Solar Gas-Phase CO2 Hydrogenation by Multifunctional UiO-66 Photocatalysts.

Solar-assisted CO2 conversion into fuels and chemical products involves a range of technologies aimed at driving industrial decarbonization methods. In this work, we report on the development of a series of multifunctional metal-organic frameworks (MOFs) based on nitro- or amino-functionalized UiO-66(M) (M: Zr or Zr/Ti) supported RuOx NPs as photocatalysts, having different energy band level diagrams, for CO2 hydrogenation under simulated concentrated sunlight irradiation. RuOx(1 wt %; 2.2 ± 0.9 nm)@UiO-66(Zr/Ti)-NO2 was found to be a reusable photocatalyst, to be selective for CO2 methanation (5.03 mmol g-1 after 22 h;, apparent quantum yield at 350, 400, and 600 nm of 1.67, 0.25, and 0.01%, respectively), and to show about 3-6 times activity compared with previous investigations. The photocatalysts were characterized by advanced spectroscopic techniques like femto- and nanosecond transient absorption, spin electron resonance, and photoluminescence spectroscopies together with (photo)electrochemical measurements. The photocatalytic CO2 methanation mechanism was assessed by operando FTIR spectroscopy. The results indicate that the most active photocatalyst operates under a dual photochemical and photothermal mechanism. This investigation shows the potential of multifunctional MOFs as photocatalysts for solar-driven CO2 recycling.

Photodynamics of azaindoles in polar media: the influence of the environment.

We have studied the relaxation dynamics of a family of azaindole (AI) structural isomers, 4-, 5-, 6- and 7-AI, by steady-state and time-resolved methods (fs-transient absorption and fluorescence up-conversion), in solvents of different polarity. The measurements in aprotic solvents show distinctive fluorescence yields and excited state lifetimes among the isomers, which are tuned by the polarity of the medium. Guided by simple TD-DFT calculations and based on the behavior observed in the isolated species, it has been possible to address the influence of the environment polarity on the relaxation route. According to the obtained picture, the energy of the nπ* state, which is strongly dependent on the position of the pyridinic nitrogen, controls the rate of the internal conversion channel that accounts for the distinctive photophysical behavior of the isomers. On the other hand, preliminary measurements in protic media (methanol) show a very different photodynamical behavior, in which the anomalous measured fluorescent patterns are very likely the result of reactive channels (proton transfer) triggered by the electronic excitation.

Influence of H-bonds on the photoionization of aromatic chromophores in water: The aniline molecule.

We have conducted time-resolved experiments (pump-probe and pump-repump-probe) on a model aromatic chromophore, aniline, after excitation in water at 267 nm. In the initial spectra recorded, in addition to the absorption corresponding to the bright ππ* excitation, the fingerprint of a transient state with the electron located on the solvent molecule is identified. We postulate that the latter corresponds to the πσ* state along the N-H bond, whose complete relaxation with a ∼500 ps lifetime results in the formation of the fully solvated electron and cation. This ionization process occurs in parallel with the ππ* photophysical channel that yields the characteristic ∼1 ns fluorescence lifetime. The observed branched pathway is rationalized in terms of the different H-bonds that the water establishes with the amino group. The proposed mechanism could be common for aromatics in water containing N-H or O-H bonds and would allow the formation of separated charges after excitation at the threshold of their electronic absorptions.

Halogen-free photosensitizers based on meso-enamine-BODIPYs for bioimaging and photodynamic therapy.

The search for efficient heavy atom free photosensitizers (PSs) for photodynamic therapy (PDT) is a very active field. We describe herein a simple and easily accessible molecular design based on the attachment of an enamine group as an electron-donor moiety at the meso position of the BODIPY core with different alkylation patterns. The effect of the alkylation degree and solvent polarity on the photophysical properties in terms of splitting absorption bands, fluorescence efficiencies and singlet oxygen production is analyzed in depth experimentally using spectroscopic techniques, including femtosecond and nanosecond transient absorption (fs- and ns-TA) and using computational simulations based on time-dependent density functional theory. The correlation between the theoretical/experimental results permits the rationalization of the observed photophysical behavior exhibited by meso-enamine-BODIPY compounds and the determination of mechanistic details, which rule the population of the triplet state manifold. The potential applicability as a theragnostic agent for the most promising compound is demonstrated through in vitro assays in HeLa cells by analyzing the internalization, localization and phototoxic action.

Revisiting the Spectroscopy of Water Dimer in Jets.

