Conformational Relaxation Dynamics of Poly(3-hexylthiophene) Photoexcited in Solution as Studied by Femtosecond Time-Resolved Stimulated Raman Spectroscopy in 1190-1550 nm Region.

When a conjugated polymer is photoexcited in solution, its effective conjugation length in the singlet exciton state often increases through the conformational relaxation of the polymer main chain and/or hopping of the excitation. We measured femtosecond time-resolved near-IR stimulated Raman spectra of poly(3-hexylthiophene) (P3HT) photoexcited in four organic solvents for understanding the dynamics of the exciton elongation through the conformational relaxation separately from that through the exciton hopping. In the ring CC stretch frequency region, a band appears at around 1415 cm-1 and decays, while a new band rises at around 1370 cm-1. The average time constant of the change is estimated to be 8.7-19 ps and correlated almost linearly with the viscosity of the solvents. These results suggest that the main chain of P3HT in the singlet exciton state relaxes from a twisted form to a planar form in the 0-100 ps range when it surmounts an activation barrier of 5.8-7.8 kJ mol-1, generated possibly by the steric effect of the hexyl side group. When the rise of the 1370 cm-1 band is analyzed in detail, it is reproduced with two exponential rise functions with time constants of 0-3.3 and 16-22 ps. The two rise components suggest that a portion of P3HT forms a cluster in solution, while the other portion of P3HT is isolated.

Energy Transfer Characteristics of Lipid Bilayer Membranes of Liposomes Examined with Picosecond Time-Resolved Raman Spectroscopy.

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.

Quest for a Rational Molecular Design of Alkyl-Distyrylbenzene Liquid by Substitution Pattern Modulation.

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 (C10 C14 ), and 2-dodecylhexadecyl at the (2,5-) substituent position. The 2,5-C10 C14 substituent pattern constructed a superior alkyl-DSB liquid with a lower viscosity, intrinsic optical properties, and high thermal- and photo-stabilities. The discovered 2,5-C10 C14 was 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.

Dynamics of electron ejection on photoionization of trans-stilbene and biphenyl in acetonitrile as observed with femtosecond time-resolved near-IR absorption spectroscopy.

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.

Direct estimation of NIR reflection spectra utilizing a snapshot-type spectrometer with photonic crystal multi-spectral filter arrays.

We fabricated 16-, 25-, 36-, and 64-channel distributed passband-type multi-spectral filter arrays by utilizing a multilayer-type photonic crystal and integrated them onto a CCD to form a snapshot-type spectroscopic sensor. Reflection spectra from target objects (fruits) under broadband light illumination were estimated directly using the Wiener estimation method. A root mean square error of the reflectivity on the order of 2∼5% was obtained under optical shot noise with 6×6 pixel binning. A number of constituent filters of 36 was sufficient for this type of fruit spectral measurement. We also visualized reflection images at specified wavelengths by applying the estimation method to a multiple filter region on the sensor.

Ultrafast Time-resolved Near-IR Stimulated Raman Measurements of Functional π-conjugate Systems.

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 ß-carotene in toluene solution. A C=C stretch band of ß-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.

Soft chromophore featured liquid porphyrins and their utilization toward liquid electret applications.

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.

Direct Observation of Structure and Dynamics of Photogenerated Charge Carriers in Poly(3-hexylthiophene) Films by Femtosecond Time-Resolved Near-IR Inverse Raman Spectroscopy.

The initial charge separation process of conjugated polymers is one of the key factors for understanding their conductivity. The structure of photogenerated transients in conjugated polymers can be observed by resonance Raman spectroscopy in the near-IR region because they exhibit characteristic low-energy transitions. Here, we investigate the structure and dynamics of photogenerated transients in a regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend film, as well as in a pristine P3HT film, using femtosecond time-resolved resonance inverse Raman spectroscopy in the near-IR region. The transient inverse Raman spectrum of the pristine P3HT film at 50 ps suggests coexistence of neutral and charged excitations, whereas that of the P3HT:PCBM blend film at 50 ps suggests formation of positive polarons with a different structure from those in an FeCl3-doped P3HT film. Time-resolved near-IR inverse Raman spectra of the blend film clearly show the absence of charge separation between P3HT and PCBM within the instrument response time of our spectrometer, while they indicate two independent pathways of the polaron formation with time constants of 0.3 and 10 ps.

Control of Electron Flow Direction in Photoexcited Cycloplatinated Complex Containing Conjugated Polymer-Single-Walled Carbon Nanotube Hybrids.

