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The effects of isoelectronic replacement of a neutral nitrogen donor atom by an anionic carbon atom in terpyridine ruthenium(II) complexes on the electronic and photophysical properties of the resulting N,C,N'- and C,N,N'-cyclometalated aryl ruthenium(II) complexes were investigated. To this end, a series of complexes was prepared either with ligands containing exclusively nitrogen donor atoms, that is, [Ru(R-1-tpy)(R-2-tpy)](2+) (R-1, R-2 = H, CO2Et), or bearing either one N,C,N'- or C,N,N'-cyclometalated ligand and one tpy ligand, that is, [Ru(R-1-(NCN)-C-Lambda-N-Lambda)(R-2-tpy)](+) and [Ru(R-1-(CNN)-N-Lambda-N-Lambda)(R-2-tpy)](+), respectively. Single-crystal X-ray structure determinations showed that cyclometalation does not significantly alter the overall geometry of the complexes but does change the bond lengths around the ruthenium(II) center, especially the nitrogen-to-ruthenium bond length trans to the carbanion. Substitution of either of the ligands with electron-withdrawing ester functionalities fine-tuned the electronic properties and resulted in the presence of an IR probe. Using trends obtained from redox potentials, emission energies, IR spectroelectrochemical responses, and the character of the lowest unoccupied molecular orbitals from DFT studies, it is shown that the first reduction process and luminescence are associated with the ester-substituted C,N,N'-cyclometalated ligand in [Ru(EtO2C-(CNN)-N-Lambda-N-Lambda)(tpy)](+). Cyclometalation in an N,C,N'-bonding motif changed the energetic order of the ruthenium d(zx), d(yz), and d(xy) orbitals. The red-shifted absorption in the N,C,N'-cyclometalated complexes is assigned to MLCT transitions to the tpy ligand. The red shift observed upon introduction of the ester moiety is associated with an increase in intensity of low-energy transitions, rather than a red shift of the main transition. Cyclometalation in the C,N,N'-binding motif also red-shifts the absorption, but the corresponding transition is associated with both ligand types. Luminescence of the cyclometalated complexes is relatively independent of the mode of cyclometalation, obeying the energy gap law within each individual series.

Femtosecond transient absorption spectroscopy was employed to determine quantitatively the ultrafast S 1 -T 1 intersystem crossing in a 2-substituted 9,10-anthraquinone derivative (3), k isc = 2.5 × 10 12 s -1 . Notwithstanding this rapid process, photoexcitation of dyad 1 is followed by competition between intersystem crossing and intramolecular charge separation, the latter leading to a short-lived (2 ps) singlet charge-transfer (CT) state. The local triplet state itself undergoes slower charge separation to populate a relatively long-lived (130 ns) triplet CT state. An earlier report about the formation of an extremely long-lived CT state (>900 μs) in 1 was found to be erroneous and was related to the sacrificial photo-oxidation of the dimethylsulfoxide solvent used in that study. Finally, some important criteria have been formulated for future experimental validation of "unusually long-lived" CT states.

This review presents some of the efforts that have been made over the last decades to produce systems in which photo-excitation leads to one or more intramolecular electron transfer events ultimately resulting in a charge-transfer (CT) excited state with a relatively long lifetime. This process is generally considered as a mimic of natural photosynthesis and is not only of relevance in relation to solar energy conversion but also in relation to perspectives such as molecular information storage, molecular electronics, and molecular photonics. A long-lived CT state in general may be considered as a weakly coupled radical (ion)pair and in this review we focus especially on the consequences of the eventual electron spin correlation in that radical (ion)pair. If substantial spin–spin interaction is still present, such as in compact dyads, CT states can be assigned pure singlet or triplet configurations (1CT, 3CT) and as we demonstrate this configuration has significant influence on the CT lifetime because charge recombination from 3CT is spin forbidden. For small spin–spin interaction such as is typical for CT states in which the radical sites are further removed from each other – e.g., in triads, tetrads, etc., – rapid interconversion of 1CT and 3CT becomes possible especially via a hyperfine interaction (HFI) driven mechanism. This HFI driven mechanism is strongly influenced by external magnetic fields, which allows sensitive detection of the actual spin–spin interaction via magnetic field effects on the electron transfer kinetics, as well as via time-resolved EPR and field-dependent CIDNP. Examples of such studies on artificial multichromophoric electron transfer systems are presented and the results are discussed.

