Diimine Triscarbonyl Re(I) of Isomeric Pyridyl-fulvene Ligands: an Electrochemical, Spectroscopic, and Computational Investigation Daniel Chartrand, Carlos A. Castro Ruiz, and Garry S. Hanan* Department of Chemistry, Universite ́ de Montre ́ al, Montre ́ al, Que ́ bec H3T 1J4, Canada * S Supporting Information ABSTRACT: The synthesis and characterization of a novel family of positively charged fac-[Re(bpy)(CO) 3 (L)]PF 6 (bpy = 2,2′-bipyridine) complexes are re- ported, where L is a pyridine functionalized in para or meta position with a fulvene moiety, namely, 4-fluoren-9-ylidenemethyl-pyridine (pFpy) and 3- fluoren- 9-ylidenemethyl-pyridine (mFpy). The complexes were prepared in high yield (86%) by direct addition at room temperature of the corresponding pyridine to the tetrahydrofuran (THF) adduct fac-[Re(bpy)(CO) 3 (THF)][PF 6 ] pre- cursor. Both ligand and complex structures were fully characterized by a variety of techniques including X-ray crystallography. The complexes did not exhibit the expected triplet mixed metal -ligand-to-ligand charge transfer (MLLCT) emission, because of its deactivation by the non-emissive triplet excited state of fulvene. The absorption profile shows that the MLLCT is overshadowed by the fulvene centered π-π* transition of higher molar absorptivity as shown by time dependent density functional theory (TD-DFT) calculations. The position of the fulvene on the pyridyl ring has a large effect on this transition, the para position displaying a much higher absorption coefficient (21.3 × 10 3 M -1 cm -1 ) at lower energy (364 nm) than the meta position (331 nm, 16.0 × 10 3 M -1 cm -1 ) ■ INTRODUCTION Rhenium triscarbonyl diimine chromophores have been studied extensively for the past 30 years; their unique characteristics (e.g., high stability, high energy excited states, capacity for reductive and oxidative electron transfer) make them ideal for photo- sensitizers. 1 They are found as photoactive components in various roles: chromophores supplying electrons to catalysts (e.g., hydrogen evolution); 2 photocatalysts reducing carbon dioxide to carbon monoxide; 3 photosensitizers permitting visible light photo- isomerization; 4 and as chromophoric building-blocks for supra- molecular assemblies. 5 One common variant involves a 2,2′- bipyridine (bpy) ligand and a secondary neutral ligand L in complexes of the type [Re(bpy)(CO) 3 L] + , with pyridine being the motif of choice for further functionalization of the rhenium chromophore through ligand L. To expand the utility of the rhenium-bpy unit, a meta and para fulvene functionalized pyridine was used as ligand L in this study, and the chemical and photophysical properties of its rhenium complexes were determined. The properties of the fulvene ligands are similar to those of stilbene molecules in that they can undergo isomerization through their triplet excited state, 6 albeit the irradiation energy necessary for this isomerization is often found only in the UV region unless highly conjugated molecules are used. In addition, the quantum yield of isomerization is usually relatively low because of the triplet state of the molecule competing with the permitted fluorescence decay of the singlet excited-state. As such, grafting a rhenium onto the fulvene core may enhance both the absorption, by causing its red-shift with a concomitant increase in the coverage of a broader spectrum of light, and the efficiency, by quenching the fluorescence to a longer-lived, higher-energy triplet state centered on the rhenium bipyridine that will act as a reservoir for the triplet state centered on the fulvene. This process is well described for Ru(II) diimine complexes containing fused polycyclic moieties. 7 Although energy transfer has been investigated using rhe- nium complexes displaying intramolecular charge transfer more than 25 years ago, 8 recent research has examined the addition of photoisomerizable ligands, containing stilbene analogues and other moieties, that once coordinated quench the luminescence of the rhenium in favor of their isomerization. 4a-i Closer to our design is the 4-styrylpyridine motif, which upon coordination to rhenium was shown to undergo photoisomerization with lower energy light and at higher efficiency by spectroscopic and high level computational studies. 4k-m In our case, no isomerization is observable because of the symmetry of the ligand, but energy transfer can still be observed because of the absence of emission of the rhenium complex, suggesting that the non-radiative decay of the excited state passes by the triplet state of the fulvene. Another interesting aspect of this study is the delocalization of the fulvene orbitals onto the rhenium core, which is seen in only one of the pyridyl isomers. ■ EXPERIMENTAL SECTION General Considerations. All of the organic reagents were ob- tained from Sigma Aldrich, rhenium carbonyl from Pressure Chemical Co. and solvents from Fischer and Anachemia and were used as received Received: July 17, 2012 Published: November 15, 2012 Article pubs.acs.org/IC © 2012 American Chemical Society 12738 dx.doi.org/10.1021/ic301559s | Inorg. Chem. 2012, 51, 12738-12747