Photochromic Dithienylethene Derivatives Containing Ru(II) or Os(II) Metal Units. Sensitized Photocyclization from a Triplet State Ron T. F. Jukes, † Vincenzo Adamo, § Frantis ˇek Hartl, † Peter Belser, § and Luisa De Cola* ,†,‡ Molecular Photonic Materials, Institute of Molecular Chemistry, UniVersity of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands, Amsterdam Nanocenter, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands, and Institute of Inorganic Chemistry, UniVersity of Fribourg, Pe ´ rolles CH-1700, Fribourg, Switzerland Received November 18, 2003 Efficient photocyclization from a low-lying triplet state is reported for a photochromic dithienylperfluorocyclopentene with Ru(bpy) 3 units attached via a phenylene linker to the thiophene rings. The ring-closure reaction in the nanosecond domain is sensitized by the metal complexes. Upon photoexcitation into the lowest Ru-to-bpy 1 MLCT state followed by intersystem crossing to emitting 3 MLCT states, photoreactive 3 IL states are populated by an efficient energy- transfer process. The involvement of these 3 IL states explains the quantum yield of the photocyclization, which is independent of the excitation wavelength but decreases strongly in the presence of dioxygen. This behavior differs substantially from the photocyclization of the nonemissive dithienylperfluorocyclopentene free ligand, which occurs from the lowest 1 IL state on a picosecond time scale and is insensitive to oxygen quenching. Cyclic voltammetric studies have also been performed to gain further insight into the energetics of the system. The very high photocyclization quantum yields, far above 0.5 in both cases, are ascribed to the strong steric repulsion between the bulky substituents on the dithienylperfluorocyclopentene bridge bearing the chelating bipyridine sites or the Ru(bpy) 3 moieties, forcing the system to adopt nearly exclusively the reactive antiparallel conformation. In contrast, replacement of both Ru(II) centers by Os(II) completely prevents the photocyclization reaction upon light excitation into the low-lying Os-to-bpy 1 MLCT state. The photoreaction can only be triggered by optical population of the higher lying 1 IL excited state of the central photochromic unit, but its yield is low due to efficient energy transfer to the luminescent lowest 3 MLCT state. Introduction In recent years much research effort has been devoted to the development of photochromic materials, that is, systems that can be converted reversibly from one form into another upon light excitation. 1-3 The interest in these compounds stems from their potential application in molecular electronics and information storage. 4-7 One of the most promising classes of photochromic materials are diarylethenes with attached thiophene rings (i.e., dithienylethenes), whose photochromic properties were first described by Irie and Mohri in 1988. 8 Among important features of this class of molecules are the good fatigue resistance, fairly high photocyclization quantum yields, high reversibility of the forward and reverse photoprocesses triggered by irradiation with light of different wavelengths, and thermal stability of the products. 9 In some cases it has been possible to quantitatively transform the system from one form to the other. 10-12 * Author to whom correspondence should be addressed. E-mail: ldc@ science.uva.nl. † University of Amsterdam. ‡ Amsterdam Nanocenter. § University of Fribourg. (1) Crano, J. C.; Guglielmetti, R. J. Organic Photochromic and Thermo- chromic Compounds; Kluwer: Dordrecht, 1999; Vol. 1. (2) Feringa, B. L. Molecular Switches; Wiley-VCH: Weinheim, 2001. (3) Du ¨ rr, H.; Bouas-Laurent, H. Photochromism: Molecules and Systems; Elsevier: Amsterdam, 2003. (4) Irie, M. E., Ed. Special edition on “Photochromism: memories and switches”. Chem. ReV. 2000, 100 (5). (5) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541- 548. (6) Raymo, F. M. AdV. Mater. 2002, 14, 401-414. (7) Balzani, V.; Credi, A.; Venturi, M. Chem. Phys. Chem. 2003, 4, 49- 59. (8) Irie, M.; Mohri, M. J. Org. Chem. 1988, 53, 803-808. (9) Irie, M. Chem. ReV. 2000, 100 (5), 1685-1716. (10) Irie, M. Tetrahedron 1997, 53, 12263-12271. (11) Gilat, S. L.; Kawai, S. H.; Lehn, J. M. Chem. Eur. J. 1995, 1, 275- 284. Inorg. Chem. 2004, 43, 2779-2792 10.1021/ic035334e CCC: $27.50 © 2004 American Chemical Society Inorganic Chemistry, Vol. 43, No. 9, 2004 2779 Published on Web 03/24/2004