On the origin of fluorescence emission in optically non-linear DCNP crystals.

We study absorption and emission spectra of optically nonlinear single crystals of 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (DCNP) at 5 K. We argue that fluorescence has a complex origin, it is emitted from the excitonic band, with the bottom at ∼18,115 cm(-1), and from trap states, and the two main traps have depths of ∼875 and ∼2465 cm(-1). The excitonic origin of the emission is confirmed by the vibrational structure of fluorescence, closely resembling vibrations observed in the Raman scattering spectrum (recorded for DCNP crystals at 295 K) and by very short decay time of the excitonic emission, as a consequence of exciton migration and trapping at deep traps.

Photoinduced water splitting with oxotitanium tetraphenylporphyrin.

Photocatalytic splitting of water was investigated in a heterogeneous system consisting of micro-crystallites of oxotitanium tetraphenylporphyrin deposited on fused silica plates, immersed in water and excited within the visible range of their absorption spectra. The water photolysis was evidenced by the spectroscopic detection of hydroxyl radicals generated in the reaction. The experimental results confirm the mechanism of water splitting and generation of OH˙ radicals proposed theoretically by Sobolewski and Domcke [Phys. Chem. Chem. Phys., 2012, 14, 12807] for the oxotitaniumporphyrin-water complex. It is shown that photocatalytic water splitting occurs in pure water, and neither pH-bias nor external voltage is required to promote the reaction.

On the nature and signatures of the solvated electron in water.

The hydrated electron is one of the simplest chemical transients and has been the subject of intense investigation and speculation since its discovery in 1962 by Hart and Boag. Although extensive kinetic and spectroscopic research on this species has been reported for many decades, its structure, i.e., the dominant electron-water binding motif, and its binding energy remained uncertain. A recent milestone in the research on the hydrated electron was the determination of its binding energy by liquid-jet photoelectron spectroscopy. It turned out that the assumption of a single electron binding motif in liquid water is an oversimplification. In addition to different isomers in cluster spectroscopy and different transient species of unknown structure in ultrafast experiments, long-lived hydrated electrons near the surface of liquid water have recently been discovered. The present article gives an account of recent work on the topic "solvated electrons" from the perspectives of cluster spectroscopy, condensed-phase spectroscopy, as well as theory. It highlights and critically discusses recent findings and their implications for our understanding of electron solvation in aqueous environments.

Biradicalic excited states of zwitterionic phenol-ammonia clusters.

Phenol-ammonia clusters with more than five ammonia molecules are proton transferred species in the ground state. In the present work, the excited states of these zwitterionic clusters have been studied experimentally with two-color pump probe methods on the nanosecond time scale and by ab initio electronic-structure calculations. The experiments reveal the existence of a long-lived excited electronic state with a lifetime in the 50-100 ns range, much longer than the excited state lifetime of bare phenol and small clusters of phenol with ammonia. The ab initio calculations indicate that this long-lived excited state corresponds to a biradicalic system, consisting of a phenoxy radical that is hydrogen bonded to a hydrogenated ammonia cluster. The biradical is formed from the locally excited state of the phenolate anion via an electron transfer process, which neutralizes the charge separation of the ground state zwitterion.

Hydrogen transfer in excited pyrrole-ammonia clusters.

The excited state hydrogen atom transfer reaction (ESHT) has been studied in pyrrole-ammonia clusters [PyH-(NH(3))(n)+hnu-->Py.+.NH(4)(NH(3))(n-1)]. The reaction is clearly evidenced through two-color R2P1 experiments using delayed ionization and presents a threshold around 235 nm (5.3 eV). The cluster dynamics has also been explored by picosecond time scale experiments. The clusters decay in the 10-30 ps range with lifetimes increasing with the cluster size. The appearance times for the reaction products are similar to the decay times of the parent clusters. Evaporation processes are also observed in competition with the reaction, and the cluster lifetime after evaporation is estimated to be around 10 ns. The kinetic energy of the reaction products is fairly large and the energy distribution seems quasi mono kinetic. These experimental results rule out the hypothesis that the reaction proceeds through a direct N-H bond rupture but rather imply the existence of a fairly long-lived intermediate state. Calculations performed at the CASSCF/CASMP2 level confirm the experimental observations, and provide some hints regarding the reaction mechanism.