Charge-Segregated Stacking Structure with Anisotropic Electric Conductivity in NIR-Absorbing and Emitting Positively Charged π-Electronic Systems.

Squarylium-based π-electronic cation with an augmented dipole was synthesized by methylation of zwitterionic squarylium. The cation formed various ion pairs in combination with anions, and the ion pairs exhibited distinct photophysical properties in the dispersed state, ascribed to the formation of J- and H-aggregates. The ion pairs provided solid-state assemblies based on cation stacking. It is noteworthy that complete segregation of cations and anions was observed in a pseudo-polymorph of the ion pair with pentacyanocyclopentadienide as a π-electronic anion. In the crystalline state, the ion pairs exhibited photophysical properties and electric conductivity derived from cation stacking. In particular, the charge-segregated ion-pairing assembly induces an electric conductive pathway along the stacking axis. The charge-segregated mode and fascinating properties were derived from the reduced electrostatic repulsion between adjacent π-electronic cations via dipole-dipole interactions.

Electron-donating curved π-electronic systems that complex with buckyballs.

Modifications in curved π-electronic systems, dipyrrolylbenzodiazepines, resulted in various derivatives with modulated electronic properties and assembly behaviour. The electron-rich pyrrole-based curved π-system exhibited C60 complexation in the form of a hydrogen-bonding cyclic hexamer, giving rise to solid-state photo-induced electron transfer as elucidated by transient absorption spectroscopy.

Laser-induced reprecipitation of pyrene at 77 K and its dynamics as studied by spectroscopic techniques.

Reprecipitation of pyrene (Py) in the glassy solution of methylcyclohexane and isopentane at 77 K was observed by the repetitive irradiation of nanosecond (ns) laser pulses at 355 nm. The dynamics and mechanism of this reprecipitation were investigated by means of time-resolved fluorescence and absorption spectroscopies. Although only the fluorescence of the Py monomer was observed before the ns laser irradiation, fluorescence of the excimer was observed during the initial one-shot laser irradiation. From the time-resolved fluorescence measurements, it was revealed that the appearance of the excimer was due to the transient melting of the glassy medium by the local temperature increase around Py, which was induced by the iterative reabsorption of the laser light by the S1 state. The time period of melting for allowing the translational diffusion of Py was limited in the time region ≤ ca. 10 ns. With an increase in laser exposure, the fluorescence intensity of the excimer increased concomitantly with the appearance and increase of the amount of Py dimer, which was also confirmed by steady-state absorption spectroscopy. Time-resolved fluorescence spectrum recorded by only the one-shot laser exposure did not show dimer emission. This suggested that the formation of the dimer was through the excimer produced by transient melting; its dissociation into monomers was prohibited in the highly viscous environment. Upon further increase in laser exposure (several 1000 shots), solidified Py was observed due to crystal formation/aggregation with the dimers as the nucleation species.

Femtosecond excited-state dynamics of fullerene-C60 nanoparticles in water.

Femtosecond excited-state dynamics of fullerene-C60 nanoparticles (nC60) having a mean size of 50 nm dispersed in pure water was studied by means of femtosecond transient absorption spectroscopy. The intermolecular charge-transfer (CT) excited state in solid C60 was directly and firstly observed by femtosecond 350 nm and 420 nm excitations, and its intrinsic lifetime of 0.35 ps was found. The CT excited state relaxed to the locally excited S1 state and excimers or directly to the ground state through geminate charge recombination. We also examined the laser fluence dependence of the CT excited-state dynamics. At a high laser fluence, the mutual interactions between neighboring CT excited states were observed immediately after the excitation. The interaction disappeared through the charge recombination in the geminate CT pair or between the neighboring CT excited states with a lifetime of 0.45 ps. After that, the locally excited S1 state decayed with a few ps lifetime independent of the fluence. In this paper, the mechanism and dynamics of the intermolecular CT excited state generated by UV light excitation is discussed in detail.

Photochemistry of fac-[Re(bpy)(CO)3Cl].

