Photoinduced Transition Research Articles

Photoinduced complete melting of spin-Peierls phase in Na-tetracyanoquinodimethane revealed by frequency doubling of coherent molecular oscillations

Photoinduced melting of the spin-Peierls phase was investigated in Na-tetracyanoquinodimethane by pump-probe reflection spectroscopy with a time resolution of 49 fs. Photoirradiation generates coherent molecular oscillation, which indicates that the spin-Peierls dimerization decreased. Increasing the excitation photon density to $g0.02$ photon/molecule doubles the oscillation frequency attributable to recovery of the original symmetry, that is, complete melting of the spin-Peierls phase. We observed a complete photoinduced transition with symmetry recovery revealed by frequency doubling of coherent oscillations.

Dynamics of ground state absorption spectra in donor-acceptor pairs with ultrafast charge recombination.

A theoretical approach to simulation of the transient spectra in molecular systems with ultrafast photoinduced nonradiative electronic transitions is developed. The evolution of the excited and ground state populations as well as the nonradiative transitions between them are calculated in the framework of the stochastic multichannel point-transition model involving the reorganization of the medium and the intramolecular high frequency vibrational modes. Simulations of transient spectra of donor-acceptor pairs excited in the charge-transfer band that are accompanied by ultrafast charge recombination into the ground state demonstrate a possibility of positive band appearance in the transient absorption spectrum caused by those systems in the ground state, which returned there from the excited state. The region of the parameters of the donor-acceptor systems where a positive ground state absorption signal can be observed is discussed. A qualitative comparison of the simulated transient spectra with the experimental data on betaine-30 is presented.

Dual self-healing abilities of composite gels consisting of polymer-brush-afforded particles and an azobenzene-doped liquid crystal.

We prepared the composite gels from polymer-brush-afforded silica particles (P-SiPs) and an azobenzene-doped liquid crystal, and investigated their inner structure, dynamic viscoelastic properties, thermo- and photoresponsive properties, and self-healing behaviors. It was found that the composite gels had a sponge-like inner structure formed with P-SiPs and exhibited good elastic property and shape recoverability. The surface dents made on the composite gel could be repaired spontaneously at room temperature. Moreover, the composite gel exhibited a gel-sol transition induced by the trans-cis photoisomerization of the azo dye, and the transition could be used as a mending mechanism for surface cracks. Consequently, we successfully developed a material exhibiting two types of self-healing abilities simultaneously: (1) spontaneous repair of surface dents by means of the excellent elasticity of the composite gel and (2) light-assisted mending of surface cracks by photoinduced gel-sol transition.

Molecular Shape Related Effects in Liquid Crystals

Liquid crystals are organic materials constituting of molecules characterized by strong shape anisotropy. Many studies unambiguously showed that some important physical effects observed in liquid crystals are strongly dependent on the shape anisotropy of their molecules.In this talk, some of the molecular shape related effects in liquid crystals, which have a potential for applications in liquid crystal displays (LCDs) and photonic devices, was overviewed. Photo-induced alignment transition from planar to vertical, flexoelectric polarization in short pitch cholesteric liquid crystals, alignment of bent core nematics as well as field driven transition from birefringent to optically isotropic state in these materials are among those effects on which this talk was focused. The role of the molecular shape was discussed with focus on the impact, which it has on the anchoring, flexoelectric, optical and electro-optical properties of the liquid crystals.

Construction, characterization and photophysical study on electrospinning composite fibers doped with phosphorescent Re(I) complex

This paper reported a diamine ligand 2-(4-bromophenyl)-5-(pyridin-2-yl)-1,3,4-oxadiazole (cited as BrPO for short) having a large conjugation chain and an electron-withdrawing group. Its corresponding Re(I) complex Re(CO)3(BrPO)Br was full characterized, including its single crystal structure and electronic transition nature. It was found that Re(CO)3(BrPO)Br took a distorted octahedral coordination mode. Its photo-induced transitions all showed a mixed character of metal-to-ligand-charge-transfer and ligand-to-ligand-charge-transfer. Experimental data suggested that Re(CO)3(BrPO)Br excited state suffered from bad geometric relaxation. To improve emission performance, Re(CO)3(BrPO)Br was doped into a polymer host using electrospinning method so that this geometric relaxation could be limited. The photophysical comparison between pure sample and composite sample was performed. Results suggested that the immobilization in polymer matrix could effectively suppress such geometric relaxation, giving improved emission performance such as emission blue shift, narrowed emission band, longer excited state lifetime and improved photostability.

