UV-Light-Induced Vibrational Coherences: The Key to Understand Kasha Rule Violation in trans-Azobenzene

Abstract

We combine sub-20 fs transient absorption spectroscopy with state-of-the-art computations to study the ultrafast photoinduced dynamics of trans-azobenzene (AB). We are able to resolve the lifetime of the ππ* state, whose decay within ca. 50 fs is correlated to the buildup of the nπ* population and to the emergence of coherences in the dynamics, to date unobserved. Nonlinear spectroscopy simulations call for the CNN in-plane bendings as the active modes in the subps photoinduced coherent dynamics out of the ππ* state. Radiative to kinetic energy transfer into these modes drives the system to a high-energy planar nπ*/ground state conical intersection, inaccessible upon direct excitation of the nπ* state, that triggers an ultrafast (0.45 ps) nonproductive decay of the nπ* state and is thus responsible for the observed Kasha rule violation in UV excited trans-AB. On the other hand, cis-AB is built only after intramolecular vibrational energy redistribution and population of the NN torsional mode.

Picosecond Switchable Azo Dyes

“…Overall, the virtually barrierless decay path to the CI, which involves a switch from the ππ* to the nπ* state, is consistent with the identification of two time constants ( τ 1 and τ 2 ) in the subpicosecond range and the early appearance of the transient absorption around 600 nm, tentatively assigned to the ground‐state cis minimum. Our mechanistic picture that involves mainly rotation is also consistent with the one described for other azobenzene derivatives, such as the parent azobenzene or recently synthesized phenylazoindoles …”

“…Similar dynamics with 3–4 time constants in the femto‐ and picosecond range have been described for push–pull‐substituted derivatives . Regarding the mechanism, recent theoretical studies favor isomerization through the torsional mechanism …”

“…The shortest time constant has been assigned to the S 2 →S 1 decay, which corresponds to a switch from the ππ* to the nπ* state. After this step, there is agreement that the population of excited molecules splits between two branches: one that isomerizes to the cis isomer and another one that returns to the trans configuration . These processes are responsible for the ≈1 and ≈3 ps time constants.…”

“…These processes are responsible for the ≈1 and ≈3 ps time constants. Most studies assume that the trans ‐to‐ cis isomerization takes place within this timescale . During this process, the population of excited molecules decays to the ground state, and the 15 ps time constant is a signature of vibrational cooling of the hot ground state formed during the decay.…”

“…After this step, there is agreement that the population of excited molecules splits between two branches:o ne that isomerizes to the cis isomer and another one that returns to the trans configuration. [34][35][36][37][38] These processesa re responsible for the % 1a nd % 3pst ime constants. Most studies assumet hat the trans-to-cis isomerization takes place within this timescale.…”