Understanding the Photoexcitation of Room Temperature Ionic Liquids

Photoexcitation of (neat) room temperature ionic liquids (RTILs) leads to the observation of transient species that are reminiscent of the composition of the RTILs themselves. In this minireview, we summarize state‐of‐the‐art in the understanding of the underlying elementary processes. By varying the anion or cation, one aim is to generally predict radiation‐induced chemistry and physics of RTILs. One major task is to address the fate of excess electrons (and holes) after photoexcitation, which implies an overview of various formation mechanisms considering structural and dynamical aspects. Therefore, transient studies on time scales from femtoseconds to microseconds can greatly help to elucidate the most relevant steps after photoexcitation. Sometimes, radiation may eventually result in destruction of the RTILs making photostability another important issue to be discussed. Finally, characteristic heterogeneities can be associated with specific physicochemical properties. Influencing these properties by adding conventional solvents, like water, can open a wide field of application, which is briefly summarized

Relaxation Dynamics of Electronically Excited C60− in o-Dichlorobenzene and Tetrahydrofuran Solution

The ultrafast response of singly negatively charged C60 fullerene in solution has been investigated by femtosecond pump-probe absorption spectroscopy and transient anisotropy in the visible and near-infrared region. Pump excitation within the near-infrared band demonstrates that this spectral feature can be described as avibrational progression associated with asingle electronic transition. Relaxation of the first electronically excited state occurs primarily by internal conversion with atime constant of 3ps, slightly depending on the solvents, tetrahydrofuran or o-dichlorobenzene, and also on the excitation wavelength. An excitation of the second electronically excited state around 530nm leads to an ultrafast internal conversion to the first excited state with apulse-limited time constant of less than 100fs. As aminor channel, stimulated emission in the spectral regime of 1150-1300nm was observed from the first electronically excited state both after near-infrared and visible excitation. After internal conversion to the electronic ground state, C60− dissipates its excess internal energy into the solvent on alonger timescale of 40-70ps. The transient anisotropy associated with directly populating the first excited state reveals an ultrafast component decaying within 100fs, which is attributed to ultrafast vibrational motions, conceivably arising from excited state pseudorotatio

Photoexcitation of Ge9−^{-} Clusters in THF: New Insights into the Ultrafast Relaxation Dynamics and the Influence of the Cation

We present a comprehensive femtosecond (fs) transient absorption study of the [Ge$_{9}$(Hyp)$_{3}$]$^{-}$ (Hyp = Si(SiMe3)3) cluster solvated in tetrahydrofuran (THF) with special emphasis on intra- and intermolecular charge transfer mechanisms which can be tuned by exchange of the counterion and by dimerization of the cluster. The examination of the visible and the near infrared (NIR) spectral range reveals four different processes of cluster dynamics after UV (267/258 nm) photoexcitation related to charge transfer to solvent and localized excited states in the cluster. The resulting transient absorption is mainly observed in the NIR region. In the UV-Vis range transient absorption of the (neutral) cluster core with similar dynamics can be observed. By transferring concepts of: (i) charge transfer to the solvent known from solvated Na$^{-}$ in THF and (ii) charge transfer in bulk-like materials on metalloid cluster systems containing [Ge$_{9}$(Hyp)$_{3}$]$^{-}$ moieties, we can nicely interpret the experimental findings for the different compounds. The first process occurs on a fs timescale and is attributed to localization of the excited electron in the quasi-conduction band/excited state which competes with a charge transfer to the solvent. The latter leads to an excess electron initially located in the vicinity of the parent cluster within the same solvent shell. In a second step, it can recombine with the cluster core with time constants in the picosecond (ps) timescale. Some electrons can escape the influence of the cluster leading to a solvated electron or after interaction with a cation to a contact pair both with lifetimes exceeding our experimentally accessible time window of 1 nanosecond (ns). An additional time constant on a tens of ps timescale is pronounced in the UV-Vis range which can be attributed to the recombination rate of the excited state or quasi conduction band of Ge$_{9}$$^{-}$. In the dimer, the excess electron cannot escape the molecule due to strong trapping by the Zn cation that links the two cluster cores

Directed Electron Transfer in Flavin Peptides with Oligoproline‐Type Helical Conformation as Models for Flavin‐Functional Proteins

To mimic the charge separation in functional proteins we studied flavin‐modified peptides as models. They were synthesized as oligoprolines that typically form a polyproline type‐II helix, because this secondary structure supports the electron transfer properties. We placed the flavin as photoexcitable chromophore and electron acceptor at the N‐terminus. Tryptophans were placed as electron donors to direct the electron transfer over 0–3 intervening prolines. Spectroscopic studies revealed competitive photophysical pathways. The reference peptide without tryptophan shows dominant non‐specific ET dynamics, leading to an ion pair formation, whereas peptides with tryptophans have weak non‐specific ET and intensified directed electron transfer. By different excitation wavelengths, we can conclude that the corresponding ion pair state of flavin within the peptide environment has to be energetically located between the S$_{1}$ and S$_{4}$ states, whereas the directed electron transfer to tryptophan occurs directly from the S$_{1}$ state. These photochemical results have fundamental significance for proteins with flavin as redoxactive cofactor

Aqueous Conversion of Fructose Phosphate Precursor Nanoparticles into Emissive C-Dot Composite Nanoparticles

