Nonlinear Optical Absorption in Nanoscale Films Revealed through Ultrafast Acoustics.

Herein we describe a novel spinning pump-probe photoacoustic technique developed to study nonlinear absorption in thin films. As a test case, an organic polycrystalline thin film of quinacridone, a well-known pigment, with a thickness in the tens of nanometers range, is excited by a femtosecond laser pulse which generates a time-domain Brillouin scattering signal. This signal is directly related to the strain wave launched from the film into the substrate and can be used to quantitatively extract the nonlinear optical absorption properties of the film itself. Quinacridone exhibits both quadratic and cubic laser fluence dependence regimes which we show to correspond to two- and three-photon absorption processes. This technique can be broadly applied to materials that are difficult or impossible to characterize with conventional transmittance-based measurements including materials at the nanoscale, prone to laser damage, with very weak nonlinear properties, opaque, or highly scattering.

Temperature dependence of the cooperative out-of-equilibrium elastic switching in a spin-crossover material.

We present a study of a molecular material, [Feiii(3-MeO-SalEen)2]PF6, undergoing cooperative reversible photo-induced transition between low-spin state and high-spin state. By using temporally multiscale pump-probe laser spectroscopy, we explore the key parameters that influence the low-spin to high-spin conversion efficiency through long range elastic intermolecular interactions during the so-called elastic step, where crystalline volume expansion takes place. We rationalize our findings using Monte Carlo simulations, and a mechano-elastic model. The experimental results and the simulations support the existence of a fast mechanism by which molecules cooperatively switch through coupling to the lattice strain. The efficiency of the coupling process is shown to depend on several parameters including the initial thermal population and the instantaneous photo-induced population among others. Far below the crossover temperature, the elastic self-amplification occurs above a threshold photo-excitation. On approaching the thermal crossover, the threshold disappears and the photo-elastic conversion increases.

Electronic and Structural Dynamics During the Switching of the Photomagnetic Complex [Fe(L222 N5 )(CN)2 ].

The [Fe(L222 N5 )(CN)2 ] compound, where L222 N5 refers to the macrocyclic Schiff-base ligand, 2,13-dimethyl-3,6,9,-12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,- 16-pentaene, is a photomagnetic FeII based coordination compound, which undergoes light-induced excited spin-state trapping (LIESST). The low spin state is hexacoordinated and the high spin state heptacoordinated. This system also serves as complex for the design of trinuclear or one-dimensional compounds made of other types of bricks with diverse coordinated metals. Here its ultrafast spin-state photoswitching dynamics are studied, by combining femtosecond optical spectroscopy and femtosecond X-ray absorption measurements at the XPP station of the X-ray free-electron laser LCLS. DFT and TD-DFT calculations are used to interpret experimental findings. These studies, performed in the solution phase, show that LIESST in [Fe(L222 N5 )(CN)2 ] occurs on the 100 fs timescale under different types of photoexcitation. In addition, coherent oscillations were observed, resulting from the structural dynamics accompanying LIESST, which were recently evidenced in more conventional octahedral FeII N6 systems.

Elastically driven cooperative response of a molecular material impacted by a laser pulse.

Photoinduced phase transformations occur when a laser pulse impacts a material, thereby transforming its electronic and/or structural orders, consequently affecting the functionalities. The transient nature of photoinduced states has thus far severely limited the scope of applications. It is of paramount importance to explore whether structural feedback during the solid deformation has the capacity to amplify and stabilize photoinduced transformations. Contrary to coherent optical phonons, which have long been under scrutiny, coherently propagating cell deformations over acoustic timescales have not been explored to a similar degree, particularly with respect to cooperative elastic interactions. Herein we demonstrate, experimentally and theoretically, a self-amplified responsiveness in a spin-crossover material during its delayed volume expansion. The cooperative response at the material scale prevails above a threshold excitation, significantly extending the lifetime of photoinduced states. Such elastically driven cooperativity triggered by a light pulse offers an efficient route towards the generation and stabilization of photoinduced phases in many volume-changing materials.