In situ spectroelectrochemical and theoretical study on the oxidation of a 4H-imidazole-ruthenium dye adsorbed on nanocrystalline TiO2 thin film electrodes.

Terpyridine 4H-imidazole-ruthenium(II) complexes are considered promising candidates for use as sensitizers in dye sensitized solar cells (DSSCs) by displaying broad absorption in the visible range, where the dominant absorption features are due to metal-to-ligand charge transfer (MLCT) transitions. The ruthenium(III) intermediates resulting from photoinduced MLCT transitions are essential intermediates in the photoredox-cycle of the DSSC. However, their photophysics is much less studied compared to the ruthenium(II) parent systems. To this end, the structural alterations accompanying one-electron oxidation of the RuIm dye series (including a non-carboxylic RuIm precursor, and, carboxylic RuImCOO in solution and anchored to a nanocrystalline TiO2 film) are investigated via in situ experimental and theoretical UV-Vis absorption and resonance Raman (RR) spectroelectrochemistry. The excellent agreement between the experimental and the TDDFT spectra derived in this work allows for an in-depth assignment of UV-Vis and RR spectral features of the dyes. A concordant pronounced wavelength dependence with respect to the charge transfer character has been observed for the model system RuIm, and both RuImCOO in solution and attached on the TiO2 surface. Excitation at long wavelengths leads to the population of ligand-to-metal charge transfer states, i.e. photoreduction of the central ruthenium(III) ion, while high-energy excitation features an intra-ligand charge transfer state localized on the 4H-imidazole moiety. Therefore, these 4H-imidazole ruthenium complexes investigated here are potential multi-photoelectron donors. One electron is donated from MLCT states, and additionally, the 4H-imidazole ligand reveals electron-donating character with a significant contribution to the excited states of the ruthenium(III) complexes upon blue-light irradiation.

Strain and temperature dependent absorption spectra studies for identifying the phase structure and band gap of EuTiO3 perovskite films.

Post-annealing has been approved to effectively relax the out-of-plane strain in thin films. Epitaxial EuTiO3 (ETO) thin films, with and without strain, have been fabricated on (001) LaAlO3 substrates by pulsed laser deposition. The absorption and electronic transitions of the ETO thin films are investigated by means of temperature dependent transmittance spectra. The antiferrodistortive phase transition can be found at about 260-280 K. The first-principles calculations indicate there are two interband electronic transitions in ETO films. Remarkably, the direct optical band gap and higher interband transition for ETO films show variation in trends with different strains and temperatures. The strain leads to a band gap shrinkage of about 240 meV while the higher interband transition an expansion of about 140 meV. The hardening of the interband transition energies in ETO films with increasing temperature can be attributed to the Fröhlich electron-phonon interaction. The behavior can be linked to the strain and low temperature modified valence electronic structure, which is associated with rotations of the TiO6 octahedra.

First evidence of light-induced spin transition in molybdenum(IV).

Photo-induced spin transition in a molybdenum-zinc complex has been evidenced and fully characterized by Squid magnetometry and several spectroscopies performed under irradiation (IR, EPR, etc.). The phenomenon has been confirmed by X-ray diffraction and DFT calculations yielding a Light-Induced Excited Spin State Trapping Effect (LIESST) on a 4d transition metal ion.

結晶構造解析による光誘起結晶－融解相転移現象の解明

結晶構造解析による光誘起結晶－融解相転移現象の解明

Photo-induced reversible structural transition of cationic diphenylalanine peptide self-assembly.

The photo-induced self-assembly of a cationic diphenylalanine peptide (CDP) is investigated using a photoswitchable sulfonic azobenzene as the manipulating unit. A reversible structural transition between a branched structure and a vesicle-like structure is observed by alternating between UV and visible light irradiation.