The photochemistry of fac-[Re(bpy)(CO)(3)Cl] (1 a; bpy=2,2'-bipyridine) initiated by irradiation using <330 nm light has been investigated. Isomerization proceeded in THF to give the corresponding mer-isomer 1 b. However, in the presence of a small amount of MeCN, the main product was the CO-ligand-substituted complex (OC-6-24)-[Re(bpy)(CO)(2) Cl(MeCN)] (2 c; bpy=2,2'-bipyridine). In MeCN, two isomers, 2 c and its (OC-6-34) form (2 a), were produced. Only 2 c thermally isomerized to produce the (OC-6-44) form 2 b. A detailed investigation led to the conclusion that both 1 b and 2 c are produced by a dissociative mechanism, whereas 2 a forms by an associative mechanism. A comparison of the ultrafast transient UV-visible absorption, emission, and IR spectra of 1 a acquired by excitation using higher-energy light (e.g., 270 nm) and lower-energy light (e.g., 400 nm) gave detailed information about the excited states, intermediates, and kinetics of the photochemical reactions and photophysical processes of 1 a. Irradiation of 1 a using the higher-energy light resulted in the generation of the higher singlet excited state with τ≤25 fs, from which intersystem crossing proceeded to give the higher triplet state ((3)HES(1)). In THF, (3)HES(1) was competitively converted to both the triplet ligand field ((3)LF) and metal-to-ligand charge transfer ((3)MLCT) with lifetimes of 200 fs, in which the former is a reactive state that converts to [Re(bpy)(CO)(2)Cl(thf)](+) (1 c) within 10 ps by means of a dissociative mechanism. Re-coordination of CO to 1 c gives both 1 a and 1 b. In MeCN, irradiation of 1 a by using high-energy light gives the coordinatively unsaturated complex, which rapidly converted to 2 c. A seven-coordinate complex is also produced within several hundred femtoseconds, which is converted to 2 a within several hundred picoseconds.

Photoswitchable fluorescent diarylethene derivatives with short alkyl chain substituents.

The sulfone derivatives of 1,2-bis(2-alkyl-6-phenyl-1-benzothiophen-3-yl)perfluorocyclopentene having various short alkyl chain substituents at reactive carbons were prepared and the effect of alkyl substitution on the fluorescence property of the closed-ring isomers was studied. Upon irradiation with ultraviolet (UV) light the derivatives exhibit a brilliant green fluorescence under irradiation with visible (> 400 nm) light, while the fluorescence disappears upon irradiation with visible (> 400 nm) light alone. The fluorescence quantum yield of the methyl substituted derivative (1b) dramatically decreases from 0.84 to 0.15 when the solvent is changed from hexane to acetonitrile, while the changes of ethyl, n-propyl and n-butyl substituted derivatives (2b)­(4b) are moderate. The quantum yields of (2b)­(4b) are kept to values close to 0.7 even in polar acetonitrile. The fluorescence lifetime measurement revealed that efficient non-radiative decay processes took place in (1b) in polar solvent, while their contribution to the deactivation was not so large in (2b)­(4b). The neighboring short alkyl chains at the connecting carbons are considered to defend the sulfone units against the attack of polar solvent molecules and weaken the solvent polarity effect.

Single-molecule fluorescence photoswitching of a diarylethene-perylenebisimide dyad: non-destructive fluorescence readout.

Single-molecule fluorescence photoswitching plays an essential role in ultrahigh-density (Tbits/inch(2)) optical memories and super-high-resolution fluorescence imaging. Although several fluorescent photochromic molecules and fluorescent proteins have been applied, so far, to optical memories and super-high-resolution imaging, their performance is unsatisfactory because of the absence of "non-destructive fluorescence readout capability". Here we report on a new molecular design principle of a molecule having non-destructive readout capability. The molecule is composed of acceptor photochromic diarylethene and donor fluorescent perylenebisimide units. The fluorescence is reversibly quenched when the diarylethene unit converts between the open- and the closed-ring isomers upon irradiation with visible and UV light. The fluorescence quenching is based on an electron transfer from the donor to the acceptor units. The fluorescence photoswitching and non-destructive readout capability were demonstrated in solution (an ensemble state) and at the single-molecule level. Femtosecond time-resolved transient and fluorescent lifetime measurements confirmed that the fluorescence quenching is attributed to the intramolecular electron transfer.

In situ preparation of highly fluorescent dyes upon photoirradiation.

