Conserving Coherence and Storing Energy during Internal Conversion: Photoinduced Dynamics of cis- and trans-Azobenzene Radical Cations

Abstract

Light harvesting via energy storage in azobenzene has been a key topic for decades and the process of energy distribution over the molecular degrees of freedom following photoexcitation remains to be understood. Dynamics of a photoexcited system can exhibit high degrees of nonergodicity when it is driven by just a few degrees of freedom. Typically, an internal conversion leads to the loss of such localization of dynamics as the intramolecular energy becomes statistically redistributed over all molecular degrees of freedom. Here, we present a unique case where the excitation energy remains localized even subsequent to internal conversion. Strong-field ionization is used to prepare cis- and trans-azobenzene radical cations on the D₁ surface with little excess energy at the equilibrium neutral geometry. These D₁ ions are preferably formed because in this case D₁ and D₀ switch place in the presence of the strong laser field. The postionization dynamics are dictated by the potential energy landscape. The D₁ surface is steep downhill along the cis/trans isomerization coordinate and toward a common minimum shared by the two isomers in the region of D₁/D₀ conical intersection. Coherent cis/trans torsional motion along this coordinate is manifested in the ion transients by a cosine modulation. In this scenario, D₀ becomes populated with molecules that are energized mainly along the cis–trans isomerization coordinate, with the kinetic energy above the cis–trans interconversion barrier. These activated azobenzene molecules easily cycle back and forth along the D₀ surface and give rise to several periods of modulated signal before coherence is lost. This persistent localization of the internal energy during internal conversion is provided by the steep downhill potential energy surface, small initial internal energy content, and a strong hole–lone pair interaction that drives the molecule along the cis–trans isomerization coordinate to facilitate the transition between the involved electronic states

Conserved Vibrational Coherence in the Ultrafast Rearrangement of 2-Nitrotoluene Radical Cation

“…The large yield of parent 2-NT + as compared to previously reported mass spectra taken with femtosecond lasers 5,17,18 is consistent with the expected adiabatic ionization process at 1300 nm excitation. 22,35,36,[38][39][40][41] with the oscillations of the smaller fragment ions, consistent with coherent excitation by the probe pulse. 22,[29][30][31][32][33][34][35][36][37][38][39] The same transient dynamics with smaller-amplitude features were observed for lower pump and probe intensities (Supporting Information, Figure S4).…”

“…shown in Figure 13 hexadiene, 65 benzonitriles, 66,67 Rydberg states of N -methyl morpholine, 60,61 and azobenzene radical cation. 38 CO loss from excited-state metal hexacarbonyls has also been reported to result in coherent vibrational motion of the remaining metal pentacarbonyl. [68][69][70] We attribute this unexpected behavior to the exceptionally fast H-atom attack within ∼ 20 − 60 fs, which maintains the localized vibrational excitation along the torsional coordinate.…”

“…We use near-infrared (1300 nm) laser pulses for ionization because strong-field excitation with wavelengths in the range of ∼1200 − 1600 nm induces predominantly adiabatic electron-tunneling ionization instead of nonadiabatic multiphoton ionization, which dominates at the Ti:Sapphire wavelength of 800 nm. 40,41 Adiabatic ionization with near-infrared pulses has been widely used to prepare a high population of ground-electronic state polyatomic cations as vibrational wave packets, 22,35,36,38,39 and produces significant improvements in the resolution of ion-yield oscillations in pump-probe experiments as compared to using 800 nm for ionization. 35,39 Interpretation of the experimental dissociation dynamics is supported by a series of density functional theory (DFT) calculations of the energies, relaxation pathways, and vibrational frequencies of the 2-NT cation and its aci-nitro tautomer, along with molecular dynamics simulations to estimate the timescale of H-atom attack.…”

Coherent Vibrational and Dissociation Dynamics of Polyatomic Radical Cations

“…In contrast, the presence of multiple coupled normalm odes leads to fast dephasing of initially prepared vibrational wave packets in polyatomic radical cations within atm ost several picoseconds of the initial ionization event. Nevertheless, coherenti on yield oscillations have been observed in many classes of polyatomic radical cations including halomethanes, [32,33] 1,3-dibromopropane, [34] azobenzene, [35] alkyl aryl ketones, [21,36,37] nitrotoluenes, [27,38] and methylphosphonates. [22,39] Although in most cases, the ion yield oscillations are attributed to dynamics on D 0 ,d ynamics on excited ionic states have also been reported.…”

Switchable Reversible Addition–Fragmentation Chain Transfer (RAFT) Polymerization with the Assistance of Azobenzenes

“…Theg round-state geometries of several species participating in the photoregulating process,including cis-/trans-Azo, cis-/trans-Azo + ,and diisobutyronitrile radical/ anion (RC/R À ), were optimized using the DFT/B3LYP/PCM method (Supporting Information, Figure S13). [21] Thegroundstate energies of the above species were calculated and the molecular orbitals showed the preferential geometry and the charge localization ( Figure 3). Theexcitation energies of cisor trans-Azo for the S 1 (n-p*) state were calculated using the EOM-CCSD method, and it is found that the energy levels of the HOMO orbitals of cis-a nd trans-Azo are lower than the LUMO orbitals of diisobutyronitrile radical (Figure 3), and almost no electronic transition absorption occurs in trans-Azo (2.95 eV with an oscillator strength of f = 0.0000).…”