Ultrafast light-induced spin-state trapping photophysics investigated in Fe(phen)2(NCS)2 spin-crossover crystal.

Few photoactive molecules undergo a complete transformation of physical properties (magnetism, optical absorption, etc.) when irradiated with light. Such phenomena can happen on the time scale of fundamental atomic motions leading to an entirely new state within less than 1 ps following light absorption. Spin crossover (SCO) molecules are prototype systems having the ability to switch between low spin (LS) and high spin (HS) molecular states both at thermal equilibrium and after light irradiation. In the case of Fe(II) (3d(6)) complexes in a nearly octahedral ligand field, the two possible electronic distributions among the 3d split orbitals are S = 0 for the LS diamagnetic state and S = 2 for the HS paramagnetic state. In crystals, such photoexcited states can be long-lived at low temperature, as is the case for the photoinduced HS state of the [Fe(phen)2(NCS)2] SCO compound investigated here. We first show how such bistability between the diamagnetic and paramagnetic states can be characterized at thermal equilibrium or after light irradiation at low temperature. Complementary techniques provide invaluable insights into relationships between changes of electronic states and structural reorganization. But the development of such light-active materials requires the understanding of the basic mechanism following light excitation of molecules, responsible for trapping them into new electronic and structural states. We therefore discuss how we can observe a photomagnetic molecule during switching and catch on the fly electronic and structural molecular changes with ultrafast X-ray and optical absorption spectroscopies. In addition, there is a long debate regarding the mechanism behind the efficiency of such a light-induced process. Recent theoretical works suggest that such speed and efficiency are possible thanks to the instantaneous coupling with the phonons of the final state. We discuss here the first experimental proof of that statement as we observe the instantaneous activation of one key phonon mode precluding any recurrence towards the initial state. Our studies show that the structural molecular reorganization trapping the photoinduced electronic state occurs in two sequential steps: the molecule elongates first (within 170 femtosecond) and bends afterwards. This dynamics is caught via the coherent vibrational energy transfer of the two main structural modes. We discuss the transformation pathway connecting the initial photoexcited state to the final state, which involves several key reaction coordinates. These results show the need to replace the classical single coordinate picture employed so far with a more complex multidimensional energy surface.

Two-Step Photon Absorption Driving the Chemical Reaction in the Model Ruthenium Nitrosyl System [Ru(py)4Cl(NO)](PF6)2·(1)/2H2O.

Various systems containing the [ML5NO] molecule, where M = Fe, Ru, ... and L = F, Cl, ..., exhibit switching under continuous light (CW) irradiation between the ground-state nitrosyl (GS), isonitrosyl (MSI), and side-on (MSII) configurations. The metastable populations, however, are often limited to a few percent. The [Ru(py)4Cl(NO)](PF6)2·(1)/2H2O system is thus a remarkable model compound as the GS to MSI transformation is nearly complete in a single crystal. A predominant two-step photon absorption process during GS to MSI switching under blue light is revealed by visible absorption spectroscopy, although a low concentration of the transient species hinders the determination of this process by the structural signature. During the depopulation of MSI, both two-step and direct processes are evidenced under red CW irradiation. Different intermediate visible spectra revealing transient species during GS to MSI and the reverse photochemical processes are discussed in relation to MSII properties.

Sequential Activation of Molecular Breathing and Bending during Spin-Crossover Photoswitching Revealed by Femtosecond Optical and X-Ray Absorption Spectroscopy.

We study the basic mechanisms allowing light to photoswitch at the molecular scale a spin-crossover material from a low- to a high-spin state. Combined femtosecond x-ray absorption performed at LCLS X-FEL and optical spectroscopy reveal that the structural stabilization of the photoinduced high-spin state results from a two step structural trapping. Molecular breathing vibrations are first activated and rapidly damped as part of the energy is sequentially transferred to molecular bending vibrations. During the photoswitching, the system follows a curved trajectory on the potential energy surface.

