Hans Riesen

Luminescence and excitation line narrowing of [Ru(2,2’‐bipyridine)3−x (2,2’‐bipyridine‐d8)x]2+ (x=0–3) in [Zn(2,2’‐bipyridine)3](CIO4)2. Unequivocal evidence for localized lowest‐excited states

Broadband and narrowed luminescence and excitation spectra at liquid helium temperatures of the title series (abbreviated as h24, d8, d16, and d24 for x=0, 1, 2, and 3, respectively) are reported. The luminescence spectra of the h24, d8, and d24 materials are found to be very similar except that the inhomogeneous broadening varies and a deuteration shift of 37 cm−1 to higher energy is observed for the d24 system. The luminescence of the d16 material is a superposition of the h24 and d24 spectra. Two distinct luminescence lifetimes can be measured in this material. The excitation spectra of the d8 and d16 materials are well represented as superpositions of the h24 and d24 spectra. The crystal structure establishes that there is only one spectroscopic site with two equivalent ligands. The third ligand has a different environment. The lowest‐excited metal‐to‐ligand charge transfer states of the two equivalent ligands are localized on an electronic time scale but very fast intramolecular excitation energy tra...

Luminescence properties of Sm2+-activated barium chloroborates

The luminescence properties of Sm2+-activated stoichiometric and non-stoichiometric Ba2B5O9Cl were investigated from 16 to 450 K. In stoichiometric Ba2B5O9Cl, the emission spectra of Sm2+ are composed of 4f6-4f6 transitions over the 16-450K temperature range studied. Luminescence was observed from Sm2+ in four inequivalent cationic sites at 16K andtwo inequivalent sites at room temperature. In the non-stoichiometric compound, the emission is characterized by 4f6-4f6 transitions at low temperature and 4f->5d1-4f6 transitions at high temperature. The Sm2+ doped into the Ca2+ and Sr 2+ analogues in the M2B5O9Cl series shows the broadband 4f->5d1-4f6 luminescence in both the stoichiometric and non-stoichiometric compounds due to the increased ligand field caused by the smaller ionic radii of the metal ions.

Exchange interactions in a trigonal chromium(III) pair

Single crystals of the title compound were studied by optical absorption, luminescence, Zeeman and MCD spectroscopy in the region of 4 A 2 4 A 2 → 2 E 4 A 2, 2 T 1 4 A 2 transitions. The ground state exchange parameters J, j could be accurately determined from the luminescence spectra. From a complete analysis of the observed electronic transitions to the singly excited pair states 2 E 4 A 2 and 2 T 1 4 A 2, the trigonal orbital exchange parameters Ja and Je could be deduced. Ja was found to represent the dominant exchange pathway. This was rationalized by an Extended Huckel calculation. The exchange interaction leads to a strong mixing of the 2 E 4 A 2 and 2 T 1a± 4 A 2 pair states. This is the main reason for the unusually large g ‖c values found in the lowest-energy excited states. The O-H stretching frequencies were found to play the major role as accepting modes in a multiphonon relaxation process.

Zero-phonon lines and luminescence line narrowing in the series Ru(bpy-d8)x(bpy)3-x(PF6)2(x=1–3). Direct evidence for localization in the two lowest excited states

Abstract Luminescence and excitation measurements were performed on single crystals of specifically deuterated compounds Ru(bpy- d 8 ) x (bpy) 3- x (PF 6 ) 2 ( x = 1–3) in the region of the sharp zero-phonon transitions at 4.2 and 2.1 K. The results are compared with the spectroscopic properties of the non-deuterated compound ( x =0). Strong evidence for the existence of localized, non-degenerate excited states is obtained. However, fast intramolecular excitation energy transfer takes place even at 2.1 K. This leads the excitation in theRu(bpy- d 8 ) x (bpy) 3- x (PF 6 ) 2 ( x =1–2) systems to be trapped on the protonated metal-ligand sub-unit(s) at temperatures below 10 K. Selective excitation allows luminescence as well as excitation features to be narrowed.

