The photoisomerization of azobenzene in solution was studied experimentally and by calculations. trans-to-cis and cis-to-trans dynamics are described through broadband transient absorption, fluorescence, and stimulated Raman spectroscopy. Transient absorption was extended to cover not only the nπ* band but also the ππ* band in the ultraviolet. Isomerization yields are used for a quantitative comparison of trans and cis transient spectra under different excitation. For the trans-to-cis path upon nπ*(S(1)) excitation, the evolution develops with 0.3, 3, and 16 ps. The first two times reflect population relaxation to a local minimum S(1t )(L) and subsequent transition to a dark intermediate S(1t)(D) over an 8 kJ/mol barrier. The existence of stationary points S(1t)(L) and S(1t)(D) is confirmed by quantum-chemical calculations. The third time corresponds to S(1t) (D) → S0 relaxation to the ground state via an S1/S0 conical intersection over a 12 kJ/mol barrier. Thus, the 16 ps time constant is attributed to the isomerization process and not to vibrational cooling, contrary to the current view and in line with the previous interpretation by Lednev et al. (J. Phys. Chem. 1996, 100, 13338). The decay of the long-lived intermediate S(1t)(D) is consistent with the hula twist rather than with the inversion mechanism. For the cis-totrans reaction following nπ* excitation, signal decay is strongly nonexponential, with 0.1 and 1 ps. The latter (1 ps) is much shorter than the 16 ps decay of the trans isomer, implying different S1/S0 conical intersections and relaxation paths for the cis-totrans and trans-to-cis reaction. New results are also obtained with ππ*(Sn) excitation. Thus, for trans-azobenzene, 50% of the population relaxes to an S1 region, which is not accessible under nπ* excitation. For cis-azobenzene, up to 30% of the excited species isomerize to trans via an Sn/S1 intersection, resulting in a mixed cis/trans S1 population. The isomerization kinetics of azobenzene shows no viscosity dependence, putting into question the torsion mechanism and suggesting the hula-twist isomerization mechanism.
Two-NIR-photon-triggered Z→E isomerization of an azobenzene was accomplished by covalently linking a two-photon-harvesting triarylamine antenna to a thermally stable ortho-fluorinated azobenzene derivative. The obtained photoswitch is fully addressable with visible and NIR light by using one-photon and two-photon excitation, respectively, with the latter offering enhanced penetration depth and improved spatial resolution.
Sum frequency mixing of fluorescence and ∼1300 nm gate pulses, in a thin β-barium borate crystal and non-collinear type II geometry, is quantified as part of a femtosecond fluorimeter [X.-X. Zhang et al., Rev. Sci. Instrum. 82, 063108 (2011)]. For a series of fixed phasematching angles, the upconversion efficiency is measured depending on fluorescence wavelength. Two useful orientations of the crystal are related by rotation around the surface normal. Orientation A has higher efficiency (factor ∼3) compared to B at the cost of some loss of spectral coverage for a given crystal angle. It should be used when subtle changes of an otherwise stationary emission band are to be monitored. With orientation B, the fluorescence range λ F > 420-750 nm is covered with a single setting of the crystal and less gate scatter around time zero. The accuracy of determining an instantaneous emission band shape is demonstrated by comparing results from two laboratories. Published by AIP Publishing. [http://dx
The terahertz (THz) absorption bands of biomolecular hydration layers are generally swamped by absorption from bulk water. Using the disaccharide trehalose, we show that this limitation can be overcome by attaching a molecular probe. By time-resolving the fluorescence shift of the probe, a local THz spectrum is obtained. From the dependence on temperature and H2O/D2O exchange, it is concluded that the trehalose hydration layer is being observed. The region of dynamic water perturbation by the disaccharide encompasses the probe and is therefore larger than the first two solvation layers.
We report on a bistable azobenzene derivative with sufficiently high 2-photon absorption to induce its photochemical isomerization and measurable excited state dynamics. Broadband transient absorption spectra were recorded and compared upon 1-photon (331 nm) and 2-photon (640 nm) excitation of the S0 → S2 transition. The spectra are different at early (t ∼ 1 ps) and late (t ∼ 100 ps) time but show similar photoisomerization behavior on a 10 ps time scale. With 2-photon excitation, strong population transfer S2 → Sn occurs due to resonance absorption of a third pump photon. Subsequent internal conversion Sn → S1 results in a very hot S1 population causing extra-broadening of the transient spectra. The resonance pump absorption is common with nonlinear excitation and should be taken into account when considering photochemical applications. The 2-photon excitation cross-section σ((2)) at 640 nm was measured to be 7 GM for the specific tetra-ortho-fluorinated azobenzene derivative and 1 GM for unsubstituted parent azobenzene. The direct 2-photon induced trans-to-cis isomerization, described herein, provides an unprecedented potential for spatially addressing P-type (bistable) azobenzene photoswitches in 3D.
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