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Molecules that contain one or more unpaired electrons delocalized within a ?-conjugated backbone are promising candidates for applications in spin electronics or simply 'spintronics'. Our group develops functional organic materials based on ?-conjugated hydrocarbon molecules, where the electrons are unpaired either in the ground state or in the excited state that is low in energy and can be populated thermally. We aim to learn how to introduce and control a multitude of properties, namely, optical, chiroptical, magnetic, and conductive, in a bulk material made of these molecules, by manipulating spin interactions between the unpaired electrons. The first model system that was developed in our group is a hydrocarbon named cethrene, which has a diradicaloid singlet ground state and a low-lying triplet excited state. In this article, the structural parameters and their impact on the properties and reactivity of cethrene are discussed within the realm of the three C's that symbolize cethrene's C-shape, chirality, and chameleonic reactivity.

Nuclear quantum dynamics beyond the Born-Oppenheimer approximation is performed using quantum trajectories. Withintheadiabaticrepresentationoftheelectronicstates, NABDY(NonAdiabaticBohmianDYnamics) is used in combination with DFT and LR-TDDFT to perform on-the-fly nonadiabatic quantum dynamics. Simple numerical test systems and current limitations of the method are discussed.

We review our recent studies of excited state structures and dynamics of chemical and biological systems with pico- and femtosecond X-ray absorption spectroscopy in the liquid phase.

A thorough theoretical description of ultrafast phenomena that occur in complex systems constitutes a formidable challenge. It not only necessitates the use of quantum mechanical methods that can describe ground and possibly even electronically excited state potential energy surfaces with sufficient accuracy but also calls for approaches that can take the real-time dynamics of a system and the coupling between its electronic and nuclear degrees of freedom fully into account. Over the last years, our group has been active in the development of mixed quantum mechanical/molecular mechanical (QM/MM) methods for the in situ simulations of dynamical phenomena in ground and excited states within the adiabatic (Born-Oppenheimer) approximation. Recently, we have extended our theoretical tools with the explicit inclusion of nonadiabatic effects in the framework of Ehrenfest dynamics and Tully's fewest switches surface hopping. These extensions allow the theoretical description of nonadiabatic ultrafast phenomena in the gas phase as well as in solution, and complex biological environments.

We present the first structural determination of the Pt2(P2O5H2)44– anion in solution by analyzing the extended X-ray absorption fine structure (EXAFS) spectrum of the Pt LIII edge. The data could be fit with a simple model involving single and multiple scattering paths to near and far P-atoms, bridging O-atoms, and the other Pt-atom in the binuclear complex. A Pt–Pt distance of 2.876(28) Å and a Pt–P bond length of 2.32(4) Å are obtained. These values are in line with distances found in previous X-ray diffraction studies. The assignment of the EXAFS spectrum of the Pt2(P2O5H2)44– anion in its ground state is required for future time-resolved X-ray absorption measurements with the goal of determining the structure and dynamics of the complex in the 1,3A2u excited states.

We discuss and illustrate by several examples how the ultrafast excited-state dynamics of a chromophore can be altered when changing its environment from a homogenous solution to a biological molecule such as proteins or nucleic acids.

The developments of the last fifteen years in the field of triplet-induced degradation of aquatic organic contaminants are reviewed. There is increasing evidence that the elusive excited triplet states of colored dissolved natural organic matter (CDOM) initiate the oxidative degradation of various contaminants. To date, phenols, phenylurea herbicides, and sulfonamide antibiotics have been shown to be affected by such an oxidation. The parallel study of aqueous oxidation of target contaminants induced by excited triplet states of aromatic ketones, taken as model systems, has been useful in understanding the action of triplet CDOM and in developing predictive tools to assess aquatic oxidation rates of such contaminants.

Some of the recent research in optical sensor and switch molecules at Queen's University of Belfast is out-lined. The ion-induced absorption spectral changes of molecules with ICT (Internal Charge Transfer) excited states are used to demonstrate superposed logic where multiple logic configurations are simultaneously observable. Similar spectral effects in multi-receptor structures give rise to XOR logic. The ion-induced luminescent spectral changes of molecules with ICT excited states can be combined with the phenomenon of Photoinduced Electron Transfer (PET) to exemplify tristable systems. Insights can also be gained into the luminescence effects initiated in designed molecules by transition metal ions in non-aqueous media. The ion-induced luminescent spectral changes of PET systems with one or two receptors display YES and AND logic respectively. The latter yield coincidence sensors for the direct detection of ion pairs. Overall, interesting optical sensors and switches can be produced from rather small molecules when the latter are fitted with ion-receptors.