Luminescence properties of Mn2+ in KMgF3 and KZnF3 perovskite crystals: Evidence of a dynamical Jahn-Teller effect

Abstract The luminescence spectra of KZnF 3 : Mn 2+ and KMgF 3 : Mn 2+ are presented and analysed. At T = 5 K, both spectra consist of one zero phonon line from which a phonon sideband centered at about 600 nm is formed. The existence of the dynamical Jahn-Teller (JT) effect in the first 4 T 1g excited state is evidenced through a reduction of the spin-orbit splitting displayed by the zero phonon lines. JT energies of 530 and 450 cm -1 associated with electron-phonon couplings carrying e g type distortions around Mn 2+ are estimated for KZnF 3 : Mn 2+ and KMgF 3 : Mn 2+ , respectively. Vibronic sidebands are interpreted in terms of lattice modes rather than local vibrations of the MnF 4- 6 complex. The magnetic and electric dipole character of the 4 T 1g → 6 A 1g transition is analysed by the temperature dependence of the luminescence spectra and their lifetimes.

Stark-swept transient hole burning and resonant luminescence line narrowing in [Zn(2,2′-bipyridine)3](ClO4)2:Ru(II)

Abstract Resonant luminescence line narrowing and Stark-swept transient hole-burning experiments were performed in the electronic origins of the lowest excited states of the title material. The energies of the two lowest excited states are well correlated. No fine structure was observed in the range 300 MHz–30 GHz for the resonantly and non-resonantly narrowed origin II and I, respectively. This is at variance with the interpretation of ODMR experiments. At 4.2 K an upper limit of ≈ 25 MHz is deduced for the homogeneous line width of origin II. A large pseudo-Stark splitting of the origins is observed corresponding to |Δμ| ≈ 5.5 D. This value is used to calibrate Stark-swept transient hole-burning experiments which needed ± 33 V/cm to measure the entire hole shape. A homogeneous line width of 14 ± 2 MHz is found for origin II at 1.5 K. The large Stark effect is the first direct evidence for the charge-transfer character of the lowest excited states in [Ru(bpy)3]2+. The results are consistent with the localized nature of the lowest excited states.

Mechanochemical preparation of nanocrystalline BaFCl doped with samarium in the 2+ oxidation state.

We report a facile mechanochemical preparation method for nanocrystalline BaFCl doped with samarium in the 2+ oxidation state by ball milling BaCl2, BaF2, and SmI2 under a nitrogen atmosphere. The resulting phosphors were characterized by powder X-ray diffraction; electron microscopy, X-ray photoelectron spectroscopy; and photoluminescence, photoexcitation, cathodoluminescence, and diffuse reflectance spectroscopy. This is the first report of a direct preparation method of Sm(2+) doped alkaline earth fluorohalides at room temperature and points to a significant potential for the preparation of a wide range of related X-ray storage phosphors containing rare earth ions in divalent and trivalent cationic states by mechanochemical methods.

Intramolecular transfer of3MLCT excitation energy in [Ru(bpy)3]2+

Abstract The rate of 3 MLCT excitation energy transfer between the two crystallographicaly equivalent metal—ligand subunits of [Ru(bpy) 3 ] 2+ in the [Zn(bpy) 3 ] (ClO 4 ) 2 lattice has been directly measured by time-resolved luminescence line-narrowing experiments. At 1.8 K the transfer rate is ≈ 1 × 10 8 s −1 in the second-lowest-excited level II. The energy transfer is incoherent as the inhomogeneous width is much larger than the excitation exchange interaction. The measured rate is in accord with the value estimated from hole-burning experiments.

Advances in High-Resolution Laser Spectroscopy of Coordination Compounds

Abstract Advanced laser spectroscopies are shown to be invaluable in providing unambiguous detail concerning the electronic structure of coordination compounds. The basic principles of two techniques which overcome the effect of inhomogeneous broadening, namely fluorescence line narrowing and hole-burning spectroscopy, are discussed. Their use and limitations in transition-metal complexes along with examples from our own work are given. These examples present a full range of d-d, ligand-centered singlet-triplet and charge-transfer transitions in both crystal-line and amorphous hosts.

Luminescence and excitation line narrowing in tetragonal Cr3+ complexes: Confirmation of a large 2Eg splitting

Abstract A long standing controversy concerning the 2Eg splitting in [Cr(NH3)5H2O](CLO4)3, [Rh(NH3)5Cl]Cl2:Cr3+ 1% and [Cr(NH3)5Cl]Cl2 is resolved by the use of luminescence, excitation line narrowing techniques and site selective spectroscopy. Large 2Eg splittings for these tetragonal Cr3+ complexes are confirmed. A conventional ligand field calculation is clearly inadequate for this class of compounds. Non-spherical contributions to the electronic repulsion probably explain the observed splittings. Large non-spherical electronic repulsions require low symmetry distortion in the first coordination sphere.