Photoinduced State Research Articles

Photophysical processes of the spectroscopic RNA probe 2-(1-ethynylpyrene)-adenosine (PyA)

We examine the photoinduced excited state dynamics of pyrene modified adenosine, a versatile probe for folding and hybridization of ribonucleic acids. Measurements in different solvents revealed complex ultrafast dynamics, but high robustness since the overall fluorescence quantum yield (Φ f ) is hardly affected. The result is a strong fluorescent RNA-probe whose spectral properties change in a defined way upon environmental changes.

Emission wavelength variation with changes in excitation in a Re(i)–bisthiazole ligand complex that breaks the Kasha–Vavilov rule

We report highly unusual photophysical properties of a fac-Re(CO)3–bisthiazole complex, which is shown to break the Kasha–Vavilov rule. Herein, we show that such a complex has unusual emission wavelength variation due to the presence of photo-induced isomerisation from the stable, ground-state N,N-bound thiazole complex to S,N- and S,S-donation in the photo-induced excited state. A triple excitation, triple emission profile along with extensive DFT calculations support this theory to showcase an unusual circumstance of suulfur donation to the fac-Re(CO)3 core upon luminescence. fac-Rhenium tricarbonyl complexes are extensively studied due to their fluorescent properties, high stability, and ready incorporation into conjugates with broad applicability in imaging and targeting. The spectroscopic features of the complex under study have broad fundamental interest while also offering potential for design of novel agents for use in biological imaging studies.

Biphenyl bridged hexadentate N6-ligands – a rigid ligand backbone for Fe(ii) spin crossover complexes

The novel hexadentate nitrogen based ligand N,N'-bis-(2-(1H-pyrazol-1-yl)pyridine-6-ylmethyl)-2,2'-biphenylenediamine (3) was synthesized and used for the preparation of iron Spin Crossover (SCO) complexes [Fe(3)][BF4]2 (4) and [Fe(3)][ClO4]2 (5), which differ only by the respective counter ion. These complex salts show different spin transition temperatures T(1/2) (135 and 157 K, respectively). This effect was studied by the investigation of the solid state structure of different low- and high-spin isomers. All complexes of this series show closely related crystal packing regardless of the counter ion, metal (Zn/Fe) and spin state. The isomer exhibiting the lower transition temperature (4) was also investigated in respect to its photomagnetic behaviour. The LIESST process could be monitored for this complex, but no reverse-LIESST was observed. The relaxation of the photo-induced state occurs at ca. 80 K, showing a complex, three-state relaxation mechanism.

Signal Discrimination Between Fluorescent Proteins in Live Cells by Long-Wavelength Optical Modulation

Fluorescent proteins (FPs) have revolutionized molecular and cellular biology; yet, discrimination over cellular autofluorescence, spectral deconvolution, or detection at low concentrations remain challenging problems in many biological applications. By optically depopulating a photoinduced dark state with orange secondary laser co-excitation, the higher-energy green AcGFP fluorescence is dynamically increased. Modulating this secondary laser then modulates the higher-energy, collected fluorescence; enabling its selective detection by removing heterogeneous background from other FPs. Order-of-magnitude reduction in obscuring fluorophore background emission has been achieved in both fixed and live cells. This longwavelength modulation expands the dimensionality to discriminate FP emitters based on dark state lifetimes and enables signal of interest to be recovered by removing heterogeneous background emitter signals. Thus, AcGFP is not only useful for extracting weak signals from systems plagued by high background, but it is a springboard for further FP optimization and utilization for improving sensitivity and selectivity in biological fluorescence imaging.

Heteroleptic FeII Complexes of 2,2′‐Biimidazole and Its Alkylated Derivatives: Spin‐Crossover and Photomagnetic Behavior