[ZrO]$^{2+}$[F6P]$^{2}$− and [Eu(OH)]$^{2}$+[F6P]$^{2}$− precursor nanoparticles (F6P: D-fructose-6-phosphate) are converted in a one-pot, aqueous approach to C-dot@[ZrO]$^{2+}$+[HPO$_{4}$]$^{2-}$ and C-dot@[Eu(OH)]$^{2+}$[HPO$_{4}$]$^{2-}$ composite nanoparticles. Herein, the C-dots (2–3 nm) are embedded in a dense zirconyl/europium phosphate matrix. The resulting composite nanoparticles (40–50 nm) are well-dispersible in water and show blue and red emission. A one-pot, fully water-based synthesis of blue- and red-emitting C-dots is presented. To this concern, [ZrO]$^{2+}$[F6P]$^{2}$− and [Eu(OH)]$^{2}$+[F6P]$^{2}$− precursor nanoparticles (F6P: D-fructose-6-phosphate) are prepared in water and converted to C-dot@[ZrO]$^{2+}$+[HPO$_{4}$]$^{2-}$ and C-dot@[Eu(OH)]$^{2+}$[HPO$_{4}$]$^{2-}$ composite nanoparticles in boiling water (100 °C) via microwave heating. Composition, structure, and fluorescence of the composite nanoparticles are validated by different analytical methods (e. g., FT-IR, EA, TG, DLS, SEM, TEM, EDXS). The resulting aqueous suspensions are characterized by high colloidal stability and intense emission. Specifically, C-dot@[Eu(OH)]$^{2+}$[HPO$_{4}$]$^{2-}$ exhibits Eu$^{3+}$-type red emission in water. The one-pot water-based synthesis with fructose-containing precursor nanoparticles and the structure of the phosphate-stabilized C-dot composite nanoparticles are reported for the first time

Intersystem Crossing Rates in Photoexcited Rose Bengal: Solvation versus Isolation

We compare the intersystem crossing rate, kISC, of Rose Bengal (RB) in an aqueous pH 12 solution with the corresponding relaxation rates of four different RB-derived anion and dianion species isolated in the gas phase: the doubly deprotonated dianion ([RB-2H]2–), the singly deprotonated monoanion ([RB-H]−), and the corresponding singly negatively charged sodium and cesium adducts ([RB-2H + Na]− and [RB-2H + Cs]−, respectively). Each of them was probed following photoexcitation of their first singlet excited states (S1) at or near room temperature. The solution was studied by transient absorption spectroscopy, whereas the mass-selected anions were characterized by time-resolved photoelectron spectroscopyall with ca. 50 femtosecond temporal resolution. [RB-H]− shows an S1 lifetime of ca. 80 ps; the solution ensemble, thought to consist primarily of solvated dianion chromophores, shows a similar lifetime of ca. 70 ps. By contrast, the isolated dianion, [RB-2H]2–, has a much longer lifetime. Superimposed on S1 decay attributable mainly to intersystem crossing, all four isolated anions also show some rapid oscillatory features of the transient photoelectron signal on a 4–5 ps timescale after excitation. Interestingly, an analogous phenomenon is also seen in the transient absorption measurements. We attribute it to a librational oscillation as the S1 state, initially populated in the S0 geometry, relaxes into its excited state equilibrium structure. Some implications of these observations for RB photophysics and interpretation of solution measurements are discussedalso in terms of density functional theory and time-dependent density functional theory calculations of ground and excited states

Sequence-independent activation of photocycloadditions using two colours of light

We exploit two reactive chromophores to establish sequence-independent photochemical activation, employing ortho-methyl benzaldehyde (oMBA) and N,N-(dimethylamino)pyrene aryl tetrazole (APAT) with N-(2-hydroxy)ethyl maleimide (NHEM), without any additives. Critically, the order of the irradiation sequence is irrelevant, as the shorter wavelength does not activate the higher wavelength activated species. Therefore, full sequence-independent λ-orthogonality is achieved through differences in both the reaction quantum yields (Φr,oMBA and Φr,APAT) and wavelength-dependent reactivity profiles of the employed chromophore

Phase‐Dependent Long Persistent Phosphorescence in Coumarin‐Phosphine‐Based Coinage Metal Complexes

A coumarin functionalized aminodiphosphine has been introduced as a bidentate ligand in coinage metal chemistry. Mono-, di-, and trimetallic copper and silver complexes were synthesized with this ligand. The hybrid character of the ligand led to compounds with rich luminescence properties. These include coumarin-based blue fluorescence, observed as a sole emission in solution at room temperature, and green phosphorescence, which is efficient at low temperatures and dominates the spectra of the metal complexes. In the rigid environment of frozen solutions, the green phosphorescence shows an unusually long (for metal complexes) decay on the seconds timescale in high quantum yield. In addition, a red phosphorescence, which may be assigned to the triplet state localized in the phosphine-M$_{3}$Cl$_{2}$ (M=Cu, Ag), is observed for the trinuclear complexes at low temperature. Neither the second-long phosphorescence nor the red emission is observed for the coumarin ligand, thus they must be a result of the coordination to coinage metal clusters. The excited states in these compounds were also investigated by femtosecond transient absorption spectroscopy and quantum chemical calculations

Transient anisotropy in degenerate systems: a semi-classical approach

A semi-classical model for transient anisotropy in degenerate excited states is developed on the basis of a more general ansatz presented recently [3]. This is the first model that can treat both rotational dephasing and the dynamics in degenerate systems, which is a prerequisite for a comprehensive theory to describe gas phase anisotropy experiments in small, highly symmetric systems. In the present contribution, it is shown that this model covers most of the features of the full quantum dynamical treatment and helps to give insights into the physical processes that are underlying these dynamics. \ua9 by Oldenbourg Wissenschaftsverlag, M\ufcnchen.Peer reviewed: YesNRC publication: Ye