Jan W. Verhoeven

The emission spectrum and the radiative lifetime of Eu3+ in luminescent lanthanide complexes

Although luminescent complexes of lanthanide ions and organic ligands have been studied intensively, relatively little attention has been paid to the natural (or ‘radiative’) lifetime of the lanthanide centered luminescent state in these systems. Here, the applicability of the well-known Judd–Ofelt theory to the emissive properties of Eu3+ complexes is investigated. Moreover, it is demonstrated experimentally that the radiative lifetime of the 5D0 excited state of Eu3+ can be calculated directly from its corrected emission spectrum, without using Judd–Ofelt theory. We also discuss briefly the possibility of finding the natural lifetimes of lanthanide ions other than Eu3+.

FLUORESCEIN AND EOSIN AS SENSITIZING CHROMOPHORES IN NEAR-INFRARED LUMINESCENT YTTERBIUM(III), NEODYMIUM(III) AND ERBIUM(III) CHELATES

Near-infrared luminescent ytterbium(III), neodymium(III) and erbium(III) chelates containing organic chromophores derived from fluorescein and eosin have been synthesized and studied spectroscopically. The complexes can be efficiently excited with visible light and show intense lanthanide luminescence at low concentrations (2≈ 10−6 mol l−1) in D2O as a result of energy transfer from the dye moiety to the rare earth ion. Quenching of the luminescence of the complexes by molecular oxygen reveals information on the rate of energy transfer from the “antenna” chromophore to the lanthanide ion.

Distance dependence of photoinduced electron transfer through non-conjugated bridges

Abstract Picosecond time-resolved emission studies of a series of molecules containing an electron donor-acceptor pair interconnected by a series of rigid non-conjugated bridges reveal the occurrence of very fast photoinduced intramolecular electron transfer. The length of the bridge was varied to provide donor-acceptor centre-to-centre separations ranging from 8.1 to 13.3 A (edge-to-edge 5 to 10.2 A). At centre-to-centre separations up to 10.7 A the rate of photoinduced electron transfer exceeded 5×10 10 s −1 (τ 10 s −1 (τ = 68 ps).

Charge-transfer absorption and emission resulting from long-range through-bond interaction; exploring the relation between electronic coupling andelectron-transfer in bridged donor-acceptor systems.

Abstract The electronic absorption- and emission spectra of seven (D)onor-(A)cceptor systems are studied with the general structure D-bridge-A, where the bridge consists of an extended, rigid, saturated hydrocarbon skeleton that separates D and A by distances ranging from 3 to 12 CC σ-bonds. Across bridges with a length up to six σ-bonds sufficient electronic interaction occurs to cause a detectable perturbation of the electronic absorption spectra and for lower homologues this leads to the appearance of discrete intramolecular charge-transfer absorptions with an intensity that is strongly enhanced by intensity borrowing from symmetry-matched local transitions. In the fluorescence spectra discrete charge-transfer (CT) type emission has been detected for bridge lengths up to ten σ-bonds. The radiative transition probability of this CT emission provides a direct measure for the electronic coupling matrix element (H da ) between the charge-separated- and the groundstate. The magnitude of H da is found to decrease exponentially with the number of intervening σ-bonds from 850 cm −1 at 3-bond separation to 17.6 cm −1 at 10-bond separation. Furthermore the rate of charge-recombination in the compounds studied is found to be proportional to the square of H da , thus providing an experimental verification of this often implied “golden rule” relation.

TETRAAZATRIPHENYLENES AS EXTREMELY EFFICIENT ANTENNA CHROMOPHORES FOR LUMINESCENT LANTHANIDE IONS

The ability of novel tetraazatriphenylenes to sensitise the luminescence of different lanthanide ions is reported. An example of the type of lanthanide complex formed is depicted. The influence of the ligand-structure variation on the absorption characteristics and other photophysical properties of the sensitiser was investigated.

Spectral Tuning, Fluorescence, and Photoactivity in Hybrids of Photoactive Yellow Protein, Reconstituted with Native or Modified Chromophores

Photoactive yellow proteins (PYPs) constitute a new class of eubacterial photoreceptors, containing a deprotonated thiol ester-linked 4-hydroxycinnamic acid chromophore. Interactions with the protein dramatically change the (photo)chemical properties of this cofactor. Here we describe the reconstitution of apoPYP with anhydrides of various chromophore analogues. The resulting hybrid PYPs, their acid-denatured states, and corresponding model compounds were characterized with respect to their absorption spectrum, pK for chromophore deprotonation, fluorescence quantum yield, and Stokes shift. Three factors contributing to the tuning of the absorption of the hybrid PYPs were quantified: (i) thiol ester bond formation, (ii) chromophore deprotonation, and (iii) specific chromophore-protein interactions. Analogues lacking the 4-hydroxy substituent lack both contributions (chromophore deprotonation and specific chromophore-protein interactions), confirming the importance of this substituent in optical tuning of PYP. Hydroxy and methoxy substituents in the 3- and/or 5-position do not disrupt strong interactions with the protein but increase their pK for protonation and the fluorescence quantum yield. Both deprotonation and binding to apoPYP strongly decrease the Stokes shift of chromophore fluorescence. Therefore, coupling of the chromophore to the apoprotein not only reduces the energy gap between its ground and excited state but also the extent of reorganization between these two states. Two of the PYP hybrids show photoactivity comparable with native PYP, although with retarded recovery of the initial state.