Conjugated polymers incorporated with cycloplatinated complexes (P1-Pt and P2-Pt) were used as dispersants for single-walled carbon nanotubes (SWCNTs). Significant changes in the UV-vis absorption spectra were observed after the formation of the polymer/SWCNT hybrids. Molecular dynamics (MD) simulations revealed the presence of a strong interaction between the cycloplatinated complex moieties and the SWCNT surface. The photoinduced electron transfer processes in these hybrids were strongly dependent on the type of the comonomer unit. Upon photoexcitation, the excited P1-Pt donates electrons to the SWCNT, while P2-Pt accepts electrons from the photoexcited SWCNT. These observations were supported by results from Raman and femtosecond time-resolved transient absorption spectroscopy experiments. The strong electronic interaction between the Pt complexes and the SWCNT gives rise to a new hybrid system that has a controllable photoinduced electron transfer flow, which are important in regulating the charge transport processes in SWCNT-based optoelectronic devices.

Experimental and theoretical investigation of fluorescence solvatochromism of dialkoxyphenyl-pyrene molecules.

We investigated the fluorescence properties of dialkoxyphenyl-pyrene molecules experimentally as well as theoretically. Our experiments confirmed fluorescence solvatochromism in 2,5-dimethoxyphenyl-pyrene and, in contrast there was no significant solvent-effect on the emission properties of the isomers, 3,5- and 2,6-dimethoxyphenyl-pyrene. This clear difference in the solvent-dependence would reflect the difference in character of the excited-state between the isomers, which differ only in the substitution positions of the two methoxy groups. The positional effects of the di-substituted molecules are successfully explained theoretically by the topologies of the highest occupied molecular orbital of the phenyl group that are governed by the relative positions of the two substituents, though it is somewhat contradictory to the meta-effect for the mono-substituted molecules. Theoretical calculations were also used to analyze the character of the excited states; 2,5-dimethoxyphenyl-pyrene alone exhibited an intramolecular charge transfer character for the excited state, which was responsible for the solvatochromism effect. The dynamics of the excited states were analyzed using time-resolved fluorescence measurements, in which a characteristic increase of the fluorescence intensity was observed for 2,5-dialkoxyphenyl-pyrene; this observation was supported by the theoretical calculations as well.

Vibrational relaxation dynamics of ß-carotene and its derivatives with substituents on terminal rings in electronically excited states as studied by femtosecond time-resolved stimulated Raman spectroscopy in the near-IR region.

The electronic and vibrational relaxation of carotenoids is one of the key processes in the protection of living cells as well as in the functions of proteins involved in photosynthesis. In this study, the electronic and vibrational relaxation dynamics of ß-carotene and its derivatives with substituents on the terminal rings is investigated using femtosecond time-resolved absorption and stimulated Raman spectroscopy in the near-IR region. The carbonyl substituent induces low-frequency shifts of the steady-state and transient absorption bands, decreases of the excited-state lifetimes and the acceleration of vibrational relaxation of the conjugated main chain, whereas the hydroxyl substituent only slightly affects them. The effects of the carbonyl group in the electronic relaxation dynamics are explained well by the lengthening of effective conjugation by the carbonyl group through a partial conjugation between the main chain and the terminal ring. Time-resolved near-IR stimulated Raman spectroscopy demonstrates the significance of the peripheral substitution with the carbonyl group for the vibrational energy relaxation of ß-carotene derivatives in the lowest excited singlet state.

The impact of metal complex lipids on viscosity and curvature of hybrid liposomes.

A morphology transformation of hybrid liposomes was shown to occur from spherical vesicles to tubular micelles when increasing the ratio of the metal complex lipid present. Phase transition temperatures increased while viscosities decreased, indicating that the hybrids exhibit stronger interaction between heads but weaker interaction between alkyl chains than occurs in pristine liposomes.

A Guide to Design Functional Molecular Liquids with Tailorable Properties using Pyrene-Fluorescence as a Probe.

Solvent-free, nonvolatile, room-temperature alkylated-π functional molecular liquids (FMLs) are rapidly emerging as a new generation of fluid matter. However, precision design to tune their physicochemical properties remains a serious challenge because the properties are governed by subtle π-π interactions among functional π-units, which are very hard to control and characterize. Herein, we address the issue by probing π-π interactions with highly sensitive pyrene-fluorescence. A series of alkylated pyrene FMLs were synthesized. The photophysical properties were artfully engineered with rational modulation of the number, length, and substituent motif of alkyl chains attached to the pyrene unit. The different emission from the excimer to uncommon intermediate to the monomer scaled the pyrene-pyrene interactions in a clear trend, from stronger to weaker to negligible. Synchronously, the physical nature of these FMLs was regulated from inhomogeneous to isotropic. The inhomogeneity, unexplored before, was thoroughly investigated by ultrafast time-resolved spectroscopy techniques. The result provides a clearer image of liquid matter. Our methodology demonstrates a potential to unambiguously determine local molecular organizations of amorphous materials, which cannot be achieved by conventional structural analysis. Therefore this study provides a guide to design alkylated-π FMLs with tailorable physicochemical properties.