Three iron(II) complexes, [Fe(TPMA)(BIM)](ClO(4))(2)⋅0.5H(2)O (1), [Fe(TPMA)(XBIM)](ClO(4))(2) (2), and [Fe(TPMA)(XBBIM)](ClO(4))(2)⋅0.75CH(3)OH (3), were prepared by reactions of Fe(II) perchlorate and the corresponding ligands (TPMA=tris(2-pyridylmethyl)amine, BIM=2,2'-biimidazole, XBIM=1,1'-(α,α'-o-xylyl)-2,2'-biimidazole, XBBIM=1,1'-(α,α'-o-xylyl)-2,2'-bibenzimidazole). The compounds were investigated by a combination of X-ray crystallography, magnetic and photomagnetic measurements, and Mössbauer and optical absorption spectroscopy. Complex 1 exhibits a gradual spin crossover (SCO) with T(1/2) =190 K, whereas 2 exhibits an abrupt SCO with approximately 7 K thermal hysteresis (T(1/2) =196 K on cooling and 203 K on heating). Complex 3 is in the high-spin state in the 2-300 K range. The difference in the magnetic behavior was traced to differences between the inter- and intramolecular interactions in 1 and 2. The crystal packing of 2 features a hierarchy of intermolecular interactions that result in increased cooperativity and abruptness of the spin transition. In 3, steric repulsion between H atoms of one of the pyridyl substituents of TPMA and one of the benzene rings of XBBIM results in a strong distortion of the Fe(II) coordination environment, which stabilizes the high-spin state of the complex. Both 1 and 2 exhibit a photoinduced low-spin to high-spin transition (LIESST effect) at 5 K. The difference in the character of intermolecular interactions of 1 and 2 also manifests in the kinetics of the decay of the photoinduced high-spin state. For 1, the decay rate constant follows the single-exponential law, whereas for 2 it is a stretched exponential, reflecting the hierarchical nature of intermolecular contacts. The structural parameters of the photoinduced high-spin state at 50 K are similar to those determined for the high-spin state at 295 K. This study shows that N-alkylation of BIM has a negligible effect on the ligand field strength. Therefore, the combination of TPMA and BIM offers a promising ligand platform for the design of functionalized SCO complexes.

W-Band Time-Resolved Electron Paramagnetic Resonance Study of Light-Induced Spin Dynamics in Copper–Nitroxide-Based Switchable Molecular Magnets

Molecular magnets Cu(hfac)(2)L(R) represent a new type of photoswitchable materials based on exchange-coupled clusters of copper(II) with stable nitroxide radicals. It was found recently that the photoinduced spin state of these compounds is metastable on the time scale of hours at cryogenic temperatures, similar to the light-induced excited spin state trapping phenomenon well-known for many spin-crossover compounds. Our previous studies have shown that electron paramagnetic resonance (EPR) in continuous wave (CW) mode allows for studying the light-induced spin state conversion and relaxation in the Cu(hfac)(2)L(R) family. However, light-induced spin dynamics in these compounds has not been studied on the sub-second time scale so far. In this work we report the first time-resolved (TR) EPR study of light-induced spin state switching and relaxation in Cu(hfac)(2)L(R) with nanosecond temporal resolution. To enhance spectral resolution we used high-frequency TR EPR at W-band (94 GHz). We first discuss the peculiarities of applying TR EPR to the solid-phase compounds Cu(hfac)(2)L(R) at low (liquid helium) temperatures and approaches developed for photoswitching/relaxation studies. Then we analyze the kinetics of the excited spin state at T = 5-21 K. It has been found that the photoinduced spin state is formed at time delays shorter than 100 ns. It has also been found that the observed relaxation of the excited state is exponential on the nanosecond time scale, with the decay rate depending linearly on temperature. We propose and discuss possible mechanisms of these processes and correlate them with previously obtained CW EPR data.

Three-way switching in a cyanide-bridged [CoFe] chain

Bistable compounds that exist in two interchangeable phases under identical conditions can act as switches under external stimuli. Among such switchable materials, coordination complexes have energy levels (or phases) that are determined by the electronic states of their constituent metal ions and ligands. They can exhibit multiple bistabilities and hold promise in the search for multifaceted materials that display different properties in different phases, accessible through the application of contrasting external stimuli. Molecular systems that exhibit both thermo- and photoinduced magnetic bistabilities are excellent candidates for such systems. Here we describe a cyanide-bridged [CoFe] one-dimensional chiral coordination polymer that displays both magnetic and electric bistabilities in the same temperature range. Both the electric and magnetic switching probably arise from the same electron-transfer coupled spin-transition phenomenon, which enables the reversible conversion between an insulating diamagnetic phase and either a semiconducting paramagnetic (thermoinduced) or a type of ferromagnetic single-chain magnet (photoinduced) state.

Photoinduced magnetic bound state in an itinerant correlated electron system with a spin-state degree of freedom

The photoexcited state in a correlated electron system with a spin-state degree of freedom is studied. We start from the two-orbital extended Hubbard model where the energy difference between the two orbitals is introduced. The photoexcited metastable state is examined based on the effective model Hamiltonian derived using the two-orbital Hubbard model. Spin-state change is induced by photoirradiation in the low-spin band insulator near the phase boundary, and it is found that a high-spin state is stabilized by creating a ferromagnetic bound state with photodoped hole carriers. An optical absorption occurs between the bonding and antibonding orbitals inside the bound state. The time evolution of photoexcited states is simulated in the time-dependent mean-field scheme. It is found that pair annihilations of photodoped electrons and holes generate the high-spin state in a low-spin band insulator. We propose that this process is directly observed by the time-resolved photoemission experiments.

