Excited State Spectroscopy and Excited State Dynamics of Rh(III) and Pd(II) Chelates as Studied by Optically Detected Magnetic Resonance Techniques

Abstract

In this paper we review optically detected magnetic resonance (ODMR) investigations of a series of Rh 3+ (4d 6 ) and Pd 2+ (4d 8 ) complexes in the lowest excited electron spin triplet state. Starting with a brief survey of the technique of optical detection of magnetic resonance, zerofield and low-magnetic field ODMR results are reviewed for the tris-diimine chelates [Rh(phen) n (bpy) 3-n ] 3+ , where phen = 1,10-phenanthroline, bpy = 2,2¢-bipyridine, and n = 0, 2, or 3, and for the mixed cyclometalated chelates [Rh(thpy) x (phpy) 2-x (bpy)] + , with thpy = 2,2¢thienylpyridinate, phpy -= 2-phenylpyridinate, and x = 1, or 2. The ODMR data reveal fine structure splittings in the phosphorescent excited state of the complexes comparable in magnitude to those known for the nonchelated ligand molecules in the excited triplet state. Anisotropy studies of the ODMR spectra for the single crystals in low magnetic fields show that the lowest electronic excitation in the complexes is a triplet state indeed and that this state is localized on a single ligand molecule per metal ion site. From microwave recovery experiments, performed under conditions that the spin-lattice relaxation can be neglected (T≤ 2 K), detailed information concerning the triplet sublevel lifetimes is obtained. The lifetimes are on the millisecond timescale, i.e., three orders of magnitude shorter than for the nonchelated ligand molecules. The lifetime shortening as well as the observed spin-selective radiative decay of the triplet sublevels of the ligand molecule are discussed in detail on the basis of enhanced spin-orbit couplings caused by the central (heavy) metal ion. Optically detected spin coherence experiments (transient nutation and spin echo decay) are also discussed. The results show that the homogeneous line broadening of the ODMR transitions of the metal complexes in the emissive triplet state is approximately 100 kHz. The homogeneous broadening is attributed to the effects of flip-flop motions of ligand proton spins that randomly modulate the triplet electron spin levels on account of dipolar electron spin -nuclear spin couplings. Finally, recent ODMR and PMDR (phosphorescence microwave double resonance) experiments performed for the Pd 2+ -chelates, Pd(thpy) 2 and Pd(qol) 2 (with qol -= 8-hydroxyquinolinate) in Shpol'skii matrices are discussed. The lowest excited electronic state in these molecules is also emissive and ODMR spectra at zero-and low magnetic fields have been observed. For Pd(thpy) 2 only one zero-field ODMR transition could be measured, but it is argued that this transition originates in an excited triplet state. The results of the microwave recovery experiments could be related to time-resolved emission experiments in high magnetic fields. Spin selectivity in the vibronic line emission is demonstrated by means of PMDR.

Organometallic Pt(II) Compounds. A Complementary Study of a Triplet Emitter Based on Optical High-Resolution and Optically Detected Magnetic Resonance Spectroscopy

“…The optical high-resolution measurements were accomplished with experimental equipment 22 and an optical setup as specified previously. The ODMR spectrometer used was the same as also described elsewhere. , Optical excitation for these ODMR experiments was at 340 mm (∼29400 cm -1 ) using a 100 W high-pressure mercury lamp and an UG11 filter. ODMR spectra were recorded for the sample in a bath of liquid helium pumped down to 1.4 K. Small magnetic fields (up to 400 G) were applied by means of superconducting Helmholtz coils immersed in the liquid helium bath.…”

“…Whereas the small ZFS between substates I and II could in principle contain contributions arising from spin−spin couplings (usually on the order of 0.1 cm -1 ; compare refs and ), a ZFS of 3.8 cm -1 is clearly a consequence of spin−orbit coupling according to Pt 5d and/or MLCT character in the low-lying triplet state. ,, The importance of spin−orbit coupling for Pt(thpy)(CO)(Cl) is also evidenced by the observation of relatively intense excitation peaks at the electronic origins corresponding to absorptions from the singlet ground state to the triplet substates (Figure d).…”

Photophysical Properties and OLED Applications of Phosphorescent Platinum(II) Schiff Base Complexes

“…2 + , with zero-field splittings of the lowest triplet state being 60, [17,18] 76, [28] and 211 cm À1 , [17,18] respectively. In contrast, complexes such as [PdA C H T U N G T R E N N U N G (thpy) 2 ] (thpy = 2-(2'-thienyl)pyridyl), [16,29] [Pd(q) 2 ] (q= 8-quinolinato), and [Pt(q) 2 ], [30] as well as [RhA C H T U N G T R E N N U N G (bpy) 3 ] 3 + [31] and [PtA C H T U N G T R E N N U N G (bpy) 2 ] 2 + [17,18] , are characterized by ZFS values significantly below 1 cm À1 . Thus, it is concluded that these complexes emit only from perturbed ligand-centered triplet states.…”

Spectroscopy and Photophysics of Self-Organized Zinc Porphyrin Nanolayers. 3. Fluorescence Detected Magnetic Resonance of Triplet States

“…Emission light was detected using an S20 (EMI) photomultiplier after dispersion through a colored glass cutoff filter (OG 540) and a monochromator (Jobin-Yvon, model HR1000). Microwaves were applied using the setup described previously. , The sample was mounted inside a slow-wave helix coupled to a semirigid cable in the cryostat. Microwave radiation was generated by a sweep oscillator (HP-8609B) using appropriate plug-in units (HP-86999B for 100−4000 MHz and 8691B for 1000−2000 MHz) and Hughes 1177H or Varian VZC-6961/VTC-6160/VA-615M traveling wave tube amplifiers; the amplified microwave output (∼20 W) was coupled by coaxial lines to the semirigid cable.…”

“…This paper focuses on the nature and properties of the photoexcited porphyrin triplet states in these films as studied by means of optically detected magnetic resonance (ODMR). In particular, we have applied microwave-induced fluorescence intensity measurements, i.e., fluorescence detected magnetic resonance (FDMR), in the absence of a magnetic field, to these films at 1.4 K. The ODMR technique and its theoretical background have been treated in several excellent reviews. − …”