangement of a molecule. This work uses ultrafast spectroscopy to probe the effects of confinement on trans → cis photoisomerization following ππ* excitation of 4-propyl stilbene and 4-propyl azobenzene encapsulated in a supramolecular host−guest complex. Transient absorption spectroscopy of the encapsulated azobenzene derivative reveals the formation of two distinct excited-state species with spectral signatures resembling the cis and trans isomers. Formation of the cis species indicates a direct excited-state isomerization channel that is not observed in cyclohexane solution. Comparison with the stilbene analogue suggests that this "hot" excited-state isomerization pathway for encapsulated azobenzene involves primarily in-plane inversion, whereas a 10-fold increase of the excited-state lifetime for the trans isomer suggests that crowding in the capsule hinders isomerization from the relaxed S 1 geometry of the trans isomer. This work provides new mechanistic insight on the relative roles of inversion and rotation in the ultrafast photoisomerization of azobenzene derivatives.

Iminothioindoxyl as a molecular photoswitch with 100 nm band separation in the visible range

“…4c, shows a fast rise component, on a timescale of a few hundred femtoseconds, and a slower decay, occurring within 12–16 ps. The timescale of the anisotropy decay is in line with what has been observed for azobenzene in solution 44 . The rise of the anisotropy within the initial 500 fs indicates that a significant charge redistribution rapidly occurs once the molecule starts to move on the excited state potential energy surface towards the conical intersection region, in line with the computed large difference in transition dipole moments for the Z and E forms (Table 1).…”

“…The remaining SADS are assigned to the S 1 state (black line), to the hot Z isomer (red curve) and the E isomer (blue curve). The very short S 2 lifetime is again similar to what is known for azobenzene, for which a value of 50 fs has been recently determined 44,46 . The decay of the broad S 1 excited state band within 320 fs and the rise of anisotropy on the same timescale indicate that ITI reaches the conical intersection region on a time scale competing with vibrational relaxation in S 1 .…”

“…The negligible change in the excited state relaxation time scale observed in solvents with different viscosity (see Supplementary Fig. 38) in first instance favors an inversion mechanism, although most probably the simple vision of motion along a single reaction coordinate is not realistic, as recently pointed out for azobenzene 44 .…”

Light‐Responsive (Supra)Molecular Architectures: Recent Advances

“…Despite the large amount of investigations, dating back to the first time the Z isomer was isolated and identified by Hartley, the detailed mechanism of isomerization of azobenzene and its derivatives has been the subject of much controversy . Indeed, this problem has challenged experimentalists and theoreticians for many years and remains a debated issue even today. The isomerization mode (thermal vs light induced), irradiation wavelength, solvent properties, substitution pattern, temperature, and pressure markedly influence the isomerization mechanism and quantum yield of the photoprocess.…”

Metastable Chiral Azobenzenes Stabilized in a Double Racemate

“…However,the lack of space in the synergistically generated cavity inside the crystal of the OBPDI host might hinder the rotation of the trapped AZO molecules into the cis form. Nevertheless,t he twisted AZO conformation provides insights into the favorable pathway an AZO molecule tends to prefer when it gains energy from an external source and begins to undergo conformational change,t hereby emerging as aw ay to shed light on the old debate [37][38][39][40][41][42][43] regarding the mechanism of the trans-cis isomerization of azobenzene (Table S1, Supporting Information).…”