Photoinduced Phase Transition in Strongly Electron-Lattice and Electron–Electron Correlated Molecular Crystals

Strongly electron-lattice- and electron-electron-correlated molecular crystals, such as charge transfer (CT) complexes, are often sensitive to external stimuli, e.g., photoexcitation, due to the cooperative or competitive correlation of various interactions present in the crystals. These crystals are thus productive targets for studying photoinduced phase transitions (PIPTs). Recent advancements in research on the PIPT of CT complexes, especially Et2Me2Sb[Pd(dmit)2]2 and (EDO-TTF)2PF6, are reviewed in this report. The former exhibits a photoinduced insulator-to-insulator phase transition with clearly assigned spectral change. We demonstrate how to find the dynamics of PIPT using this system. The latter exhibits a photoinduced hidden state as an initial PIPT process. Wide energy ranged time-resolved spectroscopy can probe many kinds of photo-absorption processes, i.e., intra-molecular and inter-molecular electron excitations and intramolecular and electron-molecular vibrations. The photoinduced spectral changes in these photo-absorption processes reveal various aspects of the dynamics of PIPT, including electronic structural changes, lattice structural changes, and molecular deformations. The complexities of the dynamics of the latter system were revealed by our measurements.

Optically controllable bistable reflective liquid crystal display

This Letter demonstrates a photo-addressable, bistable reflective liquid crystal display that is based on a dye-doped liquid crystal (DDLC). Bistable bright and dark states can be attained using the 45 deg twisted nematic (TN) and photo-induced isotropic states (PHI) of the DDLC, respectively. Both the 45 deg TN and PHI states can exist stably for tens of hours, and each can be rapidly switched to the other by the isomerization effect using UV and green light. A bistable reflective liquid crystal display is simply fabricated, easily operated, and rapidly switched. It therefore has the potential to be used in portable information systems.

Carbon Nanohorn–Porphyrin Dimer Hybrid Material for Enhancing Light-Energy Conversion

The covalent grafting through a rigid ester bond of a dimeric porphyrin [(H2P)2] and carbon nanohorns (CNHs) was accomplished. The newly formed CNH–(H2P)2 hybrid was found to be soluble or dispersible in several organic solvents. Application of diverse spectroscopic techniques verified the successful formation of the CNH–(H2P)2 hybrid material. In addition, thermogravimetric analysis revealed the amount of (H2P)2 loading onto CNHs, and TEM studies showed the characteristic secondary spherical superstructure morphology of the hybrid material. Efficient fluorescence quenching of (H2P)2 in the CNH–(H2P)2 hybrid suggests that photoinduced events occur from the photoexcited (H2P)2 to CNHs. Nanosecond transient absorption spectroscopy revealed the formation of transient species such as (H2P)2•+ and CNH•– by photoinduced charge separation in CNH–(H2P)2. Additional proof for the photoinduced charge-separated state CNH•––(H2P)2•+ was obtained, from which the electron mediates to added hexyl viologen dication (HV2+...

Bayesian analysis of the evidence for minor components in crystallographic models: an alternative to the Hamilton {\cal R} test

A simple test based on Bayesian statistics for the presence of minority populations in single-crystal structure refinement is presented. The test is illustrated by analysis of photocrystallographic experiments on single crystals of ruthenium-sulfur-dioxide-based complexes. In data sets collected after irradiation with light, conventional refinements of the populations of different metastable states to values below 4% are shown to be statistically significant. The results also confirm that the photo-induced states are absent from data collected in the dark.

Utilization of Photoinduced Charge-Separated State of Donor–Acceptor-Linked Molecules for Regulation of Cell Membrane Potential and Ion Transport

The control of ion transport across cell membranes by light is an attractive strategy that allows targeted, fast control of precisely defined events in the biological membrane. Here we report a novel general strategy for the control of membrane potential and ion transport by using charge-separation molecules and light. Delivery of charge-separation molecules to the plasma membrane of PC12 cells by a membranous nanocarrier and subsequent light irradiation led to depolarization of the membrane potential as well as inhibition of the potassium ion flow across the membrane. Photoregulation of the cell membrane potential and ion transport by using charge-separation molecules is highly promising for control of cell functions.

Excitation wavelength dependence of photo-induced intramolecular proton transfer reaction of 4′-N,N-diethylamino-3-hydroxyflavone in various liquids

Excitation wavelength dependence of photo-induced intramolecular excited state proton transfer reaction of 4′-N,N-diethylamino-3-hydroxyflavone in various liquids has been investigated by steady-state and time-resolved fluorescence measurements. It was found that the relative fluorescence intensity of the tautomer excited state to that of the normal excited state significantly decreases in ionic liquids with changing the excitation wavelength from 380 to 470nm. The initial proton transfer rate excited at 470nm was different from that obtained at 400nm excitation. The excitation wavelength dependence was discussed in relation with the inhomogeneous distribution of the solute in the ionic liquids.