Photoinduced electron transfer is a widely applied method to convert photon energy into a useful (electro)chemical potential, both in nature and in artificial devices. There is a continuing effort to develop molecular systems in which the charge-transfer state, populated by photoinduced electron transfer, survives sufficiently long to tap the energy stored in it. In general this has been found to require the construction of rather complex molecular systems, but more recently a few approaches have been reported that allow the use of much more simple and relatively small electron donor-acceptor dyads for this purpose. The most successful examples of such systems seem to be those that apply "electron spin control" to slow down the spontaneous decay of the charge-transfer state, and these are reviewed in this minireview, with a discussion of the underlying principles and a critical evaluation of some of the claims made with regard to using a pronounced "inverted-region effect" as an alternative method to prolong the lifetime of charge-transfer states.

The target donor-acceptor compound forms an acridinium-like, locally excited (LE) singlet state on illumination with blue or near-UV light. This LE state undergoes rapid charge transfer from the acridinium ion to the orthogonally sited mesityl group in polar solution. The resultant charge-transfer (CT) state fluoresces in modest yield and decays on the nanosecond time scale. The LE and CT states reside in thermal equilibrium at ambient temperature; decay of both states is weakly activated in fluid solution, but decay of the CT state is activationless in a glassy matrix. Analysis of the fluorescence spectrum allows precise location of the relevant energy levels. Intersystem crossing competes with radiative and nonradiative decay of the CT state such that an acridinium-like, locally excited triplet state is formed in both fluid solution and a glassy matrix. Phosphorescence spectra position the triplet energy well below that of the CT state. The triplet decays via first-order kinetics with a lifetime of ca. 30 micros at room temperature in the absence of oxygen but survives for ca. 5 ms in an ethanol glass at 77 K. The quantum yield for formation of the LE triplet state is 0.38 but increases by a factor of 2.3-fold in the presence of iodomethane. The triplet reacts with molecular oxygen to produce singlet molecular oxygen in high quantum yield. In sharp contradiction to a recent literature report, there is no spectroscopic evidence to indicate the presence of an unusually long-lived CT state.

In the recent literature a simple 9-aryl-acridinium ion was claimed to undergo an intramolecular, photoinduced charge shift to produce an extremely long-lived and very high energy charge-transfer state. The possible consequences of this observation are discussed and the tenability of the claims made is investigated via time resolved spectroscopy of a closely related system with spectroscopic characteristics allowing more solid identification of the actual photophysical events taking place. From the results obtained it appears likely that the long-lived species observed earlier in solution cannot be charge transfer in nature but must instead be identified as the lowest triplet state of the acridinium chromophore.

A long-lived intramolecular charge-separated state in an electron-donor-acceptor molecule is characterized by time-resolved visible and infrared absorption spectroscopy. Bands that can be assigned to the negatively charged acceptor chromophore can be clearly observed in the time-resolved IR spectrum. In contrast, the electronic absorption spectrum of the radical anion lacks characteristic absorption bands. The IR spectra can be interpreted on the basis of harmonic vibrational frequencies and intensities calculated using B3LYP ab initio methods for model systems of the separate electron donor and acceptor units. This result indicates that the charge-separated state, in spite of the small distance between electron donor and acceptor, can be represented well as an intramolecular radical ion pair.

A new family of bidentate phosphine ligands based on the biscarbazole backbone has been synthesized and applied in the ruthenium- and rhodium-catalyzed asymmetric hydrogenations of methyl acetoacetate and dimethyl itaconate. The nitrogen atoms in these BICAP ligands allow facile introduction of substituents providing structurally similar, but electronically different ligands, which were used to fine-tune these reactions to 98% and 55% ee, respectively.

