Tomohisa Takaya

When a conjugated polymer is photoexcited in solution,its effectiveconjugation length in the singlet exciton state often increases throughthe conformational relaxation of the polymer main chain and/or hoppingof the excitation. We measured femtosecond time-resolved near-IR stimulatedRaman spectra of poly(3-hexylthiophene) (P3HT) photoexcited in fourorganic solvents for understanding the dynamics of the exciton elongationthrough the conformational relaxation separately from that throughthe exciton hopping. In the ring CC stretch frequency region, a bandappears at around 1415 cm(-1) and decays, while anew band rises at around 1370 cm(-1). The averagetime constant of the change is estimated to be 8.7-19 ps andcorrelated almost linearly with the viscosity of the solvents. Theseresults suggest that the main chain of P3HT in the singlet excitonstate relaxes from a twisted form to a planar form in the 0-100ps range when it surmounts an activation barrier of 5.8-7.8kJ mol(-1), generated possibly by the steric effectof the hexyl side group. When the rise of the 1370 cm(-1) band is analyzed in detail, it is reproduced with two exponentialrise functions with time constants of 0-3.3 and 16-22ps. The two rise components suggest that a portion of P3HT forms acluster in solution, while the other portion of P3HT is isolated.

A number of biochemical reactions proceed inside biomembranes. Since the rate of a chemical reaction is influenced by chemical properties of the surrounding environment, it is important to examine the chemical environment inside the biomembranes. Although the energy transfer characteristics are a basic and important property of a reaction medium, experimental investigation of the thermal conducting capabilities of the biomembranes is a challenging task. We have examined the energy transfer characteristics of lipid bilayer membranes of liposomes, a good model system for the biomembrane, with picosecond time-resolved Raman spectroscopy. The cooling kinetics of the first excited singlet (S₁) state of trans-stilbene solubilized within the lipid bilayer membranes is observed as a peak shift of the 1570 cm^-1 Raman band of S₁ trans-stilbene. The cooling rate constant of S₁ trans-stilbene is obtained in six lipid bilayer membranes formed by phospholipids with different hydrocarbon chains, DSPC, DPPC, DMPC, DLPC, DOPC, and egg-PC. We estimate the thermal diffusivity of the lipid bilayer membranes with a known correlation between the cooling rate constant and the thermal diffusivity of the solvent. The thermal diffusivity estimated for the liquid-crystal-phase lipid bilayer membranes is 8.9 × 10^-8 to 9.4 × 10^-8 m² s^-1, while that for the gel-phase lipid bilayer membranes is 8.4 × 10^-8 to 8.5 × 10^-8 m² s^-1. The difference in thermal diffusivity between the two phases is explained by a one-dimensional diffusion equation of heat.

Achieving the lowest phase transition temperature with minimal chemical modification in highly crystalline π‐conjugated molecules is a universal problem in related research fields. This paper reports room‐temperature liquefication of diphenylpyrene isomers by introducing bulky yet flexible branched alkyl chains through molecular desymmetrization. Six isomers with different symmetries depending on the positions of the phenyl groups and alkyl groups were synthesized, and three of the isomers were found to be liquids at 25 °C, a state in which they have remained for more than five years. Although it is generally believed that the lower the symmetry of a molecule, the less likely it is to crystallize, one molecule with a relatively high molecular symmetry unexpectedly did not crystallize, which was evidenced by the kinetic inhomogeneity of this amorphous material (practically stable liquid) assessed by rheological analysis.

Alkyl‐π functional molecular liquids (FMLs) are of interest for fabricating soft electronic devices due to their fluidic nature and innate optoelectronic functions from the π‐conjugated moiety. However, predictable development of alkyl‐π FMLs with the desired liquid and optoelectronic properties is challenging. A series of alkyl‐distyrylbenzene (DSB) liquids was studied in terms of the substituent position effect by attaching 2‐octyldodecyl chains at (2,4‐), (2,5‐), (2,6‐), and (3,5‐). The effect of the alkyl chain length was investigated by attaching 2‐hexyldecyl, 2‐decyltetradecyl (C₁₀C₁₄), and 2‐dodecylhexadecyl at the (2,5‐) substituent position. The 2,5‐C₁₀C₁₄substituent pattern constructed a superior alkyl‐DSB liquid with a lower viscosity, intrinsic optical properties, and high thermal‐ and photo‐stabilities. The discovered 2,5‐C₁₀C₁₄was applied to dicyanostyrylbenzene and comparable liquid physical and optical superiorities were confirmed. This molecular design is useful for creating alkyl‐π FMLs with the aforementioned advantages, which are applicable for deformable and flowable optoelectronic devices.