Development of a femtosecond time-resolved near-IR multiplex stimulated Raman spectrometer in resonance with transitions in the 900-1550 nm region.

Charge transfer and charge delocalisation processes play key roles in the functions of large biomolecular systems and organic/inorganic devices. Many of the short-lived transients involved in these processes can be sensitively detected by monitoring their low-energy electronic transitions in the near-IR region. Ultrafast time-resolved near-IR Raman spectroscopy is a promising tool for investigating the structural dynamics of the short-lived transients as well as their electronic dynamics. In this study, we have developed a femtosecond time-resolved near-IR multiplex stimulated Raman spectrometer using the Raman pump pulse at 1190 nm and a broadband probe pulse covering the 900-1550 nm region. Spectral and temporal instrument responses of the spectrometer are estimated to be 5 cm(-1) and 120 fs, respectively. Time-resolved near-IR stimulated Raman spectra of poly(3-dodecylthiophene) (P3DDT) are recorded in toluene solution for investigating its structural changes following the photoexcitation. The spectra strongly indicate conformational changes of P3DDT in excited states associated with the elongation of its effective conjugation length. The results on P3DDT fully demonstrate the effectiveness of the newly developed femtosecond time-resolved near-IR stimulated Raman spectrometer.

Relaxation mechanism of ß-carotene from S2 (1Bu(+)) state to S1 (2Ag(-)) state: femtosecond time-resolved near-IR absorption and stimulated resonance Raman studies in 900-1550 nm region.

Carotenoids have two major low-lying excited states, the second lowest (S2 (1Bu(+))) and the lowest (S1 (2Ag(-))) excited singlet states, both of which are suggested to be involved in the energy transfer processes in light-harvesting complexes. Studying vibrational dynamics of S2 carotenoids requires ultrafast time-resolved near-IR Raman spectroscopy, although it has much less sensitivity than visible Raman spectroscopy. In this study, the relaxation mechanism of ß-carotene from the S2 state to the S1 state is investigated by femtosecond time-resolved multiplex near-IR absorption and stimulated Raman spectroscopy. The energy gap between the S2 and S1 states is estimated to be 6780 cm(-1) from near-IR transient absorption spectra. The near-IR stimulated Raman spectrum of S2 ß-carotene show three bands at 1580, 1240, and 1050 cm(-1). When excess energy of 4000 cm(-1) is added, the S1 C═C stretch band shows a large upshift with a time constant of 0.2 ps. The fast upshift is explained by a model that excess energy generated by internal conversion from the S2 state to the S1 state is selectively accepted by one of the vibronic levels of the S1 state and is redistributed among all the vibrational modes.

Two different charge transfer states of photoexcited 9,9'-bianthryl in polar and nonpolar solvents characterized by nanosecond time-resolved near-IR spectroscopy in the 4500-10,500 cm(-1) region.

Transient absorption spectra of 9,9'-bianthryl (BA) in heptane, in acetonitrile, and in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (bmimTf(2)N) are observed with a nanosecond time-resolved near-IR absorption spectrometer for the wavenumber range of 4500-10,500 cm(-1) (2200-950 nm). In nonpolar heptane solution, a broad absorption band is observed at 6700 cm(-1) (1500 nm), in addition to a strong absorption band of the locally excited (LE) state centered at 9800 cm(-1) (1020 nm). The broad band is assigned to a partial charge transfer (PCT) band. The decay time constants of the PCT band and the LE band are both (13 +/- 1) ns. The agreement of the two decay constants strongly suggests that the PCT state is in equilibrium with the LE state in heptane. In acetonitrile, an absorption band of the charge transfer (CT) state is observed at 8000 cm(-1) (1250 nm). This band decays in (41 +/- 2) ns. In bmimTf(2)N, the CT band appears at 8500 cm(-1) (1180 nm) and decays in (34 +/- 1) ns. The difference in peak position for the CT bands in acetonitrile and in bmimTf(2)N, and the PCT bands in heptane, is explained well by the model based on the charge resonance between the two equivalent electronic structures of the CT state.

Femtosecond time-resolved absorption anisotropy spectroscopy on 9,9'-bianthryl: detection of partial intramolecular charge transfer in polar and nonpolar solvents.

