Johann Strasser

Low-lying electronic states of [Rh(bpy)3]3+, [Pt(bpy)2]2+, and [Ru(bpy)3]2+. A comparative study based on highly resolved and time-resolved spectra

Abstract Optical emission and excitation spectra of [Rh(bpy-h 8 ) 3 ] 3+ , [Rh(bpy-h 8 ) 2 (bpy-d 8 )] 3+ , [Rh(bpy-d 8 ) 3 ] 3+ , [Pt(bpy-h 8 ) 2 ] 2+ , [Pt(bpy-h 8 )(bpy-d 8 )] 2+ , [Pt(bpy-d 8 ) 2 ] 2+ , [Ru(bpy-h 8 ) 3 ] 2+ , [Ru(bpy-h 8 ) 2 (bpy-d 8 )] 2+ , and (Ru(bpy-d 8 ) 3 ] 2+ are discussed. A series of trends—also including [Os(bpy) 3 ] 2+ —is uncovered. These trends, which are connected with an increase in metal-d or MLCT character in the lowest triplet states, can for instance be seen in transition energies, emission lifetimes, zero-field splittings, rates of spin-lattice relaxation, vibronic satellite structures, and changes of nuclear equilibrium. positions on excitation. Moreover, the compounds investigated are also appropriate for consideration of the concept of “dual emission”, the relation between MLCT character and covalency, as well as the current models of localization/delocalization. In particular, a comparison of the properties of [Rh(bpy) 3 ] 3+ - and [Ru(bpy) 3 ] 2+ -doped [Zn(bpy) 3 ](ClO 4 ) 2 provides illustrative and very strong evidence for covalent delocalization in the lowest excited states of [Ru(bpy) 3 ] 2+ , while ligand-centered localization is found in the states of [Rh(bpy) 3 ] 3+ . Misinterpretations might occur if the aggregation effects of the chromophores are not taken into account.

Triplets in metal-organic compounds. Chemical tunability of relaxation dynamics

Abstract Triplets of metal–organic or related compounds of the platinum metal group split into substates. The amount of splitting at zero magnetic field (zfs) is mainly determined by the effective spin-orbit coupling, which is, for example, induced by metal-d and/or MLCT participations in these triplets. The total zfs can be tuned chemically over a very wide range from about 0.1 cm−1 to more than 200 cm−1 (see Fig. 1 Download : Download high-res image (180KB) Download : Download full-size image Fig. 1 . The diagram symbolizes the significance of metal character for the lowest triplet. The corresponding metal admixture correlates with the total splitting of the triplet at zero magnetic field (zfs). Properties of the compounds A, B, C, and D are presented in detail as case studies. ). After excitation, the relaxation time between the substates can be as long as hundreds of nano-seconds to many micro-seconds at low temperature. This relaxation, the spin-lattice relaxation (slr), depends on the splitting pattern of the triplet substates, further on temperature, and on the matrix surrounding the chromophore. Four compounds [Pt(bpy)2]2+, Pt(2-thpy)(CO)(Cl), Pt(2-thpy)2, and [Ru(bpy)3]2+ with strongly different zero-field splittings are selected as case studies, to investigate the dynamics of slr according to the direct, the Orbach, and the Raman process. Temperature dependent studies and investigations at low temperature (T≤2 K) under application of high magnetic fields up to B=10 T and high pressure up to p=20 kbar, respectively, allow us to develop a deeper insight into the relaxation mechanisms. Moreover, several effects are pointed out that result from slow spin-lattice relaxation and that can be important at low temperature, like the non-validity of a Boltzmann distribution for closely lying states, the occurrence of super-imposed emission spectra from different excited states, the dependence of emission decay properties on excitation and detection wavelengths, effects of spectral shifts with time, and a specific behavior of radiationless energy transfer. In an outlook, a number of further transition metal complexes is presented to underline the general importance of the effects of relatively slow spin-lattice relaxation.

Triplet sublevels of metal organic complexes – temperature dependence of spin–lattice relaxation

Abstract Triplets of transition metal complexes with organic chelate ligands can act as important pathways in photo-redox processes. Detailed information on these states is available from highly resolved optical spectra and time-resolved investigations. The lowest triplets are often zero-field split into sublevels by several cm −1 (zero-field splitting, ZFS) due to spin–orbit interactions. Interestingly, the relaxation between these sublevels can be very slow (nanoseconds up to thousands of nanoseconds) at low temperatures. This is reflected in the emission decay times and even in the emission spectra. The population dynamics and the relaxation times are governed by the interaction between the triplet sublevels and the surrounding matrix (spin–lattice relaxation, SLR). Due to a low phonon density of states at energies of the size of the ZFS, the relaxation between the triplet sublevels is slow. It is possible to understand the relevant relaxation processes (direct, Orbach, Raman) in detail by investigating the temperature dependence of the emission decay behavior and thus of the SLR. In order to take various ZFS patterns correctly into account, an extended description for the Orbach process is derived. In the present investigation, three compounds, Pt(2-thpy) 2 , Pt(2-thpy)(CO)(Cl), and Pt(phpy) 2 ,are analyzed as case studies. Important data that describe the emission properties of the triplet substates are derived. In particular, it is possible to determine the relative importance of the three different relaxation processes for these systems. The SLR data are in accordance with qualitative models for the chromophore–cage interactions.

