Excited-State Properties of the Ligand-Localized 3 ππ* State of Cyclometalated Ruthenium(II) Complexes S. Kimachi, R. Satomi, H. Miki, K. Maeda, and T. Azumi* Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Sendai 980-77, Japan M. Onishi Department of Industrial Chemistry, Faculty of Engineering, Nagasaki UniVersity, Nagasaki 852, Japan ReceiVed: April 3, 1996; In Final Form: July 8, 1996 X We report the results of an investigation on the absorption spectra, phosphorescence spectra, phosphorescence lifetimes, and magnetic properties of [Ru(bhq)(CO) 2 Cl(L)], where bhq - is the C-deprotonated forms of benzo- [h]quinoline (bhqH) and L is either PEt 3 , p-toluidine, or piperidine. The lowest singlet states of the Ru(II) complexes are metal-to-ligand charge-transfer 1 dπ* states. Vibrational structures of the phosphorescence spectra observed in the crystalline states at 4.2 K are similar to the structures of the phosphorescence spectra and the magnitude of the free bhqH ligand. Zero-field splittings indicate that the lowest triplet states of all the Ru(II) complexes are mainly characterized as ligand-localized 3 ππ* states. However, the phosphorescence lifetimes are significantly shorter for Ru(II) complexes as compared with free bhqH. This result suggests that the lowest triplet state of the Ru(II) complex includes 3 dπ* character due to configurational mixing with the bhq-localized 3 ππ* state. By intersystem crossing from the singlet excited state, the in-plane y sublevel is the most populated for bhqH, whereas the out-of-plane x sublevel is the most populated for the Ru(II) complexes. This dramatic change of the intersystem crossing route is satisfactorily interpreted within the framework of the theory of the intersystem crossing by considering the difference of the character of the lowest singlet state. 1. Introduction Ru(II) complexes with R,R′-diimine type ligands such as 2,2′- bipyridine (bpy) and 1,10-phenanthroline (phen) have been extensively studied. For example, for [Ru(bpy) 3 ] 2+ and [Ru- (phen) 3 ] 2+ , the photophysical and photochemical properties and excited-state dynamics have been well elucidated. 1 The lowest triplet states (T 1 ) of these Ru(II) complexes are the metal-to- ligand charge-transfer ( 3 dπ*) states. Although the T 1 ’s of most of Ru(II) complexes are assigned as 3 dπ* states, some Ru(II) complexes have T 1 ’s that are characterized with ligand-localized ( 3 ππ*) states. Available examples 2-4 are [Ru(i-biq) 3 ] 2+ (i-biq ) 3,3′-biisoquinoline), [Ru(bpy)(CNMe) 4 ] 2+ , and protonated [Ru(bpy)(CN) 4 ] 2+ . However, there has been little research on the 3 ππ* states of Ru(II) complexes. In order to make a detailed analysis, we try to synthesize a series of Ru(II) complexes that have the lowest triplet state of ππ* nature. In this respect, we focus our attention on cyclometalating ligands, which have strong ligand field strength. When the cyclometalating ligand is coordinated to a metal ion, the 3 dπ* state should be pushed to higher energy, and therefore, there appears to be a possibility that T 1 is changed from the 3 dπ* state to the 3 ππ* state. In this paper, we have chosen benzo[h]quinoline (bhqH) as the cyclometalating ligand, hoping to obtain the 3 ππ*T 1 state for Ru(II) complexes. We synthesized [Ru(bhq)(CO) 2 Cl(L)] (bhqH ) benzo[h]quinoline; L ) PEt 3 , p-toluidine, and piperidine). For these three complexes, we measured the absorption spectra, phosphorescence spectra, and time-resolved EPR spectra and determined the zero-field splitting (ZFS) parameters and relative intersystem crossing rates, P i , to sublevels. We then discuss the properties of the T 1 ’s of these Ru(II) complexes. We further discuss the mechanism of the intersystem crossing from the lowest singlet state to the lowest triplet state (S 1 f T 1 ISC). 2. Experimental Section Benzo[h]quinoline (bhqH) was purified by vacuum sublima- tion. The syntheses of [Ru(bhq)(CO) 2 Cl(L)] (L ) PEt 3 , p-toluidine, and piperidine) were described previously. 5 Good crystals were obtained by a slow diffusion of n-hexane to CH 2 - Cl 2 solution at room temperature for a few days. Excitation was carried out by the 313-nm line of a 500-W high-pressure Hg lamp, and the phosphorescence spectra were observed with a Spex 1702 monochromator equipped with a Hamamatsu R3896 photomultiplier tube. Phosphorescence decays were measured with excitation by a Molectron UV-24 N 2 laser. The steady-state EPR experiment was carried out only for free bhqH; for the complexes, steady-state EPR signals were not detected due to short triplet lifetimes. The excitation was carried out by a 500-W Hg lamp through a Toshiba UV-D33S glass filter and a 10-cm-thick water filter. The temperature was maintained at 90 K using a Bruker B-VT 2000 variable- temperature unit. For time-resolved EPR experiments, a Lumonics EX-600 excimer laser (XeCl, 308 nm) was used as the exciting light source with a repetition rate of 30 Hz. The transient EPR signals were detected by a Bruker ESP 300E spectrometer without field modulation. A PAR Model 162 boxcar averager was used and integrated between 0.5 and 1.0 µs after the laser excitation. The temperature was controlled by an Oxford Model ITC4 temper- ature controller and an Oxford CF935 continuous He gas flow cryostat for the time-resolved EPR experiments. The steady- state and time-resolved EPR experiments were carried out for ethanol solution at a concentration of 1 × 10 -3 M. 3. Results We assume that the spin axes of free bhqH are the same with those of free phen as shown in Figure 1a. The molecular X Abstract published in AdVance ACS Abstracts, December 15, 1996. 345 J. Phys. Chem. A 1997, 101, 345-349 S1089-5639(96)01018-3 CCC: $14.00 © 1997 American Chemical Society