The role of ligand-field states in the ultrafast photophysical cycle of the prototypical iron(II) spin-crossover compound [Fe(ptz)(6)](BF(4))(2).

Light-induced excited spin-state trapping (LIESST) in iron(II) spin-crossover compounds, that is, the light-induced population of the high-spin (S=2) state below the thermal transition temperature, was discovered thirty years ago. For irradiation into metal-ligand charge transfer (MLCT) bands of the low-spin (S=0) species the acknowledged sequence takes the system from the initially excited (1) MLCT to the high-spin state via the (3) MLCT state within ca. 150 fs, thereby bypassing low-lying ligand-field (LF) states. Nevertheless, these play a role, as borne out by the observation of LIESST and reverse-LIESST on irradiation directly into the LF bands for systems with only high-energy MLCT states. Herein we elucidate the ultrafast reverse-LIESST pathway by identifying the lowest energy S=1 LF state as an intermediate state with a lifetime of 39 ps for the light-induced high-spin to low-spin conversion on irradiation into the spin-allowed LF transition of the high-spin species in the NIR.

Ultrafast photoswitching in a copper-nitroxide-based molecular magnet.

Molecular compounds with photoswitchable magnetic properties have been intensively investigated over the last decades due to their prospective applications in nanoelectronics, sensing and magnetic data storage. The family of copper-nitroxide-based molecular magnets represents a new promising type of photoswitchable compounds. We report the first study of these appealing systems using femtosecond optical spectroscopy. We unveil the mechanism of ultrafast (<50 fs) spin state photoswitching and establish its principal differences compared to other photoswitchable magnets. On this basis, we propose potential advantages of copper-nitroxide-based molecular magnets for the future design of ultrafast magnetic materials.

Observation of a Picosecond Light-Induced Spin Transition in Polymeric Nanorods.

Spin transition (ST) materials are attractive for developing photoswitchable devices, but their slow material transformations limit device applications. Size reduction could enable faster switching, but the photoinduced dynamics at the nanoscale remains poorly understood. Here, we report a femtosecond optical pump multimodal X-ray probe study of polymeric nanorods. Simultaneously tracking the ST order parameter with X-ray emission spectroscopy and structure with X-ray diffraction, we observe photodoping of the low-spin-lattice within ∼150 fs. Above a ∼16% photodoping threshold, the transition to the high-spin phase occurs following an incubation period assigned to vibrational energy redistribution within the nanorods activating the molecular spin switching. Above ∼60% photodoping, the incubation period disappears, and the transition completes within ∼50 ps, preceded by the elastic nanorod expansion in response to the photodoping. These results support the feasibility of ST material-based GHz optical switching applications.

100†picosecond diffraction catches structural transients of laser-pulse triggered switching in a spin-crossover crystal.

We study by 100 picosecond X-ray diffraction the photo-switching dynamics of single crystal of the orthorhombic polymorph of the spin-crossover complex [(TPA)Fe(TCC)]PF(6), in which TPA = tris(2-pyridyl methyl)amine, TCC(2-) = 3,4,5,6-Cl(4)-Catecholate(2-). In the frame of the emerging field of dynamical structural science, this is made possible by using optical pump/X-ray probe techniques, which allow following in real time structural reorganization at intra- and intermolecular levels associated with the change of spin state in the crystal. We use here the time structure of the synchrotron radiation generating 100 picosecond X-ray pulses, coupled to 100 fs laser excitation. This study has revealed a rich variety of structural reorganizations, associated with the different steps of the dynamical process. Three consecutive regimes are evidenced in the time domain: 1) local molecular photo-switching with structural reorganization at constant volume, 2) volume relaxation with inhomogeneous distribution of local temperatures, 3) homogenization of the crystal in the transient state 100 µs after laser excitation. These findings are fundamentally different from those of conventional diffraction studies of long-lived photoinduced high spin states. The time-resolution used here with picosecond X-ray diffraction probes different physical quantities on their intrinsic time-scale, shedding new light on the successive processes driving macroscopic switching in a functionalized material. These results pave the way for structural studies away from equilibrium and represent a first step toward femtosecond crystallography.

