Photoswitchable Association of an Azobenzen+Bipyridinium Diad to Eosin: Photostimulated "On-Off zyxwvuts " Guest Binding ** zyxwv By Itamar Willner*, Sharon Marx, and Yoav Eichen Photoswitchable devices evoke continuous interest as in- formation storage systems and signal amplification units of optical signals.". 21 Recent activities in photostimulation of macromolecules' functions[31 have demonstrated that physi- cal properties of photochromic polymers such as permeabil- ity,l41 volume,t51 or viscosity[61 can be controlled by light. Similarly, catalytic['] and binding properties of proteins con- taining photochromic components anchored to their back- bones''' can be activated by light (i.e., phototriggered). Photostimulated "on-off' chemical reactivity at the molecular level has, however, been less explored. Photocon- trolled binding of ions to photochromic ionophores such as azobenzene crown ethers['] or "pol yether spiropyran"["] represent phototriggered chemical processes at the mo- lecular level. Here we report on the photostimulated on- off binding of zyxwvutsr N-methyl-N'-[4-(phenylazo)benzyl]-4,4- bipyridinium (Azo-V", 1) to eosin Y (Eo2-, 2). The diad trans-1 was synthesized by the reaction of trans- 1-(bromomethyl)-4-phenylazobenzene, with N-methyl-4, 4-bipyridinium. The resulting diad exhibits photochromic properties: Illumination of trans-1 zyxwvut (A = 366 nm) results in isomerization to cis-1, and upon illumination of cis-1 zyxwvut (A > 400 nm) trans-1 is restored. Recently, we reported h = 366nm zyxwvutsrqpo trans-1 cis-1 on the structural and chemical properties of donor-acceptor complexes formed between bipyridinium salts and xanthene dyes, such as eosin.1' '1 We found that electrostatic interac- Br 2 tions as well as x--71 donor-acceptor and charge-transfer in- teractions play important roles in stabilization of these com- plexes. ['I Prof. Dr. I. Willner Institute of Chemistry and the Farkas Center for Light-Induced Processes The Hebrew University of Jerusalem Jerusalem 91 904 (Israel) [**I This research was supported by the Basic Research Foundation adminis- tered by the Israel Academy of Sciences and Humanities. The Farkas Center is supported by the Bundesministerium fur Forschung und Tech- nologie and the Minerva Gesellschaft. Trans-1 as well as cis-1 form 1 : 1 complexes with 2, but the complexes differ substantially in their association constants. Figure 1 shows the spectral changes of 2 upon addition of cis-1. By applying the Benesi-Hildebrand relation," the derived association constant K,(cis) is determined to be 38000 M-'. The association constant K,(trans) of trans-1 to 2 is 3000 M- '. Thus, functionalization of the bipyridinium I zyxw AA zyxwv 1 lnml - 1 lnml - Fig. 1. Left: Spectral changes of Eo2- (2, c = zyx 1.48 x M)at different cis-1 concentrations. These concentrations for the different experiments are a) 0 M, b)2.5~10-~~, c)5.02~10-'~, d)7.5~10-'~, e) ~ . O X ~ O - ~ M , f)1.25x M. Right: difference spectra of 2 upon addition of cis-1. A = Absorption units. component by an azobenzene unit results in a substantial difference in its association properties with 2, and the associ- ation constant of 2 with cis-1 is about twelve times greater than that with trans-1. A possible origin for this difference can be the changes in dipole moment associated with the isomerization of azobenzene. Isomerization of trans-azoben- zene (0.5 D) is accompanied by an increase in the dipole moment (cis-azobenzene, 3.0 D). This larger dipole moment of the cis-azobenzene can participate synergetically in the stabilization of the charge-transfer complex between eosin and the bipyridinium unit. The association constant K, of N,N'-dimethyl-4,4'-bipyridinium with eosin (in H,O) is 4500 M-'. This value implies that the enhanced binding of cis-1 to eosin is associated with the increase in dipole mo- ment of the azobenzene component. The difference in the association constants of trans-1 and cis-1 to eosin allows us to design a molecular switch for association (Fig. 2). An aqueous solution of eosin (c = 1.48 x M) and trans-1 (c = 6.2 x lo-' M) results in a system where the two components exhibit only weak mutu- al recognition and are essentially in a separated form. Upon illumination (A = 366 nm), isomerization of trans-1 to cis-1 occurs. This results in the formation of a complex between cis-1 and eosin, as evidenced by the decrease in the ab- sorbance of the eosin dye [Eq. (a)]. Back isomerization of 1=366nm 2 trans-1 nm Eo2- + trans-Azo-VZC [EoZ-. . cis-Azo-V2+] (a) Fig. 2. Periodic complexation and decomplexation of diad 1 to and from eosin, followed by absorption differences of free and associated eosin. o: complexed Eo2- in the presence of cis-1; m: free EoZ- in the presence of trans-1. Angew. Chem. Int. Ed. Engl. 1992, 31, No. 9 0 VCH Verlagsgesellschaft mbH, W-6940 Weinheim, I992 0570-0833/92/0909-1243 $3.50+ ,2510 1243