Pressure-Induced Enhancements of Luminescence Intensities and Lifetimes Correlated with Emitting-State Distortions for Thiocyanate and Selenocyanate Complexes of Platinum(II) and Palladium(II)

Abstract

The luminescence properties of thiocyanate and selenocyanate platinum(II) and palladium(II) complexes show strong variations with temperature and pressure. The d-d luminescence band maxima for [Pt(SCN)(4)](PPh(4))(2) (1), [Pt(SCN)(4)](n-Bu(4)N)(2) (2), and [Pt(SeCN)(4)](n-Bu(4)N)(2) (4) complexes are centered at ca. 14500 cm(-1) whereas those of the [Pd(SCN)(4)](n-Bu(4)N)(2) (3) and [Pd(SeCN)(4)](n-Bu(4)N)(2) (5) complexes are approximately 2000 cm(-1) lower in energy. Low-temperature luminescence spectra from single-crystal samples have broad bands with highly resolved vibronic structure indicating large displacements of the emitting-state potential energy minimum along several metal-ligand normal coordinates. The largest displacements involve the totally symmetric (a(1g)) stretching modes with frequencies of 295 cm(-1) (1), 303 cm(-1) (2), 274 cm(-1) (3), 195 cm(-1) (4), and 185 cm(-1) (5). The lower frequencies of these dominant progression-forming modes for the selenocyanate complexes lead to luminescence bands that are narrower by ca. 500 cm(-1) (fwhm) than those observed from the thiocyanate complexes. Under external pressures, the room-temperature luminescence intensities and lifetimes show considerable enhancement at pressures up to 40 kbar. This effect is largest for the palladium(II) complexes with lifetimes increasing from approximately 350 ns at ambient pressure up to 62 micros at 30 kbar, an increase by more than 2 orders of magnitude. The platinum(II) complexes exhibit a significant, but noticeably lesser increase of luminescence lifetimes and intensities with increasing pressure. The temperature- and pressure-dependent luminescence decay behavior is rationalized using the emitting-state molecular geometry determined from the resolved low-temperature luminescence spectra combined with the strong-coupling limit of radiationless decay theory.

Interpreting Effects of Structure Variations Induced by Temperature and Pressure on Luminescence Spectra of Platinum(II) Bis(dithiocarbamate) Compounds

“…Other square-planar complexes with d 8 configured metal centers and +2 oxidation states show similar shifts, such as +24 cm −1 /kbar for (nBu 4 N) 2 Pt(SCN) 4 21 and +29 cm −1 /kbar for (n-Bu 4 N) 2 Pd-(SCN) 4 . 21 Variable-pressure absorption spectra also show similar shifts, such as +13 cm −1 /kbar for Ni[(C 2 H 5 ) 2 P(CH 2 ) 2 P-(C 2 H 5 ) 2 ]I 2 35 and +13 cm −1 /kbar for Ni(isopropyl-PPh 2 ) 2 Br 2 . 35 According to these numerical values, the shift of the luminescence maxima observed for Pt(MeDTC) 2 is quite surprising, being significantly higher with a value of +47 ± 3 cm −1 /kbar.…”

Cyclometalated Platinum(II) Complexes as Highly Sensitive Luminescent Switch‐On Probes for Practical Application in Protein Staining and Cell Imaging

“…Excitation of 1 a (60 mm in H 2 O) at l = 410 nm gave a weak emission at 563 nm, which is attributed to the 3 MLCT excited state. [7] Complex 1 b has its emission l max at 522 nm, whereas 2 and 3 exhibit a weak emission, with l max at 656 nm in DMSO/H 2 O and 631 nm in H 2 O, respectively. Owing to the extended p-conjugation of the naphthyl group, the 656 nm emission of 2 is assigned to have a 3 IL (intra-A C H T U N G T R E N N U N G ligand) character.…”

“…[7] In previous work, we and others have demonstrated that significant changes in the emission properties of platinum(II) complexes in solutions can be induced by the presence of biomolecules. [4,8,9] Further study on the complex [PtA C…”

Excited States and Optical Spectroscopy of Nitronyl Nitroxides and their Lanthanide and Transition Metal Complexes

“…A promising direction for future spectroscopic work is the application of external perturbations, such as pressure, which could affect the magnetic order of the title materials and simultaneously greatly enhance the luminescence intensity, as has been shown recently for diamagnetic complexes of palladium(II) and platinum(II) [48,49]. Attractive compounds for future studies include recently described bimetallic complexes with radical ligands [50] and nickel(II) complexes where the imino nitroxide radical ligand shows linkage isomerism [51].…”