Laser spectroscopy in jets is one of the main sources of structural data from molecular aggregates. Consequently, numerous and sophisticated experimental systems have been developed to extract precise information, which is usually interpreted in the light of quantum mechanical calculations. However, even with the most sophisticated experiments, it is sometimes difficult to interpret the experimental results. We present here the example of water dimer and how after almost 70 years, the assignment of its mass-resolved IR spectrum still generates controversy that extends toward the mechanism of ionization of water aggregates.

An nπ* gated decay mediates excited-state lifetimes of isolated azaindoles.

Aiming to serve as a guide to understand the relaxation mechanisms of more complex aza-aromatic compounds, such as purine bases, we have studied the non-radiative channels of a set of azaindole structural isomers: 4-, 5-, 6- and 7-azaindole (AI). The relaxation of the isolated molecules, after excitation at the low energy portion of their spectra, has been tracked by femtosecond time-resolved ionization, and the decay paths have been obtained with MS-CASPT2//TD-DFT calculations. Although the ultrashort measured lifetimes for 5- and 6-AI are in contrast to the long-living excited state found in 7-AI, the calculations describe a common relaxation pathway. Along it, the initially excited ππ* states decay to the ground state through a conical intersection accessed through an nπ* state that functions as a gate state. The work reveals that the position of the nitrogen atoms in the purine ring determines the barrier to access the gate state and therefore, the rate of the non-radiative relaxation.

Dynamics of Pyrroles Excited to the 3s/πσ* State.

The available experimental data provided by ultrafast dynamics studies of pyrrole and its derivatives in excited states of mixed Rydberg/valence 3s/πσ* character are strongly affected by the interaction with the laser pulses. Understanding these data has constituted an endeavor for several groups during the past few years. Here we apply a simple theoretical model that, including the interaction with the laser pulses, allows one to clarify some aspects of the discrepancies between measurements monitoring different experimental observables. New experimental data on pyrrole, 2,4-dimethylpyrrole, and 2,5-dimethylpyrrole are also provided to check the validity of the model and to gain more insight into the excited state dynamics of pyrrole systems.

Methylthio BODIPY as a standard triplet photosensitizer for singlet oxygen production: a photophysical study.

A complete photophysical study on the iodinated-BODIPY, 3,5-dimethyl-2,6-diiodo-8-thiomethyl-pyrromethene (MeSBDP), demonstrated that it is an excellent triplet photosensitizer for singlet oxygen production in a broad range of apolar and polar solvents. Besides its absorption and fluorescence emission spectra, the dynamics of its excited states including its intersystem crossing rate was characterized by femtosecond transient experiments. The photophysical study of its triplet state by nanosecond transient absorption spectroscopy and phosphorescence emission concluded to a diffusion-controlled quenching of 3MeSBDP by O2 and to a fraction of triplet state quenching by O2 close to unity. The high (>0.87) and solvent-insensitive singlet oxygen quantum yield φΔ measured by singlet oxygen phosphorescence emission, together with the noticeable photostability of MeSBSP, as well as the absence of quenching of singlet oxygen by MeSBDP itself, allows claiming it as an alternative standard photosensitizer for singlet oxygen production, under excitation either in the UV or in the visible range.

Photophysical characterization of new and efficient synthetic sunscreens.

The photoprotective capabilities of a family of compounds have been investigated. Their relaxation mechanisms have been explored by fluorescence and transient absorption measurements, and the minimum energy relaxation pathways were modeled by CASSCF/CASPT2 methods. This study demonstrates their excellent properties as sunscreens, and provides novel mechanistic insights for the rational design of new species.

Excited state dynamics of aniline homoclusters.

We have investigated the relaxation, following excitation in the 290-235 nm region, of neutral aniline homoclusters (An)n formed in a supersonic expansion by femtosecond time resolved ionization. The applied method permits isolation of the dynamics of the dimer from that originated in bigger species of the generated distribution. Interestingly, and differently from the monomer and (An)n≥3 clusters, the dimer does not present a N-H dissociative 1πσ* channel. This fact can be explained in terms of the symmetric structure adopted, in which each molecule establishes two N-Hπ interactions, destabilizing the H dissociation channel. The observations permit relating the photophysics to the interactions established by the aniline units and confirming previous observations and theoretical predictions on the structure of aniline aggregates.

Singlet Fission Mediated Photophysics of BODIPY Dimers.