Triplet-sensitized generation of a long-lived intramolecular charge-separated excited state is described in an electron donor-acceptor molecule with a short distance between the donor and the acceptor. Time-resolved UV-Vis optical absorption spectroscopy shows that the lifetime of this triplet excited state is 1.4 micros in acetonitrile at 298 K, i.e. five orders of magnitude greater than that of the corresponding singlet charge-separated state. In slightly less polar alkane nitrile solvents, the local and CT triplet states coexist, which allows determination of their relative energies.

The nonradiative decay pathways are studied of the 1- cyano- 4- cyclohexylidenenaphthalene chromophore and electron donor acceptor compounds containing the same chromophore. It is argued that the rapid decay at room temperature of the locally excited singlet state of the electron acceptor chromophore involves twisting of the double bond via a low barrier, and a polar transition state. In non- polar solvents the charge transfer excited states also decay mostly via a thermally activated decay process, possibly via a similar pathway as the locally excited state of the acceptor unit. In more polar solvents, decay via direct charge recombination to the ground state obeys the energy gap law, but in contrast to the prediction of semiclassical Marcus theory an activation energy of 1 - 2 kcal mol(-1) is found.

Sensitised excitation of the DMN[3]DCME donor[bridge]acceptor system allows population of a triplet CT state with a microsecond lifetime in polar solvents. This is more than five orders of magnitude longer than the lifetime of the corresponding singlet CT state which is populated exclusively upon direct excitation. The picture indicates the spin-forbidden nature of the charge recombination between the acceptor (red) to donor (green) moieties.

The photophysical properties of a bicyclohexylidene (1DA) and a bicyclohexyl (2DA) substituted with an anilino electron donor and a dicyanoethylene electron acceptor have been studied. Quenching of local donor emission is observed for these compounds as well as quenching of the "pseudo-local" acceptor emission. Transient absorption spectra show dialkylanilino-type radical-cation and dicyanoethylene-type radical-anion absorptions. These results show that intramolecular charge separation takes place in 1DA and 2DA. This was corroborated by time-resolved microwave conductivity measurements from which large excited-state dipole moments were found for both 1DA and 2DA. Time-resolved fluorescence spectroscopy revealed that in the charge-separated state in cyclohexane for 2DA, molecular folding takes place on a nanosecond timescale. For 1DA in cyclohexane, either charge separation takes place in a (fully) folded conformation or very rapid (subnanosecond timescale) folding takes place subsequent to charge separation. In addition to this difference in conformational behavior, the presence of the exocyclic double bond between the cyclohexyl-type rings results in efficient quenching of the anilino donor triplet state and acceleration of the charge recombination rate by a factor of 20.

The electron transfer kinetics of several completely rigid dyads and triads which contain N,N-dimethylaniline (DMA) and dimethoxynaphthalene (DMN) as possible donors and the dicyanovinyl group (DCV) as acceptor were studied by means of (sub)picosecond time-resolved transient absorption spectroscopy. In the dyads DMN[n]DCV, the rate of charge separation decreases exponentially with the number of σ-bonds n in the bridge, the ‘‘damping factor’’ being 0.8 per bond in tetrahydrofuran solvent. In the triads DMA[4]DMN[8]DCV, the primary electron transfer from DMN to DCV occurs in solvents of low and medium polarity within 10 ps in both isomers (syn and anti). The rates of the secondary electron transfer step (formation of the fully charge separated state, DMA+[4]DMN[8]DCV−) and the following deactivation processes depend, however, strongly on the conformation. In acetonitrile, the primary electron transfer involves the two donor groups yielding preferentially DMA+[4]DMN−[8]DCV. In the anti-conformer this state is fairly long-lived; in the syn-conformer, however, it decays rapidly, in part to locally excited triplet states.

The rate of photoinduced charge separation is measured as a function of solvent for four donor (D)−bridge−acceptor (A) systems: DMN[10]DCV, DMN[10nb]DCV, DMN[10cy]DCV, and DMAN[10cy]DCV. In the first three members of this series, the D/A pair is kept constant and contains the strong dimethoxynaphthalene (DMN) donor which enables detection of electron transfer over a wide range of solvent polarity. In the fourth member, DMN is substituted by a dimethoxyanthracene (DMAN) unit, which decreases the driving force for photoinduced charge separation by about 0.58 eV and thereby limits the occurrence of electron transfer to polar solvents. In all systems the bridge is held at a length of 10 σ bonds. The configuration of the bridge is, however, varied to increase its bending in the series, which leads to center-to-center D/A distances decreasing from 13.4 A in the first system to 9.54 A in the second, and 7.50 A in the latter two. In DMN[10nb]DCV, the rate of intramolecular charge separation over 9.54 A is always...