Photoionization in solution is a basic but complex phenomenon involving a solute, an ejected electron and surrounding solvent molecules. It may seem obvious that an electron is released immediately after the parent molecule is excited to an electronic state that directly leads to the electron dissociation. However, it has been reported that the radical cations are formed in 17 ps, 24 ps, and 38 ps for trans-stilbene and 20 ps for biphenyl, based on time-resolved Raman and visible absorption measurements. For understanding this intriguing phenomenon, we observe the solvation process of electrons ejected from trans-stilbene and biphenyl with femtosecond time-resolved near-IR spectroscopy covering 900 to 1550 nm. We find that the near-IR absorption signals of the ejected electrons rise in 0.28 ± 0.01 ps for trans-stilbene and 0.33 ± 0.04 ps for biphenyl. The parent molecules release electrons in about 0.3 ps, not in a few tens of picoseconds, after the photoirradiation. The delayed appearance of the radical cation signals strongly suggests that the radical cation is formed initially in a highly excited state, electronically and vibrationally, that would not give a clear signal of Raman or absorption transitions. It then relaxes to the radical ground state in a few tens of picoseconds. We clarify the electron dissociation process associated with the photoionization of aromatic molecules with fast time-resolved spectroscopy.

Picosecond time‐resolved Raman spectroscopy is an experimental method effective for studying the structure and dynamics of short‐lived transient species. Spectral resolution of less than 10 cm⁻¹ and high stability of the laser light source are both critically important for extracting detailed information on the structure and dynamics of the transient species from the observed picosecond time‐resolved Raman spectra. However, it is often technically challenging to obtain stable picosecond pulses that have a spectral width of 10 cm⁻¹ or smaller, which is only achieved if the time‐energy product of a light pulse is close to the Fourier‐transform limit, from a picosecond light source. We present a new scheme for a picosecond time‐resolved Raman spectrometer with a stable femtosecond laser as its light source. Femtosecond light pulses from the laser are successfully converted to stable picosecond light pulses with a volume‐grating notch filter. The spectral and temporal widths of the prepared picosecond pulse are 6.0 cm⁻¹ and 3.2 ps, respectively, close to the Gaussian Fourier‐transform limit. The vibrational cooling dynamics of S₁ trans‐stilbene prepared with the photoexcitation at 316 and 266 nm has been observed with the newly developed picosecond time‐resolved Raman spectrometer, demonstrating a high capability of the proposed method.

Femtosecond time-resolved stimulated Raman spectroscopy is a promising method of observing the structural dynamics of short-lived transients with near infrared (near-IR) transitions, because it can overcome the low sensitivity of spontaneous Raman spectrometers in the near-IR region. Here, we describe technical details of a femtosecond time-resolved near-IR multiplex stimulated Raman spectrometer that we have recently developed. A description of signal generation and optimization, measurement, data acquisition, and calibration and correction of recorded data is provided as well. We present an application of our spectrometer to analyze the excited-state dynamics of β-carotene in toluene solution. A C=C stretch band of β-carotene in the second lowest excited singlet (S₂) state and the lowest excited singlet (S₁) state is clearly observed in the recorded time-resolved stimulated Raman spectra. The femtosecond time-resolved near-IR stimulated Raman spectrometer is applicable to the structural dynamics of π-conjugate systems from simple molecules to complex materials.

Optoelectronically active viscous liquids are ideal for fabricating foldable/stretchable electronics owing to their excellent deformability and predictable π-unit–based optoelectronic functions, which are independent of the device shape and geometry. Here we show, unprecedented ‘liquid electret’ devices that exhibit mechanoelectrical and electroacoustic functions, as well as stretchability, have been prepared using solvent-free liquid porphyrins. The fluidic nature of the free-base alkylated-tetraphenylporphyrins was controlled by attaching flexible and bulky branched alkyl chains at different positions. Furthermore, a subtle porphyrin ring distortion that originated from the bulkiness of alkyl chains was observed. Its consequences on the electronic perturbation of the porphyrin-unit were precisely elucidated by spectroscopic techniques and theoretical modelling. This molecular design allows shielding of the porphyrin unit by insulating alkyl chains, which facilitates its corona-charged state for a long period under ambient conditions.

Sensing systems of nonpolar gas molecules without functional groups such as natural gas and liquefied petroleum gas (LPG) remain difficult to develop because of lacking selective detection of such molecules over other gas molecules. Here we report a supramolecular fluorescence sensor for LPG using a 2-nm-sized cube-shaped molecular container i.e. a nanocube self-assembled from six molecules of gear-shaped amphiphiles (GSA) in water. The nanocube selectively encapsulates LPG, while it does not bind other gas molecules. Upon encapsulation of LPG in the nanocube, the fluorescence from the nanocube is enhanced by 3.9 times, which is caused by the restricted motion of the aromatic rings of GSA in the nanocube based on aggregation-induced emission. Besides the high selectivity, high sensitivity, quick response, high stability of the nanocube for LPG, and easy preparation of GSA satisfy the requirement for its practical use for an LPG sensor.