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Fast photodynamics of azobenzene probed by scanning excited-state potential energy surfaces using slow spectroscopy.

Tan EM, Amirjalayer S, Smolarek S, Vdovin A, Zerbetto F, Buma WJ - Nat Commun (2015)

Bottom Line: Azobenzene, a versatile and polymorphic molecule, has been extensively and successfully used for photoswitching applications.For S1(nπ*), we find that changes in the hybridization of the nitrogen atoms are the driving force that triggers isomerization.In combination with quantum chemical calculations we conclude that photoisomerization occurs along an inversion-assisted torsional pathway with a barrier of ~2 kcal mol(-1).

View Article: PubMed Central - PubMed

Affiliation: Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands.

ABSTRACT
Azobenzene, a versatile and polymorphic molecule, has been extensively and successfully used for photoswitching applications. The debate over its photoisomerization mechanism leveraged on the computational scrutiny with ever-increasing levels of theory. However, the most resolved absorption spectrum for the transition to S1(nπ*) has not followed the computational advances and is more than half a century old. Here, using jet-cooled molecular beam and multiphoton ionization techniques we report the first high-resolution spectra of S1(nπ*) and S2(ππ*). The photophysical characterization reveals directly the structural changes upon excitation and the timescales of dynamical processes. For S1(nπ*), we find that changes in the hybridization of the nitrogen atoms are the driving force that triggers isomerization. In combination with quantum chemical calculations we conclude that photoisomerization occurs along an inversion-assisted torsional pathway with a barrier of ~2 kcal mol(-1). This methodology can be extended to photoresponsive molecular systems so far deemed non-accessible to high-resolution spectroscopy.

No MeSH data available.


Rotational contour of bands in the S1(nπ*)←S0 excitation spectrum near the forbidden 0–0 transition.(a) Experimentally observed contours. (b) Simulated contours assuming a vibronic Bu←Ag transition, a rotational temperature of 9 K and a homogeneous linewidth for individual rotational transitions of 0.4 cm−1.
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f3: Rotational contour of bands in the S1(nπ*)←S0 excitation spectrum near the forbidden 0–0 transition.(a) Experimentally observed contours. (b) Simulated contours assuming a vibronic Bu←Ag transition, a rotational temperature of 9 K and a homogeneous linewidth for individual rotational transitions of 0.4 cm−1.

Mentions: Figure 3 shows fits of the rotational contour of bands in the initial part of the excitation spectrum that start from the vibrationless ground state and use rotational constants and electronic transition moments as calculated. Excellent agreement between the experiment and simulation is observed, leading to the conclusion that under our experimental conditions the rotational population distribution can be described by a rotational temperature of 9 K. More importantly, these simulations show that the homogeneous linewidth of the individual rotational transitions is 0.4 cm−1 corresponding to a lifetime of the lower rovibronic levels of S1 of about 13 ps (see Supplementary Note 3 for a further discussion of the fits). This lifetime is almost an order of magnitude longer than the lifetime reported so far in solution-phase experiments on the S1(nπ*) state11121416. The large difference between gas- and solution-phase experiments is remarkable, but, as argued below, is completely in line with the different experimental conditions under which the two types of experiments have been performed.


Fast photodynamics of azobenzene probed by scanning excited-state potential energy surfaces using slow spectroscopy.

Tan EM, Amirjalayer S, Smolarek S, Vdovin A, Zerbetto F, Buma WJ - Nat Commun (2015)

Rotational contour of bands in the S1(nπ*)←S0 excitation spectrum near the forbidden 0–0 transition.(a) Experimentally observed contours. (b) Simulated contours assuming a vibronic Bu←Ag transition, a rotational temperature of 9 K and a homogeneous linewidth for individual rotational transitions of 0.4 cm−1.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4308720&req=5

f3: Rotational contour of bands in the S1(nπ*)←S0 excitation spectrum near the forbidden 0–0 transition.(a) Experimentally observed contours. (b) Simulated contours assuming a vibronic Bu←Ag transition, a rotational temperature of 9 K and a homogeneous linewidth for individual rotational transitions of 0.4 cm−1.
Mentions: Figure 3 shows fits of the rotational contour of bands in the initial part of the excitation spectrum that start from the vibrationless ground state and use rotational constants and electronic transition moments as calculated. Excellent agreement between the experiment and simulation is observed, leading to the conclusion that under our experimental conditions the rotational population distribution can be described by a rotational temperature of 9 K. More importantly, these simulations show that the homogeneous linewidth of the individual rotational transitions is 0.4 cm−1 corresponding to a lifetime of the lower rovibronic levels of S1 of about 13 ps (see Supplementary Note 3 for a further discussion of the fits). This lifetime is almost an order of magnitude longer than the lifetime reported so far in solution-phase experiments on the S1(nπ*) state11121416. The large difference between gas- and solution-phase experiments is remarkable, but, as argued below, is completely in line with the different experimental conditions under which the two types of experiments have been performed.

Bottom Line: Azobenzene, a versatile and polymorphic molecule, has been extensively and successfully used for photoswitching applications.For S1(nπ*), we find that changes in the hybridization of the nitrogen atoms are the driving force that triggers isomerization.In combination with quantum chemical calculations we conclude that photoisomerization occurs along an inversion-assisted torsional pathway with a barrier of ~2 kcal mol(-1).

View Article: PubMed Central - PubMed

Affiliation: Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands.

ABSTRACT
Azobenzene, a versatile and polymorphic molecule, has been extensively and successfully used for photoswitching applications. The debate over its photoisomerization mechanism leveraged on the computational scrutiny with ever-increasing levels of theory. However, the most resolved absorption spectrum for the transition to S1(nπ*) has not followed the computational advances and is more than half a century old. Here, using jet-cooled molecular beam and multiphoton ionization techniques we report the first high-resolution spectra of S1(nπ*) and S2(ππ*). The photophysical characterization reveals directly the structural changes upon excitation and the timescales of dynamical processes. For S1(nπ*), we find that changes in the hybridization of the nitrogen atoms are the driving force that triggers isomerization. In combination with quantum chemical calculations we conclude that photoisomerization occurs along an inversion-assisted torsional pathway with a barrier of ~2 kcal mol(-1). This methodology can be extended to photoresponsive molecular systems so far deemed non-accessible to high-resolution spectroscopy.

No MeSH data available.