The application of fluorogenic probe molecules for in situ monitoring of radiation-induced polymerisation is illustrated by measurements on the polymerisation of methylmethacrylate in a cobalt gamma-ray source. The probe molecules fluoresce only when they are incorporated into growing polymer chains. The fluorescence intensity is therefore a measure of the degree of polymerisation. Changes in the fluorescence spectrum can in addition be used as a monitor of the changes occurring in the rigidity of the medium as polymerisation proceeds. The method is particularly suitable for studying the abrupt changes occurring during the `gel' or `Trommsdorff' phase of polymerisation.

Laser pulse photoexcitation of rigid donor–spacer–acceptor molecules in various solvents leads to chemically induced dynamic nuclear polarization (CIDNP) of the ground state species. This polarization is created during the simultaneous charge recombination of the intermediate ion pairs via singlet and triplet pathways. The different channels yield polarizations with opposite signs and equal magnitudes and are separated in time because the triplet contribution appears delayed. Their time dependence yields the triplet decay parameters. The CIDNP intensities depend strongly on the spacer length and on the dielectric constant of the medium. They are explained in terms of the radical pair theory combined with the known dependence of rate constants on energy gaps, and provide the exchange interaction in the ion pairs.

The likelihood of an infinitely long-lived, charge-shift state being formed by the target compound is re-assessed in light of persistent claims that such chemistry is both viable and observable.

The fluorescence of four semiflexibly bridged donor−bridge−acceptor systems was studied with a streak camera system. The information-rich streak data make a detailed analysis possible of both the kinetics and the spectral changes that take place. From the analysis, it follows that all compounds studied undergo a photoinduced harpooning process in which long-range electron transfer creates an extended charge-transfer (ECT) species, which converts to a compact charge-transfer (CCT) species due to Coulombic attraction. Both the ECT and CCT species display CT fluorescence. The harpooning process occurs even in solvents of medium polarity (e.g., diethyl ether), whereas it has previously been assumed for related compounds that this process only occurs in the gas phase and in nonpolar alkane solvents.

The chromophore of photoactive yellow protein (PYP) (i.e., 4-hydroxycinnamic acid) has been replaced by an analogue with a triple bond, rather than a double bond (by using 4-hydroxyphenylpropiolic acid in the reconstitution, yielding hybrid I) and by a "locked" chromophore (through reconstitution with 7-hydroxycoumarin-3-carboxylic acid, in which a covalent bridge is present across the vinyl bond, resulting in hybrid II). These hybrids absorb maximally at 464 and 443 nm, respectively, which indicates that in both hybrids the deprotonated chromophore does fit into the chromophore-binding pocket. Because the triple bond cannot undergo cis/trans (or E/Z) photoisomerization and because of the presence of the lock across the vinyl double bond in hybrid II, it was predicted that these two hybrids would not be able to photocycle. Surprisingly, both are able. We have demonstrated this ability by making use of transient absorption, low-temperature absorption, and Fourier-transform infrared (FTIR) spectroscopy. Both hybrids, upon photoexcitation, display authentic photocycle signals in terms of a red-shifted intermediate; hybrid I, in addition, goes through a blue-shifted-like intermediate state, with very slow kinetics. We interpret these results as further evidence that rotation of the carbonyl group of the thioester-linked chromophore of PYP, proposed in a previous FTIR study and visualized in recent time-resolved x-ray diffraction experiments, is of critical importance for photoactivation of PYP.