Soft X-ray synchrotron radiation spectroscopy study of molecule-based nanoparticles

The electronic structures of molecule-based nanoparticles, such as biomineralized Helicobacter pylori ferritin (Hpf), Heme, and RbCo[Fe(CN)6]H2O (RbCoFe) Prussian blue analogue, have been investigated by employing photoemission spectroscopy and soft X-ray absorption spectroscopy. Fe ions are found to be nearly trivalent in Hpf and Heme nanoparticles, which provides evidence that the amount of magnetite (Fe3O4) should be negligible in the Hpf core and that the biomineralization of Fe oxides in the high-Fe-bound-state Hpf core arises from a hematite-like formation. On the other hand, Fe ions are nearly divalent and Co ions are Co2+-Co3+ mixed-valent in RbCoFe. Therefore this finding suggests that the mechanism of the photo-induced transition in RbCoFe Prussian blue analogue is not a simple spin-state transition of Fe2+-Co3+ → Fe3+-Co2+. It is likely that Co2+ ions have the high-spin configuration while Fe2+ ions have the low-spin configuration.

Cooperatively Tuning Phase Size and Absorption of Near IR Photons in P3HT:Perylene Diimide Solar Cells by Bay-Modifications on the Acceptor

P3HT is a widely used and commercial polymer donor, but it cannot absorb near IR solar photons. Chemical accessibility to tune the frontier molecular orbits of π-conjugated small molecule acceptors increases the possibility to improve their near IR absorption, which is complementary to P3HT. Taking the aggregation tendency of the planar π-system into account, we herein use the traditional n-type organic semiconductor of perylene diimide (PDI) as the model backbone, showing a molecular way to cooperatively tune the aggregation tendency and absorption of the near IR photons. Practically, we replace the 2-methoxylethoxyl units from the mother PDI monomer (O-PDI-O), one-by-one, with the 4,8-bis(2-(2-ethylhexylthienyl) benzo[1,2-b′:4,5-b′]dithiophene (BDT) moieties, giving two other PDI monomers of B-PDI-O and B-PDI-B. Because of the photoinduced intramolecular charge transfer transition from the BDT unit to the PDI core, B-PDI-B exhibits a broad absorption shoulder beyond 600 nm in the dilute solution, and be...

Optical nutation in GaAs/AlGaAs core-shell quantum dot

Optical nutation in GaAs/Al0.3Gao.7As core-shell quantum dot (CSQD) shined by femtosecond laser has been analyzed theoretically. The nutation signal intensity, arising due to photo-induced electronic transitions between ground state and first excitonic state, has been calculated using time dependent perturbation theory and rotational wave approximation. The dependency of the properties of optical nutation signal viz. its maximum amplitude and the oscillation frequency of the nutation signal has been studied at various CSQD's core radius and shell thickness.

Excitation-photon-energy selectivity of photoconversions in halogen-bridged Pd-chain compounds: Mott insulator to metal or charge-density-wave state.

Ultrafast photoinduced transitions of a one-dimensional Mott insulator into two distinct electronic phases, metal and charge-density-wave (CDW) state, were achieved in a bromine-bridged Pd-chain compound [Pd(en)2Br](C5-Y)2H2O (en=ethylenediamine and C5-Y=dialkylsulfosuccinate), by selecting the photon energy of a femtosecond excitation pulse. For the resonant excitation of the Mott-gap transition, excitonic states are generated and converted to one-dimensional CDW domains. For the higher-energy excitation, free electron and hole carriers are produced, giving rise to a transition of the Mott insulator to a metal. Such selectivity in photoconversions by the choice of initial photoexcited states opens a new possibility for the developments of advanced optical switching and memory functions.

A perturbative formalism for electronic transitions through conical intersections in a fully quadratic vibronic model.

We consider a fully quadratic vibronic model Hamiltonian for studying photoinduced electronic transitions through conical intersections. Using a second order perturbative approximation for diabatic couplings, we derive an analytical expression for the time evolution of electronic populations at a given temperature. This formalism extends upon a previously developed perturbative technique for a linear vibronic coupling Hamiltonian. The advantage of the quadratic model Hamiltonian is that it allows one to use separate quadratic representations for potential energy surfaces of different electronic states and a more flexible representation of interstate couplings. We explore features introduced by the quadratic Hamiltonian in a series of 2D models, and then apply our formalism to the 2,6-bis(methylene) adamantyl cation and its dimethyl derivative. The Hamiltonian parameters for the molecular systems have been obtained from electronic structure calculations followed by a diabatization procedure. The evolution of electronic populations in the molecular systems using the perturbative formalism shows a good agreement with that from variational quantum dynamics.