Ultrafast spin-state photoswitching in a crystal and slower consecutive processes investigated by femtosecond optical spectroscopy and picosecond X-ray diffraction.

We report the spin state photo-switching dynamics in two polymorphs of a spin-crossover molecular complex triggered by a femtosecond laser flash, as determined by combining femtosecond optical pump-probe spectroscopy and picosecond X-ray diffraction techniques. The light-driven transformations in the two polymorphs are compared. Combining both techniques and tracking how the X-ray data correlate with optical signals allow understanding of how electronic and structural degrees of freedom couple and play their role when the switchable molecules interact in the active crystalline medium. The study sheds light on crossing the border between femtochemistry at the molecular scale and femtoswitching at the material scale.

Polymorphism in the spin-crossover ferric complexes [(TPA)Fe(III)(TCC)]PF6.

We have identified two polymorphs of the molecular complex [(TPA)Fe((III))(TCC)]PF(6) [TPA = tris(2-pyridylmethyl)amine and TCC = 3,4,5,6-tetrachlorocatecholate dianion]: one is monoclinic and the other is orthorhombic. By lowering the temperature both undergo a thermal spin-crossover between a high-spin (S = 5/2) and a low-spin (S = 1/2) state, which we detected by magnetic, optical and X-ray diffraction measurements. The thermal crossover is only slightly shifted between the polymorphs. Their crystalline structures consist of similar cation layers alternating with PF(6) anion layers, packed differently in the two polymorphs. The magnetic and optical properties of the polymorphs are presented.

Finite Size Effects on the Switching Dynamics of Spin-Crossover Thin Films Photoexcited by a Femtosecond Laser Pulse.

Using ultrafast optical absorption spectroscopy, the room-temperature spin-state switching dynamics induced by a femtosecond laser pulse in high-quality thin films of the molecular spin-crossover (SCO) complex [Fe(HB(tz)3 )2 ] (tz = 1,2,4-triazol-1-yl) are studied. These measurements reveal that the early, sub-picosecond, low-spin to high-spin photoswitching event, with linear response to the laser pulse energy, can be followed under certain conditions by a second switching process occurring on a timescale of tens of nanoseconds, enabling nonlinear amplification. This out-of-equilibrium dynamics is discussed in light of the characteristic timescales associated with the different switching mechanisms, i.e., the electronic and structural rearrangements of photoexcited molecules, the propagation of strain waves at the material scale, and the thermal activation above the molecular energy barrier. Importantly, the additional, nonlinear switching step appears to be completely suppressed in the thinnest (50 nm) film due to the efficient heat transfer to the substrate, allowing the system to retrieve the thermal equilibrium state on the 100 ns timescale. These results provide a first milestone toward the assessment of the physical parameters that drive the photoresponse of SCO thin films, opening up appealing perspectives for their use as high-frequency all-optical switches working at room temperature.

Capturing transient structures in the elimination reaction of haloalkane in solution by transient X-ray diffraction.

This Communication reports simultaneous tracking of structural and kinetic information for the photoinduced elimination reaction of 1,2-diiodotetrafluoroethane in solution by transient X-ray diffraction. The transient structure of .CF2CF2I is determined to be a classical mixture whereas .CH2CH2I is bridged. Compared with the gas phase reaction, the secondary dissociation of .CF2CF2I into C2F4 and I is slowed down by a factor of 6 in solution. Transient X-ray diffraction offers a complementary method for capturing transient structures in solution which might be invisible or "optically silent" in time-resolved optical spectroscopy.

Light-Induced Spin State Switching in Copper(II)-Nitroxide-Based Molecular Magnet at Room Temperature.

