Molecular Ruby under Pressure

Abstract

The intensely luminescent chromium(III) complexes [Cr(ddpd) ] and [Cr(H tpda) ] show surprising pressure-induced red shifts of up to -15 cm kbar for their sharp spin-flip emission bands (ddpd=N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine; H tpda=2,6-bis(2-pyridylamino)pyridine). These shifts surpass that of the established standard, ruby Al O :Cr , by a factor of 20. Beyond the common application in the crystalline state, the very high quantum yield of [Cr(ddpd) ] enables optical pressure sensing in aqueous and methanolic solution. These unique features of the molecular rubies [Cr(ddpd) ] and [Cr(H tpda) ] pave the way for highly sensitive optical pressure determination and unprecedented molecule-based pressure sensing with a single type of emitter.

Luminescence and Light‐Driven Energy and Electron Transfer from an Exceptionally Long‐Lived Excited State of a Non‐Innocent Chromium(III) Complex

“…Indeed, multiphonon relaxation typically plays adecisive role in non-radiative excited state decay of NIR emitters. [4,20,36,37] Thef ourth vibrational overtone of aromatic CH oscillators of pyridyl ligands (14 065 cm À1 ) [22] falls within the emission band envelope of the room temperature emission band of [Cr(tpe) 2 ] 3+ ( Figure S15). This results in an appreciable spectral overlap integral, hence promoting non-radiative deactivation by nearby CH oscillators (Scheme 3; pyridyl CH group with aC r ···Hd istance of d % 3.0 ).…”

“…[20] These favorable photophysical data of [Cr-(ddpd) 2 ] 3+ enable its application in temperature,p ressure and dioxygen sensing,a sw ell as in photocatalysis and photodynamic therapy. [21][22][23][24][25] Replacing one ddpd ligand by tpy slightly increases the excited state lifetime,y et dramatically reduces the quantum yield (Table 1), underlining the positive effect of six-membered chelate rings. [26] Photoredox catalytic cycloadditions have been reported using [Cr-(phen) 3 ] 3+ derivatives.…”

Spin Crossover and Long‐Lived Excited States in a Reduced Molecular Ruby

“…The molecular ruby [Cr III (ddpd) 2 ] 3+ (d 3 electron configuration, ddpd= N , N′ ‐dimethyl‐ N , N′ ‐dipyridine‐2‐yl‐pyridine‐2,6‐diamine) shows exceptional photophysical properties, such as high photoluminescence quantum yield up to 30 % (as deuterated derivative) and high luminescence lifetimes of the 2 T 1 / 2 E states (notation in octahedral symmetry) . These features led to applications in optical temperature and pressure sensing, circularly polarized luminescence, singlet oxygen formation and photocatalysis . Furthermore, the large ground state spin ( S= 3 / 2 ; 4 A 2 ) of the chromium(III) complex enables the utilization as molecular quantum bit with phase memory times of 4.25 and 8.4 μs in protio and deuterio solvents at 7 K …”

“…SCO of the Cr II complexes connects the low‐spin and high‐spin isomers. Excitation with light and luminescence links the 4 A 2 and 2 T 1 states of the Cr III complex . Finally, single‐electron transfer processes complete the square scheme from these two spin‐inversion reactions.…”

A Strongly Luminescent Chromium(III) Complex Acid

“…First applications of the strongly luminescent “molecular ruby” comprise optical temperature sensing in solution, in micelles, or in nanoparticles with a single dye based on its two NIR emission bands, optical pressure sensing in the solid state and in solution, and selective energy transfer to triplet oxygen, yielding singlet oxygen for organic photoinduced transformations . Further applications of 1 3+ and its derivatives are envisioned for the near future …”