Excited-state Dynamics of Radical Ions in Liquids.

Thomas Bally has acquired international recognition for his work on the photochemistry of reactive intermediates, which include radical ions. Here, we present a brief overview of our investigations of the excited-state dynamics of radical ions in liquids at room temperature, which are still poorly documented. A better understanding of these dynamics is most relevant, as open-shell ions in the excited state are being increasingly used in redox photochemistry and have been proposed to play a key role in highly exergonic photoinduced electron transfer reactions.

Ion-Pair Dynamics upon Photoinduced Electron Transfer Monitored by Pump-Pump-Probe Spectroscopy.

The excited-state dynamics of the radical anion of perylene (Pe) generated upon bimolecular photoinduced electron transfer (PET) with a donor was investigated using broadband pump-pump-probe spectroscopy. It was found to depend on the age of the anion, that is, on the time interval between the first pump pulse that triggers PET and the second one that excites the ensuing Pe anion (Pe•-). These differences, observed in acetonitrile but not in tetrahydrofuran, report on the evolution of the PET product from an ion pair to free ions. Two photoinduced charge recombination pathways of the ion pair to the neutral Pe*(S1) + donor state were identified: one occurring in a few picoseconds from Pe•-*(D1) and one taking place within 100-200 fs from Pe•-*(D n>1). Both processes are sensitive to the interionic distance over different length scales and thus serve as molecular rulers.

Localized holes and delocalized electrons in photoexcited inorganic perovskites: Watching each atomic actor by picosecond X-ray absorption spectroscopy.

We report on an element-selective study of the fate of charge carriers in photoexcited inorganic CsPbBr3 and CsPb(ClBr)3 perovskite nanocrystals in toluene solutions using time-resolved X-ray absorption spectroscopy with 80 ps time resolution. Probing the Br K-edge, the Pb L3-edge, and the Cs L2-edge, we find that holes in the valence band are localized at Br atoms, forming small polarons, while electrons appear as delocalized in the conduction band. No signature of either electronic or structural changes is observed at the Cs L2-edge. The results at the Br and Pb edges suggest the existence of a weakly localized exciton, while the absence of signatures at the Cs edge indicates that the Cs+ cation plays no role in the charge transport, at least beyond 80 ps. This first, time-resolved element-specific study of perovskites helps understand the rather modest charge carrier mobilities in these materials.

Broadband extreme ultraviolet probing of transient gratings in vanadium dioxide.

Nonlinear spectroscopy in the extreme ultraviolet (EUV) and soft x-ray spectral range offers the opportunity for element selective probing of ultrafast dynamics using core-valence transitions (Mukamel et al., Acc. Chem. Res. 42, 553 (2009)). We demonstrate a step on this path showing core-valence sensitivity in transient grating spectroscopy with EUV probing. We study the optically induced insulator-to-metal transition (IMT) of a VO(2) film with EUV diffraction from the optically excited sample. The VO(2) exhibits a change in the 3p-3d resonance of V accompanied by an acoustic response. Due to the broadband probing we are able to separate the two features.

Ultrafast long-distance excitation energy transport in donor-bridge-acceptor systems.

The excited-state dynamics of two energy donor-bridge-acceptor (D-B-A) systems consisting of a zinc tetraphenylporphyrin (ZnP) and a free base tetraphenylporphyrin (FbP) bridged by oligo-p-phenyleneethynylene units with different substituents has been investigated using ultrafast spectroscopy. These systems differ by the location of the lowest singlet excited state of the bridge, just above or below the S(2) porphyrin states. In the first case, Soret band excitation of the porphyrins is followed by internal conversion to the local S(1) state of both molecules and by a S(1) energy transfer from the ZnP to the FbP end on the 10 ns time scale, as expected for a center-to-center distance of about 4.7 nm. On the other hand, if the bridge is excited, the energy is efficiently transferred within 1 ps to both porphyrin ends. Selective bridge excitation is not possible with the second system, because of the overlap of the absorption bands. However, the time-resolved spectroscopic data suggest a reversible conversion between the D*(S(2))-B-A and D-B*(S(1))-A states as well as a transition from the D-B*(S(1))-A to the D-B-A* states on the picosecond time scale. This implies that the local S(2) energy of the ZnP end can be transported stepwise to the FbP end, i.e., over about 4.7 nm, within 1 ps with an efficiency of more than 0.2.