Molecular magnets Cu(hfac)2LR exhibit an unusual type of photoinduced magnetostructural switching in exchange-coupled copper(II)-nitroxide clusters. Such photoswitching from strongly coupled to weakly coupled spin state (SS → WS) was recently found to be ultrafast, thus enhancing the interest in these systems and the scope of their potential applications. However, to date such SS → WS photoswitching was demonstrated only at cryogenic temperatures, being limited by the absence of suitable SS states and short relaxation times at T > 100 K. In this work we selected model compound Cu(hfac)2Liso-Pr residing in the mixed SS/WS state at room temperature and investigated it using femtosecond optical spectroscopy. Photoinduced spin dynamics was detected, and an ultrafast SS → WS photoswitching was for the first time demonstrated at room temperature, constituting an important milestone in the development of copper(II)-nitroxide molecular magnets for practical purposes.

Structural dynamics of photoinduced molecular switching in the solid state.

Fast and ultra-fast time-resolved diffraction is a fantastic tool for directly observing the structural dynamics of a material rearrangement during the transformation induced by an ultra-short laser pulse. The paper illustrates this ability using the dynamics of photoinduced molecular switching in the solid state probed by 100 ps X-ray diffraction. This structural information is crucial for establishing the physical foundations of how to direct macroscopic photoswitching in materials. A key feature is that dynamics follow a complex pathway from molecular to material scales through a sequence of processes. Not only is the pathway indirect, the nature of the dynamical processes along the pathway depends on the timescale. This dictates which types of degrees of freedom are involved in the subsequent dynamics or kinetics and which are frozen or statistically averaged. We present a recent investigation of the structural dynamics in multifunctional spin-crossover materials, which are prototypes of molecular bistability in the solid state. The time-resolved X-ray diffraction results show that the dynamics span from subpicosecond molecular photoswitching followed by volume expansion (on a nanosecond timescale) and additional thermoswitching (on a microsecond timescale).

Photolysis of Br2 in CCl4 studied by time-resolved X-ray scattering.

A time-resolved X-ray solution scattering study of bromine molecules in CCl(4) is presented as an example of how to track atomic motions in a simple chemical reaction. The structures of the photoproducts are tracked during the recombination process, geminate and non-geminate, from 100 ps to 10 micros after dissociation. The relaxation of hot Br(2)(*) molecules heats the solvent. At early times, from 0.1 to 10 ns, an adiabatic temperature rise is observed, which leads to a pressure gradient that forces the sample to expand. The expansion starts after about 10 ns with the laser beam sizes used here. When thermal artefacts are removed by suitable scaling of the transient solvent response, the excited-state solute structures can be obtained with high fidelity. The analysis shows that 30% of Br(2)(*) molecules recombine directly along the X potential, 60% are trapped in the A/A' state with a lifetime of 5.5 ns, and 10% recombine non-geminately via diffusive motion in about 25 ns. The Br-Br distance distribution in the A/A' state peaks at 3.0 A.

Chopper system for time resolved experiments with synchrotron radiation.

A chopper system for time resolved pump-probe experiments with x-ray beams from a synchrotron is described. The system has three parts: a water-cooled heatload chopper, a high-speed chopper, and a millisecond shutter. The chopper system, which is installed in beamline ID09B at the European Synchrotron Radiation Facility, provides short x-ray pulses for pump-probe experiments with ultrafast lasers. The chopper system can produce x-ray pulses as short as 200 ns in a continuous beam and repeat at frequencies from 0 to 3 kHz. For bunch filling patterns of the synchrotron with pulse separations greater than 100 ns, the high-speed chopper can isolate single 100 ps x-ray pulses that are used for the highest time resolution. A new rotor in the high-speed chopper is presented with a single pulse (100 ps) and long pulse (10 micros) option. In white beam experiments, the heatload of the (noncooled) high-speed chopper is lowered by a heatload chopper, which absorbs 95% of the incoming power without affecting the pulses selected by the high speed chopper.