The photodynamics of an orthogonal BODIPY dimer, particularly the formation of triplet states, has been explored by femtosecond and nanosecond transient absorption measurements. The short time scale data show the appearance of transient features of triplet character that, according to quantitative analysis of their intensities, account for more than 100% of the initially excited molecules, which reveals the occurrence of a singlet fission process in the isolated dimers. The formation rate of the triplet correlated state 1(TT) is found to depend on the solvent polarity, pointing to the mediation of a charge transfer character state. The dissociation of the 1(TT) state into pairs of individual triplets determines the triplet yield measured in the long time scales. The kinetic model derived from the results provides a comprehensive view of the photodynamics of BODIPY dimers and permits rationalization of the photophysical parameters of these systems.

Femtosecond Excited State Dynamics of Size Selected Neutral Molecular Clusters.

The work describes a novel experimental approach to track the relaxation dynamics of an electronically excited distribution of neutral molecular clusters formed in a supersonic expansion, by pump-probe femtosecond ionization. The introduced method overcomes fragmentation issues and makes possible to retrieve the dynamical signature of a particular cluster from each mass channel, by associating it to an IR transition of the targeted structure. We have applied the technique to study the nonadiabatic relaxation of pyrrole homoclusters. The results obtained exciting at 243 nm, near the origin of the bare pyrrole electronic absorption, allow us to identify the dynamical signature of the dimer (Py)2, which exhibits a distinctive lifetime of τ1 ∼ 270 fs, considerably longer than the decays recorded for the monomer and bigger size clusters (Py)n>2. A possible relationship between the measured lifetime and the clusters geometries is tentatively discussed.

Triplet Mediated C-N Dissociation versus Internal Conversion in Electronically Excited N-Methylpyrrole.

The photochemical and photophysical pathways operative in N-methylpyrrole, after excitation in the near part of its ultraviolet absorption spectrum, have been investigated by the combination of time-resolved total ion yield and photoelectron spectroscopies with high-level ab initio calculations. The results collected are remarkably different from the observations made for pyrrole and other aromatic systems, whose dynamics is dictated by the presence of πσ* excitations on X-H (X: N, O, S, ...) bonds. The presence of a barrier along the C-N dissociation coordinate that can not be tunneled triggers two alternative decay mechanisms for the S1 A″ πσ* state. While at low vibrational content the C-N dissociation occurs on the surface of a lower (3)ππ* state reached after efficient intersystem crossing, at higher excitation energies, the A″ πσ* directly internally converts to the ground state through a ring-twisted S1/S0 conical intersection. The findings explain previous observations on the molecule and may be relevant for more complex systems containing similar C-N bonds, such as the DNA nucleotides.

Tracking the relaxation of 2,5-dimethylpyrrole by femtosecond time-resolved photoelectron and photoion detection.

The relaxation of 2,5-dimethylpyrrole after excitation in the 290-239 nm range, which covers the weak absorption of the S1 (1)A2 πσ* state, dissociative along the N-H bond, and the stronger band mostly attributed to the (1)B2 ππ* state, has been investigated by time-resolved ion and photoelectron techniques. The measurements yield an invariant lifetime of ∼55 fs for the (1)πσ* state, after preparation in its Franck-Condon region with increasing vibrational content. This ultrafast rate indicates that, contrary to the observations made in pyrrole (Roberts et al. Faraday Discuss. 2013, 163, 95-116), the molecule reaches the dissociative part of the potential without any barrier effect, although calculations predict the latter to be higher than in the pyrrole case. The results are rationalized in terms of a barrier free multidimensional pathway that very likely involves out-of-plane vibrations. Additionally, a lifetime of ∼100 fs is found after excitation along the higher (1)B2 ππ* ← S0 transition. The relaxation of this state by coupling to a very short living S1 (1)πσ* state, or by alternative routes, is discussed in the light of the collected photoelectron measurements.

Revisiting the relaxation dynamics of isolated pyrrole.

Herein, the interpretation of the femtosecond-scale temporal evolution of the pyrrole ion signal, after excitation in the 267-217 nm interval, recently published by our group [R. Montero, A. Peralta Conde, V. Ovejas, M. Fernández-Fernández, F. Castaño, J. R. Vázquez de Aldana, and A. Longarte, J. Chem. Phys. 137, 064317 (2012)] is re-visited. The observation of a shift in the pyrrole(+) transient respect to zero delay reference, initially attributed to ultrafast dynamics on the πσ* type state (3s a1 ← π 1a2), is demonstrated to be caused by the existence of pump + probe populated states, along the ionization process. The influence of these resonances in pump-prone ionization experiments, when multi-photon probes are used, and the significance of a proper zero-time reference, is discussed. The possibility of preparing the πσ* state by direct excitation is investigated by collecting 1 + 1 photoelectron spectra, at excitation wavelengths ranging from 255 to 219 nm. No conclusive evidences of ionization through this state are found.

