Orange-glowing iron at room temperature: Many photoactive coordination compounds contain precious metals. Replacing ruthenium with more–earth-abundant iron has been a long-sought goal, but iron compounds generally relax too rapidly after light absorption to channel the energy productively. Kjær et al. prepared an iron compound with an excited state stable enough to emit light for nanoseconds, or that could engage in bimolecular electron transfer (see the Perspective by Young and Oldacre). Targeting a ligand-to-metal rather than metal-to-ligand charge-transfer state was key to the achievement, as was the octahedral coordination environment rigidly enforced by two tridentate carbene ligands. Science , this issue p. 249 ; see also p. 225

A 9 ps ³MLCT lifetime was achieved by a Fe^II complex based on C(NHC)⁁N(py)⁁C(NHC) pincer ligands. This is the longest known so far for any kind of complexes of this abundant metal, and increased by almost two orders of magnitude compared to the reference Fe^II bis-terpyridine complex.

Carotenoids are involved in a variety of biological functions, yet the underlying mechanisms are poorly understood, in part because of the long-standing difficulty in assigning the location of the first excited (S ₁ ) state. Here, we present a method for determining the energy of the forbidden S ₁ state, on the basis of ultrafast spectroscopy of the short lived level. Femtosecond transient absorption spectra and kinetics of the S ₁ → S ₂ transition revealed the location of the intermediate level in two carotenoid species involved in the xanthophyll cycle, zeaxanthin and violaxanthin, and yielded surprising implications regarding the mechanism of photoregulation in photosynthesis.

Functions of biologically active molecules are frequently initiated by elementary chemical reactions such as energy and electron transfer, cis-trans isomerizations, and proton transfer. The nature of these reactions generally makes them very fast and efficient, occurring on picosecond and femtosecond timescales. Ultrafast spectroscopy has played an important role in the study of a number of biological processes and has provided unique information about several of nature's responses to light. Here I review the current understanding of light-energy collection and conversion in photosynthesis, the function of carotenoid molecules in photosynthesis, and the primary light-initiated reactions of the photoreceptors rhodopsin, bacteriorhodopsin, photoactive yellow protein, phytochrome, and a new type of blue-light receptor based on flavin chromophores.