FLUORESCENCE, PHOTOSWITCHING OF CATION COMPLEXATION AND OF INTRAMOLECULAR MAGNETIC INTERACTIONS FROM DITHIENYLETHENES

J.-P. MALVALa,I.GOSSEa, J.-P. MORANDa, R. LAPOUYADEa Y. GARCIAb,1,V.KSENOFONTOVb,P.GÜTLICHb J. OBERLEc, G. JONUSAUSKASc aLaboratoire d’Analyse Chimique par Reconnaissance Moléculaire (LACReM), Ecole Nationale Supérieure de Chimie et de Physique de (ENSCPB), 33607 Cedex, . E-mail: [email protected] bInstitut für Anorganische Chemie und Analytische Chemie, Universität Mainz, Staudingerweg 9, 55099 Mainz, Germany cCentre de Physique Moléculaire Optique et Hertzienne, UMR CNRS n°5798, Université de Bordeaux I, 351 cours de la Libération, 33405 Cedex, France 1Present address : Unité de Chimie des Matériaux Inorganiques et Organiques (CMAT), Département de chimie, Faculté des Sciences, Université Catholique de Louvain, Place L. Pasteur,1 1348 Louvain-la-Neuve, Belgique.

Photochromism is defined as light-induced reversible transformation of chemical species between two isomers having different absorption spectra (1). In fact the two photoisomers differ from each other in various physical and chemical properties which have been successfully exploited to photo-control different functions such as host-guest interactions (2), nonlinear optical properties (3) and molecular magnetism (4) amongst many others. For the above purposes, dithienylethenes are considered to be the most promising photochromic materials because of their high fatigue resistance and the thermal stability of both photoisomers (1c). In this lecture, we report three results involving dithienylethenes as the photochromic unit : a new azacrown derivative for the photoswitching of cation complexation, a new coordination compound as a room temperature photomagnetic molecular switch and a structural study of the wavelength effect on the fluorescence efficiency.

Photoswitching of cation complexation with a monoaza-crown dithienylethene photochrome

We report the synthesis of a new photochromic crown ether 1 composed by a dithienylperfluorocyclopentene, certainly the best photochromic system in terms of yield and of stability of the two forms, at positions 5 and 5’, with a p-phenylaza-15-crown-5 as the ionophore and a formyl group as a strong electron-withdrawing group. While the two thienyl groups are poorly conjugated in the open form, a push-pull polyene with the ionophore as the push group and the formyl substituent as the pull group, appears in the closed isomer from which a decreased cation complexing ability was expected. F F F F F F F F F F F F

313 nm CH 2X OHC 3 OHC 2+ S H3C S S S +Ca ,2X O 600 nm O N N Ca2+ O O O O O O

2+ 1O-Ca 1c

Indeed the electronic effect of the formyl group, in the closed form, is sufficient to switch off the cation ( log K = 5.4 and 1.7) from 1O and 1c respectively, for the complexation of Ca2+in acetonitrile). The reversibility of the process has been monitored optically (UV-visible absorption and fluorescence) and by various methods (NMR, ESI- MS, etc.) (5).

Room-temperature photomagnetic photochromic molecular switches

F F F F F F F F F F F F

S S Fe S S Fe N N Fe N N N

N NCS SCN

N N

N

SCN NN CS N

SCN N N N N NCN S N S S S S

F F F F F F F F F F F F

4.5

4 K]

-1 3.5 mol

3 820 nm UV

T[cm 3 M χ χ χ χ

2.5

2 0 100 200 300 T[K] We have prepared a new Fe(II) coordination compound, FeL2(NCS)2, including 1,2-bis[2’-methyl-5’-(pyrid-4’’-yl)thien-3’-yl] perfluorocyclopentene as the photochromic ligand (L). It appears as an intense orange powder completely insoluble in various solvents, as a result of its likely polymeric nature. A color change to dark blue was observed by exposing to UV light (λmax= 365nm). Irradiation with red light (λ>660nm) leads to the reverse coloration (blue to orange). The magnetic susceptibility was measured 3 for both forms : at room temperature a dramatic change was observed, from χMT=2.3cm -1 3 -1 Kmol for the open form (orange) to χMT=4.4cm Kmol for the closed form (blue), where χM is the molar magnetic susceptibility. The optical addressing of this polymeric coordination compound is accompanied by a dramatic change of the color and magnetic susceptibility. As it occurs in the solid state, at room temperature, it may be of interest for opto-magnetic storage and recording (magnetic image of optical objects) (6).