Effect of high pressure on the emission spectrum of single crystals of Tl[Au(CN)2]

Abstract Experimental results for the photoluminescence of Tl[Au(CN) 2 ] single crystals are reported as a function of pressure up to p ≈45 kbar at T =60 and 300 K. Pressure-induced red-shifts of the emission energy are Δ ν / Δ p=−140 cm −1 /kbar (at 60 K) and Δ ν / Δ p=−160 cm −1 /kbar (at 300 K). These large values are related to the pressure-induced increase of two-dimensional interactions between the [Au(CN) 2 ] − units and to the reduction of the band gap energy.

Dynamical processes between triplet sublevels of metal-organic Pt(II) compounds

Abstract Pt(2-thpy) 2 and Pt(2-thpy) (CO)Cl are investigated as representatives for the class of platinum metal compounds with organic ligands. At T = 1.3 K. these compounds show very interesting emission rise and decay properties due to relatively slow spin-lattice relaxations (SLR) between the low-lying triplet sublevels. These properties, which depend largely on the zero-field splittings (ZFS) of the lowest triplets, may be tuned chemically.

Spin-lattice relaxation in metal-organic platinum(II) complexes

Abstract The dynamics of spin–lattice relaxation (SLR) of metal-organic Pt(II) compounds is studied. Often such systems are characterized by pronounced zero-field splittings of the lowest-lying triplets. Previous expressions for the Orbach SLR process do not allow proper treatment of such splitting patterns. We discuss the behavior of a modified Orbach expression for a model system. Further, we present results of a fit of the temperature dependence of the SLR rate for Pt(2-thpy) 2 based on the modified expression.

Chemically tuned zero-field splittings and spin-lattice relaxation - Investigation by time-resolved emission

Abstract For the compounds studied, zero-field splittings (zfs) of the lowest triplets lie between about 0.1 and 200 cm −1 . This energy determines spin-lattice relaxation (slr) rates and thus time developments of the emission. It is possible to shift these relaxation times into pre-defined time regions. The importance of these properties is demonstrated for the representative [Ru(bpy) 3 ] 2+ .

Lowest excited triplet states in [Ru(bpy)3]2+ and [Rh(bpy)3]3+ A comparative study based on highly resolved spectra

Abstract The lowest excited states of [Ru(bpy)3]2+ (bpy: 2,2′-bipyridine) exhibit a very strong involvement of metal 4d orbitals, while the metal character is extremely small in the states of [Rh(bpy)3]3+ . This contrasting behavior is clearly displayed in the emission and excitation spectra, when ligands are isotopically marked. It is demonstrated that these states, being of metal-to-ligand charge transfer character (3MLCT) in Ru(II) compounds, are covalentely delocalized over the metal and the different ligands, while the ground and the lowest excited states of Rh(III) compounds being of 3 ππ ∗ character are centered on individual ligands.

Energy migration and up-conversion in Cs2NaErxY1-xCl6 at 1.2 K

Low temperature, high resolution emission spectra have been measured for the pure and diluted erbium elpasolites, Cs2NaErxY1-xCl6 from the 4G11/2, 2G11/2, 4S3/2, 4F9/2, and 4I9/2 excited states of Er3+ under 364 nm argon laser excitation. A number of these spectra show some unusual features which are consistent with migration of the excitation energy within the lattice leading to excitation of luminescence from defect sites. The nature of the defect sites is discussed and possible mechanisms leading to the migration of the excitation energy within the lattice are explored.

Up-conversion and energy migration in the holmium hexachloroelpasolites

Excitation into the 5I8yields5F5 transition of Cs2NaHoCl6 near 15 633 cm-1 at 300 K results in strong, long lived emission from the 5F5 state wit an almost exponential decay curve. The decay constant of this emission decreases slightly in Cs2NaY0.99Ho0.01Cl6 the decay becoming exactly exponential, the difference between the decay constants is greater at lower temperatures. This suggests that there is migration in the host lattice followed by energy transfer to defect sites with short 5F5 lifetimes in the pure Ho compound and this hypothesis is confirmed by measurements of the relaxation of the 5F5 state in Cs2NaNd0.01Ho0.99Cl6 where the decay constant is greatly increased but the decay curves are exponential. Under the same conditions and at temperatures from 1.2K to 300K there is emission in the ultraviolet, green and blue regions from Cs2NaY1-xHoxCl6. The decay curves are complex with more than one rise and decay processes reflecting a variety of up-conversion processes. The mechanisms of these processes are discussed.