Real-time structural dynamics of the ultrafast solvation process around photo-excited aqueous halides.

This work investigates and describes the structural dynamics taking place following charge-transfer-to-solvent photo-abstraction of electrons from I- and Br- ions in aqueous solution following single- and 2-photon excitation at 202 nm and 400 nm, respectively. A Time-Resolved X-ray Solution Scattering (TR-XSS) approach with direct sensitivity to the structure of the surrounding solvent as the water molecules adopt a new equilibrium configuration following the electron-abstraction process is utilized to investigate the structural dynamics of solvent shell expansion and restructuring in real-time. The structural sensitivity of the scattering data enables a quantitative evaluation of competing models for the interaction between the nascent neutral species and surrounding water molecules. Taking the I0-O distance as the reaction coordinate, we find that the structural reorganization is delayed by 0.1 ps with respect to the photoexcitation and completes on a time scale of 0.5-1 ps. On longer time scales we determine from the evolution of the TR-XSS difference signal that I0: e- recombination takes place on two distinct time scales of ∼20 ps and 100 s of picoseconds. These dynamics are well captured by a simple model of diffusive evolution of the initial photo-abstracted electron population where the charge-transfer-to-solvent process gives rise to a broad distribution of electron ejection distances, a significant fraction of which are in the close vicinity of the nascent halogen atoms and recombine on short time scales.

Ultrafast Intersystem Crossing and Structural Dynamics of [Pt(ppy)(µ-tBu2pz)]2.

Intersystem crossing (ISC) rates of transition-metal complexes are determined by the complex interplay of a molecule's electronic and structural dynamics. To broaden our understanding of these key factors, we investigate the case of the prototypical d8-d8 dimetal complex [Pt(ppy)(µ-tBu2pz)]2 using broad-band transient absorption anisotropy in combination with ultrafast fluorescence up-conversion and ab initio calculations. We find that, upon excitation of the molecule's metal-metal-to-ligand charge-transfer transition, ISC occurs in hundreds of femtoseconds from the lowest excited singlet state S1 to the triplet state T2, from where the energy relaxes to the lowest energy triplet state T1. ISC to the T2 state, rather than T1, is further rationalized through supporting arguments. Observed vibrational coherences along the Pt-Pt mode are attributed to the formation of nuclear wavepackets on the ground and excited electronic states that dephase prior to ISC because of the structural flexibility of the complex. Beyond demonstrating the relationship between the energy relaxation and structural dynamics of [Pt(ppy)(µ-tBu2pz)]2, our results provide new insights into the photoinduced dynamics of d8-d8 dimetal complexes more generally.

Ultrafast Photodynamics of an Azopyridine-Functionalized Iron(II) Complex: Implications for the Concept of Ligand-Driven Light-Induced Spin Change.

We report on the ultrafast photodynamics of an iron(II) complex with a photoisomerizable pentadentate azo-tetrapyridylamino ligand after irradiation with ultraviolet light. The results of femtosecond transient electronic absorption spectroscopy performed on the low-spin (LS) form of the title complex show that initial excitation of the ππ* state of the azopyridine unit in the ligand at λpump = 312 nm is followed by an ultrafast intersystem crossing (ISC) that leads to the formation of a metal-centered (MC) 5T state, in competition with the intended photoswitching of the azopyridine unit. Additional measurements carried out upon excitation of the singlet metal-to-ligand charge-transfer (1MLCT) transition at λpump = 455 nm suggest that this energy transfer occurs via an MLCT state. The resulting high-spin (HS) 5T state of the complex is metastable and recovers to the LS ground state with a time constant of ∼3 ns. The implications of these observations on the ligand-driven light-induced spin change concept are discussed.

Electronic Relaxation Dynamics of UV-Photoexcited 2-Aminopurine-Thymine Base Pairs in Watson-Crick and Hoogsteen Conformations.

