J. Phys. Chem. 1994,98, 60596062
6059
Effect of the Covalent Bond on the Radiative Properties of the Ligand-Localized 37rr*State of Rhodium Complexes with Benzo[blquinoline Hisayuki Miki and Tobru Ammi' Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan Received: December 13, 1993; In Final Form: March 16, 1994'
The phosphorescence0 of [Rh(bhq)zC1]2 was examined by a zero-field optical detection of magnetic resonance (zf-ODMR) method a t 1.3 K, and the results were compared with those of the C-protonated ligand bhqH. The zero-field splittings of the complex are nearly identical to those of bhqH. For the complex, both the x- and z-sublevels emit equally strongly, whereas for the C-protonated ligand only the z-sublevel is effectively emissive. The spectral location, lifetime, and zero-field splittings indicate that the phosphorescence is from the ligand localized 3rr*state somewhat perturbed by metal-to-ligand charge-transfer states. Due to the stronger ligand field provided by bhq and the negative charge of the ligand, two types of 3dr* states can configurationally mix with the bhq-localized 3rr*state. The sublevel properties are satisfactorily interpreted in the framework of this configurational mixing.
Introduction Photoexcited states of complexes of the metal ions of low spin (nd)6electronicconfiguration with orthometalating ligands have received substantial attention. Complexes containing 2-phenylpyridine (phpy), 2-(2-thienyl)pyridine (thpy), and benzo[h]quinoline (bhq) have been extensively studied.'-' The phosphorescence state of such complexes has been assigned to either a metal-to-ligand charge-transfer (MLCT) 3dr* stateor a ligandlocalized (LC) 3rr*state. The assignment has been made on the basis of the location of spectra, vibrational structures, lifetime, and zero-field splittings. Orthometalatingligands are expectedto provide strongerligand fields than diimine ligands of similar structure as a result of forming a covalent bond with the central metal i0n.l' Therefore, as compared with the complexof diimines,theligand fieldsplitting of d-orbitals is larger and hence the energies of d r * and dd states are expected to be smaller and larger, respectively. Thus, this effect results in a significant change in the configurationalmixing. In this paper, we focus attention on the ligand-localized UT* emission from the complex of an orthometalating ligand, bhq, having a structure similar to that of phen. Comparing the complexes with phen, we expect the following differences: (1) Ligand field strength; since bhq provides stronger ligand field than phen, the energies of dd and d r * states should be higher. (2) Electrostatic repulsive energy; the covalent bond which increases negative charge on the central metal ion has no effect on the energies of dd states but makes the energies of d r * states lower. We select [Rh(bhq)zCl]zas an example for investigation. This complex has been known as a 3rr* emitter. However, the short lifetime (2.5 ms) as compared with typical 3mr*phosphorescence indicates that states involving metal orbitals should mix with 3177r*.11 We further note that the lifetime is shorter than the Rh(II1) complexes with phen;10 this indicates that the influence from upper metal-involving states are larger. Additionally, the selected bhq complex shows that absorption bands assigned to 1MLCTtransitions in a lower energy region than ligand-localized %r*.11 In view of the existence of low-energy Id** states, we expect that the phosphorescence state of [Rh(bhq)zCl]z is much influenced by 3 d ~ *states; however, no detailed information is available on the emitting state. In order to study the influence of the above effects on the radiative properties of the phosphorescence state of free CAbstract published in Advance ACS Abstracts, May 1, 1994.
0022-3654/94/2098-6059%04.50/0
protonated bhq and Rh(II1) complexes of bhq, we try to apply the zf-ODMR method on the phosphorescenceof bhqH and [Rh(bhq)2Cl]z. First we analyze the results of free C-protonated bhq, comparing them with those of free phen. Next, we discuss results of [Rh(bhq)zCl]z by configurational mixing between upper-lying metal-concerning states and analyze the radiative mechanism of this complex.
Experimental Section Benzo[h ]quinoline was purified from commercial grade as reported12 and was used as methylcyclohexane solution. [Rh(bhq)2C1]zwas synthesized as reported" and was recrystallized by a diffusion method in which distilled hexane was slowly poured on the dichloromethane solution of the sample, and the crystal appeared about a week after. Phosphorescence spectra were recorded for either polycrystalline sample with a Spex 1702 monochromator equipped with a Hamamatu R3896 photomultiplier tube. The 365-nm line from a 500-W high-pressure Hg lamp was used for excitation. For the decay measurements excitation was carried out by a Nz-pulsed laser, and the resultant signal was digitized by a Kawasaki Electronica TMR- 10 transient memory. The zero-field optical detection of magnetic resonance (zfODMR) was performed for the polycrystalline sample by monitoring the 0-0 band of the phosphorescence at 1.3 K. (Previous studies on phen13 confirmed that the spin-lattice relaxation is completely suppressed at 1.3 K. The lifetime of the present complex is 2 orders of magnitude shorter than phen, and we therefore assume that spin-lattice relaxaiton is suppressed in this case also.) The equipment used for this investigation was essentially the same as that reported previously.13 Zero-field splittingsof the excited triplet state were determined by PMDR mea~urements.1~ The total decay rate constants and the relative radiative rate constants for the individual sublevels were determined by the fast-passage technique.15 The total decay rate constants for the nonemissive sublevels were determined by the method of MIDP.I6 Results
BhqH. The phosphorescence spectrum of C-protonated free ligand, bhqH, in the crystalline state at 4.2 K is shown in Figure la. The spectrum is essentially identical to that measured in solution by Gondo et al.17 0 1994 American Chemical Society
Miki and Azumi
6060 The Journal of Physical Chemistry, Vol. 98, No. 24, 1994
i 1
a) bhqH
-a2
,.1 Rh
d-orbitals on Rh
n orbitals on bhq
Figure 3. (a) Assumed directionsof the principal axes for free bhq and
its complex. (b) Assignment of the orbitals.
b) 2.2 GHz
a) 5.8 GHz ri
450
500
I
550
600
650
wavelength I nm
Figure 1. Phosphorescencespectra in the crystalline state at 4.2 K: (a) free bhqH and (2) [Rh(bhq)2Cl]~.
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2.88
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