A Strongly Luminescent Chromium(III) Complex Acid

Abstract

The synthesis, structure, reactivity, and photophysical properties of a novel acidic, luminescent chromium(III) complex [Cr(H tpda) ] (2 ) bearing the tridentate H tpda (2,6-bis(2-pyridylamino)pyridine) ligand are presented. Excitation of 2 at 442 nm results in strong, long-lived NIR luminescence at 782 nm in water and in acetonitrile. X-ray diffraction analysis and IR spectroscopy reveal hydrogen-bonding interactions of the counter ions to the NH groups of 2 in the solid state. Deprotonation of the NH groups of 2 by using a non-nucleophilic Schwesinger base in CH CN switches off the luminescence. Re-protonation by using HClO restores the emission. In water, the pK value of 2 amounts to 8.8, yet deprotonation is not reversible in the presence of hydroxide ions. Dioxygen quenches the emission of 2 , but to a weaker extent than expected. This is possibly due to the strong ion-pairing properties of 2 even in solution, reducing the energy transfer efficiency to O . Deuteration of the NH groups of 2 approximately doubles the quantum yield and lifetime in water, demonstrating the importance of multiphoton relaxation in these NIR emitters.

Ddpd as Expanded Terpyridine: Dramatic Effects of Symmetry and Electronic Properties in First Row Transition Metal Complexes

“…Concomitant with the very high quantum yields, the lifetime of the doublet excited state increases to up to 2.3 ms [42]. Similarly, deuteration of the NH groups of the analogous [Cr(H2tpda)2] 3+ complex increases the luminescence quantum yield from 8.8% to 12.0% in CH3CN due to the removal of the NH oscillators [138]. The high quantum yields and excited state lifetimes of [Cr(ddpd)2] 3+ complexes led to their description as "molecular rubies" [42,50,53].…”

Chiral Molecular Ruby [Cr(dqp)2]3+ with Long-Lived Circularly Polarized Luminescence

“…At 77 K, the thermally-driven population of the Cr( 2 T1) excited state vanishes and only the narrow Cr( 2 E→ 4 A2) phosphorescence can be recorded (Figure 4a is blue shifted, by more than 400 cm -1 , while the excited Cr( 2 T1) state of all these complexes are almost isoenergetic. 30, 33,52,55 . Consequently, the Cr( 2 T1/ 2 E) energy gap is reduced by a factor two and amounts to only 420 cm -1 in [Cr(dqp)2] 3+ (Figure 4b).…”

Molekularer Rubin unter Druck

“…Wir präsentieren druckabhängige Lumineszenzspektren von [Cr(ddpd) 2 ][BF 4 ] 3 , [Cr(ddpd) 2 ][PF 6 ] 3 und [Cr(H 2 tpda) 2 ][ClO 4 ] 3 (H 2 tpda=2,6‐Bis(2‐pyridylamino)pyridin) im Festkörper (Schema ) bis zu Drücken von 49 kbar. [Cr(ddpd) 2 ][BF 4 ] 3 wurde zudem in wässriger und in Methanol‐Lösung untersucht.…”

“…Zusammenfassend ermöglichen die hohen Quantenausbeuten der Spin‐Flip‐Lumineszenz der Ionen [Cr(ddpd) 2 ] 3+ und [Cr(H 2 tpda) 2 ] 3+ in verschiedenen Umgebungen und die starken druckinduzierten, linearen Verschiebungen bis zu −14.8 cm −1 kbar −1 optische Druckmessungen mit bislang ungekannter Genauigkeit im kristallinen Zustand und darüber hinaus auch in Lösung. Die einfache Synthese, die günstigen Ausgangsmaterialien, die hohe Stabilität und die gute Verfügbarkeit von Chrom (z. B. im Vergleich mit kritischen Rohstoffen wie Iridium oder Europium) in Kombination mit der hohen Quantenausbeute und der starken Druckabhängigkeit sollten eine breite Anwendung dieser Materialien als optische Drucksensoren ermöglichen.…”