Interfacial electron transfer from a molecular dye to a semiconductor is a keystone process in the conversion of light into electricity in dye-sensitized solar cells. The most successful devices developed so far are based on the sensitization of nanocrystalline titanium dioxide by ruthenium polypyridyl complexes. The ultrafast electron injection from the widely used RuII(dcbpy)2(NCS)2 dye in particular has been intensely studied. Several research groups, including ours, have found that this reaction apparently proceeds with a fast sub-100 fs phase, followed by a slower kinetic component with a time constant of 0.7–100 ps and accounting for 16–65% of the total yield. No convincing explanation has been provided for a clear understanding of the origin of this non-exponential kinetic behavior. In this contribution we show that aggregation of dye molecules at the interface is actually responsible for the slow kinetic component of the interfacial electron transfer. A thorough control of the dissolution of the dye and of its adsorption onto nanocrystalline oxide films allowed the reduction of the portion of dye excited states that react within the slow compartment and even made the latter completely disappear. In the absence of dye aggregates, femtosecond pump-probe studies of the sensitizer's oxidized state appearance yielded a rate constant for charge injection >5×1013 s?1, corresponding to an electron transfer time of less than 20 fs.

Previously we have employed the 7-hydroxyquinoline·(NH3)n ammonia-wire clusters to study excited state H-atom transfer along a sequence of hydrogen bonds. H-atom transfer experimentally occurs for n = 4 excited to its vibrationless S1 state, for n = 3 with additional excitation of ammonia-wire vibrations (~200cm–1). However, for the n = 2 cluster, no reaction is observed, even with additional excitation energy of 900cm–1. CIS calculations have been performed on the n = 2 and n = 3 clusters to understand the effect of the wire length on the energy profile of the reaction. The enol ? keto reaction is always exoergic, and the barrier for H transfer to the proximal ammonia controls the reaction kinetics.

Heck and Sonogashira cross-coupling reactions have been used for the functionalization of the upper rim of a tetraiodinated calix[4] arene. In this way, oligophenylenevinylene (OPV) and oligophenyleneethynylene (OPE) arrays have been constructed on the calix[4] arene core to produce covalent assemblies of four pi-conjugated chromophores. Electrochemical properties have been investigated by cyclic voltammetry in different solvent/electrolyte systems. Both OPV and OPE calixarenes show the simultaneous reduction of three out of four units, followed, at more negative potential, by the reduction of the fourth unit. This behaviour can be rationalized in view of a partial deconjugation in one OPV or OPE unit. On the other hand, these calixarenes show only oxidation processes corresponding to the exchange of one electron, thus demonstrating the lack of electrochemical equivalency of the OPV or OPE units, in contrast to the reduction behaviour. Electronic absorption and emission spectra have been recorded in solvents of different polarity ( toluene, dichloromethane, and benzonitrile). The absorption spectra of the OPV calixarene do not match to the sum of the component units as a consequence of the partial deconjugation of the OPV arms in the calixarene structure. On the contrary, fluorescence spectra, quantum yields and excited state lifetimes of the OPV calixarene are nearly identical to those of the corresponding model compound. The trend of the absorption and emission spectra for the OPE calixarene is reversed relative to that of the OPV counterpart. Absorption spectra are well-matched with those of four trimeric OPE fragments but emission spectra exhibit, besides the typical OPE monomeric fluorescence, a broader and longer-lived emission feature on the low-energy spectral side, attributable to excimer-type interactions. Therefore the arrangement of OPV or OPE units within the same calix-4-arene skeleton brings about different effects on the electronic and luminescence properties of the multicomponent system.

The absolute S1-state absorption cross-section spectrum of eosin Y in methanol has been determined by pico-second second harmonic Nd:glass laser excitation and time-delayed picosecond light continuum probing (probing wavelength region from 400 to 980 nm).

The saturable dye DDI is excited with picosecond ruby laser pulses to the S 1 singlet state and the relaxation to the ground state S 0 is determined by measuring the transmission of the exciting pulses through the sample. The relaxation time is found to be τ = 17 ± 3 ps for DDI dissolved in methanol. Admictures of rhodamine 6G, potassium iodide, or tetrabutylammonium iodide do not change the decay time.