Charge‐Transfer and Spin‐Flip States: Thriving as Complements

Abstract

Transition metal complexes with photoactive charge-transfer excited states are pervasive throughout the literature. In particular, [Ru(bpy) 3 ] 2 + (bpy = 2,2'-bipyridine), with its metal-to-ligand charge-transfer emission, has been established as a key complex. Meanwhile, interest in so-called spin-flip metal-centered states has risen dramatically after the molecular ruby [Cr(ddpd) 2 ] 3 + (ddpd = N,N'-dimethyl-N,N'-dipyridin-2-yl-pyridine-2,6-diamine) led to design principles to access strong, long-lived emission from photostable chromium(III) complexes. This Review contrasts the properties of emissive charge-transfer and spin-flip states by using [Ru(bpy) 3 ] 2 + and [Cr(ddpd) 2 ] 3 + as prototypical examples. We discuss the relevant excited states, the tunability of their energy and lifetimes, and their response to external stimuli. Finally, we identify strengths and weaknesses of charge-transfer and spin-flip states in applications such as photocatalysis and circularly polarized luminescence.

Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d6 Complexes with Cr0, MnI, FeII, and CoIII

“…146,147 Fighting nonradiative relaxation is therefore inherently more difficult when dealing with 3d 6 MLCT excited states, and this should be kept in mind when comparing photophysical figures of merit between d 6 MLCT luminophores and d 3 spin-flip (MC) emitters. 128,148 ■ PHOTOCATALYSIS Whereas 4d 6 and 5d 6 complexes and their MLCT excited states are extensively used in organic photoredox and energy transfer catalysis, 2,149,150 this is not yet the case for 3d 6 analogues. There is an increasing body of literature reporting on photocatalysis with Fe II complexes (mostly with polypyridine ligands), 151−154 though this usually implies reactivity from the lowest MC excited state.…”

“…No matter what exact design principles are applied, the photoactive MLCT excited states of d 6 complexes will likely remain considerably more strongly distorted than the spin-flip excited states in d 3 metal compounds, as becomes evident when comparing the Huang–Rhys factors of 3 MLCT states in [Ru­(bpy) 3 ] 2+ and its congeners with those of the lowest spin-flip excited states of Cr III . , Fighting nonradiative relaxation is therefore inherently more difficult when dealing with 3d 6 MLCT excited states, and this should be kept in mind when comparing photophysical figures of merit between d 6 MLCT luminophores and d 3 spin-flip (MC) emitters. , …”

Trendbericht: Photochemie

“…49) Molekulare Rubine bieten nicht nur Anwendungen in der Photo(redox)katalyse und Aufwärtskonversion, sondern auch in der Sensorik und der zirkular polarisierten Emission. 50) W…”

EuIII functionalized silica nanoparticles encapsulating chiral CrIII complexes with simultaneous unpolarized red and polarized NIR-I luminescence

“…Chiral materials displaying circularly polarized luminescence (CPL), which is the emission of polarized light with a certain degree of handedness, are attracting systems not only for fundamental purposes but also for potential applications in many different fields of biology, physics and chemistry. [1][2][3][4][5][6] Thus, many CPL-active (nano)materials have been developed and they are mainly based on chiral organic molecules (discrete or supramolecular), chiral metal complexes, chiral polymers or chiral inorganic solids. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] However, silica platforms are overlooked materials in this field, particularly, silica nanoparticles.…”