New insights into the excitation wavelength dependence relation between fluorescent efficiency and photochromic reactivity in dithienylethenes

Strong fluorescence is rarely observed from diarylethylenes because of the very fast competing reactions such as cis-trans photoisomerization or photocyclization. 1,2-bis(3- thienyl) cyclopentenes which are constrained in the cis configuration cannot photoisomerize but the photocyclisation is so fast that the fluorescence is hardly competitive. Nevertheless it has been shown that dithienylethenes exist in two different conformations : the antiparallel (ap) and the parallel (p) conformations (7) and the photocyclization reaction can proceed only from the ap conformation while the conformer p eventually fluoresces (8,9).

Excitation 1,0 Emission Absorption 0,8 F F F F 0,6 F F

CH OHC 3 0,4 S H3C S

a.u. (norm.) O N O 0,2 O O

0,0 1

250 300 350 400 450 500 550 600 650 700 solvent : acetonitrile (nm)

We have found a strongly solvatochromic fluorescence for compound 1, similar to that reported for 1,2-bis(2,4,5-trimethylthiophene-3-yl)-maleic anhydride (10), and that the fluorescence quantum yield increases 10 times while the cyclization quantum yield decreases approximately 10 times when the excitation wavelength is shifted from the absorption maximum (330 nm) beyond the excitation maximum ((λmax=375 nm) (5). Other dithienylethenes with dithieno-thiophene (11) or porphyrinic macrocycle (12) as substituents exhibit these excitation wavelength effects which were assigned to the modulation of the intramolecular energy transfer from the substituents excited state to the dithienylethene reactive core. We propose that the emission of compound 1 arises from a charge transfer excited state of the p conformer involving the ionophore branch as electron donor and the quasi-perpendicular perfluorocyclopentene as electron acceptor (5). Spectroscopic investigation of other dithienylethenes and of several model compounds shows that this effect is observed when one substituent has a low enough oxidation potential and is maintained orbitally decoupled from the perfluoro cyclopentene group by the second substituent (p conformer). This is a new structural effect which enlarges the nondestructive readout capability of dithienylethenes in optical memory media.

REFERENCES 1.(a) Dürr, H.; Bouas-Laurent, H. Photochromism : Molecules and Systems ;Elsevier: Amsterdam, 1990. (b) Crano, J.C. ; Guglielmetti, R.J. Organic Photochromic and Thermochromic Compounds, Plenum, New York 1999. (c) Irie, M. Chem. Rev. 2000, 100, 1685. 2. Takeshita, M. ; Irie, M. J.Org.Chem. 1998, 63, 6643. 3. Nakatani, K. ; Delaire, J.A. Chem. Mat. 1997, 9, 2682. 4. Boillot, M.-L. ; Roux, C. ; Audière, J.-P. ; Dausse, A. ; Zarembovitch, J. Inorg. Chem. 1996, 35, 3975. 5. Malval, J.-P. ; Gosse, I. ; Morand, J.-P. ; Lapouyade, R. J. Am. Chem. Soc. 2002, 124, 904. 6. Gütlich, P.; Lapouyade, R.; Garcia, Y. ; Ksenofontov, V. Ger. Patent No. 100 39 903.7, 2002. 7. Irie, M. ; Mohri, M. J. Org. Chem. 1988, 53, 803. 8. Miyasaka, H. ; Aira, S. ; Tabata, A. ; Nobuto, T. ; Mataga, N. ; Irie, M. Chem. Phys. Lett. 1994, 230, 249. 9. Ern, J. ; Bens, A.T. ; Martin, H.-D. ; Mukamel, S. ; Tretiak, S. ; Tsyganenko, K. ; Kuldova, K. ; Trommsdorff, H.P. ;Kryschi, C. J. Phys. Chem. 2001, 105, 1741. 10. Irie, M. ; Sayo, K. J. Phys. Chem. 1992, 96, 7671. 11.Tsivgoulis, G.M. ; Lehn, J.-M. Angew. Chem. Int. Ed. 1995, 63, 1119. 12. Norsten, T.B. ; Branda, N.R. J. Am. Chem. Soc. 2001, 123, 1784.