Chromophores inspired by the colors of fruit, flowers and wine

Anthocyanins, which are responsible for most of the red, blue and purple colors of fruits and flowers, are very efficient at absorbing and dissipating light energy via excited state proton transfer or charge-transfer mediated internal conversion without appreciable excited triplet state formation. During the maturation of red wines, grape anthocyanins are slowly transformed into pyranoanthocyanins, which have a much more chemically stable pyranoflavylium cation chromophore. Development of straightforward synthetic routes to mono- and disubstituted derivatives of the pyranoflavylium cation chromophore has stimulated theoretical and experimental studies that highlight the interesting absorption and emission properties and redox properties of pyranoflavylium cations. Thus, p-methoxyphenyl substitution enhances the fluorescence quantum yield, while a p-dimethylaminophenyl substituent results in fast decay via a twisted intramolecular charge-transfer (TICT) state. Unlike anthocyanins and their synthetic analogs (flavylium cations), a variety of pyranoflavylium cations form readily detectable excited triplet states that sensitize singlet oxygen formation in solution and exhibit appreciable two-photon absorption cross sections for near-infrared light, suggesting a potential for applications in photodynamic therapy. These excited triplet states have microsecond lifetimes in solution and excited state reduction potentials of at least 1.3 V vs. SCE, features that are clearly desirable in a triplet photoredox catalyst.

A 195Pt NMR study on zero-magnetic-field splitting and the phosphorescence properties in the excited triplet states of cyclometalated platinum(ii) complexes

The strength of zero-magnetic-field splitting in the excited triplet states of metal complexes is reflected in the metal NMR chemical shifts.

Multiplicity conversion based on intramolecular triplet-to-singlet energy transfer

The ability to convert between molecular spin states is of utmost importance in materials chemistry. Förster-type energy transfer is based on dipole-dipole interactions and can therefore theoretically be used to convert between molecular spin states. Here, a molecular dyad that is capable of transferring energy from an excited triplet state to an excited singlet state is presented. The rate of conversion between these states was shown to be 36 times faster than the rate of emission from the isolated triplet state. This dyad provides the first solid proof that Förster-type triplet-to-singlet energy transfer is possible, revealing a method to increase the rate of light extraction from excited triplet states.

A novel broad-band neutron spin filter based on dynamically polarized protons using photo-excited triplet states

The use of polarized protons as a broad-band neutron spin filter is an attractive alternative to the well-established neutron polarization techniques, namely polarized 3He gas and super mirrors, since the spin-dependent neutron proton scattering cross-section is large in a broad wavelength range. We have developed a novel neutron spin filter where we create the necessary large proton polarization in a solid with a recent method of dynamic nuclear polarization (DNP) that uses photo-excited triplet states. This requires only moderate experimental means and allows a compact design. In order to quantify the efficiency of the spin filter, we have measured the relevant spin-dependent and spin-independent terms of the neutron scattering cross-section of a naphthalene single crystal. The data allows to estimate the triplet spin filter performance over a broad wavelength range. With the recently achieved proton polarization of 80% the triplet filter compares well with a state of the art 3He filter.