IR mass-resolved spectroscopy of complexes without chromophore: cyclohexanol·(H2O)n, n = 1-3 and cyclohexanol dimer.

Mass-resolved IR spectra of cyclohexanol-water clusters and cyclohexanol dimer in supersonic expansions are presented for the first time. A combination of ns and fs IR lasers made possible recording such spectra without inclusion of a chromophore or a messenger atom. Furthermore, employment of the recently developed IR(3) technique [I. León, R. Montero, F. Castaño, A. Longarte, and J. A. Fernández, J. Phys. Chem. A 116, 6798 (2012)] allowed us to discriminate between the contribution of different species to the IR spectrum. Comparison of the experimental spectra with the predictions at the M06-2X/6-311++G(d,p) calculation level confirmed the assignment of the spectrum of cyclohexanol·(H2O)1 to a structure in which water is accepting a proton from cyclohexanol's OH group, and those of cyclohexanol·(H2O)(2,3) to structures with cyclic hydrogen bond networks. A comparative analysis of the results obtained with those reported on other aromatic alcohols is also offered.

Ultrafast Nonradiative Relaxation Channels of Tryptophan.

The nonradiative relaxation channels of gas-phase tryptophan excited along the S1-S4 excited states (287-217 nm) have been tracked by femtosecond time-resolved ionization. In the low-energy region, λ ≥ 240 nm, the measured transient signals reflect nonadiabatic interactions between the two bright La and Lb states of ππ* character and the dark dissociative πσ* state of the indole NH. The observed dynamical behavior is interpreted in terms of the ultrafast conversion of the prepared La state, which simultaneously populates the fluorescent Lb> and the dissociative πσ* states. At higher energies, after excitation of the S4 state, the tryptophan dynamics diverges from that observed in indole, pointing to the opening of a relaxation channel that could involve states of the amino acid part. The work provides a detailed picture of the processes and electronic states involved in the relaxation of the molecule, after photoexcitation in the near part of its UV absorption spectrum.

Ultrafast evolution of imidazole after electronic excitation.

The ultrafast dynamics of the imidazole chromophore has been tracked after electronic excitation in the 250-217 nm energy region, by time delayed ionization with 800 nm laser pulses. The time-dependent signals collected at the imidazole(+) mass channel show the signature of femtosecond dynamics, originating on the πσ*- and ππ*-type states located in the explored energy region. The fitting of the transients, which due to the appearance of nonresonant coherent adiabatic excitation requires a quantum treatment based in the Bloch equations, yields two lifetimes of 18 ± 4 and 19 ± 4 fs. The first is associated with the πσ* ← ππ* internal conversion, while the second reflects the loss of ionization cross-section as the system evolves along the dissociative πσ* surface. This study provides a comprehensive picture of the photophysics of the molecule that agrees with previous experimental and theoretical findings.

Femtosecond evolution of the pyrrole molecule excited in the near part of its UV spectrum.

The evolution of the isolated pyrrole molecule has been followed after excitation in the 265-217 nm range by using femtosecond time delayed ionization. The transients collected in the whole excitation range show the vanishing of the ionization signal in the femtosecond time scale, caused by the relaxation along a πσ(∗) type state (3s a(1)←π 1a(2)), which is the lowest excited electronic state of the molecule. This surface is dissociative along the NH bond, yielding a 15 ± 3 fs lifetime that reflects the loss of the ionization cross-section induced by the ultrafast wavepacket motion. Although a weak πσ(∗) absorption is detected, the state is mainly reached through internal conversion of the higher bright ππ(∗) transitions, which occurs with a 19 ± 3 fs lifetime. In addition to its resonant excitation, the intense ππ(∗) absorption extending in the 220-190 nm interval is also out-of-resonance populated at energies far to the red from its absorption onset. This coherent adiabatic excitation of the ππ(∗) transition should follow the excitation pulse (coherent population return effect), but instead the system relaxes toward the lower πσ(∗) surface through a conical intersection during the interaction time, leading to the population of πσ(∗) state at wavelengths as long as 265 nm. According to the observed behavior, the time evolution of the system in the full excitation range studied is modeled by a coherent treatment that provides key insights on the photophysical properties of the molecule.