Superior Z → E and E → Z photoswitching dynamics of dihydrodibenzodiazocine, a bridged azobenzene, by S₁(nπ*) excitation at λ = 387 and 490 nm

Abstract

The ultrafast Z→E and E→Z photoisomerisation dynamics of 5,6-dihydrodibenzo[c,g][1,2]diazocine (1), the parent compound of a class of bridged azobenzene-based photochromic molecular switches with a severely constrained eight-membered heterocyclic ring as central unit, have been studied by femtosecond time-resolved spectroscopy in n-hexane as solvent and by quantum chemical calculations. The diazocine contrasts with azobenzene (AB) in that its Z rather than E isomer is the energetically more stable form. Moreover, it stands out compared to AB for the spectrally well separated S(1)(nπ*) absorption bands of its two isomers. The Z isomer absorbs at around λ = 404 nm, the E form has its absorption maximum around λ = 490 nm. The observed transient spectra following S(1)(nπ*) photoexcitation show ultrafast excited-state decays with time constants τ(1) = 70 fs for the Z and <50 fs for the E isomer reflecting very fast departures of the excited wave packets from the S(1) Franck-Condon regions and τ(2) = 270 fs (320 fs) related to the Z→E (resp. E→Z) isomerisations. Slower transient absorption changes on the time scale of τ(3) = 5 ps are due to vibrational cooling of the reaction products. The results show that the unique steric constraints in the diazocine do not hinder, but accelerate the molecular isomerisation dynamics and increase the photoswitching efficiencies, contrary to chemical intuition. The observed isomerisation times and quantum yields are rationalised on the basis of CASPT2//CASSCF calculations by a S(1)/S(0) conical intersection seam at a CNNC dihedral angle of ≈96° involving twisting and torsion of the central CNNC moiety. With improved photochromism, high quantum yields, short reaction times and good photostability, diazocine 1 and its derivatives constitute outstanding candidates for photoswitchable molecular tweezers and other applications.

Nonadiabatic ab initio molecular dynamics of photoisomerization in bridged azobenzene

“…Experimentally, an ultrashort lifetime of the initally excited FC state in S 1 sate for 1Z isomer was about 70 fs. 17 The time scale of this isomerization in our simulations was about 450 fs on average.…”

“…In the experiments done by Siewertsen et al ultrafast excitedstate decays with time constant t < 50 fs for the 1E isomer reflecting very fast departures of the excited wave packets from the S 1 Franck-Condon (FC) regions. 17 The time scale of the trans-cis isomerization in our simulations was about 450 fs. Figure 4 shows the time dependence of selected geometrical parameters for reactive trajectories starting from the 1E isomer.…”

“…The quantum yield for cistrans reaction was estimated to be (66 ± 6)%, in good agreement with the reported experimental value of (72 ± 4)%. 17 The time-dependent population, averaged over 35 trajectories, is shown in Figure 3(b) in the dynamics starting from the 1Z isomer. The decay times of the S 1 state in the cis form varied from 31.8 fs to 67.7 fs with a mean value of 40.4 fs for all 35 trajectories and the population of S 1 state decayed to zero within 68 fs.…”

“…The quantum yield for 1E-1Z reactions in our calculations was valued to be (54 ± 8)%, showing very good agreement with the experimental value of (50 ± 10)%. 17 Standard deviations were computed as σ = √ φ(1 − φ)/N t in the quantum yield calculation, in which ϕ and N t are the quantum yield and the number of trajectories in the dynamics simulations, respectively. Figure 3(a) shows the time-dependent population of the S 0 and S 1 states averaged over 35 trajectories for dynamics starting from the 1E isomer.…”

“…As a derivative of azobenzene, bridged azobenzene has been very recently recognized to be a molecular photoswitch by Siewertsen et al 16,17 and turns out to have superior photochromic properties compared to azobenzene. The ultrafast photoisomerization dynamics have been studied by femtosecond timeresolved spectroscopy in n-hexane as solvent.…”