A comparison of sensitized Ln(III) emission using pyridine- and pyrazine-2,6-dicarboxylates--part II.

The synthesis, X-ray structures and photophysical properties of several new Ln(III) complexes with the dianion of pyrazine-2,6-dicarboxylic acid (H(2)PYZ) that demonstrate excellent stability and solubility in non-aqueous solution are reported, and compared to structurally analogous complexes derived from pyridine-2,6-dicarboxylic acid (H(2)DPA). The Eu(III) and Yb(III) complexes demonstrate efficient metal centered luminescence in the visible and Near Infra-Red (NIR) regions, respectively. Low temperature (77 K) phosphorescence measurements using the corresponding Gd(III) complex has allowed the photophysical properties of the sensitizer to be rationalized, together with corresponding TD-DFT studies for a model complex. Lastly, we have evaluated the sensitization efficiencies for these complexes, and have undertaken femtosecond transient absorption (TA) measurements in order to evaluate the relative importance of the intersystem crossing and energy transfer processes involved with sensitized Ln(III) emission via the antennae effect.

A donor-acceptor tetrathiafulvalene ligand complexed to iron(II): synthesis, electrochemistry, and spectroscopy of [Fe(phen)2(TTF-dppz)](PF6)2.

The synthesis and photophysical properties of the complex [Fe(phen)(2)(TTF-dppz)](2+) (TTF-dppz = 4',5'-bis-(propylthio)tetrathiafulvenyl[i]dipyrido[3,2-a:2',3'-c]phenazine, phen = 1,10-phenanthroline) are described. In this complex, excitation into the metal-ligand charge transfer bands results in the population of a high-spin state of iron(II), with a decay lifetime of approximately 1.5 ns, in dichloromethane, at room temperature. An intraligand charge transfer state can also be obtained and has a lifetime of 38 ps. A mechanism for the different states reached is proposed based on transient absorption spectroscopy.

A spectroscopic and computational study of a photoinduced cross-dehydrogenative coupling reaction of a stable semiquinone radical.

Norrish-type-II reaction on a semiquinone radical: stable semiquinone radicals serve as novel molecular platforms on which a Norrish-type-II photoreaction can be initiated. A detailed reaction scheme involving a 1,5-hydrogen transfer followed by a cyclization step that finally leads to a new C-C bond formation could be verified. Transient absorption spectroscopy and DFT calculations trace convincingly the intermediates and transition states along the reaction path.

Excited-state dynamics of Wurster's salts.

The excited-state dynamics of a series of Wurster's salts (p-phenylenediamine radical cations) with different subtituents on the nitrogen atoms was investigated under a variety of experimental conditions using a combination of ultrafast spectroscopic techniques. At room temperature, the lifetime of the lowest excited state of all radical cations is on the order of 200 fs, independently of the solvent, that is, water, nitriles, alcohols, and room-temperature ionic liquid. On the other hand, all cations, except that with the bulky nitrogen substituents, become fluorescent below 120 K. The observed dynamics can be accounted for by the presence of a conical intersection between the D(1) and D(0) states. For the cations with a small nitrogen substituent, this conical intersection could be accessed through a twist of one amino group, as already suggested for Wurster's Blue. However, this coordinate cannot be invoked for the cation with bulky nitrogen subtituents, and more probably, pyramidalization of the nitrogen center and/or deformation of the phenyl ring play an important role. Consequently, the excited-state dynamics of these structurally very similar Wurster's salts involves different decay mechanisms.

Photophysics of the galvinoxyl free radical revisited.