Synthesis and characterization of porphyrin electrochromic and photovoltaic electropolymers

New asymmetric porphyrins, with electroactive functionalities were synthesized. 5,15-Bis[4(-N,N-diphenylamino)phenyl]-10,20-bis[3-(N-ethylcarbazoyl)]porphyrin and its Zn(II) derivative allow the electrochemical formation of conjugated, conducting and stable polymeric films over semitransparent indium–tin oxide surface. The spectral changes observed upon oxidation of the films produced different colorations. The polymeric materials showed the generation of photoinduced charge separation states and charge migration upon porphyrin excitation, making them promising candidates for application in optoelectronics devices.

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.

A long-lived photo-induced metastable state of linkage isomerization accompanied with a spin transition

In addition to the generally observed LIESST phenomenon, polymorph D of trans-[Fe(II)(abpt)(2)(NCS)(2)] exhibits a long-lived photo-induced metastable state through linkage isomerization accompanied with a spin crossover transition, which is stable up to 108 K.

On the role of interband surface plasmons in carbon nanotubes

We review the properties of collective surface excitations—excitons and interband plasmons—in single-walled and double-walled carbon nanotubes. We show that an electrostatic field applied perpendicular to the nanotube axis can control the exciton-plasmon coupling in individual small-diameter (≲nm) singlewalled nanotubes, both in the linear excitation regime and in the non-linear excitation regime with the photoinduced biexcitonic states formation. For double-walled carbon nanotubes, we report a profound effect of interband surface plasmons on the inter-tube Casimir force at tube separations similar to their equilibrium distances. Strong overlapping plasmon resonances from both tubes warrant their stronger attraction. Nanotube chiralities possessing such collective excitation features will result in forming the most favorable innerouter tube combination in double-walled carbon nanotubes. These findings pave the way for the development of new generation of tunable optoelectronic and nano-electromechanical device applications with carbon nanotubes.

Protein–Ligand Structure and Electronic Coupling of Photoinduced Charge-Separated State: 9,10-Anthraquinone-1-sulfonate Bound to Human Serum Albumin

To elucidate how the protein-ligand docking structure affects electronic interactions in the electron-transfer process, we have analyzed time-resolved electron paramagnetic resonance spectra of photoinduced charge-separated (CS) states generated by light excitation of 9,10-anthraquinone-1-sulfonate (AQ1S(-)) bound to human serum albumin at a hydrophobic drug-binding region. The spectra have been explained in terms of the triplet-triplet electron spin polarization transfer model to determine both the geometries and the exchange couplings of the CS states of AQ1S(2-•)-histidine-242 radical cation (H242(+•)) and AQ1S(2-•)-tryptophan-214 radical cation (W214(+•)). For the CS state of the former, it has been revealed that, due to the orthogonal relationship between the singly occupied molecular orbitals of AQ1S(2-•) and H242(+•), the electronic coupling (5.4 cm(-1)) is very weak, contributing to the prevention of energy-wasting charge recombination, even at a contact edge-to-edge separation.

Influence of geometric and electronic characteristics of TiO 2 electrodes with nanotubular array on their photocatalytic efficiencies

The present work compares the efficiencies of titania nanotubes (NT) formed by anodizing of titanium at 20 V for 45 min in three kinds of electrolytes: ethylene glycol–ammonium fluoride–water (NT 1), tetrabutylammonium fluoride–formamide–water (NT 2) and sodium sulfate–ammonium fluoride–water (NT 3). The geometric characteristics were investigated by scanning electron microscopy (SEM). The photoactivities of the three layers were compared through photo-potential, photo-current measurements in an alkaline medium and in the presence of methanol as a hole scavenger. Impedance spectrometry was used for investigation of conduction and charge storage properties of the titania layer. Finally the photocatalytic properties were compared through the degradation rate of an azo dye, such as Amido Black (AB). In comparison to NT 2 and NT 3, the NT 1 layer showed not only the highest photopotential, but also the highest photocurrent response. Photocatalytic results showed that the degradation of dye proceeds much faster in the presence of NT 1 as compared with other TiO 2 nanotube samples. Nevertheless photoinduced surface states were found to be more important in NT 2, in comparison with the two other layers. All these results are discussed taking into account the geometric characteristics of the nanotubular layers, not only the specific surface on one hand, but also the solid fraction of the external surface, which determines the level of light absorption able to generate photo-charges.