Jan Boyke Schönborn

Unusual photochemical dynamics of a bridged azobenzene derivative

In a large-scale simulation study of ultrafast photochemical dynamics for an azobenzene compound with an additional ethylenic bridge we have found unexpected features: while the dynamics starting from the Z isomer follow a barrierless path with steep gradients, the dynamics starting from the E isomer proceed through a different conical intersection surrounded by a rather flat potential energy landscape and then encounter a sizeable barrier in the electronic ground state that markedly influences the reaction behavior. Direct comparisons with experimental static UV spectra, quantum yields, and transient absorption spectra show good agreement and reveal signatures of this unusual behavior.

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

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.

Photochemical ring-opening of cyclohexadiene: quantum wavepacket dynamics on a global ab initio potential energy surface.

We have assembled a global CASSCF potential energy surface for the excited 2A state of the cyclohexadiene -hexatriene system, in two degrees of freedom, with full relaxation in all other degrees of freedom. Quantum wavepacket dynamics on this surface yields simple interpretations of recent experimental data on the ultrafast photochemical ring-opening of cyclohexadiene as well as predictions on preferred product configurations. The feasibility of this system as a model for fulgide molecular switches is discussed.

Photochemical dynamics of E-iPr-furylfulgide

As an important theoretical step towards unraveling the mechanistic details of the photochemical switching processes in molecules of the fulgide type, we carried out a large-scale, full-dimensional computational study of the ring closure reaction of E-iPr-furylfulgide. Simulated static UV spectra and femtosecond transient spectra are in good agreement with their experimental counterparts. Using surface-hopping photodynamics simulations, we identify three major de-excitation pathways and their interplay. The dominant photochemical pathway (70% of the trajectories) allows for ring closure, while the two minor pathways involve E-Z double bond isomerization rather than cyclization. The relative abundance of the pathways is rationalized by arguments linking structure with dynamics. It should be emphasized, however, that the distinction into three pathways is only a simplified interpretational model, since the actual dynamical trajectories do not strictly follow these idealized pathways but often show mixed behaviour, evolving along two or three of them during the course of the simulation.

Brownian molecular rotors: Theoretical design principles and predicted realizations

We propose simple design concepts for molecular rotors driven by Brownian motion and external photochemical switching. Unidirectionality and efficiency of the motion is measured by explicit simulations. Two different molecular scaffolds are shown to yield viable molecular rotors when decorated with suitable substituents.