The photophysical properties of the free neutral radical galvinoxyl were studied by a combination of femtosecond time-resolved spectroscopy and quantum chemical calculations. The electronic absorption spectrum is dominated by an intense band at 430 nm that is ascribed to the D(9,10)← D(0) transitions. Upon photoexcitation at 400 nm, the population of the D(9,10) states decays within less than 200 fs to the electronic ground state. This ultrafast internal conversion does not involve intramolecular modes with large amplitude motion as the measured dynamics does not show any significant dependence on the environment, but is most probably facilitated by a high density of electronic states of different character. Depending on the solvent, a weak transient band due to the galvinoxylate anion is also observed. This closed-shell species, which is fluorescent although its deactivation is also dominated by non-radiative decay, is generated upon biphotonic ionization of the solvent and electron capture. The ultrashort excited-state lifetime of the galvinoxyl radical precludes photoinduced disproportionation previously claimed to be at the origin of the formation of both anion and cation.

Photochemical properties of UV Filter molecules of the human eye.

PURPOSE: To compare the photochemical properties of UV filter molecules present in the human lens (kynurenine, KN; 3-hydroxykynurenine, 3OHKN; 3-hydroxykynurenine O-ß-D-glucoside, 3OHKG; 4-(2-aminophenyl)-4-oxobutanoic acid, AHA; and glutathionyl-kynurenine, GSH-KN) with the use of the following parameters: excited singlet lifetime τ(S), fluorescence quantum yield Φ(fl), triplet quantum yield Φ(T), and photodecomposition quantum yield Φ(dec). METHODS: The excited singlet lifetimes were measured with the use of fluorescence upconversion (time resolution, 210 fs) and pump-probe transient absorption (time resolution, 200 fs) methods. The fluorescence quantum yields were determined relative to an aqueous solution of quinine bisulfate. The triplet quantum yields were measured with the use of nanosecond laser flash photolysis. The photodecomposition quantum yields were determined by steady state photolysis followed by the high-performance liquid chromatography analysis. RESULTS: The secondary UV filters--AHA and GSH-KN are better photosensitizers than the primary ones--KN, 3OHKN and 3OHKG: the singlet state lifetimes of the secondary UV filters are longer, and the quantum yields of fluorescence and triplet state formation are higher. CONCLUSIONS: With aging, the ratio primary/secondary UV filters in the human lens decreases from approximately 10:1 to 2:1. The obtained results demonstrate that the quality of the secondary UV filters is inferior compared to the primary ones, which may result in a higher susceptibility of old lenses to UV light. That might be an important factor for the development of the age-related cataract.

Photoinduced electron transfer reactions: from the elucidation of old problems in bulk solutions towards the exploration of interfaces.

The activities of our research group in the field of photoinduced electron transfer reactions are discussed and illustrated by several examples.

Effect of the addition of a fused donor-acceptor ligand on a Ru(II) complex: synthesis, characterization, and photoinduced electron transfer reactions of [Ru(TTF-dppz)2(Aqphen)]2+.

The synthesis and the photophysical properties of the complex [Ru(TTF-dppz)(2)(Aqphen)](2+) (TTF = tetrathiafulvalene, dppz = dipyrido-[3,2-a:2',3'-c]phenazine, Aqphen = anthraquinone fused to phenanthroline via a pyrazine bridge) are described. In this molecular triad excitation into the metal-ligand charge transfer bands results in the creation of a long-lived charge separated state with TTF acting as electron donor and anthraquinone as terminal acceptor. The lifetime of the charge-separated state is 400 ns in dichloromethane at room temperature. A mechanism for the charge separation involving an intermediate charge-separated state is proposed based on transient absorption spectroscopy.

Photophysics and photochemistry of the UV filter kynurenine covalently attached to amino acids and to a model protein.

The photophysics and photochemistry of kynurenine (KN) covalently bound to the amino acids lysine, cysteine, and histidine, the antioxidant glutathione, and the protein lysozyme have been studied by optical spectroscopy with femto- and nanosecond time resolution. The fluorescence quantum yield of the adducts of KN to amino acids is approximately 2 times higher than that of the free KN in solution; KN attached to protein exhibits a 7-fold increase in the fluorescence quantum yield. The S(1) state dynamics of KN-modified lysozyme reveals a multiphasic decay with a broad dispersion of time constants from 1 ps to 2 ns. An increase of the triplet yield of KN bound to lysozyme is also observed; the triplet state undergoes fast intramolecular decay. The obtained results reveal an increase of the photochemical activity of KN after its covalent attachment to amino acids and proteins, which may contribute to the development of oxidative stress in the human lenses-the main causative factor for the cataract onset.

