We have investigated the ultrafast dynamics of o-, m- and p-bisazobenzenes, which represent elementary building blocks for photoswitchable multiazobenzene nanostructures. The connectivity pattern within bisazobenzenes and the ensuing complex interactions between the individual azobenzene units determines the ultrafast dynamics of these compounds and their photochemical properties. While retaining a relatively high E → Z isomerization quantum yield, o-bisazobenzene exhibits a very high thermal relaxation rate (half-life of 1.6 ms). Our theoretical calculations reveal that the geometry allows intramolecular excitonic interaction between the azobenzene units, which is reflected in the femtosecond transient absorption data via the simultaneous bleaching of the two excitonic bands. In contrast, the properties of m-bisazobenzene are very similar to the monomeric azobenzene, with the two units acting nearly independently from each other. The highest degree of π conjugation extending over the two azobenzene units was observed for p-bisazobenzene, which results in strong planarity of the molecule, reduced excited state lifetime and relatively low isomerization quantum yield. Multiphotochromic systems bridge the gap between molecular photoswitches and macroscopic function and thus, understanding the properties of bisazobenzenes opens the way to the design and development of new structures with extensive and versatile applications.

The new class of microbial rhodopsins,c alled xenorhodopsins (XeRs), [1] extends the versatility of this family by inward H + pumps. [2][3][4] These pumps are an alternative optogenetic tool to the light-gated ion channels( e.g. ChR1,2), because the activation of electrically excitable cells by XeRs is independent from the surrounding physiological conditions.In this work we functionally and spectroscopically characterized XeR from Nanosalina (NsXeR). [1] The photodynamic behavior of NsXeR was investigated on the ps to st ime scale elucidating the formation of the Ja nd Ka nd ap reviously unknown long-lived intermediate.T he pH dependent kinetics reveal that alkalization of the surrounding medium accelerates the photocycle and the pump turnover.Inpatch-clamp experiments the blue-light illumination of NsXeR in the Ms tate shows ap otential-dependent vectoriality of the photocurrent transients,s uggesting av ariable accessibility of reprotonation of the retinal Schiff base.Insights on the kinetically independent switching mechanism could furthermore be obtained by mutational studies on the putative intracellular H + acceptor D220.

Photochromic switches are essential for the control and manipulation of nanoscale reactions and processes. The expansion of their application to aqueous environments depends strongly on the development of optimized water-soluble photoswitches. Here we present a femtosecond time-resolved investigation of the photochromic reactions (transition between the open and the closed form) of a water-soluble indolylfulgimide. We observe a pronounced effect of the protic nature of water as a solvent on the ultrafast ring-opening reaction. Typically, the excited state of the closed form has a larger dipole moment than the ground state, which leads to stabilization of the excited state in polar solvents and hence a lifetime (3 ps) longer than in non-polar solvents (2 ps). However, in water, despite the increased solvent polarity and the increased excited state dipole moment, the opposite trend for the excited state lifetime is observed (1.8 ps). This effect is caused by the opening of a new excited state deactivation pathway involving proton transfer reactions.