Photoswitchable or photoactivatable fluorescent dyes are potentially applicable to ultrahigh density optical memory media as well as super-resolution fluorescence imaging when the dyes are highly fluorescent and have large absorption coefficients. Here, we report on highly fluorescent photochromic dyes, which are initially nonluminous in solution under irradiation with visible light but activated to emit green or red fluorescence upon irradiation with ultraviolet (UV) light. The dyes 5a-9a are sulfone derivatives of 1,2-bis(2-ethyl-6-phenyl(or thienyl)-1-benzothiophen-3-yl)perfluorocyclopentene. It was found that substitution of phenyl or thiophene rings at 6 and 6' positions of the benzothiophene-1,1-dioxide groups is effective to increase the fluorescence quantum yields of the closed-ring isomers over 0.7 and absorption coefficients over 4 × 10(4) M(-1) cm(-1). The phenyl-substituted derivatives 5a-7a undergo photocyclization reactions to produce yellow closed-ring isomers 5b-7b, which emit brilliant green fluorescence at around 550 nm (Φ(F) = 0.87-0.88) under irradiation with 488 nm light. Any absorption intensity change of the closed-ring isomers was not observed even after 100 h storage in the dark at 80 °C. The closed-ring isomers slowly returned to the initial open-ring isomers upon irradiation with visible (λ > 480 nm) light. The ring-opening quantum yields (Φ(C→O)) were measured to be (1.6-4.0) × 10(-4). When the phenyl substituents are replaced with thiophene rings, such as compounds 8a and 9a, the absorption bands of the closed-ring isomers shift to longer than 500 nm. The closed-ring isomers exhibit brilliant red fluorescences at around 620 nm (Φ(F) = 0.61-0.78) under irradiation with 532 nm light. The ring-opening reactions are very slow (Φ(C→O) < 1 × 10(-5)). The fluorescence lifetimes of these sulfone derivatives were measured to be around 2-3 ns, which is much longer than the value of the closed-ring isomer of 1,2-bis(2-methyl-1-benzothiophen-3-yl)perfluorocyclopentene (τ(F) = 4 and 22 ps). The closed-ring isomer 8b in 1,4-dioxane exhibits excellent fatigue resistant property under irradiation with visible light (λ > 440 nm) superior to the stability of Rhodamine 101 in ethanol.

Confinement of photopolymerization and solidification with radiation pressure.

Controlling chemical reactions within a small space is a significant issue in chemistry, and methods to induce reactions within a desired position have various potential applications. Here we demonstrate localized, efficient photopolymerization by radiation pressure. We induced a one-photon UV polymerization of liquid acrylate solutions in the optical-trapping potential of a focused near-IR (NIR) laser beam, leading to the confinement of solidification to a minute space with dimensions smaller or equal to one-fifth of the wavelength of the NIR laser. Our approach can produce solidification volumes smaller than those achievable with conventional one-photon polymerization, thus enabling the production of tiny polymeric structures that are smaller than the diffraction limit of the trapping light. This is the first demonstration of a radiation pressure effect on a photochemical reaction.

One-color reversible control of photochromic reactions in a diarylethene derivative: three-photon cyclization and two-photon cycloreversion by a near-infrared femtosecond laser pulse at 1.28 µm.

One-color control of colorization/decolorization reactions of diarylethene molecules was attained by using nonresonant high-order multiphoton absorption processes with a near-infrared (NIR) femtosecond laser pulse at 1.28 µm with 35 fs full width at half-maximum (fwhm). The intensity of a rather weak laser pulse (<1 nJ/pulse) can induce the simultaneous three-photon absorption leading to the colorization, while much weaker intensity induces two-photon absorption resulting in the decolorization. The spatial patterning concomitant with higher-order multiphoton absorption processes was also demonstrated.

Ion-pairing π-electronic systems: ordered arrangement and noncovalent interactions of negatively charged porphyrins.