Photoinduced tautomeric transformations of xanthurenic acid.

The properties of xanthurenic acid (XAN) in ground and photoexcited states have been studied using steady-state and time-resolved optical methods as well as quantum chemistry calculations. In neutral aqueous solution and in alcohols, XAN is present in a single tautomeric form (keto form), whereas in aprotic solvents and probably in basic aqueous solutions, more than one tautomeric form is present. UV irradiation of aqueous and alcoholic solutions of XAN results in a very rapid solvent-assisted tautomerization to the enol form, the later undergoes solvent-assisted transformation back to the keto form. The photolysis of XAN in aprotic solvents gives rise to the formation of numerous intermediate forms of XAN in both triplet and ground states. Under intense laser irradiation, XAN undergoes biphotonic ionization, the precursor for ionization being the excited singlet state.

Ultrafast excited-state dynamics of a series of zwitterionic pyridinium phenoxides with increasing sterical hindering.

A series of pyridinium phenoxides that differ by the dihedral angle between the pyridinium and the phenoxide rings because of substituents with increasing steric encumbrance has been investigated by ultrafast spectroscopy. Like the related betaine-30, these molecules are characterised by a zwitterionic electronic ground state and a weakly polar S(1) state. Their fluorescence lifetime was found to lie between 200 to 750 fs, decreasing with increasing dihedral angle, and increasing with solvent viscosity. This was assigned to a non-radiative deactivation of the emissive state coupled to a large amplitude motion involving the dihedral angle. The transient absorption spectra suggested that emission occurs from the Franck-Condon S(1) state, which decays to a dark excited state, that itself most probably corresponds to the relaxed S(1) state. Finally, this relaxed state decays to the vibrationally hot ground state through an intramolecular charge separation process with a time constant ranging between 0.4 and 3 ps, increasing with the dihedral angle and with the solvent relaxation time. These variations were discussed in terms of the Jortner-Bixon model of electron transfer, where the charge separation dynamics depends on both electronic coupling and solvent relaxation. The results suggested that charge separation slows down with increasing dihedral angle.

Ultrafast decay of the excited singlet states of thioxanthone by internal conversion and intersystem crossing.

The experimental ultrafast photophysics of thioxanthone in several aprotic organic solvents at room temperature is presented, measured using femtosecond transient absorption together with high-level ab initio CASPT2 calculations of the singlet- and triplet-state manifolds in the gas phase, including computed state minima and conical intersections, transition energies, oscillator strengths, and spin-orbit coupling terms. The initially populated singlet pi pi* state is shown to decay through internal conversion and intersystem crossing processes via intermediate n pi* singlet and triplet states, respectively. Two easily accessible conical intersections explain the favorable internal conversion rates and low fluorescence quantum yields in nonpolar media. The presence of a singlet-triplet crossing near the singlet pi pi* minimum and the large spin-orbit coupling terms also rationalize the high intersystem crossing rates. A phenomenological kinetic scheme is proposed that accounts for the decrease in internal conversion and intersystem crossing (i.e. the very large experimental crescendo of the fluorescence quantum yield) with the increase of solvent polarity.

A compactly fused pi-conjugated tetrathiafulvalene-perylenediimide donor-acceptor dyad.

The synthesis and structural characterization of a tetrathiafulvalene-fused perylenediimide molecular dyad is presented. Its largely extended pi-conjugation provides intense optical absorption bands over a wide spectral range. The planar functional molecule exhibits a short-lived nonluminescent excited state attributed to intramolecular charge separation.

Protease responsive nanoprobes with tethered fluorogenic peptidyl 3-arylcoumarin substrates.

Protease responsive nanosensors were obtained by the attachment of unique green fluorescent bifunctional 3-arylcoumarin-derived fluorogenic substrates to poly(acrylamide-co-N-(3-aminopropyl)methacrylamide) nanoparticles, in which proteolysis results in substantial signal amplification.