Crystal melting by light: X-ray crystal structure analysis of an azo crystal showing photoinduced crystal-melt transition.

Trans-cis photoisomerization in an azo compound containing azobenzene chromophores and long alkyl chains leads to a photoinduced crystal-melt transition (PCMT). X-ray structure analysis of this crystal clarifies the characteristic coexistence of the structurally ordered chromophores through their π···π interactions and disordered alkyl chains around room temperature. These structural features reveal that the PCMT starts near the surface of the crystal and propagates into the depth, sacrificing the π···π interactions. A temporal change of the powder X-ray diffraction pattern under light irradiation and a two-component phase diagram allow qualitative analysis of the PCMT and the following reconstructive crystallization of the cis isomer as a function of product accumulation. This is the first structural characterization of a compound showing the PCMT, overcoming the low periodicity that makes X-ray crystal structure analysis difficult.

Iron(II) Spin‐Crossover Complexes in Ultrathin Films: Electronic Structure and Spin‐State Switching by Visible and Vacuum‐UV Light

The electronic structure of the iron(II) spin crossover complex [Fe(H2bpz)2(phen)] deposited as an ultrathin film on Au(111) is determined by means of UV-photoelectron spectroscopy (UPS) in the high-spin and in the low-spin state. This also allows monitoring the thermal as well as photoinduced spin transition in this system. Moreover, the complex is excited to the metastable high-spin state by irradiation with vacuum-UV light. Relaxation rates after photoexcitation are determined as a function of temperature. They exhibit a transition from thermally activated to tunneling behavior and are two orders of magnitude higher than in the bulk material.

Textures of cholesteric droplets controlled by photo-switching chirality at the molecular level

Ultra-sensitive detection of chirality is demonstrated experimentally by investigating photo-induced structural transitions in sessile droplets of cholesteric liquid crystals.

Solvation dynamics monitored by combined X-ray spectroscopies and scattering: photoinduced spin transition in aqueous [Fe(bpy)3](2+).

We have studied the photoinduced low spin (LS) to high spin (HS) conversion of aqueous Fe(bpy)3 with pulse-limited time resolution. In a combined setup permitting simultaneous X-ray diffuse scattering (XDS) and spectroscopic measurements at a MHz repetition rate we have unraveled the interplay between intramolecular dynamics and the intermolecular caging solvent response with 100 ps time resolution. On this time scale the ultrafast spin transition including intramolecular geometric structure changes as well as the concomitant bulk solvent heating process due to energy dissipation from the excited HS molecule are long completed. The heating is nevertheless observed to further increase due to the excess energy between HS and LS states released on a subnanosecond time scale. The analysis of the spectroscopic data allows precise determination of the excited population which efficiently reduces the number of free parameters in the XDS analysis, and both combined permit extraction of information about the structural dynamics of the first solvation shell.

Transient spectral hole burning observed on the single-molecule level in terrylene-doped biphenyl

We use the method of fluorescence correlation spectroscopy to analyze the single-molecule (SM) spectroscopy data earlier recorded for a special type of terrylene SM impurity center (referred as “spectrally confined unstable molecule”, SCM) in an incommensurate single crystal of biphenyl. The SCM's SM line seems to be chaotically jumping around within a broad “spectral envelope” and was first considered being subject to a peculiar spectral diffusion behavior. However, our correlation analysis reveals that all the features observed for SCM at 1.8K are consistent with an assumption that this SM center participates in a process of reversible (transient) spectral hole burning (THB) earlier observed for terrylene-doped polycrystalline biphenyl. No observations of THB processes on SM level have been so far reported for this impurity system, partially due to a low concentration of relevant impurity centers. Another reason making searching for such centers experimentally challenging is an unusual SM line behavior: the photoinduced transition to a metastable “dark state” leads to the SM line saturational broadening, which is much stronger than the triplet broadening. Hence required prolonged observation is often prevented by an SM act of persistent spectral hole burning.