Femtosecond time-resolved absorption anisotropy spectroscopy by multichannel detection has been developed. The charge transfer (CT) character and dynamics of the UV-photoexcited 9,9(')-bianthryl (BA) in heptane, acetonitrile, and ethanol are revealed with this method. The transient absorption spectra are decomposed into two absorption components with different anisotropy values by the absorption anisotropy spectra. The decomposition results show two absorption bands having different anisotropy values or different directions of the transition dipole moment. One band that has the transition dipole perpendicular to the central C-C bond has almost an identical spectral shape with transient absorption of anthracene in the singlet excited state. This band is assigned to a transition in a locally excited anthracene ring. The other band is broad and structureless. This band is assigned to partial charge transfer (PCT) absorption because its transition dipole moment is parallel to the central C-C bond. Because the PCT band is observed in a nonpolar solvent heptane as well as in polar solvents, the PCT occurs in both nonpolar and polar solvents. The PCT band rises within the instrumental response, indicating that the PCT takes place immediately after the photoexcitation. In acetonitrile, the CT component shows a significant blueshift, indicating the formation of the stabilized CT state from the PCT state. In ethanol, the CT band does not show a spectral shift, suggesting that the stabilization is smaller than in acetonitrile. From these results, a new kinetic model on the intramolecular CT in BA is discussed.

UV excitation of single DNA and RNA strands produces high yields of exciplex states between two stacked bases.

Excited electronic states created by UV excitation of the diribonucleoside monophosphates ApA, ApG, ApC, ApU, and CpG were studied by the femtosecond transient-absorption technique. Bleach recovery signals recorded at 252 nm show that long-lived excited states are formed in all five dinucleosides. The lifetimes of these states exceed those measured in equimolar mixtures of the constituent mononucleotides by one to two orders of magnitude, indicating that electronic coupling between proximal nucleobases dramatically slows the relaxation of excess electronic energy. The decay rates of the long-lived states decrease with increasing energy of the charge-transfer state produced by transferring an electron from one base to another. The charge-transfer character of the long-lived states revealed by this analysis supports their assignment to excimer or exciplex states. Identical bleach recovery signals were seen for ApA, (A)(4), and poly(A) at delay times >10 ps after photoexcitation. This indicates that excited states localized on a stack of just two bases are the common trap states independent of the number of stacked nucleotides. The fraction of initial excitations that decay to long-lived exciplex states is approximately equal to the fraction of stacked bases determined by NMR measurements. This supports a model in which excitations associated with two stacked bases decay to exciplex states, whereas excitations in unstacked bases decay via ultrafast internal conversion. These results establish the importance of charge transfer-quenching pathways for UV-irradiated RNA and DNA in room-temperature solution.

A new class of intramolecularly hydrogen-bonded dendrons based on a 2-methoxyisophthalamide repeat unit.

The synthesis and conformational properties of folded dendrons based on a 2-methoxyisophthalamide (2-OMe-IPA) repeat unit are described. The hydrodynamic properties of dendrons preorganized via the syn-syn conformational preference of 2-methoxyisophthalamide are compared with 2,6-pyridinedicarboxamide (2,6-pydic) analogues. The effect of subtle differences in the nature of the conformational equilibria that exist within the 2-OMe-IPA and 2,6-pydic repeat units on the global structural properties of the corresponding dendrons was explored computationally, by (1)H-DOSY NMR spectroscopy and time-resolved fluorescence anisotropy (TRFA) measurements. Whereas the syn-syn preference of the 2-OMe-IPA branched repeat unit is stabilized entirely by intramolecular hydrogen-bonding interactions, this preference in the 2,6-pydic system is a consequence of both intramolecular hydrogen-bonding and dipole minimization effects. However, nonspecific solvophobic compression is more important in determining hydrodynamic properties than solvent-dependent shifts in the conformational equilibria of the repeat unit for both dendron series.

Charge resonance character in the charge transfer state of bianthryls: effect of symmetry breaking on time-resolved near-IR absorption spectra.

We study the effects of symmetry breaking on the photogenerated intramolecular charge transfer (CT) state of 9,9'-bianthryl (BA) with femtosecond time-resolved near-IR spectroscopy. The time-resolved near-IR spectra are measured in acetonitrile for a symmetric substituted derivative of 10,10'-dicyano-9,9'-bianthryl (DCBA) and asymmetric substituted derivatives of 10-cyano-9,9'-bianthryl (CBA) and 9-(N-carbazolyl)anthracene (C9A), as well as nonsubstituted BA. The transient near-IR absorption spectrum of each compound at 0 ps has a locally excited (LE) absorption band, which agrees with the transient absorption band of the corresponding monomer unit. At 3 ps after the photoexcitation, the symmetric compounds show a broad charge transfer (CT) absorption band, whereas no absorption peak appears in the spectra of the asymmetric compounds. The broad CT absorption at 1250 nm only observed for the symmetric compounds can be attributed to the charge resonance transition associated with two equivalent charge separated states.