The fluorescence spectrum and fluorescence quantum yield of C70 were determined in a number of solvents at room temperature. A well resolved fluorescence excitation spectrum, which matches the absorption spectrum, was obtained over a broad wavelength region (250 to 580 nm. in n-hexane). The fluorescence lifetime (627 ps in benzene) was determined, thereby settling the uncertainty about the singlet lifetime Of C70. It is shown that the quenching of the fluorescence by organic electron donors involves both dynamic and static mechanisms. The contribution of the latter is much more pronounced than can be explained from the degree of ground-state charge transfer complexation. Analysis in terms of a Perrin model indicates that long-range intermolecular electron transfer is important, with a radius of action that increases sharply when the donor oxidation potential is lowered.

The behavior is reported of three fluorescent D-bridge-A systems that display a fascinating temperature dependence in glass forming solvents over the temperature range between 77 and 293 K. In two of these systems, a rigid, saturated alkane bridge maintains an extended conformation, and as a result, the charge-transfer (CT) state is of giant dipolar nature. This causes the position of the CT fluorescence to be an extremely sensitive probe for the reorientation polarization of the surrounding medium. As a result, the thermochromism of the continuous CT fluorescence maximum in 2-methyltetrahydrofuran (MTHF) covers the full visible region. In the higher temperature domain (above ca. 145 K), this thermochromism can be quantitatively described via the Lippert-Mataga relation. At lower temperatures, solvent relaxation slows down sufficiently to detect exceptionally large dynamic Stokes shifts of the fluorescence maximum on time scales up to greater than or equal to40 ns. The third D-bridgc-A system studied features a U-shaped ground state conformation. Remarkably, this system displays a significant thermochromic shift over a narrow temperature region around 175 K in the nonpolar methylcyclohexane (MCH) in which the other systems display only very minor thermochromism. In this U-shaped system therefore,one monitors the temperature dependence of an internal reorganization instead of a medium relaxation. Extensive ab initio calculations indicate that this internal reorganization must be related to an electrostatically driven conformational collapse of the U-shaped system in the CT state.

International audience; It is shown that the successful sensitization of lanthanide luminescence by azatriphenylenes can be expanded from sensitization in non-hydroxylic solvents to aqueous media by the addition of two or more carboxylate functionalities on the triphenylene skeleton. Furthermore, it is reported that a fluorescent donor– bridge– acceptor system incorporated in a thin polymer film allows the production of blue electroluminescence that can be tuned from blue to yellow by exposure to solvent vapours.

International audience; Rare earth ions, with relatively long luminescence lifetimes, have significant advantages for application in fields as varied as diagnostics and optical amplification. In diagnostics the long luminescence lifetimes allow for extremely sensitive time-gated detection, where the difference in temporal behavior of scatter and background fluorescence and the long-lived rare earth luminescence is utilized. In optical amplification the long excited-state lifetime makes it easier to obtain population inversion, a requirement for effective stimulated emission. Unfortunately the absorption cross section of rare earth ion transitions is extremely low. However, via sensitized excitation by means of a suitable organic molecule, efficient excitation is obtained. It is shown that excitation in the visible part of the spectrum can be used to excite rare earth ions which luminesce in the near-IR, such as ytterbium, neodymium, and erbium, via a fluorescein-derivative as sensitizer. The advantages of this approach are manifold. Low-cost light sources are available for the visible part of the spectrum, and interferences from the matrix (scatter, absorption) are minimal. Detection in the near-IR is almost interference-free. For optical amplification the wavelength regions around 1300 and 1550 nm, which can be covered with the neodymium and erbium complexes, respectively, are the most important for applications in optical telecommunication.

Via full correction for the instrumental response function, the fluorescence excitation spectrum of C60 in methylcyclohexane between 240 and 580 nm is shown to match the absorption spectrum. This proves that, in contrast to earlier reports, also after UV excitation into higher S(n) states, deactivation proceeds virtually completely via the S1 state, in accordance with the Kasha-Vavilov rule.

A calix[8]arene derivative has been found to form a water soluble complex with fullerene-C60, but not with C70, which allows extraction of C60 from e.g. toluene into the aqueous phase. Charge transfer character of the complex is evident from strong charge transfer absorption around 440 nm measured in water. Molecular modeling calculations indicate a core-shell structure for the complex.