title: Connectivity matters - ultrafast isomerization dynamics of bisazobenzene photoswitches.
abstract: 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.
authors.affiliation: Institute of Physical and Theoretical Chemistry, Goethe University, Max-von-Laue-Str. 7, 60438 Frankfurt am Main, Germany. wveitl@theochem.uni-frankfurt.de.
authors.forename: Chavdar
authors.initials: C
authors.lastname: Slavov
authors.affiliation: Theoretical and Computational Chemistry, Interdisciplinary Center for Scientific Computing (IWR), University of Heidelberg, Im Neuenheimer Feld 368, 69120 Heidelberg, Germany.
authors.forename: Chong
authors.initials: C
authors.lastname: Yang
authors.affiliation: Institute of Organic Chemistry, Justus-Liebig University Giessen, Heinrich-Buff-Ring 17, 35392 Giessen, Germany. Hermann.A.Wegner@org.Chemie.uni-giessen.de.
authors.forename: Luca
authors.initials: L
authors.lastname: Schweighauser
authors.affiliation: Institute of Physical and Theoretical Chemistry, Goethe University, Max-von-Laue-Str. 7, 60438 Frankfurt am Main, Germany. wveitl@theochem.uni-frankfurt.de.
authors.forename: Chokri
authors.initials: C
authors.lastname: Boumrifak
authors.affiliation: Theoretical and Computational Chemistry, Interdisciplinary Center for Scientific Computing (IWR), University of Heidelberg, Im Neuenheimer Feld 368, 69120 Heidelberg, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Dreuw
authors.affiliation: Institute of Organic Chemistry, Justus-Liebig University Giessen, Heinrich-Buff-Ring 17, 35392 Giessen, Germany. Hermann.A.Wegner@org.Chemie.uni-giessen.de.
authors.forename: Hermann A
authors.initials: HA
authors.lastname: Wegner
authors.affiliation: Institute of Physical and Theoretical Chemistry, Goethe University, Max-von-Laue-Str. 7, 60438 Frankfurt am Main, Germany. wveitl@theochem.uni-frankfurt.de.
authors.forename: Josef
authors.initials: J
authors.lastname: Wachtveitl