The fluorescent analogue 2-aminopurine (2AP) of the canonical nucleobase adenine (6-aminopurine) base-pairs with thymine (T) without disrupting the helical structure of DNA. It therefore finds frequent use in molecular biology for probing DNA and RNA structures and conformational dynamics. However, detailed understanding of the processes responsible for fluorescence quenching remains largely elusive on a fundamental level. Although attempts have been made to ascribe decreased excited-state lifetimes to intrastrand charge-transfer and stacking interactions, possible influences from dynamic interstrand H-bonding have been widely ignored. Here, we investigate the electronic relaxation of UV-excited 2AP·T in Watson-Crick (WC) and Hoogsteen (HS) conformations. Although the WC conformation features slowed-down, monomer-like electronic relaxation in τ ∼ 1.6 ns toward ground-state recovery and triplet formation, the dynamics associated with 2AP·T in the HS motif exhibit faster deactivation in τ ∼ 70 ps. As recent research has revealed abundant transient interstrand H-bonding in the Hoogsteen motif for duplex DNA, the established model for dynamic fluorescence quenching may need to be revised in the light of our results. The underlying supramolecular photophysical mechanisms are discussed in terms of a proposed excited-state double-proton transfer as an efficient deactivation channel for recovery of the HS species in the electronic ground state.

Ultrafast excitation energy transfer in a benzimidazole-naphthopyran donor-acceptor dyad.

The excited-state dynamics of a donor-acceptor dyad composed of 1-propyl-2-pyridinyl-benzimidazole (PPBI) as donor and the photochromic molecular switch diphenylnaphthopyran (DPNP) as acceptor linked via an ester bridge has been investigated by a combination of static and time-resolved spectroscopies and quantum chemical calculations. The UV absorption spectrum of the dyad is virtually identical to the sum of the spectra of its individual constituents, indicating only weak electronic coupling between the donor and acceptor in the electronic ground state. After selective photoexcitation of the PPBI chromophore in the dyad at λpump = 310 nm, however, a fast electronic energy transfer (EET) from the donor to the acceptor is observed, by which the lifetime of the normally long-lived excited state of PPBI is reduced to a few ps. Enabled by the EET, the acceptor switches from its ring-closed naphtopyran form to its ring-opened merocyanine form. The singular value decomposition-based global analyses of the measured femtosecond time-resolved transient absorption spectra of the dyad and its two building blocks as reference compounds allowed us to determine a value for the EET time constant in the dyad of τ = 2.90 ± 0.60 ps. For comparison, Förster theory predicts characteristic FRET times between 1.2 ps ≤ τ ≤ 4.2 ps, in good agreement with the experimental result.

Efficient intersystem crossing in 2-aminopurine riboside probed by femtosecond time-resolved transient vibrational absorption spectroscopy.

The photophysical dynamics of 2-aminopurine riboside (2APr) in CHCl3 have been studied following excitation at λpump = 310 nm by means of femtosecond transient vibrational absorption spectroscopy (TVAS) aided by quantum chemical density functional theory (DFT) and ab initio calculations. The experiments identified numerous vibrational marker bands in the regions of the NH2 stretch and the 2AP ring vibrations which could be assigned to the bleach of the S0 electronic ground state (GS) and to transient populations in the 1ππ* and 3ππ* excited electronic states. The temporal evolution of the transient vibrational bands shows that the decay of the 1ππ* population is accompanied by a partial recovery of the GS and a concurrent population of the 3ππ* state with a time constant of τ2 = 740 ± 15 ps. The ensuing electronic relaxation is concluded to proceed via the 1nπ* state as intermediate state. The absence of observable transient vibrational bands of this state hints at an upper limit for its lifetime of τ < 100 ps. The triplet quantum yield is found to be φT = 0.42 ± 0.07.

Resonance dynamics of DCO (X̃ 2A') simulated with the dynamically pruned discrete variable representation (DP-DVR).

Selected resonance states of the deuterated formyl radical in the electronic ground state X̃ 2A' are computed using our recently introduced dynamically pruned discrete variable representation [H. R. Larsson, B. Hartke, and D. J. Tannor, J. Chem. Phys. 145, 204108 (2016)]. Their decay and asymptotic distributions are analyzed and, for selected resonances, compared to experimental results obtained by a combination of stimulated emission pumping and velocity-map imaging of the product D atoms. The theoretical results show good agreement with the experimental kinetic energy distributions. The intramolecular vibrational energy redistribution is analyzed and compared with previous results from an effective polyad Hamiltonian. Specifically, we analyzed the part of the wavefunction that remains in the interaction region during the decay. The results from the polyad Hamiltonian could mainly be confirmed. The C=O stretch quantum number is typically conserved, while the D-C=O bend quantum number decreases. Differences are due to strong anharmonic coupling such that all resonances have major contributions from several zero-order states. For some of the resonances, the coupling is so strong that no further zero-order states appear during the dynamics in the interaction region, even after propagating for 300 ps.

Ultrafast dynamics of the ESIPT photoswitch N-(3-pyridinyl)-2-pyridinecarboxamide.

Molecular switches based on proton transfer that are photochromic and can be interconverted by light at different wavelengths back and forth between two thermodynamically stable tautomeric states in solution at room temperature are rare to date. We report on a study of the ultrafast conversion of the bistable proton transfer switch N-(3-pyridinyl)-2-pyridinecarboxamide (NPPCA) to a corresponding iminol after photoexcitation at λpump ≈ 265 nm by means of femtosecond time-resolved broad-band and single-colour transient electronic absorption spectroscopy (TEAS), transient fluorescence spectroscopy (TFLS), and transient vibrational absorption spectroscopy (TVAS) in acetonitrile solution. The interpretation of the data was accompanied by ab initio quantum chemical calculations of the excited electronic states and the vibrational frequencies of the reactant and product in their ground electronic state. The TEAS experiments provided four time constants, τ1 = 0.09 ± 0.01 ps, τ2 = 0.61 ± 0.01 ps, τ3 = 5.10 ± 0.80 ps, and τ4 = 20.0 ± 1.0 ps. The first two agree well with the measured TFLS lifetimes, τ1,TFL < 0.18 ps and τ2,TFL = 0.50 ± 0.01 ps. τ1 is related to the relaxation of the initially excited Franck-Condon (FC) state of the pyridinecarboxamide, followed by the excited-state intramolecular proton transfer (ESIPT) step to the neighbouring pyridine. The subsequent return of the molecules to the electronic ground state takes place within τ2, mediated by a conical intersection (CI) at a twisted configuration of the pyridinecarboxamide moiety. The main components in all TEAS time profiles feature a rise with τ2 and a decay with τ4 and describe subsequent molecular transformations in the electronic ground state. τ3 is ascribed to vibrational cooling of the molecules. The final iminol exhibits a permanent UV absorption at λ = 247 nm, where its absorbance is stronger than that of the carboxamide reactant. The iminol structure is unambiguously identified by the TVA spectra, which show the build-up of corresponding vibrational bands with τ4,TVA = 23 ± 2 ps after the initial bleach of the reactant vibrational bands, in excellent agreement with the TEAS data. Its lifetime is >10 ns.

Ultrafast dynamics of UV-excited trans- and cis-ferulic acid in aqueous solutions.

The ultrafast UV-induced processes of the neutral, anionic and dianionic forms of trans- and cis-ferulic acid (FA) in aqueous solution were studied by static and femtosecond time-resolved emission and absorption spectroscopy combined with quantum chemical calculations. In all cases, initial excitation populates the first 1ππ* state. For the dianionic cis-isomer cFA2-, electronic deactivation takes place with a time constant of only 1.4 ps, whereas in all other cases, excited-state deactivation happens more than ten times slower, on a time scale of ≈20 ps. The data suggest sequential de-excitation pathways, where initial sub-picosecond solvent rearrangement and structural changes are followed by internal conversion to an intermediate excited electronic state from which deactivation to the ground state proceeds. Considering the time scales, barrierless excited-state pathways are suggested only in the case of cFA2-, where the observed formation of the isomerisation photoproduct tFA2- provides clear evidence for a cis ⇄ trans isomerisation coordinate. In the other cases, pathways with an excited-state energy barrier, presumably along the same coordinate, are likely, given the longer excited-state lifetimes.

Ultrafast Electronic Deactivation Dynamics of Xanthosine Monophosphate.

Ultrafast energy dissipation is a crucial factor for the photostability of DNA and RNA, but even some of the key electronic deactivation pathways in monomeric nucleic acid building stones are still controversial. Here, we report on the excited-state dynamics of the rare nucleotide xanthosine monophosphate as a function of deprotonation state (XMP vs. XMP - ) and excitation wavelength ( λ pump = 278-243 nm) by femtosecond time-resolved fluorescence and absorption spectroscopy. We show that the predominating relaxation channel leads to a return of the photo-excited molecules to the electronic ground state in τ∼1 ps. The mechanism likely involves an out-of-plane deformation of the five-membered ring, different from the main electronic deactivation pathways in the canonical purine bases adenine and guanine. The results are discussed in terms of the structural and electronic differences of XMP compared to the canonical nucleotides.

Probing the excited state relaxation dynamics of pyrimidine nucleosides in chloroform solution.

Ultrafast transient electronic and vibrational absorption spectroscopy (TEAS and TVAS) of 2'-deoxy-cytidine (dC) and 2'-deoxy-thymidine (dT) dissolved in chloroform examines their excited-state dynamics and the recovery of ground electronic state molecules following absorption of ultraviolet light. The chloroform serves as a weakly interacting solvent, allowing comparisons to be drawn with prior experimental studies of the photodynamics of these nucleosides in the gas phase and in polar solvents such as water. The pyrimidine base nucleosides have some propensity to dimerize in aprotic solvents, but the monomer photochemistry can be resolved clearly and is the focus of this study. UV absorption at a wavelength of 260 nm excites a 1ππ* ← S0 transition, but prompt crossing of a significant fraction (50% in dC, 17% in dT) of the 1ππ* population into a nearby 1nπ* state is too fast for the experiments to resolve. The remaining flux on the 1ππ* state leaves the vertical Franck-Condon region and encounters a conical intersection with the ground electronic state of ethylenic twist character. In dC, the 1ππ* state decays to the ground state with a time constant of 1.1 ± 0.1 ps. The lifetime of the 1nπ* state is much longer in the canonical forms of both molecules: recovery of the ground state population from these states occurs with time constants of 18.6 ± 1.1 ps in amino-oxo dC and ∼114 ps in dT, indicating potential energy barriers to the 1nπ*/S0 conical intersections. The small fraction of the imino-oxo tautomer of dC present in solution has a longer-lived 1nπ* state with a lifetime for ground state recovery of 193 ± 55 ps. No evidence is found for photo-induced tautomerization of amino-oxo dC to the imino-oxo form, or for population of low lying triplet states of this nucleoside. In contrast, ∼8% of the UV-excited dT molecules access the long-lived T1 (3ππ*) state through the 1nπ* state. The primary influence of the solvent appears to be the degree to which it destabilizes the states of 1nπ* character, with consequences for the lifetimes of these states as well as the triplet state yields.

Switching of an Azobenzene-Tripod Molecule on Ag(111).

The trans-cis isomerization makes azobenzene (AB) a robust molecular switch. Once adsorbed to a metal, however, the switching is inefficient or absent due to rapid excited-state quenching or loss of the trans-cis bistability. We find that tris-[4-(phenylazo)-phenyl]-amine is a rather efficient switch on Ag(111). Using scanning tunneling and atomic force microscopy at submolecular resolution along with density functional theory calculations, we show that the switching process is no trans-cis isomerization but rather a reorientation of the N-N bond of an AB unit. It proceeds through a twisting motion of the azo-bridge that leads to a lateral shift of a phenyl ring. Thus, the role of the Ag substrate is ambivalent. While it suppresses the original bistability of the azobenzene units, it creates a new one by inducing a barrier for the rotation of the N-N bond.

Femtosecond Time-Resolved Dynamics of trans-Azobenzene on Gold Nanoparticles.

We report a first femtosecond time-resolved transient absorption study of the photoinduced ultrafast dynamics of trans-azobenzene (AB) on gold nanoparticles (AuNPs). The observed changes in optical density following excitation at λ = 357 nm were analyzed by using temperature-dependent Mie theory and by Lorentzian band fitting to disentangle the ultrafast relaxation of the local surface plasmon resonance (LSPR) excitation of the Au core and the electronic deactivation of the attached AB ligands. The analysis of the dynamics associated with the AB photochrome yielded lifetime constants of τ1 = 1.2 ± 0.2 ps and τ2 = 4.7 ± 1.1 ps. Both values together indicate surprisingly little difference in the dynamics of the AB ligand on the AuNPs vs in solution. Our results thus highlight the extraordinarily efficient electronic decoupling of the azo chromophore and the Au core by the alkyl linker chain.

Ultraviolet Absorption Induces Hydrogen-Atom Transfer in G⋠C Watson-Crick DNA Base Pairs in Solution.

Ultrafast deactivation pathways bestow photostability on nucleobases and hence preserve the structural integrity of DNA following absorption of ultraviolet (UV) radiation. One controversial recovery mechanism proposed to account for this photostability involves electron-driven proton transfer (EDPT) in Watson-Crick base pairs. The first direct observation is reported of the EDPT process after UV excitation of individual guanine-cytosine (G⋅C) Watson-Crick base pairs by ultrafast time-resolved UV/visible and mid-infrared spectroscopy. The formation of an intermediate biradical species (G[-H]⋅C[+H]) with a lifetime of 2.9 ps was tracked. The majority of these biradicals return to the original G⋅C Watson-Crick pairs, but up to 10% of the initially excited molecules instead form a stable photoproduct G*⋅C* that has undergone double hydrogen-atom transfer. The observation of these sequential EDPT mechanisms across intermolecular hydrogen bonds confirms an important and long debated pathway for the deactivation of photoexcited base pairs, with possible implications for the UV photochemistry of DNA.

Ultrafast Z → E photoisomerisation of structurally modified furylfulgides.

Femtosecond broadband transient absorption spectroscopy has been used in a comparative study of the ultrafast photo-induced Z → E isomerisation reactions of four photochromic furylfulgides with selected structural motifs in n-hexane as solvent. The results show that all studied Z-fulgides exhibit fast and direct processes along barrierless excited-state pathways involving a conical intersection (CI) between the S1 and S0 electronic states. The excited-state lifetimes range from τ1 = 0.18 ps for the methyl derivative to τ1 = 0.32 ps for the benzofurylfulgide. The impulsive rise of the absorption by vibrationally hot Z- and E-isomers back in the electronic ground state following electronic deactivation and isomerisation indicates that the initially prepared wave packet persists even after passage of the CI. Furthermore, the results provide qualitative evidence for a quickly dephasing vibrational coherence in the electronic ground state. In contrast to the significant changes observed for the corresponding E- and C-isomers [Renth et al., Int. Rev. Phys. Chem., 2013, 32, 1-38], the excited-state dynamics of the Z-isomers is not affected by varied sterical hindrance from methyl and isopropyl substituents at the central hexatriene unit, or by intramolecular bridging, and remains unaltered upon extension of the π-electron system in a benzannulated furyl fulgide.

Photoisomerisation and ligand-controlled reversible aggregation of azobenzene-functionalised gold nanoparticles.

The photochemical behaviour of functionalised gold nanoparticles (AuNPs) carrying azobenzenethiolate-alkylthiolate monolayers was investigated. Repeated trans-cis and cis-trans isomerisation cycles could be performed in all cases with high efficiency. Reversible photoinduced aggregation was observed when azothiolates with long alkyl spacers (≥C7) were combined with short (C5) alkylthiolate coligands. The choice of a coligand thus offers control over the aggregation properties of the nanoparticles.

Femtosecond spectroscopy reveals huge differences in the photoisomerisation dynamics between azobenzenes linked to polymers and azobenzenes in solution.

Femtosecond fluorescence up-conversion spectroscopy of two azobenzenes covalently attached to the side chain or linked by covalent bonds at each end into the main chain of polybutylmethacrylate polymer colloids with different cross-linking ratios reveals dramatic differences in the excited-state dynamics compared to the monomer chromophores in solution due to strong mechanical forces in the complex micronetworks. For the azobenzene derivative DR1 in the polymer side chain, the measurements determined an increase of the mean excited-state lifetime after irradiation at λ = 475 nm to 〈τ〉 = 5.5 ps from 〈τ〉 = 0.5 ps for the monomer. For the cross-linked BAAB in the polymer main chain, an increase of 〈τ〉 was found of more than a factor-of-20. Moreover, with a lifetime of τ = 430 ps, ≈12% of the molecules in the tightly (1 : 10) cross-linked polymer were found to remain in the excited state about 100 times longer than observed for the monomer chromophore. These results are of high relevance for applications of photoswitchable polymer materials.

Characterization of the covalent binding of allyl isothiocyanate to ß-lactoglobulin by fluorescence quenching, equilibrium measurement, and mass spectrometry.

Reversible binding of small compounds through hydrophobic interactions or hydrogen bonding to food proteins (e.g. milk proteins) is a thoroughly researched topic. In contrast, covalent interactions are not well characterized. Here, we report a rare form of positive-cooperativity-linear binding of allyl isothiocyanate with ß-lactoglobulin, resulting in the cleavage of a disulfide bond of the protein. We compared three methods (i.e. fluorescence quenching, equilibrium dialysis, and headspace-water equilibrium) to characterize the binding kinetics and investigated the molecular binding by mass spectrometry. The methodologies used were found to be comparable and reproducible in the presence of high and low ligand concentrations for fluorescence quenching and equilibrium-based methods respectively.

Ultrafast photo-initiated molecular quantum dynamics in the DNA dinucleotide d(ApG) revealed by broadband transient absorption spectroscopy.

The ultrafast photo-initiated quantum dynamics of the adenine-guanine dinucleotide d(ApG) in aqueous solution (pH 7) has been studied by femtosecond time-resolved spectroscopy after excitation at lambda = 260 nm. The results reveal a hierarchy of processes on time scales from tau < 100 fs to tau > 100 ps. Characteristic spectro-temporal signatures are observed indicating the transformation of the molecules in the electronic relaxation from the photo-excited state to a long-lived exciplex. In particular, broadband UV/VIS excited-state absorption (ESA) measurements detected a distinctive absorption by the excited dinucleotide around lambda = 335 nm, approximately 0.5 eV to the blue compared to the maximum of the broad and unstructured ESA spectrum after excitation of an equimolar mixture of the mononucleotides dAMP and dGMP. A similar feature has been identified as signature of the excimer in the dynamics of the adenine dinucleotide d(ApA). The lifetime of the d(ApG) exciplex was found to be tau = 124 +/- 4 ps both from the ESA decay time and from the ground-state recovery time, far longer than the sub-picosecond lifetimes of excited dAMP or dGMP. Fluorescence-time profiles measured by the up-conversion technique indicate that the exciplex state is reached around approximately 6 ps after excitation. Very weak residual fluorescence at longer times red-shifted to the emission from the photo-excited state shows that the exciplex is almost optically dark, but still has enough oscillator strength to give rise to the dual fluorescence of the dinucleotide in the static fluorescence spectrum.

Broken symmetry of an adsorbed molecular switch determined by scanning tunneling spectroscopy.

An asymmetric turn: Scanning tunneling spectroscopy has been used to analyze the structure of tris[4-(phenylazo)phenyl)]amine on a Au(111) surface. A degenerate marker state serves as a sensitive probe for the structure of the adsorbed molecules.