In this study, charged π-electronic species are observed to develop stacking structures based on electrostatic and dispersion forces. i π- i π Interaction, defined herein, functions for the stacking structures consisting of charged π-electronic species and is in contrast to conventional π-π interaction, which mainly exhibits dispersion force, for electronically neutral π-electronic species. Establishing the concept of i π- i π interaction requires the evaluation of interionic interactions for π-electronic ion pairs. Free base (metal-free) and diamagnetic metal complexes of 5-hydroxy-10,15,20-tris(pentafluorophenyl)porphyrin were synthesized, producing π-electronic anions upon the deprotonation of the hydroxy unit. Coexisting cations in the ion pairs with porphyrin anions were introduced as the counter species of the hydroxy anion as a base for commercially available cations and as ion-exchanged species, via Na+ in the intermediate ion pairs, for synthesized π-electronic cations. Solid-state ion-pairing assemblies were constructed for the porphyrin anions in combination with aliphatic tetrabutylammonium (TBA+) and π-electronic 4,8,12-tripropyl-4,8,12-triazatriangulenium (TATA+) cations. The ordered arrangements of charged species, with the contributions of the charge-by-charge and charge-segregated modes, were observed according to the constituent charged building units. The energy decomposition analysis (EDA) of single-crystal packing structures revealed that electrostatic and dispersion forces are important factors in stabilizing the stacking of π-electronic ions. Furthermore, crystal-state absorption spectra of the ion pairs were correlated with the assembling modes. Transient absorption spectroscopy of the single crystals revealed the occurrence of photoinduced electron transfer from the π-electronic anion in the charge-segregated mode.

Multiphoton-gated cycloreversion reactions of photochromic diarylethene derivatives with low reaction yields upon one-photon visible excitation.

Cycloreversion processes of three photochromic diarylethene derivatives with extremely low one-photon reaction yields (5.0 x 10(-5) to 1.5 x 10(-2)) were investigated by means of femtosecond and picosecond laser photolysis methods. Femtosecond visible laser photolysis revealed that the excited state of the closed form in these three derivatives decayed into the ground state with 0.7-8 ps time constants and with low cycloreversion yields that were consistent with those obtained by steady-state light irradiation. On the other hand, the cycloreversion reaction was drastically enhanced by picosecond 532 nm laser excitation for all of the three derivatives. From excitation intensity effects of the reaction yield and dynamic behavior, it was found that the successive two-photon absorption process leading to higher excited states opened an efficient cycloreversion channel, with reaction yields of 0.3-0.5. These results are discussed from the viewpoint of the one-photon inerasable but two-photon erasable photochromic system.

Fluorescence photoswitching of a diarylethene-perylenebisimide dyad based on intramolecular electron transfer.

A fluorescent photochromic molecule, which is composed of a photochromic diarylethene (DE) and a fluorescent perylenebisimide (PBI), was synthesized and its fluorescence photoswitching was studied. The fluorescence quantum yield of the open-ring isomer is constant irrespective of solvent polarity, while that of the closed-ring isomer decreases with an increase in the dielectric constant of solvents. Femtosecond time-resolved transient and fluorescence lifetime measurements revealed that the fluorescence quenching of the closed-ring isomer is attributed to the intramolecular electron transfer from the PBI chromophore to the DE unit.

Coherent dynamics and ultrafast excited state relaxation of blue copper protein; plastocyanin.

Ultrafast transient absorption measurements in the femtosecond to picosecond time region were carried out for a blue copper protein, plastocyanin (Pc). To compare the dynamical profiles after photoexcitation upon the ligand-to-metal-charge-transfer (LMCT) band and the d-d transition band, the pump wavelength was set at wavelengths of 597 and 895 nm, respectively. The results were nearly identical, indicating that the transition from the LMCT to the lower ligand field (LF) states takes place in an ultrafast time regime of less than 40 fs. Subsequently, relaxation in the LF state occurs with a time constant of 90 fs and the system returns to the ground state with that of 250 fs. The longest time constant of 1.8 ps was attributed to the vibrational cooling in the ground state. Several wavepacket motions were observed, including Franck-Condon type motion at approximately 510 nm and a Herzberg-Teller type motion at 660-720 nm. Critically damped low-frequency oscillation of approximately 30 cm(-1) was also observed with both excitation wavelengths with the strongest amplitude around 600 nm. This oscillation could be due to the motion of the protein that is ballistically stimulated by ultrafast relaxation.

