Photophysics of hydrogen bonded diarylethene dimers in the liquid phase S. Kuehn a,⇑ , S. Friede a , M. Zastrow b , K. Schiebler b , K. Rueck-Braun b , T. Elsaesser a a Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max Born Str. 2A, D-12489 Berlin, Germany b Institut für Chemie, Technische Universität Berlin, Str. d. 17. Juni 135, D-10623 Berlin, Germany article info Article history: Received 24 September 2012 In final form 7 November 2012 Available online 16 November 2012 abstract Hydrogen bonded diarylethene dimers are formed through a carboxylic acid ligand, as is evident from concentration dependent OH stretching infrared spectra. Dimer formation leads to a red shift, enhance- ment and broadening of OH stretching absorption characteristic for cyclic carboxylic acid dimers. We compare the electronic spectra and photoinduced ring-opening and -closing kinetics of dimers and monomers. The S 0 –S 1 absorption band of the closed form undergoes a slight red-shift when going from the monomer to the singly closed dimer and the doubly closed dimer. The photoreaction kinetics of dimer and monomer are identical. Consequently, the photochrome retains its original photophysical properties in the dimer state. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Photochromism is the ability of a molecule to undergo a revers- ible photochemical reaction between optically, chemically or elec- tronically distinct states [1]. Among the broad manifold of photochromes, diarylethenes stand out with excellent thermal sta- bility, unsurpassed fatigue-resistance [2], fast response times and good photoreaction quantum yields. Upon ultraviolet photoexcita- tion, diarylethenes undergo a transition from an open-ring (o) to a closed-ring (c) isomer which can be fully reversed by irradiation with visible light. This class of molecules has received intense attention and is regarded as a highly promising candidate for fu- ture technological applications. Ultrafast all-optical information processing and molecular-level high density data storage are two visionary examples. A large body of knowledge has been gathered on the basic pho- tochromic process of the diarylethenes by studying highly dilute li- quid solutions. However, the realization of devices based on photochromes requires their integration on surfaces and at high densities. Under these circumstances, couplings between neigh- bouring photochromes become crucially important. Under favour- able conditions, the intermolecular coupling can lead to an improved photoreaction yield as has been observed for diaryleth- ene crystals [3] and silyl bridged dimers [4]. It can, however, also become detrimental to the function in through-conjugated diary- lethene oligomers where the photoconversion of one molecule ren- ders the neighbouring molecule unswitchable [1,5]. Apart from steric interactions, the intermolecular couplings can broadly be di- vided into those acting through space and those induced by the chemical bond which is utilized to control the separation between the pair of photochromes. The chemical bond may lead to an orbi- tal overlap and introduces a level of complexity [6] which makes it hard to single out contributions from different coupling mecha- nisms. It may even be arguable to what extent the monomer and the oligomer are comparable or whether they represent different chemical species. In this Letter, we present new results on hydrogen bonded di- mers of diarylethenes formed via a carboxylic acid ligand attached to the chromophore. The occurrence of dimers with a cyclic O– HO binding geometry is evident from the concentration depen- dent infrared spectra of the OH stretching vibration. The absence of an orbital overlap between the associated molecules provides access to the through-space contribution of the coupling. Analysis of the electronic spectra, the fluorescence properties and the pho- toreaction kinetics of the dimers points to a negligible electronic coupling via this route. Likewise, minor changes in the electronic transition energies in the singly and doubly closed dimer do not cause any detectable difference in the photophysical properties of dimer and monomer. Thus, this particular dimer geometry al- lows for linking chromophores in a well-defined geometry while retaining the photoswitching behavior of the monomers. 2. Experimental The analyte (1-(2,5-dimethyl-3-thienyl)-2-[2-carboxyl-5- (4-methoxyphenyl)-3-thienyl]perfluorocyclopentene (DTE, Scheme 1a) was synthesized according to a published procedure [7,8]. All solutions of DTE were prepared in degassed carbon tetra- chloride (Merck, spectroscopy grade) under a nitrogen atmosphere (<1% oxygen). The open-form isomer (o-DTE) was generated by irradiating the solution with a red power LED source emitting at 625 nm. The absorption spectrum was then inspected (Perkin-El- 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.11.012 ⇑ Corresponding author. Fax: +49 30 6392 1489. E-mail address: skuehn@mbi-berlin.de (S. Kuehn). Chemical Physics Letters 555 (2013) 206–211 Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett