Iodobenzene Molecules Research Articles

Basic studies toward ultrafast soft x-ray photoelectron diffraction; its application to probing local structure in iodobenzene molecules.

Ultrafast x-ray photoelectron diffraction (UXPD) for free molecules has a promising potential to probe the local structures of the molecules in an element-specific fashion. Our UXPD scheme consists of three steps: (1) near-infrared laser (NIR) with ns pulse duration aligns sample molecules, (2) ultra-violet laser with fs pulse duration pumps the aligned molecules, and (3) soft x-ray free-electron laser (SXFEL) with fs pulse duration probes the molecules by measuring x-ray photoelectron diffraction (XPD) profiles. Employing steps of (1) and (3), we have measured I 3d XPD profiles from ground state iodobenzene aligned by the NIR laser with the SXFEL. Then, we have intensively calculated I 3d XPD profiles with density functional theory, taking degrees of alignments of the molecules into account, to extract a distance between C and I atoms in iodobenzene from the experimental I 3d XPD profiles. Although we have failed to determine the distance from the comparison between the experimental and theoretical results, we have succeeded in concluding that the degeneracies of the initial state eliminate the sensitivity on molecular structure in the I 3d XPD profiles. Thus, the observation of fine structures in the XPD profiles could be expected, if a nondegenerate molecular orbital is selected for a probe of UXPD. Finally, we have summarized our criteria to perform UXPD successfully: (1) to use SXFEL, (2) to prepare sample molecules with the degree of alignment higher than 0.8, and (3) to select a photoemission process from a nondegenerate inner-shell orbital of sample molecules.

Theoretical evidence for the sensitivity of charge-rearrangement-enhanced x-ray ionization to molecular size

It was recently discovered that molecular ionization at high x-ray intensity is enhanced, in comparison with that of isolated atoms, through a phenomenon called CREXIM (charge-rearrangement-enhanced x-ray ionization of molecules). X-ray absorption selectively ionizes heavy atoms within molecules, triggering electron transfer from neighboring atoms to the heavy atom sites and enabling further ionization there. The present theoretical study demonstrates that the CREXIM effect increases with the size of the molecule, as a consequence of increased intramolecular electron transfer from the larger molecular constituents attached to the heavy atoms. We compare x-ray multiphoton ionization dynamics of xenon, iodomethane, and iodobenzene after interacting with an intense x-ray pulse. Although their photoionization cross sections are similar, iodomethane and iodobenzene molecules are more ionized than xenon atoms. Moreover, we predict that the average total charge of iodobenzene is much larger than that of iodomethane, because of the large number of electrons in the benzene ring. The positive charges transferred from the iodine site to the benzene ring are redistributed such that the higher carbon charges are formed at the far end from the iodine site. Our first-principles calculations provide fundamental insights into the interaction of molecules with x-ray free-electron laser (XFEL) pulses. These insights need to be taken into account for interpreting and designing future XFEL experiments.

Ultrafast Photodissociation Dynamics of Highly Excited Iodobenzene on the C Band.

The photodissociation dynamics of highly excited iodobenzene from the C band absorption has been studied by femtosecond time-resolved ion yields techniques. Detailed photodissociation routes are discussed with the aid of high-level, spin-orbit resolved ab initio calculations of 1D potential energy curves. Upon 200 nm excitation within the C band, iodobenzene molecules on 7B2 and 7B1 states decay to 7A1 and 8B2 states through internal conversion in 75 fs, with electronic energy converted into high vibrational energy of 7A1 and 8B2 states. Subsequently, 7A1 and 8B2 states decay through internal vibrational energy redistribution in 540 fs, accompanied by the excited C-I mode and the resulting cleavage of the C-I bond. The overall time for the reaction starting from the phenyl-type modes and ending in final C-I fragmentation for I(2P3/2) production is 1.2 ps.

Adsorption of polyiodobenzene molecules on the Pt(111) surface using van der Waals density functional theory

Adsorption of aromatic molecules on surfaces is widely studied due to applications in molecular electronics. In this work, the adsorption of iodobenzene molecules on the Pt(111) surface has been studied using density functional theory. Iodobenzene molecules, with various numbers of iodine atoms, have two non-dissociative adsorption minima. One structure exhibits chemisorption between the ring and the surface (short-range) and the other structure exhibits chemisorption between the iodine ions and the surface (long-range). Both structures have a strong van der Waals interaction with the surface. In general, the adsorption energy increases as the number of iodine atoms increases. The dissociated structure of monoiodobenzene was investigated, and the dissociation barrier and the barrier between the short- and long-range states were compared.

Laser-field-free orientation of state-selected asymmetric top molecules

With combined electrostatic and shaped laser fields with a slow turn on and rapid turn off, laser-field-free orientation of asymmetric top iodobenzene molecules with higher degrees of orientation has been achieved. In order to further increase the degrees of orientation, state-selected molecules are used as a sample. It is confirmed that higher degrees of orientation are maintained in the laser-field-free condition for 5--10 ps, which is long enough to study femtosecond-attosecond dynamics in molecules, after the rapid turn off of the laser pulse. The observation of the slow dephasing time of 5--10 ps ensures future prospects in molecular orientation techniques. This accomplishment means not only that a unique molecular sample has become available in various applications but also that the present technique can be used as a spectroscopic technique to investigate ultrafast rotational dynamics of molecules.

Temperature effect-driven assembly of [CunXn]-based coordination polymers from CuX and 1,4-bis(imidazol-1-yl)butane

Three [CunXn]-based (X=Br, I) coordination polymers, {[Cu2(μ-I)2(bimb)2]·iodobenzene}n (1), {[Cu4(μ3-I)4(bimb)2][Cu2(μ-I)2(bimb)]}n (2) and {[Cu2Br(bimb)2]Br}n (3) were prepared from solvothermal reactions of CuX (X=I, Br) with 1,4-bis(imidazol-1-yl)butane (bimb) at different temperatures. These compounds were characterized by elemental analysis, IR, and single crystal X-ray diffraction. Compound 1 contains a rhombic [Cu2(μ-I)2] dimer that acts as a planar four-connected node to link four equivalent ones via bimb bridges to form a 2D (4,4) network. Each layer is further stacked by π–π interactions to form a 3D structure with 1D rhombic channels occupied by the guest iodobenzene molecules. Compound 2 consists of [Cu2(μ-I)2] dimers and [Cu4(μ3-I)4] tetramers which are interconnected by bimb bridges to generate another 2D network. Compound 3 contains [Cu2Br(bimb)2]+ cations that are linked by bimb ligands to form a 1D cationic chain. We also investigated the solid state photoluminescent properties of 1–3 at ambient temperature. The results provide an interesting insight into how reaction temperatures exert a great effect on the assembly of [CunXn]-based coordination polymers.

An experimental and molecular-statistical study of the adsorption of the iodobenzene, 2-iodothiophene, and isomeric iodoadamantane molecules on the graphite basal face surface

The thermodynamic characteristics of adsorption of iodobenzene, 2-iodothiophene, and 1- and 2-iodoadamantanes on the surface of graphitized thermal carbon black were determined experimentally. The influence of the special features of the molecular structure of the adsorbates on the thermodynamic characteristics of adsorption was studied. The atom-atom approximation of the semiempirical molecular-statistical theory of adsorption was used to calculate the thermodynamic characteristics of adsorption of the adsorbates using the newly determined potential function parameters of pair intermolecular interaction (φ(r)) of I with C atoms of the basal graphite face. For the example of isostructural monohalogenated benzenes, thiophenes, and adamantanes, a comparative analysis of the contributions of the F, Cl, Br, and I atoms to the thermodynamic characteristics of adsorption was performed for the nonspecific adsorption of these compounds on a plane graphitized carbon black surface.

Nσ* and πσ* excited states in aryl halide photochemistry: a comprehensive study of the UV photodissociation dynamics of iodobenzene

A recent review (Ashfold et al., Phys. Chem. Chem. Phys., 2010, 12, 1218) highlighted the important role of dissociative excited states formed by electron promotion to σ* orbitals in establishing the photochemistry of many molecular hydrides. Here we extend such considerations to molecular halides, with a particular focus on iodobenzene. Two experimental techniques (velocity mapped ion imaging (VMI) and time resolved infrared (IR) diode laser absorption) and electronic structure calculations have been employed in a comprehensive study of the near ultraviolet (UV) photodissociation of gas phase iodobenzene molecules. The VMI studies yield the speeds and angular distributions of the I((2)P(3/2)) and I*((2)P(1/2)) photofragments formed by photolysis in the wavelength range 330 ≥λ≥ 206 nm. Four distinct dissociation channels are observed for the I((2)P(3/2)) atom products, and a further three channels for the I*((2)P(1/2)) fragments. The phenyl (Ph) radical partners formed via one particular I* product channel following excitation at wavelengths 305 ≥λ≥ 250 nm are distributed over a sufficiently select sub-set of vibrational (v) states that the images allow resolution of specific I* + Ph(v) channels, identification of the active product mode (ν(10), an in-plane ring breathing mode), and a refined determination of D(0)(Ph-I) = 23,390 ± 50 cm(-1). The time-resolved IR absorption studies allow determination of the spin-orbit branching ratio in the iodine atom products formed at λ = 248 nm (ϕ(I*) = [I*]/([I] + [I*]) = 0.28 ± 0.04) and at 266 nm (ϕ(I*) = 0.32 ± 0.05). The complementary high-level, spin-orbit resolved ab initio calculations of sections (along the C-I bond coordinate) through the ground and first 19 excited state potential energy surfaces (PESs) reveal numerous excited states in the energy range of current interest. Except at the very shortest wavelength, however, all of the observed I and I* products display limiting or near limiting parallel recoil anisotropy. This encourages discussion of the fragmentation dynamics in terms of excitation to states of A(1) total symmetry and dissociation on the 2A(1) and 4A(1) (σ* ← n/π) PESs to yield, respectively, I and I* products, or via non-adiabatic coupling to other σ* ← n/π PESs that correlate to these respective limits. Similarities (and differences) with the available UV photochemical data for the other aryl halides, and with the simpler (and more thoroughly studied) iodides HI and CH(3)I, are summarised.

Quantum-state selection, alignment, and orientation of large molecules using static electric and laser fields

Supersonic beams of polar molecules are deflected using inhomogeneous electric fields. The quantum-state selectivity of the deflection is used to spatially separate molecules according to their quantum state. A detailed analysis of the deflection and the obtained quantum-state selection is presented. The rotational temperatures of the molecular beams are determined from the spatial beam profiles and are all approximately 1 K. Unprecedented degrees of laser-induced alignment (<cos2 theta2D>=0.972) and orientation of iodobenzene molecules are demonstrated when the state-selected samples are used. Such state-selected and oriented molecules provide unique possibilities for many novel experiments in chemistry and physics.

Laser-Induced Alignment and Orientation of Quantum-State-Selected Large Molecules

A strong inhomogeneous static electric field is used to spatially disperse a supersonic beam of polar molecules, according to their quantum state. We show that the molecules residing in the lowest-lying rotational states can be selected and used as targets for further experiments. As an illustration, we demonstrate an unprecedented degree of laser-induced one-dimensional alignment (cos;(2)theta_(2D)=0.97) and strong orientation of state-selected iodobenzene molecules. This method should enable experiments on pure samples of polar molecules in their rotational ground state, offering new opportunities in molecular science.

Surface Chemical Reaction Enhanced by Adsorbate-Electron Interaction

We have observed two type of bonding configuration of Iodobenzene (C6H5I) molecule upon room temperature adsorption on the Si(111)-7×7 surface and investigated an effect of Adsorbate-Electron interaction on a bonding configuration change of the Iodobenzene molecules on the Si(111)-7×7 surface using scanning tunneling microscope (STM). With the direct-current heating, a conversion rate of the bonding configuration as a function of the lattice temperature, K(Tl), has been determined. The plot of ln(K(T)) vs. T shows a deviation from a Arrhenius straight line, which suggests that the current accelerates the molecule's reaction by heating the adsorbate preferentially above the lattice temperature. Though tunneling current induced chemical reaction has been reported, we believe this is the first report of a direct observation that the interaction between an adsorbate and lattice current causes a chemical reaction.

Halogen Effect on the Photodissociation Mechanism for Gas‐Phase Bromobenzene and Iodobenzene

The velocity imaging technique combined with (2+1) resonance-enhanced multiphoton ionization (REMPI) is used to detect the halogen fragments in the photodissociation of bromobenzene and iodobenzene at 266 nm. With the aid of potential energy curve calculations by Lunell (Y. J. Liu, P. Persson, S. Lunell, J. Phys. Chem. A 2004, 108, 2339-2345.), the Br fragmentation is proposed to stem from excitation of the lowest excited singlet (pi-pi*) state followed by predissociation along a repulsive triplet (n-sigma*) state. The slowed dissociation rate leads to production of the isotropic Br fragments and 93 % internal energy deposition. Only the ground state Br((2)P(3/2)) is detectable. In contrast, when iodine is substituted, the iodine effect stabilizes the repulsive states associated with the I-C6H5 bond rupture and the subsequent dissociation channels become more complicated. 84 % of the iodobenzene molecules obtained follow a direct dissociation channel, while the remaining undergo a predissociative process. Both routes result in rapid dissociation with anisotropy parameters of 0.7+/-0.2 and 0.9+/-0.2 as well as 70 % and 26 % in the fractions of translational energy deposition, respectively. The relative quantum yields of I* and I are 0.35 and 0.65 and their related photodissociation pathways are discussed in detail.

Assembly of Linear Clusters of Iodobenzene Dimers on Cu(110)

Low-temperature STM observations of the low-coverage chemisorption behavior of iodobenzene on Cu(110) are presented at two annealing temperatures. When a random distribution of isolated iodobenzene molecules deposited at T < 100 K is annealed to approximately 170 K, the undissociated molecules assemble into chainlike clusters composed of identical subunits. An STM tip-induced dissociation reaction is used to determine that the individual units in the chains are composed of pairs of iodobenzene molecules. A model is proposed in which iodine atoms on each member of the pair are directed toward one another. The driving force for the formation of such clusters is suggested to be the dispersion interactions between the polarizable iodobenzene molecules.

Alignment enhancement by the combination of a short and a long laser pulse

The alignment dynamics of polyatomic molecules subject to two temporally overlapping nonresonant laser pulses is studied theoretically and experimentally. We examine the potential advantage of combining a long (compared to the molecular rotational periods) with a short pulse, where both laser fields are linearly polarized with common polarization direction. Experimentally, iodobenzene molecules are irradiated by a $2\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ pulse at $800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ synchronized to the peak of a $9\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ pulse at $1064\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The alignment dynamics following the short laser pulse is measured using time-resolved Coulomb explosion with a delayed $25\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$-long intense laser pulse. The alignment attained with the two-pulse combination is significantly enhanced compared to that attainable with either a short or a long pulse alone under nonionizing conditions. Numerically, we solve the time dependent Schr\odinger equation nonperturbatively for asymmetric top, symmetric top, and linear molecules subject to similar combinations of long and short excitation pulses. The alignment dynamics, and in particular the alignment enhancement, are explained and their generality is tested.

Nonadiabatic laser-induced alignment of iodobenzene molecules

Nonadiabatic alignment of an asymmetric top molecule induced by a short, moderately intense laser pulse is studied theoretically and experimentally. Numerically, we solve nonperturbatively the time-dependent Schr\odinger equation for a general asymmetric top molecule subject to a moderately intense laser field, and analyze the dependence of the alignment dynamics on the field strength and on the rotational temperature. Experimentally, we use time-resolved photofragment imaging to measure the time-dependent angular distributions of the spatial orientation of the molecules. Our studies, using iodobenzene as a test molecule, focus on the short-time alignment dynamics, during and after the pulse.

Observation of Enhanced Field-Free Molecular Alignment by Two Laser Pulses

We show experimentally that field-free alignment of iodobenzene molecules, induced by a single, intense, linearly polarized 1.4-ps-long laser pulse, can be strongly enhanced by dividing the pulse into two optimally synchronized pulses of the same duration. For a given total energy of the two-pulse sequence the degree of alignment is maximized with an intensity ratio of 1:3 and by sending the second pulse near the time where the alignment created by the first pulse peaks.

Selective bond breaking of single iodobenzene molecules with a scanning tunneling microscope tip

Controlled step-by-step dissociation of single iodobenzene molecules has been performed at Cu(1 1 1) step-edges using tunneling electrons from a scanning tunneling microscope (STM) tip at 12 K. We show that the threshold tunneling electron energies to break a single bond inside a polyatomic molecule can be determined by using I–V spectroscopy of single molecules. Electron energies of 1.5±0.1 eV are necessary to break the σ C–I bond. This selective bond breaking procedure is possible due to the inherent bond energy differences inside the molecule. It requires electron energies of 3.1±0.1 eV or higher to further disrupt the π ring.

Welding, Organizing, and Planting Organic Molecules on Substrate Surfaces—Promising Approaches towards Nanoarchitectonics from the Bottom up

The rapidly evolving field of nanotechnology[1] has so far primarily benefited from the development of instrumentation by the physical community, which enables visualization and manipulation of matter down to the atomic level.[2] These major advances have propelled us chemists into the unique position of gaining fundamental insights into the dynamics and reactivity of single molecules[3] and furthermore to exploit this knowledge for the bottom-up approach to nanofabrication, which promises to revolutionize future device miniaturization and electronics.[1,4] Our ability to custom-design and synthesize programmed molecular objects will be crucial to construct functional nanoscale architectures. These molecules will serve as the functional building blocks of molecular construction sets[5] and inherit the desired electronic or magnetic properties, adopt a well-defined shape, and incorporate specific handles such as reactive groups and recognition motifs. In order to assemble these individual building blocks into larger integrated systems, several issues will have to be addressed: 1) The surface should be locally activated, either by inducing a chemical process in individual molecules or by changing the substrate directly; 2) the molecules should preferably be organized on the substrate by self-assembly; 3) the generated features should be stabilized and permanently fixed to the substrate. Recent advances in these areas utilizing organic molecules will be highlighted here from a synthetic chemist©s point of view.[6] The concept of inducing a chemical reaction by using tunneling electrons of a scanning tunneling microscope (STM) tip was suggested by Ho in the late 1990s.[7] However, there are still few experiments which demonstrate singlemolecule reactivity in the sense of a classical bond-forming event.[8] In a key experiment, Rieder and co-workers recently carried out an TMUllmann reaction∫, which involved the homocoupling of two iodobenzene molecules to afford biphenyl, step by step at the step edge of a Cu(111) surface using a low-temperature, ultrahigh-vacuum STM (Figure 1).[9] First, selective breaking of the weakest bond (the C-I bond) was induced by applying the appropriate sample bias. Single tunneling electrons presumably cause C I bond dissociation and the resulting phenyl radical intermediate is stabilized by p interaction with the surface as well as s interaction

An in Situ Scanning Tunneling Microscopy and Cyclic Voltammetric Study of Iodobenzene and Iodoheptane Molecules Adsorbed on a Pt(111) Electrode

We employed in situ scanning tunneling microscopy (STM) and cyclic voltammetry to study the adsorption of iodobenzene and iodoheptane molecules onto a well-ordered Pt(111) electrode in 0. 1 M HClO 4 . These molecules were adsorbed intact without noticeable decomposition at potentials negative of 0.9 V (vs a reversible hydrogen electrode), whereas degradation became prominent at potentials positive of 0.9 V. The coverage of the iodobenzene monolayer is estimated to be about 0.45 from the amount of charges involved in its reductive desorption in KClO 4 (pH 10) solutions. This value nearly equals that of iodine atoms at Pt(111). In situ STM was used to probe the spatial arrangements of these adsorbates as a function of potential in 0.1 M HClO 4 . A well-packed (7 x 7)R19.1°-iodobenzene structure predominated at 0.3 V, which readily rearranged into a (3 x 3) adlattice as a result of a slight increase of coverage at more positive potentials. The STM appearances of these two adlattices resemble those of iodine atoms on Pt( 111), suggesting that the contrast in STM arises mainly from the iodine headgroups. More negative potentials resulted in desorption of iodobenzene molecules and a disordered adlayer. In contrast, iodoheptane molecules were disordered at potentials negative of 0.7 V, but reorganization occurred to produce local (7 x 7)R19.1° at more positive potentials. These phase transitions were reversible to the modulation of potential between 0 and 0.9 V. The electrochemical potential dominated not only the chemical nature but also the spatial arrangements of the alkyl or aryl iodide molecules. The tip-and-sample interaction was noticeably stronger for iodoheptane, resulting in disordering of the adlayer.

Photodissociation of laser aligned iodobenzene: Towards selective photoexcitation

Iodobenzene molecules, having their symmetry axis aligned to the polarization direction of a strong, linearly polarized nanosecond laser pulse, are photodissociated into phenyl and iodine radicals with a 1.5 ps long laser pulse at 266 nm. The yield of I photoproducts, detected by resonant multiphoton ionization, is enhanced up to a factor of 2.7 when the dissociation laser is polarized parallel instead of perpendicular to the alignment laser polarization. In particular, the high-velocity distribution of I products, corresponding to excitation of an (n,σ*) repulsive surface, is enhanced by a factor of 3.3, when comparing parallel and perpendicular polarizations, whereas the low-velocity distribution of I products, corresponding to excitation of predissociative (π,π*) surfaces is only enhanced by a factor of 2.2. The difference is explained by the different directions of the transition dipole moment for the two transitions. We discuss the perspectives for selective photoexcitation using three dimensionally aligned molecules.