Ultrafast delocalization of cationic states in poly(N-vinylcarbazole) solid leading to carrier photogeneration.

Femtosecond measurements of the transient dichroism and near-IR time-resolved spectra revealed the ultrafast delocalization of the cationic state in poly(N-vinylcarbazole), leading to carrier photogeneration.

Nanosecond laser photothermal effect-triggered amplification of photochromic reactions in diarylethene nanoparticles.

Intense ns pulse laser excitation to nanoparticle colloids of a photochromic diarylethene induced an amplified cycloreversion reaction. The mechanism was explained as a 'photosynergetic response' coupled with nanoscale laser heating and the photochemical reaction in nanoparticles.

Ultrafast photodissociation dynamics of a hexaarylbiimidazole derivative with pyrenyl groups: dispersive reaction from femtosecond to 10 ns time regions.

The photodissociation dynamics of a hexaarylbiimidazole (HABI) derivative with two pyrenyl groups was investigated by time-resolved transient absorption spectroscopy and fluorescence measurements. Transient absorption spectroscopy revealed that photodissociation took place in the wide time region of <100 fs to 10 ns. On the other hand, fluorescence time profiles showed the dynamic red shift in the time region <100 ps. The apparent dispersive photodissociation process was attributed to the increase in the interaction between the pyrenyl moiety in the excited state and the other moiety in the ground state, resulting in the gradual increase of the activation energy for the crossing between the attractive potential surface of an excited pyrenyl unit and the repulsive potential surface.

Ultrafast charge transfer process of 9,9'-bianthryl in imidazolium ionic liquids.

Excited state dynamics of 9,9'-bianthryl (BA) in 1-butyl-3-methylimidazolium ionic liquids with three different types of anions was investigated by means of femtosecond to nanosecond transient absorption (TA) spectroscopy and time-resolved fluorescence (TRF) measurements. TA spectroscopy revealed that charge transfer (CT) occurred multiexponentially in the time scale of hundreds of picoseconds while TRF measurement revealed that red-shift of the BA fluorescence peak extended into the nanosecond regime. It is concluded that an energy relaxation process slower than the CT reaction, which could be the solvation dynamics in the product state, is observed.

Efficient Cycloreversion Reaction of a Diarylethene Derivative in Higher Excited States Attained by Off-Resonant Simultaneous Two-Photon Absorption.

Off-resonant excitation of the closed-ring isomer of a photochromic diarylethene derivative at 730 nm induced the efficient cycloreversion reaction with a yield of ∼20%, while the reaction yield was only 2% under one-photon excitation at 365 nm. Excitation wavelength dependence of the one-photon cycloreversion reaction yield under steady-state irradiation in a wide wavelength range showed that the specific electronic state leading to the large cycloreversion reaction yield, which is originally forbidden in the optical transition but partially allowed owing to the low symmetry of the molecule, is spectrally overlapped with the electronic state accessible by the allowed one-photon optical transition in the UV region. Femtosecond transient absorption spectroscopy also revealed that the off-resonant two-photon excitation preferentially pumped the molecule into the specific state, leading to the 10-fold enhancement of the cycloreversion reaction.

Femtosecond Pump-Probe Microspectroscopy of Single Perylene Nanoparticles.

We have developed a femtosecond pump-probe light scattering microspectroscopic system in which the output of a femtosecond Ti:sapphire oscillator (1 W, 82 MHz) was used as a light source; the pump light is the second harmonics (395 nm) of the laser output, and the probe light is a femtosecond white-light continuum (490-900 nm) generated with a photonic crystal fiber. Detection of the backscattered light from single nanoparticle on a glass substrate allowed us to obtain higher gain of the transient signals by ∼20 times in comparison with the conventional transmittance-mode experiment. This high-sensitivity of the backscattering detection makes it possible to examine ultrafast relaxation dynamics of excited states in organic nanoparticles, which, in general, are lower photodurability than the inorganic one. We applied the system to single nanocrystals of α-form perylene and then succeeded in direct observation of the excimer formation dynamics on a picosecond time scale. Single nanoparticle measurements for the perylene nanocrystals having a size range of 100 to 500 nm suggested that the excimer formation time became short from 2 ps to <0.3 ps for decreasing of the size.