Strong effects of the bridge configuration on photoinduced charge separation in rigidly linked donor-acceptor systems

Abstract Photoinduced electron-transfer rates are reported for two pairs of rigid bichromophoric molecules 1 (6)/ 2 (6) and 1 (8)/ 2 (8). In the first pair electron donor and acceptor are separated by six, in the second pair by eight, carbon—carbon σ bonds. While these σ bonds provide an all-trans coupling path in 1 (6) and 1 (8), that path contains s-cis elements in 2 (6) and 2 (8), which - as shown by X-ray structure data and by spectroscopic evidence - leads to a slight decrease in the effective, spatial donor-acceptor separation. Nevertheless, photoinduced electron transfer in each of the “stretched” compounds is about one order of magnitude faster than in the corresponding “bent” compound. This remarkable effect is interpreted as resulting from the unique ability of an all-trans array of σ bonds to mediate electronic through-bond interaction (TBI). Interestingly the solvent dependence of the rate of photoinduced electron transfer is significantly larger in the “bent” systems, thus indicating that superexchange via solvent molecules becomes competitive with TBI if an all-trans array is not available.

Long-range exchange contribution to singlet-singlet energy transfer in a series of rigid bichromophoric molecules

Abstract Intramolecular singlet-singlet energy transfer is reported in a series of compounds containing a 1,4-dimethoxy-naphthalene chromophore as the energy donor and a cyclic ketone as the energy acceptor connected by rigid, elongated, saturated hydrocarbon bridges with an effective length of 4, 6, and 8 CC σ bonds. The rate of energy transfer is found to be proportional to the spectral overlap - as varied by solvent variation - and to show an exponential distance dependence while its magnitude significantly exceeds that predicted for a dipole-dipole coupling mechanism. From this it is concluded that energy transfer occurs predominantly via an exchange mechanism. Exchange integrals of 60, 10, and 2.5 cm −1 across 4, 6, and 8 σ bonds are calculated. The magnitude of these is proposed to signify through-bond exchange interaction between symmetry-matched donor (ππ*) and acceptor (nπ*) states.

On the nature of sigma-coupled transitions : Through-bond interactions in 1-aza-adamantane derivatives

Abstract 1-Aza-adamant-4-one and a number of its derivatives, in which the CO function is modified, show absorption in the near UV region which is attributed to a sigma-coupled transition. From absorption and emission spectroscopic data it is shown that this transition has to be charge transfer in character and that it derives its intensity mainly from a local π-π* transition in the (modified) CO group. From the relative basicities, the IR spectra and the 13 C NMR spectra it is concluded that the amount of charge transfer in the electronic ground-state is very small for the compounds studied.

Mechanism and transition-state structure of hydride-transfer reactions mediated by nad(p)h-models

Abstract The energy to transfer one electron from NAD(P)H and related 1,4-dihydropyridines to a series of substrates is calculated and compared with the experimental activation energy for transfer of a hydride equivalent between these species. It is concluded that single electron-transfer (SET) cannot occur as a primary step in the overall hydride-transfer process except for substrates with very strong one-electron oxidizing properties. A simple valence-bond configuration mixing (VBCM) model is presented, that rationalizes the general occurrence of concerted hydride transfer as the lowest energy reaction-pathway and furthermore explains why the activation energy of such a concerted pathway is often linearly related to that of a -hypothetical- SET process. For one intramolecular and two related, intermolecular hydride-transfer reactions the temperature dependence of the primary kinetic isotope effect (TDKIE) was studied. For the intramolecular reaction, where a face to face orientation of the reactants is enforced, the TDKIE parameters suggest the occurrence of a bent hydride-transfer pathway. For both intermolecular reactions, however, a linear transition-state geometry is indicated. MNDO calculations of the reaction profile for hydride transfer from a 1,4-dihydropyridine to either a positively charged substrate (i.e. the pyridinium-ion) or to a neutral substrate (i.e. 1,1-dicyanoethylene) confirm, that a linear transition-state geometry is favoured, unless the system is geometrically restrained to prevent such a geometry. The MNDO calculations furthermore indicate that in a linear transition-state almost unimpeded rotation can occur about the C...H...C axis. This rotation interconverts the relative orientation of the reactants between parallel-exo and tilted-endo, which may have important consequences for the interpretation of the stereochemical outcome of reactions involving (pro) chiral reactants.